Effects of changing rim widths on contact patch 3

[TMcRally]

For off-road use.

For a given tyre size - 35" X 12.5" X 18", when the air pressure is dropped to 10psi from 40psi- am I better off with an 8" wide rim or a 12" rim for straight line traction and which one will wrap around ground irregularities like rocks better?

 

[GregLocock]

Using a narrow rim will give you a more flexible sidewall for a given psi, but perhaps with the wider rim you can run lower pressures. I do have a tire simulator, which says that for a light truck tire 26570 on 7.5" rims, at 20 psi, vertical stiffness would be 228 N/mm, and increasing the rim width to 8.5 would increase that to 243, 6.5" would be 212, and then pumping that up to 23 psi would give 227 again.

I'd trust that as far as I can throw a wheel, and we are far into extrapolation (I don't think I have a single test result below 30 psi), but at least it gives you an idea of the sort of sensitivity.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[CapriRacer]

For every tire size, there is an allowable rim width range. For a 35X12.50R18 it is 8 1/2" to 11". Within that range, any performance difference is minimal.

But if you use too narrow a rim, the tread face will arch and ride in the center. And if you use too wide a rim, the tread face will de-arch and ride on the shoulders. Neither of which is good for handling irregularities in the road surface nor for straight line traction.

The biggest problem here is that a tire's footprint is designed to operate within a window of load/inflation combinations. That means that if 40 psi is correct for on road usage, using 10 psi for off-road usage is a compromise to allow the tire to work successfully in those conditions - albeit not optimally.

So the question you are trying to ask is: How badly do I want to compromise the on road performance in order to get off road performance - using rim width and inflation pressure as variables? To answer that question, you need to know more than what you have published here. You need to know what the vehicle weighs (both front and rear weights!), the amount of on road vs off road usage you are going to see, etc.

 

[TMcRally]

Hi Capriracer, 100% only looking for off-road capability, I disagree the difference will be minimal. The tyre manufacturer provides tyre data based on 40psi to meet street acceptable performance however recommends 15psi for some off-road use so I assume has allowed for this in the design. I lower pressures further as the benifits in traction improve exponentially as the pressure drops and I hate getting stuck . I would say at 40psi the tyres are not fit for purpose and start to become suitable below 20psi.

I do quite a lot of sand driving/towing and I have formed the opinion that a longer thinner contact patch provides more straight line traction than a wider/shorter one. I haven't done any credible testing it's just an opinion based on experience. I feel this is from having a better moment of effort to float the tyres and a lower % of flexing as the tyre goes down and pulls up from the contact patch. A bit like mimicking a taller tyre.

I am convinced not only the area of the patch improves traction and floatation but the shape (dynamically), I think the rim width will influence the shape. I take your point about the side wall controlling the flatness of the patch at various pressures and as Greg Locock has said above the side wall verticle stiffness is affected by the rim/tyre width ratio.

The car weighs approximately 2500kgs loaded and loaded would be roughly 50/50 f/r (guestimate) so approx 625kgs per corner.

 

[CapriRacer]

First, I notice the mixed units: psi vs kg

Second, I disagree that you are 100% interested in off road capability. Unless you are trailering your vehicle to the site, or it is permanently stationed there, there is some part of the journey where you are on paved roads - and you have to be interested in what happens there. Tire failures can have tragic results.

The load carrying capacity of a 35X12.50R18LT at 40 psi is 1150 kg - which means at 625 kg, it is running at 54%. That sounds like the tire pressure is way too high. At 25 psi, the load carrying capacity is 825 kg = 76%. That seems more like it.

So I gotta ask, where did the 40 psi come from?

A related question is, does your vehicle have a vehicle tire placard that lists the original tire size and inflation pressure? Placards are usually on the doorframe, in the glove box, of the fuel filler door, or on the trunk lid. The combination of the original tire size and pressure gives a clue as to how the vehicle manufacturer viewed this inflation pressure vs load thing. (Unless your vehicle is heavily modified.) There may also be a sticker that gives the front and rear GAWR's (Gross Axle Weight Ratings). That would also be a clue for the weight distribution.

And my understanding of what happens when you use a lower inflation pressure with the same load is that the footprint pressure distribution shifts towards the shoulders and away from the center.

Also my understanding of traction in soft terrain is that you want pressure distribution to be as even across the tread face as possible. One of the problems associated with sorting this all out is that most people can't control enough variables when they do seat of the pants testing.

Ya' see, soft terrains can vary quite a bit. Sometimes there is harder material below the surface that the footprint can penetrate down to. Think of what happens in snow where the tire can penetrate down to the road surface. The traction of the road surface is way, way more than snow, so a skinny footprint will feel like there is more traction, when if the snow is deep enough, it actually has less traction.

So if lowering the pressure helps traction, that is away from a long narrow footprint and towards a wide footprint.

 

[SwinnyGG]

there is some part of the journey where you are on paved roads - and you have to be interested in what happens there.

