Reducing Tire Width

[CapriRacer]

Originally Posted by Meph

.........Im very much in limbo as to what I should do.......

I'm sorry, but I am going to add to your confusion.

I just redid the regression analysis, using only 3 parameters - Section Width, Aspect Ratio, and Rim Diameter - and got much less of a size affect on RRC. While I am still examining the analysis to see if I can find any errors, one thing did remain the same. It still seems to point to load carrying capacity as being a major driver in RRC: Larger = Better.

I'm going to take a few days to ponder this new analysis and will likely update my web site to reflect the new analysis (deleting the old one!) So if you go there, please be aware that it may change in a few days.

Originally Posted by euromodder

Or pick the higher load index versions (XL) of a given tyre size.

That'd help too, wouldn't it ?

No, because XL versions have the same load carrying characteristics as SL versions, except they allow the use of higher inflation pressures. When a tire is applied to a vehicle, the load is the same regardless of the tire size or the load rating of the tire.

[robchalmers]

Its also one hell of a compound change, as a racer you must realize it'll be a little twitchy!

[euromodder]

Originally Posted by CapriRacer

First, no one is going to be able to answer your question with any degree of accuracy. To understand why, you need to read this:
Barry's Tire Tech
and this:
Barry's Tire Tech

Is it certain that the different tyres were indeed tested with a different load ?
If I was to study the effects of tyre size, I'd study the effect of all allowed tyre sizes on a given vehicle (i.e. always with the same vehicle weight).

In that light, I'd come to the opposite conclusion - and each and every car manufacturer is complying with that conclusion as they're fitting their eco-models with skinny tyres.
If wider tyres were beneficial, surely they'd all fit those instead of narrower tyres ?


The study also has a graph (p. 35) correlating RR (actual force, not coefficient) to LI, with higher LI also resulting in higher RR.
(83% correlation, and even 86% to max. sidewall load)
Higher LI (and max sidewall load) usually goes together with wider, lower profile tyres.

Same with tyre weight, overall diameter - which both show a 75% correlation with RR - where increasing weight and diameter would usually indicate a wider, lower profile tyre.

All these then result in the graph on page 30, clearly indicating higher RR (actual force) for the wider tyres.

(What I'm missing in the study, is a graph of RR versus profile height.)

In the end, you need to overcome the actual RR force of a tyre at a given load, not a RR coefficient.


The study concludes :
Rolling resistance rolling forces and rolling resistance coefficients did not correlate.

But I fail to see where it comes to the conclussion that wider is better.

[euromodder]

Originally Posted by euromodder

Or pick the higher load index versions (XL) of a given tyre size.
That'd help too, wouldn't it ?

I'd like to withdraw this statement.
See previous post.

[Frank Lee]

Something seems seriously amiss here for I recall reading from what I think is a credible source that rr is least when the tire has the FEWEST plies it can get away with for the load. And that intuitively makes sense to me if I agree with the statement that it is in the tread not the sidewalls where most of the losses originate.

[rfdesigner]

Well I have experience of going narrower and in my case it made a difference.

I was driving a Merc W124 E-Class coupe a bit like this one:
File:Mercedes W124 Coupe rear 20071022.jpg - Wikimedia Commons fairly slippery but heavy and it had low profiles on it.. as the car aged and I didn't want to spend all my money on tyres I decided to back to the narrower and higher profile stock tyres. This added ~5% to my FE.

So if you drive, say, a large but unloaded Van then your Cd will dominate, if however you drive, as I do, a relatively aerodynamic but heavy car then Crr will show itself.

Added to that is speed, if you always drive well above the point where Cd>Crr then dropping your Crr will not make much difference.

Derek

[CapriRacer]

Originally Posted by euromodder

Is it certain that the different tyres were indeed tested with a different load ?....

Yes, that is how it is always done.

Originally Posted by euromodder

....If I was to study the effects of tyre size, I'd study the effect of all allowed tyre sizes on a given vehicle (i.e. always with the same vehicle weight).

In that light, I'd come to the opposite conclusion - and each and every car manufacturer is complying with that conclusion as they're fitting their eco-models with skinny tyres..........

I think if you did that, you'd find that RRC was very close regardless because the tires are pretty closr load carrying capacity-wise. It's only when you make a major move upwards in load carrying capacity that you see significant changes,

Originally Posted by euromodder

............If wider tyres were beneficial, surely they'd all fit those instead of narrower tyres ?............

Except that there are cost, packaging, and ride issues to consider. Plus, isn't that what is happening?

Originally Posted by euromodder

..........The study also has a graph (p. 35) correlating RR (actual force, not coefficient) to LI, with higher LI also resulting in higher RR. (83% correlation, and even 86% to max. sidewall load)
Higher LI (and max sidewall load) usually goes together with wider, lower profile tyres...........

Don't forget the RRF (Rolling Resistance Force) is meaningless unless the load on the tire is the same from size to size - which it isn't. When you apply a tire the load is determined by the vehicle, so RRC is the proper graph to look at.

Originally Posted by euromodder

..........Same with tyre weight, overall diameter - which both show a 75% correlation with RR - where increasing weight and diameter would usually indicate a wider, lower profile tyre.

All these then result in the graph on page 30, clearly indicating higher RR (actual force) for the wider tyres.........

Be careful looking at RRF. It will lead you to incorrect conclusions.

Originally Posted by euromodder

.........(What I'm missing in the study, is a graph of RR versus profile height.)..........

It embedded in there if you want to study it. Which is why I've redone the regression analysis.

Originally Posted by euromodder

........In the end, you need to overcome the actual RR force of a tyre at a given load, not a RR coefficient.........

Sorry, but while you are correct the it is the actual force that needs to be overcome, that value is generated by multiplying the load on the tire by RRC.

Originally Posted by euromodder

....The study concludes :
Rolling resistance rolling forces and rolling resistance coefficients did not correlate......

I thought that was an odd way to phrase it. I think would they meant was the both RRF and RRC did not correlate to something independent that could be used as a regulatory meme.

Originally Posted by euromodder

............But I fail to see where it comes to the conclussion that wider is better.

It doesn't, which is why I did the regression analysis. What I found most striking was that the RRC values were significantly lower for very large tires - and that lead me to do a deeper look.

[SlideWRX]

On my Mustang, switching to 255 fronts and 285 rears (from 235 width all around) resulted in less than 1 mpg change on highway trips. The car is rated 15City/23Highway, and on flat terrain I could get 27 pretty consistently. Switching to wider tires didn't seem to change that, although I only have one big trip on the new tires for comparison. Anecdotal I know, but it didn't appear to change things much.

Different tires on different wheels; there's a lot of changes to consider. For me it didn't change much.