Why do wider tires help with contact patch?
07-04-2019, 12:07 AM
#1
Burning Brakes
Thread Starter
Why do wider tires help with contact patch?
I am an engineer by degree and one thing has puzzled me as I see a lot of varying explanations as to "why does a wider tire increase the contact patch."
My intuition and observation suggests wider tires help, but why?
Someone posted an equation (I think on here) that stated simply that (static) contact patch = car weight / tire pressure. Makes sense and the units of measure work out, but with this formula, static contact patch area is independent of tire width. Tire width will obviously influence the length (or width) of one side, but the area remains the same so long as the tire pressure is the same.
If you believe that line of thinking, then the "reason" that wider tires helps must have something to do with dynamic contact patch (or heat dissipation) or at least that's what I thought. But I never saw an equation or quantitative explanation that explained WHY the wider tire helped. I saw a lot of qualitative explanations, some bordering on superstition and voodoo.
Interestingly enough, I found the article below, where they actually found that the patch pressure is different than the tire pressure at different loads due to deflection.
In their experiments, they did measure that at a 550 lb load the contact patch for the tires tested (245F and 315R) were equal (67 sq. inches). In fact, they also found that the narrower tire had a LARGER contact patch at loads from 100 lbs to 500 lbs than the wider tire.
However, as the loads increased beyond 500 lbs, the contact patch increased for the wider tire but not directly in proportion to width.
As an example, the contact patch at 992 lbs and 28 psi was 81.5 and 88.6 sq. in for the 245 and 315 tires, respectively. Thats an 8.7% increase in contact patch for 28.7% increase in tire width.
They also found that if you lowered the pressure too much the contact patch actually gets smaller!!! Again, because the patch pressure does not necessarily follow the tire pressure.
I think this is why it is so important to follow the engineer's recommendations for cold and hot tire pressures. Not just for tire reliability but also potentially for maximum contact patch. In some cases, lowering the pressures will actually reduce contact patch.
It also confirms what our intuition tells us - wider tires have a larger contact patch at increasing loads (cornering). Their research also shows that the simple formula that I offered up and have seen before is too simplistic in that it does not take into account the effects of tire design and sidewall stiffness on actual patch pressure which is what sets contact patch. Wider tires do help but probably not as much as you might think and less so for lighter cars.
TLDR: Wider tires have a larger contact patch but not directly in proportion to the increased width and offer smaller gains as the car gets lighter and lighter. Also, follow the manufacturers recommended tire pressures.
https://www.enginebasics.com/Chassis...t%20Patch.html
My intuition and observation suggests wider tires help, but why?
Someone posted an equation (I think on here) that stated simply that (static) contact patch = car weight / tire pressure. Makes sense and the units of measure work out, but with this formula, static contact patch area is independent of tire width. Tire width will obviously influence the length (or width) of one side, but the area remains the same so long as the tire pressure is the same.
If you believe that line of thinking, then the "reason" that wider tires helps must have something to do with dynamic contact patch (or heat dissipation) or at least that's what I thought. But I never saw an equation or quantitative explanation that explained WHY the wider tire helped. I saw a lot of qualitative explanations, some bordering on superstition and voodoo.
Interestingly enough, I found the article below, where they actually found that the patch pressure is different than the tire pressure at different loads due to deflection.
In their experiments, they did measure that at a 550 lb load the contact patch for the tires tested (245F and 315R) were equal (67 sq. inches). In fact, they also found that the narrower tire had a LARGER contact patch at loads from 100 lbs to 500 lbs than the wider tire.
However, as the loads increased beyond 500 lbs, the contact patch increased for the wider tire but not directly in proportion to width.
As an example, the contact patch at 992 lbs and 28 psi was 81.5 and 88.6 sq. in for the 245 and 315 tires, respectively. Thats an 8.7% increase in contact patch for 28.7% increase in tire width.
They also found that if you lowered the pressure too much the contact patch actually gets smaller!!! Again, because the patch pressure does not necessarily follow the tire pressure.
