corner balance questions

[GMR911]

I have my car pretty well balnces now but I can not get my rear ride heights within specs as outlined in the Bentley manual states as 8 mm max difference in trim height left to right sides. My car weights and trim heights are as follows- left front 572lb/109mm trim, right front 586 lb/107 tim, left rear 826lb/7 mm trim, and right rear 839 lb/ 21 mm trim. The difference in rear trim is 14 mm but only 8 lb difference in weight. How significant is this, if any, and any suggestions in getting this closer trim height closer without getting the weights ou of whack, any thoughts, ideas. Thanks Jerry

[911st]

Have you checked your air pressure and disconected your sway bars?

What are your goals or use for the car?

[Flieger]

Don't worry about the heights (from the fender to the tire) unless you are getting tire rub in the turns.

Go with the corner weights- equal fronts for braking stability or equal diagonals for even handling in the turns.

The only height measurements that matter are the torsion bar to wheel centerline distances the factory uses.

[911st]

I am not an expert but from the numbers, I believe that is the measurement he is talking about.

I believe the factory measurements set the chassis square to the ground and put the car higher than the so called "euro height" which seems to have become the norm.

Further, most now set up the car with rake relative to the ground, set it up lower (especially in the front), and the early "S" specs are not of much use for most unless setting up a show car to stock specs.

The allowable side to side differential however should still be relevant.

Equal front weights should have an effective limited slip to prevent premature rear wheel lock up on one side. Having said that the weights look great. If the car is to be tracked, weight should be added representing the driver during the set up.

If the tire pressures are balanced, the sway bars disconnected, and or it dose not have a bad gas pressurized shock. There may be damage like a twisted chassis or failed bushing.

[GMR911]

The measurements I am using are the centerline of torsion bars relative to centerline of wheel. would like to get the rears closer w/o upsetting the balance I have achieved, Thanks jerry

[911st]

Did you check you tire air pressures beforee the CB?

Did you disconnect you sways before your CB?

And one more silly question, it the floor flat where you did your CB?

[Flieger]

The weights effect handling dynamics more than the amount of height variation he has here.

As long as they are not WAY off such that the instant centers for suspension movement get way off, which in the rear takes a lot of work.

Your setup looks good.

[bob deluke]

Gerry and I did the corner balancing over a 4 day/night period. It seems that no matter what we do to the rears, the proportions (7mm-21mm) between the rear tires stays the same. I suspect a bad torsion bar. We changed out the original 23mm to a used set of 25mm bars. We installed new spring plates/bushings and initially set up spring plate to 31.5 degrees the first time. Rear wheel heights were 4mm-18mm. Redid spring plate angle to 32.5 degrees, same thing happened after weight jacking and measuring c/l of torsion bar/wheel center per Porsche specs. Relationship of rear wheel height remained the same (about 13-14mm difference). I told Gerry the used torsion bars may be playing a part in this issue. The corner balancing came out great, and ride height difference between the two rear tires is about 1/2". Not a big deal and hardly noticeable. The car sits nicely and seems to be properly balanced. Weight distribution is 41% front, 59% rear. We are both happy with the results, and will be doing my car next. I'll post my results and see if they differ from what Gerry has. This has been an interesting and somewhat frustrating project, but hopefully the end result is a better handling car.

[Flieger]

What kind of suspension bushings do you have?

It could be old rubber is cold-flowing and settling at that uneven ride height.

[aston@ultrasw.c]

As I understand it:

If you have adjustable links, disconnect the sways and re-install them so they do not impart any force in the neutral condition.

If you do not have adjustable links, leave the sway bars attached.

[911st]

Left front is 2mm high, right rear is 14mm high. Thus, net out of balance is about 12mm assuming the surface was truly flat and measurements accurate.

Did you accurately set the alignment before or with the CB?

I bet rear camber can have a significant effect on rear trim heights and front can have some effect.

Each deg of rear camber has an effect of about 6-7mm in height.

Thus, it might be the neg camber differs from side to side in the rear by about 1.7-2 deg.

For example, if one side is set at -.8 deg of camber and the other at -2.5 this might account for your variance.

