Want to get some track wheels/tires....questions
10-12-2004, 02:19 AM
#21
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Assuming those are cold pressures, and ...
The tires don't heat up much in those one minute sessions, you may gain 2 psi in the front, 1 psi in the rear, for a
46 front/41 rear hot
To get in the ballpark, I'd bump rear pressure up to the front (44/45 cold) and go from there. That would get rid of a chunk of understeer, but if the car's too skittish, maybe drop front and rear together.
Also, don't forget that a tire is an air spring, and that every 1 psi changes it's spring rate by ~25 lb/inch. When you have your front tires 5 psi higher than rear rears, you've increased the front spring rate by 125 lb/in relative to the rear, and that would only increase understeer. With our race car, we fine tune in only 1/2 psi increments; and the effect on handling balance is still noticeable.
46 front/41 rear hot
To get in the ballpark, I'd bump rear pressure up to the front (44/45 cold) and go from there. That would get rid of a chunk of understeer, but if the car's too skittish, maybe drop front and rear together.
Also, don't forget that a tire is an air spring, and that every 1 psi changes it's spring rate by ~25 lb/inch. When you have your front tires 5 psi higher than rear rears, you've increased the front spring rate by 125 lb/in relative to the rear, and that would only increase understeer. With our race car, we fine tune in only 1/2 psi increments; and the effect on handling balance is still noticeable.
10-13-2004, 12:57 AM
#23
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bollocks
we haven't even begun to discuss camber effects of wider tyres, finding the correct wheel offset, compensating for the higher spring rate of low profile tyres, nor compensation for the decrease in track, and the tendancy to tramline. nor have we discussed the effect of increased unsprung weight and rotational inertia.
nor have we discussed better better slip angles, or sharper turn-in.
do you want to have an intelligent dicussion on these points, or do you have nothing to add?
nor have we discussed better better slip angles, or sharper turn-in.
do you want to have an intelligent dicussion on these points, or do you have nothing to add?
10-13-2004, 01:23 AM
#24
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Re: We're 1/3 there :-)
[dave]
2. "i am also saying that the long narrow contact patch will *not* provide the same cornering grip as the short fat one, but it will provide much better longitudional grip (braking and traction)..."
[randy]
Any car provides better longtitudingal than lateral grip, but it's not the result of the tire's width. It's only because all(?) cars have a wheelbase longer than the track is wide. Under max braking, tire loads are more uniform front-to-rear than they are under max cornering left-to-right. Even with a perfectly round friction circle, this results in better braking than cornering, and only because the coefficient of friction varies with vertical load. This is the entire basis of Weight Distribution Revisited where I calculate that an S8 can corner at 0.88 g but brake at 0.95 g.
[dave again]
that is not my point. keeping compounds the same (remembering that most lower profile tyres use softer compunds to increase grip), the frictional forces are dependent only on the frictional coefficient of the materials and the force holding them together. and the force isn't changing (same car). therefore the changed shape (not size) of the contact patch decreases traction/braking by *exactly* the same amount as the increase in lateral (cornering) grip. immmutable fact.
[dave]
3. "if the key for better laptimes is better cornering grip, then you could well be better off with a wider tyre, provided the time you lose in traction and braking is accounted for."
[randy]
Not really ... Because the contact patch of a wide tire has a more uniform load distribution in lb/in2 than the contact patch of a narrow tire, the wider tire will always provide both better cornering and braking. It's the exact same reasoning that in all cases results in better braking than cornering.
[dave again]
no, the load distribution is identical - it has to be (otherwise please explain your agreement that the contact patch size doesn't change against your claim that "wide tire has a more uniform load distribution in *lb/in2* than the contact patch of a narrow tire"???
the only way to change load distribution is to transfer load under braking and cornering. for example braking will increase load on the front and so increase grip. in the same way as the contact patch stays the same size, so does the load distribution with wider tyres.
2. "i am also saying that the long narrow contact patch will *not* provide the same cornering grip as the short fat one, but it will provide much better longitudional grip (braking and traction)..."
[randy]
Any car provides better longtitudingal than lateral grip, but it's not the result of the tire's width. It's only because all(?) cars have a wheelbase longer than the track is wide. Under max braking, tire loads are more uniform front-to-rear than they are under max cornering left-to-right. Even with a perfectly round friction circle, this results in better braking than cornering, and only because the coefficient of friction varies with vertical load. This is the entire basis of Weight Distribution Revisited where I calculate that an S8 can corner at 0.88 g but brake at 0.95 g.
