You know how your phone sometimes freezes when you really need it to work? Well, imagine that, but with your car’s brakes. That’s apparently what some owners of Xiaomi’s shiny new SU7 electric sedan are experiencing, and folks, it’s not great.
For those who haven’t been following the “tech companies building cars” trend, smartphone manufacturer Xiaomi jumped into the EV game with their first car earlier this year, the SU7. It launched with some genuinely impressive specs for the top trim model, the SU7 Max. It boasts up to 673 horsepower, and can sprint to 60 mph in under 3 seconds. Xiaomi made a big fuss about its performance credentials, boasting it could outperform the Porsche Taycan Turbo in many respects.
Here’s the thing about performance cars—people expect them to perform. The problem with the Xiaomi SU7 is that, when it comes to track use, it might not have the stopping power to match its hefty amount of go.
Hold Up
Since the SU7 launched earlier this year, a number of owners and enthusiasts have been putting the vehicle through its paces. While it’s the debut vehicle for Xiaomi, the SU7 is built under contract by BAIC, a Chinese manufacturer with many decades of experience. Thus, you would expect a certain basic level of quality—that the vehicle would go and stop as commanded. Indeed, the vehicle has won many fans, including Ford CEO Jim Farley himself.
[Ed Note: Per our Chinese car-market expert, Tycho, BAIC wasn’t actually involved in the development. From Tycho:
Xiaomi: BAIC is not the manufacturer. It is complicated. Initially, Xiaomi didn’t have a car-producing license, so they piggybacked on BAIC’s license. This is allowed in China for start-up automakers. That usually means that the start-up pays a fee per car produced to the established automaker. BAIC has not actually been involved in making the SU7. The factory is, and has always been, fully owned and operated by Xiaomi. Earlier this year, Xiaomi got its own car-production license so it doesn’t have any ties with BAIC anymore.
Interesting stuff! -DT]
Few doubt the SU7’s ability to fling itself down the road at great velocity. It’s plenty capable of fast launches, a party trick shared with many other luxury performance EVs. However, the problems appear to crop up when the SU7 is tested more broadly as a sports car during actual track use.
Videos of an on-track crash first surfaced in April. A Xiaomi SU7 Max was driven at the Shanghai Tianma circuit, with influencer Tang Zhu Liao Che (堂主撩車) at the wheel. In a video of the incident, we see that as a tight left approaches, the car goes straight on instead of turning, barely losing speed as it plows into the barriers. The crash had occurred after several hot laps of the circuit.
The crash in question. You can watch the full original Chinese video—which racked up over 13 million views—here. Embedded player removed due to autoplay.
The car was significantly damaged in the crash, with both front airbags deployed. Thankfully, no serious injuries were suffered, thanks in part to the tire wall at the circuit. The vehicle was recovered by crane. As reported by Sina.cn, in a video after the event, Tang Zhu explained that he’d had little warning something was wrong. He’d heard a loud grinding noise in the previous braking zone, indicative of severely worn pads. He had—perhaps naively—hoped he could still complete his current lap, only to find in the next braking zone that he could not stop the car.
Video shared by Tang Zhu from after the crash shows the SU7’s Brembo brakes in a severely degraded state. The pads can be seen worn down almost to the backing plates. The vehicle had done five laps prior to the incident. We don’t know how many miles the vehicle had done before this. Regardless, given this happened in April, the vehicle was only a month or so old at most.
The crash caused quite a stir in the Chinese EV world. The video quickly racked up millions of views on Chinese website Bilibili, pushing Xiaomi to release a statement on the matter. The company basically came out and stated that the vehicle was not built for track use. Translated, it reads as below:
We strongly remind you that Xiaomi SU7 Max is a high-performance luxury car for road use. Please do not try extreme driving such as on professional racing circuits.
The braking system of Xiaomi SU7 Max uses NAO friction pads, which are suitable for daily driving. They are mainstream products that balance braking noise and braking performance and meet the needs of public road driving.
In addition to meeting national standards, NAO friction pads also meet special performance standards and performance verification for braking performance:
• AMS performance: after 10 consecutive 100km/h-0 full-force braking events under full load conditions, the braking distance decay is less than 2m;
•100km/h-0 braking distance is 33.3m, which is at the leading level in the industry;
Under the simulated conditions of ordinary driving, the wear of the brake pads remains at an acceptable level.
Due to the high speed, large kinetic energy accumulation, sharp braking, and rapid wear of components under continuous high load and high temperature under track conditions, all core components of the vehicle are required to meet requirements far beyond those of daily conditions. Professional drivers need to perform system-level professional maintenance on their vehicles before driving on track.
Track driving is the most extreme and demanding test for both drivers and vehicles, and requires a deep accumulation of vehicle expertise and professional driving. We once again call on people not to take the challenge of driving the Xiaomi SU7 on track lightly. Thank you for your understanding and support.
