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Roll centre/cog

Motorsport Wheel Alignment Fundamentals

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If I wanted to work out the roll centre/cog of my own vehicle. Is this possible without the use of computer programmes etc ...

Can I take measurements from points on the car and plot it out ?

Hi J P. Yes, this is certainly possible! In fact, in general, you can do many things in motorsport engineering without computers, they just help to speed up our jobs.

Two different parts to your question, I'll split it up.

The geometric roll center is a kinematic concept, all that means is it is a theoretical point that is something to do with the way the suspension moves. You can plot this out quite well-using graph paper with a pencil and ruler. Before you do that you need to measure all your hard points in space, as in where all the pivot points in your suspension relative to a datum (like the ground for example). This part can be a bit tricky depending on the car but it's always doable.

In practice, a happy medium to make use of the concept of a roll center is to use a very simple spreadsheet-based tool. The advantage is they are easy to use and also cheap! There are plenty out there, a very popular one is called Susprog.

For CoG, without having the whole car defined inside a CAD package it can take a bit of work. Finding the forward/aft position of the CoG is easy with some scales, but I assume you are talking about the vertical position of the CoG. The means lifting the car up either from the front or side and finding the tipping point. You know the vertical height of the CoG at the point where the car is about to tip because at the point where the car will tip the CoG MUST be above the pivot point. You can also do this by lifting the car less than the tipping point and taking scale measurements but I think for this to be sufficiently accurate (in my experience CoG testing is always harder/more work than you think) you have to lift the car so high you might as well tip it anyway.

We will be releasing a suspension fundamentals course this year that will cover both of these concepts in a lot more detail but I hope that helps to start with!

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