RTAB on Z4M: bushings or limiter kit?

[TomK]

The limiter kit is designed to be used with OEM rubber bushes, not with a PU bush. A PU bush effectively acts like a limiter kit in some respects, although having had both I much prefer the oem + limiter to PU.
And yes the kit is effectively 4 big washers. http://www.turnermotorsport.com/p-964-turner-motorsport-rear-trailing-arm-bushing-limiter-kit-rtab-shim-kit.aspx

 

[Yorkie Z]

The limiter kit is designed to be used with OEM rubber bushes, not with a PU bush. A PU bush effectively acts like a limiter kit in some respects, although having had both I much prefer the oem + limiter to PU.
And yes the kit is effectively 4 big washers. http://www.turnermotorsport.com/p-964-turner-motorsport-rear-trailing-arm-bushing-limiter-kit-rtab-shim-kit.aspx

Might have to hop on the lathe and make up some aluminium ones.

 

[GuidoK]

A PU bush effectively acts like a limiter kit in some respects,

A PU bushing in the shape how powerflex makes them acts like a limiter kit in all the respects :wink:

 

[TomK]

A PU bush effectively acts like a limiter kit in some respects,

A PU bushing in the shape how powerflex makes them acts like a limiter kit in all the respects :wink:

Well it certainly didn't feel like that when I drove them back to back. I'm not talking about noise, I'm talking about the way the car handled.

 

[TomK]

At the very least poly will add bind during significant suspension movement, and this can make the handling feel odd (unexplained oversteer) as well as add significant stress to the suspension-to-chassis mounts (which is how they can fail over time).

Quoted from an earlier post in this thread. This kind of explains what I felt.

 

[GuidoK]

Then you must have had an install issue. If it's one thing the PU bushing doesnt do compared to oem busings is 'add bind'. It's in the nature of the design.

And how does the PU bushing 'add significant stress' precisely? :scratchhead:
The failure rate for oem bushings is much much higher than for the powerflex ones.

 

[TomK]

Only repeating a (potentially biased) technical description of what I felt http://www.vorshlag.com/product_info.php?cPath=1_106_111&products_id=59

Both my PU bushes and oem + limiters were installed by a very respected race shop over here who have done them both many many times.

They also don't use PU bushes in the RTABS for their race cars.


ETA: have you driven a car with brand new OEM bushes and the limiter kit? You might have felt what I mean then??

 

[GuidoK]

ETA: have you driven a car with brand new OEM bushes and the limiter kit? You might have felt what I mean then??

No, but what you're describing isn't a natural behaviour for the PU bushings, so there must have been something wrong in the install.
I don't have that problem, so... either I did it correctly and your shop didnt, or I did it wrong and it had the perculiar side effect that the things you're complaining about dont occur with that.

The vorschlag piece is just some marketing s**t for the stuff they make and sell..... I'm sure every manufacturer will say their product is the best and their competitors product is s**t...
The piece has no evidence.

The PU bushing acts as a bearing. It has no bind at all. It allows for full travel, if there wasnt a chassis on top, it would allow for 360deg travel.
It's the closest you can get to a pillowball without the harshness.

And it's no secret that PU bushings last much longer than the OEM ones that are known to fail. Somehow vorschlag says otherwise. So where do all those OEM bushings then come from? That alone says enough about the level on which that piece was written...

 

[TomK]

Well yes my shop could have done it incorrectly, or the many other people out there on the internet who have found the same thing also installed incorrectly. I find that unlikely though as you know how easy the PU bushes are to install. It's practically impossible to install them incorrectly. Clearly Turner motorsport (who I'm sure you'd agree a very respectable M car tuner) also don't know what they're talking about according to you?
Would you say this quote is completely factually incorrect "In the Rear Trailing Arm Bushing (RTAB) location on the rear suspension of an E36 or E46 BMW the bushing has to pivot on two axis. Unlike other suspension bushing locations that only have one axis of rotation, swapping in excessively stiff replacement bushings using materials such as Polyurethane can and will cause suspension bind. "

Also they're not saying that the PU bushes fail themselves, they're saying that that the additional loads cause the chassis components to fail.

Either way, I've driven both back to back and I hated the way The PU RTAB bushes made the car feel so much that I swapped them out after a month and went OEM + Limiter. Far happier with the way car handles and I accept the OEM bushes will wear out much faster, but I don't really care tbh.

 

[GuidoK]

Yes I know the arm rotates on 2 axis. but let me guess..... lateral movement is what? 2 degrees or so? Its minute
And it's polyurethane!. It's compliant. It's not solid like delrin or aluminium bushings.

