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How does traction compare to the width of a tire?

  1. Mar 17, 2012 #1
    Please help settle this argument I had with a friend. We both have fast FWD cars, and on road courses we both have problems with oversteer (where the back of the car slides out on corners). He says if I put wider tires on the rear of my car, I will have less oversteer.

    My understanding of this is that wider tires by themselves would not help. I think the wider tires, although having more surface area, will have the same or perhaps even less traction than the tires I have now (my tires are about 8" wide, and the absolute maximum I could fit is 10"). I say that's because the wider tires will conform to the track less than narrower tires because they would have less downward force per square inch, and not grip as well because they are are not forced as deep into the valleys in the pavement.

    His argument is that when he went from all season tires of the stock width to high performance tires on a wider rim, he had much less oversteer.

    All other things are equal, the weight distribution in the car, same tread compound and tread design, same weight wheel+tire assemblies (but wider, just "magically" the same weight), same diameter tires, etc.. The only change is that the tires are wider.

  2. jcsd
  3. Mar 17, 2012 #2


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    Wide tires are used in race cars for reduced load per surface area, reduced heat (due to reduced load and more surface area) and reduced wear (due to reduced load), which allows softer compounds to be used. Unless the replacment tire compound has more grip (usually a softer compound) than your current tire, there's almost no advantage to a wider tire (assuming your current tires are not getting overheated, or worn out during a run).
  4. Mar 17, 2012 #3
    Generally the more tire is touching the pavement, the MORE traction you have. This relationship is by no means linear, nor does it follow the textbook friction formula, but in the real world that's what tends to occur. This is why lighter, lower cars generally handle better.
  5. Mar 17, 2012 #4
    Your friend is right.

    A wider tire in the same compound will increase traction because of the change in contact patch shape from long and narrow to short and wide. If the tire pressure remains the same, the contact patch area will be the same.

    To decrease oversteer: stiffer front sway bar and/or front springs, softer rear sway bar and/or springs, wider tires (with the proper width rims), more rear weight percentage (changes the effective spring rate), optimized rear camber setting, tire pressure balance front to rear. Best way to proceed with a FWD car is to get the widest tires you can use, move as much weight to the rear as possible (battery at the least), replace all the springs to lower the car, get the camber and tire pressures sorted so that the tire temp is even across the width of the tread, use the biggest front sway bar you can find for your car, then balance the car with rear sway bar size.

    Typically, cornering balance changes from understeer to oversteer as speed increases so you'll have to decide what the ideal compromise is for your track and which corner is the most important. A slight amount of oversteer on the fastest corner is good if the car is still controllable.

    For your information, tires use three methods of generating the total friction: adhesion, meshing and tearing of the rubber. Softer compounds increase adhesion and meshing.

    Head over to the automotive section in Mechanical Engineering, lots of good info there.
    Last edited: Mar 17, 2012
  6. Mar 17, 2012 #5
    Or more correctly, the lower the unit load on the tire, the more traction it will have. Tire load graphs show that quite well.
  7. Mar 17, 2012 #6


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    Wouldn't that require a lower tire pressure or softer sidewalls? Also is there a point of diminshing returns on tire width? I wasn't sure if going from 8" wide tires to 10" wide tires would change the contact patch area enough to have that much effect on oversteer. Not mentioned if the problem oversteer occurs under braking (corner entry) or under acceleration (corner exit).

    Back to the original post, if you want more grip, then getting a performance tire with a softer compound like a Toyo Proxes R888 will help, but it's tread life is about 1/2 that of a typical tire, with a tread wear rating of 100. Beyond that you have near racing compound DOT tires with tread rating of 60, like Yokohama Advan A048. Going the other direction, Bridgestone Potenza RE070 have a tread wear rating of 140, and the Khumo Ecsta XS or Bridgestone Potenza RE11 have a tread wear rating of 180. You should probably ask the people at the track(s) you run at for recommendations.
    Last edited: Mar 17, 2012
  8. Mar 17, 2012 #7


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    "all other things are equal" is not possible. When you make the tire wider while holding the diameter constant, you lower the ratio of sidewall height to contact patch width, and that improves cornering.

