Wider Tires = Less Grip ????????????????
Feb 17, 2010, 03:57 PM
#46
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Not picking on any particular poster, but opinions are pretty worthless. Look for data. Autocross cars live on grip. Street Prepared is faster than Stock and they always go to bigger wheels & tires. And the stock guys generally put as much rubber on the car as possible.
And if looking to autocross, only look at the best. The competitive guys at the SCCA National Championships. Like stock mustangs running 295's on 8" (way undersized) rims and going super quick.
Ignoring snow, there is no argument. Bigger is better.... Okay, compound's real important too.
And if looking to autocross, only look at the best. The competitive guys at the SCCA National Championships. Like stock mustangs running 295's on 8" (way undersized) rims and going super quick.
Ignoring snow, there is no argument. Bigger is better.... Okay, compound's real important too.
i built one of the top 5 fastest RAW time cars at SCCA nationals this last year running a 6inch wide tire. we COULD use a wider tire, but we picked the narrower one to get it up to temp quicker (which for us was more beneficial then the added contact patch of the wider tire).
(edit, and we actually tested different tires, compounds and sizes on a tire tester in house, and also had a labratory testing tires as well... anyone in FSAE the last 5 years or so would know what i'm talking about)
Last edited by KevinD; Feb 17, 2010 at 04:03 PM.
Feb 17, 2010, 05:05 PM
#47
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heres a little more scientific reasoning:
remember a few things, all your forward acceleration, and lateral acceleration is determined by one thing. the force of friction (denoted as Ff)
Ff = mu * Fn
where mu is the coefficient of friction (strictly determined by materials, i.e. soft tire has high mu, hard tire has low mu... if the pavement is ice, mu is extremely low, if pavement is concrete, its higher. mu does change if it is static or kinetic though, but lets set that aside for later).
Fn is the normal force, or in other words the weight on the tire.
if you notice, nowhere in that equation is there area (or contact patch). so the force of friction is not related to the surface contact patch directly. so putting a wider tire of the exact same compound doesn't change traction at all. you do increase contact patch, but you reduce the normal force per sqft. so you might have more area, but the force per area is less. it all comes out to be the same if you were to integrate the load over the area.
surface area DOES play an effect on softer tires though. imagine a new hoosier A6, the tread is gewy, you can stick your finger nails in it, and it feels very soft. this soft rubber has a very low shear strength. basically the material rubs apart very easily. this is why they wear out so fast, the soft rubber has a high mu, but low shear. shear DOES matter on area though, if you distribute the load over a larger area, you reduce the shear stress, and are less likely to rip apart the rubber.
so you now have two situations where you can "lose" traction. one is where the mu isn't enough the two surfaces slip (static to kinetic). this is typical with a very hard rubber, low normal force, or on a slippery surface. there is usually little to no rubber transfered to the pavement. (think of it like rubbing your hand across a smooth laminate counter, you hand slips easily, no matter how hard you push down, you can still slide you hand over it)
the second situation is where the tire grips really well with the pavement, but the tire itself comes apart (shear stress exceeded). this is why race tracks usually have rubber at all the turns or at the start of say a drag strip. the soft tires grip so well the force of friction is greater then the shear stress. (this would be more like rubbing your hand across a sheet of heavy grid sandpaper.... very high mu, and if you push down hard and try slipping your hand over it, more then likely it will cut you or scratch the outerlayer of skin off your hand... so your skin shear stress is less then the force of friction)
so when you wonder why large wings make a difference, the downforce generates more normal force, which increases the force of friction of the tire against the pavement. it doesn't help you at all from a standstill though.
alignment plays a huge part because if a tire is pointed in the wrong direction (bad toe, bump steer etc..) you end up getting two lateral forces pushing against each other constantly. if these lateral forces are large, it reduces the remaining force of friction or shear stress pushing you longitudinally, or in a turn less in the direction you actually want to go). it also increases tire wear, and increases temperatures.
the last consideration was already mentioned, was heat. all tires work best warm. a huge tire will never get up to temp in one lap of an autocross, so running a narrower tire can actually be more effective because it is easier to get up to temp. if your tracking your car and the tires are overheating, a wider tire is most likely needed to distribute the load better and reduce the temp.
if you know why you have no traction, thinking this through can greatly increase you logic when choosing a tire. more often then not, a better tire in the same size will yeild better results then throwing the widest tire you can find on there.
remember a few things, all your forward acceleration, and lateral acceleration is determined by one thing. the force of friction (denoted as Ff)
Ff = mu * Fn
where mu is the coefficient of friction (strictly determined by materials, i.e. soft tire has high mu, hard tire has low mu... if the pavement is ice, mu is extremely low, if pavement is concrete, its higher. mu does change if it is static or kinetic though, but lets set that aside for later).
