Weight transfer and traction

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I do not remember if I asked this question before, since I have asked so many, but here goes!






Does more weight on a tire increase its traction with the road? It would seem so since when you hit the brakes your front tires bite into the pavement and you get excellent traction. But does there ever come a point where you overload the tire with weight transfer?








CAUTION! Here is why I ask. When I was riding sportbikes, I often wondered if the new lighter and improved bikes had better traction. I thought they have less weight to help the tire dig in, but also less weight/energy/inertia to have to slow down.
 
Yes, you absolutely increase traction with more weight. The suspension geometry also changes when weight transfers, further aiding it. There's a racing technique known as trail-braking where you intentionally brake while turning, which keeps the front tire contact patch as large as possible to help rotate the car: https://en.wikipedia.org/wiki/Trail_braking

It's also why you see race cars with a lot of spoilers and other clever bodywork to generate downforce. You want to push the car down onto the road at higher speeds and reduce lift.

There is a point at which this becomes counterproductive, however. Too much downforce (or weight, really) will increase friction as the contact patch gets larger, reducing acceleration and top speed. It escapes me at the moment, but one of the big car magazines was testing the new Corvette Z06 with the Z07 aerodynamic package, and they kept fiddling with the rear spoiler to improve traction -- and lost several MPH on the straight sections of track as a result.
 
Originally Posted By: mjoekingz28


CAUTION! Here is why I ask. When I was riding sportbikes, I often wondered if the new lighter and improved bikes had better traction. I thought they have less weight to help the tire dig in, but also less weight/energy/inertia to have to slow down.


The most traction is when you're popping a wheelie, as all the weight is on the drive tire. Lighter bikes have better *accelleration* and with a decent sized tire have better traction than a comparable heavier one, b/c the mass of the bike is fighting back less against the forces of the tire trying to push it. If you took a sport bike and tried to tow a school bus with it, it would be spinning its sticky tire.

There's a reason the Bugatti Veyron is AWD, and it's not snow.
wink.gif
 
Originally Posted By: dparm
There is a point at which this becomes counterproductive, however. Too much downforce (or weight, really) will increase friction as the contact patch gets larger, reducing acceleration and top speed. It escapes me at the moment, but one of the big car magazines was testing the new Corvette Z06 with the Z07 aerodynamic package, and they kept fiddling with the rear spoiler to improve traction -- and lost several MPH on the straight sections of track as a result.


Its not the weight on the suspension that causes drag. Its the wind force on the spoiler. Too big a spoiler or too aggressive an attack angle creates lots of aero resistance.
 
Originally Posted By: turtlevette
Originally Posted By: dparm
There is a point at which this becomes counterproductive, however. Too much downforce (or weight, really) will increase friction as the contact patch gets larger, reducing acceleration and top speed. It escapes me at the moment, but one of the big car magazines was testing the new Corvette Z06 with the Z07 aerodynamic package, and they kept fiddling with the rear spoiler to improve traction -- and lost several MPH on the straight sections of track as a result.


Its not the weight on the suspension that causes drag. Its the wind force on the spoiler. Too big a spoiler or too aggressive an attack angle creates lots of aero resistance.



I think it will depend on how aggressive the spoiler is and the design of it. The ones you see on purpose-built race cars, or track cars (think Dodge Viper ACR), are going to be pressing down on the rear tires VERY hard. McLaren and others actually use the downforce as an air-brake to help slow the car down under heavy braking. Some manufacturers quote how much downforce is being generated at speed, for example.
 
Lighter weight sport bikes get going fast not due to traction but the physics law of inertia. Lighter weight is easier to get it moving even if it has less traction.
 
As others have said, you can overload a tire.

The example I think of is understeer in a FWD car. You are trying to turn at speed. You start turning and even though you have all that weight up front and perhaps even more on the outside front tire as you turn, the car begins to push (understeer).

Not enough traction to make the car turn the way you want it to turn.

No matter the weight on the tire, you can get to the point where you are asking too much of the tire with respect to traction.
 
Don't have time to get into it now, but with any vehicle it all comes down to "The circle of traction"
 
The collision analyst rule of thumb is that 65% of your braking comes from your front wheels. I'd have to do a bit of digging, but I think there's a slight modification with that number with respect to bikes, but it's been a long time, and I'd have to look back to see if that's the case.
 
When it comes to tires and traction, you have to remember that the old rule from high school physics does NOT apply: That the load on a tire isn't directly proportional to the grip. That's because tires penetrate the macrotexture of the pavement just a bit and the ultimate grip is achieved when the rubber is being torn off - 10% to 15% slip.

Plus, the geometry of a tire mounted on a wheel limits how much tire can be in contact with the road surface.

So, No! The size of a tire's footprint can NOT be determined by knowing the inflation pressure and the load.

This also means it is possible to saturate the tire - to ask for more grip that the tire is capable of handling (We call that sliding!) - AND - increasing the load on a tire doesn't proportionally increase its grip, so there is an upper limit to grip as well.

One of the problems in trying to figure this out is that vehicles - whether it be street cars, race cars, motorcycles, trucks - have tires that are proportionally sized: Look at a lighter car/bike and the tires are generally smaller, too.

Not to mention that tires come in a variety of grip levels.
 
The basic answer is that tire grip doesn't increase in a directly proportional relationship with increasing downforce on the tire.
So adding stiffer springs on the rear end of a car, increases the downforce on the outside rear tire when in a corner, but since the outside rear tires grip doesn't increase in a direct relationship, the outside rear tire is now more likely to slide than the outside front tire. Giving more oversteer than with the softer rear springs.
 
Originally Posted By: IndyIan
The basic answer is that tire grip doesn't increase in a directly proportional relationship with increasing downforce on the tire.
So adding stiffer springs on the rear end of a car, increases the downforce on the outside rear tire when in a corner, but since the outside rear tires grip doesn't increase in a direct relationship, the outside rear tire is now more likely to slide than the outside front tire. Giving more oversteer than with the softer rear springs.

32.gif
Made me think of this:
 
Originally Posted By: CapriRacer
When it comes to tires and traction, you have to remember that the old rule from high school physics does NOT apply: That the load on a tire isn't directly proportional to the grip. That's because tires penetrate the macrotexture of the pavement just a bit and the ultimate grip is achieved when the rubber is being torn off - 10% to 15% slip.


quoted because it needs quoting.

As to aerodynamics, they can increase the vertical load on the tyre without adding mass to the vehicle...so they tyre/traction can be related to a two tonne vehicle with the inertial forces of a one tonne vehicle.

Analogous to the lift/drag in an aircraft, same thing with vehicles...want downforce, you get aerodynamic drag.
 
And with a (racing) motorcycle, the contact patch of the front tyre is designed to increase under braking, it flattens out, decreasing under trail braking and normal profile on corner exit.
 
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