OK, chassis question. i need informed advice [Archive] - SaabCentral Forums

: OK, chassis question. i need informed advice


ragtopcav
6th October 2004, 06:01 PM
Check out the snap below
http://img.photobucket.com/albums/v377/ragtopcav/Rock05.jpg

So what! you say? look closely, front axle is giving positive camber [OK under big load] the rear is still holding a negative camber, almost an exagerated negative, is this due to the 'floppyness' Saab built into the rear beam? and back to the front, i've got poly bushes in the inner front position the other 2 are rubber, time to stiffen? Thoughts please.

Adrian W
6th October 2004, 06:24 PM
In the font end the upper suspension/shock mounting rolls with the body of the car. The camber relative to the car remains at spec, while the camber relative to the road becomes positive on one side and more-negative on the other.

Can you see how the roll of the body, to which the suspension is attached, will cause the suspension to roll as well?

The only way I know to correct that is to use VERY negative camber, such as in a race car, or to seriously reduce body roll either by spring travel or swaybars.

Some MacPhearson strut setups are designed to change goemetry enough under load to compensate. However, if you then throw five people in the car and go on a long journey the extra load will put the front wheels at a SERIOUS negative camber and prematurely wear your tires. So it's a compromise.


The rear end is a semi-independant suspension setup and therefore keeps a fairly constant geometry relative to the road surface even as the body of the car rolls significantly. That's what allows it to keep negative camber in hard driving.

You've just un-wittingly discovered why race cars use hugely exagerrated negative camber when running a stock MacPearson strut suspension.

Some Double A-arm setups can vary the goemetry under load enough to allow lots of body roll without compromising stock-street camber settings ... but only sometimes.

That's probably why everyone loves the C-900 Chassis so much. Double A-Arms have that advantage. They also have the advantage of reduced torque steer without a "rough ride". Primarily because the suspension has more mounting points, so you can use rubber bushings, which give a better ride, and still not get too much suspension deflection under hard accelleration.

Adrian~

SaabScott
6th October 2004, 09:40 PM
The car looks good there Iain!

I'm suprised to actually see the dual anti-sway bars under there (I know you did it, I just didn't think they would be that obvious - not bad mind you, just noticable).

Can you do something about that trailer hitch though? :oops:
It kind of takes something away from the fear you would otherwise be striking into the hearts of the other drivers. :wink: :cheesy:

Sorry ... couldn't resist. :oops: :oops:

Do you have more pics? Full size pics? :wink: :wink:

Adrian W
6th October 2004, 10:05 PM
When you go for track day you could always run a super negative camber. I'm afraid there isn't much to do about it otherwise. :oops:

Adrian~

bkrell
6th October 2004, 10:50 PM
I've got negative camber at standstill on my rear wheels! :o Noticeable! Would just shimming leave bad suspension geometry or be okay?

I really don't trust the "new" saab dealer to mess with it. Maybe the nice Germans down the road, though?

I wouldn't mind it if I didn't drive so far daily.

Sorry, now back to Raggy's thread. :lol:

ragtopcav
7th October 2004, 04:38 AM
I've got negative camber at standstill on my rear wheels! :o Noticeable! Would just shimming leave bad suspension geometry or be okay?

Well this is the thing, it also ties up with Scotts thread about geometry in the workshop forum as i recall, I had my alignment checked by laser, whooo there was issues [some of which finally manifested in severed bushes] the guy commented on excessive negative camber on the rear beam, but he added that it will help in high speed cornering but may causes wear on the inside of the tyres [funny but rear tyre wear is even :-? ]

But the problem here is what to do about the front? it is not really feasable to adjust the camber on these cars, other j-type derivatives have had adjustable top mounts designed by tuners but not for the 900. I can still fit some more poly bushes to further reduce flex, but some of the flex that is visible i'm sure is down to the subframe flexing too :roll: it may be brace time too.

SaabScott
7th October 2004, 06:01 AM
I've got negative camber at standstill on my rear wheels! :o Noticeable! Would just shimming leave bad suspension geometry or be okay?

Well this is the thing, it also ties up with Scotts thread about geometry in the workshop forum as i recall, I had my alignment checked by laser, whooo there was issues [some of which finally manifested in severed bushes] the guy commented on excessive negative camber on the rear beam, but he added that it will help in high speed cornering but may causes wear on the inside of the tyres [funny but rear tyre wear is even :-? ]

But the problem here is what to do about the front? it is not really feasable to adjust the camber on these cars, other j-type derivatives have had adjustable top mounts designed by tuners but not for the 900. I can still fit some more poly bushes to further reduce flex, but some of the flex that is visible i'm sure is down to the subframe flexing too :roll: it may be brace time too.


I guess it really comes down to what you want to do with the car.
I you attend a track day 3 times a year and the rest of the time the car is on public roads, shuffling you and yours back and forth, then I would say that you don't worry about it.
What did your alignment guy say about the front end? I'm assuming it was fine at standstill since you haven't said anything about it.
I'm sure that you could tighten things up by changing more bushes and adding a brace (or 2), but how will that affect your day to day driveability?

