Tires - power is knowledge!
07-19-2003, 01:33 AM
#31
Race Director
I need a new title, too many addicts around here... 
"Side wall flexes at speed @ turns. Agreed? To much and handling suffers - agreed?"
Sidewall flex is a complex issue and stiffer sidewalls aren't always the best. It depends upon the rims, the type of vehicle and the driving-style. In general, pure racing tires like slicks, tend to have very thin and soft sidewalls (no threat of parallel parking into kerbs). Soft sidewalls allow the casing to flex, yet the tread remains flat on the ground. So in cornering, it would look like this /__/ or \__\. If the sidewalls were stiff, it would tend to keep the entire cross-section square and lift up the tread under cornering.
The real issue is going with street cars and street tires. Typically you have people using too wide of a tire on too narrow of a rim. This causes an incredible amount of lateral flexing of the sidewalls, around 2-3". This large amount of flex also distorts the contact patch and you end up with a cup in the middle (no contact). A super-stiff sidewall on a street tire would tend to keep this flex to a minimum and keep the tread flat. But you end up having a lot more longitudinal flexing of the because of a stiff sidewall (like bias-ply tires).
So the ultimate solution is to have a soft supple tire on a wide rim (at least as wide as the tread-width). What this rim-width does is prevents the lateral shifting of the sidewalls under cornering. Imagine the typical wide-tire/narrow rim combo on a street car. It's coming towards you on the track and it takes a turn. You'll all seen photos of this where it looks like the tire is gonna be ripped off the rim towards the inside.
This is a dynamic turn-in situation and you have to take it frame-by-frame to see what happens. A tire only works in tension, in fact, it's an inside-out suspension-bridge. As the driver initiates the turn, the rim points in but the body and momentum of the car still wants to go straight. The angle of the tire rolling inwards pulls the tread inwards which pulls on the sidewall. The sidewall is pulled from its position outside the rim-edge to being even with the rim, it undergoes compression. Then is finally pulled inside the rim-edge. Only when the tread has pulled the sidewall all way inside the rim-edge does the sidewall undergo tension forces, and it finally pulls on the rim, which pulls on the suspension which pulls on the car and makes it deviate from a straight line. This sidewall compression-to-tension lateral movement can be as much as 3" on a street car and takes time.
A better scenario is where you'd want the rim wide enough so that sidewalls of the tire are parallel and even with the rim edges. Then any lateral forces from cornering would immediately tension the sidewalls and you'd get an instant tug on the rim. So imagine a standard tire that looks like this on a narrow rim |__|, it would have to be distorted into this shape /__/ before can pull on the rim. If the tire was like this instead \__/, the sidewalls would be under tension immediately.
I had a surprize introduction to this when I swapped rims at Laguna Seca a couple years ago. I had a new set of Dunlop SP8000s on my stock Fuchs 16x7/8" rims. Ran the first two sessions and they felt much better than the old tires that were on there, but they were no where near a R-compound or slick. So I bought a used set of HRE 16x10" wheels from a fellow racer and swapped rims (with the same tires) over the lunch break. On the next session out, I litterally flew off the inside of turn-2 because the turn-in was so quick and much more responsive than before. There was absolutely zero sidewall flex with immediate response to steering input. I even had to let out some air because the steering just felt too nervous.
"Side wall flexes at speed @ turns. Agreed? To much and handling suffers - agreed?"
Sidewall flex is a complex issue and stiffer sidewalls aren't always the best. It depends upon the rims, the type of vehicle and the driving-style. In general, pure racing tires like slicks, tend to have very thin and soft sidewalls (no threat of parallel parking into kerbs). Soft sidewalls allow the casing to flex, yet the tread remains flat on the ground. So in cornering, it would look like this /__/ or \__\. If the sidewalls were stiff, it would tend to keep the entire cross-section square and lift up the tread under cornering.
The real issue is going with street cars and street tires. Typically you have people using too wide of a tire on too narrow of a rim. This causes an incredible amount of lateral flexing of the sidewalls, around 2-3". This large amount of flex also distorts the contact patch and you end up with a cup in the middle (no contact). A super-stiff sidewall on a street tire would tend to keep this flex to a minimum and keep the tread flat. But you end up having a lot more longitudinal flexing of the because of a stiff sidewall (like bias-ply tires).
So the ultimate solution is to have a soft supple tire on a wide rim (at least as wide as the tread-width). What this rim-width does is prevents the lateral shifting of the sidewalls under cornering. Imagine the typical wide-tire/narrow rim combo on a street car. It's coming towards you on the track and it takes a turn. You'll all seen photos of this where it looks like the tire is gonna be ripped off the rim towards the inside.
This is a dynamic turn-in situation and you have to take it frame-by-frame to see what happens. A tire only works in tension, in fact, it's an inside-out suspension-bridge. As the driver initiates the turn, the rim points in but the body and momentum of the car still wants to go straight. The angle of the tire rolling inwards pulls the tread inwards which pulls on the sidewall. The sidewall is pulled from its position outside the rim-edge to being even with the rim, it undergoes compression. Then is finally pulled inside the rim-edge. Only when the tread has pulled the sidewall all way inside the rim-edge does the sidewall undergo tension forces, and it finally pulls on the rim, which pulls on the suspension which pulls on the car and makes it deviate from a straight line. This sidewall compression-to-tension lateral movement can be as much as 3" on a street car and takes time.
