Want to get some track wheels/tires....questions
10-10-2004, 11:07 PM
#11
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some physics
1) the amount of rubber on the road (size) is determined solely by dividing the vehicle weight by the pressure of the air in the tyre. divide by 4 and you get your contact patch size.
2) you will note the absence of tyre size in the above equation. the reason is simple, it doesn't matter. you have the same amount of rubber on the road with a 205 tyre as with a 275 one. the only thing that changes is the shape of the contact patch, not the size.
3) generally speaking (compounds being the same), you trade off longitudional forces (braking, traction) against lateral forces as you increase the width of a tyre (i.e. worse longitudional, better lateral). an important fact in high speed formula is also the increased aero drag of wider tyres.
caveats:
1) compounds tend to get softer as tyres get wider whcih increases grip at the expense of tyre life.
2) tyre pressure has a huge effect on tyre performance.
hth
2) you will note the absence of tyre size in the above equation. the reason is simple, it doesn't matter. you have the same amount of rubber on the road with a 205 tyre as with a 275 one. the only thing that changes is the shape of the contact patch, not the size.
3) generally speaking (compounds being the same), you trade off longitudional forces (braking, traction) against lateral forces as you increase the width of a tyre (i.e. worse longitudional, better lateral). an important fact in high speed formula is also the increased aero drag of wider tyres.
caveats:
1) compounds tend to get softer as tyres get wider whcih increases grip at the expense of tyre life.
2) tyre pressure has a huge effect on tyre performance.
hth
10-11-2004, 03:11 AM
#12
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I want to increase my cornering ability
So what would you suggest? The car pushes badly and skips the front end when cornering hard.
The braking aspect of my car is perfect, the thing is a monster!
So would a 265 do anything for me in your opinion? Thanks.
The braking aspect of my car is perfect, the thing is a monster!
So would a 265 do anything for me in your opinion? Thanks.
10-11-2004, 03:25 AM
#13
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Except for one little (actually huge) detail ...
Though a 205/65-16 and a 305/35-18 have the same outside diameter, common sense says that the wider tire will provide much more grip than the narrow tire, in all directions; and common sense is correct. This is a good comparison because 205 is the width of the OE tire for Audi A6; while 305 is the width of the tire used on the SCCA <A HREF="http://www.world-challenge.com/">Speed World Challenge</a> Audi RS6.
This SCCA racing series is interesting because at the beginning of the season, macro adjustments are made to a car's specs to make a variety of cars as "equal" as possible, which is followed by micro tweaking during the season, including changing minimum weight for every race! To make the RS6 competitive, a macro adjustment allowed it to be the only car to have a nearly 4" track increase to get a 305/35-18 on an 11" wheel to fit under the fenders (which I think look silly in the pic at the end of the post).
When you realize that the tire is actually an air spring, skiwi comes to the correct conclusion that the size of the contact patch is essentially the same for both narrow and wide tires, but ...
<b>The pressure distribution of the load across the tire patch itself is more uniform with the wider tire.</b>
Assume (and exaggerating) that you're trading a long narrow 12" X 1"contact patch with the 205/65-16 for a short wide 1" X 12" patch with the 305/35-18. Simply because a tire is round, the load on any contact patch will be lower on the leading and trailing edges and higher in the center, but the distribution with the skinny tire will be worse simply because the contact patch wraps further up around the tire. That may sound like a nit-picking detail, but the effect on grip is huge.
<img src="http://pictureposter.audiworld.com/17157/tiregrip.jpg">
In addition,
1. Combining the chart with the fact a tire's grip follows a friction circle and is essentially the same in all directions explains the almost perfect match with real world performance in <A HREF="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution Revisited</a>.
2. The chart also explains how sway bars work - Stiffening the bar at one end of a car increases the load difference across the tires at that end, reducing their combined grip. The load distribution across the tires at the other end of the car becomes more uniform, increasing the combined grip of these two tires. The result: a stiffer rear sway bar reduces understeer.
3. The details of a tire's construction can also affect pressure distribution. For example, Bridgestone brags that their top-of-the-line S-03 uses "Consistent Surface Contact technology, a pattern of tread blocks designed for uniform contact pressure, improving dry and wet handling."
4. And finally, this is why race cars use slicks. The lack of tread grooves reduces the load on the rubber of the contact patch and significantly increases grip.
So ... wider provides more grip, always (at long as it's not raining, another story).