Fn is the normal force, or in other words the weight on the tire.
if you notice, nowhere in that equation is there area (or contact patch). so the force of friction is not related to the surface contact patch directly. so putting a wider tire of the exact same compound doesn't change traction at all. you do increase contact patch, but you reduce the normal force per sqft. so you might have more area, but the force per area is less. it all comes out to be the same if you were to integrate the load over the area.
surface area DOES play an effect on softer tires though. imagine a new hoosier A6, the tread is gewy, you can stick your finger nails in it, and it feels very soft. this soft rubber has a very low shear strength. basically the material rubs apart very easily. this is why they wear out so fast, the soft rubber has a high mu, but low shear. shear DOES matter on area though, if you distribute the load over a larger area, you reduce the shear stress, and are less likely to rip apart the rubber.
so you now have two situations where you can "lose" traction. one is where the mu isn't enough the two surfaces slip (static to kinetic). this is typical with a very hard rubber, low normal force, or on a slippery surface. there is usually little to no rubber transfered to the pavement. (think of it like rubbing your hand across a smooth laminate counter, you hand slips easily, no matter how hard you push down, you can still slide you hand over it)
the second situation is where the tire grips really well with the pavement, but the tire itself comes apart (shear stress exceeded). this is why race tracks usually have rubber at all the turns or at the start of say a drag strip. the soft tires grip so well the force of friction is greater then the shear stress. (this would be more like rubbing your hand across a sheet of heavy grid sandpaper.... very high mu, and if you push down hard and try slipping your hand over it, more then likely it will cut you or scratch the outerlayer of skin off your hand... so your skin shear stress is less then the force of friction)
so when you wonder why large wings make a difference, the downforce generates more normal force, which increases the force of friction of the tire against the pavement. it doesn't help you at all from a standstill though.
alignment plays a huge part because if a tire is pointed in the wrong direction (bad toe, bump steer etc..) you end up getting two lateral forces pushing against each other constantly. if these lateral forces are large, it reduces the remaining force of friction or shear stress pushing you longitudinally, or in a turn less in the direction you actually want to go). it also increases tire wear, and increases temperatures.
the last consideration was already mentioned, was heat. all tires work best warm. a huge tire will never get up to temp in one lap of an autocross, so running a narrower tire can actually be more effective because it is easier to get up to temp. if your tracking your car and the tires are overheating, a wider tire is most likely needed to distribute the load better and reduce the temp.
if you know why you have no traction, thinking this through can greatly increase you logic when choosing a tire. more often then not, a better tire in the same size will yeild better results then throwing the widest tire you can find on there.
.... I love youBut I have one question, based on the video that was posted at the first page: why did they put wider tires afterall when they could have just put better compound??? I mean, the test wasnt for long (endurance race or so)!
Another question: does your explanation mean that most of the Evo owners here choose wider wheels and tires ONLY for the looks?? apart from the durability??
Feb 17, 2010, 07:19 PM
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not always true.
i built one of the top 5 fastest RAW time cars at SCCA nationals this last year running a 6inch wide tire. we COULD use a wider tire, but we picked the narrower one to get it up to temp quicker (which for us was more beneficial then the added contact patch of the wider tire).
i built one of the top 5 fastest RAW time cars at SCCA nationals this last year running a 6inch wide tire. we COULD use a wider tire, but we picked the narrower one to get it up to temp quicker (which for us was more beneficial then the added contact patch of the wider tire).
But back on topic I'll bet you can't name one stock or street prepared winner who elected to use small tires. These cars are much more comparable to our Evos.
BTW, your tech analysis is pretty good, but the reason your math doesn't favor the wider tire is you're considering mu to be unrelated to tire size. In actuality, mu has a very slight variance with unit stress and is somewhat non-linear. It's enough to favor a larger tire, all other things being equal. You probably use Milliken...I think you'll find it in there. Carroll Smith covered it too. It's not much, but just like weight transfer math, it's the little non-linearities that make things work out.
Feb 18, 2010, 05:43 AM
#50
not always true.
i built one of the top 5 fastest RAW time cars at SCCA nationals this last year running a 6inch wide tire. we COULD use a wider tire, but we picked the narrower one to get it up to temp quicker (which for us was more beneficial then the added contact patch of the wider tire).
(edit, and we actually tested different tires, compounds and sizes on a tire tester in house, and also had a labratory testing tires as well... anyone in FSAE the last 5 years or so would know what i'm talking about)
i built one of the top 5 fastest RAW time cars at SCCA nationals this last year running a 6inch wide tire. we COULD use a wider tire, but we picked the narrower one to get it up to temp quicker (which for us was more beneficial then the added contact patch of the wider tire).
(edit, and we actually tested different tires, compounds and sizes on a tire tester in house, and also had a labratory testing tires as well... anyone in FSAE the last 5 years or so would know what i'm talking about)
I haven't found anything that is 'too wide' for an evo. Our car runs 285's and I'd kill to be able to fit something wider in our class.
