Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[Roadcone]

I run 31psi in my 245/40R17 PSS on a 17x9" wheel. I've tried as low as 27 and as high as 36. You have two issues. too high of pressures and living in Ontario i'm assuming its cold... these tires are not made to work under about 50 degrees F. It also sounds like you're "driving style" is a factor...

learn to drive and quit reading the internet... if you're really concerned about it call michelin and talk to their help line, that pressure on the door sill is for stock tires for comfort and tire wear, not performance. PSS will have a different load capability at the same pressure as the Primacy HP... I was advised to run 31psi in mine and its pretty much dead on...

[Dave-ROR]

Oh Suberman.. I wonder what you'll call your next account.

[jvincent]

Originally Posted by Dave-ROR

Oh Suberman.. I wonder what you'll call your next account.

I sense a contest!

[Dipstik-sportech]

Originally Posted by Dave-ROR

Oh Suberman.. I wonder what you'll call your next account.

Can we pick it for him?!

[D_Thissen]

uberknob?

[Captain Snooze]

Originally Posted by Captain Snooze

I don't understand why changing the diff oil would make a difference given it's a Torsen type diff, no friction plates are involved.

Originally Posted by Ubersuber

Not quite true. The variability of torque bias depends both on gear angles (primarily) and end thrust plate materials. Some manufacturers will sell you different thrust "washers" to change the bias ratio.

What do gear angles have to do with changing the oil viscosity? In @Ferrari's case s/he changed the diff oil; no other variables involved.

[ZDan]

Originally Posted by wparsons

There's a difference between what pressure is required to hold up a given weight for a given tire size, and what pressure is required when increasing weight without changing tire size.

Yeah, more load will require more pressure.

You increase pressure with heavier loads on the same tire because less pressure results in too much sidewall deformation and excess tire heat.

Yes.

That doesn't change the relationship between weight, pressure and contact patch area in the slightest though.

No, I don't think anyone has suggested that.

If a vehicle weighs X lbs and has Y sq/in of desired contact patch area, then it will require Z psi in the tires. Z is going to be roughly X/Y.

Roughly, yeah.

If you increase weight to double, then you'll need roughly double the contact patch area at the same pressure to support the vehicle. If you want the same contact patch size, then you'll need roughly double the pressure.

Double the weight, and yeah, you might roughly/approximately double the contact patch area. Which is bad. So you increase pressure.

If you increase the weight and don't touch anything the contact patch area will be larger. Now if the tires aren't happy with that much sidewall deformation, you either need to change tires to wider ones so you get more contact patch area with less sidewall deformation

At the same pressure, that will be about the same contact patch area, but with less deformation or squish like you say.

or you need to increase the pressure.

Yup.

Contact patch width is basically fixed for a given tire, regardless of inflation pressure and load.

Contact patch area is usually going to be somewhat narrower than the tread width (particularly with some camber. Add load, it will get wider. Add pressure, it will get narrower. Somewhat dependent on the specific tire of course.

Bigger contact patches will support more weight at a given pressure,

But didn't you just say that contact patch area is load divided by pressure?!

A wider tire carries the load at a given pressure with roughly the same contact area but with less squish. Less deformation, less heat, just like you said before. But the contact patch area is roughly the same, not bigger.

A taller tire with a larger OD doesn't flex a given section of tire as often per mile as a shorter tire of the same width and profile. So at the speed rating, the taller tire is not heating up as much despite the same squish.

The above two points are why bigger (wider *and* taller) tires are inherently better for higher loads. It's not due to a "bigger contact patch".

[drewbot]

Originally Posted by Dave-ROR

Oh Suberman.. I wonder what you'll call your next account.

UbersteerFTL

[wparsons]

Bigger contact patches will support more weight at a given pressure,

Originally Posted by ZDan

But didn't you just say that contact patch area is load divided by pressure?!

Eff. You're saying the same thing, just not following how to re-arrange an equation.

I said more area will support more weight at a given pressure.

You said at the same weight and pressure a wider tire will have the same contact patch area but with less squish.

pressure = weight/area

or

area = weight/pressure - guess what happens to area if you increase weight but not pressure?

and, area = width * length.

So if you add weight and don't increase pressure, you either increase the width or length (or both) of the contact patch. Increasing length means more squish, increasing width does not.

At the end of the day, if you increase weight but do nothing else the contact patch is going to grow. That's a bigger contact patch supporting more weight at the same pressure, like I've said all along.

Now if the tire can't deal with that much squish, you go to a wider tire to get the contact patch area without too much squish.

I'm not claiming that at the same weight and pressure a wider tire will have a bigger contact patch at all, but it will have a different shape.

As an aside, when I said width is basically fixed for a given tire and any mention of contact patch size/shape, I meant at rest. Your image of a corner is dead on, BUT, if you compare the area of all four to the area while resting they'll be the same. Just shifted around with the load (weight). Trying to figure out what is dynamically happening in a corner is nearly impossible without a ton of datalogging.

[Tcoat]

WOW this is still going on!??
With Ubersurber gone aren't you all just agreeing with each other at this point?
Do we need to send this to Mythbusters or something?

[Dipstik-sportech]

Yeah guys I think we can close this thread now that uberdouche is gone

[7thgear]

Originally Posted by Dave-ROR

Oh Suberman.. I wonder what you'll call your next account.

Mr.Plow

[ZDan]

Originally Posted by wparsons

Eff. You're saying the same thing, just not following how to re-arrange an equation.

???!!! I'm exactly following how to "rearrange the equation". If we assume inflation pressure is load divided by contact patch area, then contact patch area is *invariant* with load at a given pressure. Big tire or small tire.

I said more area will support more weight at a given pressure.

More weight increases contact patch area, yes. The way you're saying it implies that if you have more contact area you can support more weight. Perversely suggesting that decreasing pressure, which would by God add contact patch, would allow you to support more weight.

So if you add weight and don't increase pressure, you either increase the width or length (or both) of the contact patch. Increasing length means more squish, increasing width does not.

Yes! Also, increasing sidewall height decreases squish relative to it, which reduces the strain energy in the tire carcass (lateral deformations spread over a larger distance, less extreme bending).

At the end of the day, if you increase weight but do nothing else the contact patch is going to grow. That's a bigger contact patch supporting more weight at the same pressure, like I've said all along.

That's all *I've* said all along!

Looking back to where I entered the thread, it was stonenewt's comment after all, not yours, that I took issue with:

Originally Posted by stonenewt

To increase the contact patch area. The heaver the load the larger an area you need on the road to provide grip.

That's not right at all. That's the only point I've been arguing against, the idea that you go to a bigger tire to get a bigger contact patch area. You're already getting a bigger contact patch area with the same tire. But more sidewall deformation.

I'm not claiming that at the same weight and pressure a wider tire will have a bigger contact patch at all, but it will have a different shape.

Agree!

As an aside, when I said width is basically fixed for a given tire and any mention of contact patch size/shape, I meant at rest. Your image of a corner is dead on, BUT, if you compare the area of all four to the area while resting they'll be the same. Just shifted around with the load (weight). Trying to figure out what is dynamically happening in a corner is nearly impossible without a ton of datalogging.

I was just trying to show that with reduced load, contact patch width is reduced. The lightly loaded inside front is clearly narrower, the heavily loaded outside front clearly wider.

[wparsons]

Originally Posted by ZDan

That's not right at all. That's the only point I've been arguing against, the idea that you go to a bigger tire to get a bigger contact patch area. You're already getting a bigger contact patch area with the same tire. But more sidewall deformation.

You go to a bigger tire to get a bigger contact patch with acceptable deformation, which was my point all along.