I think this is why it is so important to follow the engineer's recommendations for cold and hot tire pressures. Not just for tire reliability but also potentially for maximum contact patch. In some cases, lowering the pressures will actually reduce contact patch.
It also confirms what our intuition tells us - wider tires have a larger contact patch at increasing loads (cornering). Their research also shows that the simple formula that I offered up and have seen before is too simplistic in that it does not take into account the effects of tire design and sidewall stiffness on actual patch pressure which is what sets contact patch. Wider tires do help but probably not as much as you might think and less so for lighter cars.
TLDR: Wider tires have a larger contact patch but not directly in proportion to the increased width and offer smaller gains as the car gets lighter and lighter. Also, follow the manufacturers recommended tire pressures.
https://www.enginebasics.com/Chassis...t%20Patch.html
07-04-2019, 12:24 AM
#2
Rennlist Member
Sidewall height also has an influence on patch size both in relation to pressure and dynamic deformation (largely longitudinal G loads).
There's a reason that racing 911s have taller rear sidewalls than their street cousins - part of it is due to suspension tuning (spring rate) and part is due to transient increased patch = grip under throttle.
There's a trade-off of course with the incrementally higher mid-corner grip that comes with wider tires. At what cost re. rotating weight, friction, and aero drag?
I would read Paul Haney's excellent book, The Racing & High Performance Tire.
There's a reason that racing 911s have taller rear sidewalls than their street cousins - part of it is due to suspension tuning (spring rate) and part is due to transient increased patch = grip under throttle.
There's a trade-off of course with the incrementally higher mid-corner grip that comes with wider tires. At what cost re. rotating weight, friction, and aero drag?
I would read Paul Haney's excellent book, The Racing & High Performance Tire.
07-04-2019, 01:01 PM
#5
Rennlist Member
This would be a good case for some FEA analysis as it is a question that we intuitive know the answer to but can’t really prove.
07-04-2019, 01:04 PM
#6
Burning Brakes
Thread Starter
Originally Posted by Jabs1542
As an engineer (structural) I agree with this. The entire tire, sidewalls, shoulder, tread base, all present structural strength properties the affect the way the tire will flex. The balloon principle oversimplifies what’s going on with the rather thick elements of the tire.
This would be a good case for some FEA analysis as it is a question that we intuitive know the answer to but can’t really prove.
This would be a good case for some FEA analysis as it is a question that we intuitive know the answer to but can’t really prove.
07-04-2019, 01:34 PM
#7
Rennlist Member
Trending Topics
08-13-2019, 03:43 PM
#8
One of the empirical reasons, is that with wider tires, you can put less pressure into the tires, which will increase the contact patch and, thus, the overall braking efficiency,. Skinnier tires require higher pressure..
08-13-2019, 04:59 PM
#9
Drifting
1. It runs cooler, and/or
2. it makes more efficient use of its contact patch by having a greater percentage adhering, and/or
3. it can run at lower inflation pressure and therefore actually have a larger contact patch, and/or
4. it can have greater lateral stiffness at a given pressure and therefore keep its tread planted better, and/or
5. it can use a softer, stickier, faster-wearing compound without penalty in longevity.
Summarized from Mark Ortiz Chassis Newsletter, full text here: http://www.auto-ware.com/ortiz/Chass...vember2004.htm
Had to dig into the archives for that one, 2004, yikes! time flies...
2. it makes more efficient use of its contact patch by having a greater percentage adhering, and/or
3. it can run at lower inflation pressure and therefore actually have a larger contact patch, and/or
4. it can have greater lateral stiffness at a given pressure and therefore keep its tread planted better, and/or
5. it can use a softer, stickier, faster-wearing compound without penalty in longevity.
Summarized from Mark Ortiz Chassis Newsletter, full text here: http://www.auto-ware.com/ortiz/Chass...vember2004.htm
Had to dig into the archives for that one, 2004, yikes! time flies...