Tire width vs. Rolling Resistance (canned post)
 

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
https://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:
https://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:
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Tire rolling resistance by model:
https://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: https://www.miata.net/garage/tirecalc.html

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

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!

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%.

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

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.

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.

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

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.

You could chaulk the right rear, then change the setup and chaulk the tire on the left rear. Or you could just reapply more chaulk. No need to wash the tire, because then you'd have to wait till it was dry before you could do any more testing.

-Jay

That was fun:D

It actually worked better than I imagined, I got my laptop and scanner, took it outside, jacked the car up and put in a sheet of 12mm not 10mm tempered glass from Stands Unique, this glass is rated for weight up to 350kg

I sat the glass on some two 2 by 2 beech wood and slide the scanner under the glass, the first scans looked really dark as it needs to be as close as possible to the items being scanned.

Well I roped my friend into this as well.

This was his tires contact patch when dry, its a Dutch Tire which he claims is great for FE

https://www.saabphotos.com/gallery/al..._Dry.sized.jpg

This is from my car, its a Dunlop SP9000.

https://www.saabphotos.com/gallery/al...0Dry.sized.jpg

You can't actually tell what is touching the glass, How much do you think is actually touching the glass?

I was totally wrong in my guess. I used a thinned down water solution to get as little skin as possible and sprayed the glass plate before lowering the jack so all the cars weight was on the glass.

This is all that touched the glass for my friends tire.

https://www.saabphotos.com/gallery/al..._023.sized.jpg

This is how much of my tire touched the glass.

https://www.saabphotos.com/gallery/al..._020.sized.jpg

I need to print this out and calculate the actual contact area but its not much, way less than I expected, I always thought the whole width of the tyre touched the ground, it does not appear to here, yet I know the road is not the same as a glass surface, its probably going to have less contact in reality.

Wow! That is some AWESOME work! You sir are very cool. I commend you on the effort! :D I had no idea that such supplies were readily available to you. This is the kind of experimentation and measurement that we need more of. Thank you!!!!!

I am at least as surprised as you are about the contact patch, as measured by the water. I'm not sure that the water is accurate. What about a thicker liquid, or at least colored water, that could be squished out by the tire? Take a look at the pictures on https://www.tirerack.com/tires/tirete...e.jsp?techid=3 to see what I mean. I suspect you have to try to immerse the whole thing to really see where the contact patch is.

Next, if you could squeeze a measuring tape between the glass and the scanner, or at least mark the glass for measuring in both dimensions, that would help quantify what we're looking at; then it's time to alter pressure and see how that affects contact patch, and it would get a little more complicated to determine the part about width vs. contact patch length.

I've had a bee in my bonnet about getting a 65 series tire vs the a 70 for quite a while, since I saw an old old pirelli ad advertising economy benefits of the "new" 65 series tires.

Anyway with that calculation above and looking at a chart that turned up on ecomodder, it appears that perfect size on 15 inch rims would be 210, and 195 would be perfect on 14s. This probably explains why my stock size should be 195/75R14, but these are made out of platinum plated unobtanium these days, so have had 205/70s on it as what all the tire shops recommend.

However, I've had these lighter 15 inch rims hanging around for months that I'm itching to put on, and have basically 3 choices of tire size to get the right diameter, 215/65, 215/60 and 225/60 with the risk that 225s might rub during "energetic" cornering. So knocking around on ecomodder somewhere is a graphic that shows RR against tire size, and 215/65R15 comes out VERY good, it's in the top 5. Think it was a Californian study. It showed a 10% RR reduction over a 205/70R14...

So... looks like I can go with "better" 215 tires on 15inch rims and get a RR saving at the same time.

Well, the charts I saw (and I don't remember specifically the one you mention) were all very inconsistent. However, the trend (and logic) that I found indicated that upsizing the wheel will almost always increase RR regardless of what tire you put on (unless you go to a tire with a significantly larger outside diameter). Regardless of that, I'm all for upsizing wheels anyway -- you will probably get better handling with shorter sidewalls, allowing you to carry more of your momentum through turns.

195/75/R14 isn't that hard or expensive to buy. I put a set of new tires on my Regal last year and I think they were only about $40/ea. I think the tires were even Michelins. They were a lot cheaper than my old tires were. I used to buy the Goodyear Invicta GAL with the double whitewall before they were discontinued. I used to spend about $110 each on those.

-Jay

Posted in another thread:

That's some pretty good data, and a great test procedure. The only issues confusing the test could be that the tire is taller (same outside diameter but taller sidewalls with smaller rims) and probably a different model of tire (maybe with different wear levels).