Huge tires for autoX...whats the deal? ATTN chet, smithsr, etc
Huge tires for autoX...whats the deal? ATTN chet, smithsr, etc
As you all say, grip is mostly a function of compound. Why then do autocrossers tend to run such huge tires? Its almost the opposite of drift...recently I saw a post on zilvia regarding 245/45/16s on a 16x7 wheel. This seems way too big. Since autocross courses are short, heat cannot be a factor. How does the bigger tire help? As a side note, I've also seen posts regarding changing the balance of a car through tire size. In particular, one member went from 245/40 on 17x9 all around to 235/45 front and 255/40 rear on the same wheels to reduce oversteer at high speeds at HDPEs. How does this work if grip is based on compound only?I understand the physics of friction, how its based on normal force and coefficient of friction only, etc. I'm just trying to look for evidence of this in real world situations.
Nobody says these people are right. The best explanation I've heard or read is the idea that:
people often associate a larger tire with that of a softer tire. Larger (wider) does not mean softer.
Here is how this applies to me:I've started with 205/55-16 in the stickiest tire readily available. RA1 with treadwear 40 and also available in dozens of larger sizes.
I start with this OEM size and find the limit of adhesion with tire at operating temp, for braking, acceleration, and hard cornering.
Depending on lap times / cornering speeds, and also how much reserve the driver feels, one can dial wider tires of the same compound(same RA1 in wider sizes in my case) to lengthen the patch, which is often said to give better lateral grip. Why people in actual driving condition(not looks for shows) go wider R, narrower F.
Common theory is, wider tire in any compound increases lateral grip because of shape change of contact patch. Probably some holes in the above, it's late.
people often associate a larger tire with that of a softer tire. Larger (wider) does not mean softer.
Here is how this applies to me:I've started with 205/55-16 in the stickiest tire readily available. RA1 with treadwear 40 and also available in dozens of larger sizes.
I start with this OEM size and find the limit of adhesion with tire at operating temp, for braking, acceleration, and hard cornering.
Depending on lap times / cornering speeds, and also how much reserve the driver feels, one can dial wider tires of the same compound(same RA1 in wider sizes in my case) to lengthen the patch, which is often said to give better lateral grip. Why people in actual driving condition(not looks for shows) go wider R, narrower F.
Common theory is, wider tire in any compound increases lateral grip because of shape change of contact patch. Probably some holes in the above, it's late.
Re: Huge tires for autoX...whats the deal? ATTN chet, smithsr, etc (turtl631)
i was thinking wider contact patch = more surface area in contact with pavement. more potential surface area = greater possibility for increased traction.
on hi- hp cars this is i guess the way to go. but since the 240 in stock trim isn't exactly hi-powered, tread width must be kept in moderation. theres a point where its over kill and you'll probaby lose time due to other factors.
fwiw i still run stock alloys w/ azenis as you can see below. but i wish falken made them in a 215 or 225 size for 15", just so i could compare what a little more tread width would be like. however in my case the rim width is going to be a limiting factor
on hi- hp cars this is i guess the way to go. but since the 240 in stock trim isn't exactly hi-powered, tread width must be kept in moderation. theres a point where its over kill and you'll probaby lose time due to other factors.
fwiw i still run stock alloys w/ azenis as you can see below. but i wish falken made them in a 215 or 225 size for 15", just so i could compare what a little more tread width would be like. however in my case the rim width is going to be a limiting factor
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Q45tech
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Re: Huge tires for autoX...whats the deal? ATTN chet, smithsr, etc (eddiec)
Tire slip angles are a function of loaded weight vs total reserve weight [the load index].........the more air volume the higher the load index [generally].
Most oem designs have inadequate front tires to promote some understeer, the tires get overloaded from braking and turning forces and overheat [internally] quickly and wash out in a matter of seconds.
Assuming identical designed tires just different load indexes.....you can gain roughly half the ratio of load index increase as usable G.
10% stronger tire might raise ultimate G ability by 5%.........due to decreased slip angle [not going over the top and into the downward part of curve].
