Porsche 911UK Forum

Welcome to the @Porsche911UK website. Register a free account today to become a member! Sign up is quick and easy, then you can view, participate in topics and posts across the site that covers all things Porsche.

Already registered and looking to recovery your account, select 'login in' and then the 'forget your password' option.

New Suspension rebuild. Problem with rear of car

Jamesx19

Well-known member
Joined
10 Jul 2015
Messages
627
Hi All,

Just finished (well, thought I had) my suspension rebuild, and have a rather large issue that's presented itself today after having the Geo / alignment done.

RyxBXXf.jpg


The rear of the car is incredibly unstable. Either at low speed over bumps, and definitely at higher speeds where the rear went into a uncontrolled weaving motion when coming off the power, and hit a bump.

The feeling is very much a lateral shimmy, like the rear of the car is moving side to side as well as up and down.

Some backround to this is:

I bought the car last October. It hasn't been used much over the last 4 years, and the previous owner has proved, (in the fullness of the time since, I have found) to have been rather quiet about several issues, and I suspect a hooky MOT. No matter.

I have undertaken a full strip down of the Suspension and removed and replaced Coffin arms (Meyle and modified with Polyurethane in the voids) , Diagonal control arms, Rear track arms, Anti roll bar bushes, all the drop links. The bolts and nuts have all been replaced with new Porsche camber bolts, etc.

I have fitted H&R lowering springs. I visually assessed the dampers (original B4 Mo30 units I'm guessing, because that is what came fitted from the factory, and the spring paint codes confirm Mo30) Decided they weren't leaking, the rod's chrome looked in good nick, so painted them and refitted.

Today I went to 9 Excellence and they set the the alignment. No issues X74 spec. for reference.

I hadn't driven the car much since I bought it, as it needed all the work. However it has pretty much felt like this, though now its worse.

I have already replaced the engine mounts as part of the suspension rebuild as I though they might have something to do with it.

So, the question is what is wrong? The Chap at 9 Excellence said its one of the worst feeling cars he's ever experienced!

My initial thoughts are:

1.) Rear dampers shot?
2.) Something I have not tightened up properly?
3.) Meyle bushes with the extra Polyurethane in the bushes causing an issue?
4.) I greased the camber bolts inc the faces of the sub frame mounts to ensure easy adjusting, are they slipping?

I drove straight back to 9 Excellence, and they popped the car straight back on the ramp and checked the Geo again for me, which was exactly the figures they left it at, and all the bolts were tight. Which suggests to me that nothing is slipping.

Is the reason its now worse, or reached a tipping point, is the rear springs are a higher poundage and are taxing the dampers more?

Does anyone have any other ideas at all?

Many thanks, James
 
Some things to consider ..

gearbox Mount

Tyres / wheels buckled

Dampers .. did you check they actually worked when you had them off the car ? is it possible that theres no actual fluid left in them ?

Top strut mounts replaced ?

Was it a full geo or just back end ?

Check your front suspension .. might be theres a fault there ie front dampers.

Dont know modified parts im afraid .. i do standard stuff so cant comment there .

Obviously nothing loose as it was the same settings after they first did it.

Chassis out of alignment or the suspension frame out of alignment but pretty unlikely .
 
Thanks Demort,

Looks like I've plenty of checking to do.

Gearbox mount didn't look too bad when I was under the car. ie not awful enough to stick a screwdriver in there and check for play, so Ill do that.

Tyre's are brand new on the rear, and the fitter didn't comment that the wheel was buckled, and they balanced up. Still I can take them to a different fitter and get them to spin them up and check.

Top mounts original. All appeared well with them.

I turned all the dampers over and compressed them. All appeared strong in bounce, rebound was reasonably quick? Bilstein did comment that Dampers are only designed to work the right way up, so checking them this way, they can feel weird. Balance Motorsport is in Henfield, not too far away. I think they may have a shock absorber test machine. I will probably resort to stripping the dampers off and testing.

