Suspension

The Z3 uses a rear suspension design referred to as Semi-Trailing Arm suspension. This design was also used in the E30 3 series (including the E30 M3 of course). Older Porsche 911’s used this design until the 993 Generation of the 911 was introduced with a multi-link rear. The newer e36 3 series and e46 3 series uses a variation of a multi-link rear design called the Z link.

The Semi-Trailing Arm rear suspension is a variation of a trailing arm suspension. Another variation is called the swing axle. These designs basically use one “arm” to locate the wheel and place all suspension loads (axial, bending and torsion) on this arm. The design has a bit of a weight penalty compared to a multi-link setup, and has built-in geometry control limitations.

The main thing to understand about this design is what happens to camber and toe settings as the semi-trailing arm moves through its travel. The rear wheel on a semi-trailing arm suspension gains negative camber as the suspension compresses. Also the rear suspension gains toe-in as it compresses past the horizontal position of the trailing arms. The gain in camber and toe-in actually is beneficial during cornering while power is being applied.

But what happens when we hit the brakes and cause the arm to unload instead of compress?

As the rear trailing arm moves downwards from horizontal we loose negative camber (i.e. gain positive camber) as the rear wheels droop. Similar effect is happening to toe-in. Lifting the throttle during hard cornering causes the front to dive and the rear to lift due to weight transfer. This happens in all cars of course and even in the best handling cars this will induce oversteer, but with semi-trailing rear suspension, the problem is compounded by the fact that the outside rear tire steers (toe angle) towards the outside of the turn, in effect creating even more oversteer. That’s why some people call our car twitchy or "tail happy". What semi-trailing arm suspensions are susceptible to is “trailing throttle oversteer”. It doesn't happen at 9/10ths driving, but right *at* 10/10ths when there's a bit of slide in the rear. The inexperienced driver will suddenly lift off the throttle and this sends the car spinning tail-first. The key is *not* to lift, except very gradually, when this occurs, and it can be caught.

A semi-trailing arm arrangement is usually used because it's compact layout allows a lot of room for other things, like passengers and luggage, not because it offers superior chassis dynamics. Most agree the reason for its use in the Z3 was due to compactness.

Onto a comparison with the e30 m3:

One of the main differences between the M-Coupe rear trailing arms and the earlier E30 arms is the factory addition of an "A-arm" type reinforcement between the two members of the trailing arm. This was probably done to strengthen the trailing arms so that they could cope with the much higher output of the M-Roadsters and M-Coupes There is also an additional gusset added to the M-Coupe arm. Some suggest that this gusset was added to prevent high cornering loads from flexing the trailing arm and allowing the camber setting to change (dynamic camber.) Also the subframe is reinforced compared to e30.

I’ve been told that the Parts CD shows that the part numbers for the M Coupe eccentric and regular trailing arm bushings are the same as those for the E30 chassis

My own personal comments:

So what high performance cars today use our suspension or similar?

Porsche 911: moved to multi-link rear with the 993 generation..  Which was about 1993 I think

BMW M3: moved to multi-link rear with the release of the new e36 chassis...so since about 1994 as well (on m3's...3 series released in 92)...

Corvette C5 uses SLA (short- and long-arm) suspension with aluminum wishbones and knuckles at each comer

semi-trailing arm rear suspension "was" the dominant rear suspension design until the 1990's. Until the move to multi-link rears...

Is our suspension obsolete?

Obsolete? in a way yes because the only reason to use it is really for cost concerns. And even that is somewhat untrue in today's world...look at the new MINI...that pretty much has a rear suspension design very very simliar to that used in the e46 3 series and your talking $20,000 sticker on Cooper S.

Another stated advantage is packaging benefits...but even that is no longer the case...Honda has done a great job of making the rear suspension of the s2000 compact and even the MINI has achieved the same goal...

There is a reason that multi-link designs are favored and are superior to the semi-trailing arm design. The multi-link designs allow each link to be placed in pure tension or compression resulting in structural superiority. It also offers much more flexibility in terms of suspension kinematics (camber ,toe ,etc). The downside is added cost and usually packing problems (need more space for all these links)

I know we all love our cars but it doesn’t mean I don’t give respect to the competition…realistically speaking the Honda S2000 has a superior suspension design. (suspension…I’m not talking about the whole car now J )

And when I talk about suspension I’m referring to the overall effectiveness of the suspension and not simply referring to a skidpad number from a magazine. One can get high skidpad numbers from pretty much any setup with the proper tires and suspension tuning. Good suspension design is able to cope with more than a smooth skidpad surface to generate high “g” numbers.

We use our cars in the real world. Full of bumps, dips, ruts, etc. When driving aggressively on real world roads that aren’t butter smooth, the semi-trailing arm design shows its limitations. I can take a bumpy corner at a higher rate of speed and with greater ease and smoothness in an e36 or e46 car than I can with my M-coupe. The e36 and e46 are easier to drive faster and remain more composed under conditions that would find the e30 derived setup faltering.

Even on the track…road course or auto-x….there are some limitations but it has less to do with road surface conditions and more to do with adjusting your technique to deal with how our cars like to put the power down. This doesn’t mean our setup cannot be made to work well. It definitely can, but under certain conditions it just doesn’t equal the performance of the new designs. This is why the new z4 will use a design that is similar to the e46.

Can we bring it up to "par" with newer designs?

basically you can't...
we can however make it work better through a certain range of travel. We can improve it to a certain degree by aftermarket shocks and springs which help to control and minimize the camber and toe changes.


I have heard that the good drivers would rather have oversteer than understeer, they seem to like a twichy rear, Am i out in right field or just talking to older drivers who are set in their ways?

there are basically three steady state conditions that can exist

Understeer
Neutralsteer
Oversteer.

We don't want neutral steer because although it would seems the car is perfectly balanced the car is really susceptible to road imperfections and driver inputs and would send the car into oversteer.

Most people think that a good driver wants oversteer. This is somewhat of a misconception. You don't want a car that is setup up to oversteer as part of its design (by this I mean that the suspension design is specifically tuned to be balanced toward oversteer) oversteer is pretty much an unstable condition.

What you want is a car that has "slight" steady state understeer because this has a stabilizing effect. And i mean "slight", This is not to be confused with the severe understeer that most cars seem to have these days. A car that is setup for slight understeer at the limit can be controlled with the throttle to get oversteer..

What's important to understand is that the conditions of understeer or oversteer can happen through many parts of the turn....on corner entry, apex or corner exit depending on how the driver is negotiating the turn (speed, braking, steering angle, etc)

if we want to talk about our car specifically...you basically want to make sure that your entry speed into a turn is not too fast. That way you can avoid corner entry understeer by coming in too fast and avoid the transition into trailing throttle oversteer because you just realized you came into fast and hit the brakes causing the rear end to rise and now the back end want to come around.

If you come in at the right speed and can progressively apply power through the turn which our car will like because under compression we gain camber and toe-in and ultimately we can get on the power earlier and really power out of the corners. And if you are really good you might be able to do this with a hint of oversteer

basically our cars like the "slow in/fast out" method of driving...of course we have to make adjustments to decreasing, constant, or increasing radius turns...