Most oem designs have inadequate front tires to promote some understeer, the tires get overloaded from braking and turning forces and overheat [internally] quickly and wash out in a matter of seconds.
Assuming identical designed tires just different load indexes.....you can gain roughly half the ratio of load index increase as usable G.
10% stronger tire might raise ultimate G ability by 5%.........due to decreased slip angle [not going over the top and into the downward part of curve].
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Q45tech
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Re: (corn322)
Really no testing done [that can be linked on web] as to the exact GAIN of using a stronger tire.
When you study the tirerack site you can allude to the gains in the same size with different tire construction.
The info I have pertains to say a Michelin Pilot H4 [same design same construction] in going up from 95H [1521 pounds] to 98H [1640 pounds].
A 4.5% gain [say .80G yields 0.836G] roughly........the slip angles stay the same just at a higher load on the stronger tire.........take a slalom say 1.5 mph faster.
Only fair to say that in identical brands/designs a stonger tire will probably handle better........what is key is longevity.......a 8% stronger tire will last at least 8% longer under the same static load and driver.
We have done lots of study on 215/65/15* vs 225/60/15* vs 235/60/15 and 245/50/16* vs 255/50/16
* 1521 pounds vs 1640 pounds
Whether that correlates to your sizes and brands is unknown?
When you study the tirerack site you can allude to the gains in the same size with different tire construction.
The info I have pertains to say a Michelin Pilot H4 [same design same construction] in going up from 95H [1521 pounds] to 98H [1640 pounds].
A 4.5% gain [say .80G yields 0.836G] roughly........the slip angles stay the same just at a higher load on the stronger tire.........take a slalom say 1.5 mph faster.
Only fair to say that in identical brands/designs a stonger tire will probably handle better........what is key is longevity.......a 8% stronger tire will last at least 8% longer under the same static load and driver.
We have done lots of study on 215/65/15* vs 225/60/15* vs 235/60/15 and 245/50/16* vs 255/50/16
* 1521 pounds vs 1640 pounds
Whether that correlates to your sizes and brands is unknown?
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Onizuka
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89 Nissan S14 hatch SR20DE
One theory is that a wider tire contains much more material and is therfor mor resistant to over heating. Now this can be applied to autocross in two ways:
-Street tires are made of compounds that work better with a semi-hot operating temperature. Runs in autocross are close enough that, for the most part, a street tire can overheat in just one run and wont have time to cool down to optimum operating temperature before the next run. Adding tire mass will allow the tire to absorb more energy without heating up quite as much.
-Race tires like hoosiers need high temperatures to stick, such that some times it is appropriate to run with as little tire mass as required. Some will decrease tire pressure to make the tires heat up faster, or run the car with two drivers (making twice the runs at twice the pace) to keep the tires hot. Wider is not necessarily better in this case.
So in conclusion the only practicle reasons for mounting wider tires for auto cross is for heat resistence (or if you are in dire need of lateral stability).
-Street tires are made of compounds that work better with a semi-hot operating temperature. Runs in autocross are close enough that, for the most part, a street tire can overheat in just one run and wont have time to cool down to optimum operating temperature before the next run. Adding tire mass will allow the tire to absorb more energy without heating up quite as much.
-Race tires like hoosiers need high temperatures to stick, such that some times it is appropriate to run with as little tire mass as required. Some will decrease tire pressure to make the tires heat up faster, or run the car with two drivers (making twice the runs at twice the pace) to keep the tires hot. Wider is not necessarily better in this case.
So in conclusion the only practicle reasons for mounting wider tires for auto cross is for heat resistence (or if you are in dire need of lateral stability).
Re: (J-Spec Tuner)
I'm going to restate Jspecs more recent quote I liked:
"Contact patch area does not vary with tire size, repeat this to yourself 100 times"
It varies by load (weight) and inflation.
Wider tires do have more surface area, and in theory would have more ability to absorb heat. This argument is kind of nullified by a few things:Ambient temp, car weight, driving style, and suspension settings can all affect the heat buildup of a tire, thusly, running a wider tire to "reduce the heat buildup" in the tire would seem to be a band-aide in my opinion when what you should be doing is looking at a different compound, tire type, tread design, suspension setting (using a pyrometer to check temp differences across the tread face) and other variables before just blindly increasing tire width.
