I'd had my eye on the Z4M for many years but was always put off by reports of wayward handling. My recent car history includes the likes of a Lotus Evora and a Megane R26.R - I like light, precise cars with a high degree of tactility, and always assumed a Z4M was the complete opposite.
However the lure of the S54 was too much and when a Z4MR came up for sale in April that ticked all of the right boxes I dived in with an impulse purchase. It was quickly apparent that more or less everything I had been led to believe was true. On stock suspension the car is a fine cruiser and perfectly serviceable up until about 7/10ths. Push harder and the experience degrades, with the well-documented sensation of the front and rear ends being disconnected magnified by the seating position a long way away from the front axle.
I like to know exactly what the front axle is doing and the OEM setup didn't give me confidence when pushing on. Meanwhile the rear was unforgiving over bumpy lanes, bouncing around a fair bit. It was obvious that the rear was too harshly sprung and the front too soft.
Looking into options KWs and Bilstein B16s are well reviewed and I have a good experience with B16s fitted to my Impreza, but they and KWs are three times the prices of BCs. The Bilstein Pro-Kit was ruled out right away as I did not want to lower the car (ground clearance is important for UK roads, plus I don't like how lowering messes with geometry unless you install supporting mods such as roll-centre adjustment track-rods).
Furthermore I wanted the ride height adjustability offered by coilovers to change the OEM stance from negative rake (nose up) to slightly positive rake (nose down), which has the effect of reducing nose lift on hard acceleration as well as reducing understeer by moving the roll centre backwards, and allowing you to run softer springs at the rear.
BCs get mixed reviews but I run them successfully on my Clio, and I managed to obtain a brand-new set on this forum for a very good price (the entry-level, non-digressive BR versions). Installation was straightforward thanks to the many excellent guides available online. Having owned an E39 M5 for 9 years I was reminded what a pleasure it is to spanner BMWs of this vintage: good-quality, non-rusted fastenings and sensible designs made for taking apart. A far cry from say some French models designed to confound the amateur mechanic.
At the same time as installing the BCs I took the opportunity to put in new wishbones, lollipops, inner and outer track-ends, Powerflex RTABs and trailing-arm upper ball joints. I did all of this in one go so can't comment on the individual impact of each change.
I took a moment to bask in the afterglow of shiny new bits. I'll let the pics tell the story (taken before ride-height adjustments were made). Before you ask, the arch wet patches are fresh ACF-50 which I applied before fitting coilover covers.
I left things a little late getting the work done, with a road trip looming. Conventional wisdom says 100-200 miles to allow suspension to settle before checking ride height. I only managed about 20. Ride height set to 600mm rear (about 5mm higher than OEM) and 595mm front (10mm lower than OEM), measured lower rim edge to arch. Alignment was done the day before I set off for Plymouth to catch the Brittany Ferry to Santander.
Caster is not shown on the reading, but is approximately 6.5 degrees each side. More camber is available via the BC top mounts but I decided to go close to OEM camber settings to begin with.
Cue 2,500 miles most of which were making very good progress on the best roads in Iberia, ergo the world. This is a regular trip that I've been doing for many years with a group of other keen drivers, in a variety of machinery. It was immediately obvious that the BCs (on 6 kgs F / 10 kgs R spring rates) had cured the front-rear disconnect. The car immediately felt more organic and more fluid. Damper settings front and rear right in the middle of the 30-click BC adjustment felt good, but on ultra-smooth Iberian roads I felt confident to stiffen the fronts and ended up using five clicks from full hard. I feel the front end would benefit from being stiffer still, and may look into swapping out the 6 kgs springs to 8 kgs (you can typically go +/- 2 kgs on coilovers without needing to re-valve the damping). Rear felt good on the default middle setting and most of the bounciness on imperfect surfaces had gone, albeit sitting so close to the rear axle you'll always feel the bumps.
Having been accustomed to more 'accessible' machinery that you can get in and drive fast right away it was clear that the Z4MR (for me) was not that kind of car, even with the revised suspension. It took me a couple of days to get properly into the groove. You can't throw a Z4 into a bend like a hot hatch and neither can you floor it before the apex like the LSD-equipped Clio and the GC8 Impreza I also own. The front end is still not as confidence-inspiring as I would like, and I am looking into strut braces which I believe may help, but it's much better now than stock. And if you take a leap of faith, push past the slight vagueness, the front grips hard and doesn't let go. The limits (on 255 rears and 225 front) are much higher than you expect, at least on warm, dry and ultra-grippy Iberian tarmac.
It's still more GT than sports car and it's still a heavy brute IMO, but most importantly, I have gained a fluidity that is absent from the OEM setup. Rather than discouraging you from pushing hard, my car now encourages.
All in all I'm very happy with the BCs, especially at their price point. The only thing I'd do differently is specify harder front springs than the 6 kgs items that came with my set. After my trip the rear suspension seems to have settled another 5 mm or so and I'll be sorting that (easy job thanks to the BC adjustable spring perches). I may try adding front camber to see if that helps further reduce the front-end vagueness.
Bear in mind that if you own a non-///M E85 that the front coilovers have different mounting points and it may not be possible to have the same ride-height adjustment on the front. I don't know for sure, but was chatting with a chap on Facebook who thinks that there may not be enough adjustment on the non-///M E85 BC front coilover to achieve OEM ride height, so caveat emptor if that's important to you. No problem at all on the ///M version - in fact you could go considerably higher than OEM if desired.