Thought about chassis and sub frame, but as the wheels all lined up ( 4 Wheel Laser alignment) I'd have thought we would have trouble getting it set up. Also no signs of old crash damage on the inside bodywork or sub frame.

The first thing I might do is hook out the new polyurethane in the bush voids, but I think unlikely to be the culprit, as NordicMatter has had good results. Waiting to see how MisterCorn's install works.

On a different matter, I have changed the ignition switch due to weird faults such as windscreen washers not working. I'm hoping the battery drain might be something to do with that, as I haven't got round to checking for rogue trackers yet.

Cheers James
 
Ok ..

Bent chassis would be extreme but it might not show up on a geo if the thrust angles are correct .. think x1 wheel closer to the front than the other on 1 axle .. it can still be adjusted on a geo .. probably asking for trouble saying that as im sure people will disagree but its a thought .. pretty extreme and i think it would show up on body work .. so we wont go there atm .

If we look at this logically .. it had a problem before .. you have overhauled the rear end and made it tighter and now its worse .. you have made it better but not fixed the fault so the fault has to lie else were ..

im thinking front end .. im no expert here im afraid , one of the other guys at work specialises on this .. but if a front shock was shot at the front then perhaps it would show itself more on the rear end as thats were all the weight is .

Tyres and wheels are out as you have had them checked .

I think you would have noticed if it was rear shocks so thats out .

Top mounts .. it would be obvious i feel so thats out .

Bent suspension / shocks would show on a geo check so thats out .

Gearbox mount .. bit of a long shot really .. i was trying to think what could cause a weight imbalence .. engine moveing side to side .

springs and ride height .. hmm .. thats a moding area so im not familiar with that but could be a possible area .. need other people here to help on that one .

Front end suspension check is next i feel .

Do a wiggle test front and rear .. top / bottom and side to side for any movement .
 
Thanks Demort,

Will try a wiggle test, and think about the front susp.

Will also measure front to rear hub distances on both sides to be sure. No signs of buckling in the floor pan.

Will come back to you with results

Cheers
 
Hi james
I am sorry to hear about your problems as I also spent many nights trying to work out my similar problem.
Lateral movement felt in the car can only be a few things.
Left to right movement when looking from the back of the car on the coffin arms will generally be felt whilst cornering and will cause a weave sensation as the rear track is altered quite dramatically as the bush is compressed.

Weave on acceration is the coffin arm centre bush being over loaded and causes a weave under hard acceration or at high speed corners 80mph plus.

Rear subframe will need to be inspected especially around the bush mounts where it meets the body. Gearbox mount does make a big difference with drive take up and marginal improvement with high speed stability.

I suspect the car will need to be put to standard ride height otherwise you might be chasing your tail. The 996 rear set up is basically a reversed front end without the steering rack and two extra tie arms to reduce the camber arc and add stiffness this set up may be susceptible to bump steer like the front is. Did you replace the tie / link arms with genuine ? As the bushing might be too soft. You are welcome to pm your number if you need to bounce off ideas.

The poly is removeable if a little effort but as Porsche fit a solid set up ong gt3 and later 997 this is very unlikely to cause a problem.

Mark
 
Hi Mark,

Many thanks for your thoughts.

All the Coffin arms are new. That doesn't rule them out of course, but the issue feels the same as before I replaced everything, only worse. There is no side to side during strong acceleration and in a constant turn the car feels ok, its mainly upset by bumps.

As you say, I think filling the voids in the bushes is unlikely to be the problem, for the reasons you mention.

Thinking of Gearbox mount, the drive take up is fine, and the engine mounts all appear as I installed, and the bolts torqued to the correct setting.

If the sub frame was moving, I'd expect to see changes in the car Geo when we re-measured, but all were spot on. I'll check them.

Re the rear tie link arms, I replaced with a secondhand set that had been polybushed. The ball joints felt good.

Your and Demort's input is great, because its helping me to narrow down things to what I feel is most likely.

