Z4M Feeling at 70MPH

Thanks for the feedback chaps.
Went out for a blast (sticking to 70 :driving: ) and I think the issue is probably above the national speed limit. If the car wasn't planted it wander easily and that it definitely doesn't do. I believe is more to do with aero lift as mentioned...
 
My ///M felt decidedly dodgy at "motorway speeds". Light and floaty, like it was being blown by moderate crosswinds when there was no crosswind.

I changed the tyres which improved things significantly as there was a mismatch between my front and rear axles in terms of the tyre sidewall loading.

I then changed the rear trailing arm bushes which seems to be a common issue and my car now feels totally planted. I have not tested it at crazy speeds but "usual motorway speeds" it is now excellent.

I am of the opinion that the rear has to be absolutely spot on with the Zed because you are sitting so far back in the car you are almost over the rear axle.

The slightest wander or anything else around the back feels decidedly nasty with these cars.
 
Removing the slack in the RTAB bracket by fitting stiffer bushes or a Limiter Kit will certainly improve the handling of an otherwise OEM Z4M, but the floating and vague steering, especially under acceleration on poor roads, is the consequence of the load-shifting that can occur with the OEM suspension.
 
exdos said:
Lower said:
At constant speed? How do spring rates affect lift at a constant speed?
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I don't know what the effective spring rates are of the OEM suspension because they're "progressive" springs and not linear, but the front springs are made from 11mm diameter wire and the rear springs are of 19mm wire; that's some difference front to rear. In comparison, the diameter of the wire of the ACS Racing suspension that I have on my Z4MC are also progressive springs but made from 14mm wire at the front and 16mm wire at the rear, so it seems obvious that the OEM front springs are pretty soft, especially compared to the OEM rear springs. Therefore, the front end of the car will have more vertical movement (under bump/rebound) on its OEM 11mm springs than it will on, say, 14mm springs (i.e. aftermarket suspension), and once the nose of the car rebounds upwards, thus reducing positive rake angle, more air will get under the car creating lift. On a billiard table smooth road this wouldn't be a problem because there would be no movement of the front suspension, but such roads don't seem to exist in the UK.
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Roads that you can drive at a constant 70mph are going to be smooth enough that the front end won't be moving enough to change the rake angle.

A light front end will be more down to toe settings or castor angles.
 
Lower said:
A light front end will be more down to toe settings or castor angles.
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Which do you reckon makes a light front end: toe-in or toe-out and/or greater or less castor angle?
 
Mikey_Boy said:
Less castor is the dominant effect for steering wander at speed... :thumbsup:
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Castor is fixed for the Z4M so if that's the cause of the floaty steering it would be impossible to dial out without fitting catsor/camber plates even after fitting aftermarket suspension. But the floaty steering can be removed without fitting castor/camber plates, so castor isn't the cause in the Z4M.
 
Lower said:
exdos said:
Lower said:
A light front end will be more down to toe settings or castor angles.
Click to expand...
Which do you reckon makes a light front end: toe-in or toe-out and/or greater or less castor angle?
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Toe out and less castor will give a light front end
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It will make the steering much sharper but not lighter (as in, less load on the front end).
 
There's a definite front end lift at extremely high speeds with fully OEM suspension. This is characterised by a vague front end that drifts and wanders as well as an imprecise responses to steering inputs. This markedly worsened when my driver's side rear spring lost 3/4 of a turn and more or less resolved with Eibachs and OEM dampers. More front negative camber makes for twitchy steering and almost exaggerated response to steering inputs.
Now I have KW Clubsports, with bump and rebound set too stiff the car becones skittish on all four wheels on rough surfaces and very crashy over potholes. I've backed off the rebound on the rear compared with the settings suggested by KW and the car is now very well-mannered (as long as I drive round the bigger irregularities in the road surface).
To me this suggests that the extremely high speed front end vagueness with fully OEM suspension is secondary to front end lift not geometry changes.
 
exdos said:
It will make the steering much sharper but not lighter (as in, less load on the front end).
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Less toe in or even toe out reduces resistance to turning the steering wheel so the steering will feel lighter.

With caster you effectively lift the front of the car slightly when you turn the wheel as you change the camber of the wheels. Therefore the less castor you have the less self centring effect you have and again lighter steering.
 
Lower said:
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Less toe in or even toe out reduces resistance to turning the steering wheel so the steering will feel lighter.

With caster you effectively lift the front of the car slightly when you turn the wheel as you change the camber of the wheels. Therefore the less castor you have the less self centring effect you have and again lighter steering.
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Lower, I understand absolutely what you are saying, but I've been referring to aerodynamic lift, something completely different to a "lighter" steering feel. The OEM geometry has fixed castor and set with some toe-in.
 
Mine was tramlining until I had the toe in checked. Once the drivers side was corrected it now feels like it's glued to the floor.. Check your tire pressures which my be a little high.
 
exdos said:
Mikey_Boy said:
Less castor is the dominant effect for steering wander at speed... :thumbsup:
Click to expand...

Castor is fixed for the Z4M so if that's the cause of the floaty steering it would be impossible to dial out without fitting catsor/camber plates even after fitting aftermarket suspension. But the floaty steering can be removed without fitting castor/camber plates, so castor isn't the cause in the Z4M.
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Didn't realise it was non adjustable on the Z4... Something new learnt!
 
Bigpow said:
Mine was tramlining until I had the toe in checked. Once the drivers side was corrected it now feels like it's glued to the floor.. Check your tire pressures which my be a little high.
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Yep! lots of clever complicated options here but you've just mentioned the one very important thing that can vary a fair chunk: tyres pressure!
 
Fast3000 said:
Hi All,

My Z4MC just came back from a couple of hours of tracking from BMW. Feels a lot better and doesn't veer to the left as much. My query was how your Z4M felt at speed. As mine feels slightly lighter then I expected. The steering feels light and I can make minor adjustments to the steering very easily. is this normal? or should it feel heavier and more 'planted'?

thanks
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If your car is aligned correctly it will feel planted most of the time as mine did all the way up to 160mph, see link in my sig.
I did notice sometimes it could feel a bit vague at 60+ mph when driving up steep inclines, the air flow hits the car differently and I could feel the difference on the front! it generally feels like the front has a bit more slack and can be a bit unnerving.

Good luck
 
Also guys, it can be a combinations of all the things that have been mention, I had a limiter kit installed on RATB,s. This took the slack out if the rear, but also improved the front end, because the platform felt more stable during turns.
 
exdos, anyone for that matter,

anyone go the Tq spec for the RTAB bracket to chassis bolt spec, the 3 bolts that mount the carrier to the car??

76nm i THINK but don't want to over TQ, want to fit new RTA bush for track day on the 19th....
 
I'm still not convinced that the electronic steering does adjust down on mine when in Sports Mode. I really don't notice any difference. I find the front very light at anything around and over 70.
 
Beedub said:
exdos, anyone for that matter,

anyone go the Tq spec for the RTAB bracket to chassis bolt spec, the 3 bolts that mount the carrier to the car??

76nm i THINK but don't want to over TQ, want to fit new RTA bush for track day on the 19th....
Click to expand...

Very close! 77Nm according to the TIS. :thumbsup:
 
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