And if you had done the installation yourself, if you had hands on experience, if you had moved the suspension without shocks/springs/wheels brakes, like I have, you would have known that the suspension wouldnt bind up. You would have known that the movement is smooth.
So yes, I'd say that that quote is factually incorrect. Not that it pivots on 2 axis (where the lateral axis is a very minute movement), but that polyurethane is excessively stiff and would bind. Because I know... first hand

It's like saying: the trailing arm moves on 2 axis. so it will bind on the limiter spacers..... True or false?
If someone would say that and I would quote that, it would still be a bunch of bullshit. If other people say something, it's not automatically true. Most people are full of s**t, so it's most likely s**t too :lol:

And about turner... you know that's that 'other complany' that makes limiter kits do you?

Fine that you don't like PU bushings, I don't care, everybody has their preferences, but don't try to underpin that with technical arguments that arent correct.
Or what other people say for that matter...most people on the internet say what other people have said. That way you dont have to think for yourself.

 

[TomK]

Yes I know the arm rotates on 2 axis. but let me guess..... lateral movement is what? 2 degrees or so? Its minute
And it's polyurethane!. It's compliant. It's not solid like delrin or aluminium bushings.

And if you had done the installation yourself, if you had hands on experience, if you had moved the suspension without shocks/springs/wheels brakes, like I have, you would have known that the suspension wouldnt bind up. You would have known that the movement is smooth.
So yes, I'd say that that quote is factually incorrect. Not that it pivots on 2 axis (where the lateral axis is a very minute movement), but that polyurethane is excessively stiff and would bind. Because I know... first hand

It's like saying: the trailing arm moves on 2 axis. so it will bind on the limiter spacers..... True or false?
If someone would say that and I would quote that, it would still be a bunch of bullshit. If other people say something, it's not automatically true. Most people are full of s**t, so it's most likely s**t too :lol:

And about turner... you know that's that 'other complany' that makes limiter kits do you?

Fine that you don't like PU bushings, I don't care, everybody has their preferences, but don't try to underpin that with technical arguments that arent correct.
Or what other people say for that matter...most people on the internet say what other people have said. That way you dont have to think for yourself.

All I'm going on is what I felt when I had them installed. There's no way they were installed incorrectly, and to me as I've said I found they negatively impacted the handling of my car.
I'm not suprised the suspension didn't bind up when you moved things by hand on the ramps, that's hardly the same scenario as a full loading generated in cornering. To be honest I'm more inclined to believe the 'factual' explanation from these 'two bit' companies just trying to sell me s**t than you guido. Perhaps I am the idiot, but I would suggest you try the car with the limiter solution before being so definitive about it.

 

[GuidoK]

I don't have the problems you mentioned, so why would I try the limiter rings?
IF there would be bind, there would be wear. And there isn't. It's that simple.
Also your own quote says that the bind would come from 2 axis pivot. The forces of the suspension dont have an effect on that. it;s either there or it isnt. Or the force would immensely bend the trailing arm. That;s rather unlikely.
And if there was binding, you think those forces, or the thousends of pounds of spring pressure would be influenced by that? If there was binding, those forces would tear the polyurethane to pieces. And as that doesnt happen......
You mustn't believe information from companies who have an agenda. Only a fool does that.
You're much better off using your own intelligence..

This has nothing to do with what preference you have, but all with what reasons you give, based on 'what other people say'.

 

[Scooba_Steve]

I don't have an M. But I do have poly RTABs and I'm very happy with them. No noise and much better.

All I'd say is I take a pinch of salt with what the manufacturer of an expensive limiting kit says about poly competitors who do an almost identical job at a lower price point.

Doesn't mean the limiters aren't better. My purchase reason was I couldn't be bothered with the pre-load faff and cost.

 

[TomK]

Well each to their own I guess.
If you are worried about cost then use the rogue ones... Significantly cheaper than their PU rtabs. Perhaps that's why they don't market them as much?
Undoubtedly it is much more of a faff to install oem rtabs than PU rtabs, most shops don't have the tool.
http://www.rogueengineering.com/rogue/S_BUSH/RTAB_SHIM.html

 

[Scooba_Steve]

I picketed up poly RTABs for about £45 - so with shipping, customs and new OE RTABs there probably wouldn't be much in it.

I was DIY'ing so I couldn't be bothered with preload! As you say, each to their own but at least this place helps inform decisions.

 

[GuidoK]

Significantly cheaper than their PU rtabs.