I'm thinking Dampers, and as Demort says, possibly front ones. What would be handy is if I could get hold of a "known good" spare set, someone could lend for a week or two. That would then, as you say, return things to standard and rule mine in or out.

Cheers
 
My money's on the rear wheels/tyres. With the greatest of respect for the other knowledgeable views above, I have experience of buckled wheels on other cars and this sounds similar. The problem would now be worse because you have tightened and stiffened everything up, so any wobble is transmitted to the chassis more directly. I know new tyres have been fitted, but was the fitter actually looking for damage?

Check for any flat spots, particularly to the inside edge of the rim. Check also for any run out of the rim edge. Get the wheel in the air, set up a datum at the rim edge (just balance a pen on a brick or something) and turn the wheel, watching for any change in the gap between the datum and the rim during the course of a rotation. Even a small distortion can be felt keenly, particularly at certain speeds.

Good luck...
 
Do you know if the arms are same batch? I have read that some Meyle arms a while ago were machined a bit shorter than OEM and it's since been sorted. However if you have one long and one short it would have different toe geometry either side through the suspensions movement. This creates a very odd loose feeling back end.

I had a very similar issue on an Elise that somehow had an S1 arm and S2 arm on the back creating this issue and then again with a Z3M Coupe that had a front lower arm replaced with a standard 3 series one for an MOT. Both cars dialled up correctly on an alignment rig and both felt awful to drive. Good luck!
 
Had you not driven it previously to the suspension mods for reference?
What condition are the engine mounts, this was my first thought without reading about your suspension upgrades. it may be the engine moving about as this can affect the handing if you have all that weight wobbling about and can create a pendulum effect.
 
Many thanks for all your replies.

I've just come back from the tyre fitters. We put the rears on a balancing machine, spun them at different speeds, and checked for both run out and flat spots on the rim. Both appeared fine with no buckling or damage, and both still in balance.

Tyre pressures all round are fine.

I also measured the distance between the front and rear hubs, ie R/o/s to F/o/s and R/n/s to F/n/s. There is a 2-3mm discrepancy, but I reckon that is within whatever tolerance these things are measured in, especially as the steering may have been slightly one way or other, and the distance is around 2300 millimeters.

Itsdashy - Thanks, All the arms were delivered at the same time. I will check them again for completeness, but I already checked them against the OEM ones that came off, and they were exactly the same length.

Infrasilver - Yes, I had driven a small amount. It was like this before, but not quite as bad. Engine mounts brand new. 964RS items. Checked them for security, and they appear fine.

The subframe mounts look OK, I will put a torque wrench on them and check for tightness.

Any other ideas? I'm thinking of stripping off the dampers tomorrow and getting them all checked....

Cheers
 
Try some alternative tyres.

My C2 had a fairly noticeable pull under acceleration, and a shimmy/pull the other way on lift off due to the nsr tyre having 4mm less tread than the osr even after a full set of new arms, and geo set up by CG.

8mm diameter tyre difference makes the diff work hard, which gave the behaviour it did. As soon as I swapped the rear tyres, the problem went away.
 
Cheers,

Both rears are brand new Michelin sport pilots.....
 
Far from an expert but I reckon getting ALL the dampers checked is a good place to start.
As a separate note when I have had instability problems with my car or bikes (road & track) it is often the other end that is causing the problem. I.e. Rear instability may likely be a front suspension problem.
If the dampers are not the issue then I suspect you may need to go back to a standard geo to help with trouble shooting. Is the car attitude set correctly?

Good luck with finding a solution, it will be interesting to discover what the problem is, small things can make a big difference to handling.
 
You might want to consider booking the car in at Center Gravity.
They will systematically check it all over and a road test to diagnose and then rectify the situation.

Might be a long wait list, but highly recommended. Contact them via their web site or phone. Web site contact is usually quickly followed up. Phone may take longer as both Pete and Chris are in the shop floor working on cars.
 

New Threads

Forum statistics

Threads
124,354
Messages
1,439,445
Members
48,708
Latest member
JLav211
Back
Top