Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[stonenewt]

Originally Posted by ZDan

I don't see where I misunderstood, a little help?
Here's how it appeared in the thread:


You get a larger contact patch for free with the SAME SIZE tires under a higher load at a given pressure. Having a larger tire at the same pressure doesn't necessarily give you greater contact patch area (also, greater contact patch area doesn't necessarily imply greater GRIP). In fact, greater contact patch area means greater deformation and greater heat into the tire, which is why you increase pressure for a given tire under high loads. It *reduces* the contact patch area but INCREASES allowable load.

Greater contact patch area does not equate with greater load-carrying capacity.

Greater tire volume does give greater load-carrying capacity at a given pressure, even though contact patch area is roughly the same vs. a smaller tire at the same pressure.
http://www.bridgestonetrucktires.com...4/v3i4Tech.asp
You could say it's the number of air molecules inside that determines the maximum load. Higher pressure means more air molecules and higher capacity. And, a bigger tire can contain more air molecules-and therefore has more capacity-than a smaller tire at the same pressure. And there's one other factor.

http://www.goodyeartrucktires.com/pd...tread_S5_V.pdf
A tire requires proper air pressure
to adequately carry the load placed on
it. The “container volume,” material
properties and inflation pressure determine
the load carrying capacity of the tire.

So why does this happen
Take the maximum contact patch at 90% sidewall deformation on a 265/30 tyre at R22, R21, R20 & R19. Then multiply the maximum load of the 265/30R22 tyre by the surface area % of the lower diameter tyres.

		    Load (kg)
Tyre		Rated	calculated
265/30R22	 730	   730
265/30R21	 710	   715
265/30R20	 670	   696
265/30R19	 650	   655

As the maximum contact patch area reduced the maximum load falls in almost direct proportion.

We also know that the weight supported by the tyre is contact patch area multiplied by tyre pressure. See what just happened? We've show the direct link between the maximum tyre load, outer radius of the tyre & the sidewall height!

If we work off volume we get

		    Load (kg)
Tyre		Rated	calculated
265/30R22	 730	   730
265/30R21	 710	   697
265/30R20	 670	   645
265/30R19	 650	   579

Hum, not a great correlation there. I'm sure you can find something with a square root or two but nothing that's elegant or based on such basic physics as documented above.

I can't make truck tyre load ratings make sense from a geometrical standpoint so I'm going to assume that there are other factors going on. I suspect that thermal influences have a far bigger impact on truck tyres than car tyres.

[ZDan]

Originally Posted by wparsons

We're not talking about extremely heavy loads here, we're talking about passenger vehicle tires.

Same principals apply.

If tire width doesn't matter, why don't you see BMW X5's driving around on 165's? The reason is because to get the contact patch area needed to support the heavy truck a 165 would need a very long contact patch (and a very flat looking tire), or extremely high pressures.

Who said width doesn't matter? Heavier load => bigger tire with greater volume is required. Of course you wouldn't run 165/70-13 tires on a 5000 lb. behemoth, you need much bigger tires than that. Wide, tall tire with tallish sidewalls. Base X5 tire is 255/55-18. Big weight => big tire.

Take one car and two sets of wheels/tires. One set is a 215/45/17, the other is 215/50/16. Same tire make/model, so construction will be very similar. Both tires will have the same contact patch, and both have to support the same weight. Will the desired pressure in the 16's be lower because of higher volume?

Nobody makes a 215/50-16, but it is interesting to compare 225/45-17 vs. 225/50-16 tires at TireRack. Same width, same diameter, 225/45-17 is rated for 1356 lb. load while 225/50-16 is rated for 1389 lb for most tires ("XL" tires are rated for additional load). The tire with a bit more internal volume is rated for a bit more load at the same pressure.

[ZDan]

Originally Posted by stonenewt

So why does this happen
Take the maximum contact patch at 90% sidewall deformation on a 265/30 tyre at R22, R21, R20 & R19. Then multiply the maximum load of the 265/30R22 tyre by the surface area % of the lower diameter tyres.

