Poly bushes - good or bad?

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Bertrand
Joined: Fri 14 Mar, 2014 17:34
Posts: 16

  Z3 roadster 2.8
Location: Loughborough

Poly bushes - good or bad?

Post by Bertrand »

Just had Poly bushes fitted to Z3 2.8 to replace the rubber diff mounting bush and rear beam bushes.
I've had one garage - Indy BMW specialist - say that this is causing a knocking sound coming from the rear of the car.
The garage that fitted them says that the bushes will have made things better by tightening up the back end and removing the flexing from the rubber bushes.

Anyone else had any experience of fitting the Poly bushes - good or bad?
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Dino D
Joined: Fri 10 Feb, 2012 16:59
Posts: 376

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by Dino D »

I have done the same, night and day difference in handling, gearshift and 'pull'. My bushes were very worn.

Even from new the Z3 had a 'steer from the rear' feeling where you have to wait for the rear to settle in a corner (as it feels like it moves by itself rather than your input, not a trait I ever liked). This is all gone and the car is confidence I spring (I do have different suspension and tyres mind).

Wether it is 'good' or 'bad' is purely subjective. For me the pluses out weigh the negatives (more road noise, diff noise).

How do you find it? That's what matters at the end if the day!
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pingu
Joined: Fri 30 Apr, 2004 16:01
Posts: 3412

  M roadster S50

Re: Poly bushes - good or bad?

Post by pingu »

My thinking is that bushes are meant to flex.

Ask yourself "What would happen if you welded the diff to the crossmember?" This is effectively what you do if the bush is too hard.

Hard bushes are good if the body they are mounted to is strong enough to take it. They are also OK for bushes that are only used to support a twisting rod or pin (ARB bushes and the bushes at the rear of the lower front wishbones).

I wouldn't use a hard bush on the diff as I don't think the crossmember is strong enough to take the undamped forces.
Pingu
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Southernboy
Joined: Thu 07 Oct, 2010 12:39
Posts: 6436

  Z3 roadster 3.0i
Location: Johannesburg

Re: Poly bushes - good or bad?

Post by Southernboy »

I go with Pingu..... bushes are meant to flex. If you want to achieve less roll and travel, fit sport shocks... :wink:
"Normal is overrated"
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deni2s
Joined: Mon 25 Mar, 2013 22:02
Posts: 761

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by deni2s »

I know that on bimmerforums.com they came to conclusion (Randy Forbes also supported), that on rear diff stock bush is better than poly, because poly causes too much vibration. Actually I have poly bush on my cars diff, because stock bush wasn't available, when it needed to be replaced and I can say, that I clearly feel much more vibration on idle. I can compare the feel to diesel engine running in older cars.

Everywhere else poly bushes (I used Powerflex standard bushes, not the hard black ones) definitely improved feedback to the wheel and overall driving experience. Driving on uneven road I was even worrying that something is wrong because my steering wheel vibrated much more than usual, but was just better feedback to the steering wheel.

I think, maybe now when you stiffened your suspension with poly bushes, some other weak spot showed itself and that is what that indy is hearing...
Dino D
Joined: Fri 10 Feb, 2012 16:59
Posts: 376

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by Dino D »

I have sports shocks and springs. That doesn't remove the movement of the rear subframe when cornering...in fact I think it highlighted it as the car was planted but this strange moment would occur.
Coming from great handling FWD cars I put it down to RWD but turns out that's not the case. My Z is very confidence inspiring now, so planted and the rear follows the front perfectly now.

I've read the debates and theories about the diff bush but the reality (as seen on the forums) is that it is the cars with standard bushes that suffer popped welds...

Powerflex purple are not solid bushes. I believe the black are basically solid. I'm using purple.

Whilst there is increased diff whine it pleasant rather than overpowering like on WRC car. I have no vibrations coming through, it justy feels more mechanical and direct but no vibes...not sure why yours does deni2s....

Just re-read the OP and I see you have a noise after the fitting of the bush?
I had a rattle before fitting and that was due to the worn standard bush. The rattle is gone but with the Powerflex purple there is 'clunk' noise when engaging gears only noticeable at low speeds- it's there on the standard cars, even from new but it's more noticeable with the poly bush.

I think Southernboy and Lee were talking about this 'clunk' and had traced it to some slack inherent in the drivetrain.

I had this clunk on a fairly new e46 330i too, it was barely audible but I knew it was there, just part if the charm I thought!
Del
Joined: Sat 19 Nov, 2011 18:35
Posts: 2136

  Z3 roadster 1.9

Re: Poly bushes - good or bad?