Secondly, as explain in the FAQ, a wider tire does, at least in theory, provide a bit more lateral stability due to its contact patch shape. The biggest mistake here is looking at a vehicle with wider tires on the rear (or wider tires than yours) and thinking that this would be beneficial for you.
Sometimes a wider tire carries a higher load index (take for instance the corvette 245-45-17 and 275-40-18 split setup) and is thusly used to create a better handling of the load/weight transfer under power by a FR (or, even more pronounced) a MR car (Ferrari F360 is a 415-45-18 and 275-40-18...), the reasoning behind this lies in what Q45 tech said, along with looks, and the aforementioned heat/lateral stability issue.
Another facet often overlooked by most people is "ideal rim width" for a certain tire...a 245-45-16 is fat too wide, and low of an aspect ratio for a 7" rim, and will cause 'crowning' of the tire, thereby effectively increasing its aspect ratio and decreasing the tires ability to handle loads because of the contact patch distortion. There’s a reason you want to stay plus or minus .5" of an "ideal" rim specification for a tire size people.... optimum contact patch, and tire characteristics.
For you drifters, with the 215-40-17 on a 9" rim, consider this:
What you are actually doing (and not realizing it) is effectively decreasing the aspect ratio of the tire, leading to quicker response (less sidewall flex) but a reduced ability of the tire to do its job properly, because this will inevitably lead to contact patch distortion (along with bead stress) and shorten the tires life, as well as subject it to more heat and such than it would normally be subject to...not a good idea.
Likewise, the opposite happens with a 245-40-17 (or so) on a 17x7 rim...
Turtle:"the auto crossers on zilvia insist that wider tires work better, but they can't really explain why"...
...
That should have been your first clue that it’s a "someone told me it handled better, and I think it does"
I can almost guarantee you none of the guys tauting such things have lap times or other evidence to point to.
That said, it has been proven a lower aspect ratio and slightly wider tire can help lap times (to a point), as exemplified in a test by GRM a few months ago. They used the same tire type (compound and design) but did neglect to tell you that the 2 plus-sized tires were Z rated, while the 'stock' was V.... which could account for some of the results right there alone. There is evidence in testing and a lower aspect ratio tire can produce a higher cornering force for a given slip angle, but there is a rate of diminishing returns. The stipulation being the tire has to have an equivalent, if not higher, load index, and lots of other things...
I'm rambling now, but hopefully I made enough clear for you to get a bead on what you needed between myself, Dennis and Smith's posts.
Thanks!-Chet
"Contact patch area does not vary with tire size, repeat this to yourself 100 times"
It varies by load (weight) and inflation.
Wider tires do have more surface area, and in theory would have more ability to absorb heat. This argument is kind of nullified by a few things:Ambient temp, car weight, driving style, and suspension settings can all affect the heat buildup of a tire, thusly, running a wider tire to "reduce the heat buildup" in the tire would seem to be a band-aide in my opinion when what you should be doing is looking at a different compound, tire type, tread design, suspension setting (using a pyrometer to check temp differences across the tread face) and other variables before just blindly increasing tire width.
Secondly, as explain in the FAQ, a wider tire does, at least in theory, provide a bit more lateral stability due to its contact patch shape. The biggest mistake here is looking at a vehicle with wider tires on the rear (or wider tires than yours) and thinking that this would be beneficial for you.
Sometimes a wider tire carries a higher load index (take for instance the corvette 245-45-17 and 275-40-18 split setup) and is thusly used to create a better handling of the load/weight transfer under power by a FR (or, even more pronounced) a MR car (Ferrari F360 is a 415-45-18 and 275-40-18...), the reasoning behind this lies in what Q45 tech said, along with looks, and the aforementioned heat/lateral stability issue.
