khooni

Hi,

I got a specialist to do alignment and corner weighting on my 7.2rs.

After putting the car on the scales, the feedback was as follows:

the original alignment was in line with factory spec.s.

But when the car was put on the scales (assuming they disconnected the anti-roll bars), the shop discovered that:

they could equalise the FL and FR weights but the RL and RR were offset by 80kg. Loosening the factory rollcage reduced the imbalance by 14 kg so the car is still 66 kg off at the rear. Now the theory is that perhaps i have faulty springs and they are intending to remove them for testing.

what do you guys think? My race instructor feels that getting the FR and FL weights equal, camber/caster/toe and ride height within tolerance is more important that static weight distribution and cross weight distribution. In fact, he's successfully raced cars with up to 55 kg of rear weight imbalance and won races.

I am a newbie at suspension and would like to deepen my understanding re corner weight and alignment. I think the current plan of action is to test the springs and if not faulty, set the car up with the front weights equal, adjust the cross weights and static weight distribution as best as possible without creating too much ride height imbalance and then proceed to alignment of camber, caster and toe.

Any thoughts on the above? The technicians checked the car over for any sign of accident repair. They couldn't find any and they also deem the chassis tobe straight. Has anyone encountered such a big different in rear weight imbalance before? finally on a near 1400kg car, 80 kg doesn't actually that much (5.7%).

Thanks

analogmike

The drivers side (left usually but right in the UK) will always weigh more, due to the controls and the driver's weight in the seat.

Find a new shop that understands corner balancing.

J richard

+1

jdistefa

Clearly the car is inherently defective and you should give it to me.

(ps. you will never get a street car to be corner balanced to the level of precision you're worried about)

(pps. read Fred Puhn's classic book then go drive your car and enjoy it )

spg993tt

7.2 hrs? they might struggle to redo a setup in between on-track sessions.
...what analogie said.

khooni

LF 283 RF 301
LR 387 RR 490

that's after setting the geometry and ride height.

I understand having a 30-35kg difference in the rear is ok but after adjustment the shop got it to 72kg

anyway, am testing the dampers and shocks in the RR now and will see if anything is out of order

analogmike

Do the math and you will see what the correct weight distribution is but that ASSUMES
that the weights were taken on a totally level surface, with correct tire pressures and sway bars disconnected.

LF 283 RF 301
LR 387 RR 490

left weight = 670
right weight = 791
total weight = 1461
front % = 283 + 301 / 1461 = 40%, rear must be 60%

If your scales are correct, you will always be the same weight off on each corner, and one diagonal
will be heavy and the other light.

So correct corner weights are
RF = 40% x 791 = 316.4 (you are 15 kg light on RF)
LF = 40% x 670= 268 (15 kg heavy there)
RR = 60% x 791 = 475 (again 15 kg heavy, RR/LF diagonal)
LR = 60% x 670= 402 (15 kg light on RF LR diagonal)

So you just need to adjust a bit to correct for the 15 kg unbalance.

You need to raise the RF / LR diagonal and lower the LF / RR diagonal slight
equal amounts. This will keep the ride height and alignment the same.

Good luck!

khooni

does it not matter that the left weight distribution is 45.86%?

analogmike

Does not matter, it is what it is.

If you want to move things around like battery, etc, to improve weight distribution, that's a totally different thing from corner balancing. You can't move weight around by adjusting springs. Even raising the front by a foot will not add noticeable weight to the front end.

Corner balancing is to take what you have and make it work best.

The Fred Puhn book mentioned above is good, it's old but physics have not changed since then.

khooni

I have also heard that it is important for the front weights ie LF and RF to be near perfectly balanced as it affects turn in. You don't seem to care about that.

analogmike

Some successful people set up the car like that with equal weights on FL and FR. It's great under braking but the car may understeer to the right and oversteer to the left, or vice-versa, as the rear tires will not be supporting their actual weights.

My '73 911 in F class needs a lot of ballast, which I put in the passenger's side, so I can get the car 50% left to right and that makes corner balancing math trivial, just set the fronts the same and the rears don't even need to be measured.

khooni

can it be that my RR spring and shock set is faulty which is why i am seeing so much weight towards the rear? Sorry to be a pain but whilst i wait for wednesday when my shop is sending the damper unit for bench testing, i wanted to be educated on the subject.

so Mike, assuming the damper/spring in RR is not faulty and nothing in the suspension is binding, would you still advocate maintaining cross weights rather than getting FL and FR equal? Thanks

analogmike

The car is what it is, does not matter what shocks and springs you have unless they are binding. But your suspension and scales seem to be working fine as the corner weights all make perfect sense. I was thinking they were 15 pounds off but it's kilograms, that's not close enough for me.

Print out the calcs above and show them to your shop and they should understand.

amso3

I was crew chief for a number of years on a 997 Cup. We corner balanced using Mike's math method. On occasion we might add "wedge" to get the front's closer but that was not the standard starting point. Also make sure the car has driver weight added to the driver seat during set-up.

It is just as important to measure ride height as corner weights during set-up. If you don't manage ride height, the weights might be ok but the chassis platform might not be lever, thus affecting geometry of suspension movement. I'd re-iterate the earlier comment "find someone who knows how to corner balance (a Cup Car)"

por29

you definitely need a shop tht knows how to corner balance. Looking at your numbers it's easy. pm me