It might not be quite that extreme. Production variances and bushing sag may be contributing.


*** GMR911 or bob deluke -- I will ask a third time***:

Was the tire air pressure checked before the CB?

Were the sway bars disconnected before the CB?

If none of this fits, it is probable there is suspension damage.

[JP911]

Did you add weight (or yourself) to the driver's seat when you set up the car? This makes a big difference.

[DohertyCM]

I was wondering about corner balancing because i just readjusted my torsion bars. Although they were not perfect i was able to adjust the height of the front torsion bars to match which made the rear torsion bar height match up as well.
I have 24.75 in the rear and 24.58 in the front...

By the conversation, are you saying that I did a corner balance?

I thought you would have to bring in scales?

[GMR911]

Thanks for all the responses. The tire were set to 34lb rear and 30 lb front. The sway bars were not disconnected. I have read where the sway bars do not have to be disconected if they are non-adjustible type? The suspension is completely new with new stock a-arms(2 years old) and new stock rear soring plates. The car has not been alligned yet. The car has bilstein shocks all around. Bob and I took the car out for a short spin tonite and when we got back the car sat considerable lower than before we took it out? Probably a good inch at all 4 corners. Wonder if this is a function of the bilsteins? We are going to check the heights again after a nite of sitting. Anxious to see it they heights raise back up again. Thanks Jerry

[mmack]

I just went through this with my car as well. New (within a year) torsion bars (22/28), new bilsteins, new Neatrix bushings. With 200lbs in the driver's seat, my car's diagonal balance is within 13 lbs (that's the good news). The frustration is that the driver's front sits about 3/4 of an inch lower than the passenger front. Like the OP, I/we (my mechanic) couldn't get it happy at anything less that that type of difference in ride height (measuring top of fender to ground).

My sways are non-adjustable, so they were not disconnected, but I'm curious as to whether there's some preload pushing one of the fronts down a little harder than the other. (Maybe GMR911, that's what you're dealing with too...)

Next step is to disconnect it and see if the weight redistributes. If it does, then I'll chase down some adjustable end links for the front sway and neutralize it.

If not, I'll just get used to tilting my head when I look at the car, or just close my eyes 'til I'm in the seat. It drives beautifully.

Mark

[911pcars]

Originally Posted by mmack

I just went through this with my car as well. New (within a year) torsion bars (22/28), new bilsteins, new Neatrix bushings. With 200lbs in the driver's seat, my car's diagonal balance is within 13 lbs (that's the good news). The frustration is that the driver's front sits about 3/4 of an inch lower than the passenger front. Like the OP, I/we (my mechanic) couldn't get it happy at anything less that that type of difference in ride height (measuring top of fender to ground).

My sways are non-adjustable, so they were not disconnected, but I'm curious as to whether there's some preload pushing one of the fronts down a little harder than the other. (Maybe GMR911, that's what you're dealing with too...)

Next step is to disconnect it and see if the weight redistributes. If it does, then I'll chase down some adjustable end links for the front sway and neutralize it.

If not, I'll just get used to tilting my head when I look at the car, or just close my eyes 'til I'm in the seat. It drives beautifully.

Mark

Do you realize you can't perform accurate corner balancing while the sway bars are connected? Each wheel must be independent while adjusting each corner. That means disconnecting the SB drop link. Otherwise, any height difference from side to side will affect the opposite wheel. Disconnecting the sway bars after the fact will relieve any weight jacking, however slight, but it won't compensate for any corrective adjustments performed with the sway bars attached.

Sherwood

[mmack]

At the time I witnessed the corner balancing process, I wasn't wise enough to think about the effect of the sway bars. Only after I came away with the type of ride height difference I have, did I dig into the forum and read up on it.

Most have said, if the sways are not adjustable, leave them connected, but if they're adjustable, disconnect them. Thinking that was a decent rule of thumb, I didn't question after the fact why my mechanic left them attached.

"Disconnecting the sway bars after the fact will relieve any weight jacking, however slight, but it won't compensate for any corrective adjustments performed with the sway bars attached."