Another facet often overlooked by most people is "ideal rim width" for a certain tire...a 245-45-16 is fat too wide, and low of an aspect ratio for a 7" rim, and will cause 'crowning' of the tire, thereby effectively increasing its aspect ratio and decreasing the tires ability to handle loads because of the contact patch distortion. There’s a reason you want to stay plus or minus .5" of an "ideal" rim specification for a tire size people.... optimum contact patch, and tire characteristics.
For you drifters, with the 215-40-17 on a 9" rim, consider this:
What you are actually doing (and not realizing it) is effectively decreasing the aspect ratio of the tire, leading to quicker response (less sidewall flex) but a reduced ability of the tire to do its job properly, because this will inevitably lead to contact patch distortion (along with bead stress) and shorten the tires life, as well as subject it to more heat and such than it would normally be subject to...not a good idea.
Likewise, the opposite happens with a 245-40-17 (or so) on a 17x7 rim...
Turtle:"the auto crossers on zilvia insist that wider tires work better, but they can't really explain why"...
...
That should have been your first clue that it’s a "someone told me it handled better, and I think it does"
I can almost guarantee you none of the guys tauting such things have lap times or other evidence to point to.
That said, it has been proven a lower aspect ratio and slightly wider tire can help lap times (to a point), as exemplified in a test by GRM a few months ago. They used the same tire type (compound and design) but did neglect to tell you that the 2 plus-sized tires were Z rated, while the 'stock' was V.... which could account for some of the results right there alone. There is evidence in testing and a lower aspect ratio tire can produce a higher cornering force for a given slip angle, but there is a rate of diminishing returns. The stipulation being the tire has to have an equivalent, if not higher, load index, and lots of other things...
I'm rambling now, but hopefully I made enough clear for you to get a bead on what you needed between myself, Dennis and Smith's posts.
Thanks!-Chet
Re: (Exar-Kun)
i'm a total noob when it comes to the science behind all this
but check this out, one thing no one here mentioned other than tire width is the offset
by having a tire thats 1 inch wider than a stock rim, but the new width is completly on the outside of the wheel, you now essentialy increased your wheel base by 2 inches
so now when the car leans there is more resistence for it to do so, or to put it in another way, there is now more tire to support the car.
thats just a joe blow observation
but check this out, one thing no one here mentioned other than tire width is the offset
by having a tire thats 1 inch wider than a stock rim, but the new width is completly on the outside of the wheel, you now essentialy increased your wheel base by 2 inches
so now when the car leans there is more resistence for it to do so, or to put it in another way, there is now more tire to support the car.
thats just a joe blow observation
Re: (7thGear)
....its an observation that's completely wrong, that's why no one mentioned it.
Weelbase is not calcuated from the outside of the tire, it is calculated (and rightly so) from the centerline of hte wheel/tire assembly..just like track (which is part of the wheel base calc.)
So no, you're not doing anything to wheelbase by changing the tire width. Go read more, post less.
Also, beacuse it seems you've got confusion, tire width does not affect the offset of a wheel in any way.
Also, "there is now more tire to support the car" isn't accurate either, since both premises you were basing it on are wrong, the ability to support the car (as far as tire/weheel assemblies are concerned) is based on load index/strength of the tire and strength of the wheel.
Increasing a cars track will decrease weight transfer, benefiting handling to a certain degree... but nothing you mentioned would do that.
Thanks for trying. And please quit slaughtering perfectly good logic by basing it on misinformation and conjecture.
oh, I probably offended you...But at least you (hopefully) learned something in the process.
-Chet
Weelbase is not calcuated from the outside of the tire, it is calculated (and rightly so) from the centerline of hte wheel/tire assembly..just like track (which is part of the wheel base calc.)
So no, you're not doing anything to wheelbase by changing the tire width. Go read more, post less.
Also, beacuse it seems you've got confusion, tire width does not affect the offset of a wheel in any way.
Also, "there is now more tire to support the car" isn't accurate either, since both premises you were basing it on are wrong, the ability to support the car (as far as tire/weheel assemblies are concerned) is based on load index/strength of the tire and strength of the wheel.
Increasing a cars track will decrease weight transfer, benefiting handling to a certain degree... but nothing you mentioned would do that.