[dave again]
that is not my point. keeping compounds the same (remembering that most lower profile tyres use softer compunds to increase grip), the frictional forces are dependent only on the frictional coefficient of the materials and the force holding them together. and the force isn't changing (same car). therefore the changed shape (not size) of the contact patch decreases traction/braking by *exactly* the same amount as the increase in lateral (cornering) grip. immmutable fact.
[dave]
3. "if the key for better laptimes is better cornering grip, then you could well be better off with a wider tyre, provided the time you lose in traction and braking is accounted for."
[randy]
Not really ... Because the contact patch of a wide tire has a more uniform load distribution in lb/in2 than the contact patch of a narrow tire, the wider tire will always provide both better cornering and braking. It's the exact same reasoning that in all cases results in better braking than cornering.
[dave again]
no, the load distribution is identical - it has to be (otherwise please explain your agreement that the contact patch size doesn't change against your claim that "wide tire has a more uniform load distribution in *lb/in2* than the contact patch of a narrow tire"???
the only way to change load distribution is to transfer load under braking and cornering. for example braking will increase load on the front and so increase grip. in the same way as the contact patch stays the same size, so does the load distribution with wider tyres.
10-13-2004, 03:46 AM
#25
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We're getting closer, I think I found the problem !!
[dave again]
no, the load distribution is identical - it has to be (otherwise please explain your agreement that the contact patch size doesn't change against your claim that "wide tire has a more uniform load distribution in *lb/in2* than the contact patch of a narrow tire"???
the only way to change load distribution is to transfer load under braking and cornering. for example braking will increase load on the front and so increase grip. in the same way as the contact patch stays the same size, so does the load distribution with wider tyres.
[randy]
I'm not talking about the load distribution on the four tires, but the load or pressure distribution under the contact patch of one individual tire. THIS IS THE KEY !!!
Lets assume we have 700 lb on the corner of a car with a narrow tires, and that the result is a long narrow 7" x 1" contact patch and an average pressure on the patch of 100 lb/in<sup>2</sup>. Take a look at Figure 7 on page 15 in the pdf <A HREF="http://www.control.lth.se/articles/article.pike?action=fulltext&artkey=gaf%2B03">Tire Model for Braking and Cornering</a>. The example shows 7 different vertical loads in one patch, and they vary by nearly 50% from max to min. The average pressure may be 100 lb/in<sup>2</sup>, but it varies from maybe 75 to 150 lb/in<sup>2</sup>
Now lets assume we have a wide tire with a short wide 1" x 7" contact patch. In this simplistic case, the pressure distribution across the wide patch is a uniform 100 lb/in<sup>2</sup>. Obviously the pressure distribution is not uniform in one of those square inches, but the variation is less because a one inch long patch is smaller relative to the diameter of the tire than a 7" long patch.
And this brings us back to that chart of lateral grip versus vertical load. The total grip available from those two 7 in<sup>2</sup> patchs is significantly different, and the wide patch with it's more uniform load distribution will provide the most grip.
The chart of vertical load versus lateral grip is really the key to understanding much of a car's handling. It explains most or all of ...
1. Why nose-heavy cars tend to understeer.
2. Why tail heavy cars are preferred on track.
3. Why wider tires provide more grip than narrow tires.
4. Why stiffer front springs increase understeer.
5. Why a stiffer anti-sway bar increases oversteer.
6. Why handling is so sensitive to tire air pressure.
7. Why <A HREF="http://www.ozebiz.com.au/racetech/theory/shocktune1.html">Shock Tuning</a> may affect handling in one phase of a corner but not others.
8. Why cars brake at higher g's than they can corner.
And probably a few other things that slipped my mind. If the coefficient of friction remained constant and followed the blue line, a car would be much simpler, but not as much fun :-)
<img src="http://pictureposter.audiworld.com/17157/tiregrip.jpg">
Did I convince you yet ?
no, the load distribution is identical - it has to be (otherwise please explain your agreement that the contact patch size doesn't change against your claim that "wide tire has a more uniform load distribution in *lb/in2* than the contact patch of a narrow tire"???
the only way to change load distribution is to transfer load under braking and cornering. for example braking will increase load on the front and so increase grip. in the same way as the contact patch stays the same size, so does the load distribution with wider tyres.
