Suspension Jacking Forces

by huwcdavies on August 2, 2013

If you’ve been following my blog you’ll probably be aware of my complaints of understeer this year. At this point in time it feels like the limiting factor of the car. Straight line speed is good, the weight of the car is down with more to lose over the winter, the car no longer wants to spin out when a rear wheel touches a curb but now it feels like it has more understeer than a family hatchback. Just take a look at the Rockingham video and see how wide I’m exiting, how many apexes are missed, how much steering lock is being applied and how much time I’m loosing entering the corners when there are other cars in frame. Really, if I tried to enter some of those corners at the same speed as my competitors I’d just carry straight on into a tire wall, field or marshall’s post! Something has to be done.

Enter (once again) the Suspension Geometry Calculator. Wondering what could be causing this phenomenon I entered the front suspension dimensions into the calculator and within a minute I had some possible answers:

Suspension Jacking

sgc_jackingAs I understand it, suspension jacking occurs when there is a distance between the vehicle’s center of gravity (CoG) and the roll center (RC). Think of the CoG as the center of rotation when the chassis rolls and the RC as the handle of a lever that you will use to rotate the chassis. The greater this distance is, the easier it will be to rotate the car, right? So in summary, the greater this distance the more the chassis will roll.

But what about if you don’t want the chassis to roll? In my experience, I’ve had to increase the the wheel frequency and increase the anti roll bar stiffness to compensate. What generally happens when you essentially lock-up the suspension movement? You lose grip leading to understeer.

So, what ya going to do about it?

In terms of me dealing with my specific issue I’ve found that I can move the chassis locating points of the lower control arms without too much difficulty and after some modeling using the Suspension Geometry Calculator I’ve found a set-up that will move my RC up, reduce the chassis roll which in turn turn will allow me to have a more compliant front suspension set-up.

sgc_jacking_2Updates to the SGC

This analysis has not only proved useful for my racing aspirations but also for this site as it has highlighted a few areas that could be improved within the calculator, namely:

* A means of comparing two suspension models by way of overlaying the new model on the old one. Expect this one quite soon.

* A way of measuring the jacking force would be great in future and with integration with the Weight Distribution Calculator (weight and CoG) as well as the Wheel Frequency Calculator this could become a very powerful tool. Expect this in the future.

Suspension Geometry Calculator

If you would like to check your own suspension set-up follow this link to the Suspension Geometry Calculator. It’s quick and easy to use using the drag and drop interface and for any dimension you know the values of you can enter them using the form input. Try it today, it’s free!



Comments (6)

Racing Aspirations March 23, 2017 at 7:46 am

I need to revisit this article as it is a few years out of date. The distance between the CoG and the RC is the control you have as a designer to manage the kinematic lateral load transfer. The distance between the CoG and the roll axis (or ground) affects the total lateral load transfer. You can manage the kinematic lateral load transfer as well as the elastic lateral load transfer which enables you to control the total lateral load transfer.

The distance between the roll axis and the RC affects suspension jacking.