Suspension Geometry and its association to Brake efficiency

[BruceA]

As we all know, under braking, weight is transferred towards the front of the vehicle.
Is there any relationship between suspension geometry/centre of gravity and the transfer of weight to a particular point on the front of the car? To get an idea of where Im coming from, does the weight transfer appear at a point (or line) across the vehicle eg: the tire/road contact patch, or does this/can this point appear ahead or behind the front axle line? Or is there even such a thing?

I was thinking about brake efficiency, when it would be best to have all the transferred weight appearing on the front tyre/road contact patch, and not ahead or behind this.
I know there is anti-dive that 'locks up' the ball joints to put this transfer on the wheels, but can such a thing be measured?

Am have been wondering if there is some sort of vector diagram that could illustrate this, or am I in fairyland on this one?

 

[Craig Gillingham]

what are you trying to achieve.....better lap times.....or just great brakes.....or both??

(or are you designing a car/chassis from the off)

all elements of the cars geo works with and mostly also against each other...
an optimum set up must be reached which is also usually a compromise of the variables

work on corner weights, spring rates, rebound, tyre geo, brake set up to get better lap times and also driving ability plays a big part. (each track is different, weather makes a difference)

ive never considered centre of gravity on the radical and trying to make a theoretical centre of gravity appear under brake load would seem crazy in a world of so many variables as it is.....it sits where it sits and transfers where it transfers.....as long as the times are good whats it matter.....might aswell redesign the whole car........

wouldnt say fairyland......would say get the car out more....

 

[Dave Bilyk]

Hi Bruce,
I drew up a diagram which explains the weight transfer effect in its simplest form.
The CG height is cgh, and the deceleration force acting on the CG is the car mass times deceleration as shown.
I took summation of forces, and moments about a point on the line between contact patches and directly below the CG location to get the following.

The static weight distribution is given by the first part of the equation,
weight x b/wheelbase on front and weight x a/wheelbase on rear.

The weight transfer from the rear contact patch onto the front contact patch is given by the second part, mass x decel x cgh/wheelbase.

For a GT40, assuming the cgh to wheelbase ratio is about 20% ?? means that under 1g braking the weight transfer is about 20% or about 200kg.

This is quite interesting as it shows that;
The lower the cgh height, the lower the weight transfer.
If the cg was at ground level (impossible but aids understanding) there would be no weight transfer.
If the cg was below ground level(crazy but again aids understanding) the weight transfer would be from front to back.
Again as an aid to understanding the mechanics of the effect, you could make a model on a raised track with an underslung weight to illustrate this.

This is very simplistic and takes no account of suspension movements during braking, but illustrates the basic effect.
Hope this helps

Dave

 

[Craig Gillingham]

i stand corrected...its me that needs to get out more...

 

[Mike Pass]

Anti dive does not lock up the balljoints - they are always free to pivot excepting a small amount of friction. The idea of anti dive is to set the inner pivots of the wishbone at an angle to the horizontal so that when brakes are applied there is a component of the braking force which acts vertically to lift the front of the car to counteract the weight transfer to the front which tends to compress the springs. The opposite applies at the rear to prevent squat by angling the wishbone pivot axis to the horizontal so that that there is a component of the accelerating force which tries to push up the rear of the car.
However the action of anti dive reduces the effective weight transfer to the front under braking which will reduce the max braking force available but the car will remain more level in f/r attitude which may stop the chassis hitting the road if the springs at the front are soft.
As the braking force is found from the load on the wheel times the coefficient of friction then (1) the greater the load on the tyre the more braking force available and (2) the greater the coefficient of friction the greater the braking force (sticky tyres give more braking force.)
Stability under braking is an important factor and transferring all the weight to the front may not give you the best braking in real life as the friction between tyre and road varies and if one wheel locks with 100% of the weight on the front things might get a bit hairy! Also a rolling wheel has more grip as it is using static friction - as soon as it skids it is using dynamic friction which is always less which is why avoiding lock ups is so important and why anti lock brakes are so good. In the wet the rear tyres are running on road which has been cleared by the front tyres so get a better surface to try to get a grip on so drivers wind the brake bias to the rear in the wet to take advantage ( unless you have already lost it and are heading backwards off the road!)
Hope this helps
Cheers
Mike

 

[Chris P.]

RamboLambo - A few things to consider.

For a given acceleration on the mass of the car (braking) and the wheelbase and CG height, a car will have a resulting weight transfer.

Tire grip is not a linear function with load. So, when braking, the increase in grip on the front tires is less than the decrease in grip on the rears. This means that the more weight that is transferred, the less overall grip the car will have for braking. This is the exact same situation as lateral grip in a turn.

Obviously, as you increase braking acceleration (or decleration, however you want to name it), more weight and hence grip will appear on the front and less on the rear up to the point of maximum braking where, in an ideal situation, all four wheels will lock. This is not a simple equation to just plug in a few numbers and get an answer.

I have written this in Excel for the purposes of designing my own car, but it would take some time to extract it out (it's part of a bigger file that has a lot more information about the car) and make it easy to use for someone who didn't write it. (Even I, after writing it some time back and looking at it recently, took some time to understand what I was thinking.) If people are interested, I might find some time to tackle this. It makes assumptions on a tire grip/load model (which can be changed) and allows many variables to be changed such as brake master / caliper sizes, rotor size, tire size, brake pad coefficient, pedal force and ratio, dry / wet, bias... it will tell you if the front and rear are locked, and if you have the Solver add in for Excel (which I think comes standard now in all levels of MS Office), Solver can be set up to find the best master / caliper piston sizes and bias for a given pedal force and wet / dry conditions. I would be very curious actually to see someone use it on their car and correlate to real world results.

Any interest?

 

[BruceA]

Thanks for all the informative replies. This stems from the fact that I can lock my wheels at around 50mph with hard braking. No, its not cheap tyres or a wet road! So hence I was considering the fact that more weight on the front wheels from weight transfer would help.

SO it looks like CG (centre of gravity) height is perhaps one of the biggest performers for increasing brake efficiency, amoungst a heap of other things it helps out with. Therefore we dont really want weight transfer, or as little as we can have would be the best option.

 

[Cliff Beer]

Bruce, it's also good to note that weight transfer depends upon conditions as well - for example, in the wet there's less weight transfer so a little more rear bias is acceptable whereas in the dry there will be greater weight transfer so a little more front bias is helpful - a variable brake bias in the cockpit can be helpful for racing (just set it and leave it for street time).

 

[Russ Noble]

Thanks for all the informative replies. This stems from the fact that I can lock my wheels at around 50mph with hard braking. No, its not cheap tyres or a wet road! So hence I was considering the fact that more weight on the front wheels from weight transfer would help.

Bruce,

Just put more brakes on the back! QED. Simple, effective, and uncomplicated.

Cheers,

Russ