Dynamic and static friction on tires 2

[Tunalover]

In physics and statics they always told us that the friction force F is given by F=µN where N=the force normal to the contact surface and µ=the coefficient of friction. The friction force is independent of the contact surface area. This seems over-simplistic because intuition and observation show that wider tires on cars provide better stopping and starting. What does the friction force REALLY depend on? I know that a bicycle tire width on a car will not stop the car faster than the a wider tire! Somebody correct me if I'm wrong about this.



Tunalover

 

[gruntguru]

Unfortunately the wing does nothing at the point where extra acceleration makes the most difference to ET (launch) and is most effective where the extra acceleration is least useful (finish line). Top fuel is one of the few categories which are traction limited for most of the 1/4 mile and therefore able to gain some benefit from the wing.

Last time I checked, top fuelers have about 1000lb of downforce at zero road-speed. This is jet effect from the upswept exhausts. Power has gone up since then so the 1000lb has as well. This is probably one reason they use clutch-slip and ignition retard (not throttling) for low speed traction control - it is important to maintain mass-flow through the engine to generate this DF.

Engineering is the art of creating things you need, from things you can get.

 

[Lou Scannon]

Last time I checked, top fuelers have about 1000lb of downforce at zero road-speed. This is jet effect from the upswept exhausts. Power has gone up since then so the 1000lb has as well. This is probably one reason they use clutch-slip and ignition retard (not throttling) for low speed traction control - it is important to maintain mass-flow through the engine to generate this DF.

Makes sense, gg... in top fuel, nothing goes to waste!

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[tbuelna]

TF dragsters transfer close to 100% of their 2250lb weight to the the rear tires at launch. The pressure from the exhausts also contributes a useful amount of downforce at launch. The massive rear wing provides no help at launch, but about 1.5 to 2.0 seconds into the run when the car exceeds 150mph the rear wing adds huge amounts of downforce to the rear tires.

 

[hydrae]

Back to the original question:
Why do wide tires work better than skinny tires in normal uses?
It is NOT due to friction, it is due to shearing and wear.
When you put the tires to the limit they shred, which is a function of shear strength.
Hardness is proportional to strength including shear strength.
Harder tires have a lower coefficient of friction between the tires and the road but do not shred as easily, so they last a long time.
Softer tires have a higher coefficient of friction between the tires and the road and shred easily.
To have both a high coefficient and strength the area in contact with the road must increase.
Hence the huge tires on top fuel that are really soft, at low air pressure so they deform, are heated by the burnout so they are softer yet and will turn into a glue between the tire and pavement. One other function is if the contact area has time to set with perfect seal between the tire and road, a vacuum will form increasing the normal force but just for the very start of the launch,
Please also note top fuel tires do not last very long…

Hydrae