The statement specifically calls out the NAO (non-asbestos organic) compound brake pads for being optimized for reduced noise as opposed to performance. As covered by PowerStop.com, these pads are known as being particularly soft compared to other compounds. Indeed, brake manufacturer Bendix notes that NAO pads aren’t ideal for performance vehicles driven hard.
Unfortunately, not everyone heeded Xiaomi’s call. Further influencers took the SU7 to the track, with predictable results. One driver shared a video in June this year, with the crash itself later reposted in October by YouTube channel HandsOnTheWheel.
In much the same way, the driver found they had no brakes at the end of a straight. This time, however, the driver lost control and spun out as they attempted to slow the car, with the rear of the car hitting the wall instead. The driver can be seen being flung around the cabin during the impact. Amazingly, despite the apparent severity of the crash, the vehicle was able to limp away from the incident with plenty of damage to the rear end.
In the wake of these crashes, photos of the Xiaomi SU7’s brakes were shared far and wide across Chinese social media channels. They’re quite surprising if you’re familiar with performance brakes. While the calipers themselves look big on the outside, they are only four-piston units, and are fitted with relatively small brake pads.
When it comes to brakes, size matters. On performance vehicles, brake pads, rotors, and calipers are typically quite large. This isn’t because bigger brakes can slow a car down faster. In fact, braking performance is limited by tire grip in most cases, and even moderately-sized brakes can lock up a wheel beyond the tire grip limit.
Instead, performance brakes are large because of heat. Larger rotors, pads, and calipers have greater thermal mass, so they take longer to heat up. They’re also typically better at shedding heat to keep the brakes in a safe operating range. That’s important on track. Bigger brakes let you turn hot laps for longer before they get critically hot and lose braking capability.
In this regard, it’s worth comparing the SU7 Max to a similar vehicle—like the Porsche Taycan Turbo. It weighs 5,119 pounds and has 616 hp. Those are very similar figures to the SU7 Max which weighs 4861 pounds and has 673 hp. However, they have very different brake setups. Xiaomi doesn’t include the size of the SU7’s brakes on its official spec sheets, but more on that later. What we do know is that it’s rocking four-piston brakes up front with those rather small pads and cast iron ventilated rotors.
In contrast, the Taycan Turbo has larger six-piston calipers up front, and four-piston calipers at the rear, with 410 mm (16.14 inch) front rotors, as a guide. The Taycan Turbo S doesn’t add any more power, but it steps up to 10-piston calipers up front and larger 420 mm (16.5-inch) ceramic rotors for even better performance. At the bottom end of the range, even the 321-horsepower base Taycan has bigger brakes than the SU7 Max.
A Taycan Turbo S has similar weight and power to a Xiaomi SU7. It’s also got a much beefier set of brakes.
Indeed, Xiaomi has recognized this limitation. While the SU7 Max is a fast road-going vehicle, it’s simply not cut out for track use. Thus, enter the SU7 Ultra. This model is yet to be released, but a prototype version smashed the Nurburging lap record last month. Xiaomi has equipped the race-tuned vehicle with massive 430 mm (16.9 inch) carbon ceramic brake discs up front, and 410 mm discs at the rear. These brakes can handle temperatures up to 2372 F, according to Xiaomi, and are far less subject to brake fade during track driving.
As shared by Fast Technology, there’s a shot comparing the SU7 Ultra’s brakes with what appear to be those of the lower-end model. Taking a rough measurement from the image, I think I’ve got a rough idea of the regular SU7’s brake size. Take it with a grain of salt, but they look to be 74% the size of the larger ceramic discs. If the SU7 Ultra brake is 430 mm in diameter, the lower SU7 models are using discs just 318 mm (12.5 inches) in diameter. That’s pretty titchy (small) for a car weighing almost 5,000 pounds with over 600 horsepower.
To give you an idea, you’d find brakes of similar size on something like a Honda CR-V. That’s a regular road-going SUV, which weighs under 4,000 pounds with maybe one-third the power of the Xiaomi SU7 Max. In contrast, something like a Tesla Model Y Performance with 530 hp has much beefier 385mm (15.16-inch) rotors—note how that’s much closer in size to the Porsche Taycan than the Xiaomi SU7 Max.
Basically, it comes down to this: The SU7 Max is a heavy electric car. It’s really heavy. And it’s got lots of power, and really small brakes. It’s also got soft brake pads that are built explicitly for road use. Fling that electric heifer down the track, and you’ll enjoy its great acceleration—but you’ll also be chewing through your brake pads in no time quick. They’ll be turning to dust every time you try to haul it up for a corner. When they run out, when you hear that first little grind? You’re flirting with disaster. Pull it to a halt, don’t try to run hot into the next hairpin. You’ll regret it.
If you absolutely have to take one out on track, you’ll want to consider some serious brake upgrades. That is, of course, if you’re intending to keep it out of the wall.
