Hacker News Reader

Comment by toxik | Hacker News Reader

While true that tire stiffness is a major player in vibration transfer, it also trades off friction. As we all know, stiffer tires (higher pressure) is faster. I think you're reducing a complicated mechanical system to a few variables; mountain bikes, for example, have dedicated shock absorbers. Granted, they're for much harder impacts, but something similar could be said of high-frequency shocks.

View on HN

Too high pressure actually decreases efficiency. While you have less deformation of the rubber, it causes the bike to bounce over irregularities more. The energy from this bouncing is lost. On rougher roads, often pressures below 90 PSI are best.

As a casual cyclist 50 psi already feels like "this is amazing I just hope my tires aren't gonna burst". So it seems "higher is better" is solid advice for most people, who aren't gonna go over 90 psi anyway.

On a road bike, you will get a pinch flat from running tires that low, where the tube gets cut between the rim and the ground.

Recommended pressure always varies. With wider tires, you typically must run them lower, since a too-high pressure will cause the tire to blow off the rim.

Ok, yes, assuming traction isn't lost, stiffer is better -- really, smaller contact area is better, but that translates into stiffness. Then there is heat loss to tire deformation, of course.

Stiffness in a tire is bad. That's why more expensive tires are suppler. Energy lost due to rolling resistance on a paved road is primarily viscous dissipation due to flex of the tire.

If the tire is a stiffer material, this dissipates more energy. In addition, suppler tires deform over obstacles more readily, reducing bouncing and vibration.

More or less, your contact patch is pressure/weight, since the tire can't support much weight on its own. It's going to be almost the same no matter what (slick) tire you are running. Wider tires have less rolling resistance for the same pressure (to a point), since the tire deflects less to maintain the same contact patch. Two tires of the same width, one stiff and one supple, will have to bend in the same way, and have nearly identical contact patches. However, the supple one will dissipate less heat doing this.

> As we all know, stiffer tires (higher pressure) is faster.

This is not correct. Optimum speed comes are optimum pressure, which is usually quite a bit lower than most cyclists imagine. Going too high results in worse rolling resistance, AND worse handling AND worse comfort.

As a ballpack figure for a road bike, a 25c tire will usually have optimum rolling resistance at around 90-100psi for typical male rider mass and road surfaces.

And the faster, better handling tires also have less rolling resistance, but less durability.

> As we all know, stiffer tires (higher pressure) is faster.

We're living in interesting times for this "conventional" wisdom! One school of thought argues that while there might be a theoretical relationship between high pressure and low rolling resistance, but the real-world effect has been exaggerated by poor testing methodology, which usually involves a circular drum as a substitute for a flat road. And that the benefits of achieving low rolling resistance are offset by the losses due to increased vibration.

Of course, this school of thought is largely the product of one man, Jan Heine of Compass Bicycles, but it's interesting to read and consider. Personally, I made the jump from 23mm road tires at 120psi to 48mm slick tires at ~40psi this year, and I have to say, my average speed didn't take a hit at all on the road and am much happier and more comfortable.

https://janheine.wordpress.com/2018/05/02/testing-tires-isnt...

https://janheine.wordpress.com/2016/03/09/tire-pressure-take...

Just curious, with the same set of tires, didn't you ever notice how much easier it is biking uphill when you don't neglect to inflate the tires before the ride? I'm not a competitive cyclist, but I'm on a bike every day and I notice it on my bikes (mountain, thick heavy tires and road, slick tires).

Sure. There's an optimal tire pressure for each tire, and the wider the tire, the lower the pressure. The point I was making was that a wide tire at optimal (low) pressure is not significantly slower than a narrow tire at optimal (high) pressure. However, any particular tire can be underinflated relative to its optimum, and become sluggish.

On my mountain bike it's very noticeable when I'm below 30 psi on a road, but the difference between 30 and 50 is not noticeable in terms of speed (hard cornering does feel a bit different).

I do, the difference is "woah this thing powers itself" kind of noticeable. Sure I could get the same average speed on my ride, but at the cost of so much more effort on low pressure tires.

Bicycle Quarterly put forward the idea that higher presure tyres feel faster because they transmit more vibration. Normally we would feel that vibration increasing as we go faster.

However,even though they feel faster, they’re actually slower. That extra vibration is coming from vertical deflection that a more supple (lower presure) tyre would allow to stay as forward momentum.

Personally I keep “25mm” (GP4Ks always seem to measure closer to 27mm for me) somewhere between 70-80 psi but then I’m the sort of madman that thinks I can feel the difference with latex inner tubes.

Reply on news.ycombinator.com