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Re: Traction-force as a function of Power and Speed.
Posted: 13 Dec 2009, 08:05
by xpensive
Great graph, Jon! Considering our assumption of a constant Power of 480 kW, it looks pretty realistic as well.
Re: Traction-force as a function of Power and Speed.
Posted: 16 Dec 2009, 00:10
by mep
Making some predictions about tyre characteristics just with the cars acceleration and top speed capability is not a very accurate science. You just get to many assumptions into your calculations.
Here are some F1 tyre data. They are from a 25.0 x 9.0-13 Goodyear F1 Front Tyre.
Source: Race Car Vehicle Dynamics.
I just converted the data into the metrical system.
With this data I made some other graphs that show you the friction coefficient dependant from the slip angle and the vertical tyre load.
You clearly see that the coefficient is well below 2 and how it is dependent from the slip angle and the load.
I hope this will help you.
Maybe we could ask one of the tyre suppliers to give us more detailed data of some of their old tyres for scientifically researches.
Re: Traction-force as a function of Power and Speed.
Posted: 16 Dec 2009, 21:02
by Richard
Forgive my ignorance....
I thought the friction during wheel spin was considerably less than no-wheelspin.
So, I'd expect a very low mu at the start, then as velocity increased, then mu would increase. Maybe I'm misreading this, but are you assuming a constant mu with the friction force varying linearly with down force?
Oh, and I'd expect a lot of tyre smoke at the start!
Re: Traction-force as a function of Power and Speed.
Posted: 17 Dec 2009, 00:42
by xpensive
As I said at the very start of this thread Richard, we had quite a number of simplifications, mu being one of them. Static friction is obviously higher than dynamic such, one of the reasons for traction-control and anti-lock brakes, but I think the dynamic friction should stay pretty constant regardless of amount of wheel-spin?
Mu is clearly not constant with contact-pressure, Load over area, which is why cars have wide tyres in the first place. But on the other hand, contact area, or patch, increases with load, why there is a trade-off I think.
Re: Traction-force as a function of Power and Speed.
Posted: 17 Dec 2009, 01:16
by Scotracer
xpensive wrote:As I said at the very start of this thread Richard, we had quite a number of simplifications, mu being one of them. Static friction is obviously higher than dynamic such, one of the reasons for traction-control and anti-lock brakes, but I think the dynamic friction should stay pretty constant regardless of amount of wheel-spin?
Mu is clearly not constant with contact-pressure, Load over area, which is why cars have wide tyres in the first place. But on the other hand, contact area, or patch, increases with load, why there is a trade-off I think.
Given that rate of deceleration
reduces when the tyres are locked up, I'm inclined to disagree with your statement.
Re: Traction-force as a function of Power and Speed.
Posted: 17 Dec 2009, 07:48
by xpensive
"Static friction is obviously higher than dynamic such,..."
"Given that rate of deceleration reduces when the tyres are locked up, I'm inclined to disagree with your statement."
Xcuse me Scot, but isn't that what I said, when dynamic friction is what you get after lock-up or wheel-spin?
Re: Traction-force as a function of Power and Speed.
Posted: 17 Dec 2009, 10:34
by autogyro
I can only suggest you try driving on your figures.
The result will be simple and rapid.
No forward motion followed by no tyres and then no engine.
Bye
Re: Traction-force as a function of Power and Speed.
Posted: 17 Dec 2009, 15:13
by Scotracer
xpensive wrote:"Static friction is obviously higher than dynamic such,..."
"Given that rate of deceleration reduces when the tyres are locked up, I'm inclined to disagree with your statement."
Xcuse me Scot, but isn't that what I said, when dynamic friction is what you get after lock-up or wheel-spin?
Yeah, I read that wrong. Woops.
Re: Traction-force as a function of Power and Speed.
Posted: 15 Jan 2010, 08:13
by cwatson
Scotracer wrote:Here's what I came up with:
I had to use a larger frontal area than you, Ciro, as 0.9m^2 was giving me very small values.
*Ignore the stupid unit mess up in the last column
Important values:
Coefficient of Lift: -2.3
Coefficient of drag: 1.0
Mass: 605kg
Rho: 1.2kg/m^3
Planform area: 3.5m^2
Frontal area: 1.5m^2
Coefficient of Rolling Resistance: 0.05
Here are some pretty graphs:
Total drag including rolling resistance and aero drag
Power comparison. Top speed is the cross-over point, obviously - High Downforce Setup
^ Shows how efficient these cars are, given that they have open wheels.
To calculate the wheelspin problem we need to know the torque about the back wheel and the force resisting that torque (which will be a function of mu, wheel radius and downforce I reckon...).
Hello everyone. First post here. It looks like that L/D is pretty high. I noticed your coefficients were 2.3 and 1.0 but you used different areas. My understanding is that coefficients are ussually quoted based on the same area (part of the reason I really think cl*A and cd*A should be used when applying aero forces to vehicle dynamics). Correct me if I'm wrong, but high downforce open wheel cars have L/D's on the order 2-3 depending on configuration with close wheeled cars getting 4+. (source: aero data @ mulsannescorner.com)