Xiaomi likely hasn’t said anything we can find that indicates it believes it’s done anything wrong. Some enthusiasts may disagree. Personally, I’d say if you go out boasting how your 600-horsepower car is better than a Porsche… you should probably expect that somebody’s going to test that on a track at some point. For their own safety, and to avoid your own embarrassment, you should equip it with suitable brakes to handle that circumstance. Just saying.
Image credits: Xiaomi, various via YouTube screenshot
I can buy better pads, rotors, and calipers.
I’m FAR more concerned with how that driver’s seat just folded like a wet noodle on impact.
This!
Honestly, that’s genuinely shocking how that chair frame assembly buckled like that. That wasn’t even that violent (relative term acknowledged) of a collision, and that thing flopped his ass around like a carp. That’s crazy terrible safety engineering.
Why did the chinese video auto play when I opened this page? Please don’t go down that road.
This, first time it’s happened to me.
Sorry—Bilibili has a terrible embedded player. Removed.
You probably shouldn’t embed anything Bilibili, ByteDance, or Taobao considering there’s been reports of all three using them as locators for scrapers. Their web scrapers aren’t known to respect the robots.txt of websites and could bring the site down just from sheer traffic.
Most cars need upgraded brake pads and fluid to run at the track. I’ve got a buddy who ran his ND Miata at a karting track with stock pads and fluid and he said the brakes were useless after a few laps.
It should be pointed out that brake pads suitable for track use suck for normal every day driving. If you’re driving patterns, include a few minutes of not using your brakes and then a pedestrian walking in front of you, track pads are not a good thing. Especially if you’re that pedestrian..
Also, measuring brakes by diameter is lazy and misleading. Swept area is what matters.
Hardly think it’s lazy and misleading when we’re talking about brake fade and heat capacity.
I’m not passing this off as a full, in-depth engineering study. However, it’s easy to see that if a heavy sports car has brakes a fraction of the size of its rivals, that’s worth considering when the cars are ending up in the wall.
Agreed. There is a difference between rotor/caliper size, and pad compound.
A high end sports car can be built with large rotors and pad area, but with soft pads acceptable for road use. The user can switch them to harder high-temperature pads for track use.
Building this sports car with tiny brakes and pads (and likely fake Brembos?), is stupid no matter how you look at it.
100% – high performance brake pads are noisy and have less available friction when used under the ideal operating temp.
Additionally, since this is an EV the motor(s) are capable of applying their full torque in braking (negative torque). Meaning the vehicle effectively has 2 braking systems, the mechanical brakes and the drive motor(s). Of course it depends on how the controls engineers decided to use the available motor negative torque and how they blend with the mechanical brakes.
It doesn’t look like they quit working due to heat but instead they just completely wore the pad out. If they had used a better pad with a harder compound this problem could have probably been avoided. A bigger pad would have also lasted longer but I think the soft compound of these just wasn’t enough.
Harder pads would be unsafe for driving in normal traffic around pedestrians etc.
Teslas along with most other cars come from the factory with ceramic pads. Explain how using these is dangerous?
High performance pads require heat to reach full stopping power. On the racetrack they get heat quickly and retain it from corner to corner so they are always ready. On the street, they might be stone cold. You jump on the binders and they will start generating heat so their stopping power ramps up. Street pads are softer and designed to apply peak stopping power immediately.
I understand that. I was replying to Hughs comment about ceramic or semi metallic pads being unsafe in traffic. It’s obvious what happened here. The manufacturer used organic pads because they are softer and quieter for everyday use. Plus being an electric car it has regenerative braking. This would have fine for street use. When they got on the track with them they wore out super quick because they’re organic and soft. They also probably got too hot because organic pads are worse as displacing heat and that led to even faster pad use. I feel that all of this could have been avoided with a harder pad material like ceramic or semi-metallic. No, they’re not a “track” pad but they would have held up better than the organic.
Honest question: If pad material exists, but is entirely heat soaked and unable to dissipate the heat required for stopping, will the brakes do anything?
Not an engineer, but my gut says no, as the primary function of stopping is converting kinetic energy into thermal energy that dissipates via the properties of the braking surfaces. If the pad and rotor have no more capacity to absorb and dissipate thermal energy, it will not provide stopping power, whether material exists or not. It’s probably not as binary as this, but the high level point is, it still wouldn’t be safe and the accident still would have happened.
but is entirely heat soaked and unable to dissipate the heat required for stopping,
Entirely heat soaked means melted or vaporized. I get what you’re saying and 2 things would happen, the brake fluid boils rendering it useless, that happens first, after that, like if you could prevent that from happening, some part of the metal structure would go “plastic” and mounts would sheer off, or the caliper would deform rather than apply pressure to the disk, something like that. Material goes plastic at fairly low temps, like the name implies the metal loses its rigidity. Brake fade, something common in car testing and track use, is that “heat soak” you’re taking about, it’s caused by the fluid boiling, but the reason the fluid gets so hot is the brake parts can’t shed the heat.
It’s OK though because if you ignore it the car crashes then the brakes can cool off.
