Crazy oversteer / wheelspin with Michelin Pilot Super Sport

[wparsons]

Originally Posted by 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

What I find amazing is how often he contradicts himself, often minutes apart.

[Dipstik-sportech]

That's what makes it amazing!!!!

[troek]

more volume does mean less pressure is required to support the same amount of weight. given the tire construction is the same/similar. try putting 120 psi in a car tire would be quite dangerous, however in a bicycle tire, which is significantly weaker, will handle it no problem. how that applies to slighty increasing the size of tires on a car, i dont know, i just felt like contributing something lol.

[stonenewt]

Originally Posted by troek

more volume does mean less pressure is required to support the same amount of weight. given the tire construction is the same/similar. try putting 120 psi in a car tire would be quite dangerous, however in a bicycle tire, which is significantly weaker, will handle it no problem. how that applies to slighty increasing the size of tires on a car, i dont know, i just felt like contributing something lol.

Unless you're on a really smooth surface then with tyres under 35mm you actually want to run the lowest pressure you can without the risk of pinch punctures. This includes when you're lent over for long downhill sweeper at say 40mph, which gives less deflection scope than when upright.

As for strength, think about this. If I span a 20cm gap with a straw that can only just support 50g in the middle what happens when I make that gap 2m? The straw collapses. So for the same loading the bike tyre doesn't need to be nearly as strong to start with.

With this in mind let's take a 19mm track tyre with a maximum pressure of 220psi, its bead to bead tension will be about 380lbs. On a 215/45 tyre it would be about 3540lbs! At 45psi, the maximum pressure for the tyres on my car, the tension is around 725lbs. For a 2.5" bike tyre, a typical size for a maximum of 45psi, the tension is about 260lbs.

So basically you can't really compare bike tyre & car tyre pressures.

[stonenewt]

Originally Posted by Ubersuber

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.

Non I didn't lie, I simply summarised what they said.

The types of surface you drive on, the exact specification of your car, the sorts of journeys you do, your driving style & the raw materials used in your tyres' batch will all effect the tyre pressure you should run.

There's one thing in that, non-exhaustive, list they can't actually quantify now. The exact properties of the raw materials used to create the tyres in question. This is what makes it a difficult thing to do. In the extreme when racing at the level that every 10th of a second counts sometimes a particular set of tyres requires a marginally different suspension setup!

The question becomes how close do you want to be to the ideal pressure - 3psi? 1psi? .25psi?

[troek]

Originally Posted by stonenewt

Unless you're on a really smooth surface then with tyres under 35mm you actually want to run the lowest pressure you can without the risk of pinch punctures. This includes when you're lent over for long downhill sweeper at say 40mph, which gives less deflection scope than when upright.

As for strength, think about this. If I span a 20cm gap with a straw that can only just support 50g in the middle what happens when I make that gap 2m? The straw collapses. So for the same loading the bike tyre doesn't need to be nearly as strong to start with.

With this in mind let's take a 19mm track tyre with a maximum pressure of 220psi, its bead to bead tension will be about 380lbs. On a 215/45 tyre it would be about 3540lbs! At 45psi, the maximum pressure for the tyres on my car, the tension is around 725lbs. For a 2.5" bike tyre, a typical size for a maximum of 45psi, the tension is about 260lbs.

So basically you can't really compare bike tyre & car tyre pressures.

All that may be true but it dosnt change the fact that 30 psi in a larger container exerts significantly more pressure than on the walls than in a smaller container. Its the reason why it only takes a few psi to blow the windows out of a plain or why 1 inch piping can handle way more pressure than 2 inch piping of the same wall thickness. I may be wrong, but the square inches of surface are in the container, the more pressure, hence pounds per square inch. Maybe were saying the same thing.

Edit: pipe was a bad reference, it had more to do with the radius of the pipe wall.

[wparsons]

Originally Posted by troek

more volume does mean less pressure is required to support the same amount of weight. given the tire construction is the same/similar. try putting 120 psi in a car tire would be quite dangerous, however in a bicycle tire, which is significantly weaker, will handle it no problem. how that applies to slighty increasing the size of tires on a car, i dont know, i just felt like contributing something lol.

