Rear axl bump steer?

[Rene_Z4MC]

Hello fellow Z drivers,

I'm new to this forum so quick intro;
Z4 M Coupe 2006 owner for 6 months, car is original except for Eibach Pro kit and CDV delete (both DIY), has ~40kmls on the clock.
Here a snap of me in my ///M;
http://i.imgur.com/3KbYxWJ.jpg

Now my question; from the first day of ownership I been struggling with the handling of the car, it just doesn't seem right.
It is difficult to describe but I'll give it a try; the car is really sensitive to uneven road conditions, there is a strange wobble in the chassis when you hit a small pothole or even when cross the paint of road indications (not sure what they are called). It's worse when you are in a corner, feels like bump steer in the rear axl.

I've replaced the rear trailing arm bushes (RTABs) and as mentioned before installed Eibachs. Also I got new tyres (PS2), this reduced the effect a little but didn't cure it. Recently had a full geo but effect is still there.

I'm beginning to think that it might be the 'character' of the M setup.

Does this sound familiar to any of you?

René

ps. sorry for the poor English, its not my native language.

 

[Rene_Z4MC]

Nobody? :?

 

[Beedub]

I'd replace the RTAB ..... While your Thier stick in the turner bush limiters


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[exdos]

+1 for the rear trailing arrm bush limiter kit. There's a lot of space between the arm and the bracket which permits a lot of unwanted lateral movement. I fitted this kit and I posted an illustrated "how to" for the Turner kit here (half way down the page): http://www.z4-forum.com/forum/viewtopic.php?f=9&t=38918&start=105

There's a Rogue version too, which is cheaper and simpler to fit, which should do the job equally as well.

 

[dario]

DOn't forget that the PS2 from new needs lots of time to bed in. Mine used to take between 500 and 1000 miles before i hand any confidence in them. after that

 

[Rene_Z4MC]

Wow thanks guys

I've already replaced the RTABs since I suspected them to be the cause of the issue but the 'old' bushes I took out where as good as new so I didn't expect it to do much (which was true), if only I had known about the limiters, will fit them for shure.

Is this strange RTAB wobble feeling a 'known thing' then within the BMW community? I'm new to the scene so I wouldn't know.
My previous car was an Elise S1. This car is, as you might know, very precise so maybe I'm too critical on the ///M.

question for Exdos;
Whats the best way to prevent pre-loading of the bushes when tighting the main RTAB bolt. I did put some markings on the bush vs bracket when the car was still on its wheels but found this difficult to align once the bracket was out. Any tips?

again thanks for the replies!

 

[andysat]

Hi and welcome

 

[Rene_Z4MC]

Thanks Andy

 

[Beedub]

Wow thanks guys

I've already replaced the RTABs since I suspected them to be the cause of the issue but the 'old' bushes I took out where as good as new so I didn't expect it to do much (which was true), if only I had known about the limiters, will fit them for shure.

Is this strange RTAB wobble feeling a 'known thing' then within the BMW community? I'm new to the scene so I wouldn't know.
My previous car was an Elise S1. This car is, as you might know, very precise so maybe I'm too critical on the ///M.

question for Exdos;
Whats the best way to prevent pre-loading of the bushes when tighting the main RTAB bolt. I did put some markings on the bush vs bracket when the car was still on its wheels but found this difficult to align once the bracket was out. Any tips?

again thanks for the replies!

i really dont get this wobble feeling..... i turn the car goes.... hitting a bump mid corner doesnt upset that car as such, however in my track setup car it will skip across the road on a big enough bump...... but its all solid movements, no wobble as such, look towards your tyre pressures, this car is reallly sensitive to changes, but equally, get it up in the air and check everything over, MY arb brackets were FINGER tight after a few trackdays.........

 

[exdos]

Is this strange RTAB wobble feeling a 'known thing' then within the BMW community? I'm new to the scene so I wouldn't know.
My previous car was an Elise S1. This car is, as you might know, very precise so maybe I'm too critical on the ///M.

question for Exdos;
Whats the best way to prevent pre-loading of the bushes when tighting the main RTAB bolt. I did put some markings on the bush vs bracket when the car was still on its wheels but found this difficult to align once the bracket was out. Any tips?

I know exactly what you mean. Personally, I hate the OEM suspension set up because IMHO it's totally unbalanced and I knew that I couldn't live with it in OEM format long before I bought one and I replaced it with AC Schnitzer Racing adjustable suspension within 2 months of buying my Z4MC.

You won't preload the bushes because when you torque the bolt through the bracket, the metal sleeve inside the RTABs takes all the load. The limiters simply sit inside the brackets to stop the rubber bushes moving laterally on their metal sleeves that you are trying to correct. IMHO, the brackets should have been made narrower, as if they had limiters. EDIT > Once I had refitted the trailing arm into the bracket and loosely tightened it in place, I then loosely fitted the bracket to the chassis and then lifted the trailing arm so that it was at a position that I thought was about correct for the normally loaded position and I marked the bracket accordingly, then I properly tightened the trailing arm into the bracket in that position, which should ensure that the preload on the rubber is neutral(ish). < EDIT

Before I undid the three bolts that retain the RTA bracket, I painted a few spots on the edge of the bracket which also marked the chassis in the same places, so that when I refitted the brackets, it was just a simple job of lining up the paint marks and rebolting the brackets in the exact same position.

