Suspension Explained

Understanding how the Suspension of your E30 fits together is one thing. Understanding why it operates in this way is another. If you want to make any changes to your car such as lowering the suspension or widening the track, then you'll need to know what effect that will have on the vehicle.

We'll try to explain all that to you, without being too technical. Here goes...

Overview

All of the parameters of your suspension is known as the geometry. These settings determine how the wheels move when the suspension's doing it's job - how they sit when the car is static, and how they bounce when the car goes over a bump.

The E30 is a particularly sophisticated design for its age, featuring independent suspension all round, with struts up front and an elegantly simple semi-trailing arm setup at the rear. This keeps the weight of the suspension low, but also giving it excellent characteristics in terms of Toe, Camber and Caster.

Since the design is so simple, it means that there is little room for adjustment in the factory setup, and any problems or faults usually require replacement parts rather than minor tweaks of bolt positions. If you think something's wrong with your suspension, learn how to check the toe and camber. It is essential that things are straight and true, as the angle of each wheel in all three dimensions is critical to ensuring your 1300kg of car doesn't go careening off the road into a tree, and if those angles go out of alignment by a single degree, there can be disastrous real-life consequences.

So to understand how bouncy-bouncy up-and-down translates to grip and performance, here's how the suspension works.

Weight

Your E30 weighs a tonne. Well, closer to 1.2 tonnes, and that's without your lardy arse weighing down the driver's seat. That's a lot of metal, plastic and flesh to go barrelling down the road, and even at relatively low speeds the smallest rock will have a serious effect on your car if you don't have some way of absorbing the shock.

That's where your suspension comes in. By disconnecting the wheels from the rest of the weight and "suspending" them on wobbly springs, you're allowing your wheels to jiggle and bounce over the stones and holes in our roads without having too much effect on the rest of the car.

Think of it like a game of conkers. If you've got two conkers of equal weight, and you smash one into the other, then that second conker is going to get pulverised. But if the first conker is only the size of a pea, and the second one is the size of an apple, then the second conker will barely move, because the little conker doesn't have enough mass, and therefore weight, to make a huge difference. We'll call the apple the sprung weight of the car, and the pea the unsprung weight.

Sprung vs Unsprung

The sprung weight of a car is, unsurprisingly, the weight that's carried by your springs. That means everything above the suspension mechanism, and which doesn't seriously react to the lumps and bumps in the road. Don't let things like rubber bushes confuse you; those are usually vibration dampers rather than proper suspension parts, so the sprung weight will include the engine, the drivetrain and the rear beam of the car.

In contrast, the unsprung weight of the car is any bit that bounces up and down. That's obviously the wheels and tyres, but also the brakes and the driveshafts too, since they have to pivot around the fixed differential. Suspension-wise, all the wheel hubs, the front wishbones and the rear trailing arms contribute to the unsprung weight of the car.

What does this all mean? Let's go back to our pea-vs-apple conker match. Our E30's body, engine and drivetrain is the apple, and that has a relatively fixed weight. We can rob a few kilos here and there between body shells, but it's still going to weigh around 1000kg. That's our sprung weight.

In contrast, all our wheels and suspension are our unsprung weight, and every kilo counts. If you swap your E30's 14" steel wheels for some fat 17" rimzz, you're trading an air-filled tube for solid aluminium, and that adds kilos to the unsprung weight, especially if the wheels are cheap, heavy alloys. Similarly, beefier brakes and bigger discs will pile on the pounds. Little by little, your upgrades will increase the size of your little conker. Sure, it won't be huge, but it'll start to leave bruises when it hits, and if you're driving down a British road then you'll get a lot of hits.

It's because of this that the best E30s left the factory with aluminium wishbones - anything to shave an ounce here and there added to the comfort of the car, as well as the response time of the suspension. Your shocks and springs will react a lot quicker if they're only trying to dampen 35kg instead of 40, so a lighter unsprung weight can react to road conditions quicker too.

Geometry

The Geometry characteristics of the car describe how the wheels are positioned relative to the body of the car. In your head, it's simple; the wheels are vertical, and they all roll forward, just like a Matchbox car. Oh, dear reader, if only that were true - car design would be as simple as welding two long poles under the car and bolting the wheels to the end, which is pretty much how the Americans have been doing it for the last hundred years.

In reality, where we have funny things called corners, it all becomes dramatically more complex. When we look at how your wheels are in relation to the car, we need to look at the Toe, Camber and Caster - the three dimensions in which a wheel moves, and how that affects the Thrust Angle.

Toe

Toe settings: Front 0°06' to 0°12'. Rear -0°04' to 0°26'.

Look down at your feet, and try and get them parallel, so that from heel to toe they point both ahead. Got it? Now bend your feet in, as if you really needed the bathroom, so that the toes are closer together than the heels. That angle change is called Toe, and by pointing your feet inwards you've created toe in.