A better scenario is where you'd want the rim wide enough so that sidewalls of the tire are parallel and even with the rim edges. Then any lateral forces from cornering would immediately tension the sidewalls and you'd get an instant tug on the rim. So imagine a standard tire that looks like this on a narrow rim |__|, it would have to be distorted into this shape /__/ before can pull on the rim. If the tire was like this instead \__/, the sidewalls would be under tension immediately.
I had a surprize introduction to this when I swapped rims at Laguna Seca a couple years ago. I had a new set of Dunlop SP8000s on my stock Fuchs 16x7/8" rims. Ran the first two sessions and they felt much better than the old tires that were on there, but they were no where near a R-compound or slick. So I bought a used set of HRE 16x10" wheels from a fellow racer and swapped rims (with the same tires) over the lunch break. On the next session out, I litterally flew off the inside of turn-2 because the turn-in was so quick and much more responsive than before. There was absolutely zero sidewall flex with immediate response to steering input. I even had to let out some air because the steering just felt too nervous.
07-19-2003, 04:33 AM
#32
Burning Brakes
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By the way everyone. I just spoke to my friend at Porsche Stuttgart technical department.
Porsche rates Bridgestone SO2 as all-round tyre and it rates Pirelli PZerro as a sports tyre.
I think this says it all.
...
I hope no-one comes on here saying Continental is any good. They rate it as a comfort tyre.
Porsche rates Bridgestone SO2 as all-round tyre and it rates Pirelli PZerro as a sports tyre.
I think this says it all.
...I hope no-one comes on here saying Continental is any good. They rate it as a comfort tyre.
07-19-2003, 05:23 AM
#34
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Similar to Simon I have fitted Pirelli Pzero Rossos to my 951 - they are excellent and make the behaviour of the car very predictable.
However I am running the same tires on my Impreza WRX and recently experienced these tires overheating very easily on track (much easier than the previous Bridgestone S02s) - after about 5 laps. Once hot they start letting go extremely verociously - no warning whatosever - once they start sliding at speed you simply can't build up enough grip to catch the slide anymore. Just something to keep in mind when tracking these tires...they are among the very best for the street - but not suited for sustained track use.
However I am running the same tires on my Impreza WRX and recently experienced these tires overheating very easily on track (much easier than the previous Bridgestone S02s) - after about 5 laps. Once hot they start letting go extremely verociously - no warning whatosever - once they start sliding at speed you simply can't build up enough grip to catch the slide anymore. Just something to keep in mind when tracking these tires...they are among the very best for the street - but not suited for sustained track use.
Last edited by Swedeboy; 07-19-2003 at 07:20 AM.
07-19-2003, 08:38 AM
#38
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Originally posted by SimonK
Swedeboy, we better keep our P0Rosso cool a?
Swedeboy, we better keep our P0Rosso cool a?
Its not a major issue if you are aware of it - only a real issue on shorter, twistier circuits with only short straights to cool down the tires. On for example a street-type circuit like the Nurburgring which arent as hard on the tires it doesnt seem to be an issue.
On the shorter circuits I'd say the first 2 laps the Pirellis are on par with or better than decent (cold) R tires. After that when the tires get warm - the R tires get better and the Pirellis degrade rapidly. Playing with tire pressures when the Pirellis are hot seems to help quite a bit as well.
07-20-2003, 08:45 AM
#40
Burning Brakes
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Hey - what about this?
They just filled my tyres with gas (nitrogen). It worms up to a certain temperature and then it stays there!!!
I am happy, if it will work? Will test and report.
They just filled my tyres with gas (nitrogen). It worms up to a certain temperature and then it stays there!!!
I am happy, if it will work? Will test and report.
07-20-2003, 11:17 AM
#42
Race Director
Hmm, yeah, you have to find some way to get rid of all the moisture (water-vapor) in your tire. Might take a couple purges of nitrogen to do it, and even then they'll be trace amounts.
The whole pressure-rise with air and water is because of the partial vapor-pressure of water dissolved in the air. At temperatures approach the boiling-point of water, the partial-pressure of the water-vapor component increases tremendously! This throws off your finely-tuned pressures of course...
The whole pressure-rise with air and water is because of the partial vapor-pressure of water dissolved in the air. At temperatures approach the boiling-point of water, the partial-pressure of the water-vapor component increases tremendously! This throws off your finely-tuned pressures of course...
07-21-2003, 12:12 AM
#43
Burning Brakes
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Hey it works!
Testing time;
Tyres 2.5 bar cold.
Went for a blast - brakes cooking, tyres smelling...
Measured again;
2.6 bar hot.
Not tried it at track yet but hey it kind of works...
Testing time;
Tyres 2.5 bar cold.
Went for a blast - brakes cooking, tyres smelling...
Measured again;
2.6 bar hot.
Not tried it at track yet but hey it kind of works...
07-21-2003, 12:23 AM
#44
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It's very tough to overheat tires on the street.
You might want to try slightly lower pressures - a friend with a 968 CS was running the Pzeros at Spa and was recommended 2.2/2.1 by Pirelli engineers.
You might want to try slightly lower pressures - a friend with a 968 CS was running the Pzeros at Spa and was recommended 2.2/2.1 by Pirelli engineers.