I'm sat here reading loads of gumpf on designing your own suspension (oh dear...) and one of the first things nearly all the different sources have told me I need to do is work out roll centres and centre of gravity to get the roll couple and... ahhh...

I have no car to measure the c of g on, and I want to work out the suspension, so I can work out how much room I'll have to put other bits n pieces, which will of course alter the centre of gravity...

Also need to take into account the weight distribution... ah....

I hate suspension geometry

So... where do you start? Do you just make a "best guess" for c of G and weight distribution and hope you'll be able to dial out any glaring cock ups on your in-built cock-up-dial-out-adjusters?

What's the trick I'm missing here...

Look at similar cars and work out the static roll centres and roll axis

Roll centres are only a design tool, they aren't carved in tablets of stone, provided you don't do anything too silly with them you can tune handling by changing roll stiffness at each end.

unfortunately no-one with a similar car will let me near it, especially when I get a tape measure and inclinometer out

If your building a low slung sports type car, then it's a fair bet that the C of G will be somewhere between hub height and a max of 16". Your roll centres should be calculated on a arcuate of about 8 or 9 feet. The front should be lower than the back, "ie" 3" front, 4" rear. The hardest part is calculating body roll allied to suspention travel, and maintaining your roll centres, (near enough), through this travel. If you can locate one "Andre Jutes" "Building Special Cars" is well worth a read, goes into suspention design very well. HTH Ray

Give us a clue as to what you're building (be amazed if no one on here has not done something vaguely similar) & we can probably help.

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Originally posted by Chippy
If your building a low slung sports type car, then it's a fair bet that the C of G will be somewhere between hub height and a max of 16". Your roll centres should be calculated on a arcuate of about 8 or 9 feet. The front should be lower than the back, "ie" 3" front, 4" rear. The hardest part is calculating body roll allied to suspention travel, and maintaining your roll centres, (near enough), through this travel. If you can locate one "Andre Jutes" "Building Special Cars" is well worth a read, goes into suspention design very well. HTH Ray

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Where are you located, just say's south east, if reasonably local may be able to help. Cheers Ray

Many Thanks Chippy, I'm not too far from you - in Southampton.

Since posting I've had a bit of help from Nasty Bob (many thanks NB!) who's provided me with details of his basic setup, so I can get a better feel for how it all works - much easier when you can do a virtual creation! Obviously it's going to need some adjusting but as it's all in CAD at the moment with nothing real I can make plenty of mistakes without it costing money...

CoG is at about 38-41% of the total height for most cars.

Is that true of even very low slung cars? With a ride height of 100mm and total height of 1100mm, it's not a great deal taller than the engine!

I tend to believe so. If I remember correctly the CoG of the McLaren F1 is known on the internet. So check the theory!

thanks, I'll have a search about for that.