Winter is here in the northern hemisphere, and for many people, that means snow. You’ll naturally want to have your daily prepped for the weather with a solid set of winter tires, that much goes without saying. But what should you do about tire pressures?
Here to answer that question is Jonathan Benson, the professional test driver behind Tire Reviews on YouTube. He set out for the Pirelli proving grounds, armed with a Mk 7 Volkswagen Golf wearing Pirelli Cinturato Winter 2 tires. That gave him a beautifully prepared, evenly-graded snowy surface upon which to execute his testing.
This holiday season, you could set your tire pressures based on the advice of any number of uncles, grandmothers, or scary cousins. Alternatively, you could watch the video below, understand what’s going on, and decide for yourself.
The test regime was simple. Benson set about timing acceleration runs from 5 km/h to 35 km/h (3-21 mph). For braking, the test involved measuring the distance taken to brake from 40 km/h (25 mph) down to 5 km/h (3 mph).
The results will be familiar to anyone who has done a lot of driving on soft surfaces. For example, it’s well-known that driving on sand is easier at lower tire pressures. This is because a tire at lower pressure has a larger contact patch, reducing ground pressure and allowing the car to “float” more on top of the surface rather than digging in.
“The lower the pressure, the better the grip is,” says Benson. “That’s something I’m very happy to tell you.” The clearest gains were found in the acceleration tests. The Golf accelerated to 35 km/h (21 mph) almost a full second quicker at 10 psi than at 50 psi. Meanwhile, braking distances from 40 km/h (25 mph) were 3.2 feet shorter than at 50 psi, though the improvement dropped off below 20 psi.
The video also includes a handling test. Benson times how long it takes him to complete a simple course on a snowy track. He completed runs at 50 psi, 35 psi, 20 psi, and 10 psi, noting how the car’s behavior changed at each level.
As you might imagine, 50 psi is simply too much. “I started at 50 psi, what did I notice? The car felt pretty uncomfortable,” says Benson. “Turn in felt pretty positive and quick, but then it very quickly break into understeer.” Dropping pressures down instantly improved the feel, however. “At 35 psi… the car now feels much more compliant over the bumps… brakes feel more positive,” he says.
There are yet more gains to be had, though. “20 psi feels awesome!” Benson exclaims, as he heads out on the snow circuit once more. The changes are instantly obvious to feel through the steering wheel. “Everything does happen a little bit slower, the steering definitely doesn’t want to react how it did before, but when you’re turning and when you’re braking, and the traction out of the corners in particular is just outstanding.” Lap times were seconds faster with the tires at lower pressures, too.
At 10 psi? “As for the handling, coming out of corners you can really get on the throttle…” says Benson. “There is a bit of slip, but you can feel that extra traction.” He notes the biggest benefit is when accelerating in a striaght line. “When you’re turning, I don’t know if I’m rolling onto the sidewall… but you do seem to lose a little bit [of grip] laterally.” Still, the lap time was another 0.7 seconds faster than running the tires at 20 psi. Not bad!
All these tests were done on snow, which raises an obvious question—what about ice?
Accelerating on ice showed that it’s possible to go too low with your tire pressures, even negating concerns about rolling a partially-deflated tire off the rim. The tires did still perform better at lower pressures down to 20 psi, but going one step further to 10 psi was less helpful. At 10 psi, the test car took a full 5.23 seconds to reach the test speed of 30 km/h (18.6 mph), versus 4.67 seconds at 20 psi and 4.96 seconds at 50 psi. Results were less conclusive when it came to braking, however. The car performed best at 10 psi, and worst at 30 psi.
Overall, though, Benson advises against going too low with your tire pressures. “I think we’ve pretty much solidly proven that the lower pressure is the better pressure,” “However, don’t go down to 10 psi, it’s not worth the small advantage.” He notes that at this point, you risk rolling a tire off a rim and getting stranded. The important thing to remember is that you need to reinflate your tires back up when the snow conditions are over. “Make sure you reinflate your tires [because otherwise] you’re going to get pretty serious uneven wear on the outer shoulders at such a low pressure,” he explains.
Benson’s conclusions concur with others in the field. Notably, the rally experts at Team O’Neil did a similar test some years ago. Their assessment was the same—lower pressures create a larger contact patch which aids grip in the snow.
What should you take away from all this? Basically, if you’re having trouble with grip in the snow, start with winter tires, and consider dropping your pressures lower than usual. Just know that once you start getting below 20 psi or so, your chances of causing damage or pulling the tire off the bead increase, which could cause you a lot of hassle. In any case, drive to the conditions and look after your car this winter season to minimize your chances of having an accident in the snow.
Image credits: via YouTube screenshot
Would be interesting to know what is the variation in the results. And what happens with the different pressures when you hit thick slush… Although it would be difficult to test.
I test mine by doing the WalMart parking lot test.
And yes obviously I can feel a 1.5 lb difference in pressure.
And using shoppers, and their carts, as my imaginary pylon cones…YMMV
Another way low pressure seems to help is that the extra flexing of the tread cleans out the areas between tread blocks. I say seems because this is simply my anecdotal observation from the first time I aired down some WinterForce tires on my old Subarus’ 13” wheels. I couldn’t get up a hairpin to the site of Audie Murphy’s plane crash as earlier snow had melted, then refrozen, and there was fresh snow on top. My tires were smooth surfaces with pretty patterns. Let them down to low 20s, and the snow & ice chunks were ejected from the low points on the backside of rotation allowing some bite and therefor progress.
I will say that 185/70/13s get a >bit< squishy for hard cornering on pavement at low pressures, though. I carry a small compressor now—but of course we haven’t had snow in a couple years. Fingers crossed!
I run my Blizzaks at the pressure on the door sticker. I mostly drive on dry pavement at home and ice, slush or hard pack in the mountains. I rarely encounter fresh snow so messing with pressure seems like overkill.
The correct solution to all winter conditions is to apply more throttle.
This is, of course, assuming you’re driving RWD as God intended.
-Resident Canadian
I watched this video this weekend, ah the algorithm. As someone who airs down for off roading, it did not seem like he had ever done that for off roading. Good video. Lucky guy to have an winter track for himself for a day.