Tire size and FE with a Jeep

[CapriRacer]

Some thoughts:

1) Differences between tires - meaning make and model - can have a HUGE effect on rolling resistance.

2) Increases in tire size improves rolling resistance, but it's effect is small compared to the difference between tires (See #1)

3) Aero effects caused by tire size differences are even smaller than differences in rolling resistance caused by tire size differences.

4) Changing from P metric to LT metric or Flotation sizing is going the wrong direction for rolling resistance. LT metric and Flotation tires are built out of similar materials and RR is hardly a consideration. If it ever becomes a consideration, they will always be worse than P metric tires because of the loads they are designed to carry.

I support all of that here:

Barry's Tire Tech

So if you are going to report on tire size changes, be aware that the largest effect in rolling resistance is going to be the tires themselves and not the size change. (Please note: I am not commenting on the effect tire size may have on engine rpm and the change in fuel economy derived from that change. I am not aware that there is any data on that!)

[stovie]

I was thinking that a good way to increase your FE in that jeep is to get class d tires i think they Are because there able to hit 80 psi and i figured that with LRR tires all they've done is increased tire pressures, i mean think about it this way my jeep is 5300 pounds ok 2750 in front 2550 in the back if you take though's and divide them by 2 and then your tire pressure it gives you square inches that your tire is on then you divide by tire width and you have inches from front to back and the higher the pressure the less tire contacting the road and less RR.

there you happy now Frank

[Frank Lee]

dr vna gv ths gy sm vwls k? tnks

[CapriRacer]

Originally Posted by stovie

I was thinking that a good way to increase ur fe in that jeep is to get class d tires i think they r b/c there able to hit 80 psi and i figured that with lrr tires all they've done is increased tire pressures, i mean think about it this way my jeep is 5300 pounds k 2750 in front 2550 in the back if u take though's and divide them by 2 and then ur tire pressure it gives u square inches that ur tire is on then u divide by tire width and u have inches from front to back and the higher the pressure the less tire contacting the road and less RR.

First, LRR tires are all about the materials used. You can test differing tires of the same size at the same load and inflation pressure and get HUGE differences in RR.

Clearly increasing the inflation pressure does decrease rolling resistance. But what are labeled as LRR tire is connected to the RR when you compare them at the same conditions - meaning size, load, and inflation pressure.

And, no, the formula you've provided is not valid. You can not calculate the area of the footprint just given the load and the inflation pressure. Put another way, the average ground pressure of a tire's footprint is NOT the same as the inflation pressure.

And in case you are thinking that the friction with the road is what RR is all about, that would be incorrect. RR is mostly about the internal friction the material in a tire generate.

[stovie]

Check out this explination on tires and there psi to weight figures http://auto.howstuffworks.com/tire4.htm
it says u can guess pretty close to the weight of your vehicle just by looking at the area of your contact patch and yes i understand that your not going to be exact or anything with that calculation but i think you would get pretty dang close. I don't think that friction with the road is all RR is about but if you increase the tire pressure not only are u decreasing the contact with the road but the amount of deforming the tire does therefore less energy converted to heat in the tire now if you could get class D tires with 80Psi max and with the low rolling resistance material then you'll have really good FE gains. i got the idea from watching mythbusters the other night they took tires that had a max psi of 35 and increased it to 40 and got a 6.5% increase in FE and thats 2 points better then the best LRR tire i've seen so far but if you find a better one then let me know ok.(and yes i know i'm not goin psi to psi ok but it steal works)

[CapriRacer]

Originally Posted by stovie

Check out this explination on tires and there psi to weight figures HowStuffWorks "How Tires Work"
it says u can guess pretty close to the weight of your vehicle just by looking at the area of your contact patch and yes i understand that your not going to be exact or anything with that calculation but i think you would get pretty dang close. I don't think that friction with the road is all RR is about but if you increase the tire pressure not only are u decreasing the contact with the road but the amount of deforming the tire does therefore less energy converted to heat in the tire now if you could get class D tires with 80Psi max and with the low rolling resistance material then you'll have really good FE gains. i got the idea from watching mythbusters the other night they took tires that had a max psi of 35 and increased it to 40 and got a 6.5% increase in FE and thats 2 points better then the best LRR tire i've seen so far but if you find a better one then let me know ok.(and yes i know i'm not goin psi to psi ok but it steal works)

Sorry, it's an old wife's tale that a tire's contact pressure and inflation pressure are somehow related in a linear way. Here's a link that is a lot more detailed on the subject:

Fact or Fiction? Tire contact patch and air pressure.

As you can see, the contact pressure and the inflation pressure don't seem to be very close at all.

And as far as Mythbusters is concern. I saw that episode, too. They measured the fuel economy at steady state - and the effects on inflation pressure are a lot more easy to measure - which is why they did it.

But let's start off with this as a basis:

http://www.energy.ca.gov/transportat...%20Testing.pdf

Bruce Lambillotte of Smithers Scientific Services is reporting a 60% difference between the best tire and the worst tire in 2 different sizes - pages 11 and 12.

That is HUGE - and much bigger than any test results I have seen for inflation pressure increases. Don't get me wrong - I am not saying inflation pressure increases don't improve fuel economy, but compared to what the differences can be between tires, changes in tire size are small, and changes in inflation pressure are next, but differences in tires is the largest of them all.

And you have to be careful when you change tire types. P metric tires are constructed quite different than LT metric tires - and as a general rule, LT metric tires - even with their 80 psi pressure - can be completely out classed by an efficient P metric.

[Frank Lee]

I did my own measurements of pressure vs contact area and indeed there was no direct correlation.

[stovie]

so then technically contact size has nothing to do with efficiency and only the deforming of the rubber on the tires and the internal friction but increasing tire pressure would reduce the tires deformation therefore increasing FE by reducing the heat generation right???(and also tire material for internal friction)man i hate when people publish bad info on the web but thanks for the redirect

[CapriRacer]

Originally Posted by stovie

so then technically contact size has nothing to do with efficiency and only the deforming of the rubber on the tires and the internal friction but increasing tire pressure would reduce the tires deformation therefore increasing FE by reducing the heat generation right???(and also tire material for internal friction)man i hate when people publish bad info on the web but thanks for the redirect

Not exactly. The size of the contact patch does relate to rolling resistance. However, the size of the contact patch is not linearly related to the inflation pressure nor is rolling resistance.

Also, you will find many old wife's tales on the internet. It is no better than the average group discussion.

[stovie]

"Not exactly. The size of the contact patch does relate to rolling resistance. However, the size of the contact patch is not linearly related to the inflation pressure nor is rolling resistance."

Sorry i was trying to get to this conclusion but i was in a hurry. i was also just wondering if the bulg of the tread could have a pretty big factor in RR for example my dad has 31 inch tires with 1/4 inch bulg will i have 29's with 3/4 inch bulg both pairs are new?? Thanks agian