Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[Ubersuber]

Originally Posted by stonenewt

To summarise a lot of very lengthy reply from one tyre manufacture.



My take home from this - tyre pressure is a bit of a black art & the answer needs to be constantly re-evaluated.

And you guys have mastered this black art and the Michelin man hasn't.

Amazing.

[CSG Mike]

Originally Posted by Ubersuber

And you guys have mastered this black art and the Michelin man hasn't.

Amazing.

There are blanket recommendations for the masses, and the experiences of the guys who actually go, and know what to follow and what not to follow.

[Ubersuber]

Originally Posted by wparsons

Define significant. I doubt the difference in volume is as much as you think it is. What about different wheels (same diameter and width, but different barrel shape)? That's likely to have just as big of volume difference as 225/45 to 215/45 will.



Why does volume matter for tire support? You're adding pressure to support the carcass. Sidewall construction would have a greater difference on pressure need than air volume.

I see you believe in the balloon theory of tire inflation.

Sure, you do the math, it isn't that tricky. You just need to compare the internal volume of two torus. Then change the pressures and compare.

Before you spend too much time on the maths I can estimate it is about 5%. That would put the 225/45 x 17 at 33-34 psi or so if 35 psi were recommended for 215/45x17.

You may also wonder why Porsche built hollow alloy wheels for their 959.

Then you'd have to think hard about the effect of air pressure and wonder why air pressure inside the tire is so different from air pressure outside the tire. Also, why runflat tires are so tricky to make and why they handle so poorly. Give some thought to semi-pneumatic tires and you'll get the idea, possibly. But then why do solid rubber tires not work well and tires with some air in them work great?

Food for thought, even for you.

[stonenewt]

Originally Posted by Ubersuber

And you guys have mastered this black art and the Michelin man hasn't.

Amazing.

What was said is they realise that there are so many unknowns that they can't actually answer the question accurately! Instead they gave a huge amount of advice and procedures on dialling in finding correct pressure.

[wparsons]

Your stupidity knows no bounds!

Originally Posted by Ubersuber

I see you believe in the balloon theory of tire inflation.

What are you even trying to suggest?

Originally Posted by Ubersuber

Sure, you do the math, it isn't that tricky. You just need to compare the internal volume of two torus. Then change the pressures and compare.

Before you spend too much time on the maths I can estimate it is about 5%. That would put the 225/45 x 17 at 33-34 psi or so if 35 psi were recommended for 215/45x17.

Even if the volume is bigger, that doesn't mean the tire needs less pressure. Air volume doesn't provide support, pressure does. There is a lot of air outside, but I wouldn't jump off a roof hoping it will support me.

Based on your theory (add 5% volume, remove 5% pressure) since the atmosphere is essentially an infinite amount of air, it should require almost zero pressure to support me, right? I realize that statement is idiotic, but it outlines the flaw in your logic.

Originally Posted by Ubersuber

You may also wonder why Porsche built hollow alloy wheels for their 959.

It would definitely have nothing to do with weight savings at all. Idiot.

Originally Posted by Ubersuber

Then you'd have to think hard about the effect of air pressure and wonder why air pressure inside the tire is so different from air pressure outside the tire. Also, why runflat tires are so tricky to make and why they handle so poorly. Give some thought to semi-pneumatic tires and you'll get the idea, possibly. But then why do solid rubber tires not work well and tires with some air in them work great?

What does that have to do with VOLUME? My post was about volume of air inside the tire not changing the pressure requirements to support the carcass. At no point did I claim you needed no pressure for a tire to work.

You're either illiterate, or so hell bent on posting dribble that you don't even see that your points aren't even arguments to what you're trying to argue.

Originally Posted by Ubersuber

What is the recommended cold tire pressure for the Michelin SS in 215/45 x 17 size on the BRZ?

You're the expert, you tell us.

[reardrv]

Originally Posted by Ubersuber

The 225/45 x 17 has significantly more internal air volume than the stock 215/45x17. Subaru has no recommendation for the tire pressure for this size since they don't fit the size to their car. Michelin will. Anyone actually asked Michelin whether they recommend lowering the tire pressures from their recommended cold settings to improve performance? Thought not.

