Try this one.
I have a 97 Mercedes E320 Avantgarde (sports suspension - basically hard springs & shocks) which has a sometimes nasty twitch from the rear end. The back will twitch to the left when hitting a pothole with the n/s wheels, and sometimes when hitting a different surface on the n/s wheels from the o/s. It doesn't do it with a bump or if both n/s & o/s wheels hit the same object (trough or bump).
I have had it checked by a main Mercedes dealer, a small independent, and a middling-size general but very experienced garage, and none can find anything wrong, either with the suspension set-up (arms, bushes etc etc) or even can they reproduce the problem. I have new shocks all round, which did nothing to help, and a full 4-wheel alignment by a company who definitely knew what they were doing. The guy who road-tested after the 4-WA did notice the twitch (so its not a figment of my imagination, and even my wife notices it), but he had no suggestions other than "well, now we know it's not the alignment". Tyres are 50% worn, evenly, standard fit P6000s.
My theory is that the rear toe-in (which is about 14 minutes toe-in each rear wheel) seems quite significant when you look along the car - you can see the wheel angles - and with the fairly beefy tyres, if one wheel "unsticks" itself (i.e. loses contactdue to a dip, or hits a slippery surface (drain cover), the toe of the other wheel pushes the back end over.
Anyone got any ideas?
If not, what I feel I need is someone experinced in setting up performance car suspension - such behaviour would not be permitted on a track car for example. I'm in hertfordshire, so any suggestions?
Hope someone can help
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Cliff_G
(no relation to Ali)
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Cliff
Can't help you with an explanation, but might be able to help with leads to local expertise.
First, there's my mobile service mechanic, who's on holiday currently; he's an MB and other German car specialist and I recommend him wholeheartedly (he does all my cars, including the E320 Coupé). If you want to contact him, send me an e-mail via the mods.
Secondly, PTS in Luton are heavily into high-performance engine tuning. If they don't touch suspension systems, they'll probably be able to tell you who to go to.
PTS
Unit D
Kingsway Industrial Estate
Kingsway
Luton
LU1 1LP
Tel. 01582 731733
Thirdly, Chiltern Tyre & Exhaust are (I think) the only outfit in the area with laser-alignment kit:
Unit 7
Walter Lawrence Estate
Brewers Hill Road
Dunstable
Tel. 01582 609600
Hope this helps.
Roger Jones
Harpenden
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Roger,
sorry, I can't find a messaging function on this forum. Is it normal to send e-mails via the moderators?
[Yours isn't the tidy dark green E320 Coupe I have seen around H'den, is it?]
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Cliff_G
(no relation to Ali)
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Cliff
On a very occasional basis, I have found the mods happy to forward e-mails. Like most others, I don't want to expose my e-mail address to any more miscreants than have already abused it as a result of "harvesting" it from discussion groups and the like.
My E320 Coupé is silver. So's my Capri 2.8i. The Golf VR6 is black.
RJ
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>>Is it normal to send e-mails via the moderators?
we're more than happy to do so. You send me an e-mail, I forward it on, he replies to you - then you each have each other's e-mail address.
Many people use their work e-mail address and I fully understand why they would not want to expose that to all and sundry - especially given who some of them work for.
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Thirdly, Chiltern Tyre & Exhaust are (I think) the only outfit in the area with laser-alignment kit:
FWIW: The Service Centre of Vx Dealer Camden Motors in Leighton Buzzard have a full suspension pit, with all the kit this implies, and in my experience, know how to use it. They were the only outfit who found an elusive suspension problem that I had with my Vectra GSi, and quickly and easily put it right.
As a happy customer, who was also treated well, I specifically asked "If you routinely use your equipment and knowledge for diagnosing problems on other makes of car", and was informed "Yes".
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many years ago, i once had a problem on my herald that sounds very similar to your problem.
when i hit a bump,pothole,etc, i also experienced the rear end twitch, which was quite un-nerving, i also got it under hard braking, however, on inspection nothing appeared to be wrong or loose.....until you jacked the wheel off the floor, then we found the axle tie-bar was loose where it attached to the chassis member (can`t remember if the weld had gone or the bolt threads had stripped).
don`t know if "mercs" have axles these days!, but may be of some help.
billy.
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Cliff G, I think your theory is basically correct.
According to my MB database for your car the rear toe-in is 3.9mm, which corresponds to about 33 minutes of arc. This is quite a bit of toe-in. For comparison the front toe-in is only 1.8mm; half as much.
Rear toe-in is used to improve high-speed straight-line stability and also has the effect of reducing the tendancy to oversteer in a curve (the weight transfer to the outer rear tyre means that its toe-in tends to 'steer' the back end into the curve).
Some manufacturers design compliant bushings so that the toe-in of the outer rear tyre increases in a curve due to weight transfer. This is a safety feature to help 'over-enthusiastic' drivers out of potential oversteer situations.
Unless there is something obviously wrong with your suspension then this may be something that you have to live with. Firm springing will make the car a little 'skittish' on road imperfections and during short periods when grip is lost at one side you will get some rear-end 'steering'. You say that it would not be acceptable on a 'track' car - well, your car is set up very differently from a track car so you would not expect it to behave like one. Track cars do not have to be held in-lane on a motorway for example.
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Aprilia,
Thanks for this. It's the first explanation I have had that combines what I observe, what I have worked out as an engineer myself, the repeated reports on inspection that there is nothing wrong, and a reason why. Things started to get clearer when I found a (rather alarming) drift once, on a motorway at 70 mph, when the n/s and o/s wheels saw different grip surfaces, hence my theory. I and a mate had originally gone for something like a knackered arm bushing.
Yes, I probably can live with it. I've had the car 6 months now, and know when it does it, so now I have a reason that there is nothign wrong with the car, I am happier. When I first experienced it, I suddenly thought "what have I bought here?!"
[An interesting CV on your profile, Mr A.]
Thanks again.
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Cliff_G
(no relation to Ali)
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Cliff_G
While you experience the problem when your rear wheel goes over a bump, it is possible that the cause is not the rear suspension.
If cars had three wheels (bear with me on this!...), they would always sit properly on the road, and the vertical load at each wheel would balance properly - like a three legged stool. once you have more than three wheels, i.e., four, it is easy for diagonal pairs of wheels to take more than their fair share of the load - like a four legged chair or table which doensn't sit properly, and rocks.
So, it is possible that misaligned front suspension, or, if you are v. unlucky, a twisted bodyshell would give this sort of non-symmetric behaviour.
Checking the corner weights of the car would enable this hypothesis to be tested.
I hope this helps,
number_cruncher
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