effects of rear suspension on front suspension

[TEAM78]

how does the design on the rear suspenion effect the front end suspension? i have read that the roll axis should slope down from the rear in order to increas the traction on the rear, how ever how does this work and why should the Rear RC height affect the front RC location. also how shoul dthe camber paths of the front and rear suspension be correlated to each other?

cheers

[phoenix]

The height of the front a rear roll centres are not affected by the design of the suspension at the opposite end of the car. The front roll centre height is determined only by the design of the front suspension and the rear roll centre height is determined only by the design of the rear suspension.

As you say, it is normal to have the the front roll centre lower than the rear so that the roll axis is inclined downwards in that direction.

To remain neutral - i.e. no net gain or loss of camber, any camber compensation designed into the suspension at either end of the car should attempt to reflect the amount of roll and it's effect on the camber on the wheel/tyre. If the chassis roll would cause the wheel/tyre to gain 2 degrees of positive at a certain cornering force, the design should permit the loaded suspension to move sufficiently at the same cornering force so that 2 degrees of negative camber is added to the wheel/tyre.

If it is desired that more negative camber should be added so that the tyre is able to generate more cornering force, then negative camber should be added faster by the suspension than positive camber is added by the chassis rolling.

If, when cornering, negative camber is added by the suspension at different rates front and rear then the dynamic balance of the car is likely to change. For example, if, as cornering speed/load rises, the rear has more grip added than the front, the car will understeer more (or oversteer less) the higher the cornering loads.

[TEAM78]

how is it best to set up this? also am im right in saying it is only possible to have the same camber rate front and rear at one set steering angle?

[phoenix]

It is virtually impossible to answer your first question without knowing the suspension arrangement on the car you are talking about and a whole load of other things - like how much camber do your tyres need to work best (this varies widely), does the car generate any downforce, how high is the centre of mass, what is the weight distribution, what is the front and rear track, what is the wheelbase, do you run anti-roll bars. A huge number of variables.

The answer to the second question is no. On the front wheels, castor can be set to be add negative camber, which will be a fixed amount of camber for a fixed steering angle. But there will also be camber changes due to the suspension moving under cornering load. So at one speed the steering angle may add 1 degree of negative camber but the suspension, compressing due to cornering load, may add another 1/2 a degree. At a higher speed the same amount is added by the castor setting, but the suspension being compressed more may add a total of 1 degree.

As I said previously, what goes on at the rear can be quite different to the front. Best example is if you have a solid rear axle where there would be no change in camber angle due to the suspension during normal bump travel.

[TEAM78]

Thanks for your reply. So for the front suspension design the RC height should be in the region of half the height of the rear in order to place a larger proportion of the rolling moment on the front as 55% of the weight is on the rear along with dynamic camber on the front to move at the same rate as the dynamic camber gain on the rear. the inclination of the front upper wishbone and caster should be used to achieve this camber gain to conincide with the body roll as the camber gain on the rear is purely induced by the body roll of the car. the body roll is upto 2° at the very max.
it is important to ensure camber gain rate on the front and rear is the same or this will cause adverse vehicle characteristics. the vertical load on the tyres should be evenly distributed front to rear during cornering hence enabling the opportunity of a balanced car.
How ever how does this work with the rear end being driven as more will be expected from the rear tyre?

should the camber gain rate be the same front to rear? or when should it be this?

The aim is to achieve the same slip angles on all tyres through out all phases of cornering..

thanks Phoenix for your help just need to detail my aims and objectives better for designing the front suspenion. Ive completed the rear suspension design with having a roll camber coefficient of 0.55 on the rear. The circuit though is tight and twisty

[phoenix]

Originally Posted by TEAM78

Thanks for your reply.How ever how does this work with the rear end being driven as more will be expected from the rear tyre?

Depending on the weight distribution, the difference is commonly resolved by running wider rear tyres with greater grip potential. This allows cornering & (some) acceleration loads to be carried without leading to oversteer.