Additional related links:
World Challenge RS6 <A HREF="http://www.world-challenge.com/competitors/vts/2004-VTS-AUDI-RS6-REV3.pdf">Vehicle Specs</a>
World Challenge <A HREF="http://www.world-challenge.com/competitors/bulletins/04-gt-appendixa-1.pdf">Tire Specs</a>
<A HREF="https://www.audiworld.com/model/rs6/03/03rs6.pdf">Audi RS6 Specs</a>
<A HREF="http://www.championracing.net/">Champion Racing</a>
<img src="http://www.championracing.net/Audi_RS6/archive/RS6_2004/pics/AtlGT_14.jpg">
This SCCA racing series is interesting because at the beginning of the season, macro adjustments are made to a car's specs to make a variety of cars as "equal" as possible, which is followed by micro tweaking during the season, including changing minimum weight for every race! To make the RS6 competitive, a macro adjustment allowed it to be the only car to have a nearly 4" track increase to get a 305/35-18 on an 11" wheel to fit under the fenders (which I think look silly in the pic at the end of the post).
When you realize that the tire is actually an air spring, skiwi comes to the correct conclusion that the size of the contact patch is essentially the same for both narrow and wide tires, but ...
<b>The pressure distribution of the load across the tire patch itself is more uniform with the wider tire.</b>
Assume (and exaggerating) that you're trading a long narrow 12" X 1"contact patch with the 205/65-16 for a short wide 1" X 12" patch with the 305/35-18. Simply because a tire is round, the load on any contact patch will be lower on the leading and trailing edges and higher in the center, but the distribution with the skinny tire will be worse simply because the contact patch wraps further up around the tire. That may sound like a nit-picking detail, but the effect on grip is huge.
<img src="http://pictureposter.audiworld.com/17157/tiregrip.jpg">
In addition,
1. Combining the chart with the fact a tire's grip follows a friction circle and is essentially the same in all directions explains the almost perfect match with real world performance in <A HREF="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution Revisited</a>.
2. The chart also explains how sway bars work - Stiffening the bar at one end of a car increases the load difference across the tires at that end, reducing their combined grip. The load distribution across the tires at the other end of the car becomes more uniform, increasing the combined grip of these two tires. The result: a stiffer rear sway bar reduces understeer.
3. The details of a tire's construction can also affect pressure distribution. For example, Bridgestone brags that their top-of-the-line S-03 uses "Consistent Surface Contact technology, a pattern of tread blocks designed for uniform contact pressure, improving dry and wet handling."
4. And finally, this is why race cars use slicks. The lack of tread grooves reduces the load on the rubber of the contact patch and significantly increases grip.
So ... wider provides more grip, always (at long as it's not raining, another story).
Additional related links:
World Challenge RS6 <A HREF="http://www.world-challenge.com/competitors/vts/2004-VTS-AUDI-RS6-REV3.pdf">Vehicle Specs</a>
World Challenge <A HREF="http://www.world-challenge.com/competitors/bulletins/04-gt-appendixa-1.pdf">Tire Specs</a>
<A HREF="https://www.audiworld.com/model/rs6/03/03rs6.pdf">Audi RS6 Specs</a>
<A HREF="http://www.championracing.net/">Champion Racing</a>
<img src="http://www.championracing.net/Audi_RS6/archive/RS6_2004/pics/AtlGT_14.jpg">
10-11-2004, 03:54 AM
#14
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Questions/comments
What cold/hot tire pressures did you run? I'm certain that the tires won't heat up at an autocross like they do at Mid-Ohio, so cold pressures should be higher. "The car pushes badly and skips the front end when cornering hard." -- sounds like pressure is too high in the front and too low in the rear.
Did anyone hear anything back from ECS Tuning after they requested a list of performance mods that would interest us? I had asked for an even stiffer rear sway bar.
And yes, 265 will help, and 305 would be better :-)<ul><li><a href="https://forums.audiworld.com/a8/msgs/75515.phtml">ECS Tuning post</a></li></ul>
Did anyone hear anything back from ECS Tuning after they requested a list of performance mods that would interest us? I had asked for an even stiffer rear sway bar.
And yes, 265 will help, and 305 would be better :-)<ul><li><a href="https://forums.audiworld.com/a8/msgs/75515.phtml">ECS Tuning post</a></li></ul>
10-11-2004, 10:57 PM
#16
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physics <> common sense
common sense has nothing to do with it randy. physics is physics, and you can't change the facts.
also when you say "grip" you need to specify what you mean. as i said in my post, the thing that often changes with lower profile, wider tyres is that the compound is made softer (often seen in tyres that don't last very long). this makes more overall grip available, but also changes the contact patch size, so the trade-offs which i mentioned in my post still apply.
is anyone wants more reading on this, i would recommend chris daniels book "chassis dynamics".
also when you say "grip" you need to specify what you mean. as i said in my post, the thing that often changes with lower profile, wider tyres is that the compound is made softer (often seen in tyres that don't last very long). this makes more overall grip available, but also changes the contact patch size, so the trade-offs which i mentioned in my post still apply.
is anyone wants more reading on this, i would recommend chris daniels book "chassis dynamics".
