BMW Steering & Suspension Buyer's Guide and Problems

 

Steering

Older BMWs tend to use worm and sector steering which sits within a steering box at the end of the steering column. Even some fairly modern large BMWs do this as there are a lot of space and packaging advantages. Worm and sector feels less precise than rack and pinion and can suffer a lack of feel in the straight ahead position.

For example the six cylinder E39 five series models use rack and pinion but the V8s use worm and sector, it's often been said the 530i is the best handling E39 as a result of this and it's superior weight distribution. Both systems have two end tie rods but worm and sector also has centre tie rod taking the place of the rack. One end of this centre tie rod is connected to the steering box and other to an idler arm.

All current and most older BMWs have power assisted steering. The hydraulic pump is generally driven by a belt from a dedicated crankshaft pulley on older cars or by a common serpentine belt on modern engines. The power steering fluid is Dexron III ATF.

BMW Efficient Dynamics models have electric rather than hydraulic power steering. This puts less parasitic drag on the engine and saves fuel as a result. A torque sensor measures the force the driver applies to the steering wheel and adjusts the steering characteristics to accordingly. This gives a more immediate and direct driving experience. Some models have a sport mode button that modifies the steering feel. BMW's systems are judged to be superior to most others and preserve the steering feel you'd expect in a sporty car.

M-Tech 1	M-Tech 2

Steering Wheels

The steering wheel is held on by a single central bolt. There are several styles of mounting and some BMW wheels are interchangeable with other models. Cars from the E28/E30 onwards tend to use the same spline pattern but the recess to the steering column shroud differs between models. Basic cars used to get plastic steering wheels but an upgrade to a leather wheel is a simple and desirable option. Several diameters are normally available with the smaller //M-Tech versions being the most sought after.

In recent times BMWs have fitted stereo and other controls to the wheel, not an idea I'm in favour off when it's overdone. I can see the point for radio volume and cruise control but the current wheels look too busy by far. Alpina have an extremely well crafted leather wheel with the up and down shifter buttons for their Steptronic transmission.

From the early 90s BMW made airbag equipped wheels optional and later standard equipment. These must be treated with great respect by the amateur mechanic to avoid explosive deployment! The SRS (secondary restraint system) airbag warning lights needs a special tool to reset the warning light, this is a safety feature. US cars have larger airbags as they can't rely on the occupant wearing a seat belt.

One trend I've never understood is the fitting of after market wheels with metal supports and little or no padding. In the event of an accident you might as well fit a large spike to the steering wheel.

E46 Sport wheel, feels much nicer than normal version.

Alignment

BMW dealers use an alignment technique called KDS. This is very accurate and involves weighting the car down then mounting wheel plates to get the most repeatable results. It's expensive but perhaps cheaper than premature tyre wear caused by bad alignment. KDS is even more important if you're running 17" or larger wheels. As is often the case the BMW solution isn't the cheapest but is technically superior to a simple alignment at your local tyre vendor.

Generally front camber isn't adjustable although you can retrofit upper strut mounts from an M car or buy fully adjustable mounts. On Models with the later Z axle there is a lot more to adjust on the rear axle, older systems need offset bushings for some adjustments.

It the car seems unstable or wanders it's often a good idea to have the tracking checked. My E36 handled dreadfully after a woman in a Renault Clio decided to hit me instead of using her brakes. The light impact had been enough to throw the rear end alignment out, the difference was incredible.

F30 wheel seems a bit too "busy" a design.

Steering Faults and Problems

The steering tends to suffer wear and tear over the years but little else goes wrong. Worn tie rod ends or centre tie rod ends are often first noticeable as a judder under braking which can be mistaken for warped brake discs. The steering column joints are very reliable but on bigger engined cars (and especially E30 M3s converted to right hand drive) they can dry out so an oiling now and then is good preventative maintenance.

Steering squeal can often be traced to a slack drive belt. Hard steering can be worn strut top bearings on extremely high mileage cars but can also be due to a blocked fluid reservoir filter. On older BMWs (E23, E24) they're replaceable but on most others you need to back flush or replace the reservoir in extreme cases.

If your steering is hard when cold it's time for a fluid change. Steering fluid should be the colour of cherries, if it's dark brown it's overdue for a change.

Many E36s had a new lower steering column fitted under warranty due to corrosion problems, there was a recall for this. On E28 and E24 models the steering box support can break causing potentially serious problems, it wasn't really strong enough for the six series.

E30 Macpherson strut front suspension.

E30 semi-trailing arm rear suspension.

Suspension

BMWs have always been famed for their superb handling, together with the wonderful engines these two things define what a BMW is all about. The big breakthrough for BMW came with the Neue Klasse saloon in the early 60's. At a time when most cars had a rigid rear axle the BMW 1500 introduced independent rear semi trailing arm suspension coupled with front Macpherson struts and thus invented the compact affordable sports saloon! The Neue Klasse was a revelation in handling terms. The system is similar in principle to the later E30 system show in the above diagrams.