That's not due to volume, it's due to contact patch. Higher pressure is required because the contact patch is smaller, so less tire is holding up more weight. To prove it, think of what would happen if you made the sidewalls taller without touching width (and rims smaller to keep the outer diameter the same) to the point where the volume is the same. Since you haven't changed the contact patch size, you would still need the same pressure, despite having a lot more volume.

Originally Posted by troek

All that may be true but it dosnt change the fact that 30 psi in a larger container exerts significantly more pressure than on the walls than in a smaller container. Its the reason why it only takes a few psi to blow the windows out of a plain or why 1 inch piping can handle way more pressure than 2 inch piping of the same wall thickness. I may be wrong, but the square inches of surface are in the container, the more pressure, hence pounds per square inch. Maybe were saying the same thing.

Edit: pipe was a bad reference, it had more to do with the radius of the pipe wall.

That's also factoring in surface area (contact patch). The piping is a good example of why volume doesn't matter. Does 10 feet of 1" pipe hold more pressure than 100 feet of 1" pipe? There's a significant difference in volume there.

Think of pressure/volume like this. If you have an object that has a footprint of 1 square inch and weighs 10lbs, it exerts a pressure of 10psi on whatever it sits on. If you have a container with 1L of air at 10psi or 1000L of air at 10psi with a footprint of 1 square inch, both will support that 10lb object the same. Now if you change the footprint, the pressure will change.

Now if someone wants to calculate (or measure) the change in contact patch size from a 215/45/17 to a 225/45/17 I'll definitely agree that if there is a significant change in size (there won't be), then pressure will need to be adjusted in relation to that change.

If it was volume alone, then a 205/40/17 would need more pressure than a 205/45/16, but since the outer diameter is VERY close (within 1/4") the contact patch size won't change (assuming same tires) which means the pressure requirements don't change.

[reardrv]

Originally Posted by Ubersuber

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

Huh, that's strange. Because on my actual car that I drove to work only about an hour ago it reduces the tendency to oversteer.

[stonenewt]

Originally Posted by wparsons

If it was volume alone, then a 205/40/17 would need more pressure than a 205/45/16, but since the outer diameter is VERY close (within 1/4") the contact patch size won't change (assuming same tires) which means the pressure requirements don't change.

Your car weights 3000lbs, you're tyres are at 30psi, it requires 100 sq" of area to support that weight, or 25 sq" per tyre. On a 215 wide tyre that's something around 3.75" of tread length on the road assuming the contact patch is close to an ellipse. For this reason even on very large radius changes or tyre width changes you shouldn't see much increase of contact area.

[troek]

this has greatly diverged from the topic, but im enjoying learning about this. what makes taller tires have higher load ratings? IE a 205/55r16 vs a 205/65r16.

[wparsons]

Originally Posted by troek

this has greatly diverged from the topic, but im enjoying learning about this. what makes taller tires have higher load ratings? IE a 205/55r16 vs a 205/65r16.

I don't know that it's the taller sidewall that gives it the higher load rating as much as the tires constructed for higher load ratings are typically for trucks/SUV's which need taller sidewalls for stock fitment.

[AsianStyle]

I have 235/45/17 MPSS on 17x8 wheels, what should the tire pressure be for standard street use be if I want to optimizing tread wear over maximum grip?

[7thgear]

Originally Posted by AsianStyle

I have 235/45/17 MPSS on 17x8 wheels, what should the tire pressure be for standard street use be if I want to optimizing tread wear over maximum grip?

high as possible before over inflation....

[stonenewt]

Originally Posted by troek

this has greatly diverged from the topic, but im enjoying learning about this. what makes taller tires have higher load ratings? IE a 205/55r16 vs a 205/65r16.

From what I understand is the lowest weight at which one of the following conditions occurs:
1 - 70% static side wall deformation occurs, where 100% sidewall deformation is the difference in the sidewall height at maximum pressure & 0psi with a specific load on the tyre (100kg think).
2 - the tyre overheats at the speed rating & max tyre pressure

So low speed rating tall tyres will tend to be able to support more load as they'll be overheating at max PSI before they run out of tyre deformation. Lower profile sports tyres will tend run out of deformation first.