Tire width vs. Rolling Resistance (canned post)

[theholycow]

Note: I posted this in another thread, but I realized it makes more sense to just post it here and link to it when it's needed in other threads. I plan to edit this as time goes on and I refine it.

Tire Width vs. Rolling Resistance
A canned post by theholycow
Originally posted at
http://www.gassavers.org/showthread.php?t=7713
Please include the link if you post/distribute this elsewhere.

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Edit 2009-10-18: An interesting discussion on tire width, traction, and friction by people who study physics:
http://www.physicsforums.com/showthread.php?t=330790
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Edit: New data has come to light that breaks my theories. I am much less sure that what I say below is true than I was before. Scroll down to a post by user slogfilet with a very interesting link. I can't say for sure that the new data is correct, or even analyzed correctly, nor can I say if it's wrong. I hope to do some of my own testing.

Another edit: Additional study brings out another question and supports wider tires as having lower RR. RR is dependent on pressure and load. A wider tire at the same height (not the same ratio, mind you) has a higher load capacity, so with a given weight is loaded to a lower percentage of its capacity. Also, there's this quote found from someone else's analyzation:

Michelin Tiger Paw AWP P225/60R16 at .00683 - a 25lb tire. On both these model lines, the smaller/lighter/narrower the tire gets, the higher its RRC/4.

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Tire rolling resistance by model:
http://www.gassavers.org/showthread.php?p=110700
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It is commonly thought that narrower tires have less rolling resistance and a smaller contact patch. This is not entirely true. They do offer weight and aerodynamic advantages, though.

Given that everything else is equal (construction, materials, tread pattern, air pressure), the wider tire will have LESS rolling resistance. Rolling resistance comes mainly from sidewall deformation -- look at where the tire meets the road, it's partially flattened instead of perfectly round, and the sidewall bulges to make that happen.

Some people are concerned about flex in the tread blocks causing RR; comparing most car tires, tread squirm should not make much difference in RR, though for knobby off-road truck tires compared to normal road tires the difference would be large. Perhaps even particularly aggressive treads on high performance road tires could have a little extra RR, but I doubt it would be much.

Now, on to why wider tires have less RR. The contact patch size is determined by the weight on the tire and the pressure in it. At 50psi (pounds per square inch) with a 500 pound load, that's a 10 square inch contact patch. If that tire is 5 inches wide, it will have a 2 inch long contact patch -- so only 2 inches of sidewall must deform. If the tire is 2 inches wide, it will have a 5 inch long contact patch -- so 5 inches of sidewall must deform. The narrower tire in that extreme example will have far more RR.

How and why did I learn this? I was an avid member of rec.bicycles.tech and a bicyclist with tired, achy legs. When it's your own legs, sweat, and pain on the line, it becomes much more important to really understand and reduce losses than when it's just fuel economy. There's some very knowledgable folks on that newsgroup who have done the in-depth scientific research about things like this. On a bicycle the narrower tire is usually rated for higher pressure, so it ends up with equal or better RR anyway. On a car, that's not usually the case.

The other advantage for wider tires is better cornering, so you can carry more of your momentum through the corner.

Note: This all applies well when you're choosing between the same model of tire to put on the same rim; if you're going to change to a different manufacturer/model tire it probably still applies well, but if you're changing the size of your rims at the same time all bets are off. A larger rim means a shorter sidewall, which will have to deflect more sharply, which probably means increased RR. I do not have any research or deep thought into that question, though, so I could be wrong about that.

Also of interest when choosing tires, this tire size calculator will help you determine the height of a different size tire, and the effect it has on your gearing, speedometer, and odometer: http://www.miata.net/garage/tirecalc.html