The system was developed continuously and worked well until being replaced with the Z axle multi-link rear suspension pioneered on the Z1 and 8 series. Although a good system the semi-trailing arm suspension has some geometry shortcomings under hard cornering, these are resolved by the more complex multi-link system. One of the few BMWs not to use either system was the race bred M1. Most E36s have the Z axle but interestingly the Z3 and Compact kept the old system to save space (and money).

Newer BMWs tend to have the rear shocks and springs separately mounted, a boon for the the amateur mechanic. But many older models such as the E28 and E34 tend to have the spring over the shock absorber.

Self Leveling Suspension

Some larger BMWs had self leveling rear suspension, even as long ago as the early 70's E3 saloons. It's not advantage unless you often carry heavy loads in the back, I've no idea what BMW thought they were adding to some late M6's by fitting it! If you have any trouble with it then ditch it and fit normal shocks from a standard car.

Electronic Aids

Dynamic Stability Control (DSC) detects oversteer and attempts to neutralise it by controlling the throttle and applying individual brakes. If the car has electronic power steering it will make minor changes to that too. It uses the ABS sensors to detect wheel spin and has a yaw sensor to detect the angular motion of the car. It also has a steering wheel angle sensor (the SZL).

In the newer cars DSC incorporates DTC (Dynamic Traction control). DTC must be manually activated by the driver. It allows the wheels to spin in situations where this may offer more traction, such as in snow or gravel. The system will never cut engine power. Under spirited driving in good conditions DTC will still use the brakes to try and prevent power oversteer. It's kind of a less nanny version of DSC that still tries to stop you doing anything too stupid.

Also worth mentioning is EDC, Electronic Damper Control. This is best known from the E34 M5 and other high performance models. The system can adjust the damping to suit hard cornering or motorway travel. On most versions there is a control for sport or comfort mode in the cabin, but some M5s have a totally automated version which is not obvious unless you look for the control box under the rear seat. Only down side is that new shocks are very expensive (around 600 GBP!), bear this is mind if looking at an EDC equipped car.

As BMW is fond of reminding drivers in it's manuals, these systems are good but cannot break the laws of physics.

Suspension Faults and Problems

As with the steering general wear and tear are the main enemies. Shocks will last at least 100-140k before needing replacement. But rear shock mounts can fail sooner and cause a tinny rattle over low speed bumps, cheap to fix.

On higher mileage cars the rear subframe mounts can fail after 10 years or so. Two rubber bushings hold the subframe to the car, not fun to change without the correct tool. Failed bushings make the back end of the car wander about, you can feel it when changing lane.

Rattles, knocks or squeaks from either end can be traced to failing anti-roll bar links, around 11 GBP each as a rule. E38s were notorious for wearing front ones and squeaking annoyingly, but they're easy to change. Another cause of knocking can be a failed trailing arm bushing, two are used to locate the front of each trailing arm to the subframe.

A bad judder under braking or around 55mph could be worn front lower control arm bushings. BMW have always fitted too soft a bushing on non-M cars in order to give a smoother ride, downside is they wear faster. E46s were terrible for this. Solid poly replacements are a good idea and not any dearer.

Modified Suspension

The factory produced many sports models and made M-Tech suspension available as an option on others. This is generally an excellent setup which offers improved body control without undue harshness. For everyday use this is preferable to most of the harsher after market systems. I have this on my E30 318iS and can't fault it, it's 15mm lower than a normal E30 which isn't overdone.

Lowering a pre-90s BMW with semi-trailing arm rear suspension more than about 25mm will result in too much negative rear camber. Not only will this look odd but you'll wear out the inner edges of the rear tyres quickly. A proper lowering kit for such a car will use offset rear bushings to cancel out some of the camber change.

A kit from a supplier such as Alpina, AC Schnitzer or Dinan will include shocks with custom valving, springs, anti-roll bars and possibly bushings. This will have been track and road tested and will add value to the car. But expect to pay around 1000 GBP for an Alpina suspension kit. For 500ish you'll get a set of lowering springs and quality Bilstein dampers, not a bad option but it may be harsher. Anything cheaper is to be treated with caution. If you just lower the car by cutting the springs you can expect to bottom out the shocks fairly soon.

If all you want to do is reduce understeer then fitting a stiffer rear anti-roll bar from a higher model (i.e. 325i bar in a 328i) is a cheap option. My E36 328i had an M3 Evo rear anti-roll bar and it was perfect. If you don't want to lower the car and are replacing worn shocks I'd suggest Boge Turbo Gas, not dear and they work well with the stock springs. I had them on my E36 coupe.

For track use you can replace the suspension bushings with polyeurathane equivalents, they transmit too much noise for road use though unless it's just on the lower control arms. Another common mod for E30s is to fit E30 M3 offset front control arm bushings to improve turn in and feel.

Older BMWs benefit from the addition of a front strut tower brace, I had one on my 635CSi (E28 Sparco bar fits). But on a modern BMW there is little to be gained by this as the body is more rigid, I'd not waste money on one for an E30 or E36 road car unless I did a lot of track days. One exception is the front subframe "X" cross brace, a lot of E36 318ti drivers say great things about this mod.