79 SC corner balance

[hbkramer]

79 CS Coupe (Fatty) Corner Balance issues
Here is my data and observations /comments are welcome.The Left side bias needs correctiing I think,although the car handles quite well.(for a street car)
Per my Bentleys Manual:The max. allowable wt differential=20kg(44#)

So,The question is,How do I un weight the LR corner??and for a street car with a b+ driver will the result be noticable.Should I mess with it?
The ride height is a comprimise between DOT and EURO to prevent tire rubbing,so to "unweight" the LR I would have to re index the LR TBar ,hence LOWER the LR corner BUT the effect on the FR corner would be to increase it? correct.

LF=640# (20%) RF=664# (21%)
LR=976 (31%) RR=884# (28%)

Data was double checked due to the LR reading ,and the one thing I forget to set was air pressure !!

Front/ Rear Wt.Bias= 41% / 59%
left side /right side Bias= 51% /49%

LoadMeasurements were taken with Rugglles scales(4:1) ,full tank gas, driver in car:
Car/gas/driver=3164#
gas 15gal*7=105
driver 195#

Thanks

[Wil Ferch]

Whatever you do to one corner....say...increase its weight....will also happen on its diagonal. However....ANY TWO ADJACENT wheels will ( in their total) always stay the same. So you will affect all 4....kinda a tail chasing game. Your fr/rr total....and left/right total....will always stay the same regardless of what you do. Unless you actually move "stuff" around....like re-locating batteries, etc.

[Flat Six]

Up on LF; this will increase LF and RR while decreasing RF and LR (after resetting tire pressures, of course).

Get you more evenly balanced LF to RF, LR to RR, and diagonals (LF/RR to RF/LR).

D

[hbkramer]

Flaxt six ,Will,
Thanks,I see your method of raising LF,to inc corner weight @ both LF and RR and correct the balance,thanks.The resultant inc in LF ride height in front may not be good.

What about looking at the LR and considering re-indexing to lower it( LR) and decrease BOTH he LR and RF corner loads.Is that a possibility? Which approach is better? The LR could be re-indexed 1-2 splines without messing up ride height(It is about 1/4 "-3/8") Higher than the RR is .
Thouhgts?

Thanks

[Flat Six]

I can't speak to how much effect re-indexing rear torsion bars would have on ride height / corner weighting; outside my level of expertise. But 1/4" to 3/8" difference LR-RR doesn't seem like a lot to me, especially depending on how/where you measure it and considering you may have some tire pressure differences that haven't yet been taken into account.

If you're dead set against raising LF you could lower RF and achieve about the same effect, I think. And much less difficult than re-indexing / adjusting LR. Just my $.02.

Dale

[Wil Ferch]

I agree if the "bothersome" amount of height difference is on the order of 3/8". The "angle calculator" will tell you what your different free-hanging spring plate angle has to be to make such a change...and then you can see the "degrees " difference free-hanging would be required...very small.

Porsche's own spec/tech books speak of this relationship in this fashion---> "1 degree change in spring strut inclination = approx 7 to 9 mm change in car height". This is about 0.27" to 0.35" change. My own add'l caveat---> they mean "while at static ride height with car weight on the wheels". They do not mean as a free-hanging spring plate angle change...as these two condtions don't relate.

[911pcars]

Ignoring any ride height changes, lowering LR and RF will reduce the weight at those corners and increase the weight supported at RR and LF and bring L and R proportions closer to ideal. A slight ride height adjustment should do it, confirmed by the weight scales.

As for setting desired ride heights, this will be a compromise with corner balance. If the chassis is straight, shouldn't be a problem. Remember to disconnect one of the sway bar drop links from each sway bar before measuring.

Sherwood

[hbkramer]

Thanks Sherwood,
Consensus rules.
I think I have to dig into the LR torsin bar to do it correctly.i dont see the correction happening if I lower the RF only. Thanks for the reminder on loosening the sways .

[Wil Ferch]

Lowering RF only will lessen the weight on the RF....and also a like-amount on the LR. However, remember what I said about ANY TWO adjacent wheels ( total weight) always remaining the same?... LF+RF.... RF+RR......RR+LR.....LR+LF.....any of these pairings as a total weight of 2....will be unchanged.

So...back to the problem. If you lower the RF and lessen it X lbs....you should see X lbs less on LR. BUT both front totals will remain the same...remember. Therefore the LF will show an increase....but because it does...so will the RR. It gets to be a tail-chasing game.

[Grady Clay]

Keep in mind the goal:
Same L-R bias in the front as the L-R bias in the rear.

You can’t change the overall F-R bias unless you reposition things in the car.
Similarly, you can’t change the overall L-R bias unless you reposition things in the car.

Now you have cross-corner bias: your Rf-LR is ‘heavy’ and your LF-RR is ‘light’.

You are desiring the same L-R bias at the front AND L-R bias at the rear as the overall L-R bias (51%-49%).

As Sherwood (Pelican 911pcars) properly points out, the sway bars (one side F&R at least) need to be disconnected. When re-connected, they need to be adjusted so the weights do not change.

All this needs to be done within the desired ride height at each corner and toe/caster/camber in the front while centering the steering and toe/camber in the rear with the rear center pointing to the same as the front center.

Even the most experienced chases this for a while.

Best,
Grady

[911st]

I am not a corner balance expert but am happy to offer my two cents anyway.

I believe you have to have your sway bars disconnected to get a usable corner balance or you could have a garbage in -- garbage out situation.