10-11-2004, 11:13 PM
#17
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in the context of your post, i would consider understeer a setup issue to be cured, and not a tyre issue (i.e. push may be masked by wider tyres, but it will still be there - just at higher limits).
therefore i would suggest an approach based in order of cost and ease:
1) work with your pressures front and rear.
2) tyre compounds will also help on existing rims
3) next i'd look at sway bars
4) given the physics of tyres, wider tyres will help lateral grip [penalties are more drag and poorer traction/braking]. therefore they can help if you can make up more time in corners (generally the secret to lap time is cornering speed, not straightline speed).
5) high-bias torsen diff to allow more power spread.
so, imho, only 2 of these approaches has the potential to solve understeer, the rest just move it to a higher plane.
also, iirc, the stasis web-site had a very useful piece from their experience in setting up their quattro [dialing out understeer] for production car racing.
one last thing: understeer is not necessarily a bad thing - and certainly doesn't mean a car can't be fast (or that it is inherent in quattro, and not in rwd).
therefore i would suggest an approach based in order of cost and ease:
1) work with your pressures front and rear.
2) tyre compounds will also help on existing rims
3) next i'd look at sway bars
4) given the physics of tyres, wider tyres will help lateral grip [penalties are more drag and poorer traction/braking]. therefore they can help if you can make up more time in corners (generally the secret to lap time is cornering speed, not straightline speed).
5) high-bias torsen diff to allow more power spread.
so, imho, only 2 of these approaches has the potential to solve understeer, the rest just move it to a higher plane.
also, iirc, the stasis web-site had a very useful piece from their experience in setting up their quattro [dialing out understeer] for production car racing.
one last thing: understeer is not necessarily a bad thing - and certainly doesn't mean a car can't be fast (or that it is inherent in quattro, and not in rwd).
10-12-2004, 12:23 AM
#18
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In this case they agree,
and I'm not sure what physics or facts you mean.
A long narrow contact patch physically cannot provide the same grip (resistance to horizontal load) as a short wide contact patch; and the only reason is that the pressure distribution is more uniform with the short wide patch. I never explicitly said it in the original post, but the average coefficient of friction is lower with the narrow tire even when it uses the same rubber compound. (see middle chart)
Do you really believe that an RS6 would be competitive on 205/65-16 tires using the same compound as a 305/35-18? Why did Audi lobby strongly for these wider tires if they're just heavier with more aero drag?
<img src="http://www.gururacing.net/ImagesMisc/TireEfficiency.jpg">
A long narrow contact patch physically cannot provide the same grip (resistance to horizontal load) as a short wide contact patch; and the only reason is that the pressure distribution is more uniform with the short wide patch. I never explicitly said it in the original post, but the average coefficient of friction is lower with the narrow tire even when it uses the same rubber compound. (see middle chart)
Do you really believe that an RS6 would be competitive on 205/65-16 tires using the same compound as a 305/35-18? Why did Audi lobby strongly for these wider tires if they're just heavier with more aero drag?
<img src="http://www.gururacing.net/ImagesMisc/TireEfficiency.jpg">
10-12-2004, 01:02 AM
#19
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we are talking at cross purposes
i am saying that the size of the contact patches is equal on both your rs6 cases 305/30 18 and 205/65 16. if you don't believe me, work it out yourself.
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) - and superior wet weather performance. 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.
re-read my original post.
you are obscuring the fundamental issue with other data. the base case is as i say. you can't change physics.
peace huh?
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) - and superior wet weather performance. 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.
re-read my original post.
you are obscuring the fundamental issue with other data. the base case is as i say. you can't change physics.
peace huh?
10-12-2004, 01:35 AM
#20
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We're 1/3 there :-)
1. <b>"i am saying that the size of the contact patches is equal on both your rs6 cases 305/30 18 and 205/65 16. if you don't believe me, work it out yourself."</b>
I've always agreed.
2. <b>"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)..."</b>
Any car provides better longitudingal 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 <A HREF="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution Revisited</a> where I calculate that an S8 can corner at 0.88 g but brake at 0.95 g.
3. <b>"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."</b>
Not really ... Because the contact patch of a wide tire has a more uniform load distribution in lb/in<sup>2</sup> than the contact patch of a narrow tire, the wider tire will <i>always provide both better cornering and braking.</i> It's the exact same reasoning that in all cases results in better braking than cornering.
I've always agreed.
2. <b>"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)..."</b>
Any car provides better longitudingal 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 <A HREF="https://forums.audiworld.com/a8/msgs/36145.phtml">Weight Distribution Revisited</a> where I calculate that an S8 can corner at 0.88 g but brake at 0.95 g.
3. <b>"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."</b>
Not really ... Because the contact patch of a wide tire has a more uniform load distribution in lb/in<sup>2</sup> than the contact patch of a narrow tire, the wider tire will <i>always provide both better cornering and braking.</i> It's the exact same reasoning that in all cases results in better braking than cornering.