Lowering the body

[ausrobbo]

If you read through the article I posted a link to, it concludes that wider tires primarily are for wear reasons.  As we all know, a rubber tire sacrifices itself to provide the friction (all the gum balls on your front tire after a track session prove this is so).  A small tire dealing with significant friction demands, and in drag racing probably being overloaded will wear fast.  So if you had the smallest tires that could support the weight of the car and did a run, you would likely destroy them due to friction heat and mechanical stress.  If you were really serious about winning a drag, you could have single pass tires that had just enough strength for a run and then die.  They would be smaller and lighter.  But you would change tires every single pass.  So the extra rubber is for wear and handling of forces - vertical, lateral, roll on the sidewalls/carcass, flex to absorb acceleration and deceleration shock, bumps and so on.  It doesn't provide more friction or grip.

[Jaybird180]

If a fully heated race tire balloons then it makes sense. But since there is still some flex in the carcass I think there's something else going on.

I will concede that I don't know what or why. I'll get back to you after I study more on the lack of relationship that surface area has to tire grip.

[jcw]

5 hours ago, ausrobbo said:

If you read through the article I posted a link to, it concludes that wider tires primarily are for wear reasons.  As we all know, a rubber tire sacrifices itself to provide the friction (all the gum balls on your front tire after a track session prove this is so).  A small tire dealing with significant friction demands, and in drag racing probably being overloaded will wear fast.  So if you had the smallest tires that could support the weight of the car and did a run, you would likely destroy them due to friction heat and mechanical stress.  If you were really serious about winning a drag, you could have single pass tires that had just enough strength for a run and then die.  They would be smaller and lighter.  But you would change tires every single pass.  So the extra rubber is for wear and handling of forces - vertical, lateral, roll on the sidewalls/carcass, flex to absorb acceleration and deceleration shock, bumps and so on.  It doesn't provide more friction or grip.

Drag racers rebuild their engines after each run.

You really think cost is preventing them from running a skinny tire if they could gain a tenth?

No.

 

I have corresponded with the author of the article you link and while he might understand physics, I'm not convinced he understands motorcycle dynamics in a practical sense.

 

 

The truth lies somewhere between the two. I have yet to find the answer. But the closest reasoning I've read that the coefficient of friction rather than being a constant can vary with temperature. Large contact patch might resist temperature change hence resist changes in coefficient of friction. Additionally, coefficient of friction does not accurately describe a rolling and cornering tire that operate with some slip angle.  The tire is not stationary but not sliding either. 

 

No, it is too simplistic to say contact patch doesn't matter.

 

From a practical motorcycling sense, much of Code's teaching talks about contact patch and friction. Based on all that practical experience of thousands of riders, there is some truth to the statement.

 

Just my opinion.

[ausrobbo]

1 minute ago, jcw said:

From a practical motorcycling sense, much of Code's teaching talks about contact patch and friction. Based on all that practical experience of thousands of riders, there is some truth to the statement.

I agree that managing the contact patch, for all practical purposes, is the way to improve cornering performance.  It may or may not be the size of the contact patch vs the forces you apply on the patch, but the logic to balance the chassis and the weight on each tire gives the same result.  I don't see this is contradicting anything of the Code principals.

[Lnewqban]

On 8/12/2017 at 3:20 PM, ausrobbo said:

Slightly off topic, but this is not correct.  The contact patch size has no impact on traction.  It is a complex topic, and I like this explanation best http://www.stevemunden.com/friction.html.

The second law of friction states "Friction has no relation to contact surface area".  Friction depends entirely on the Force (not pressure) applied between the two surfaces. If we assume the rubber compound and road surface of both tires are the same at any given point in time, the grip on front and rear has nothing to do with the size of the contact patch and only to do with the amount of force applied.   Moving around on the bike changes the forces applied to the tire and the MotoGP style uses less of the available friction than other styles - it is not because the contact patch is different specifically.  The grip at all angles of lean on the same tire on the same surface is the same.