I've seen pictures of rogue rtabs without knurling on the inside. They market their 'grease trap', a small chamfered ridge on the outside, but a quality manufacturer like Powerflex has knurling inside the bushing, that traps grease where it matters (the inside pivot point, right on the inner stainless core). The outside edge of the bushing basically functions the same as the limiter spacers. lubrication on there is less of an issue (but it cant hurt obviously)
AKG (another american rtab producer) also lacks that knurling. Maybe it's a difficult or expensive thing to achieve, I don't know.
Imho powerflex is still (one of) the best poly bushing manufacturer. Strongflex also uses the knurling and is a cheaper alternative with a larger product range for the z4 (they also have bushings for the other rear suspension arms).

I suspect that rogue, akg and the like don't manufacture their products (the poly bushings I mean) themselves and go to a generic supplier that does things to their specs. Their range of poly bushings is obviously tiny compared to powerflex or strongflex (they make thousends of different bushings). Maybe that experience shows in the know how put in the products.

 

[TomK]

The PU bushing acts as a bearing. It has no bind at all. It allows for full travel, if there wasnt a chassis on top, it would allow for 360deg travel.
It's the closest you can get to a pillowball without the harshness.

http://www.elephantracing.com/techtopic/binding.htm

I suppose they could be talking bu###hit as well? I'm no engineer so all I can say, as before, is that I felt these characteristics when I had PU bushes (powerflex). Seems I'm not the only one otherwise why would people make these things?

 

[GuidoK]

Well, I don't feel it, and I've checked it first hand. What can I say? I had no problem moving the suspension over the complete travel. Either it binds like s**t and I'm hulk hogan or it doesnt bind.
Remember, not all PU bushings are solid, most are compliant just as normal rubber.
So how did you feel this? did your suspension didnt spring back? I mean when it binds, it also locks and seizes. And it would make a heck of a noise.
In theory the suspension can also bind on the limiter rings when the movement is too big (althoug the chances are slim, as I said the sideway movement is probably less than 2 degrees)

But yes, that link talks utter bullshit. They talk about polyurethane as if that is one substance with 1 physical property. And if you do that, you're stupid as s**t, or you're trying to convince people that are stupid as s**t.

A polyurethane bushing with shore80A or so is about as soft as normal suspension rubber. So it will act exactly the same.
A polyurethane bushing with shore90A is a bit stiffer but still compliant.
A polyurethane bushing with shore75d (or higher) is hard as rock (well, hard as delrin or nylon) and is hardly compliant.
all of the above should always be considered when changing bushings.

But hey, stupid people always look for simple straightforeward answers that give a single answer. Please no nuances. that requires thinking!
Oh and let me guess... that shop sells rubber bushings and no polyurethane bushings. Doh!

If it would bind, the core would tear itself into the polyurethane and slowly destroy the bushing. Suspension forces are huge when stopped (by binding or otherwise), and I mean HUGE. No material can withstand that, so it will break or damage. And the fact is that that doesnt happen.

 

[TomK]

I'm sure they could sell PU bushings if they wanted to.
ETA:
I have no issues with using PU anywhere else on the car, just the RTAB with it's 2 axis movement that you don't think is a problem.

Sorry to answer your question, the way I felt it was with less confidence (predictability) in the rear at high loads. I can't describe it any better than that I'm afraid, but it was a direct back to back compare.

 

[GuidoK]

I'm sure they could sell PU bushings if they wanted to.

Maybe, but they dont. And it shows in their explanation that lacks any nuance. They regard pu as solid. And it isn't. that makes it very biassed.
You can make it virtually solid with PU, but you can also make it as compliant as normal rubber, or anywhere inbetween. And they completely pass over that important fact, as it dictates if you have a solid busing or a bushing that is as compliant as rubber.
Would I advise pu bushings harder than shore 95A? never. Actually nowhere on the car. both the suspension and chassis mounting points are not build for that. In fact I wouldnt use anything over shore80a directly on the chassis (rear subframe bushings and front arb bushings)
Tears in the chassis and on welds (metal fatigue) are difficult and expensive to repair.

I have no issues with using PU anywhere else on the car, just the RTAB with it's 2 axis movement that you don't think is a problem.

There are more places on the car that have 2 axis movement. In fact all the rear suspension arms on all the hingepoints.
And I've completely polybushed all of them! (with shore90a)
And it still moves smoothly on the complete arm travel. Smoother than with rubber bushings.

I have no predicability issues nor have I ever heard that from anyone who has driven my car.
My experience was that with polybushing the rear the car wants to steer/turn more eager (not like oversteer btw, no grip issues). Like the stock suspension drags behind the car. If that's an effect of the tramlining with the stock bushings or some other aspect of the geometry I dont know.