A further thought. Are your tyre pressures correct? From my experience of Falken 452s on my Z3MC, I know that when the car is turning hard on track, I can feel the car moving laterally more than the tyres because the rear tyre walls sway laterally, and once they're fully distorted, the car feels as though it has rear-wheel steer by the throttle.
Hope this helps,

 

[geepee]

I think they all do it to one extent or another.
Have a read of this article and compare to your own experience.

http://www.evo.co.uk/carreviews/evolongtermtests/46372/bmw_z4_30i.html

But is the Z4 really a sports car? Now that's where the controversy lies. Our Z4 featured BMW's sports suspension as standard, and I really wish it hadn't: the ride's simply dreadful. It dominated every journey and became incredibly irritating over time. There seemed to be no handling benefits on offer to offset the constant pummelling that drivers and passengers received when on the move. It had none of the precision you get with a Boxster, for example, nor the control you'd expect from such a firm set-up. In fact, Honda's recently revised S2000 felt like a limo in comparison.

The other problem was that unless the tarmac was super-smooth, the Z4 seemed to wander from bump-to-bump, sending confusing messages to the driver as to what was really going on. It felt like an advanced form of tramlining and became christened the 'Thrill of Writhing' in the office. At first I thought it had to be the fault of the standard run-flat tyres, so after the Z4's disappointing performance at last year's eCOTY (063), we swapped the Bridgestones (having covered 11,000 miles, they were around 60 per cent worn) for a set of Vredestein Ultracs. The initial ride coarseness certainly improved on the Ultracs, while wet grip was transformed. But even so, the Z4 never felt really relaxed unless the road was glassy smooth.

The real culprit was finally revealed on the Mercedes SLK launch last month. Mercedes brought in a Z4 for evaluation during the SLK's development and had noticed the same strange chassis behaviour that we'd experienced. After putting the car on a chassis rig, the explanation seemed to be a noticeable change in rear toe angle when the Z4's rear suspension moved up and down, meaning it would start to 'steer' from the rear on undulations. Now, increased toe-in on compression is great on track as it tucks the car into the apex, killing understeer in the process, but pretty useless on our lumpy roads as the change in toe angle can cause the constant wandering we'd experienced as the rear wheels steered their own independent course to the fronts.

 

[Rene_Z4MC]

@exdos;
Thanks for the edit, that is the way I did it when replacing the RTABs.
I've read your thread about tweaking the suspension, really interresting stuff. Nice to hear about a fellow Z4 M owner questioning the OEM setup, I noticed that its a big NO NO to have comments on the standard setup ("you think you know better than the M engineers, blah blah"), its almost like swearing in church.

@geepee;
"...the explanation seemed to be a noticeable change in rear toe angle when the Z4's rear suspension moved up and down, meaning it would start to 'steer' from the rear on undulations."
This exactly describes what I feel when driving the Z, hench the title of this thread.

I did not expect this kind of behaviour when buying an M car (wobbling, kicking, nose lift/dive, vague steering), was expecting a precision tool but he, with a little help from this forum who knows where it will end :wink:

Thanks for the feedback!

 

[Rene_Z4MC]

i really dont get this wobble feeling..... i turn the car goes.... hitting a bump mid corner doesnt upset that car as such, however in my track setup car it will skip across the road on a big enough bump...... but its all solid movements, no wobble as such, look towards your tyre pressures, this car is reallly sensitive to changes, but equally, get it up in the air and check everything over, MY arb brackets were FINGER tight after a few trackdays.........

Are you running OEM setup or did you mod anything to get the handling you describe?
I'm not sure what you mean by 'finger tight brackets', can you elaborate on that?

 

[Rene_Z4MC]

What I don't understand is that in most reviews you read/hear complaints about the Z4M's handling while in other reviews they rave on about how fantastic the E46 (CSL) is. I know it's a different car but the bits that matter (suspension components) are almost identical.
One difference I noticed is the lower wishbone bush mounting to the rear trailing arm, on a CSL this is a proper ball-joint, has anyone tried that component in a non CSL car? just a thought.

 

[Beedub]

i really dont get this wobble feeling..... i turn the car goes.... hitting a bump mid corner doesnt upset that car as such, however in my track setup car it will skip across the road on a big enough bump...... but its all solid movements, no wobble as such, look towards your tyre pressures, this car is reallly sensitive to changes, but equally, get it up in the air and check everything over, MY arb brackets were FINGER tight after a few trackdays.........

Are you running OEM setup or did you mod anything to get the handling you describe?
I'm not sure what you mean by 'finger tight brackets', can you elaborate on that?