Now imagine you're above your car, looking down at a pair of wheels. You can set the wheels to do the same thing, so that for two wheels on the same axle, their fronts will be closer together than their rears. Correspondingly, if we twist them the other way, they'll have toe out, also known as negative toe.

The toe of a wheel is actually set to the centre line of the car, not to the opposite wheel, so it's perfectly possible to have one wheel toeing in, and the other toeing out.

On a normal car, controlling the toe means making a decision between straight-line stability and precision cornering. If you're doing a lot of motorway miles, you don't want twitchy steering, so we set the front wheels to toe in slightly. That way the wheels will gently push each other towards the centre line, keeping the car stable. In contrast, if we're setting up a car for the track we want the car to turn in to the corners as rapidly and dynamically as possible, so we set the fronts to toe out.

The rear of the car is a little different. It's almost always better to have toe in at the back, as it adds straight-line stability and makes the car's behaviour a lot more predictable when things get out of hand, such as hitting bumps in the road. The E30 has its rear toe is designed to be pretty much 0, and it's not adjustable unless you make some significant modifications to the rear beam.

Camber

Camber settings: Front -1°10' to -0°10'. Rear -2°30' to -1°30'.

Camber is how the wheels lean when they're stood up. If it helps, imagine Stone Henge; you've got two big vertical slabs with another flat bit perched on top. If we shorten that top stone then the tops of the verticals will be closer together; they'll be leaning towards each other like a pair of drunks.

Those verticals are your wheels. If you look at your car from the rear, you'll see that the tops of the wheels do lean a bit into the arches, almost like a sumo wrestler planting his feet wider for support.

And like a sumo wrestler, camber helps your wheels get more grip when being pushed sideways, which is what happens when cornering. If your wheels were perfectly vertical then they'd start to tip over as your car cornered, which would reduce the amount of rubber touching the ground. If they lean into the car, then at worst they'll be pushed into a vertical position, which will keep the maximum amount of tyre in touch with the road.

This leaning is actually called negative camber - if the wheel leaned away from the car it would have positive camber, which you see on the back axles of heavy trucks. Positive camber is only set if you're expected to drop a heavy load into the car; that way, as the weight pushes the car down, the wheels will be pulled vertically and, ultimately, into a negative camber position if the load is heavy enough.

Negative camber therefore seems good, but you can have too much of a good thing. Tilt your wheels too far, and the inside edges of the tyres will be carrying most of the weight. This will lead to some serious inner tyre wear; a common problem when lowering the car. Unfortunately on an E30, there's no way to adjust this at the rear of the car without making some significant modifications to the rear beam.

Caster

Caster settings: Front 8°00' to 9°00'.

When your wheel hits a bump, it bounces upwards to absorb the shock; that's the whole point of the suspension. But it doesn't bounce straight up. Instead, it'll actually bounce backwards a bit too. If you look at your wheel face-on and imagine it's a clock, then you'd expect it to bounce to 12 o'clock. In fact, it'll bounce closer to 1 o'clock, since the suspension is angled backwards relative to the centre of the wheel. This is known as caster, and is exactly the same caster as you get on shopping trolley wheels.

On a shopping trolley we have negative caster. That means that as the trolley moves forward, the wheel is dragged along behind it, pulling it into a straight line. Positive caster is a lot more like a chopper motorbike, where the wheel is pushed ahead of the bike. On a car, instead of the vehicle straightening the wheel, the wheel will straighten the vehicle, which is what we want.

If you look at your wheel on the road, you'll see it flatten at the bottom to carry the weight of the car. This flat area is the contact patch. Now, draw an imaginary line from the top of your shock absorber down through the middle of your wheel and out the other side, and you should find that the line touches the road just in front of that contact patch. The idea of this invisible line is that it shows you the "force" of the steering - how strongly the wheels are pulling the car into line. The bigger the distance between the contact patch and the caster line, and the more stable the car will be, especially at high speed. That's why those chopper bikes have such incredibly long front forks. But just like bikes, the bigger the caster, the bigger the force needed to make a turn, which is makes turning the steering wheel particularly heavy.

Thrust Angle

Tolerance: -0.15° to 0.15°

Draw a line right through the middle of the car, pointing forwards. That ought to be the direction of travel when the car's on a straight road. But what about the force that's actually pushing the car along - the thrust? That comes from the two rear wheels, and if those wheels aren't running parallel then the car will be pushed slightly sideways. The actual line of thrust won't run through the middle of the car, but will instead point to the left or right. This difference between where the car should be heading and where the wheels are pushing it is called the thrust angle, and it should be as close to 0° as possible.

When your thrust line is out of true, it will push you car down the road at an angle. This is known as crabbing, and feels like the car is constantly wanting to turn in one direction. The biggest cause of crabbing is excessive rear toe on one side, or worse, one wheel has toe in and the other toe out. However, crabbing can also occur with worn rear beam bushes, bent trailing arms, fitting a welded differential, or with severe damage to the body shell.

