tyre pressure effects

[funbobby]

can some one explain the effects of lowering/raising pressure in relation to understeer/oversteer on a rwd car?

[TheEnd]

taking the correct or best pressure for a tyre as the norm, lower pressures will increase grip from a larger contact area, but increase drag and friction from the saggy rubber, higher pressures will reduce grip by making the contact patch smaller.

[Sam_68]

It's a bit more complicated than that.

For example, lowering the pressure will do a couple of things apart from increasing the contact patch:

• It will lower the spring rate of the tyre (which acts as an undamped spring in sequence with the road springs).
• It will reduce the effective carcase stiffness, which will both increase the tyre's slip angle for a given load and cause the cascase to generate more heat.

Lower the tyre pressure enough and the heat generated by the second point will cause it to explode... which definitely decreases the amount of grip available.

Sadly, there are no easy and simple rules, because the interaction with other aspects of the car's design and behaviour are too complex.

[jon-]

What sam said.

If you want to see how it effects your car get used to it at the factory settings, then try 2psi either way.

[funbobby]

so will raising the pressure increase understeer or the other way round? what about doing say just front or backs what are the implications there?

[jon-]

funbobby said:

so will raising the pressure increase understeer or the other way round? what about doing say just front or backs what are the implications there?

I guess the EXTREMELY SIMPLE base line rule is more psi, less grip, but that's such an over simplified view on the huge complexities involved with tyres it's pretty useless, hence the recommendation just to play with it yourself.

If you've a front driver, I'd start with just the front as that will make by far the most change.

[JJM]

Is it normal to increase tyre pressures for track days over normal road driving?

In a few weeks I am taking my car to North Weald for one of the Car Limits handling days, and am sure they suggest taking your tyres up to their max pressure.

[jon-]

JJM said:

Is it normal to increase tyre pressures for track days over normal road driving?

In a few weeks I am taking my car to North Weald for one of the Car Limits handling days, and am sure they suggest taking your tyres up to their max pressure.

No, and of all the trackdays i've done I've never heard that advice.

Generally you let 4-5 PSI OUT of the tyres before starting the first session, the reason being the tyres get much hotter on track than they would on the road so they can increase by as much as 15-20psi. You then check the hot pressures and adjust accordingly.

[JJM]

jon- said:

JJM said:

Is it normal to increase tyre pressures for track days over normal road driving?

In a few weeks I am taking my car to North Weald for one of the Car Limits handling days, and am sure they suggest taking your tyres up to their max pressure.

No, and of all the trackdays i've done I've never heard that advice.

Generally you let 4-5 PSI OUT of the tyres before starting the first session, the reason being the tyres get much hotter on track than they would on the road so they can increase by as much as 15-20psi. You then check the hot pressures and adjust accordingly.

Hi jon

Thanks for the response, that fits with my understanding...

Just found the link on their site...

http://www.carlimits.com/script/viewVenue.php?venu...

"Please note that as your vehicle will be cornering at speed, you may need to inflate your tyres more than you would normally. Vehicles such as the Lotus Elise, Caterham etc are usually ok as they are lightweight and designed for high speed cornering, but cars such as Subaru Imprezas, Porsches, Madzas, TVRs etc are recommended to inflate their tyres to the maximum stated on the tyre itself to protect the side wall of the tyre. There is a garage just off the M11 roundabout (on the A414 towards Harlow) where you can do this before the event. We usually have a tyre pressure gauge so that you can decrease the tyres before your journey home!"

[RobCrezz]

JJM said:

jon- said:

JJM said:

Is it normal to increase tyre pressures for track days over normal road driving?

In a few weeks I am taking my car to North Weald for one of the Car Limits handling days, and am sure they suggest taking your tyres up to their max pressure.

No, and of all the trackdays i've done I've never heard that advice.

Generally you let 4-5 PSI OUT of the tyres before starting the first session, the reason being the tyres get much hotter on track than they would on the road so they can increase by as much as 15-20psi. You then check the hot pressures and adjust accordingly.

Hi jon

Thanks for the response, that fits with my understanding...

Just found the link on their site...

http://www.carlimits.com/script/viewVenue.php?venu...