Right! Thats the crux in this story. This didn’t just happen without warning and then the guy crashed. This is a problem with EVs. They are easy to drive, fairly affordable, have gobs of power, and weigh a ton. That means a lot people can afford to go really fast. And the more people you have doing something, the more you have that shouldn’t be doing it.
It’s also worth remembering this car costs about 1/3 the Taycan (from what I could find) and that extra $60K isn’t just for the badge.
Yup. It’s kinda bonkers the Taycan can be had with 10 piston calipers. 10! That’s more pistons in one caliper than my car has total; engine and brake pistons combined!
Ceramic pads (not carbon ceramic rotors) are more about low noise and low dusting. They are still formulated for the temp range they are expected to be driven in. A street pad is made for lower temps since they rarely see the heat a track pad will see.
I know this. But in the article it says this car is equipped with ORGANIC pads, which would be even worse for the track.
They come with pads that if not for ABS would be able to lock up the wheels stopping on dry pavement while cold. Harder pads suitable for track use won’t.
I know this from unpleasant experience where a someone doing a restoration assumed I was going to drive the car on the track. Granted this was on an old Porsche with unboosted brakes, but still , brakes you have to warm up for a panic stop are kind of a bad idea.
Isn’t one of the points/benefits of EVs regenerative braking? I would think the smaller pads and brakes for the comparative performance would be okay if the car as a system were operating properly and it wasn’t relying so much on the friction brakes as it obviously was…
The taycan turbo has some of porsche’s largest brakes ever despite also having regen. Same goes for the ioniq 5n, claimed as hyundai’s most powerful braking system. I’d guess that consistent fast and hard braking stands a risk of damaging the regen system, or something similar
The regen system definitely operates, but it’s capped at a certain charge rate that the battery/electronics can handle.
In normal road driving, the power of regenerative braking definitely lessens the need for powerful friction brakes, and as a result most EVs have tiny friction brakes.
However, in track use where everything gets pushed to the extreme, regen braking is not strong enough (it’s limited by the recharge rate of the battery) to lock the wheels but still definitely helps, but its strength may even end up decreasing over time due to the battery getting hot and maximum charge power derating (though idk if this actually happened here, the cooling system may have kept up).
While regen braking does reduce friction braking needs, it’s likely fully offset by the increased weight of an EV along with the better acceleration in the context of track driving, meaning that ‘full size’ brakes are needed on the track. These SU7s just have too little friction brake power for track needs.
“A Xiaomi SU7 Max was driven at the Shanghai Tianma circuit, with influencer Tang Zhu Liao Che (堂主撩車) at the wheel….”
Nice try, Liao Che.
I watched the onboard video. There used to be a joke about auto journalists driving high performance cars with inadequate skills. I think it goes triple for “influencers”.
Yes, the car is at fault. Yes, the brakes failed and caused the car to pivot but then the driver panicked, overcompensated, and squandered any chance at avoiding a crash.
Any car you strap into can fail in any way. If you are going to be track testing a 600hp lead sled at triple digit speeds you might want to learn some high performance driving skills before firing up your GoPro.
I’m pretty sure he was attempting to slow the car down by seesawing the wheel back and forth.
Maybe don’t equip the car intended only for street use with over 600 hp then?
Anything with that much power implies a level of performance and the rest of the car needs to be built to handle that, street or track is immaterial IMO.
Time for my 3 cents (thanks inflation) as someone who worked in brake caliper design in the past. Looking at the pictures I’m more inclined to believe this is more of a “used the wrong brake pads” and “don’t ignore the pad wear indicator” than a fundamental design issue with the brakes.
With Brembo specifically, I wouldn’t be surprised that Xiaomi picked an off-the-shelf caliper that fits their wheel packaging and works good enough for them, which tends to work for mass produced and cost-conscious vehicles. But there’s a ton of factors that go into the size and shape of brake pads: like rotor OD and ID, piston count, diameter, spacing, the stuck chocolate bar in the vending machine, and my favorite (/s) chasing the NVH dragon. But regardless, it shouldn’t be a surprise that daily driver pads got chewed up doing something they weren’t designed to do.
Also to touch on the caliper size: since those are fixed calipers with pistons acting on both pads (rather than floating calipers like the soccer mom CUV which only has pistons acting on the inner pad and the caliper slides back and forth), fixed calipers generally don’t require as much radial packaging space (height) but to maintain stiffness it takes up more tangential space (width). This also means one can get away with a larger diameter rotor relative to the wheel packaging at the expense of more $$$.
As you’re likely knowledgeable, is it true that pads are chamfered on the ends to keep them from squealing as they break in?
-I replied that to someone on page 2, and don’t like spreading misinformation
Usually those cutouts and chamfers originate from NVH throughout the life of the brake pad, especially brake noise
Dottie, thank you for the excellent comments – much appreciated. My gut said that the overall braking area for these is about what I expect for a car of this size and performance – those are very large rotors by traditional standards.
Quality commenting.
That caliper size to pad size ratio is wild.
I would have expected those to be some serious 6-piston calipers from a quick glance.
