BMW M135i
Our first tweaks have us in suspense
WHY WE’RE RUNNING IT To delve into the world of aftermarket modifications and discover how to respectfully improve on factory efforts
Perhaps the single biggest upgrade we’ll make to our used M135i is also the first – the suspension. Out goes BMW’S Electronic Damper Control equipment in favour of a set of Eibach springs with passive Bilstein dampers, both tuned specifically for the M135i.
Birds, the specialist we’re working with on this project, recruited former racing driver James Weaver and experienced race engineer Peter Weston to tune the new suspension. Between them, Weaver and Weston chose appropriate off-the-shelf springs for the car while also specifying their own damper curves to Bilstein, which built the damper kits accordingly. The result is that the Birds suspension upgrade for the M135i is proprietary. You can’t get it anywhere else. The new suspension took half a day to fit, costing £1554.23 with installation, but not VAT (which takes the total to £1865.08).
“I drove the standard car and it was a bit of an eye-opener to say the least,” says Weaver. “The ride quality was pretty poor. The problem is that in Germany new cars have to be able to do 124mph, five-up and with a fully loaded boot, normally on smooth roads.”
Consequently, as Weston explains, modern cars are desperately overdamped most of the time, when they only have one or two occupants. They are also often not optimised for the UK’S broken, bumpy road network. It means that most cars, the M135i included, offer plenty of room for improvement, which certainly tallies with my own experience of the car.
Weaver and Weston test drove a standard M135i using a loop of about four miles – mostly made up of narrow, bumpy lanes – near Birds’ Buckinghamshire headquarters. They found that the ride quality was poor, mostly because there was far too much rebound in the standard set-up, and the car also lacked grip, traction and steering precision.
“The damping I’ve incorporated into the shocks is what we’ve learned over the years with James in a racing environment, with a highperformance car on a bumpy circuit like Sebring,” says Weston. “That experience taught us an awful lot about how to support a car on the springs and yet have damping that allows you to go over bumps. It blows the theory that for a car to be good over bumps you need soft springs – that isn’t always the case.”
The set-up the pair eventually arrived at includes springs that are 15% stiffer at the rear and about 10% stiffer at the front, with a 10mm drop in ride height over the front axle. The retuned damping has much less rebound, allowing a given wheel to quickly drop out of its wheel arch rather than hanging up in there. Whether driving over a bump in the road, a pothole or sunken drain cover, less rebound damping will massively improve ride quality.
I’ve covered a few hundred miles on the new set-up now and it’s definitely a big step forward. What it certainly isn’t is a very stiff, track-focused set-up. Instead, the car now feels much more settled on bumpy roads at speed. It no longer leaps about with the tyres losing contact with the ground. It’s more comfortable now too, the new suspension rounding off potholes and smothering broken, rough patches of tarmac. Body control is also much better and the car no longer feels as though it’s going to bounce itself off the road over crests and undulations.
The steering upgrade, meanwhile, is simply a pair of 10mm spacers on the front axle that subtly adjust the geometry. The steering now feels sharper and more direct, although it’s not a night-and-day improvement.
The next upgrade will be a Quaife limited-slip differential to further improve traction and make the car easier to control on the throttle right at the limit of grip. Only then will we go anywhere near the engine.