Further, you say that you have negative camber in the rear (as do several of us).
Mine hasn't caused any premature wear on my tires either. :o
What I do have is the lovely "cupping" effect and the horendous noise! :evil:
Once I replace my ball joints and radius arms and anti-sway bar links in the front, I am going to get a 4-wheel alignment done.
Then I am going to take the car into a tire specialist and get his assessment on what else I may need to do.

Tweek's Turbos
8th October 2004, 08:06 AM
My car has this same problem...I eat tires like oreos. The only thing I can think, is the Abbott rear axle to correct this.

ragtopcav
10th October 2004, 04:09 AM
I guess it really comes down to what you want to do with the car.
I you attend a track day 3 times a year and the rest of the time the car is on public roads, shuffling you and yours back and forth, then I would say that you don't worry about it.
What did your alignment guy say about the front end? I'm assuming it was fine at standstill since you haven't said anything about it.
I'm sure that you could tighten things up by changing more bushes and adding a brace (or 2), but how will that affect your day to day driveability?The front was equally terrible when I had the allignment looked at, since then I had 2 bushes fail in grand style, so the car has new radius arms and the inner front bushes are now poly [the oem rubber ones allow a massive amount of deflection] I shall not be having alignment looked at again until i've finished bush swapping. With the few bits of poly fitted so far the car is quiter on the road with less creaks and groans and of course a little more responsive

Further, you say that you have negative camber in the rear (as do several of us).
Mine hasn't caused any premature wear on my tires either. :o
What I do have is the lovely "cupping" effect and the horendous noise! :evil:
. Cupping?

My rear beam is not just high on negative camber [the chassis guy said not to worry about this unless it was doing damage to the tyres] the entire beam is slightly twisted to the body, the rear bushes do deform with age [and allow about 1/4" of movement] and if one is worse than the other then the whole beam will be in the wrong place relative to the car.

SaabScott
11th October 2004, 12:01 PM
Cupping?



See here. (http://www.kaltire.com/commercial/medium_truck_tires/cupping.php)

Of course, this suggests there could be other causes to my problems than just the alignment. :o :cry:


See the light and dark patches on the tread?
http://img.photobucket.com/albums/v299/SaabScott/Sway%20bar%20mod/Stacked_installed_1_smaller.jpg
http://img.photobucket.com/albums/v299/SaabScott/Sway%20bar%20mod/Swaybarmodandcleaning039small.jpg
That's what I'm talking about.
I'm starting to wonder if it may be my rear struts that are to blame ...
They do look original, don't they? :o :cry:

G96nt
11th October 2004, 12:44 PM
that tread-pattern is from un-even toe.

you have your two rear tires fighting against eachother.
one is aimed "straight" And the other in/out.
This is why your tread-blocks, if you look closely, look like they're smeared. there will even be a tiny little "lip" at the inner-most side of the blocks.

un-fortunately, you'll need a rear alignment to fix it.

bkrell
12th October 2004, 07:54 AM
Yeah, those are the original sachs/boge struts. Toe usually isn't a problem on the rears, just the front. Though it is conceivable if the axle is bent. More often, it seems, our rear problems ARE from excess negative camber. That type of wear will be pretty uniform wear on the inside. Excess toe in/out will manifest itself in the form of a fethered tread pattern on the inside and also leaves the inner edge feeling lumpy.

The test for toe in misalignment is to run your hand along the inner edge of the tire from front (over the top) to the back. You should feel some resistance from the fethering, which is as if the inner treads have formed a thin lip. What it is a result of is the fact that the tire is not completely forward facing when driving straight ahead. This causes the tires to "scrub" the pavement a bit as they role along.

SaabScott
12th October 2004, 07:31 PM
Yeah, those are the original sachs/boge struts. Toe usually isn't a problem on the rears, just the front. Though it is conceivable if the axle is bent. More often, it seems, our rear problems ARE from excess negative camber. That type of wear will be pretty uniform wear on the inside. Excess toe in/out will manifest itself in the form of a fethered tread pattern on the inside and also leaves the inner edge feeling lumpy.

The test for toe in misalignment is to run your hand along the inner edge of the tire from front (over the top) to the back. You should feel some resistance from the fethering, which is as if the inner treads have formed a thin lip. What it is a result of is the fact that the tire is not completely forward facing when driving straight ahead. This causes the tires to "scrub" the pavement a bit as they role along.


And of course that would cause the howl I hear whenever I get above 15 mph or so. :cry:

ragtopcav
20th October 2004, 07:49 PM
Can you do something about that trailer hitch though? :oops:
Oi! that's my parking distance sensor mate! when it hits things i know I can't go back any further :lol: .

One way or another some of us will be spending money on our rear axles :roll: .

SaabScott
20th October 2004, 11:11 PM
Can you do something about that trailer hitch though? :oops:
Oi! that's my parking distance sensor mate! when it hits things i know I can't go back any further :lol: .

One way or another some of us will be spending money on our rear axles :roll: .


I don't think I will be the first though now that I have to sort out my wife's car. :cry: