Advice on suspension/ ride quality please

[Pond]

So my 3 litre (2001) Z3 has a terrible ride quality.
It crashes over any imperfection in the road then bounces for a while after. It is tiring to drive and I find myself looking at the road surface more than I should and dodging everything I see. There are a couple of bits of road near my house that I won't drive down as the car feels so bad over them. Not ideal!

My car has small wheels (15") and large sidewall tyres; 65% of 215mm. As they are 215/65. I believe the 'bounciness' is due to the tyres, which I can't do much about apart from changing the pressures, which I have done constantly with little effect.
My car also has new BC coilover springs and dampers. These were necessary to reduce the arch gaps after the new body was fitted. The dampers are fairly basic, so only have adjustment for rebound (or is it pre-load?). The adjustment is on top of the mounts. I have tried the dampers on hard, soft and everywhere in between with little difference. For now I have the dampers dialed right back to their softest settings but the ride is still bad. I think the springs are very stiff, as this type of suspension is aimed at the 'sporty' market.

So I am now thinking the main problem must be with other parts of the suspension; ie pivot joint bushes and things like the beam bushes. I bought a new pair of beam bushes but haven't got them fitted as yet.

I don't have any benchmark for how the car should ride as I never drove it before changing everything. I don't know how good or bad the Z3 should be, but I'm sure it shouldn't be this bad.

I am planning on booking the car in to have the suspension 'checked'. The problem is at present I have to believe everything they will tell me, as I don't know any better.

So....my question is what are the most likely causes of the crashiness and poor ride quality for a 25 year old Z3 (taking the new springs and dampers into account), which presumably hasn't been touched for a long while, if ever? If it helps the front seems worse than the rear, but that may be just because that is the end that hits the road imperfections first?

Also....is there anything I can check myself (without a lift)?

 

[Peter1450]

Would not changing the weight of the car with the aftermarket body kit have any effect as the coil overs are designed for a standard car?
I refreshed my wife’s 3.0 a couple of years ago and it drives very smoothly on 17” wheels and new shocks/ bushes all round.

 

[Pond]

Would not changing the weight of the car with the aftermarket body kit have any effect as the coil overs are designed for a standard car?

It's not radically different in weight.
I am leaning towards suspension linkages/ bushes being worn and 'tight' but don't know at present.

 

[IainP]

The bushes in the rear arms don’t rotate in the arms, effectively they’re another spring. One of the reasons people prefer poly bushes, they’re fitted using silicone grease, (or should be), so both the bush and sleeve can rotate.

ALL aftermarket coilover packages use the same e36 spring rates, irrespective of engine size, or model. No matter what the adverts/tech spec say. They’re too hard for the rear of a Z3. Single adjustable dampers alter both bump and rebound, sounds like you need more rebound, which you can’t get because it’s giving you more bump (upward movement of the suspension), all that will do is make it really uncomfortable.
I suspect you need softer springs, probably closer to standard rate. Certainly at the rear. Then altering the dampers will actually have an effect on the springs. Going harder on the dampers will then resist the upward movement of the spring preventing the softer springs bottoming out, bump, but it’ll also resist it extending again quickly, rebound, and it won’t ‘boing’, as they’re doing at the moment.

I can’t see it being the tyres, they’d need to be balloon tyres at low pressure to ‘boing’ enough for the feeling you’re outlining.

 

[Delk]

I have BC coil overs on mine with 18 inch wheels and it drives fine. It is set for sporty so a bit stiffer then the other half's on Bilstein's.
Bouncing after sounds like worn out dampers but you say these are new. The adjustments on the BC dampers is pretty fine so it will not make a huge difference. Its more to balance the car and not make major changes. As an example my fronts are stiffer then the rear to get the back to not slide out as much.
Maybe your damper and ride height adjustments need to be looked at? There is a lot of videos online how to set them up.
The rear is pretty standard with coils and an adjustable length damper. You can always swap in any damper and give it a try since its independent of the spring. BC do make different spring rates for coils.

 

[Pingu]

I'd put your problem down to your tyres.

I've spent the last couple of days trying to find an animated example of what is going on, but I can't.

Here's an image. Hopefully you can imagine the effect of different values of spring and damper on the bounce of the car...

The force from a bump on the road pushes on the tyre compressing kt and is damped by ct. The resultant force pushes on the suspension which compresses ks and is damped by cs. Damping only happens when things are moving, so this can't be tested statically. The resultant force is "felt" by the car as a vertical acceleration.

Basically, your 15" tyres are too bouncy. You can test this by bouncing them on the ground. I expect your tyre to bounce higher than a low-profile tyre. The damping deterimines how long they bounce for. The spring determines how high they bounce. The bounce frequency is the resonant frequency. You need to try to match your suspension's frequency to oppose this.

Your standard suspension is set for low profile tyres, which are, in crude terms, almost a solid. You need to match your car's suspension's resonant frequency to oppose your tyre's resonant frequency, and provide the appropriate ride quality. This is a VERY complicated thing to do by design. I'd do it using different spring rates, variable damper settings and something like Belgian pavé road surface. Rears are easy to swap out, but fronts will be much more problematic.

Alternatively, just live with it .

 

[Duncodin]

If a car suddenly got bouncy I'd say dampers. But because you've changed things like tyres sidewalls etc then that's a possibility.

I had what I'd call 'crashiness' in my car too and that was down to the rear beam bushes. Going over a normal pot hole and the back would kinda skit and thump. I thought is was dampers so changed those but didn't help. Changed the beam bushes now that's gone.