    Reductio ad adsurbdum: If tire width didn't improve cornering, Formula One cars would all run tall skinny bicycle tires to minimize air resistance while not sacrificing any cornering ability. But we see the exact opposite - tires as wide as practical within the rules.
  9. Mar 17, 2012 #8
    Yes,to keep the contact area constant. A lighter car with the proportionate tire pressure will have more traction than a heavier car, again with the proportionate tire pressure and the same tires.
    Actually the graph goes the other way; it's steeper at lower loads. 10" from 8" might be sufficient to balance his car but it would have to be tested.
    Oversteer on exit with a FWD points to some serious issues! Typically the oversteer is under braking into a corner because of the weight transfer; brake bias usually takes care of that, leaving mid-corner or high speed as the most likely situation for oversteer to occur.
    Just a note, it's unwise to mix tire compounds (use the same tire all around; different sized if need be) and changing the compound of all four tires won't change the cornering balance.
  10. Mar 17, 2012 #9


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    Well Formula 1 cars do run skinnier, shorter, front tires than LMP (enclosed body) type race cars (their front tires are about as wide and tall as their rear tires), because of air resistance. However there's more than just tire width involved here. Having a wider tire with lower pressure allows a softer compound to be used, reducing load factor, reducing wear factor, providing more heat dissipation surface, ...

    I meant to swap all 4 tires. Running lower pressure in the rear tires (and/or higher pressure in the front tires) should help with oversteer (adjusting tire pressures is one of the methods used for balancing cars in some forms of racing). Do the suspension mods mentioned if you have the budget for this.
  11. Mar 18, 2012 #10
    Thank you all for the replies.

    That's what I was thinking. The tires don't get hot, and one set lasts a whole summer. Road courses, drag strips (no burnouts), and also as my primary daily driver until this year.

    I've got General Exclaim UHPs on it now, I went with them as a good balance between track and street. I'll probably get some really grippy tires for this year because I just bought another car so I can take my time building up the engine in my fast car. I'll keep it as a daily driver so I don't have to take wet traction or treadwear into account on my next set of tires.

    For this situation, my friend and I were assuming the same sidewall to width ratio by having a smaller diameter wheel on the wider tire. We were discussing only the width of the tire as it relates to the contact area. I could squeeze 20" wheels on my car and have a very short sidewall, nearly eliminating sidewall flex with the width I have now.

    With so much weight on a bicycle tire, the tire would wear out so fast they'd have to pit every lap, would have to be solid hard rubber, and would probably melt off the rim. Having wide tires lets them go further between pit stops.

    What I have always heard and read is what rcgldr said in the first reply. The tires are wider only to allow a softer compound (which gives the extra traction) that won't wear out after a few laps.

    I did adjust the tire pressures to where they all have nearly the same footprint. I raised the car on my hoist and put white paper where the tires would be lowered. I measured the marks left on the paper and adjusted until they were all the same, then lowered the rear another 2PSI due to the weight transfer while braking. I'll experiment with an accelerometer on the track and adjust until I get the most lateral Gs. I missed the quote in multiquote, but to whoever asked, the oversteer is only on braking or occasionally entering the corner too fast. I adjusted the rear brakes down a bit, but can't go any further because of how hot the front brakes get (bigger brakes coming). Suspension mods will be next year, I'm doing the engine and maybe brakes this year. I like drag racing better because I don't have to worry nearly as much about another driver hitting me, so cornering isn't a huge concern.

    I was just looking to settle the disagreement we had, but I do appreciate all the tips and input from everyone.
  12. Mar 18, 2012 #11


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    I'm not sure of all the details, but the guys that race Porsches claim that the 18" wheels are best. My guess it the reason that 19" wheels aren't popular for racing is a combination of unsprung weight and perhaps sidewalls that are too stiff (could also be due to more tire choices with 18" wheels). Using a wheel with larger diameter and/or width, will increase unsprung weight. In a 3,000+ lb car, there's less effect than there is with a 1500 to 2500lb race car.