I thought the same. Shocking to see the actual pad area vs the caliper’s potential.
Tech company making cars. What could go wrong?
Not even the established Westerners like Apple could successfully pull it off, let alone a Chinese company with zero automotive expertise.
See also: Car companies doing tech.
I can see two sides to this. On one hand if the car meets expectations on the use case its designed for, then that’s acceptable, but on the other hand it’s a 600hp machine. That exceeds what’s necessary for on road driving by a large margin, so all the other systems should probably be robust enough to perform better under conditions where that power is useful.
I thought one of the big advantages of EVs was regenerative braking and therefore the friction brakes could be smaller.
I suspect that advantage applies to on road driving, especially in traffic, and the advantage evaporates on track where you’re less likely to give it time to spin the generator when slowing down.
No. More like the friction brakes last a lot longer for road use.
Also, the current EV’s can’t recharge the battery that quickly. Not to the level of absorbing all the current from the motors/generators in a track environment. Big brakes are still needed.
Railroad locomotives have big resistors on the roof to dump regenerated current. In the mountains at night I remember seeing them glowing red on downhill grades.
I was going to add that a capacitor bank for absorbing the current from regen braking would be good for a track car. But that gets into the issues of wiring size for the amps needed to be carried and capacitor lifespan. Electronics hate being hot.
I think you’d also get into packaging issues.
Whatever happened with that Porsche 911 GT3 R Hybrid that used the Williams F1 flywheel KERS?
That seemed like the ideal solution.
I’m not a battery expert so I can’t say for sure whether a battery that can suck up 180kW during a fast charge can safely soak up all the power the generators can deliver for the few seconds it takes to slow the car from high speed. It can certainly deliver enough power to impressively accelerate the car though.
My remote control hobby revolves around electric powered vehicles and I daily an EV. I know some about the current batteries. Definitely not an expert, but a knowledgeable amateur.
With that said, the current batteries can discharge far faster than recharge. As an example, the packs for my cars take about 10-20 minutes to discharge and 60-80 minutes to recharge at a safe rate. EV batteries have much better controllers than my RC batteries. That still doesn’t get around the charge rate limits. Some of that kinetic energy will have to be scrubbed off as heat through the brakes in a track environment. On the road the regenerative braking rate is fine since the vehicle isn’t slowing quite as quickly or as often.
Well I guess its time for some math:
The battery in this thing is reported to be able to take a maximum average charge of 218kW for19 minutes between 20-80%
https://evkx.net/models/xiaomi/su7/su7_max/
Which by the back of my envelope works out to a charging speed of 11.4kW/min or 0.19kW/sec. Note this is an average sustained charging speed so it’s not out of the question for the battery to accept more for a very brief bit of emergency braking.
The car weighs 2200 kg so its kinetic energy at say 200 kph is again by the back of my envelope 3,395,062 J or 0.94 kWh
So the battery should be able to easily suck up all the kinetic energy of this car from 200 kph to 0 in 4.9s or less with no friction braking at all. That’s pretty damn good!
Math maths. Disregard my comment.
Slight correction. The new guesstimate is 15.5s.
This is why I like to have people check my work. Please feel free to do so.
Car weight: 2200 kg
Total motor power: 495kW (This is greater than the fast charge speed so IMO the motor is probably not the weak link)
https://en.wikipedia.org/wiki/Xiaomi_SU7
Kinetic energy of 2200 kg car at 200 kph = 3395062 J
https://www.calculatorsoup.com/calculators/physics/kinetic.php
3395062 J = 0.943073 kWh
Average fast charging speed of battery = 218kW
https://evkx.net/models/xiaomi/su7/su7_max/
0.943073 kWh/218kW = 0.004326h
0.004326h = 15.6s
Not as impressive as 4.9s but still I think enough to do the job.
Kudos to you for doing the math to work this out!
So, it’s plausible that friction brakes were downplayed for regen. But they didn’t expect the Spanish Inquisition!
No-one ever does.
This approach to the math is not the correct one to take, or at least it does not correspond to people’s experience of braking. Brakes are best looked at as something where you modulate the torque at the wheels, and play it against the limit of grip. This results in much, much more energy dissipated in the brakes during the high-speed portion of a stop than in the low speed portion. Regenerative braking is going to be (as you and others have noted) limited by something to do with the batteries and electronics. This will lead to much, much less available decelerating wheel torque at high speed than at low speed. Drive an EV and you will feel this. It’s pretty obvious, even with the nannies that compensate for it. The upshot with respect to the calculation you’ve done using a constant power deceleration is that the stopping time may be correct, but the stopping distance is much longer for a constant power deceleration.
All US-sold EVs (I’m told) are required to have braking systems that are sufficient for safe use independent of regenerative braking, because that is an incredibly obvious rule to have. Regenerative braking straight up doesn’t work at high states of battery charge, and a bunch of other conditions that wouldn’t allow a zillion watts to be safely shoved into a reversible chemical reaction. Anybody who has had an EV can tell you that fast charging doesn’t work more often than it does work. It would be insanity to rely on the fast charging mechanisms for a critical safety feature.