Thanks for trying. And please quit slaughtering perfectly good logic by basing it on misinformation and conjecture.
oh, I probably offended you...But at least you (hopefully) learned something in the process.
-Chet
Re: (Exar-Kun)
should have brought my flame suit.
your not a senior engineer by any chance are you? i cant tell you dont do social work.
kudo's
your not a senior engineer by any chance are you? i cant tell you dont do social work.
kudo's
Re: (7thGear)
Nope. And yes, you should have brought your flame suit...
I spent 1.5 yreas studying Mechanical engineering, and rest of the knowledgei;ve gained has been from getting slapped around by those more knowledgable than myself and going and finding out where they learned it, beacuseI want to know more.
I've spent roughly the last year more or less obsessed with understanding the car cahssis and suspsnsion as a "system" and not just one or two isolated things, and trying to learn to a higher degree how it all works and why...
Feel free to ask any questions (unless its so redundant I can't stand it...), I will inevitably provide an answer if I have it, even if I sound rude.
Enjoy learning more about the car you love, and let me know if I can help.-CHet
I spent 1.5 yreas studying Mechanical engineering, and rest of the knowledgei;ve gained has been from getting slapped around by those more knowledgable than myself and going and finding out where they learned it, beacuseI want to know more.
I've spent roughly the last year more or less obsessed with understanding the car cahssis and suspsnsion as a "system" and not just one or two isolated things, and trying to learn to a higher degree how it all works and why...
Feel free to ask any questions (unless its so redundant I can't stand it...), I will inevitably provide an answer if I have it, even if I sound rude.
Enjoy learning more about the car you love, and let me know if I can help.-CHet
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Q45tech
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Re: (turtl631)
Consider the equation for weight transfer:
Center of G in inches / trackwidith in inches x spung weight x Gforce.
1 inch lower C of G is much better than 1" wider track width
20/60 vs 19/60 or 20/61
Center of G in inches / trackwidith in inches x spung weight x Gforce.
1 inch lower C of G is much better than 1" wider track width
20/60 vs 19/60 or 20/61
Re: (turtl631)
serously?turtl631 wrote:chet, I'm curious as to how you would set up your car ideally for what you use it for. and will the z32 brakes clear my stock wheels?? what offset will look flush?? how low do tein HEs go? oh wait, you said NOT redundant....
I like cheeze.
If you want a response to your first Q, I'll give it out, but its got a ton of qualifiers...you're in ATL right? If you're in the Marietta area, buy me a beer, and I'll answer questions all night :p-Chet
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skylndrftr
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2010 Ariel Atom (pending...)
one of the more valid reasons I have heard (kinda along the lines of this discussion here) for people running wider tires, was the latent heat contained in the air inside the tire. Iwas talking to one of the mechanics for skip barber about a year back and discussing this and the fact that they seemed to run wider (as well as harder but thats seperate reasons) tires at the school as compared to those run in the actual skip barber series. What he gave me as an answer was that they have problems with people unfamiliar with the cars doing a lot of sipping sliding skidding and locking of brakes...In there case, they do it to create a larger air volume that must be heated. Basically a higher thermal inertia. This of course is going to basically invert when applied to autocross and getting tires hot quickly.
my $.02
my $.02
Re: (Q45tech)
That's a pretty usefull formula, but how easy is it to lower the center of gravity by an inch compared to making the track width an inch bigger? I have no idea how you would go about doing this in an already finished car, as in engine placement is already determined. Also, when you say center of gravity in inches, is that distance from the ground or the center of the wheels?Q45tech wrote:Consider the equation for weight transfer:
Center of G in inches / trackwidith in inches x spung weight x Gforce.
1 inch lower C of G is much better than 1" wider track width
20/60 vs 19/60 or 20/61
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Q45tech
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Re: (vvaffle)
The center of gravity is sprung thus you use lowering springs.........the down side is you must change suspension pick up points to maintain ideal camber and bump steer/toe curves.....[something never done by street guys] thus the problems.
Track width center of tire to center of other tire on same axle. Wheel base center of tires front to rear.
