Tire width vs. Rolling Resistance (canned post)

[Jay2TheRescue]

Originally Posted by theholycow

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

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

[ShadowWorks]

Originally Posted by 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.

That was fun

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



This is from my car, its a Dunlop SP9000.



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.



This is how much of my tire touched the glass.



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.

[theholycow]

Wow! That is some AWESOME work! You sir are very cool. I commend you on the effort! 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 http://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.

[GasSavers_RoadWarrior]

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.

[theholycow]

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.

[Jay2TheRescue]

Originally Posted by RoadWarrior

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.

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

[theholycow]

Posted in another thread:

Originally Posted by Lug_Nut

Narrow width and high pressure don't always offset the higher rolling resistance.
I did some low speed coast down tests in advance of the 2005 Tour de Sol with the Passat I had at the time. From a sandstill on a slight slope I'd release the brakes and see how far I coasted before coming to a stop. Four Continental T135-80-15 at 60 psi did not coast as far as four 185-70-14 Michelins at their max of 32 psi. The best of the temporary tire roll-down distances was not as good as even the worst of the full size tires' distance.

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).