A selling point for the Porsche Taycan is that the brakes are intentionally sized to be car-appropriate for high-performance use. It’s a hard choice between those and the suspension, but they are one of the best features of that car. They are truly amazing. Laying into the brake pedal on that car feels more like hitting “stop” on a 718 than it has any right to. As V10omous says somewhere else here, sure this isn’t a track car, but seriously: Dusting the entire volume of the brake pads in 5 laps? That is ridiculous under-engineering.
The point was that EVs don’t only depend on their friction brakes. EVs can and usually do absorb a good chunk of kinetic energy back into the battery. I simply wanted to see how quickly the battery could suck up all that kinetic energy based on its measured average fast charging speed. I think if it can reliably handle 19 minutes of 218 kW input it can handle a few seconds of the same from the motors. The answer I think indicates regen braking SHOULD have taken a good amount of the load off the friction brakes.
However the story claimed “the driver found they had NO brakes at the end of a straight” (emphasis mine).
Why? What happened to the regen brakes? Was regen turned off or did track driving cause that to fail too? If regen was off I think that would go a long way to explain the crash.
Side question, why didn’t Jim Farley and Ford criticize the brakes on their own SU7 test mule? Certainly over the past 6 months they must have tested it’s brakes on their own test track.
I understand your point and your math, but both are wrong. Under hard braking regen brakes will do very little work, and this is well-known. It’s freshman physics. I’m trying to be nice and not all professor-y, and probably failing. An automotive engineer who would count on regeneration to undersize brakes because they are counting on regen to contribute in hard braking would probably be found criminally negligent if it came to that. There is a very big fundamental difference between the kinematics of the two systems.
Under hard braking regen brakes will do very little work, and this is well-known.
I dunno, maybe that was true in the early days of EVs when the batteries were smaller, set up for level 1 or 2 charging only, voltages were lower and there were longevity and safety concerns of exposing batteries to sudden bursts of high power. This car however clearly has a large battery and electronics capable of handling such power and motor/generators capable of converting that kinetic energy into electrical energy (on paper at least).
It’s not even THAT much energy to regenerate, less than 1 kWh when moving at 200 kph and a quarter of that when at 100 kph. This is an AWD vehicle so all the wheels are capable of regen.
Unfortunately as I said earlier I’m not an EV engineering expert so sure, I might be looking at this all wrong. Or maybe you are basing your argument on old tech or RWD EVs where regen is less capable.
Perhaps this will be a future Autopian story. I’d be nice to get an EV battery expert to weigh in on the matter.
I would be very unsurprised if the SU7 could regen brake 180kW or more, but that’s still very low compared to what’s needed for maximum braking (capable of locking the wheels) at high speeds; braking requires constant torque, and Power = torque x rpm, so even your typical commuter car’s brakes output 1000s of hp when emergency braking.
Based on its curb weight of 2200 kg and its listed average fast charging speed of 218kW (between 20-80% charge) I estimated the charging system can easily suck up the entirety of this car’s kinetic energy from 200-0 kph in under 5 seconds and probably much less time that that in an emergency. See below for details.
I think most electric cars also provide the ability to turn off/turn down regenerative braking. I’m assuming that for track use, you would probably want to turn off regenerative braking completely since you need to control/modulate braking into the turns (and regenerative braking could interfere with the feel and use).
Was that mentioned and I missed it?
I believe you’re thinking of lift-off engine-braking-like regen and/or one pedal driving, and I’d definitely turn off the latter on track. However, even with thise settings turned off, regenerative braking is still fully utilized when applying the brake pedal.
As others have said, that probably only applies to normal road use, not track. My bolt will only charge at 55 kW peak (if I’m lucky), but regens up to 70 kW, but only for short periods of time. It is enough for ~0.3g of braking, so I rarely ever use the mechanical brakes in normal driving. I would think that your quick calcs would apply to stopping from a highway cruise, but with track use that could all change. In 0F and below type of temperatures, the bolt will entirely disable regen, and when “fast” charging will kick the fan/cooling on at not very high ambient temps. I wouldn’t be surprised if track use with extreme discharge/charge of the battery would heat the battery to the point that the car would disable regen to preserve the battery and keep it from overheating, at which point all braking would be from the mechanical brakes.
I did the math below. In short the charging system including the battery is capable of sucking up the entirety of the car’s 200-0 kph kinetic energy in 4.9s without a sweat.
Again, it’s not 0.3g of braking at all speeds. That 70kW of decelerating power is a lot fewer g’s at 100mph than at 30mph. Friction brakes will give you all the g’s of deceleration your tires can handle at all speeds, they just heat up faster at high speeds. As somebody else mentioned, hitting the brakes at high speed is briefly putting thousands of horsepower through the brakes on even a regular old car.
Why are we surprised that a heavy vehicle burned through its brake pads during a track day?