[randy]
I'm not talking about the load distribution on the four tires, but the load or pressure distribution under the contact patch of one individual tire. THIS IS THE KEY !!!
Lets assume we have 700 lb on the corner of a car with a narrow tires, and that the result is a long narrow 7" x 1" contact patch and an average pressure on the patch of 100 lb/in<sup>2</sup>. Take a look at Figure 7 on page 15 in the pdf <A HREF="http://www.control.lth.se/articles/article.pike?action=fulltext&artkey=gaf%2B03">Tire Model for Braking and Cornering</a>. The example shows 7 different vertical loads in one patch, and they vary by nearly 50% from max to min. The average pressure may be 100 lb/in<sup>2</sup>, but it varies from maybe 75 to 150 lb/in<sup>2</sup>
Now lets assume we have a wide tire with a short wide 1" x 7" contact patch. In this simplistic case, the pressure distribution across the wide patch is a uniform 100 lb/in<sup>2</sup>. Obviously the pressure distribution is not uniform in one of those square inches, but the variation is less because a one inch long patch is smaller relative to the diameter of the tire than a 7" long patch.
And this brings us back to that chart of lateral grip versus vertical load. The total grip available from those two 7 in<sup>2</sup> patchs is significantly different, and the wide patch with it's more uniform load distribution will provide the most grip.
The chart of vertical load versus lateral grip is really the key to understanding much of a car's handling. It explains most or all of ...
1. Why nose-heavy cars tend to understeer.
2. Why tail heavy cars are preferred on track.
3. Why wider tires provide more grip than narrow tires.
4. Why stiffer front springs increase understeer.
5. Why a stiffer anti-sway bar increases oversteer.
6. Why handling is so sensitive to tire air pressure.
7. Why <A HREF="http://www.ozebiz.com.au/racetech/theory/shocktune1.html">Shock Tuning</a> may affect handling in one phase of a corner but not others.
8. Why cars brake at higher g's than they can corner.
And probably a few other things that slipped my mind. If the coefficient of friction remained constant and followed the blue line, a car would be much simpler, but not as much fun :-)
<img src="http://pictureposter.audiworld.com/17157/tiregrip.jpg">
Did I convince you yet ?
10-13-2004, 05:22 AM
#26
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Actually ...
Yes, I will add that your "shut up, I know better than you" approach is nowhere near as appropriate as randy's helpful "let me teach you" approach, and the sooner you adapt and adopt, the sooner you'll make it farther in your real life.
One approach attracts learners and the other attracts catcalls.
One approach attracts learners and the other attracts catcalls.
10-13-2004, 10:03 AM
#27
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errrr....
as far as i am concerned a discussion is occuring.
your post is the only one here which is dismissive, adds no information, and provides catcalls.
if you have nothing to add, don't.
your post is the only one here which is dismissive, adds no information, and provides catcalls.
if you have nothing to add, don't.
10-13-2004, 10:40 AM
#28
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Re: We're getting closer, I think I found the problem !!
in your example, a 7x1 contact patch vs a 1x7 one, you are assuming that the load distribution varies over the surface area.
this is simply not so. as the pressure inside the tyre is uniform, therefore the load distribution over the contact patch area is also uniform. it has to be. this is another of these things where "common sense" is wrong.
don't forget that the only thing supporting the car is the tyre pressure (pressure=weight/area). this does not vary within the tyre - so there is no way load distribution can change within the contact patch.
the only thing that would change this is irregularities in the sufrace you are running on (and camber but that is a separate discussion).
heat is a factor though, which is why the compounds are often varied with lower profile tyres.
this is simply not so. as the pressure inside the tyre is uniform, therefore the load distribution over the contact patch area is also uniform. it has to be. this is another of these things where "common sense" is wrong.
don't forget that the only thing supporting the car is the tyre pressure (pressure=weight/area). this does not vary within the tyre - so there is no way load distribution can change within the contact patch.
the only thing that would change this is irregularities in the sufrace you are running on (and camber but that is a separate discussion).
heat is a factor though, which is why the compounds are often varied with lower profile tyres.