More important is "polar moment of interia" ~~= to how much weight is beyond the spring points [front and rear overhang] why things in trunk affect handling so much vs rear/front seat passengers.
With a IRS [independent rear suspension] you must consider that the rear center of gravity in inches is higher than the front and varies with fuel load.
Track width center of tire to center of other tire on same axle. Wheel base center of tires front to rear.
More important is "polar moment of interia" ~~= to how much weight is beyond the spring points [front and rear overhang] why things in trunk affect handling so much vs rear/front seat passengers.
With a IRS [independent rear suspension] you must consider that the rear center of gravity in inches is higher than the front and varies with fuel load.
Re: (Q45tech)
What is needed to change the suspension pick up points?Q45tech wrote:the down side is you must change suspension pick up points to maintain ideal camber and bump steer/toe curves.....[something never done by street guys] thus the problems..
Regarding the overhangs, I very much understand this. Besides for just trying to move everything as close as possible to the center of the car, do you have any suggestions on how to reduce overhang weight? Thanks for all the info so far.
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Q45tech
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Re: (vvaffle)
The lower suspension arm must be parallel with the ground at static ride height with the as used weight inside the car [oem ride height].
)-------------{}tire
Lowering the body makes the suspension arm lower than parallel. Also makes the [steering] tie rod lower [thus bump steer].
Thus 99% of body lowering is wrong from a suspension curve standpoint.....but oem takes some of this into consideration as passengers may be fat and lower body by 1"..............only takes ~~500 pounds inside car cabin [like a Q45] to lower body 1".
Assumed to be 1 driver and the owner sitting opposite rear reading newspaper while being driven to work [meetings].
Front battery and AC compressor, ABS, etc are positioned opposite driver to help balance front.......but rear is symetrical [left/right in balance without rear seat passenger........just in case car is driven alone.
What is gained [1.5-2-2.5%] from lowering is usually offset by weird camber and steering responses.
)-------------{}tire
Lowering the body makes the suspension arm lower than parallel. Also makes the [steering] tie rod lower [thus bump steer].
Thus 99% of body lowering is wrong from a suspension curve standpoint.....but oem takes some of this into consideration as passengers may be fat and lower body by 1"..............only takes ~~500 pounds inside car cabin [like a Q45] to lower body 1".
Assumed to be 1 driver and the owner sitting opposite rear reading newspaper while being driven to work [meetings].
Front battery and AC compressor, ABS, etc are positioned opposite driver to help balance front.......but rear is symetrical [left/right in balance without rear seat passenger........just in case car is driven alone.
What is gained [1.5-2-2.5%] from lowering is usually offset by weird camber and steering responses.
Re: (Exar-Kun)
i need to go to Georgia!Exar-Kun wrote:If you want a response to your first Q, I'll give it out, but its got a ton of qualifiers...you're in ATL right? If you're in the Marietta area, buy me a beer, and I'll answer questions all night :p-Chet
im torn on what to do for a suspension setup...
Re: (Q45tech)
Yep. And thus, mine, Smiths, and others immense hatred for the "lower is better" crowd is aptly described by Dennis in better words than I could. Thanks dennis!Q45tech wrote:The lower suspension arm must be parallel with the ground at static ride height with the as used weight inside the car [oem ride height].
)-------------{}tire
Lowering the body makes the suspension arm lower than parallel. Also makes the [steering] tie rod lower [thus bump steer].
Thus 99% of body lowering is wrong from a suspension curve standpoint.....but oem takes some of this into consideration as passengers may be fat and lower body by 1"..............only takes ~~500 pounds inside car cabin [like a Q45] to lower body 1".
Assumed to be 1 driver and the owner sitting opposite rear reading newspaper while being driven to work [meetings].
Front battery and AC compressor, ABS, etc are positioned opposite driver to help balance front.......but rear is symetrical [left/right in balance without rear seat passenger........just in case car is driven alone.
What is gained [1.5-2-2.5%] from lowering is usually offset by weird camber and steering responses.
PS: this is also why I dislike lowering over 1.5" without adjsutable arms and things to correct the control arm angle issues...
-Chet