Post by Del »

Most Z3s are now so old that they would benefit from replacing every bush (OE rubber or otherwise) underneath. I've fitted poly bushes (powerflex purple) to the main beam and differential because the still "MOT compliant" 1998 bushes were in a poor state and the powerflex ones are very easy to fit. The powerflex bushes struck me as very well made and when examined closely they are quite flexible - the powerflex diff bush was almost as easy you manipulate and bend by hand as the bush that came out. The rear beam bushes really made a big difference to the car - it feels more like an "arrow" now on motorways and feels much more stable - the last remnant of tramlining has disappeared. With respect to Randy Forbes neither he nor other layperson is in a position to categorically confirm that my rear diff is any more likely to tear itself from the car because of replacing these slightly less "bendy" bushes. The BMW mantra of course (for legal, rather than any engineering reason) will be that OE is best - this would even apply to their seat bushes. :D
Bertrand
Joined: Fri 14 Mar, 2014 17:34
Posts: 16

  Z3 roadster 2.8
Location: Loughborough

Re: Poly bushes - good or bad?

Post by Bertrand »

Appreciate all the replies - even thoughI am now slightly more confused than before!
Before the poly bushes were fitted I did have a clunk when selecting gears - especially going from first to second.
After the poly bushes were fitted the sound is like a bag bolts when I select a gear - to be honest it is too unpleasant to drive.
I also noticed a whine that I hadn't heard before - one of the replies mentions this is due to the diff/poly bushes.
I don't actually drive the car that hard, I bought it more for the styling and engine than performance, so stiffening the backend wasn't really an issue for me, it was more to get rid of the rough gear change.

On balance, I think I will get the garage to remove the poly bushes and fit new standard BMW rubber bushes.
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Dino D
Joined: Fri 10 Feb, 2012 16:59
Posts: 376

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by Dino D »

I think you issue is more than bushes. Based on my experience of the Powerflex Purple I removed the rattle and have a gearshift like new. Sure there is more diff whine and a more mechanical feel to engagement but certainly not making it unejoyable.

I'd say in any even the diff bush is not for you if the small increase in diff whine is unpleasant for you.
I think there must be sometime else needing attention, maybe the 'giubo', also a item the needs renewing at this age.

As for the rear beam bushes, those help the feel of rear to no end but only contribute a bit of road noise in my experience.
As it is many hours labour (costs!) to get those in and out I would say leave those until you have the gearshift sorted and see from there.

Do you know which brand of bushes were fitted?
deni2s
Joined: Mon 25 Mar, 2013 22:02
Posts: 761

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by deni2s »

Bertrand wrote:Appreciate all the replies - even thoughI am now slightly more confused than before!
Before the poly bushes were fitted I did have a clunk when selecting gears - especially going from first to second.
After the poly bushes were fitted the sound is like a bag bolts when I select a gear - to be honest it is too unpleasant to drive.
I also noticed a whine that I hadn't heard before - one of the replies mentions this is due to the diff/poly bushes.
I don't actually drive the car that hard, I bought it more for the styling and engine than performance, so stiffening the backend wasn't really an issue for me, it was more to get rid of the rough gear change.

On balance, I think I will get the garage to remove the poly bushes and fit new standard BMW rubber bushes.
Clank when selecting gears - that for sure was worn rear beam bushes, and I mean really worn! Had the same problem, powerflex purple poly bushes fixed that all.

I would only change diff bush to bmw rubber bush, and look for other problems elsewhere. If beam bushes are fitted correctly, they are not the problem source you are looking for.
Bertrand
Joined: Fri 14 Mar, 2014 17:34
Posts: 16

  Z3 roadster 2.8
Location: Loughborough

Re: Poly bushes - good or bad?

Post by Bertrand »

If anyone is still interested....
In the end I had the poly bushes replaced by the genuine BMW ones. No more whine.
I also took the advice of the garage and replaced the gearbox (breakers one) and everything is now fine!
I had been told by everybody that the gearbox in these cars was bullet proof, so was reluctant to change it initially.
Car has been to France and Italy and despite extreme heat, no problems at all.
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littlefeller
Joined: Sun 28 Apr, 2013 18:06
Posts: 683

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Re: Poly bushes - good or bad?

Post by littlefeller »

did you try driving it with the rubber bushes and before the box change? if yes what was it like in comparison? I fitted polly to the front but not to the rear (hadn't gotten around to it yet) suppose you could argue that if polly is better then why doesn't bmw ( or anyone else come to that) fit them from new. I noticed an instant improvement after I swapped the fronts, but the original rubbers were shot so anything would have been better, I also needed to change the drivers side lower arm hence the swap to polly ( its really difficult getting the rubber bushes into that steel carrier, I tore one just trying).
Bertrand
Joined: Fri 14 Mar, 2014 17:34
Posts: 16

  Z3 roadster 2.8
Location: Loughborough

Re: Poly bushes - good or bad?