		    Load (kg)
Tyre		Rated	calculated
265/30R22	 730	   730
265/30R21	 710	   715
265/30R20	 670	   696
265/30R19	 650	   655

As the maximum contact patch area reduced the maximum load falls in almost direct proportion.

What assumptions are you making to get a contact patch area? An ellipse 265mm wide (or some fraction of 265mm) with length dictated by 90% sidewall deformation for each size? I don't think that's valid. Contact patch area should be roughly equal in each case, given same make/model tire with the same internal construction, ply count, sidewall stiffness, etc.

The larger-diameter tires with the same profile have greater internal volume and hence greater load-carrying capacity.

We also know that the weight supported by the tyre is contact patch area multiplied by tyre pressure.

OK, so contact patch area is weight divided by pressure. So for a given inflation pressure and the same weight, the tires you list above should have the *same contact patch area*. The reason the load-carrying capacity is different is due to internal volume. The larger-diameter tires have greater internal volume, and are rated for higher load.

[wparsons]

You guys are mixing up maximum load capacity with pressure required to maintain the contact patch...

This all started as a discussion on if you need to decrease pressure with larger volume, not which tire is rated for the most load.

When you're looking at how much pressure is required to maintain the contact patch, volume does not matter. Sure the tire with bigger volume might be able to support more weight maxed out, but if you compare the max weight on a given tire compared to the vehicle weight you would need to almost double the weight of the car to max out the tires (stock tires max out at 1201lbs).

Since no one here is talking about adding a ton of weight to their car, max load capacity is really an irrelevant number to look at.

[unsurety]

Legendary troll, Ubersuberman.

[ZDan]

Originally Posted by wparsons

You guys are mixing up maximum load capacity with pressure required to maintain the contact patch...

Kind of an aside, the subject of load capacity vs. "contact patch" was brought up...

This all started as a discussion on if you need to decrease pressure with larger volume, not which tire is rated for the most load.

"Optimal" pressure will of course depend on a thousand things, including user wants/needs/usage. The difference between optimal for 215/45-17 and 225/45-17 won't be huge in any case, and that difference will also depend on the specific tire make/model.

35psi is surely fine, 30psi would be fine as well. Not going to be the end of the world either way.

This has probably already been covered, but going way back to the first thread regarding lack of grip with Michelin PSS tires, I've found that they aren't that great at even coolish temps. On the street at ~50F, not the grippiest tires out there. 37psi cold with the MPSS is probably worse than 32psi would be as far as street grip. In the end, on the street, probably down to personal preference IMO.

Of course any talk about how going below 35psi cold is dangerous and never a good idea can be safely ignored...

[Ubersuber]

Originally Posted by wparsons

Challenge accepted.

The link you posted:




Point number 1:





Point number 2:









I REALLY hope you're better at fact checking before using anything in a court room than you are with arguing online. Although, I would LOVE to see you in action where you present evidence to defend someone that actually incriminates them further.

Will you go away now?

None of this makes any sense.

The link I posted doesn't have a page 17 to begin with.

Oversteer is defined by reference to slip angles. Lower tire pressures result in higher slip angles for the same side force. If the car oversteers to begin with lowering the tire pressure will increase the oversteer.

The reference in the article to higher cold pressures being needed for autocross as compared to track work results from the lower temperature rise experienced in autocross than on the track. Even lower temperature rises are experienced in normal driving require even higher cold temperatures than that.

Quoting out of context can get you into all sorts of logical problems.

Reading what is actually written is a good start.

[Ubersuber]

Originally Posted by wparsons

You guys are mixing up maximum load capacity with pressure required to maintain the contact patch...

This all started as a discussion on if you need to decrease pressure with larger volume, not which tire is rated for the most load.

When you're looking at how much pressure is required to maintain the contact patch, volume does not matter. Sure the tire with bigger volume might be able to support more weight maxed out, but if you compare the max weight on a given tire compared to the vehicle weight you would need to almost double the weight of the car to max out the tires (stock tires max out at 1201lbs).

Since no one here is talking about adding a ton of weight to their car, max load capacity is really an irrelevant number to look at.