Lighter cars will do the same, especially if they’re equipped with street-duty pads, like this car was.
*Rockwell painting meme*
I think anything sold as a high performance vehicle should be able to survive more than 5 hard laps without the pads wearing to the backing plates.
They’re clearly following the computer printer model where you get a “starter” cartridge with your purchase.
Also high performance =/= track ready. I can’t speak to what’s needed to be allowed on a track but I’m under the impression track cars tend to need a few upgrades.
Different cars are different degrees of readiness out of the box in my experience, but I could take my truck or minivan onto the track and its brakes wouldn’t be this cooked in 5 laps.
Sounds like 30 minutes of hard mountain driving could send you off the cliff in this car.
I did the same with my old Cruze around Watkins Glen about a decade ago. After three pace car led laps, the then-factory brakes were smelly but otherwise fine. The high dollar high performance cars there that paid for six paced laps didn’t have that smell.
I doubt you’ll get super car track speeds on the track from your truck or minivan though. Even if you did, say by strapping a JATO rocket to the roof I think you’ll quickly discover the limits of the rest of the vehicle as well as its brakes.
About “high performance =/= track ready” yes and no, there are plenty of “high performance” cars sold today that will do plenty of laps where the tires will turn to grease before the other systems need to be upgraded. Since this car company compared themselves to Porsche I will reference them. I would trust any of their sports cars to be able to do lap after lap on stock components without failure. They make sure even base models have the cooling capacity and braking required for the power and weight of the vehicle.
Agreed re: Porsche. A lot of the perceived Porsche tax is actually due to the fact that when they specify performance figures for their vehicles it usually means that the car can perform at that level consistently and not just once. If it is spec’ed to go 0-60 in 3.2 seconds, it will probably be able to do that at least 10 times in a row if not all day. Same with braking.
And brakes are literally the worst component to fail first. Tires, engine, even steering could fail as long as you have working brakes.
Very few folks who buy Porsches ever explore the performance limits of their cars though. This comparison is for posers who buy a Porsches as boulevard cruisers and mall crawlers. For them these brakes are just fine.
Stories of Porsche owners who explore the area just outside the performance limits of their new Porsches the day after big IPOs are numerous in the Bay Area. It’s almost like they should have a warning on the traffic report about tranches of suddenly rich electrical engineers in fast Porsches. The most famous example of course was the president of Eagle computing but that was actually a Ferrari.
Strange. I’ve lived in San Jose/Sunnyvale for most of my life and I don’t think I’ve ever heard those traffic reports.
I do see a LOT of Porsches here though. I used to pass a bright greenish yellow long nose duck tail RS on my way to elementary school (I thought it was kinda dumb looking).
The Porsches I see today are usually proudly displayed in driveways, dropping kids off at soccer, in the mall parking lot, or in traffic with us plebs.
I said that they should , not that they did.
The story of Dennis Barnhart, founder of Eagle Computer used to be told frequently, and I remember some let’s see what this will do moments back when Milpitas still had farmland.
I had a 2009 GTI that I used to track a fair bit. The first track day I ever did, the car was 100% stock. Yes, I overheated my brake fluid, ruined my tires, and destroyed my brake pads. After probably 20+ hard laps of a track around a mile long. I didn’t really start experiencing brake fade until my last laps of the day, and even then, was able to drive the car over two hours home without any issue. There was even a guy there in a rental Kia Optima, which was still drivable at the end of the day. I would argue that a significant amount of regular ICE cars can do more than five laps before brake failure, like this one experienced.
I did a 2-day track day with stock pads and fluid in my MINI, and didn’t have any trouble.
If a car comes with gigantic, yellow, Brembo-labeled calipers, it should do much better than this.
I can sort of understand it – if it’s not designed to be raced then there’s no point installing race car brakes because they just add weight. Added to that a lot of retardation in EVs comes from regenerative braking anyway – think about one pedal modes on many cars. If it’s able to do ten emergency stops in a row from high speed that’s probably sufficient for road use.
That said, the tiny pads in the massive caliper is pretty shady.
Actually, performance parts are really expensive, so if it is not a race car, why spend more and making the car more expensive if buyers are not supposed to track it?
Really doubt it was a weight reduction consideration.
Yeah, the size of the caliper would imply these are much more capably brakes and that’s clearly intentional for the look.
I’m not surprised that fast and heavy of a car torched stock pads. Pads that are good on the track have compromises on the street, and I wouldn’t expect to see them offered unless there was a real “track pack” option.
See my previous comment, there is a minimum size of caliper for that design and disk diameter, and that is what these are.
I think that with that type of caliper, the size is determined by the diameter of the disk that fits inside. The older style brake calipers were much thicker because wheel offset wasn’t the issue that it is now, so they could be thicker and stiffer and they weren’t held together by the bolts at the ends. Instead they had a lot more structure outside the diameter of the disk that isn’t possible with a disk that takes up most of the space inside of the wheel rim.