Feb 18, 2010, 10:24 AM
#51
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You said car, but you mean FSAE? That's arguably closer to a kart than the Evo's we're talking about. The top three FSAE's were incredibly fast last year. If yours is one of those, congrats. I think a Texas car won...yours? I watched the competition, but didn't really know one from the other.
But back on topic I'll bet you can't name one stock or street prepared winner who elected to use small tires. These cars are much more comparable to our Evos.
BTW, your tech analysis is pretty good, but the reason your math doesn't favor the wider tire is you're considering mu to be unrelated to tire size. In actuality, mu has a very slight variance with unit stress and is somewhat non-linear. It's enough to favor a larger tire, all other things being equal. You probably use Milliken...I think you'll find it in there. Carroll Smith covered it too. It's not much, but just like weight transfer math, it's the little non-linearities that make things work out.
But back on topic I'll bet you can't name one stock or street prepared winner who elected to use small tires. These cars are much more comparable to our Evos.
BTW, your tech analysis is pretty good, but the reason your math doesn't favor the wider tire is you're considering mu to be unrelated to tire size. In actuality, mu has a very slight variance with unit stress and is somewhat non-linear. It's enough to favor a larger tire, all other things being equal. You probably use Milliken...I think you'll find it in there. Carroll Smith covered it too. It's not much, but just like weight transfer math, it's the little non-linearities that make things work out.
yes, there are definately non-linearitys as the load on the tire increases mu changes. we would deal with this when taking our aero into consideration. making over 1g of aero changes the loading on the tires significantly, which shows far more of the non-linearity then the difference between a 6 and 7" tire.
the weight of the evo is pretty significant, and the weight transfer in a turn is very significant (with inside rear tire lifting on occasion, meaning most of the weight is on the outside tire). most definately a wide tire will help in that case because the narrow tire even with a R compound isn't strong enough for those loads. the wider tire is DEFINATELY not needed for straight line acceleration or braking though. it would be interesting to see how mark berrys evo handled with a narrower tire. since his evo is so much lighter, he might not need such a wide tire (if the car is 2/3's the weight of a normal car...).
griceiv: the tires don't get warm after a skidpad run. it takes more then 10 seconds of low speed turning to get them up to temp. in fact it takes a full autocross run to get them up to temp usually. and i'll use our car as proof, wider isn't always better. we've won SCCA nationals i think every year there has been a FSAE class. at the SAE events we regularly finish at, or near the top of the autocross, and last year won the skid pad. if the 8" tire is absolutely superior then why isnt everyone beating us if we are lacking grip?
Feb 18, 2010, 10:45 AM
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yeah, i was chief engineer, and later team captain of the F06 car which won SCCA nationals in 09.
yes, there are definately non-linearitys as the load on the tire increases mu changes. we would deal with this when taking our aero into consideration. making over 1g of aero changes the loading on the tires significantly, which shows far more of the non-linearity then the difference between a 6 and 7" tire.
the weight of the evo is pretty significant, and the weight transfer in a turn is very significant (with inside rear tire lifting on occasion, meaning most of the weight is on the outside tire). most definately a wide tire will help in that case because the narrow tire even with a R compound isn't strong enough for those loads. the wider tire is DEFINATELY not needed for straight line acceleration or braking though. it would be interesting to see how mark berrys evo handled with a narrower tire. since his evo is so much lighter, he might not need such a wide tire (if the car is 2/3's the weight of a normal car...).
griceiv: the tires don't get warm after a skidpad run. it takes more then 10 seconds of low speed turning to get them up to temp. in fact it takes a full autocross run to get them up to temp usually. and i'll use our car as proof, wider isn't always better. we've won SCCA nationals i think every year there has been a FSAE class. at the SAE events we regularly finish at, or near the top of the autocross, and last year won the skid pad. if the 8" tire is absolutely superior then why isnt everyone beating us if we are lacking grip?
yes, there are definately non-linearitys as the load on the tire increases mu changes. we would deal with this when taking our aero into consideration. making over 1g of aero changes the loading on the tires significantly, which shows far more of the non-linearity then the difference between a 6 and 7" tire.
the weight of the evo is pretty significant, and the weight transfer in a turn is very significant (with inside rear tire lifting on occasion, meaning most of the weight is on the outside tire). most definately a wide tire will help in that case because the narrow tire even with a R compound isn't strong enough for those loads. the wider tire is DEFINATELY not needed for straight line acceleration or braking though. it would be interesting to see how mark berrys evo handled with a narrower tire. since his evo is so much lighter, he might not need such a wide tire (if the car is 2/3's the weight of a normal car...).
griceiv: the tires don't get warm after a skidpad run. it takes more then 10 seconds of low speed turning to get them up to temp. in fact it takes a full autocross run to get them up to temp usually. and i'll use our car as proof, wider isn't always better. we've won SCCA nationals i think every year there has been a FSAE class. at the SAE events we regularly finish at, or near the top of the autocross, and last year won the skid pad. if the 8" tire is absolutely superior then why isnt everyone beating us if we are lacking grip?
