Ground Clearance

[Dalton]

What ground clearance are most GT40 road cars running? Here in NSW (Australia) at least, the absolute legal minimum is 100mm. But practically, are most running about 125mm say?

Dalton

[40 Bud]

Because I lowered the floor I have only 3.75 inches clearance in that area. If I had no lowered floor I would have 4.75 inches. I have to watch for speed bumps and negootiate them carefully and when loading onto a trailer I must raise the front of the trailer to get a good angle so the lowered seat area clears. Other than those exceptions I drive it all over California. Longest single trip was L.A. to San Francisco.

[Canuk40]

Hey Dalton,

Good question, could be a survey on it's own. I like to think about how low can you go, although 4 inches or 101.6mm was the specified ride height.

In the first picture below we have our customer, Angelo between his two (now three) GTs. The Gulf car is the first CAV GT in Canada and was initially set to 4 inches front and 4.25 rear.

We always put a bit of wedge in to lower air pressure under the car. Angelos car has a dropped drivers floor pan so it's under three inches on that side. Surprisingly, most speed bumps are not a problem, nor railway crossings or general road conditions.

The second picture is another customer car, Chris S. His CAV GT has since been fitted with the dual nostril nose and is undergoing a colour change. It's sitting at 4.25 front and 4.50 rear. Low enough or should I say high enough!

Anyways, as long as they're sunny side up, all GT40s are gorgeous.

Cheers Mate

[Malcolm]

I run 4.5 inches front and 5 inches rear for chassis clearance. A wedge angle as referred to above is what I prefer on my car although the standard GTD setting is 5 inches front and rear. When running slicks on a smooth track I have got as low as 3 inches but would not recommend that on road cars. For minimum height road use, do not go below 4 inches as you will quite likely get caught out one day and damage the front or cills of your car.

[Tom Hudson]

I hope someone may be able to answer a question that has been perplexing me for some time i.e.

It is accepted practice to have the rear ground clearance set higher than the front. The usual reason given, as Ian Clark has said, is to lower air pressure under the car but with more ground clearance under the rear of the car the airflow in this area will slow down which, according to Bernoulli’s theory, will result in a higher air pressure in this area, thus causing rear lift

This ignores the fact that 40s do not run with side skirts so the under floor airflow will be more complicated than my simplified example but it seems logical as far as my very limited knowledge of aerodynamics goes.

Any other ideas welcome

Tom

[Mark Worthington]

Good question Tom. I would think that aerodynamic downforce on the outside of the car due to a positive rake will dominate any under-car aerodynamics.

[Malcolm]

Tom, have you got the result of the wedge back to front? If a fixed volume of air passes under the nose of the car and then passes backwards where the rear chassis is higher, that same volume of air now has to fill a bigger void so air pressure will reduce causing downforce not lift. If I am wrong on this, then I must wonder at all the diffusers you see on cars nowadays!

[Howard Jones]

The reactive downforce resulting from the wedge far out weights the bernoulli downforce created by any wing effect of chassis body pannels in these cars.

[Joe]

as malcom said, this is the idea behind a diffuser (along with the coanda effect directing air upwards on a curved diffuser like on a lotus elise) this is what good ol' wikipedia says:

Rear
A diffuser, in an automotive context, is usually a shaped section of the car underbody which improves the car's automotive aerodynamics properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand so that the boundary between the car's airflow and "external" airflow is less turbulent, and it also provides a degree of "wake infill" (the wake being a turbulent area of low pressure that is caused by the passage of the vehicle through the air; this can cause pressure drag).
As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient downforce producing device by reducing drag on the car and increasing downforce.
The aft part of a car underbody can be a diffuser. It tries to connect the underbody to the back without producing turbulences so that Bernoulli's principle applies and the pressure increases while the velocity decreases. The side and the roof end in a sharp edge, so that their pressure does not increase. Because the pressure in the back tends to equilibrate, the pressure below the car is lower than on the side and the roof of the car.

so in effect having the rear higher creates a larger volume of 'space' underneath the car at the rear.
also, the airflow over the body is directed (slightly) upwards (along with the lip at the rear) creating even more downforce. basically more downforce for a tiny bit more drag and everybodys happy! downforce for everybody!

ps i'm not an expert on this, it just seems to make sense is all.

[Jim c]

Having the rear higher does help with the weight transfer under brakes and also initiating turn in on corners.


Jim

[Bill D]

3.5" in the front, and 4" in the rear