Standard practice in the off-roading community is to drive to the trailhead at whatever street pressure you want, and then drop pressures before the off-roady bit begins. You'll occasionally change pressures in the middle of the trail if its made necessary by a change in conditions. On-road performance absolutely does not matter in this dicussion.

With that said - @TMcRally, you're overthinking it.

There is no way to calculate an exact 'perfect' off-road tire pressure. There are way too many variables, up to and including the exact tire you're using; different tires will have different levels of sidewall stiffness, tread section stiffness, and corner stiffness - all of which will affect how much the tread section will 'cup' due to a change in pressure, or load, or rim width.

The shape of the tire will change drastically under load at lower pressures. This means that if you calculate some 'perfect' tire pressure for when the vehicle is perfectly level, that pressure will not longer be 'perfect' once the vehicle is on an incline in any direction, because load will shift. The tires, and the shape of their contact patches with the ground, will change in response to that change in load.

The only way to determine what works best for your exact vehicle/load/specific tire/rim combination is to experiment. Which, it appears, you are doing already.

 

[TMcRally]

Hi SwinnyGG

Agreed on tyre pressures regarding trial and error. On sand, the lower the pressure the more motive force (not sure if that's a correct use of the term, but it's descriptive for me for floatation and straight-line drive). I have occasionally lowered to 6psi to get out of trouble but air up straight after. Between 10 & 12 seems a good working pressure (previous tyres have needed some variations of this) and the tyre at that pressure looks and feels to have close to minimum acceptable sidewall stiffness. Much less than this and you can see and feel the lack of support. I am always mindful of rolling a tyre off the bead when turning or traversing a hill and intend to get some form of Beadlock wheels this time with a heavier vehicle.

My main concern at present is which rim width will give the best "motive force". I see pros and cons for narrow and wide options. I initially thought narrow rim would give me best results but Greg has bought up a good point, that I will likely be able to run less pressure with a wider rim for the same sidewall stability.

CapriRacer, I had a good look at my tracks in wet sand yesterday with 10psi expecting to see deeper penetration at the shoulders but the track was pretty even right across.

40psi is the minimum I use on tarmac with the current 4x4 anything much less than that and the slip angles when cornering are too high even at moderate speeds. My usual pressures are between 40 and 45 it can be difficult to distinguish between the tall tread lugs moving around and the sidewall flexing but in that range works for me. The vehicle recommend pressures are 26/39 f/r (Nissan D22 twincab ute) it would be terrible to drive using those pressures. Science V's seat of pants.

I am a bit old school on seat of pants testing and I see benifits in a mix of it and the on paper calculations, but as stated above "we are far into extrapolation" can be the downfall of the calculations. The bottom line is the driver has to be comfortable with the vehicle and have confidence in it.

 

[SwinnyGG]

My main concern at present is which rim width will give the best "motive force". I see pros and cons for narrow and wide options. I initially thought narrow rim would give me best results but Greg has bought up a good point, that I will likely be able to run less pressure with a wider rim for the same sidewall stability.

I should've gone all the way in my previous post...

What you really care about, and are attempting to control (as you've already acknowledged) is the shape of the contact patch. How you get to the 'ideal' contact patch shape is a mixture of exact tire choice, rim width, and pressure. You've already figured out that on your current rig, 10-12 psi is the ideal tire pressure value based on the experimentation you've done up to now.

What I was getting at, although I didn't really say it.. is that the rim width doesn't matter all that much; all that will happen is that your 'ideal' tire pressure will change. If you change the rim width, you'll need to re-experiment to find the new ideal tire pressure in various conditions.

So we can't unequivocally tell you that one rim width or the other is going to yield best results. The variable you're asking about is, in my opinion, the one that matters the least in this particular equation.

If I were in your shoes, I'd probably make a decision like this based on other factors. IE is a 12" wide rim actually going to fit? On your Porsche, probably, since I'd assume it came with wide wheels/tires from the factory.. but does it cause other issues (tires rubbing, loss of useful suspension travel, etc etc). Is an 8" wide wheel cheaper/lighter/stronger?

PS - I don't think I've mentioned it in any of your other threads.. but 'unconventional' off road rigs are one of my favorite things.

When this thing is done I demand pics.

 

[CapriRacer]

Have we established that you are driving to the off road location and lowering the pressure there, then just before getting back on the pavement, raising it back up? If not, let us know what you are doing.

I'm going to assume that is the case.

So what I know about rim width is that the wider you get, the more you take the sidewall out of the situation. It turns from acting like an flexy arched spring to a stiff cantilever spring - not good!

Too narrow and the sidewall also starts to act like a cantilever spring, albeit involving the tread as part of that spring.

I think the rim width needs to be within the allowable rim width range so the sidewall acts as intended. But if you force me to choose, narrow would be better than wide.

And just so you know, what happens to the footprint when you drop pressure (for the same load) is that the footprint lengthens way, way more than the pressure distribution changes. That's why using low pressure in off road situations doesn't have large negative affects.