[Goran Malmberg]

This is a large answer question. First we could talk in general and then about a specific car. We may optimice the cornering power for a certain condition or we might build the car to be easy to "read" for the driver and therby easy to adjust-set up for different condition. As a general I prefere a very much "zero" car that has no agressive cambercompensation curve, low forcelines, longer A-arms and less SAI (and litle scrub). That way the car can be adjusted at the track for different and tire condition, like using static camber setting opposed to camber compensation A-arms.

We should watch the forcelines for staying the same angle during wheeltravel-roll, not altering the geometric weight transfer that confuse the calculation of spring-roll setting. I find it better using close to ground forceline (Rc) and only work with springs-rollbars, that can be adjusted, and then hopefully without influence from jacking forces which makes it easier for the crew.

We dont need to pay attention to where the forcelines intercept sideways, it is the angle that is important and each wheel could be calculated with separatley with its froceline angle where the outer wheels forceline angle is the moost important. The car will acually physically roll about where the outer wheel forceline crosses the centre of the car independent from where the the forcelines interception are located sideways. We could use a sloped roll axis,lower front, but i dont find it to important.

Building the front and rear suspension geometrically more alike is an easier way not to make balance-handling misstakes. If not knowing exactley what we are doing.

Regards
Goran Malmberg

[TEAM78]

Hi thanks for your replys guys, Goran what do you mean by forcelines?? Also why have long wishbones?? I have found that by having long wishbones this gives a very linear camber curve and theres not alot you can do about it even with dramatically shortening the upper wishbone. The design of the FS car has forced me to use long wishbones on the rear where as I would of prefered short wishbones also they are diverging which actually helps to lower the RC so do so many cars go for a horizontal lower wishbone?

Cheers

Matt

[Lukin]

Originally Posted by Goran Malmberg

We should watch the forcelines for staying the same angle during wheeltravel-roll, not altering the geometric weight transfer that confuse the calculation of spring-roll setting. I find it better using close to ground forceline (Rc) and only work with springs-rollbars, that can be adjusted, and then hopefully without influence from jacking forces which makes it easier for the crew.

Building the front and rear suspension geometrically more alike is an easier way not to make balance-handling misstakes. If not knowing exactley what we are doing.

So you put the RC as close to the ground as you can so you only have to concentrate on elastic weight transfer? That's a little different. On our cars we have no control over the rear roll centre (watts link) but still, I think if it's possible, it is a very good tuning tool given the way it loads the tyres differently. Not always good, but not always bad either.

The Ortiz article in the back of RCE deals with roll axis inclination this month. Interesting read.

[Goran Malmberg]

Originally Posted by Lukin

So you put the RC as close to the ground as you can so you only have to concentrate on elastic weight transfer? That's a little different. On our cars we have no control over the rear roll centre (watts link) but still, I think if it's possible, it is a very good tuning tool given the way it loads the tyres differently. Not always good, but not always bad either.

The Ortiz article in the back of RCE deals with roll axis inclination this month. Interesting read.

First 78 ask for forceline explanations. Fl is a line drawn from instant centre back to the centre of the tire grip. Instant centre is projection line extension from the A-arm to their interceptions.

I should explain my theory a little closer, I am just going to put words on it, hmmm. Anyway, if we are stuck to Rc limitations we must use the ways that are still open. I havent read the Ortiz last article so I cant make any comments, but Ortiz has very good ideas moost of the time.

Even if the Rc is way of the grond, we should se to that the frocelines is not altering its angle to much during wheel travel. That way balance stays more stable. The difference using jacking forces as a balance instrument is that it is UNSPRUNG therby hitting the tire grip hard. But the balance itself may be ok but with less grip marginal during road iregularities..
Regards
Goran Malmberg

[Lukin]

Oh ok, I didn't realise you were talking about RC migration during suspension travel. I do agree with you on that case.