Anything wrong with 100 PSI in 700x32 tires on a road bike?

Question

Independent of the manufacturer of my new road bike, I wanted to ask if I am overkilling the purpose or intent of 700x32 tires by using 100 PSI. The air volume is greater for 700x32, so I believe the additional volume will cushion more than e.g. 700x26 and that I don't need to go down to e.g. 70 PSI to provide more cushion. I am not sure the rolling resistance or drag/friction/contact area when using 100 PSI will be significantly lower from that when using 70 PSI.

Specifically, with regard to manufacturer, I was able to purchase an unused 2021 Trek Domane SL7 that was still in inventory at a great savings. The original 700x32 OEM Bontrager tires had a max PSI of 70. But I like Conti GP 5000s so much (max PSI 102), I put a set of 700x32 on the carbon wheels in about 30 minutes yesterday (without a prying tool) and during a ride the bike felt considerably faster and lighter.

Should I knock down the PSI in the GP 5000s to 70 or so for a more cushiony ride, or is leaving PSI near 100 okay?

Answer 1

The Trek Domane SL7 comes with wide, modern rims with 25mm internal width. Using 32mm wide tyres at 100 PSI (6.9 bar) should be perfectly safe with such rims. As you note, the tyres are also rated for that pressure.

However, using such a high pressure probably increases your rolling resistance compared to a more moderate pressure. Even if you are very heavy (>100kg) and/or have luggage I wouldn’t use such a high pressure. Something around 5 bar sounds much more reasonable and is probably around the sweet spot for rolling resistance as well as improving comfort.

All the vibrations from the road you can feel in your muscles and body are wasted energy. Running high-performance tyres at lower pressure wastes a small bit more power in the deformation of the tyre but at the same time reduces vibrations and vertical movement of the whole system a lot.

Answer 2

There are a few points here, that need to be cleared up:

The air volume is greater for 700x32, so I believe the additional volume will cushion more than e.g. 700x26

This is wrong. The slimmer your tire, the more cushioning effect you have at the same pressure. This is due to the fact, that the contact patch is longer and slimmer (same area!) with a thin tire. As such, a small bump in the road does not exert as much of a force on the tire as it would with a wider contact area.

I am not sure the rolling resistance or […]/contact area when using 100 PSI will be significantly lower from that when using 70 PSI.

The contact area will be smaller by a factor of 70/100. That's simple physics: Your bike is only supported by the air pressure above the contact area, so your contact area is inversely proportional to the air pressure in your tires. You can simply calculate your contact area by dividing the weight on your tire by the pressure in your tire.

I am not sure the rolling resistance or drag/friction[…] when using 100 PSI will be significantly lower from that when using 70 PSI.

Nobody knows: It depends on your road conditions. Tire pressure is a compromise between energy loss due to tire deformation, and energy loss due to bumping off an uneven road surface or displacement of road material (soft sand). As such, on perfectly smooth asphalt or concrete, the highest allowable pressure for your tires probably rolls best. However, on a washed out sand road, where there are stones sticking out of the surface pretty much everywhere, you are very likely to get the best performance with the lowest possible tire pressure. You have to decide, where you want to strike this compromise. Hint: When your hands get sore from the constant vibrations of your handle bar, your tire pressure is probably too high.

Answer 3

Tyre pressure is always a compromise.

As the air pressure inside increases, the size of the contact patch decreases. A 200 pound rider+bike with 100 PSI air in the tyres will have a ground contact patch of 2 square inch in total, split between both wheels in accordance with the front/back balance, so perhaps 1.2 square inches on the rear wheel and 0.8 square inches on the front.

Sorry for imperial units, but in this specific case imperial PSI is more intuitive than Bar.

High pressures produce progressively less rolling resistance BUT the contact patch decreases so grip drops.
At the same time, Low pressures suck more of your power in distorting the tyre while it rolls, but the increased ground contact patch gives more grip.

If you underinflate, you work harder and risk pinch flats.

If you overinflate, the small contact patch means cornering gets increasingly sketchy. (I once tried riding on 125 PSI and it felt like riding on ice, even in the dry. NOT recommended! )

The best pressure is the one that suits you, your riding style, the bike and the conditions. And your total weight.

Personally I'm 95 kg plus bike and load, so 115kg. I run my 28mm wet-bike tyres at 70-75 PSI for the added grip. And my 25mm dry road bike are at 90/100. All are Conti, gatorskins on the wetbike and gp4k/5k on the road bike.
But I know I tend toward harder, because as a kid I rode on No-more-flats which were brutally hard and I got accustomed to that.

I would go for 75-80 PSI in a 32mm road tyre, and then adjust up or down as you see fit. "Squirm" means it is low, add air. While a particularly harsh ride and feeling like you're riding on ice means pressure is too high, drop some.

Answer 4

That's slight overkill. I use 100 PSI on 28mm tires, but then again I'm over 100kg and have to put lot of pressure because I reinflate rarely, once per two weeks (at which point it ain't no longer 100 PSI).

I notice that at 100 PSI, ride accuracy on some sand roads gets lower, which is restored by waiting for the tires to lose pressure and riding again on the same road.

I think the main allure of 32mm tires is that you don't have to use high pressures. You can have high quality low rolling resistance tires that aren't usually available wider than 32mm, but you can use a pressure that allows accurate riding on sand roads.

I would put 85 PSI on 32mm tires for my weight. If your weight is lower, as low as 65 PSI might be acceptable.