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Is the maximum tire pressure always the fastest on a road bike? I've seen the Michelin chart on tire pressure from another question ("What pressure should I run my Road Bike tyres at?"), but they don't mention whether it is optimized for speed, safety, comfort, or some combination of these.

I have heard that the more pressure a tire has, the faster it goes. But this is not always true on a mountain bike, depending on the road/trail surface. Could it be that a smoother, less jittery ride at 105 psi might be faster than 120 or 130 psi on a road bike?

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I think this question is badly worded and that you're trying to ask, "Why does the chart suggest using less than the tires' maximum pressure, when the rider's weight is lighter? What are the advantages of inflating a tire to less than its maximum pressure? I understand that it improves comfort, but what about speed? If you want to optimize only for speed, should you not always prefer the tire's maximum pressure?" –  ChrisW Aug 19 '11 at 13:40
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just came across this on the bicycle subreddit: bikequarterly.com/images/TireDrop.pdf –  fady May 11 '12 at 22:12
    
See also: bicycles.stackexchange.com/questions/2744/… –  amcnabb Oct 28 '12 at 2:15
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There is an example of rolling resistance on real roads increasing with increasing pressure about two-thirds of the way through this article. –  R. Chung Oct 29 '12 at 12:15

7 Answers 7

up vote 8 down vote accepted

I generally agree with Angelo's answer. Tire diameter and your weight will determine the optimal pressure. Beyond that, this is actually a controversial subject.

Let's assume that you want to minimize rolling resistance. "Rock hard" isn't necessarily the best approach. One widely cited opinion is that above a certain pressure, there's no benefit in terms of rolling resistance, and the target is a 15% "drop" (vertical deformation under load)—it is argued that even with less drop than that (i.e., higher pressure) you do not see an improvement in rolling resistance, so all you're doing with that extra pressure is making the ride harsher.

Here are a couple of articles (pdf) discussing pressure, rolling resistance, and tire drop. You can see that there's a diminishing-returns curve in terms of power savings at higher pressures. The second one includes a graph showing how much you should inflate a tire to get a 15% drop depending on how much load there is on that wheel and how wide the tire is.

I should point out that this opinion is not universally held. Jobst Brandt, for one, is skeptical, but I'm not sure what he recommends.

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Those links go to the same file. Rolling resistance in that article is linear with respect to speed because they test it on a smooth surface (wheel). I doubt if it is linear with a load on a road surface. –  xpda Aug 19 '11 at 14:34
    
Thanks, I've corrected the 2nd link. If the road surface is rough, is rolling resistance going to increase geometrically or something? I've never read anything to that effect. –  Adam Rice Aug 19 '11 at 15:20
    
In a rough road, there is a point in the tire pressure where the wheels start bouncing. This would mess up the nice linear resistance at least at that point. I'm not sure what happens beyond there. –  xpda Aug 20 '11 at 1:48
    
@xpda: the rider falls off, making rolling resistance very high (and painful). High pressure bouncing is bearable in a straight line but when cornering it results in premature loss of traction. So there's a happy medium between traction and rolling resistance that depends on weight. –  Kohi May 7 '12 at 23:43

On a perfectly smooth road you want your tires to be equally smooth and rock-hard.

On a rougher road it's more efficient if the tires have enough "give" to "levitate" you over the smaller bumps, so that you're not using a lot of your energy to make the bike bump up and down.

So on a "sorta bumpy" road you'd want to have tires that weren't quite rock-hard. On a really bumpy road you'd want tires that are more along the line of mountain bike tires, running at a significantly lower pressure. Basically as hard as possible without transmitting too many of the smaller bumps to the bike.

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Your optimal tire pressure depends most importantly on the width of the tire and your weight. There's no formula. The recommended pressure on the tire is usually a lower bound for most folks, it is not unusual to exceed it by 10 psi (and sometimes a lot more).

If you have trouble with pinch flats and your tire is already very hard to the point of being jarring, you need to get a wider tire.

Here is one trick that I've already mentioned before: since your front wheel is under less load than the rear, it is OK to put less air pressure in it than the rear. This will dampen the road shock transmitted to your hands a slight but noticeable amount.

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It depends how fast you're going.

At relatively low speeds, what matters (i.e. the biggest force slowing you down) is rolling resistance.

At relatively high speeds, what matters is aerodynamics.

At high (i.e. racing) speeds, then, you need a narrow/thin tire to minimize wind resistance. A narrower tire requires a higher pressure than a wider one (e.g. if you'll forgive my non-SI units, a 23 tire at 130 psi compared with a 32 tire at 85 psi).

I'm not sure I understand your question, though: I don't think you'd race a road bike on a "trail surface" (you'd do it on a paved road).

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I meant for a road bike tire of fixed width, say, 23mm, is the maximum pressure the fastest on an ordinary street? –  xpda Aug 19 '11 at 22:24

Professional track racers will generally run on tubular tyres @ around 250psi. Yes, that's not a mistype. That's what tubs give you over clinchers.

On that basis you can probably guess what my answer to your question would be...

(I remember seeing this number out of Graham Obree's autobiography and being abasolutely staggered. This guy held the world hour record so I am prepared to accept he knows what he's on about.)

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On a smooth track, rolling resistance is least with maximum pressure. That's why trains use steel wheels. A track is smooth, but roads have regular bumps half the diameter of the tire. If a tire has no "give", then the bike bounces instead of the tire deforming, which costs energy. So there is a tradeoff. –  xpda Oct 26 '12 at 20:10

From my own personal experience on a paved road, I found that I am generally a lot faster on my road bike when the tires are inflated to maximum pressure. I found that a lower inflated rear tire slows be down by as much as 11 minutes on a 15 mile ride. But I must add, that I was riding with a tailwind on my way back with the inflated tire. Nonetheless this 11 minute difference is accurate for the most part as I did the slower trial 15 mile run with the lower pressure tire fighting a headwind. So I guess the wind factor sort of cancel each other off.

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From my own personal experience, inflating the rear tire to the maximum would be a lot faster. They also tried with mythbusters with cars. saving fuel by over inflating tires. My tires are rated 50 PSI as recommended pressure, and 65 PSI for maximum pressure. when I go for 60 PSI the 6th gear feels like 5 and it enables me to maintain higher average speeds. As an added bonus, inflating tires near the maximum rated pressure makes it more puncture resistant. The only drawback is it a little bit bumpy. I use bike for short distances (10 KM on average), so speed is the most important factor for me. Hope that this helps.

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