"Please note that as your vehicle will be cornering at speed, you may need to inflate your tyres more than you would normally. Vehicles such as the Lotus Elise, Caterham etc are usually ok as they are lightweight and designed for high speed cornering, but cars such as Subaru Imprezas, Porsches, Madzas, TVRs etc are recommended to inflate their tyres to the maximum stated on the tyre itself to protect the side wall of the tyre. There is a garage just off the M11 roundabout (on the A414 towards Harlow) where you can do this before the event. We usually have a tyre pressure gauge so that you can decrease the tyres before your journey home!"

Might be because north weald isn't a proper track and the dodgy concrete can be harsh on tyres?

[jon-]

I wonder how much of that is North Weald specific. Their surface is extremely abrasive, and if I understand the format of the day correctly it's lots of very short stints, so no real temperature will build up.

That said, I'm sure a lot of road tyres have up to 60PSI on them, so they expect you to run 60PSI when cold, then rag your car? Stupid advice IMO.

[Daston]

JJM said:

jon- said:

JJM said:

Is it normal to increase tyre pressures for track days over normal road driving?

In a few weeks I am taking my car to North Weald for one of the Car Limits handling days, and am sure they suggest taking your tyres up to their max pressure.

No, and of all the trackdays i've done I've never heard that advice.

Generally you let 4-5 PSI OUT of the tyres before starting the first session, the reason being the tyres get much hotter on track than they would on the road so they can increase by as much as 15-20psi. You then check the hot pressures and adjust accordingly.

Hi jon

Thanks for the response, that fits with my understanding...

Just found the link on their site...

http://www.carlimits.com/script/viewVenue.php?venu...

"Please note that as your vehicle will be cornering at speed, you may need to inflate your tyres more than you would normally. Vehicles such as the Lotus Elise, Caterham etc are usually ok as they are lightweight and designed for high speed cornering, but cars such as Subaru Imprezas, Porsches, Madzas, TVRs etc are recommended to inflate their tyres to the maximum stated on the tyre itself to protect the side wall of the tyre. There is a garage just off the M11 roundabout (on the A414 towards Harlow) where you can do this before the event. We usually have a tyre pressure gauge so that you can decrease the tyres before your journey home!"

Tat sounds odd to me. Also arn't Imprezas, Porches & TVR's also designed for high speed cornering, plus the fact that TVR's dont tend to weigh much more than a Lotus.

As said I would be decreasing the pressure and checking the PSI after the first few runs.

[panic458]

Car Limits is not a track day, it is about learning car control an includes a lot of going sideways hence the need to protect the sidewall. The higher pressure also makes the car more likely to let go an hence increase your enjoyment sorry I meant education.

Daston said:

JJM said:

jon- said:

JJM said:

Is it normal to increase tyre pressures for track days over normal road driving?

In a few weeks I am taking my car to North Weald for one of the Car Limits handling days, and am sure they suggest taking your tyres up to their max pressure.

No, and of all the trackdays i've done I've never heard that advice.

Generally you let 4-5 PSI OUT of the tyres before starting the first session, the reason being the tyres get much hotter on track than they would on the road so they can increase by as much as 15-20psi. You then check the hot pressures and adjust accordingly.

Hi jon

Thanks for the response, that fits with my understanding...

Just found the link on their site...

http://www.carlimits.com/script/viewVenue.php?venu...

"Please note that as your vehicle will be cornering at speed, you may need to inflate your tyres more than you would normally. Vehicles such as the Lotus Elise, Caterham etc are usually ok as they are lightweight and designed for high speed cornering, but cars such as Subaru Imprezas, Porsches, Madzas, TVRs etc are recommended to inflate their tyres to the maximum stated on the tyre itself to protect the side wall of the tyre. There is a garage just off the M11 roundabout (on the A414 towards Harlow) where you can do this before the event. We usually have a tyre pressure gauge so that you can decrease the tyres before your journey home!"

Tat sounds odd to me. Also arn't Imprezas, Porches & TVR's also designed for high speed cornering, plus the fact that TVR's dont tend to weigh much more than a Lotus.

As said I would be decreasing the pressure and checking the PSI after the first few runs.

[jon-]

panic458 said:

Car Limits is not a track day, it is about learning car control an includes a lot of going sideways hence the need to protect the sidewall. The higher pressure also makes the car more likely to let go an hence increase your enjoyment sorry I meant education.

Fair enough, makes sense, however I still find it very odd that they recommend the maximum pressure when COLD. Few minutes of going sideways could put the tyres 20psi OVER maximum, which could result in one hell of a bang.