Thread: Wheel nut torque to wide versus thin tyres

Originally Posted by RRT

Sorry for the spelling joke, it is Tuesday

In Southern Racer English they are "tars". Refer Smokey Yunick autobiography.

As I have NEVER used a torque wrench on a wheel stud I wouldn't have a clue what the tension setting is on ANY of the 4WD I have owned, All I have ever done is to use a breaker bar and tighten until I hear the wheel nut "Creak" then drive for about an hour and retighten.
I have NEVER lost a wheel or stripped/broken a wheel stud using this method over the last 4 decades driving roughly 40,000k's a year.
I bought an 18v rattle gun a few years back and found it to be invaluable when changing wheels because it takes 80% of the work out of the job.
I use it to remove the wheel studs and also put the studs back on (Loosely) then I do the breaker bar trick

Originally Posted by V8Ian

Correct torque for a D2 is 120Nm.

Just checked RAVE, listed under suspension in the torque settings.

is 140Nm!

I understand the OP was about wheel nut torque and yes most alloy wheels are done to 140Nm. I also found the fat vs thin discussion interesting.
After a bit of a poke around I found this.... Ground Pressure
I knew tanks had a low pressure (overall) but the table in this article is intersting.

M1 Abrams Tank - 15 psi
1993 Toyota 4Runner/Hilux Surf - 25 psi (I need to wash my hands now)
Mountain Bike - 40 psi
Stiletto Heel - 471 psi

This got the cogs turning and came up with the following:

We all tend to lower our tyre (internal) pressure in certain off road situations. Therefore, when lowering this pressure and lengthening our tread contact area with the ground we also lower our ground pressure due to this larger contact area. With that in mind we know that lowering tyre pressure on sand is a good thing (lower ground pressure) but may not be absolutely necessary in some situations.

Have I got that right?

Originally Posted by ChookD2

....

Have I got that right?

I don't think that's quite right.
Not pretending to fully understand the numbers, but ground pressure alone can't be used to calculate overall grip for a set of specific conditions.
Gives you an idea of how to compare one set of conditions to another tho.

eg. say we have a vehicle .. even the dreaded Surf! ... and say we're on sand.

Their theory says that your tyre pressures is a good approximation of the ground pressure your car exerts on the ground.
They start driving over the sand with tyres at 40psi. They're not getting good grip, so stop and lower tyres to 15psi.
The important point to be mindful of in the variance in those figures is the "per square inch" part.
Vehicle weight doesn't change, so obviously the only change is the area of contact patch.

Sounds obvious of course, and stuff that we all know either through experience or observation or theory or whatever ... but maybe not enough to conceptualise visualise the actual change happening.

We know that friction is always related to surface area for given conditions as well. That is, the more surface area, the more friction .. friction = grip.

So even in mud, slush, gloop .. surface area = grip. So over the top of it you still have some grip .. just not enough to overcome the weight of the vehicle.
The theory is that the taller skinnier tyres in deep muddy slop are supposed to cut their way through to firmer ground, and that may well be spot on, but there's no guarantee that you will reach this firmer deep section either.
That is, you're more likely to bottom out more so than break through to firmer ground. So, you want more grip(ie. surface area) to begin with hoping to get through over the soft sloppy part before it digs in too far.
Technically, a fully deflated tyre will give the most grip, as it will have deformed as far as it can. This then provides the most surface area on the surface that is causing the grip issue.
Obvious alert!!! yeah, tyres spin off rim .. etc. tyre damage, etc... so the perfect off road tyre/rim set up would be run flat tyre designs.

Have you ever heard of the dreadnought wheel? used a lot in WWI in the sloppy/muddy Euroean battlefields. Basically run flat wheel design.
If you have the inclination, do a search for Big Lizzie, huge tractor in the Wimmera(Vic) that cleared thousands of acres of sandy land. Used those dreadnought type wheels very successfully.

I just did some quick math.
Going by the theory in the link you posted(tyre pressure is approximation of ground pressure)
A low to medium weight vehicle or about 2t(~4K lbs) with 40psi tyres, has about 1 sheet of A4 papers worth of contact area on the ground.
Drop the tyres to 15psi, and that changes the contact area to 2.84 sheets of the same paper.
Actual numbers are 266 sqin at 15psi, 100 sqin at 40psi. Close to 3x the area difference between tyre and ground.

Originally Posted by AK83

So, you want more grip(ie. surface area) to begin with hoping to get through over the soft sloppy part before it digs in too far.

That's precisely why wide tyres are better because the force of friction remains the same while they provide more traction due to the larger contact surface (and therefore more surface adhesion and mechanical interlocking) and because there's less chance of sinking due to the reduced pressure exerted on the ground by the wheels.

Another important factor when wheels are sunk below the surface is that the rolling resistance shoots up. Therefore more torque will need to be applied to the wheels to overcome this resistance in order to initiate forward or reverse motion. But as more torque is applied to the wheels, there is a good chance that the force will be far greater than the force of static friction that is keeping the wheels pinned to the ground. If this happens, grip will be lost and the applied torque will be wasted in spinning and sinking the wheels even further instead of moving the vehicle.

Originally Posted by NPG

.....

Another important factor when wheels are sunk below the surface is that the rolling resistance shoots up. Therefore more torque will need to be applied to the wheels to overcome this resistance in order to initiate forward or reverse motion. But as more torque is applied to the wheels, there is a good chance that the force will be far greater than the force of static friction that is keeping the wheels pinned to the ground. If this happens, grip will be lost and the applied torque will be wasted in spinning and sinking the wheels even further instead of moving the vehicle.

This is why it's a balance consideration.

Wide tyres on rocks .. almost certainly better. tyres don't dig in and create the additional force to overcome.
Taller skinnier tyres in sand .. almost certainly better once aired down .. More track like, forward/rearward spread when aired down.

This is why you read some folks claim taller skinner tyres are better .. they probably do more sand type driving.

Been there too.
I rarely venture out into the more rocky country, and prefer the central desert areas(even here in western Vic).
I recently changed from 255/70's to 245/75's simply due to my preferred use.
I still do go out to say Vic High Country, and maybe the 255's would give me an extra percentage of grip, but my balance preference is for tall/skinny.

But someone that does more rocky country off roading would be better served with wider fatter tyres .. etc, etc.

Good points as the variance is pretty much on what you want the tyre to have.

On sand you want for want of a better term buoyancy the tyre to stay on top of the surface rather than a high pressure with it cutting through to the under surface where as on silt or dust or shale or mud you want them pushing or cutting down to bedrock for grip and forward movement being key.

Maybe therefore taking pressures down on the loose stuff such as up in the high country is wrong as it decreases the pressure per sqcm and a modicum of sacred height!

Rocks on the other hand, just like with big hands all the easier to grip you with, the extra friction of greater surface area versus the tighter grip of a smaller footprint exerting greater pressure per sqcm, ahhh but which is better? is it not the one where there is less chance of wheel spin due to the tyre having traction but until there is conclusive proof,

Both fat and thin are still the best!!!!


Arty