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

[thisisntjared]

sweet, i am going for 225s on my civic then.

[ShadowWorks]

This is a good topic, my car came with 195 60 15 tires and I changed them years ago for 205 50 16, I remember thinking this will hurt my FE because the tyre is wider but it didn't happen, I just increased my tire Psi as recommended by THC and it seems great!

[slogfilet]

Divine Bovine -

While I think the "constant of contact patch" theory may be a good rule of thumb and work in some specific applications (you mentioned bicycle tires... there may be some fundamentally different variables at work there), I don't think that the categorical statement that "given the same pressure and load, the contact patch of any given tire will be approximately the same, independent of width" is necessarily true.

In the below study, tire load was doubled, and contact patch increased by approximately 25%.

http://www.performancesimulations.co...on-tires-1.htm

[theholycow]

Holy crap! That is very interesting, and forces me to question the whole foundation of my theories. I'm going to edit my main post above about it.

However, it does offer some additional comfort for people increasing their inflation who worry about the reduced contact patch.

I may do some experimentation myself. It should be easy enough. I could weigh the rear of my pickup, then go somewhere paved that I can use spraypaint. There, I'll load and unload the rear at various tire pressures, and spraypaint the bottom of the tire/ground around it, and measure the contact patch by the un-painted tire silhouette on the ground. I can also measure length of sidewall deflection using a ruler but that will be difficult to do accurately.

The hardest part will be getting ten people to come with me and climb in and out of my truckbed. I could use flat black spraypaint so it won't be visible on my tires...

[Jay2TheRescue]

Originally Posted by theholycow

Holy crap! That is very interesting, and forces me to question the whole foundation of my theories. I'm going to edit my main post above about it.

However, it does offer some additional comfort for people increasing their inflation who worry about the reduced contact patch.

I may do some experimentation myself. It should be easy enough. I could weigh the rear of my pickup, then go somewhere paved that I can use spraypaint. There, I'll load and unload the rear at various tire pressures, and spraypaint the bottom of the tire/ground around it, and measure the contact patch by the un-painted tire silhouette on the ground. I can also measure length of sidewall deflection using a ruler but that will be difficult to do accurately.

The hardest part will be getting ten people to come with me and climb in and out of my truckbed. I could use flat black spraypaint so it won't be visible on my tires...

Rather than putting paint on your tires, or on the ground do this: get a bottle of chaulk dust to refill a snapline at the hardware store. Its cheap, and a lot more Earth friendly than paint. Drive to a flat paved area, like a vacant parking lot. Rub the chaulk dust on the tread of the tire. Then move the truck a few feet. If the tire is not completely contacting the pavement these areas will still have chaulk on the tread.

[theholycow]

I thought of chalk, and I have a bottle of it in my truck now. However, it won't get pressured into going everywhere it needs to go if I throw it at the tire, and what you suggest will only work if I can move the truck without rolling the tires -- I'd need a crane to do it...

[Jay2TheRescue]

Originally Posted by theholycow

I thought of chalk, and I have a bottle of it in my truck now. However, it won't get pressured into going everywhere it needs to go if I throw it at the tire, and what you suggest will only work if I can move the truck without rolling the tires -- I'd need a crane to do it...

Don't throw it. Put on a glove and put a handful in your hand and rub it on the tread. You can get the entire tire except for what's actually touching the ground when parked. This is enough. You don't need 100% of the tread covered. That's what I did to determine my proper tire pressure. I overinflated & chaulked the tires. I then let air out a little at a time and moved the truck a few feet until the contact patch was properly formed.

[theholycow]

Oh, I see. That could work, but I'd have to wash the tire between measurements, and it would be tough to measure.

[ShadowWorks]

I have an Idea, using 10mm Glass and a scanner to actually check the contact patch of the tire, this will take an hour or two.