BMW M135i long-term review

Why we’re running it: To discover how much we can enhance this used BMW M135i with a little help from our friends at Birds

Month 1 – Month 2 – Month 3 – Specs

Life with a used BMW M135i: Month 3

Tweaking the M135i’s tyre pressures – 18 October 2017

Having replaced the standard suspension with uprated springs and dampers, I’ve been playing with tyre pressures.

I wanted to find the perfect compromise between ride quality, tyre stability and grip. I found 29psi was great for ride and grip, but the tyres squidged around too much in hard cornering.

But 37psi knackered the ride and did nothing for grip, so I’ve settled on 32psi. Perfect.

Mileage: 32,150

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Life with a used BMW M135i: Month 2

Getting to grips with the M135i’s suspension – 13 September 201​7

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 over-damped 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 high-performance 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 percent stiffer at the rear and about 10 percent 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.

Mileage: 31,706

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Life with a used BMW M135i: Month 1

Welcoming the BMW M135i to our fleet – 16 August 2017 

My new long-term test car isn’t new at all. In fact, it’s coming up to five years old.

By the standards of today’s throwaway society, that probably means it’s halfway to the scrapheap already, but we have grand plans for it nonetheless.

The BMW M135i was launched in 2012, making this 62-plate example one of the early cars. The M135i was so well received by the press and car-buying public that you couldn’t read through more than 15 internet forum posts without somebody telling you to get one. 

Five years on, the cheapest, leggiest M135is have lost more than half their value. You can pick up a 70,000-mile car today for around £14,000, which looks like very good value indeed for a modern rear-wheel-drive hatch with 316bhp.

This particular car has covered 30,000 miles and it cost £17,500. The spec is just about spot on: three doors, manual gearbox, black leather.

The M135i cost a fiver under £30,000 when it was new, which meant it wasn’t much more expensive than high-performance Volkswagen Golfs, Renault Sport Méganes and the like. But it was more powerful, it had a more prestigious badge and it was, of course, much more rear-wheel drive than any of them. Little wonder it proved so popular.

Years later, it’s still a compelling package. The turbocharged six-cylinder engine gives it strong straight-line performance, the rear-driven chassis is nicely balanced and, on the much less interesting side, it’s practical and pretty refined.

However, it’s actually somewhat flawed. I won’t say too much about the styling, except that I never much liked the big, triangular headlights, but I love that you can tell just from looking at it where the power is sent: the cabin that’s pulled backwards over the car and the distance between the base of the windscreen and the front axle line give it away.

The biggest problem, though, is to do with the dynamics. Now, the M135i is very good to drive up to about seven-tenths, but when you really start hustling it, or when the road becomes particularly difficult, it all starts to unravel.

There isn’t the precision in the steering or the tight-fisted body control you need to really fling it around with confidence.

And when you start throwing crests and little jumps and compressions into the mix, the damping feels all at sea. No matter which of the two damper modes you select, the chassis is never able to iron out lumps and bumps in the road surface while also keeping the body locked under tight control.



To be fair, only the very best cars do those two divergent things at once with any degree of flair.

In the M135i, you soon find yourself backing off because the body is heaving up and down like a boat in a storm, or because the rear end is scribing some strange circular pattern behind you as it bobs up and down while also flopping into and out of corners.

And then there’s the lack of a limited-slip differential. Mostly, that really isn’t a problem at all, but it does mean the car leans on its traction control far too often in the wet. And when you’re really getting after the thing in the dry and you’ve got those systems off, it’s frustrating to feel the unloaded inside rear wheel spin up uselessly as you try to neatly slide it out of a corner.

There is, therefore, a great deal of room for improvement. So this M135i is our new project car and, over the next few months, we’re going to upgrade all the important bits and pieces to transform it into the vehicle we reckon it should always have been.

Working with BMW tuners Birds, we’re going to replace the standard, under-specified suspension with much higher-quality Bilstein components tuned specifically for the M135i, we’re going to fit a Quaife limited-slip differential and we’re going to lift power to somewhere approaching 400bhp. We might yet make a few other modifications, too.

When it’s upgraded to the full Birds B1 specification, it should be quite a car. We tested the company’s M235i demonstrator – the upgrades are interchangeable across hatchback and coupé – and found it to be a vast improvement over the standard car.

What makes us so sure that we, a car magazine, and an independent garage, can build a better car than BMW?

Every component in a mass-produced car is built to a strict unit price. The cars are also designed for a wide cross-section of drivers and for countries on opposite sides of the world. We, however, are able to spend much more on suspension parts than BMW could justify and we also have a very specific idea of how we want the car to feel once it’s finished.

The M135i will be upgraded over time and you can follow our progress in these pages.

Please do get in touch via the usual channels to let us know what you think of our new project car.

Second opinion

The M135i promised a better drive than it delivered.

The standard car’s vertical body control is what’s in greatest need of improvement.

A slippy diff has as much chance of hurting handling balance as it has of improving it, so let’s hope it’s well set up. I look forward to finding out.

Matt Saunders

BMW M135i specification

Specs: Used Price £17,500; New Price £29,995

Test Data: Engine 6 cyls, 2979cc, turbocharged petrol; Power 316bhp at 5800rpm; Torque 332lb ft at 1250-5000rpm; Top speed 155mph; 0-62mph 5.1sec; Claimed fuel economy 35.3mpg; Test fuel economy 30.0mpg; CO2 188g/km; Faults None; Expenses Suspension upgrade (£1865)

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