It's been a long time since I started this thread but I believe I now know the main cause of my "bump steer" issues. I'm not entirely sure if what I was experiencing truly was bump steer but what I was feeling certainly seemed to fit that description. Anyway, for me, the biggest improvement by far was gained by tweaking the rebound settings on my coilovers. The out-the-box settings of "9 clicks down from max", front and rear, was too soft. Or perhaps to put it a better way, too fast. This meant that the suspension wasn't being controlled enough by the dampers, making the car bounce on the springs. So I can only assume that the stock Mini suspension is very under damped, or whatever the correct term is, and the coilovers are setup out-of-the-box to be similar. Saying that, my original suspension was leaking, so it probably wasn't working like new.
I currently have my coilovers set to 7 clicks down from max front, and 4 clicks down from max rear. So not a huge change on the front but very noticeable. Changing the rear just brought it all together and balanced out the overall feel.
I should probably point out here that on the ST XTA suspension, the rear rebound adjustment is at the top and can only be accessed by removing interior trim and drilling an access hole in the chassis on both sides. So it's not exactly quick or simple. The adjuster can then be accessed without removing trim through the little access hatches in the sides of the boot. You need an allen screw driver, patience and preferably small hands but it's a pain.
In addition to tweaking the rebound, I have made a lot of other changes from stock, including:
New front wishbones and outer ball joints.
Adjustable upper and lower rear control arms.
All new steering tie rods and ball joints etc.
Front and rear adjustable drop links.
22mm adjustable rear anti-roll bar, middle setting. Front bar still stock non-sport version.
All new poly bushes (except front anti-roll bar, which will be done soon).
Front and rear upper strut braces.
Each of those changes made small improvements, mostly helping to tighten up the feel, but tweaking the rebound made a huge difference. The car now rides bumps much smoother and I get much less bump steer. I also get much better turn in and rotation. So I'm looking forward to testing the car more this year.
I hope that helps other people experiencing similar issues.
Thanks for the update, did you ever experiment with disconnecting the front bar? I did for a while - back when I was running Bilstein B14s - and I really liked it, but we're not allowed to do that here by The Authorities. I'd probably run a standard Cooper bar if I could be arsed to get around to it. By the way, and just for the record, I found the bump steer to be improved on the B14s, as compared to the stock SS+.
I suppose they wanted the car to feel flat and firm and nippy from the factory, but the dampers let it down, and it became harsh and crashy pretty quickly, and especially with the stupid big fat 17" rims and run flats. Even on SS+, it worked so much better on 16s.
Yeah I don't think Dave's issue is bump steer by definition. Bump steer is caused by toe angle change during suspension movement, and that happens also when cornering hard on a flat surface, not only on bumps. Common thing on lowered cars, gets worse the lower you go.
Mine had very bad bump steer, scary even, when I had roll center correction spacers installed. When the weight shifted outwards after turning in to a corner the car would tighten it's line way more than expected. I raised the car a bit, but that didn't do much, so I took the spacers off, and it's much better. They increased the angle between lower control arms and steering tie rods, and that's the perfect recipe for bump steer.
Stiffer suspension and ARBs will reduce bump steer, as they reduce body roll.
Can't really help with the exact problem you're having Dave, but you're probably on the right path with the damper settings.
I haven't tried disconnecting the ARB yet. After speaking to a few people and doing some research, I decided that while it may help it wasn't the route I wanted to take. As far as I can tell. my car had the standard suspension option, so not the thicker front ARB. I already knew I wanted a thicker rear ARB, so I decided to see what the balance was like with that and and adjustable drop links. They made a definite improvement to the overall balance but it still felt bouncy before adjusting the rebound.
The other reason I decided not to disconnect the front ARB was that I realised the bouncing wasn't just happening during cornering. One day I drove over a raised steel strip across a gateway and despite having the steering straight and only going maybe 5 to 10 mph, the front end bounced and the steering went light for a split second. So in a situation like that, the ARBs aren't doing anything. After that I tried lowering the rebound on the front as I thought the tyres were skipping over bumps due to rebound being too stiff. But that made it worse. So then I went the other way and the bouncing reduced a lot. That was my eureka moment.
Another thing I changed which I forgot to mention before was I raised the suspension all round. When the coilovers were installed it was dumped pretty low. It looked great but I was getting tyre rubbing and I think it wasn't helping the bouncing for the reason JKo described. I was also getting drive shaft vibration. Now it's a bit higher, all of that's improved but I did that before changing the rebound. I got the alignment tweaked as well as raising the ride height threw that off a bit.
Now the suspension is dialled in better I've shifted my attention to tyre pressures to see how much difference that makes. I'm currently running a set of 215/45/17 Michelin CrossClimate on stock wheels which will hopefully become my winter wheels/tyres when I get some new, lighter wheels. They're currently at 36psi front and 34psi rear but I'm still experimenting.