Rather than off-the-shelf parts, Birds' suspension upgrade uses springs and dampers that have been tuned specifically for the M135i. The Iver-based company recruited former racing driver James Weaver and experienced motorsport engineer Peter Weston to develop the upgrade package. Between them, James and Peter selected the correct springs and arrived at their own damper curves. The latter are valved accordingly at the Bilstein factory - this isn't simply a matter of bolting on stock parts and fiddling with damper clicks.
But hold on a minute. What do a couple of circuit boffins know about tuning a road car chassis? I wondered that myself, but when I heard James had some initial input into the Noble M600's chassis - one of the best suspended cars I've ever driven - I left my cynicism at the door.
James - who raced single seaters, touring cars and endurance racers across a four-decade career - wasn't particularly impressed with the M135i's chassis. 'I drove the standard car and it was a bit of an eye opener to say the least,' he says. Ride quality was his main bugbear. Peter reckons the issue is that German cars are generally developed for German roads, which are typically much smoother than our own, and that they're actually optimised for a very narrow set of circumstances. 'In Europe new cars have to do 200kph five-up with a boot full of luggage, so that's what the suspension is optimised for,' he comments. 'But normally, when your car is heavily loaded you moderate your driving anyway; you're not trying to find the limits of the car. You really want the car to be at its best two-up.'
James and Peter arrived at a set-up that uses stiffer springs - 15 per cent at the rear and 10 per cent at the front - and dampers that have much less rebound. The front end also sits 10mm lower. The stiffer springs should keep the body better controlled while less rebound damping will allow each wheel to drop into and roll through negative road features - potholes, sunken drain covers and the like - vastly improving ride quality.
That's the theory, at least. Having driven the car a few hundred miles on its new set-up I reckon that's exactly what James and Peter have achieved. There's greater plushness to the ride now, the new suspension smoothing out those negative road features rather than thumping through them. Very rough patches of tarmac are also much better ironed out. On particularly bumpy stretches of road, the kind of back lane that's almost unique to the UK, the body is much more settled now, rather than skipping and bouncing around, and the car also feels better tied down over crests whereas before it felt like the body and wheels might separate entirely.
Along with the new suspension Birds also fitted 10mm spacers to the front axle, subtly changing the steering geometry to try and bring some feel and precision to the wheel. The steering is better now and the car also looks a little meaner with a touch more track width, but it's not a night-and-day improvement.
So what's next? We'll be fitting a limited-slip differential soon, which should get the car to where we want it to be in terms of dynamics. Only then will we look at obtaining more power from the turbo straight-six.
Car: BMW M135i
Run by: Dan Prosser
Bought: July 2017
Mileage: 32,400 total, 900 this month
Purchase price: £17,500
Last month at a glance: Shockingly good improvement with new suspension fitted
'You could get an M135i for that' - so we have!
Lead image Luc Lacey, all other pics Stan Papior