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Twitching Rear Suspension

Author : Brian Brown Posted on : 05/11/2004
Model : BMW, 3 - Series (E36) 1991-99, 318ti Category : Suspension
Source : http://www.318ti.org
 
The 318ti retains the semi-trailing arm suspension from the earlier E30 cars. This type of setup was also used on other BMWs through the late eighties. Basically each wheel/hub assembly is rigidly attached to the control arm. The control arm pivots along a single axis on two bushings, and it extends from the center of the car diagonally outward - hence the term 'semi-trailing'. This is as opposed to cars with pure trailing arms that extend straight back. Going the other way are cars like the Beetle with 'swing axle's' that extend directly to the side.

This is about as simple of a suspension possible. There is only one piece (the semi-trailing arm) that controls suspension geometry. The springs, shocks, and stabilizer bar are completely unrelated to the geometry of the wheel travel. The subframe's purpose is to be a rigid attachment point that is isolated from the body.

When the wheel travels up and down, it follows the arc defined by the control arm. When it's at its neutral position (when the car is sitting on the ground at rest), the wheel is basically pointing straight forward and is straight up and down (neglecting the minor alignment angles for toe and camber). When the wheel travels up, it also aims (steers) outward, and the top of the tire tips inward (more negative camber). When the wheel travels down, it aims (steers) inward, and the top of the tire tips outward (more positive camber).

A car with pure trailing arms has no steering or camber change with suspension travel. A car with swing axles has no steering with suspension travel, but it does have extreme camber changes. (You may have noticed the wheels on a Beetle tucking in and out as it goes over bumps.)

The main reason semi-trailing arms are a desirable design is that it's simple, and the camber change when the car is leaning around a corner helps keep the wheels perpendicular to the ground. The steering effects can be useful (especially going around hard, relatively low speed corners), and they are certainly entertaining.

There are basically five situations a semi-trailing arm rear suspension gets itself into:

NEUTRAL POSITION, such as when the car is going down the road in a straight line. The wheels simply point straight and roll.

CAR LEANING INTO A TURN. The outside wheel travels up, the inside wheel travels down. The outside wheel has more negative camber (keeping it perpendicular to the road), the inside wheel goes into positive camber (also keeping it perpendicular to the road). These changes in camber angles directly counteract the body lean. The outside wheel turns in the opposite direction of the front wheels, and the inside wheel also turns in the opposite direction of the front wheels. This helps the car turn sharper than would be directed by the front wheels alone (this is the fun part).

REAR OF THE CAR SQUATTING, such as when under acceleration. The camber of both wheels goes negative, and toe-out increases. Neither of these changes are particularily desirable, but they aren't very bad either, especially in the ti since it doesn't squat much under power. Also, the track width (the distance between the wheels) increases. As long as this happens equally on both sides, you don't notice it.

REAR OF THE CAR UNLOADING, such as when braking. Everything's the opposite of squatting: camber goes positive, wheels toe-in, and track width decreases. Again, not wonderful, but not really a problem.

ONE REAR WHEEL GOING OVER A BUMP OR DIP. While this does cause the wheel to change its steering angle and camber, the main effect is caused by the change in track width. The wheel that is not going over a bump tries to go in a straight line, causing the back end of the car to follow the midpoint between the two wheels as the track width changes. This results in the back of the car wiggling or "twitching".

The other situations possible are combinations of the above, and transitions between situations. The most important one to consider is that when going around a corner near the limit and then hitting the brakes, the combination of the change in rear wheel steering angle and decreased rear tire force can cause the rear to lose grip and the car to spin out. This isn't as strong of an effect in the ti compared to older BMW's and it's certainly not like old Porche 911's, but it is still there.

The multi-link rear suspension used on the other E36 models causes the wheel to travel in a fairly complicated compound arc. It minimizes toe and camber changes during squatting and unloading and virtually eliminates track width changes. It maintains proper camber compensation during body roll, but it doesn't have nearly the self-steering effect (which could be considered either good or bad).

The ti and the Z3 still use the semi-trailing arm to save space and cost. Personally, I really enjoy the driving characteristics of this design. The lower weight (compared to other new BMW's) of these cars is a benefit to their handling. I think the biggest thing the ti has going for its handling is the consideration and expertise that BMW has put into optimizing the geometry, weight balance, body structure and suspension tuning. It is so well balanced and predictable at both high and low speeds, as well as being very responsive to small corrections near the limit. Comparison articles around the world aside (both favorable and unfavorable) I personally don't think that there's another car in the U.S. anywhere near the ti's price that even come close to it's well-balanced level of performance handling.

Regards,

Brian Brown.
 

References

  • http://www.318ti.org
    •  


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