Thus, I might put the driver's weight in the car (assuming that is how it is going to be driven) and disconnect one of the rear sway bar links and see if it is preloaded. Then I would disconnect the two center point connections for the front and see if the bar is square to the chassis.

Then check the corner balance. When it is were you want it, see if the sway bars will reconnect without introducing any preload. If not you will need to shim the bars, or start over on your height adjustments until they are square (very time consuming and frustrating process).

Thus, 49/51 diagonals are probably not to bad for a street car if the sways are not introducing load in any way. Especially if you do not have adjustable rear arms.

Then, if you later have an alignment done, any change in camber or caster will most likely effect your balance.

[Wil Ferch]

911st....seems we meet again!

Quote.."I believe you have to have your sway bars disconnected to get a usable corner balance or you could have a garbage in -- garbage out situation..."

Technically...true, BUT.....

If you don't have adjustable drop links fitted, (as for a race car where this is VERY important)...tell me....what difference would disconnecting the drop-links make if after you're all done....you reconnect those bothersome/fixed-length drop links for a street car?...what did you gain? You will re-establish the improper pre-load you had before.

[911pcars]

Originally Posted by Wil Ferch

911st....seems we meet again!

Quote.."I believe you have to have your sway bars disconnected to get a usable corner balance or you could have a garbage in -- garbage out situation..."

Technically...true, BUT.....

If you don't have adjustable drop links fitted, (as for a race car where this is VERY important)...tell me....what difference would disconnecting the drop-links make if after you're all done....you reconnect those bothersome/fixed-length drop links for a street car?...what did you gain? You will re-establish the improper pre-load you had before.

If one goes to the expense of corner balancing, one would also provide some means of installing adjustable drop links. However, if the chassis is straight, getting the CB close should be fine. When the CB is way off, the car is not fun to drive or capable of what it can do. For DIY confirmation, look in the archives for "Tripod method".

Sherwood

[911st]

Hi Wil,

Yes, not having adjustable drop links is a pain but not insurmountable.

However, if one leaves the sways connected it could be that when one wheel is being adjusted to change it's load, the sway bar might be at a point where it is working against the adjustments.

For example, I think on a 3.2 the effective front sway bar rate is very close to one sides torsion bar rate. Thus you could have say 112 lbs from the torsion bar working in one direction and 140 lbs from the torson bar working it the other (from memory and could be wrong).

On my car I went to great expense to get the CB, alignment, and height at each corner near perfect so the stock sways can be hooked back up without introducing any preload. It was an effort that few would probably go to for a street car but it is possible.

If one dose not want to go to this much effort, the other way is to as I noted in my post (quote below). You can put shims under one side of the sway bar mount.

Learned this from Chuck at ER. Cool idea and would have saved me a bunch.

Originally Posted by 911st

... see if the sway bars will reconnect without introducing any preload. If not you will need to shim the bars, or start over on your height adjustments until they are square (very time consuming and frustrating process)...

I think if I was to have to do it again, I might start by using the "tripod" method that ER came up with some time ago to get the height on one side at a time correct first. However I would adjust these heights for the impact of the drivers weight.

Before I did my car I disconnected the sways and put a reference mark on each side of my car. I measured to that mark and then sat in the car to see how much each side compressed with my weight. I forget but the drivers side I think compressed about 3/8's inch more than the other. Thus, I would set the drivers side that much higher from the start.

I know, crazy stuff.

[Wil Ferch]

It all comes down to purpose....certainly a car with individual corner weights that are 100+ lbs "off" ideal...is no good. Porsche has a rather simple test of saying the left or right side should be within 20-40 lbs of one another ( depending if Turbo or non-Turbo)...and leaves it at that.

What I'm saying is that for a street car..."Close" with any of these nuances applied or not is probably going to be OK.....but I see your point about shimming...slick if you don't have adjustable drop links.

[911pcars]

911st,
Just to be historically accurate, the "tripod method" to isolate front and rear suspension settings is described in the factory repair manual. If I stumble upon the pages (PAG indexing is NG), I'll insert it here.

Sherwood

[911st]

Thx Sherwood,

Is that where you raise the front of the car at the center and then square the car to the ground and then do the same at the rear?

My 72 even had a notch for this.

This would work well if each side of the car weighed the same. However, with just a driver the drivers side weighs more so I did not find it very useful.

If this is the method your are referring to, that is different.

Chuck's thought was to lift it at the side jack point, take the wheels off that side, lower to near stock height, and measure the front and rear height some where near the torsion bar center line and compare the two sides.


If the factory and Chuck's method is the same, great minds often think alike.

[911pcars]

Originally Posted by 911st

Thx Sherwood,

Is that where you raise the front of the car at the center and then square the car to the ground and then do the same at the rear?

Yes, that's it

My 72 even had a notch for this.

This would work well if each side of the car weighed the same. However, with just a driver the drivers side weighs more so I did not find it very useful.

The weight distribution on a street vehicle is where it is unless you physically move components around or delete them. Setting the ride height will take those weight differences into consideration no matter what method you use to measure.

If this is the method your are referring to, that is different.

Chuck's thought was to lift it at the side jack point, take the wheels off that side, lower to near stock height, and measure the front and rear height some where near the torsion bar center line and compare the two sides.

If the factory and Chuck's method is the same, great minds often think alike.

Chuck's method sounds like the same principle, but any weight difference between L and R side remain what they are. I haven't tried that method.

Sherwood

[911st]

(edited)

Both methods are usable to set the final ride height goal and rough in a car's balance so an alignment can be done.

After the alignment the ride height should be checked and the final balance should probably be checked again.

Probably sufficient for a street car with some care.