Because it is very complicated, the subject of friction between rolling rubber and pavement has been and will be debatable.  It goes beyond the simple empirical determination of a coefficient between two sliding surfaces. Rubber is a material which properties change a lot under mechanical and thermal stresses. It acts like a viscous-elastic material and it suffers from elastic hysteresis.  When the same area of rubber hits and partially slides over pavement, internal chock waves develop and the original softness of the rubber has less time to recover.  It has been demonstrated that for stationary surfaces and for light pressures in the contact area between a tire and a road, the rubber will only make contact with about 5% of the road surface. As the contact pressure increases, the rubber gets squeezed into many of the smaller-sized cavities.  The coefficient of friction is not constant for different conditions.  Many universities have studies and papers explaining this.  If interested, you can research "elastoplastic contact between randomly rough surfaces".

https://en.wikipedia.org/wiki/Viscoelasticity

https://en.wikipedia.org/wiki/Hysteresis#Elastic_hysteresis

Nevertheless, let's assume that friction and area are more or less independent in this case.  The useful range of traction of any tire depends on the pressure/temperature/stress over the contact patch.  If that is true, then the rider needs to unload the front tire some, and overload the rear tire some, in proportion to the differences in the areas of both contact patches.  If the rubber compound and manufacturer of both tires are the same, he/she wants to achieve the same pressure/temperature/stress on both contact patches while cornering.

The whole idea of proper throttle control to achieve a proper weight distribution while cornering is based on that concept.

"To determine an ideal scene for traction, machine-wise, we start by simply measuring the contact patches of the tires to discover what the basic distribution of loads should be while cornering. Roughly speaking, those measurements show that 40 percent of the total load should be up front, 60 percent at the rear.......... At the point where the correct transfer of weight is achieved by the rider (10 to 20 percent rearward) by using the throttle, any big changes in that weight distribution reduce available traction." - ATOTW-2

[BikeSpeedman]

On 8/12/2017 at 4:22 PM, ausrobbo said:

If you read through the article I posted a link to, it concludes that wider tires primarily are for wear reasons.  As we all know, a rubber tire sacrifices itself to provide the friction (all the gum balls on your front tire after a track session prove this is so).  A small tire dealing with significant friction demands, and in drag racing probably being overloaded will wear fast.  So if you had the smallest tires that could support the weight of the car and did a run, you would likely destroy them due to friction heat and mechanical stress.  If you were really serious about winning a drag, you could have single pass tires that had just enough strength for a run and then die.  They would be smaller and lighter.  But you would change tires every single pass.  So the extra rubber is for wear and handling of forces - vertical, lateral, roll on the sidewalls/carcass, flex to absorb acceleration and deceleration shock, bumps and so on.  It doesn't provide more friction or grip.

RE Car tire widths...

The movement of a car tire when turning is split between the wheel and the tire. The steps involved are:

1. Turn steering wheel
2. the wheels turn while the tires remain in original position
3. the tires then let go of the road surface and twist to get back in line with the position of the wheel

This repeats in little steps over and over throughout the turn. It happens for rear tires too but it's the attitude of the car which turns the rear wheels. In both cases, the tires lag behind the wheel, let go of the road, and catch up with the wheel as long as the direction is being changed.

The way a tire performs this sideways deflection (twist) is a product of sidewall height and tire width. This is one of the main reasons race cars have low profiles and more width. It should be noted that there's a practical limit for how low you can go with profile before losing too much suspension effect from the tire. It's far more desirable to add width as much as possible without hitting suspension components or fenders. Also, you want to increase the wheels along with increases in tire because even though you can often add +5 or +10 mm for a given wheel size, it will allow the tire to twist more than if this relationship is controlled.

If the article says that more width does not give more grip (in cars), then it is wrong. I spent a lot of time a while back autocrossing and time trialing. I've been to racing schools and read dozens of books about car setup and performance. Classes are tightly controlled about all aspects of the tire including width. Getting a tire that is too wide for your current class bumps you into a different class and your lap times drop. It's easily verified. Anyone involved in racing (bikes or cars) will tell you being able to consistently run laps with little variation is important both for safety and to reliably improve the car's setup. It's not "in your head". It's a real effect.

I can't speak to bike tire widths.