Not really, you just keep reading in stuff that isn't there or taking what is there out of context.

Contact patch area has nothing to do with load capability. The tire will hold up the same load from almost flat to overinflated.

Contact patch shape is the objective of tire pressure and that is determined by load.

Heavier cars do not necessarily require larger contact patches and a vehicle of a given weight will be very tolerant of fairly wide variations in contact patch shape.

It seems likely that Michelin has a typo in the chart:

http://www.tirerack.com/tires/tirete....jsp?techid=58

[Dipstik-sportech]

@wparsons the Orville is in the microwave, I'm ready this time!!! I'm starting to wonder if he's ever driven a car at or over the limit, hell I'm willing to be he's a wheel shuffler lol

[Ubersuber]

Originally Posted by drewbot

Adding some actual experience to this thread:

I got an oil change at Toyota and they put my tire pressures back to the "driver's side door" spec.

Guess what? They filled it on the coldest day this week (in the morning) and are probably a bit higher right now just due to temperature. The car almost snapped away from me when getting on the throttle on my favourite on-ramp. Didn't happen the day before...

Tire pressures recommendations are just a starting point!

1 psi per 5 degrees C (approximately) it is actually a bit less than that but 1 psi per 5 degrees C or or 10 degrees F is close enough.

This means adjusting the cold pressure to what the cold pressure should be at the expected ambient temperature during the majority of your driving.

All the blathering on about adjusting tire pressures for competition or autocross or whatever is actually just about adjusting tire pressures for the expected operating temperature of the tires for the driving you expect to be doing.

It is dangerously misleading to suggest that you tweak the handling of your car for the street by adjusting the tire pressures away from those specified by the car manufacturer or the tire manufacturer.

Of course you must set the cold tire pressures at the correct ambient temperature.

It is very dangerous to adjust your tire pressure when the tires are hot for street driving and you should not do it unless you really know what you are doing. The objective when doing so must be to approximate the specified cold pressures they should have been set at in the first place.

Most shops in Canada set tire pressures way too low winter in their nice warm shops thinking they are setting them at spec and not allowing for ambient temperature (which is what "cold" in the spec actually means).

[D_Thissen]

Originally Posted by Dipstik-sportech

@wparsons the Orville is in the microwave, I'm ready this time!!! I'm starting to wonder if he's ever driven a car at or over the limit, hell I'm willing to be he's a wheel shuffler lol

I got my pizza and beer ready!

[D_Thissen]

Originally Posted by Ubersuber

1 psi per 5 degrees C (approximately) it is actually a bit less than that but 1 psi per 5 degrees C or or 10 degrees F is close enough.

So, yesterday it was 5C and tomorrow is supposed to be near 18C, I should adjust my cold tires by 2.5psi ..ish?

[drewbot]

Originally Posted by Ubersuber

1 psi per 5 degrees C (approximately) it is actually a bit less than that but 1 psi per 5 degrees C or or 10 degrees F is close enough.

This means adjusting the cold pressure to what the cold pressure should be at the expected ambient temperature during the majority of your driving.

All the blathering on about adjusting tire pressures for competition or autocross or whatever is actually just about adjusting tire pressures for the expected operating temperature of the tires for the driving you expect to be doing.

It is dangerously misleading to suggest that you tweak the handling of your car for the street by adjusting the tire pressures away from those specified by the car manufacturer or the tire manufacturer.

Of course you must set the cold tire pressures at the correct ambient temperature.

It is very dangerous to adjust your tire pressure when the tires are hot for street driving and you should not do it unless you really know what you are doing. The objective when doing so must be to approximate the specified cold pressures they should have been set at in the first place.

Most shops in Canada set tire pressures way too low winter in their nice warm shops thinking they are setting them at spec and not allowing for ambient temperature (which is what "cold" in the spec actually means).

Did you see the part where I said TOYOTA changed my tire pressures?

Sent from my OnePlus One

[Dipstik-sportech]

Originally Posted by drewbot

Did you see the part where I said TOYOTA changed my tire pressures?

Sent from my OnePlus One

No he glossed over that part because it doesn't help him make his case.