    Getting back to the original question about wider tires, you can do a web search and find a lot of debates about this. It's hard to isolate the difference down to just tire width, except in the case where a tire is clearly too narrow. My guess is that there is a point of diminishing returns in the case of a street compound tire. You'd have to look at all the tradeoffs between the 8" wide tires and the 10" wide tires, such as overall weight. There could be an issue with camber setting sensitivity, but that's also related to overall stiffness of the tire as well as the width.
  13. Mar 18, 2012 #12
    I've been involved in racing for over thirty years and have performed quite a bit of tire testing on cars ranging from street to NASCAR and Formula cars; it's important to know how the various tire specs affect the performance of the tire and of the overall vehicle package. Know how tires work and you'll have a big advantage over your competitors while saving time and money by not making random changes and hoping they work.

    Besides being wrong, there is much more to tires than that.

    The contact patch of a car can be found by dividing the load on the tire by the tire pressure. That means that a 2000 lb car with 20 psi in the tires has a total of 100 square inches of tire tread resting on the road, the same as a 3000 lb car with 30 psi in the tires.

    The load on the tire is the weight that the tire is supporting. For a given contact area, the higher the load, the lower the coefficient of friction (Cf). Because of the lower unit load, which is the weight supported by our two cars with those 100 square inches of contact area, the light car's tires will have a higher Cf than the tires on the 3000 lb car. The exact amount will depend on the tires but a typical difference would have the 2000 lb car cornering at 1.00 gs while the 3000 lb car cornered at 0.93 gs.

    The load on the tire affects the traction; the width does also but for a different reason. As a tire reaches its traction limit during cornering, the rearmost part of the tire starts slipping first. This starts at roughly the same distance from the leading edge of the contact patch. This means that a wide but short patch shape will have less of the total area slipping at a specific slip angle (directly related to the force on the tire) than a long narrow contact patch. As a result, a wide tire generates more grip than a narrow tire at the same unit loading.

    Tire compound is a separate issue.
    A better and much more effective way to set the tire pressures is to use a tire pyrometer. After doing a few laps or during a test session (which you should do if you want to be fast), come into the pits and quickly measure the temperature of each tire in three places across the tread of the tire: about an inch from the outside edge of the tread, in the middle, and an inch from the inside edge of the tread. If your camber and tire pressures are close, the temps will be very close to the same across the tire. Low temp indicates that the tire in that area is not contacting the road as well as it should and adjustments need to be made to get the full face of the tire working. A check of the average tire temps will also tell you whether both ends of the car are contributing the same amount; spring and sway bar adjustments are usually needed to correct that.

    You'll find that the accelerometer readings will improve as you get the tires working more efficiently, and doing it this way tells you what to work on; g readings and lap times just tell you something's wrong but not what's wrong.

    Using tire pressures to balance the car is a crutch and won't be as effective as fixing the real problem.

    Here are some articles worth reading:

    One of the better books on the subject is "The Racing and High Performance Tire" by Paul Haney; a very good place to start if you're wanting to understand those round black things holding up your car.
    Last edited: Mar 18, 2012
  14. Mar 18, 2012 #13


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    These are the issues I've read from my web search on wider tires. It seems that slip angle versus side load should be less for a wider tire. If suspension has an issue with camber changes in turns, that should affect a wider tire more (unless a very soft tire). Assuming that the camber issue was resolved, how much more grip would a 10" wide tire have than an 8" tire? What about unsprung weight tradeoff between the two tires? Would the wider tire be less forgiving?
  15. Mar 18, 2012 #14
    Be warned that my answers will be as specific as possible but addressing only the change asked about, tire width. Changing the tire width while retaining the other tire specs is rarely the case so most people wrongly attribute any changes in behavior to the width and not the real factors like sidewall height and tire construction. If my answers aren't clear, ask.:smile:
    Yes, because the wider tire will develop more total side load. At peak loading, the slip angle will be very close to that of a narrower tire at peak.
    No, assuming that the aspect ratio and rim to tire width ratio stays the same. Those factors affect tread face control much more than the width.
    There aren't any charts for this because most people use the widest tire allowed by the rules but rough calculations show an increase in grip of about 3.5% when going from an 8" to 10" tire.
    You should be able to look up the exact weight difference involved but the advantage greatly outweighs the disadvantage.
    That depends much more on the tire design than width; typically there would be no difference.

    Again, many of the things you're asking about would be caused by changes other than the tire width.
    Last edited: Mar 18, 2012
  16. Mar 20, 2012 #15
    Just to quantify the 3.5% improvement: for the average 10 turn road race track, that would be worth at the least half a second per lap.
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