Generally speaking the larger the air volume of the tire for a given vehicle weight the lower the recommended static tire pressure will be.

Autocross requires such completely different suspension tuning considerations that your selected tire pressures would not be useful nor appropriate for the street use of the original poster. Again though, your experience is entirely consistent with the trend the tire manufacturer would recommend I.e. lower static cold pressures for larger section tires of the same wheel diameter.

The objective is to keep the designed contact patch size and shape and seek and maintain the ideal tread compound temperature for best grip and best handling. For street use this means using the tire (or car) manufacturer's recommended settings and you will invariably find that increasing the tire pressure by up to 2psi (assuming recommended cold pressure of 35 psi or so) improves grip and hardens the ride.

Driving on the street you will never get the tires up to optimum temperature unless you are looking to get ticketed.

Also, you might be very surprised to find the effect ambient temperature has on cold tire pressures. One advantage of adding around 2 psi to measured cold tire pressure is that you will more often be riding at the actual recommended pressure when ambient temperatures are lower than in your garage.

34psi front and 32psi rear cold is what I run on the street. Like most people who buy PSS's my car is primarily daily driven. I did not mention in anyway about using those pressures for autocross.

[7thgear]

Originally Posted by Ubersuber

http://www.fia.com/championship/euro...s-championship

http://www.casc.on.ca/autoslalom

OK, if you say so.

what's your point?


"autocross", or autox, or solo, or solo 2, or cone chasing, what is understood to be autocross in north America, and you would have to be an idiot to confuse the use of the FRS in autocross and the dune-buggy, hill hopping dirt adventures they have in the EU.

[Dipstik-sportech]

Originally Posted by 7thgear

what's your point?


"autocross", or autox, or solo, or solo 2, or cone chasing, what is understood to be autocross in north America, and you would have to be an idiot to confuse the use of the FRS in autocross and the dune-buggy, hill hopping dirt adventures they have in the EU.

He doesn't have a point, he just picks a side and whether it's wrong or not he argues it to death with huge long convoluted responses.

[Ubersuber]

Originally Posted by reardrv

34psi front and 32psi rear cold is what I run on the street. Like most people who buy PSS's my car is primarily daily driven. I did not mention in anyway about using those pressures for autocross.

This increases the tendency for this chassis to oversteer. It really doesn't need the help.

[Ubersuber]

Originally Posted by 7thgear

what's your point?


"autocross", or autox, or solo, or solo 2, or cone chasing, what is understood to be autocross in north America, and you would have to be an idiot to confuse the use of the FRS in autocross and the dune-buggy, hill hopping dirt adventures they have in the EU.

The point is simple, even an idiot should be able to get it. Any version of autocross is irrelevant to the initial post.

[Ubersuber]

Originally Posted by stonenewt

What was said is they realise that there are so many unknowns that they can't actually answer the question accurately! Instead they gave a huge amount of advice and procedures on dialling in finding correct pressure.

They were lying to you then. Michelin can recommend the correct tire pressure for any tire they make and any vehicle that tire will fit.

Bear in mind you have to ask them the right question which is what is their recommended tire pressure for the BRZ fitted with the Michelin Pilot SuperSports in 225/45x17 size for the street. After they tell you that isn't the stock size they will be able to advise you what the recommended pressure is...for street applications.

There is no correct tire pressure for competition applications for the simple reason that you're not trying to drive on the street.

The initial post asks what can be done to improve handling on the street with the tires chosen. Frankly, 35 psi should present no problems whatsoever even if the ideal static cold pressure should be 34 psi for the slightly larger size.

For the curious, it seems obvious that the car is supported by distorting the tire carcass by a set amount proportional to the vehicle weight, the object being to maintain the contact patch in the correct shape intended by the tire manufacturer. Tire pressure for any particular tire is selected by vehicle weight.

Period.

If the internal volume of the 215 tire is x litres and the internal volume of the 225 tire is, let us estimate, x plus 5% of x, then obviously the required pressure to ensure the tire is distorted by the correct amount should be about 5% lower.

Now if you actually look at tire grip plotted against load and tire pressure you can see that raising tire pressures above those recommended for street use will generally increase overall grip for any normal street driving. This is a diminishing returns process and usually adding 2 psi above recommended will produce the best result.

