Let's talk about wheel (rim) size, and its effect on offroad performance

[LocalmotionSC]

Top line question for conversation: What is the effect of wheel size (for a given tire size) on offroad (and onroad) performance? What are the pros and cons to larger or smaller wheels given the same tire diameter?

I've been looking into a vehicle that comes stock with 11r22.5's (standard semi tires on 22.5" wheels). That's a 42x11 inch tire on a 22.5" wheel. That got me thinking, we talk a lot about different tire sizes, pros/cons, gearing etc. But how does the wheel size itself effect performance, efficiency, handling, etc, assuming the same wheel diameter.

Some initial thoughts, I "feel" (in the most unscientific sense) that a smaller wheel to tire ration (i.e. bigger sidewalls) might be better for offroad applications. It feels, like it might be better for airing down the tires, absorbing washboard or flexing over/around rocks etc. Maybe this isn't linear, maybe there is a certain minimum threshold sidewall height and beyond this offroad improvements are minimal. Or maybe my 'feeling' is just wrong/misinformed.

On the other hand, My guess is that a larger wheel saves weight and increases rigidity. That translates to less strain and wear on the axles, drivetrain, etc, and possibly to improved fuel economy. Its possible a higher wheel to tire ratio (smaller sidewalls) translates to better high speed on pavement cornering. I know that its quite popular among the Fuso/Isuzu crowd to replace their stock 16 inch wheels with larger 19.5 alloy wheels from ATW. I believe ATW claims this saves something like 110KG(?). I also know that its not a settled issue and that many people continue to feel the 16 inch wheel is more suitable for their needs.

So what do y'all think (or better yet, what do you know), what hard data is out there regarding wheel size.


extra credit question: does anyone know if these high load (6000lb+), high pressure (upto 120psi) commercial tires can be aired down to common off high PSIs, does vehicle weight play a big role in this?

 

[Joe917]

We are running 315 22.5 80 on our MB917AF. This is a 20 ply, load range L, 10000lbs per tire at 130 psi.
We run 95psi on asphalt and gravel, we air down to 70 ish for washboard and have been down to 40 for sand. A 20 ply tire is stiff but it does lengthen the tread considerably airing down. Dropping 20lbs or so takes the hard hits of washboard . The 22.5 rim and tire combination is the best on road/ bad road tire (for the heavy brigade, I do't include Fuso/Isusu or pick ups).
A similar size tire on a 20" Is the go to tire for off road mud and sand. Generally very soft sidewalls that deflate well, aggressive loud treads. Friends of ours have happily dropped their pressures to 5 psi in soft sand. just don't expect to turn and keep the rubber on.
As far as weight goes these are all heavy combos so your drivetrain has to be up to it. The 20" rubber wears a lot faster but you can generally get cheap gov surplus tires that are being sold off as they age out. Don't plan to go to South America with 20" rims, the tires are not available.

 

[buellconvert]

From my own experiences I feel the same way you do regarding off-road driving is better with more sidewall, more cushion for the pushin kinda thing and a smaller, shorter sidewall on the street if you're looking for handling aspects. Any vehicle wearing the wheel and tire size you're talking about isn't really going to handle like a sports car on the road or a trail buggy off-road regardless of either, but that said, it might benefit even more with adjustments either way based on it's size and weight.

Sent from my moto g(6) play using Tapatalk

 

[OSV]

tire availability can be a factor in picking wheel size.

15" rims for instance, don't have many tire choices.

 

[Peter_n_Margaret]

Some initial thoughts, I "feel" (in the most unscientific sense) that a smaller wheel to tire ration (i.e. bigger sidewalls) might be better for offroad applications. It feels, like it might be better for airing down the tires, absorbing washboard or flexing over/around rocks etc.

Absolutely correct, in my experience.
The original tyre fitted to my OKA was 9.00 x 16 (which became a 255/100R16 metric). They perform very well in pretty much all off road situations. They are good for a max load of about 1700kg per tyre .
Unfortunately, I needed more weight capacity than they could handle and moved to a 305/10R19.5, which is almost the same rolling diameter, but lower profile. They are OK, but clearly not as good off road as the originals were. On the black top they are great and last a very long time. They are good for a max 3075kg per tyre (at 115psi). I run them at 45psi front and 65psi rear on the black top and typically 20psi front and 30psi rear in soft sand, but that varies according to the sand. I typically air down 20% when travelling on good gravel roads and reduce speed.
My off road tyre of choice for this vehicle would be a 325/85R16 MXL. They are a tad larger in diameter compared to the others. They are good for a max of 2300kg per tyre (at 65psi). Their pressure requirement would be less than I currently use for the same weight, and that is important. The problem is that they are expensive and don't last so long on the black top, but from all reports are unbeatable in the bush.