Understood - I guess the thing to do is just start over with the sway bars disconnected, starting from a level ride height (with the driver's seat weighted), get the corner weights happy by weight jacking, hopefully without having as much difference in ride height as I have now, then reconnect the sway bars.

So at that point, if one side of the sway bar doesn't want to meet the end link, either up or down, it's time for adjustable end links so I'm not preloading the bar. Am I on the right track?

(Not intending to hijack this thread -- I think the we're chasing down the same issue...)

Mark

[Flieger]

Non-adjustable sway bars stay connected for the corner balance.

However, the alignment, torsion bar adjustment, ride height, and damper changes you made will all affect the corner balance. You must have the driver weight in the car, track tire pressures, and all of your suspension work (new bushings, bars, etc.) sorted beforehand. Disconnect any adjustable sway bars and then try to find a good compromise between alignment and corner-balance. It is nice if the heights work out but doesn't matter too much.

I thought you had done a proper corner balance and alignment. If that was the case, the height difference woulf be no big deal. There is a good chance it is due to those non-adjustable sway bars.

[911st]

Originally Posted by GMR911

Thanks for all the responses. The tire were set to 34lb rear and 30 lb front. The sway bars were not disconnected. I have read where the sway bars do not have to be disconected if they are non-adjustible type? The suspension is completely new with new stock a-arms(2 years old) and new stock rear soring plates. The car has not been alligned yet. The car has bilstein shocks all around. Bob and I took the car out for a short spin tonite and when we got back the car sat considerable lower than before we took it out? Probably a good inch at all 4 corners. Wonder if this is a function of the bilsteins? We are going to check the heights again after a nite of sitting. Anxious to see it they heights raise back up again. Thanks Jerry

Sways should never be hooked up for a proper CB.

I am confident that a combo of not having the camber set correctly and leaving the sway bars contected is why you can not meet the specs in your Bently repair manual.

Again, the rear camber dramatically changes the height of the wheel relitive to the chassis by 7 to 9mm per degree. The front also can do so but to a lesser degree.

You need to get your car height set and your car aligned before a corner ballance should be done.

This problem with your CB is componded by leaving the sway bars conected. When you are rasing up the righ rear, force is being transfered through the sway bar exadurating things.

Setting up a 911 w stock sway bars that do not have adjustable links is a real chalange. It is not that difficult to get the corner ballances correct. It is difficult to get the CB right and not add preload to the car when the sways are re-attached. Add to that the effect of adding the driver to the equation and it can take a lot of time and skill but it can be done.

You also note your car has settled. You alwas have to drive the car after you jacked it up to settle it. Tires side load and pinch the suspention and the rubber bushings may reindex them selves. Most do not think of this but the rubber bushings are part of your spring rate. If one or several are wound up the spring rate at thouse wheels will be higher.

Lets get back to having the drivers weight in the car. If you have a 200 lb driver, about 75% of that weight goes to the drivers side and 25% to the pasangers side or 150 to one side, 50 to the other. Further, that is 100 lbs more added to the drivers side. If it is a stock 3.2 911 the spring rate on rith front is 110 lbs per inch. The left rear's is 140 for a total of 250 lbs per inch. Thus when the driver gets in the car the driver side will compress about .4" or about 3/8's more than the passangers side.

I belive this is close as I marked referance lines on each side of my car and measured the compression at the doors before I hooked up my sways. After I hooked up my sways it was about half that.

Sorry for getting carried away. Just get your height where you want it. Get the car aligned, then do your CB with weigth in the passanger seat untell you get it close and the sways can be hooked back up w/o adding preload to them.

When this is done, the differances in trim between each corner will be very close (with the drivers weight).

[Flieger]

The experts say to leave the non-adjustable sways hooked up. You want the corner balance and alignment to reflect the way the car will be run when driving on the track. Thus, since you cannot adjust out the preload later, you must account for it in the actual corner balance and alignment.

The resulting corner balance and alignment will be compromised compared to a car with adjustable sway bars and end links, but it will not be screwed up the moment you drive away from the shop and hook up your sway bar, introducing unknown forces into the suspension and balance.

Leave non-adjustable sway bars hooked up for corner balance/alignment.