Roll Center

[Dave P]

Hey Guys:

I have been reading through the suspension threads. I understand the basics of roll center. (All my past work has been circle track setup with bias ply tires) Some body had metioned that an orginal 40 was around 4.6 inches front and 4.8 rear. (Which seems high to me)
Where is a good starting point for the roll center front and rear for a GT40 on radials? (turning both directions left and right)

 

[Russ Noble]

Hi Dave.

In the absence of anyone else giving you an intelligent answer, here's my $.02.

I'm unsure about the implications of the actual height for road use but generally your roll centres need to be parallel to your mass centroid (basically the longitudinal line of the CoG) which in something like a GT40 will be lower at the front than the rear.

I have no idea what the original cars had but I would bet that Trevor Booth (nota 2226) would know.

FWIW my roll centres are 1/2" front; 2" rear and I determined those as something of an average used by single seaters and racing sports cars in their day using similar suspension and tyres. I am unsure whether for road use there are any advantages in altering these heights, but bearing in mind chassis ground clearance, the position of your lower pivot points may dictate higher roll centres anyway. I don't think from a practical point of view that the actual RC height matters, it is more important to keep it relative to the GoG front and rear. I think higher RCs in road cars are used to counteract body roll when used with softer springs and bars. As I said I'm not that conversant with RCs for road use, but I'm sure if I am wrong about this, someone will take the opportunity to shoot me down in flames....

Great to see steady progress with your car. Keep up the good work.

 

[BruceA]

Russ, if you keeping the roll centre heights and centre of gravity around the same, dont you think your 1/2" / 2" is a bit lower than your CG? The further away from the CG the roll centres are the bigger the moment arm leverage that would induce body roll?
What is your estimated CG height on your car?

 

[Tom]

Raising RC to CG height in a road car will probably create jacking geometry when cornering - not useful for generating grip.

 

[Russ Noble]

Russ, if you keeping the roll centre heights and centre of gravity around the same, dont you think your 1/2" / 2" is a bit lower than your CG? The further away from the CG the roll centres are the bigger the moment arm leverage that would induce body roll?
What is your estimated CG height on your car?

Rambo, I never said RC and CoG at the same height. I said parallel to.

Obviously RCs at 1/2", 2" are much lower than CoG. Can't remember what I figured CoGs at now but the difference front to rear was 1 1/2" but that depended on the validity of many assumptions and guesstimates, so could really be anywhere. Yes to the body roll, but in my case I don't expect to get a lot of that with 800 lb/in springs.

Spud is right about jacking with high RCs.