Traction-force as a function of Power and Speed.

[autogyro]

I'd be surprised if mu was anywhere near 2.5

It is possible to gear a car to give NO wheel spin from zero to maximum speed.
It is also possible to gear a car to give constant wheel spin from zero to maximum speed.
So what exactly are you trying to say?

[xpensive]

I agree in part JT, when a mu above 1.0 seems unreasonable between any flat and solid surfaces. Though rubber against tarmac might produce more than that I could imagine, a combination of conventional friction, adhesion and "normals" between tyre and surface grooves perhaps?

[Scotracer]

I'd be surprised if mu was anywhere near 2.5

According to Bridgestone (I don't have a source) 2.5 is pretty close to the mark.

xpensive, by "Traction force" do you mean "Thrust"? That's the only way I can read your original post for it to make sense. I'll see what I can plot in Excel and get back to the thread...

I agree in part JT, when a mu above 1.0 seems unreasonable between any flat and solid surfaces. Though rubber against tarmac might produce more than that I could imagine, a combination of conventional friction, adhesion and "normals" between tyre and surface grooves perhaps?

As my old University professor explained, to get a mu >1.0 the tyres work instead on the shear forces in the rubber itself rather than friction between the rubber and tarmac.

[xpensive]

"Thrust" would actually be a better description than "Traction-force", thanks Scot. Looking forward for your graph.

[RH1300S]

I thought that mu was the coefficient of friction. It is quite easy to find many exmaples of tyres having a coefficient of friction greater than 1.0. I have read that F1 tyres exceed 2.0 - without the help of aerodynamic downforce. (BTW - somewhere my mind is telling me that I might be confusing mu with something else, after all the size of the contact patch affects grip, but I assume doesn't change mu......).

An F1 tyre against tarmac isn't a flat surface against a flat surface. Not only is there mechanical grip, but there is also chemical grip (they are sticky when warm).

To answer the wheelspin question - surely we need to know how much grip the tyre generates before downforce comes into play and how much thrust it can put to the road - then know how the increasing downforce increases the load on the tyre and what is the load sensitivity of that tyre.

[Richard]

I would say grip is not the same as friction. Velcro is very smooth with low friction, but used in the right way has fantastic grip. Not that I remember much of the physics in this area, my form of engineering uses bolts, welds and aggregate mechanical interlocking for shear.

[xpensive]

Thrust, as we call it from now on (goldstar for you Scot), will still be the propelling force from the contact between rear tyres and surface, regardless of whatever resistance.

From zero speed, the majority of said thrust will be translated into accelleration, according to the old "force equals mass times accelleration", but the faster we go, more of the same thrust will be taken up by aerodynamic resistance.

However, thrust times speed equaling power applied, remains the same.

[Scotracer]

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...).

[Jersey Tom]

I don't care what Bridgestone says, I'm calling bull on mu = 2.5. I've worked with tires with more traction than F1, and it's not that high. Greater than 1.0 certainly, but not that outrageous.

If they go purely by what they see on a flat track (like the MTS they have), then yea, they'd see numbers artificially that high. In reality, it's just not the case.

As a quick sanity check, if the tires had that much grip, then with a pitiful 1500 lbs of downforce they could take a corner at 5G. With 3000 lbs of downforce (certainly not unreasonable) they'd be going through corners at 7G +.

[Scotracer]

I don't care what Bridgestone says, I'm calling bull on mu = 2.5. I've worked with tires with more traction than F1, and it's not that high. Greater than 1.0 certainly, but not that outrageous.

If they go purely by what they see on a flat track (like the MTS they have), then yea, they'd see numbers artificially that high. In reality, it's just not the case.

As a quick sanity check, if the tires had that much grip, then with a pitiful 1500 lbs of downforce they could take a corner at 5G. With 3000 lbs of downforce (certainly not unreasonable) they'd be going through corners at 7G +.

And in reality they have 6000lb of downforce at 190mph...

I don't know how they've calculated it but I took their figure as at least representing reality in some way. With that much downforce how would load sensitivity come in to it?

[Jersey Tom]

Like I said, if Bridgestone did make that claim, it's possible its based off flat track data.. which can be very misleading (even for tire professionals).

While yes tires are load sensitive, I don't think it would be that extreme.

If you want a good feel for the real grip level of the tires, work it out from cornering levels. It's well under 2.

[xpensive]

With 480 kW at the wheels, you could never convey a thrust of more than 24 kN at 20 m/s, or 72 km/h, no matter the mu.

The air-resistance negligeable, this would give a 700 kg object an accelleration of 24000 over 700, alas 34 m/s^2, or 3.5g.

Still impressive though.

[dumrick]

Like I said, if Bridgestone did make that claim, it's possible its based off flat track data.. which can be very misleading (even for tire professionals).

Controled environments like flat tracks are very important for comparison purposes (between tyre compounds and constructions, in this case). What could be misleading would be to try to perform valid comparisons in any kind of real track. As far as flat track data being misleading in real-world situations, I'm sure it is: after all, this miu, in a real-world situation, depends on many tyre-track interaction factors (asphalt composition and grain, dust, tyre and track temperatures, actual tyre load due to having to deal with mass inertia and suspension work, and so on).

If this coeficient was usable in real racing, then electronic traction controls would be a waste of technology.

[xpensive]

Thanx Scot, most eduacative graphs. One of my points is that applying 480 kW at zero speed would lead to infinite thrust.

So, what happens to all that power at zero-speed wheel-spin? Rather simple actually, power is still force times speed, but the speed is under such circumstances is the peripheral speed of the wheel itself.

Which begs for the next question, "Friction" for dicsussion, dynamic such vs static, anyone?

Btw, thank you for giving space to old-fart thinking, Ciro.

[Scotracer]

Thanx Scot, most eduacative graphs. One of my points is that applying 480 kW at zero speed would lead to infinite thrust.

So, what happens to all that power at zero-speed wheel-spin? Rather simple actually, power is still force times speed, but the speed is under such circumstances is the peripheral speed of the wheel itself.

"Friction" for dicsussion, dynamic such vs static, anyone?

Btw, thank you for giving space to old-fart thinking, Ciro.

I'm doing a little piece for this thread at the moment. With a CVT you really shouldn't get any wheelspin as a correctly designed one will be preset for the peak torque the back wheels can take under acceleration. I am instead doing a scenario where it's a conventional gearbox. I'm having to take a few liberties on the assumptions so bear with me. I should have it up later.

I think it's safe to assume that to keep the idea of 480kW @ 0m/s possible we have a clutch involved. I don't think even tyres of mu = 2.5 are quite ready for infinite thrust