Feb 18, 2010, 10:52 AM
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I have not seen the video, with that being said from what you have stated, I believe the theory is that with suspention adjustments you can change the contact patch alone and inturn, have a lot better driveability. If you just up the width without taking into account the size of the actual contact patch[or the maximum achiveable contact patch with the size of tire], without taking into consideration the alignment, your contact patch can basically be the same, therefore making the reason you bought larger wheels and tires pointless. In a sense, he's saying with the money you bought in wheels and tires, you may as well have bought a suspention system to improve your grip. Your larger wheels and tires won't give you your better grip if your suspention setup won't allow you to achive maximum contact patch. Now some people have previously mentioned compounds and operation temperature, that is very true also, it is another variable in the equation for grip.
I went over a video on Youtube (Best Motoring International) where they were testing Honda S2000s in the Touge Battle Series.. The Amuse S2000 was the fastest, clipping corners with unbelievable speeds!!
The Video: http://www.youtube.com/watch?v=TapkhD6WHjM
After That they went over some analysis to explain how the car performed very well!! At 17:43(video time), the guy said "Even with extra wide 255 size tires that can result in grip loss due to increase in road contact area, the suspension has been setup for maximum traction for highest cornering speeds!!!!!!!"
NOW, i have always thought that the wider the tire, the more grip the car has because the car gets more road contact area, rather than slipping!!! How can fitting wider tires translate to less grip?????
I bought a set of wheels (18*9.5+22) for my IX, and I was going to put 265s to achieve what I thought "maximum grip", but now, IM SHOCKED!!
If anyone can give me better explanation on such theory, and whether this theory will apply on my car with 265 or even 255 tires, I would be more than thankful!!
-Waleed.
The Video: http://www.youtube.com/watch?v=TapkhD6WHjM
After That they went over some analysis to explain how the car performed very well!! At 17:43(video time), the guy said "Even with extra wide 255 size tires that can result in grip loss due to increase in road contact area, the suspension has been setup for maximum traction for highest cornering speeds!!!!!!!"
NOW, i have always thought that the wider the tire, the more grip the car has because the car gets more road contact area, rather than slipping!!! How can fitting wider tires translate to less grip?????
I bought a set of wheels (18*9.5+22) for my IX, and I was going to put 265s to achieve what I thought "maximum grip", but now, IM SHOCKED!!
If anyone can give me better explanation on such theory, and whether this theory will apply on my car with 265 or even 255 tires, I would be more than thankful!!
-Waleed.
Last edited by Dorikun; Feb 18, 2010 at 10:58 AM.
Feb 18, 2010, 11:24 AM
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Good to meet another FSAE member. I was chief powertrain engineer for two years, but that doesnt mean I didnt get involved with everything else on the car. I also helped set up our suspension and helped run our setup through Optimum K. You should know that based on the tire data you get, that one of those cars suspensions and chassis is set up completely around the size and the brand of tire being used. Every tire maker makes their tire differently and it will deform underload differently as a result. Everything from dive, anti dive, squat, etc is all set up based on the tire data. You can have the best tires in the world but if the suspension and chassis arent set up, you wont do as well as another team who set their car up around a skinnier tire. I know what you're saying with your car, but its hard to compare a full out race car and a production evo together. But regardless, with any car the suspension needs to be set up as best as you can around the tires being used.
, suspension was the one area i didn't do much with though, we had an awsome grad student who was a veteran of the team dealing with that. engine, tuning, electrical, and aero were the areas i spent most my time on (not really the aero design, but more of the systems engineering aspect and how it interacted with the car).. and making sure everyone was doing what needed to be done was my job
) with that said, why in the world wouldn't you setup your evo for the tires your running?
i'll have to throw a smaller turbo on mine and run some skinny tires and see how it does at autocross 
Feb 18, 2010, 11:37 AM
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BTW, as to how quickly a tire warms up...I would think the critical part is just the first couple millimeters of tread, as that's what reacts with the road. And I could envision that warming up much quicker than the overall carcass. But when you stop to read the temps, the cold carcass will quickly soak away the surface heat making it appear the tire never warmed at all. Not fact, just speculation.
Last edited by WAM; Feb 18, 2010 at 11:44 AM.
Feb 18, 2010, 12:55 PM
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So if I get proper tires like the Yukohama AD08, and then make a proper suspension setup for the tire compound and not just the width, that time i'll definately get better traction setup. Right?!?!?!?!?!
