Roll axis inclination for live axle car
Discussion
I know that the (static) roll axis should be something like 2 - 3" higher at the rear than at the front, but that's taken from an example based on a car with IRS.
I'm in the process of redesigning the front suspension on my car, which has a live axle, and therefore a very high rear roll centre (middle of the diff, so around 12"
.
What would happen if the car were to be given a fairly low front roll centre, so that the roll axis was steeply inclined?
There is a way to lower the roll centre of a live axle, but the 'Wob' link isn't practical as the ground clearance is reduced (and ground clearance is already an issue!).
Cheers,
Chris.
I'm in the process of redesigning the front suspension on my car, which has a live axle, and therefore a very high rear roll centre (middle of the diff, so around 12"
. What would happen if the car were to be given a fairly low front roll centre, so that the roll axis was steeply inclined?
There is a way to lower the roll centre of a live axle, but the 'Wob' link isn't practical as the ground clearance is reduced (and ground clearance is already an issue!).
Cheers,
Chris.
Do a search on this thread on 'roll centres' and you'll find a very detailed (and eventually quite esoteric!) discussion. It's not anything like as simple as saying that the roll centre needs to be 2"-3" higher at the back!
To stick to answering your questions, however:
You'd alter the diagonal weight transfer characteristics, therefore changing the handling characteristics completely. You could 'correct' the influence of the roll centre change to some extent by using different spring rates, anti-roll bars, etc., but you'd still be opening a whole can of worms on the handling balance front.
There is a way to lower the roll centre of a live axle, but the 'Wob' link isn't practical as the ground clearance is reduced (and ground clearance is already an issue!).
The roll centre on a Woblink is basically at the centre of the chassis pivot, so you could potentially get the roll centre down to ground clearance + half the diameter of the 'head' of the Rose joint on the bottom of the Woblink, if you wanted to; say 4" above tarmac, for reasonably practical road use. There are other linkages available to do similar things, but nothing that is clearly superior to the Woblink in all respects.
What is it that you are trying to achieve, exactly? In other words, why are you redesigning the suspension?
To stick to answering your questions, however:
chrisj said:
What would happen if the car were to be given a fairly low front roll centre, so that the roll axis was steeply inclined?
You'd alter the diagonal weight transfer characteristics, therefore changing the handling characteristics completely. You could 'correct' the influence of the roll centre change to some extent by using different spring rates, anti-roll bars, etc., but you'd still be opening a whole can of worms on the handling balance front.
chrisj said:
There is a way to lower the roll centre of a live axle, but the 'Wob' link isn't practical as the ground clearance is reduced (and ground clearance is already an issue!).
The roll centre on a Woblink is basically at the centre of the chassis pivot, so you could potentially get the roll centre down to ground clearance + half the diameter of the 'head' of the Rose joint on the bottom of the Woblink, if you wanted to; say 4" above tarmac, for reasonably practical road use. There are other linkages available to do similar things, but nothing that is clearly superior to the Woblink in all respects.
What is it that you are trying to achieve, exactly? In other words, why are you redesigning the suspension?
Edited by Sam_68 on Wednesday 10th January 18:43
Hi Sam 68.
Sorry for the generalisation of 2-3" higher at the back, I've just skimmed through a few things relating to roll axis inclination, and that was a comment made.
As to why I'm doing all this, the car is undergoing a complete rebuild due to a rotten chassis.
I want to increase the front track to at least match the rear. At present the front track is some 120mm narrower than the rear.
Also the car still has all of the original Triumph bits, and I want to replace them.
For one, the wishbones are too short, and two the upright uses a trunion on the bottom which means that if you want castor in the geometry, you have to incline the whole wishbone / upright arrangement (trunion only pivots in a single plane), which at present means 'pro-dive' geometry.
As I'm starting from the ground up I thought it would be a good opportunity to do the job right, and also learn about the subject in the process.
What I find tricky is deciding on a start point.
There's plenty of information on calculating various aspects of the suspension, making adjustments etc, but without prior knowledge what do you base your calcs on?
I know I want the car to handle well, but I can't quantify anything, such as how much camber gain in bump I need.
About the only thing I can do at the moment is analyse what the existing setup is, and then advance from there.
Wish me luck.
Sorry for the generalisation of 2-3" higher at the back, I've just skimmed through a few things relating to roll axis inclination, and that was a comment made.
As to why I'm doing all this, the car is undergoing a complete rebuild due to a rotten chassis.
I want to increase the front track to at least match the rear. At present the front track is some 120mm narrower than the rear.
Also the car still has all of the original Triumph bits, and I want to replace them.
For one, the wishbones are too short, and two the upright uses a trunion on the bottom which means that if you want castor in the geometry, you have to incline the whole wishbone / upright arrangement (trunion only pivots in a single plane), which at present means 'pro-dive' geometry.
As I'm starting from the ground up I thought it would be a good opportunity to do the job right, and also learn about the subject in the process.
What I find tricky is deciding on a start point.
There's plenty of information on calculating various aspects of the suspension, making adjustments etc, but without prior knowledge what do you base your calcs on?
I know I want the car to handle well, but I can't quantify anything, such as how much camber gain in bump I need.
About the only thing I can do at the moment is analyse what the existing setup is, and then advance from there.
Wish me luck.
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