The frame of reference of the moving road in the wind tunnel might muddy the waters, but on the road, the point of the tyre in contact with the tarmac is, for an brief instant, not moving at all (the road does not move and tyre slip is negligible). With the center of the tyre moving constantly at the same speed as the car, and the wheels averaging the same speed as the car, the top of the tire must move twice as fast.

But the wheel is still moving... & if the tyre contact patch was not rolling, such as during a brake lock-up,
the fact that it is still moving at car speed relative to the road - means it would 'flat-spot' the tyre, no?

"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

The frame of reference of the moving road in the wind tunnel might muddy the waters, but on the road, the point of the tyre in contact with the tarmac is, for an brief instant, not moving at all (the road does not move and tyre slip is negligible). With the center of the tyre moving constantly at the same speed as the car, and the wheels averaging the same speed as the car, the top of the tire must move twice as fast.

Maybe this illustrates it better.

That video is showing displacement of a point on the wheel, not velocity... but I do now see what you're saying.

Yes the issue is our chosen fixed reference - I tend to work car fixed, you're talking about road fixed. Expressing it mathematically, the tangential velocity of the wheel, , is equal to the driving speed of the car, . Using a car fixed system the the local longitudinal speed around the circumference of the wheel, is:

so as the car is fixed in space and the air and road are travelling backwards, at the contact patch (0deg), the wheel is going longitudinally backwards at the speed of the car.

In your case, as the car is driving forwards and the road and air are stationary:

so the top of the tyre (180deg) is going forwards at 2x the speed of the car, and the contact patch (0deg) is being driven backwards at the same speed the car is travelling forwards.

I find it difficult to get the language right for road fixed.... but I'll try...

In terms of compressibility where I say the air is going backwards at 2x the speed of the car, it's actually being jetted backwards at the speed of the car - i.e. the car is moving forwards at 240km/hr the air is locally being pushed backwards at 240km/hr. This is where there is flow compressibility, as the airflow opposes the direction of travel and the difference (240 - -240 = 480) of the velocities is greater than Mach 0.3.

At the top of the tyre, the thin layer attached to the surface is moving forwards at 2x the speed of the car - as it's in the same direction there is no compressibility effect, because the difference of the velocities is less than Mach 0.3 (240 - 480 = -240).

Not sure if this helped....I think I'm more confused...

#aerogandalf "There is one big friend. It is downforce. And once you have this it’s a big mate and it’s helping a lot." Robert Kubica

I might be being obtuse here, but I am seeing the problem from the point o view of the real world.
When the car is moving forward at 300km/h, the top of the wheel is moving forward at 600km/h and meeting stationary air. I call that Mach 0.6, at least locally.
So does this create compressibility effects?

Edit: I see now that the middle part of your post is meant to refer to the air under the wings. Which is fascinating. I had heard of that effect, but the sheer magnitude! Wow.

I see your point that there is no mach 0.6 difference compared to the rest of the wheel nearby, but wouldn't the relevant condition be, in some spots, relative to the stationary air?

More than happy to be educated here! #armchairaerodynamicist.

Quick question, does anyone know the sort of steer angles and yaw angles they test at in the wind tunnel i.e the largest angles they test at?

I think I've seen ±10° quoted as the limit on the system TMG use. ±15° would be the absolute upper end for F1, but ±5° would be closer to normal range. Depends on the team and facilities. Automotive testing with a turntable would be more like ±30°, but in steps like 1°,2°, 3°, 5°, 8°, 10°, 15°, 20°, 30°.

#aerogandalf "There is one big friend. It is downforce. And once you have this it’s a big mate and it’s helping a lot." Robert Kubica

Quick question, does anyone know the sort of steer angles and yaw angles they test at in the wind tunnel i.e the largest angles they test at?

I think I've seen ±10° quoted as the limit on the system TMG use. ±15° would be the absolute upper end for F1, but ±5° would be closer to normal range. Depends on the team and facilities. Automotive testing with a turntable would be more like ±30°, but in steps like 1°,2°, 3°, 5°, 8°, 10°, 15°, 20°, 30°.

Ok so is ±5° the norm for both steer angle and yaw angle?

Ok so is ±5° the norm for both steer angle and yaw angle?

Define the norm... they use constant movement systems which sweep through a range of angles, up to a limit of motion of ±10° to 15°. They may use a PDF to quote aero loads at some statistically relevant yaw angle, which would probably be around 2° or 3°, because F1 cars won't ever have huge yaw angles.

#aerogandalf "There is one big friend. It is downforce. And once you have this it’s a big mate and it’s helping a lot." Robert Kubica

Ok so is ±5° the norm for both steer angle and yaw angle?

Define the norm... they use constant movement systems which sweep through a range of angles, up to a limit of motion of ±10° to 15°. They may use a PDF to quote aero loads at some statistically relevant yaw angle, which would probably be around 2° or 3°, because F1 cars won't ever have huge yaw angles.

Ok thanks. Do you know the typical maximum steer angle they test at?