That’s true, but if you look at the inside of a Taycan caliper:
https://partslinkent.com/2020-22-porsche-taycan-4s-front-left-disc-brake-caliper-pads-assembly-white-y1a-pt111822/
the pad is probably somewhere around twice the size of the one used here. But I think what’s more key is that the pad is the same size as the raised part of the caliper, which is traditionally the part where the pistons are – and that’s not the case for the XU7.
So wait, you’re telling me that a vehicle that has a price far below anything built elsewhere might have *skimped* on the brakes? Wasn’t the labor cost of slave labor enough? Or being built in a place with pretty much zero environmental regulations? And someone else points out that the seat broke in the accident on film. Could it be that sometimes you get what you pay for?
Well Model S Plaid had similar issues. Not quite as bad, but fe Throttle House and C&D had severe issues with even moderate track use. Or even multiple runs on drag strip.
https://www.youtube.com/watch?v=Hn9QWjxFPKM
But anyho, personally I have very little faith in chinese companies after working with them on tech area for 20+ years. The culture really rewards cutting corners regarless how sketsy the results may be. Not to mention the possible cost of supporting their growing war machine.
Can we talk about the driver’s seat not providing any whiplash protection either in the HandsOnTheWheel video? The seat back collapsed hence the reason why, “The driver can be seen being flung around the cabin during the impact.”
These are the sorts of failures that would scare me away from most Chinese designed/built EVs, should they ever be available in the US. FMVSS/NHTSA testing is not fool proof, and there drastic cost delta between Chinese EVs and ones available in the US is not entirely down to labor cost. Corners are simply being cut on these cars, but in creative, subtle, and hidden ways. Calipers that look massive but are undersized, seat frames that are likely far flimsier than they should be, and who knows what else. \
Throw in the reports of cars being bricked as their companies go bankrupt in China due to being so software dependent. Ultimately these cars are compelling on the surface, but they’re far from perfect, and the general public will never know that.
Beyond cost cutting, part of it is simply a matter of experience building cars. Especially cars that perform on this level. They may have their best engineers on it, but their best engineers simply can’t amount to a team that’s as experienced as the folks working in GM/Ford/Honda/etc’s performance divisions.
Your last point is something I’ve been thinking about lately. There’s so many manufacturers making such a dizzying array of EVs in China right now, but I doubt even half of those companies will make it until 2030. Which leaves a bunch of cars with zero manufacturer support, software or otherwise. Even if a manufacturer sticks around, what does support look like when your car is a few years old?
It’s a pretty typical story though, look at how many automakers there were trying to make it in the US in the early 1900s, and how quickly that number dropped off as companies were consolidated or went bust. Wouldn’t be surprised to see that in China too.
That giant-ass caliper with the tiny pads inside is pretty sad.
It’s like they’re designed to look larger than they really are. I’d like to know if they’re really Brembo. I’ll bet they’re fakes.
Xiaomi is a real company which does a lot of work with other companies, they’re likely real. It would be way too risky for them to get caught using a supplier of fakes.
But that doesn’t mean they didn’t open the Brembo catalog, point to the cheapest possible option, and go “those ones.”
Those are Brembo’s the ones made in the Nanjing
It’s like those caliper covers that have been going on various Tesla vehicles to look “cool”
And probably much heavier than a smaller caliper capable of the same job.
But Brembo!!!!
Seriously tho, that seems pretty much normal. Most cars, even the semi sporty ones, can’t just go out and turn laps without swapping some things out. In this case, a set of pads with a proper compound and checking them often is the order of the day.
And, when anything starts making weird noises or vibrations, PIT NOW!!
Yeah, I’m not so sure this is news. My Focus STs brakes are entirely inadequate for track use too. I’ve not done it, but when it first came out, lots of people found that out very quickly. Required track pads and brake fluid that doesn’t boil. This car just exacerbates the problem by weighing probably 5000 lbs.
Exactly. My track toy is a C6 Z06 so it’s biased towards track duty a bit more than most and still needs to run a track pad for longevity. The OEM compound works well on the street and light track use but it wears very fast and a real hot shoe would be lucky to last a weekend of sessions on a set.
I sort of agree with you, but there’s a long difference between a weekend of sessions and five laps
Looking at the pads, I see very little “swept area”.
Is this the point where we question Chinese engineers? Or is it the accounting department looking at the placement of the pinto gas tank cost?
This feels strongly like accountants, especially since the higher model is getting appropriate brakes.
And what did they save with the tiny brakes? Probably $400….on a car of around $100,000.
“Fling that electric heifer…”
I need to incorporate use of this phrase in regular, day-to-day life.
I think my dental hygienist used to play double bass in Fling That Electric Heifer back in the 90s…
Isn’t that the name of the EV Hummer?
All I hear is Cuba Gooding Jr yelling “Xiaomi the braking power”.
All *I* hear is the auto-playing commercial in the video player. The mute seems to no linger work; I had to mute my computer.
Take your damn star….