How do you calculate a car's roll centre?

[mvrick208]

The car in question is a Mclaren F1 road car. I have the suspension geometry and I believe it uses a double wishbone suspension. Should I just draw lines to see where they meet and that will be the front and rear roll centres or are there any equations to find a definite answer ? Thank you.

[Black Lotus]

How to do it is shown on the internet.
You just draw the lines to the instant center (usually past the center of the car to the other side. Then back across the car to the center of the tire contact patch. Where it passes the centerline of the car--that is the roll center height.
Which governs the up or down jacking forces, and the rolling moment of the car at that end.
No calculations involved to just find the roll center, as far as I know anyway.

[mvrick208]

Here is what I have done to find the rear roll centre. I dont know how to get any numbers. How do I find out what the wishbones are angled at ? I know the rear camber is 0.75 degrees.

[mvrick208]

RC stands for Roll Centre
1472 mm is the rear track
rear camber is 0.75 degrees

I dont know what the COG for Mclaren F1 is but do I even need it?

[Black Lotus]

your drawing looks plausible--the lines are going in the right direction.
If you have the car at hand, you find the longitudinal centerline. Then measure the suspension pickup points from that. Measure both upper and lower A-arms bushings from centerline, and distance from the ground.
Hard to do accurately! I know, I've tried it.
Center of gravity or the mass centroid axis isn't necessary to find roll centers, but it's required to find out how the roll center axis matches up to the mass centroid axis and just about everything else.
You may be able to get the cars CG height and front to rear weight distribution from car publications or by youself by physically tilting the car, but you'll be hard pressed to get information about roll centers and mass centroid heights.
It's none of your business, you know.

[mvrick208]

Originally Posted by Black Lotus

your drawing looks plausible--the lines are going in the right direction.
If you have the car at hand, you find the longitudinal centerline. Then measure the suspension pickup points from that. Measure both upper and lower A-arms bushings from centerline, and distance from the ground.
Hard to do accurately! I know, I've tried it.
Center of gravity or the mass centroid axis isn't necessary to find roll centers, but it's required to find out how the roll center axis matches up to the mass centroid axis and just about everything else.
You may be able to get the cars CG height and front to rear weight distribution from car publications or by youself by physically tilting the car, but you'll be hard pressed to get information about roll centers and mass centroid heights.
It's none of your business, you know.

I dont quite understand what you have said here. How do I find the longitudinal centre line ? I know the dimensions of the McLaren F1. Also what are suspension pick up points? I know what the ground clearance of the McLaren F1 is.
I know its none of my business but I have an assignment at hand.

You have also posted a picture, I don't quite get it. Is it a software you have used to find the heigh of the RC as it states ?

[mvrick208]

Originally Posted by Black Lotus

your drawing looks plausible--the lines are going in the right direction.
If you have the car at hand, you find the longitudinal centerline. Then measure the suspension pickup points from that. Measure both upper and lower A-arms bushings from centerline, and distance from the ground.
Hard to do accurately! I know, I've tried it.
Center of gravity or the mass centroid axis isn't necessary to find roll centers, but it's required to find out how the roll center axis matches up to the mass centroid axis and just about everything else.
You may be able to get the cars CG height and front to rear weight distribution from car publications or by youself by physically tilting the car, but you'll be hard pressed to get information about roll centers and mass centroid heights.
It's none of your business, you know.

Here is a better drawing of the rear suspension, you said calculate the bushing distance from the centreline, I have represented this where the big question mark is. Is this what you meant ? btw the 3.68 means 368 mm its just a rough drawing. I estimated this figure by looking at an image of the McLaren F1 which showed the arms to take up about 20/25% of the space at the rear.