Complete suspension upgrade ... From track spec coilovers right down to the bushes ......

What I'm saying is... Things get loose with hard use, check tq specs to eliminate any loose fittings.


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[exdos]

What I don't understand is that in most reviews you read/hear complaints about the Z4M's handling while in other reviews they rave on about how fantastic the E46 (CSL) is. I know it's different car but the bits that matter (suspension components) are almost identical.

I did not expect this kind of behaviour when buying an M car (wobbling, kicking, nose lift/dive, vague steering), was expecting a precision tool

Nice to hear about a fellow Z4 M owner questioning the OEM setup, I noticed that its a big NO NO to have comments on the standard setup ("you think you know better than the M engineers, blah blah"), its almost like swearing in church.

If you've ever driven a Z3MC you'd soon realise that that car is far more of a handful than a Z4M because it also has unsuitable suspension but because it's too sloppy and there's huge amounts of lateral body roll. Most Z3MC owners who push their cars acknowledge this fact and suspension modification has now become normal practice. I think that once the Z4M is a few years older the third, fourth and fifth owners etc.will consider it more acceptable to mod their cars and then I expect to see more people upgrading their suspension. F**k! Have I just sworn in church?

 

[daz05]

I did not expect this kind of behaviour when buying an M car (wobbling, kicking, nose lift/dive, vague steering), was expecting a precision tool but he, with a little help from this forum who knows where it will end :wink:

I don't remember mine wandering at the rear to the extent your saying but it was an issue. I remember an adjustment in my alignment really helped when i reduced rear toe and rear camber. A lot of the issues come from the suspension, unbalanced as Exdos says, too soft at the front too stiff at the rear which also causes the diving and light vague steering.

Driving my E92 on the same local roads is a far easier experience that's for sure. The Zed has great character though so I can see why some chose to make improvements.

 

[ChawenHalo]

steering wobble is often a squared Wheel (potholes hitting t(he Wheel etc. Car should not fell skittish for sure. I suspect that the Eibach Pro kit installed in isolation will have afected your suspension's geometry. Worth getting it checked by a pro, although you're in good hands on this forum. good luck.

 

[bennyboysvuk]

What I don't understand is that in most reviews you read/hear complaints about the Z4M's handling while in other reviews they rave on about how fantastic the E46 (CSL) is. I know it's a different car but the bits that matter (suspension components) are almost identical.
One difference I noticed is the lower wishbone bush mounting to the rear trailing arm, on a CSL this is a proper ball-joint, has anyone tried that component in a non CSL car? just a thought.

Coming from an E46 M3, I'd say that the Z feels very different in its handling. The M3 felt very much like it slides as one piece, very neutrally and either end can be tweaked to make it under or oversteer, but overall it feels very balanced. By comparison, the Z4M feels to me like the front and rear are completely separate entities and it doesn't quite feel all-of-a-piece. That said, I've yet to set the Z4M geo up a bit more aggressively, so that will probably help.

As for the problem in the original post. I've only had one really snappy oversteer moment in the Z that seemed uncharacteristic. It was on a wet road and I had the DSC off. I was doing 60 ish whilst on the power fairly hard, the left hand corner was just starting to open up and the surface had a short compression, as the car hit the end of the compression it snapped sideways by about 30 degrees, pretty suddenly. I didn't really think about bump steer at the time, but it kind of makes sense, although having played with the Z quite a bit, I don't find it very often suddenly relents its grip. When driving it hard in the dry it feels quite flowing and I find generally that I'm managing understeer and not really worrying very much about oversteer.

 

[exdos]

Coming from an E46 M3, I'd say that the Z feels very different in its handling. The M3 felt very much like it slides as one piece, very neutrally and either end can be tweaked to make it under or oversteer, but overall it feels very balanced. By comparison, the Z4M feels to me like the front and rear are completely separate entities and it doesn't quite feel all-of-a-piece. That said, I've yet to set the Z4M geo up a bit more aggressively, so that will probably help.

I think the difference in the perception of handling characteristics between the E46 M3 and Z, with essentially the same suspension, is largely down to the seating position of the driver relative to the axis of rotation of the car with respect to yaw. In the case of the E46 M3 (and other saloon cars) the driver is sitting close to the axis of yaw, and as you say, the car handles relatively neutrally, whereas in the Z you're essentially sitting behind the axis of yaw.

If you are cornering hard in both E46 M3 and Z and you've got the tyres set at a "slip angle" of, say, 7 degrees on both the front and rear axle, then in the driver's seat of the E46 M3 you will be aware of the rotation of the car, with respect to the slip angle of the tyres, but you will experience no lateral movement of the car, with respect to yaw. However, in the case of the Z, because you are behind the axis of yaw you will experience the angular rotation of the car as lateral movement around the axis due to the front wheels being turned into the corner whilst the rear tyres permanently remain pointing in the direction of the car. Some Z owners just don't get on with the amplified sliding feeling due to the full use of slip angle, which is normal and still within the boundary of safety.