Rear axl bump steer?

[bennyboysvuk]

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.

I'm not sure I totally understand. If there is the same amount of slip at front and rear, won't the car still be running neutrally through the corner with neither the front nor rear pushing wide? I can understand it if the tail is out of line, in which case the Z driver is moving perhaps 6 inches further sideways than the M3 driver for the same amount of rear tyre slip.

 

[exdos]

I'm not sure I totally understand. If there is the same amount of slip at front and rear, won't the car still be running neutrally through the corner with neither the front nor rear pushing wide? I can understand it if the tail is out of line, in which case the Z driver is moving perhaps 6 inches further sideways than the M3 driver for the same amount of rear tyre slip.

As you know, If the front wheels are turned, then they don't take the same path as the rear wheels as per the diagram below;

Therefore, if you keep steering so that your car goes in the direction you are looking and if you have the same slip angle on the front and rear tyres, then there's still a slightly different arc taken by the rear axle to that of the front so that the rear feels as though you're drifting laterally somewhat with the tail slightly out of line (which you are).

 

[bennyboysvuk]

I'm not sure I totally understand. If there is the same amount of slip at front and rear, won't the car still be running neutrally through the corner with neither the front nor rear pushing wide? I can understand it if the tail is out of line, in which case the Z driver is moving perhaps 6 inches further sideways than the M3 driver for the same amount of rear tyre slip.

As you know, If the front wheels are turned, then they don't take the same path as the rear wheels as per the diagram below;

Therefore, if you keep steering so that your car goes in the direction you are looking and if you have the same slip angle on the front and rear tyres, then there's still a slightly different arc taken by the rear axle to that of the front so that the rear feels as though you're drifting laterally somewhat with the tail slightly out of line (which you are).

The pic shows that the fronts take a wider arc than the rears, so surely the Z would feel more like it is understeering to the driver, sitting further back in the chassis compared to the M3 driver. Is this what you meant? Or are we saying that the rears are sliding, but not enough to require opposite lock and that the fronts are partially turned into the corner, but the turn is being partially completed by the rear tyres? In that case, I understand completely. I think if we were chatting in person, I'd have grabbed a pen and pad by now and drawn loads of diagrams.

 

[exdos]

The pic shows that the fronts take a wider arc than the rears, so surely the Z would feel more like it is understeering to the driver, sitting further back in the chassis compared to the M3 driver. Is this what you meant? Or are we saying that the rears are sliding, but not enough to require opposite lock and that the fronts are partially turned into the corner, but the turn is being partially completed by the rear tyres? In that case, I understand completely. I think if we were chatting in person, I'd have grabbed a pen and pad by now and drawn loads of diagrams.

I've highlighted in green what I'm saying and you've understood exactly.

In that diagram the tracks might be the car taking that corner very slowly with virtually no slip on the tyres, but if the car is going much faster so that the front tyres take the red track and with the front tyres turned in at 7 degrees slip, then the rear wheels will tend to slip outwards, towards the track made by the front wheels, and if you're behind the axis of rotation you are aware of this lateral and angular movement. Back seat passengers in saloon cars get thrown around more than the front seat occupants - that's why your kids end up travel sick.

 

[pilchardthecat]

Stock wheels with 265/40 PSS = zero bump steer

 

[bennyboysvuk]

The pic shows that the fronts take a wider arc than the rears, so surely the Z would feel more like it is understeering to the driver, sitting further back in the chassis compared to the M3 driver. Is this what you meant? Or are we saying that the rears are sliding, but not enough to require opposite lock and that the fronts are partially turned into the corner, but the turn is being partially completed by the rear tyres? In that case, I understand completely. I think if we were chatting in person, I'd have grabbed a pen and pad by now and drawn loads of diagrams.

I've highlighted in green what I'm saying and you've understood exactly.

In that diagram the tracks might be the car taking that corner very slowly with virtually no slip on the tyres, but if the car is going much faster so that the front tyres take the red track and with the front tyres turned in at 7 degrees slip, then the rear wheels will tend to slip outwards, towards the track made by the front wheels, and if you're behind the axis of rotation you are aware of this lateral and angular movement. Back seat passengers in saloon cars get thrown around more than the front seat occupants - that's why your kids end up travel sick.

I love that feeling when the rear is helping to steer. It just feels so right and if everything is working to its maximum potential.

Lol, I'll have to bear that in mind when my boy gets a wee bit older.

 

[exdos]

I love that feeling when the rear is helping to steer. It just feels so right and if everything is working to its maximum potential.

So do I; that's when you know you're pushing hard enough and playing with the slip angle. 8)

 

[mad4slalom]

i sort of get what you mean, i am a suspesion novice in the knowledge area but though the m is good i do feel it could be so much more. just dont know how to go about it, other than doing what beedub did , what is the cost of tha ac set up , similar to kv 3 ,s

 

[exdos]

i sort of get what you mean, i am a suspesion novice in the knowledge area but though the m is good i do feel it could be so much more. just dont know how to go about it, other than doing what beedub did , what is the cost of tha ac set up , similar to kv 3 ,s

I don't know what the new price of the ACS Racing suspension kit that I have is because I bought mine secondhand at a price I simply couldn't refuse. If I'd had to pay the same price for it as the KW Clubsports, then I would've bought the Clubsports because it has linear springs at the front. Having said that, I was very pleased with the ACS kit on my recent trip to The Ring both on track and for the 1k mile round trip there. For the track, I had it set up stiff at the front and a bit softer at the rear, which is opposite to the way that OEM suspension is set up.