10-13-2004, 03:13 PM
#29
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Ooops, I meant Fig 7 not Fig 6 in the pdf
[Dave]
"as the pressure inside the tyre is uniform, therefore the load distribution over the contact patch area is also uniform. it has to be. this is another of these things where "common sense" is wrong."
[Randy]
The pressure inside the tire is uniform, but the pressure over the contact patch is uniform only if the tire is elastic in all three dimensions like a balloon. A tire, however, is only elastic in a direction normal to the axle. The tread and carcass reinforcement plies make the tire extremely rigid in the lateral and longitudinal directions.
[Gafvert and Svendenius, p10 of the pdf]
"Note that the (lateral Fy) force depends nonlinearly on the normal force Fz. This dependence is, e.g., due to variations in the pressure distribution within the contact patch and change of friction characteristics. In general the initial slope and peak value of Foy/Fz decrease with increased load."
[randy again]
So what's your opinion: For the last half century, why has the trend been to wider tires on performance cars? Why did Audi lobby to use the heavy, high drag, wide 305/35-18 on the RS6? Why do rear engine Porsches and Formula cars use extremely heavy, but wide rear tires? All for cosmetics?
"as the pressure inside the tyre is uniform, therefore the load distribution over the contact patch area is also uniform. it has to be. this is another of these things where "common sense" is wrong."
[Randy]
The pressure inside the tire is uniform, but the pressure over the contact patch is uniform only if the tire is elastic in all three dimensions like a balloon. A tire, however, is only elastic in a direction normal to the axle. The tread and carcass reinforcement plies make the tire extremely rigid in the lateral and longitudinal directions.
[Gafvert and Svendenius, p10 of the pdf]
"Note that the (lateral Fy) force depends nonlinearly on the normal force Fz. This dependence is, e.g., due to variations in the pressure distribution within the contact patch and change of friction characteristics. In general the initial slope and peak value of Foy/Fz decrease with increased load."
[randy again]
So what's your opinion: For the last half century, why has the trend been to wider tires on performance cars? Why did Audi lobby to use the heavy, high drag, wide 305/35-18 on the RS6? Why do rear engine Porsches and Formula cars use extremely heavy, but wide rear tires? All for cosmetics?
10-13-2004, 03:53 PM
#30
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we're getting there!
the only things that change the load distribution acting through the contact patch are changing the load on the tyre. as i have said - this happens only with weight changes (downforce, cornering, braking & traction), as well as with camber changes.
nothing else!
how can it? the *only* thing applying the force is the air pressure! (there is an interaction from the sidewall, but lets leave this out of the equation - as that is very complex).
if you disagree with this, please explain how air, the springing mechanism, can alter the load distribution through the contact patch - localised high pressure???
with regards to the use of wider tyres in racing - as i have stated a number of times:
1) good circuit time is often dependent on improving cornering performance (i.e. going through the corners faster)
2) cornering performance (lateral grip) increases with wider tyres
3) circuit times can improve it the tradeoff against decreased braking and traction is acceptable
in f1 this is exactly what has happened. except that the other effect of wide tyres (aerodynamic drag) is a much greater consideration. bridgestone and michelin have adopted quite different strategies with regards their tyre profiles as a result of this issue alone (bridgestone narrower tyres, michelin wider ones).
wrt quattro, clearly traction and braking are already strengths and so can be traded for better cornering grip whilst still maintaining an acceptable edge in traction/braking.
nothing else!
how can it? the *only* thing applying the force is the air pressure! (there is an interaction from the sidewall, but lets leave this out of the equation - as that is very complex).
if you disagree with this, please explain how air, the springing mechanism, can alter the load distribution through the contact patch - localised high pressure???
with regards to the use of wider tyres in racing - as i have stated a number of times:
1) good circuit time is often dependent on improving cornering performance (i.e. going through the corners faster)
2) cornering performance (lateral grip) increases with wider tyres
3) circuit times can improve it the tradeoff against decreased braking and traction is acceptable
in f1 this is exactly what has happened. except that the other effect of wide tyres (aerodynamic drag) is a much greater consideration. bridgestone and michelin have adopted quite different strategies with regards their tyre profiles as a result of this issue alone (bridgestone narrower tyres, michelin wider ones).
wrt quattro, clearly traction and braking are already strengths and so can be traded for better cornering grip whilst still maintaining an acceptable edge in traction/braking.