Suggesting that overheating the tires by driving on the street will mean the tire pressure will go above optimum is stating the obvious and to no useful purpose.

[Ubersuber]

Originally Posted by Ferrari

I bought the car new and after 1000Km I changed the tires to the supposedly much grippier Michelin Pilot Super Sport, 215/45-17, on stock wheels. The car is manual and not modified. Under braking the tires are amazing, but I break traction under light/medium throttle at very moderate speeds in a corner, so I get oversteer constantly. 3000RPM, 2nd gear, 40Km/h, if I gently accelerate out of a corner, the back slides out.

Do the MPSSs need to be warmed up before they'll grip on the road? I have them inflated to 37psi. I haven't tracked them so it's not like they were heat cycled. Baffled...

Basically, 37 psi is high for this tire in this size on this car. If you heat up the tires the tire pressure effects will be worse, not better. Having tire pressures too high front and rear will not be the cause of your problem. At low speeds in tight corners under hard acceleration with traction control off you will oversteer. Period.

[wparsons]

Originally Posted by Ubersuber

This increases the tendency for this chassis to oversteer. It really doesn't need the help.

Except, it will probably increase rear grip, not decrease it.

The front tires have to carry more weight, and need a little more pressure than the rears to work evenly.

I fail to see how you're qualified to give any advice on tire pressures and their effect on handling since your only advice is to go with what the car manufacturer suggested, and you refuse to acknowledge that there is value in playing with pressures for street driving.

You seem to completely miss the point that manufacturers do things for simplicity first, then for optimal usage. Don't believe me, look at your oil change interval vs mine. Yours is 10k km's because that's what all current Subaru's are, mine is 8k km's because that's what all current Toyota's are. Then look at what the US intervals are, they're longer than either of ours are on both brands.

Knowing that, do you really think that the 35psi is optimal for performance, or was a safe easy to remember guideline? Also keep in mind they're giving a guideline based on people not checking their pressure as often as they should (as ambient temperatures shift). Saying 35psi cold all around is MUCH easier for them to publish than to say XXpsi front, YYpsi rear set when the tire reaches a temperature of ZZ *C.

For someone that claims the suspension engineers were lazy and didn't do any work when adapting parts from the Impreza platform, you sure put A LOT of faith in how much thought went into the suggested tire pressures.

Originally Posted by Ubersuber

For the curious, it seems obvious that the car is supported by distorting the tire carcass by a set amount proportional to the vehicle weight, the object being to maintain the contact patch in the correct shape intended by the tire manufacturer. Tire pressure for any particular tire is selected by vehicle weight.

Period.

So weight distribution isn't a factor at all?

Originally Posted by Ubersuber

If the internal volume of the 215 tire is x litres and the internal volume of the 225 tire is, let us estimate, x plus 5% of x, then obviously the required pressure to ensure the tire is distorted by the correct amount should be about 5% lower.

Where are you finding anything to support this? Volume does not change the pressure required to properly support the carcass like you keep suggesting.

Contact patch size will dictate required pressure (larger contact patches will need less pressure), but contact patch and internal volume are not directly related in any way. If you're comparing a mountain bike to a road bike tire and required pressures, it's because of width and contact patch, not volume.

5L of air at 10psi has the exact same ability to support weight as 500L of air at 10psi, assuming the same contact patch (or footprint).

Originally Posted by Ubersuber

Now if you actually look at tire grip plotted against load and tire pressure you can see that raising tire pressures above those recommended for street use will generally increase overall grip for any normal street driving. This is a diminishing returns process and usually adding 2 psi above recommended will produce the best result.

Well, which is it?

Originally Posted by Ubersuber

Basically, 37 psi is high for this tire in this size on this car. If you heat up the tires the tire pressure effects will be worse, not better. Having tire pressures too high front and rear will not be the cause of your problem. At low speeds in tight corners under hard acceleration with traction control off you will oversteer. Period.

35psi + 2psi is 37psi. So is 2psi above recommended best for traction, or is it too high?

[Dipstik-sportech]

A lawyer giving advice on tire pressure and suspension setup, which ends up being completely opposite of what people with real world experience are saying. This can only end amazingly