B1 Sport Suspension Kit - BMW F20 M135i
The B1 Sport Suspension kit by Birds stands alone as the only BESPOKE suspension upgrade for the BMW M135i that has been made available off-the-shelf.
Consisting of Bilstein shock absorbers assessed and immaculately tuned by industry experts, and perfectly judged bespoke springs, this unique suspension kit is designed and guaranteed by Birds to offer the ultimate improvement in ride quality and handling dynamics on all road surfaces for your M135i.
For information on our B1 suspension kit for the previous generation 1 series, including videos, testimonials and editorials, click here.
The problem with the standard car.
The M135i comes as standard with Drive Performance Control which provides a range of driving modes from Eco Pro through to Sport+. It can also be specified with electronically controlled adaptive M Sport dampers.
The problem with the standard car is both the front to rear springs stiffness balance and the damping performance available from both the adaptive and standard shock absorbers. There are also issues with the weight balance of the car. BMW’s obsession with “near 50/50 weight distribution” has led to heavy items such as fuel tanks and batteries being placed rearwards, often behind the rear axle centreline. What this achieves, under hard acceleration, is a significantly rear-heavy weight balance. One motor manufacturer has been battling the problems caused by excessive rear axle weight for over 50 years. Futhermore, if the rear tyres are pushed beyond the grip limit, this can cause the rear end to act like a pendulum. With an excessive rear end spring stiffness, the sudden break-away can be quite difficult to catch.
On the damping side, the more expensive adaptive dampers are designed to give the driver the option of how stiffly damped he wants the car to be at any given time. The Comfort mode gives soft damping all the way from slow speeds upwards. Most drivers are fishing for the “Sport button” just as soon as speeds increase beyond 60mph. Unfortunately, this immediately re-engages the traction control, so those cars not blessed with a mechanical limited-slip differential are immediately robbed of the additional control over the rear end and power delivery that these essential devices allow.
Even so, at higher speeds, the dampers run out of talent, and the car starts to get floaty, generating a loss of confidence of the driver. You will find your right foot being automatically pulled away from the throttle…
Oddly, we believe that the standard dampers give a better account of themselves, especially al low and medium speeds, but they are cheaply made, and like the Adaptive Dampers have pretty much linear damping force curves measured against damper speed.
The B1 solution.
We were told to restrict our search for the correct shock absorber components to Bilstein. With the Bilstein shock absorber, it is possible to generate genuine damper curves across the useful speed range. It is a proven fact that soft damper forces in the low frequency range generate a comfortable ride in the critical 30-50mph speed range, even on the UKs’ crumbling tarmac. Couple that with gently stiffening response as road speeds and bump frequency increases, and you can achieve sublime body control in the upper performance reaches. Finally, it is possible to independently tune both bump and rebound forces. Of course, you could opt for the popular “adjustable dampers”. Unfortunately, these systems appear to be just like BMW’s own adaptive creations. Any damping force curves that exist are outside of the relevant frequence range, and the important areas on the curves cannot be adjusted independently. Of course the owner also has the problem of trying to understand which knob or button does what. Our suspension guru told us “show me an adjustable damper and I’ll show you 100 ways of getting it wrong”.
So, armed with the test car on the prototype springs, we set about the task of drawing damper curves. Once again, we road and dyno tested various different damper sets, found the versions that were closest to our ideal set-up, and produced drawings of our ideal curves using existing curves as a basis. Bilstein re-valved an existing damper set to our spec.
Testing and adjusting the dampers takes considerable time, effort and money. Like with all products we offer, we only do this because we know it’s right, and we don’t offer anything that doesn’t make a significant and immediately tangible improvement to the performance of the car.