A perfect thrust angle is achieved by balancing the toe angles on each wheel, so that the toe on one side cancels out the toe on the other.

Testing

If your geometry goes out of line, you might not feel it every time you go over a bump. However, you'll notice it when you check your tyres; you'll see dramatically uneven tyre wear, often with plenty of tread on one side of the tyre, but the other worn down to the cords. If that happens, you need to know what's wrong before you can fix it. Here's how to test.

Toe

Toe is the angle of the tyre relative to the thrust angle or, in layman's terms, whether the tyres point straight ahead or not. Bad toe settings lead to feathering, where the tyres get chewed up on either the inner or outer edge. This is different to normal wear - with feathering, the tyres are being dragged sideways along the tarmac because of the toe angle, which means the road actively chews the tread off the tyre, which is far more aggressive than the normal wear of rubber on tarmac. If the wear is on the inner edge of the tyre you've got too much toe out; outer wear is too much toe in. Too much toe in is normally a by product of aggressively lowering your car.

To measure toe you'll need a tape measure, a metal ruler (the type that measures from its tip), a long piece of string and various strips of packing material, such as cardboard strips or lollipop sticks.

Get the car parked on flat, level ground. If you've jacked the car up, bounce it on its shocks to get it to settle. Make sure the steering wheel is pointing straight ahead.

Now tie a length of string all the way around all four tyres in one big loop, as if it were ribbon round a Christmas present. The aim is to get the string properly taut, and with the string line running through the centre of the wheel. The string will need to go under the engine and the boot, so make sure it doesn't get snagged on anything. To ensure that it's level, use your tape measure to make sure the string is the same distance up each wheel from the ground. Pull the string so that the knot is somewhere under the engine or boot, not along the sides of the car

With the string is in place, pay attention to the tension of it as it moves across the front wheels. Your aim is to slide your packing material under the string on one side so that the string just kisses the rubber on the other side of the wheel. If your fronts have toe out you may not need to do this, but if you have toe-in you'll need to put a few millimetres to get the desired effect.

When you've done the fronts, move to the rears and do the same thing. Again, the string can't touch the sidewall of the tyres, so pack out the string to pull it out and away from the wheel.

Once that's done, get your ruler and offer it up to the lips of each wheel rim where the string passes over it. You should notice that the string is further away from one wheel of the other, and it's the difference in measures that gives you the toe for that wheel. For example, on the side of the wheel with the packing material, the string will be 11mm away from the wheel rim. On the other side of the same wheel the string will be 9mm away from the lip of the rim. That means you have a toe of 2mm across the wheel. If the packing material is on the forward-side of the wheel, you've got toe in, and if the packing's at the rear then you've got toe out.

Once you have your millimetre readings, you can convert them to degrees using this online calculator. Remember that degrees are calculated in minutes, with 60' in 1°, so 0.75° is actually 0°45'. Then compare them to the standard Toe settings.

Camber

Camber is the way the tyre leans in or out of the car. On an E30, they're supposed to lean in slightly, so that when you throw the car into a corner the roll of the body will pull the wheel vertically on the outer side, maximising the grip area of the tyre. However, as the years take their toll on the suspension the car will start to sag, increasing the negative camber until you get bad inner tyre wear. Too much camber in is also a by product of aggressively lowering your car.

How do you know if your camber is too harsh? To do this, you need two bits of wood (2" batten works well), some gaffer tape, a spirit level and a ruler. You'll also need a friend or a nail, whichever is easier to find.

Your two lengths of wood need to be exactly the length of wheel rim, so 15" on a normal E30 wheel (381mm). With the wood cut to length, hold the two pieces together and gaffer tape them at their bottom edge to make a hinge. This should make something like a film director's clapper board.

Offer this odd device up to your wheel vertically, with the hinge at the bottom, so that you have an inner and outer piece of wood. The inner piece should be tight against the wheel rim. If you have a friend get him to hold the inner piece. If you don't, bang a nail into the inner piece so that the nail sits on the bottom of the wheel rim, supporting the wood.

With the inner piece held tight, offer your spirit level to the outer piece of wood and you should see that it's clearly not vertical. Now pull the top of the outer piece away from the wheel until the spirit level is perfectly plumb. This should result in a 5-10mm gap between the tops of the two pieces of wood. Holding everything in place, use your ruler to measure exactly the size of that gap. Do it twice to double-check.

Make a note, and repeat for each wheel. These measurements are your camber, and they all denote negative camber. If you can't get it to work, repeat this test upside down, so the taped hinge on the wood is at the top of the wheel. If you can now get readings, you have positive camber, meaning something's seriously wrong with your geometry.

Once you have your millimetre readings, you can convert them to degrees using this online calculator. Remember that degrees are calculated in minutes, with 60' in 1°, so 0.75° is actually 0°45'. Then compare them to the standard camber settings.