Weight vs downforce, Stability and grip

[Isaac Zachary]

I don't think the downforce converts to more traction linearly. Maybe it would if you could size the tire as downforce is increased or decreased. In other words there's a point where traction increase isn't as much as the amount of weight added.

However, inertial weight does convert to inertia linearly. Double the weight and you double the inertia.

Why do big heavy vehicles take longer to stop even if they have enough braking power to lock the wheels? Could it be from very little tire in comparison to weight? Compare that to a sports car with wide tires. Obviously width isn't everything, but does seem to play a role on traction. Adding more weight would seem to merit bigger/wider tires in proportion to the weight increase.

Adding more downforce doesn't increase traction linearly, but doesn't add inertial weight.

[samwichse]

Originally Posted by Isaac Zachary

Adding more downforce doesn't increase traction linearly, but doesn't add inertial weight.

I mean the basic friction equation is:
friction force = (coefficient of friction)*(normal force)

Static and kinetic friction are the same equation with different values for the coefficient of friction.

So yeah, downforce (adding to the normal force) should pretty linearly increase traction.

[Isaac Zachary]

Originally Posted by samwichse

I mean the basic friction equation is:
friction force = (coefficient of friction)*(normal force)

Static and kinetic friction are the same equation with different values for the coefficient of friction.

So yeah, downforce (adding to the normal force) should pretty linearly increase traction.

Mmm.... Interesting.

But if you take a car and just add more weight over the front wheels the car will tend to understeer more (inertia overpowers traction). And if you do the opposite, add more weight over the rear wheels it tends to oversteer (again the increased inertia is more than the increased traction.)

So maybe it's linear, but at a lower increase than inertia.

[AeroMcAeroFace]

Originally Posted by Isaac Zachary

I don't think the downforce converts to more traction linearly. Maybe it would if you could size the tire as downforce is increased or decreased. In other words there's a point where traction increase isn't as much as the amount of weight added.

However, inertial weight does convert to inertia linearly. Double the weight and you double the inertia.

"This graph can provide a lot of information. As you can see, the lateral force coefficient actually decreases by increasing vertical load." http://racingcardynamics.com/racing-...lateral-force/



But those graphs suggest that there should be proportionally more tyre grip.

Originally Posted by Isaac Zachary

Why do big heavy vehicles take longer to stop even if they have enough braking power to lock the wheels?

I thought the reason heavy vehicles take longer to stop is higher tyre pressures, and harder rubber, less grippy compounds. But maybe it isn't linear, those graphs suggest better lateral grip to vertical load ratios at lower loadings. (not necessarily the same as braking but I am assuming they are)

According to those graphs, it would seem that reducing weight can increase your cornering speed due to the tyres being proportionally more grippy, and give you more grip proportionally than adding downforce. Of course downforce comes with no increase in mass, so is still better for grip.

[Autobahnschleicher]

Originally Posted by AeroMcAeroFace

This is probably not best suited to the aerodynamics section, but it is here now.

But it obviously doesn't cancel out, because if it did then removing the spare wheel would make no difference. The inertia is reduced, there is less grip, both proportionally by the same amount because the tyre grip can be assumed to be linear.

I suppose the question is really, why does a small change in weight/downforce have a big effect on some cars, yet a negligible change in others?
And why doesn't the grip loss balance, less weight at the front, less grip at the front. The only explanation I can think of is weight transfer, cg height, moment of inertia, or maybe tyre pressures too high for the new weight.

To be clear, I don't believe that the sole cause of increased stability in Julian's case was a few kg of downforce, I believe that it is mostly to do with the centres of pressure moving further back, but I think he would disagree.

The ballance issues are due to the spring rates and stabilizer beeing wrong for that and my center of gravity moving even further behind the center of pressure.
Small changes are more noticeable if the change the ballance and the car is lightweight.
Tire grip is not perfectly linear indeed, I used it as an approximation.
The coefficient of friction drops a little with ground pressure.

Anyways, I'm not going into details of suspension setups because that's too much.

[AeroMcAeroFace]

Definitely, I get way too confused with suspension, moment of rotation, spring rates, anti-roll bar stiffness etc. That is why my name is AeroMcAeroFace, and not SpringyMcSpringFace. But they all work together, I know a bit about aero and more the drag reduction side, but how aero actually affects the car handling and dynamics I know little about.