How to maintain similar weight transfer after altering weight distribution?

[Zico]

Ok, Im no chassis expert so Im hoping some of you may be able to offer some suggestions as to how the changes we have made are gonna affect the handling, and what direction we should go to try and maintain or achieve as similar to the original weight transfer ratios as possible.

We have 2 near identicle Mk2 escort Grp4 rally cars, the original and the new improved, and more powerful one which we are in the process of building.
Both have a 5 linked Atlas axle, the new one is fully floating while the old one has half shafts and because they are forrest cars we will be keeping the leaf spring set-up.
The new car has a larger diff tunnel for improved suspension travel should we choose to use softer rear springs.

With the new car we have moved the engine back 15 cm to the rear to a Front/mid position and will likely fit a carbon/kevlar sumpguard to remove some weight from this area and to improve the PMI, we also aim to move the fuel tank in the new car to a more central position than where it is right now (above the rear axle) Whilst improving PMI.. This is gonna alter the weight distribution and affect the whole balance of the car so Id really like to try and work out how it will effect everything.

My initial thoughts are that we will be able/have to to use softer springs on both front and rear but I need some formulas to work out how much softer, I'd also like to know about the other side effects this is gonna create on the dynamics of the car.

Excuse the amateurish attempt at trying to explain the situation.. All info and pointers gratefully recieved.

Cheers..

[phoenix]

Just thinking out loud here...

15 cm is about 6% of the wheelbase of the Escort.

I think I am right in saying that the engine and gearbox amount to about 33% of the cars total mass.

Overall, moving the engine back would seem to give about a 2% shift of mass relative to the wheelbaase. I am not sure what the weight distribution front/rear is, but guessing at 65/35, this would mean the c of g shifting rearwards about 3% of the wheelbase.

My thoughts are that this would not have a great impact on spring rates, though it may improve the handling a little (maybe a little less tendancy to understeer on turn-in).

Moving the tank forward would also have a relatively low impact on spring rates I would have thought.

[Lukin]

If you have gone to this much effort on your race car I would imagine you corner weight the car before each outing so you should know pretty quickly when you drop it on the scales.

If you have the same corner weights as before then the roll stiffness distribution won't change, though the transient response will change a fair bit with lower yaw resistance.

[phoenix]

Originally Posted by phoenix

Just thinking out loud here...

15 cm is about 6% of the wheelbase of the Escort.

I think I am right in saying that the engine and gearbox amount to about 33% of the cars total mass.

Overall, moving the engine back would seem to give about a 2% shift of mass relative to the wheelbaase. I am not sure what the weight distribution front/rear is, but guessing at 65/35, this would mean the c of g shifting rearwards about 3% of the wheelbase.

My thoughts are that this would not have a great impact on spring rates, though it may improve the handling a little (maybe a little less tendancy to understeer on turn-in).

Moving the tank forward would also have a relatively low impact on spring rates I would have thought.

I think I made a mistake with my maths: moving 33% of the vehicle mass rearwards by 6% of the wheelbase would, I think, shift the c of g rearwards by 9% of the wheelbase (not 2% as I said above).

If this is the case, the revised weight balance would be around 56% front and 44% rear and the net effect would be something like moving 162 lbs onto the rear and off the front. The car would require stiffer rear springs and softer fronts if you wish to keep the current wheel rates.

Moving the tank forward will slightly off-set this of course.

Someone please check my calculations!

As both you and Lukin say, the car will have a lower polar moment of inertia and should change direction a little faster; this may or may not be a good idea on the loose....... Turn in should definitely be improved though, which is no bad thing.

[Zico]

Thanks for the replies.. very helpfull, as always.