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Comment by xorcist | Hacker News Reader

Sounds like the optimum size was a little bit off what was previously thought, but not by much.

28mm is still a very narrow tire, by any other standard than racing bikes. It's narrower than any utility bike and about half than you would run on a mountain bike, and describing the difference as radical seems a bit excessive to an outsider. Those tire pressures also probably excludes anything close to a flat tire.

It's not like anyone thought harder tires were always better. Otherwise they would all be running metal or wooden tires. After all, that was what everyone did before rubber tires were invented, which they were because they were more efficient than bumping around on wood. If someone had guesstimated the optimum at 23mm and 120psi and it was measured at 28mm, was it really that bad?

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28mm is not the optimum. It's all I can fit on my bike. I'm not sure what the state of the art is -- the podcast mentions testing 50mm and wider tires, and running less than half the pressure of what people used to use. They're talking about, basically, the widest possible tire you can fit, and the lowest pressure you can go (including going tubeless) without the tire coming off the rim in corners or cracking a wheel in an impact.

It's truly a remarkable change in strategy and thinking. As you go wider it's unquestionable that the weight and aerodynamic properties get worse, which means for wider tires to be more efficient, the rolling resistance decrease has to be huge to overcome the aero loss.

There will be an optimum point, where the benefit of the wider tire is balanced out by the extra weight of the rim and extra air resistance. Maybe the rim and tire merge, with spokes or struts connecting directly to a wide rubber rim/tire.

Absolutely true; the only problem is that, quite literally, every road surface is different, so that perfect point changes from second to second, sometimes very subtly, and sometimes in much larger ways.

It’s a huge difference — you’re increasing by 20-40%!

Which means you’re dropping to 75-100 PSI... which is also a big change from 120 PSI.

1.5 the width and 0.65 the pressure is a “big” change for a part like a tire.

That it’s because we assumed something untrue about the mechanics is fascinating.

wow, this is the first time it was pointed out to me that I shouldn't be running my new fat tires at 120 PSI anymore. (face palm).

It's worth giving the podcast I linked a listen. In fact, I just listened to it a 2nd time yesterday after posting this. And it's 4 years old already!

It sounds like the primary reason you need(ed) high pressure, in addition to assumptions about rolling resistance, was to increase air volume to protect the rim. As the tire gets wider, the air volume goes up, so the required pressure to protect the rim goes down. Which is why a narrow road tire can have a rim impact at 40psi where a mountain bike tire would not have a rim impact on the same obstacle at 40psi. So, the wider tire ALLOWS you to run lower pressure and not damage the rim with impacts -- completely leaving rolling resistance aside.

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