I too have high sidewall tyres. (16") 195/65 on the front and 205/65 on the rear. But I'm currently trying "C" rated tyres. ie van tyres. Harder sidewalls to carry extra weight but, on a car, no bouncy-bouncy. Wouldn't fancy doing a track day with these on but for tootling round town they feel fine.

 

[Pond]

I too have high sidewall tyres. (16") 195/65 on the front and 205/65 on the rear. But I'm currently trying "C" rated tyres. ie van tyres. Harder sidewalls to carry extra weight but, on a car, no bouncy-bouncy. Wouldn't fancy doing a track day with these on but for tootling round town they feel fine.

I got the sizes wrong.....mine are 205/65/15.
I spent ages choosing tyres, as most of that size are van tyres. Van tyres were ruled out because of the power and speed of the car. Wasn't a good idea IMO.
There was not a lot of choice for cars in the size, so I went with Goodyear all seasons which are V rated.

 

[Pond]

Basically, your 15" tyres are too bouncy.

I think so too, but I would expect the ride to be 'soft but bouncy'. But the ride is 'CRASHY AND bouncy'. It will bounce over the slightest undulation and crash through the slightest imperfection (a 'standard' pothole is a nightmare).

Which is why I think it could be other suspension linkages being too 'stiff' (ie worn or failing). To my logic this would have a similar effect as having very stiff dampers. But I am guessing.
I can't do anything about the springs apart from release any preload by adjusting the locking collars. Then again, I don't want them to be loose and come out of their mounting buckets, especially the rears as they are separate so not really coilovers at all. They call them 'divorced coilovers' apparently!

 

[Pingu]

"Crashing" is usually down to worn out bushes.

I'd get underneath with a long lever bar and start trying to move stuff that shouldn't move.

Mine is also very hard and I need to crawl over speed bumps. I think the BC springs and coilovers are very harsh, but they are needed to allow the ride heights to be set.

 

[Pond]

"Crashing" is usually down to worn out bushes.

I'd get underneath with a long lever bar and start trying to move stuff that shouldn't move.

Mine is also very hard and I need to crawl over speed bumps. I think the BC springs and coilovers are very harsh, but they are needed to allow the ride heights to be set.

As I said I am going to book it in for a suspension 'check', as I don't really know what I am looking for and don't have a lift.
I do think the coilovers aren't helping, but I needed them as you say.

 

[Duncodin]

As I said I am going to book it in for a suspension 'check', as I don't really know what I am looking for and don't have a lift.
I do think the coilovers aren't helping, but I needed them as you say.

I'm not sure how many mechanics know how to check the back bushes because even when they're knackered they still look very snug. You can usually see them when standing next to the car looking down in front of the rear wheel. You can see the beam and the plate bolted below the beam.

If the beam is right down resting down on that plate then the bush is knackered. If they're the OEM bushes then there should be an air gap of a few mm above the plate as the rubber suspends the beam off away from that plate. If the beam is resting down on the plate then the rubber is worn out and you get that thump every time you go over a pot hole as the beam lifts and crashes back down onto the plate.

 

[Jam03]

As I said I am going to book it in for a suspension 'check', as I don't really know what I am looking for and don't have a lift.
I do think the coilovers aren't helping, but I needed them as you say.

Presumably people like this might be able to help https://www.spires-st.com/

 

[Pond]

I'm not sure how many mechanics know how to check the back bushes

I know the beam bushes need replacing. As I said, I have a new pair just haven't got them fitted yet. If other bushes need replacing I shall ask if they have the 'tool' to do the beam bushes aswell. If not, I will have to ring my local BMW specialist man, but he is very expensive and very busy.

 

[Pond]

Presumably people like this might be able to help https://www.spires-st.com/

Warwick is a bit far TBH.
I had the alignment done recently. I took it to a local place and the lad who did it seemed very knowledgeable on suspension, so I will take it back to them to start with.

 

[Duncodin]

Sorry to keep chipping in - you might find some mechanics don't want to do the beam bushes. The plates at the bottom, Schubstreben (ie push rods), have torx bolts into the chassis that usually round off and refuse to come out. I've heard on here some people say their mechanic refused to do them for fear of snapping the bolts but I've never actually heard of any snapping. But they often do need a 'rounded bolt extractor' and then replace them with new hex head bolts of equal strength.

I know it's a long long way but if I needed the rear end overhauling I'd book it in to the zed shed. You'll know it'll get done 100%

 

[Pingu]

Sorry to keep chipping in - you might find some mechanics don't want to do the beam bushes. The plates at the bottom, Schubstreben (ie push rods), have torx bolts into the chassis that usually round off and refuse to come out. I've heard on here some people say their mechanic refused to do them for fear of snapping the bolts but I've never actually heard of any snapping. But they often do need a 'rounded bolt extractor' and then replace them with new hex head bolts of equal strength.

I know it's a long long way but if I needed the rear end overhauling I'd book it in to the zed shed. You'll know it'll get done 100%

You need to make allowance for your car running at a different height...

If my car measures 600, then the value 577 should be input for the rear height. Sometimes they measure from the bottom of the rim to the wheel arch. You just need to do the maths.

I had to use alignment plates on my rear trailing arms to correct the toe and the camber. You may need them as well. They cost £80 and have to be welded to the rear beam. It's quite a bit of work to do it, and can't be done with the rear beam in situ.

The toe and camber adjustment plates are at the end of the video. (You won't need the strengthening plates)