Post by Bertrand »

littlefeller wrote:did you try driving it with the rubber bushes and before the box change? if yes what was it like in comparison? I fitted polly to the front but not to the rear (hadn't gotten around to it yet) suppose you could argue that if polly is better then why doesn't bmw ( or anyone else come to that) fit them from new. I noticed an instant improvement after I swapped the fronts, but the original rubbers were shot so anything would have been better, I also needed to change the drivers side lower arm hence the swap to polly ( its really difficult getting the rubber bushes into that steel carrier, I tore one just trying).
I actually had the bushes replaced and the gearbox done at the same time - it was Friday afternoon and I was driving the car to France the next morning!

Just my opinion, but I think that the car is designed to have some slight flex in the transmission system, hence BMW fit rubber rather than poly bushes in the first place.

i am quite happy having a slightly softer feel to the handling, it goes with the car for me as I didn't buy it to sprint round the country lanes, otherwise I would have got a more modern Z4.
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Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by Hairyscreech »

This is quite a contentious issue but I hear it a lot and there are some factors that are just never considered.

The first thing is to determine if the bush needs rotating motion or flexture, if it has to flew then hard poly bushes will actually reduce traction by creating stiction and an undamped spring rate.

Poly bushes MUST be periodically lubricated in rotating applications or rapid wear and increased friction will be the result. Prodrive for one do not like Poly bushes as an unlubricated poly bush has higher friction than a rubber bush.

For flexing bushes, Rubber has its own inbuilt version of damping, poly urethane does not have this. (well it does but it's different and a complex subject). Urethane also comes in several grades of hardness and not all poly bushes are equal.
Most polybushes are 90a which is way too hard for the application.

The Z3/E30/E36 compact application my take is to use the solid rubber E30M3 (eccentric) or Z3M (concentric) bushes for the front wishbone lower rear bushes.

The Rear beam and diff bushes work in concert to locate the rear beam, It is unwise to stiffen just one of the bushes as it will introduce extra loading on that bush and through the mountings off the diff (seen more than one cracked beam, especially where the two top diff bolts go in). The rear beam has an amount of passive rear steering built in, unfortunately this was designed initially for the E30 on 80s tyres and a Z3 or E30 with modern tyres has way more grip than the bushes can cope with, this causes the flexy rear feeling as the bushes deflect a lot more than needed.
The best solution to this is to use either 75a durometer polyurethane bushes (hard to get) or to fill a stock bush with liquid urethane (again 75a). This stiffens the bush and makes it behave like it is solid rubber (75a poly is about as stiff as the stock rubber).
I personally prefer the void filled rubber as it still keeps its internal damping, it is worth noting that for the E21 BMW motorsport use to offer solid rubber mounts for the beam and as noted above in traction limited applications some companies still stick with rubber.
This is the route I took on the E30 rear end below (Hybrid with Z3 parts) and the E36 Compact race car I built a while back, the proof in the pudding was a £6k home built S50 powered compact lapping thruxton at the same pace as big money tuning company supported Z3M.
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For the trailing arms above I stuck with rubber for the reasons above. They are so small they make no change to feel and for the friction/squeaking reasons just are not worth it.

Poly has applications but they are limited. What they are is cheap to make aftermarket and easy to fit, that's the main reason they are everywhere in the aftermarket.
Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Poly bushes - good or bad?

Post by Hairyscreech »

littlefeller wrote:did you try driving it with the rubber bushes and before the box change? if yes what was it like in comparison? I fitted polly to the front but not to the rear (hadn't gotten around to it yet) suppose you could argue that if polly is better then why doesn't bmw ( or anyone else come to that) fit them from new. I noticed an instant improvement after I swapped the fronts, but the original rubbers were shot so anything would have been better, I also needed to change the drivers side lower arm hence the swap to polly ( its really difficult getting the rubber bushes into that steel carrier, I tore one just trying).
Just a note here, everytime I have done one I have just pressed the metal cased bush into the lollypop off the car by putting them in a large vice, never had an issue that way, You have to file a tiny lead in chamfer to start the bush (like 2mm) and makes sure the lollypop is not rusty and they should just press in.
Joycey
Joined: Mon 11 Jul, 2011 22:15
Posts: 290

  M roadster S52
Location: Basingstoke

Re: Poly bushes - good or bad?

Post by Joycey »

I cant help but think your clunking is coming from somewhere else (which would sound louder with poly bushes). I fitted the Purple poly bushes to my M 25,000 miles ago and those of you who know me know that my car is driven to its full. But like with all cars if you uprate one component the stress will move to another weaker component.

About 3,000 miles ago I started getting a loud clunking noise in the rear

What I found was the pre load in the Diff pinion bearing had released. Weather it was caused by age or my driving style I don't know but the two tapper bearings had worn.

Over on .org I did a full write up and videos showing the clunk noise. I removed the diff and got a company to overhaul all the bearings and seals for £240.

We had a discussion about the clunk in the back of Z3's and it looks like this will start to be the main cause from here on out, after all they are reaching 17 years plus.

Lee
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