The benefits of airing down are closely aligned to the side wall height. The lower the profile, the less you can air down and the less the flotation gain that is achievable. Sidewall flex also increases more rapidly with a lower profile tyre as it is aired down, so the speed reduction required to stop it from overheating (and being destroyed) is more dramatic.
These are the 305/70R19.5 tyres. The pics are mine, the numbers are derived from Michelin data.

Cheers,
Peter
OKA196 motorhome

 

[LocalmotionSC]

Thank you Peter, this is great hard data and first hand information! I appreciate you sharing your knowledge. That gives me a lot to digest.

I have a couple follow up questions. First, I am under the impression that running tires (at least for heavy trucks) at less than their rated pressure reduces the load they can safely carry. Have you calculated the safe load for your tires at the pressures you are down to?

Second, If I'm understanding correctly, you are saying tires with greater sidewall height can (1) be aired down further (2) achieve greater results (3) better handle the extra heat generated from sidewall flex or maybe not generate as much heat in the first place. Am I understanding the principles at play correctly?

Lastly, I believe the conventional wisdom (and possibly a legal requirement) among on road truckers is that for on-highway driving a tire should always be properly inflated to the rated pressure (115 in the case of your tires) and operating at lower pressure is unsafe and possibly illegal (for commercial vehicles) at highway speeds, and causes tires to heat up though increased sidewall flex and increasing the likelihood of a blowout, not to mention the decrease in fuel economy. You run at roughly half the rated PSI for your tires on pavement, what led you to this decision? Are you satisfied that it is safe and sensible?

Thanks again for sharing your knowledge, its very valuable!

 

[Peter_n_Margaret]

One at a time..

I have a couple follow up questions. First, I am under the impression that running tires (at least for heavy trucks) at less than their rated pressure reduces the load they can safely carry. Have you calculated the safe load for your tires at the pressures you are down to?

Yes and no. The appropriate pressure on the highway at highway speeds is determined by the load, not the other way around.
I did not calculate the appropriate pressures to run, Michelin did that for me. To get the pressure correct you MUST know the load carried by the tyre first. In practice, the "shape of the tyre"( its footprint and its sidewall bulge) should always be the same, irrespective of the load carried, for highway speeds. I run different pressures front and rear because the axles carry different loads.
There is a huge amount of information published by Michelin. It is sometimes hard to get though, but the effort is worth it.

Second, If I'm understanding correctly, you are saying tires with greater sidewall height can (1) be aired down further (2) achieve greater results (3) better handle the extra heat generated from sidewall flex or maybe not generate as much heat in the first place. Am I understanding the principles at play correctly?

That is a generality that I believe to be true. But be careful. Some tyres have heavier sidewalls. They are also less suitable for airing down because the thicker the sidewall, the more heat generated and the less the rate of cooling, so you need to go slower so that the rate of heat generation is never higher that the rate of cooling (unless it is just for a minute, or so). Overspeed will heat the side wall and the belt very rapidly and lead to total tyre failure (maybe some time in the future, as damage may not be immediately visible).

Lastly, I believe the conventional wisdom (and possibly a legal requirement) among on road truckers is that for on-highway driving a tire should always be properly inflated to the rated pressure (115 in the case of your tires) and operating at lower pressure is unsafe and possibly illegal (for commercial vehicles) at highway speeds, and causes tires to heat up though increased sidewall flex and increasing the likelihood of a blowout, not to mention the decrease in fuel economy. You run at roughly half the rated PSI for your tires on pavement, what led you to this decision? Are you satisfied that it is safe and sensible?

No. See answer 1. I use Michelin recommendations to the letter.
I have several pages of speed/load/pressure (they are closely related) charts from Michelin for the 305/70R19.5 XDE2s that I run. I can't post them here because they are copyright and I respect that, but I did convert their data to a chart the I use constantly that is wholly derived from that data. It covers the range that I use, not the whole range for the tyre.

EDIT: You will notice that the pressure/load relationships are very close to linear, so without the full data you can come close by extrapolating the maximum load/pressure printed on the tyre for highway use. Just be a bit conservative and add maybe 10% pressure.
For airing down it is a bit harder to get right.
Cheers,
Peter
OKA196 motorhome