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      Basic BMW Z3 Suspension Modifications by Mike Fishwick
      Article reproduced with kind permission from Mike Fishwick

      Body Bracing

      Back in the days when cars had a separate chassis it was not unusual for the driver to see that the radiator and bonnet were moving relative to the scuttle – the front of the body’s rigid area, on which the windscreen was mounted – as the chassis flexed. This phenomenon was known as ‘Scuttle Shake,’ and largely passed away with the advent of monocoque body structures.

      Many motoring writers, however, still claim that the Z3 suffers from scuttle shake, this being a catch-all term for any suspension-related problem, from a harsh ride on poor surfaces to tramlining, while their more technically aware colleagues like to blame a lack of torsional rigidity. A high torsional rigidity value is not, however, guaranteed to provide the ultimate in handling qualities.

      When the body is fitted to a torsional measurement jig, it is bolted down via the suspension mounting points – in terms of a Z3 this means the threads into which the rear cross-member studs attach, and those onto which the front cross-member is bolted. One end is held stationary while the other is twisted, the resultant angle of movement and effort being recorded to give a figure in kilogrammes per degree.

      The test is therefore based on the application of equal loads at each corner of the car, a situation which does not occur in normal driving, and it does not apply loads into the structure where it will absorb suspension loadings from the springs.

      As a test of the body structure, this does not include any additional flexibility resulting from the suspension and its mountings, and in terms of handling qualities means little in terms of a complete car, on which the overall rigidity is dictated by that of the most flexible components. Major sources of such flexibility on a Z3 are the space between the front suspension strut mounts and the studs onto which the rear suspension cross-member is mounted.

      In this case, one could be excused for wondering why car bodies are now much stiffer, if the other components have remained flexible. The answer is simply that the great increase in body stiffness over recent years has been driven more by crash safety requirements in roll-over situations rather than improved handling, which is not very high in the priorities of most drivers. Remember – a BMW survey found that less than 4% of Z3 buyers were concerned with trivia such as handling and performance!

      Although the basic handling deficiencies of a Z3 are minor ones, capable of being easily cured by stiffening the main rubber bushes at either end of the car, like any other structure it can be usefully stiffened if it is desired to bring handling qualities towards a more optimum level.

      Strut Brace:

      The most popular modification is undoubtedly a strut brace, many owners claiming that it can make a vast difference in terms of ‘scuttle shake,’ steering response and general stiffness. While I would never suggest such claims to be exaggerated, I do wonder if they may be in proportion to the amount of glitter a polished brace will add to one’s engine bay . . .

      These claims do however contain quite a bit of truth, as any reinforcement in the space between the suspension struts must resist their tendency to lean inwards under suspension loads, and so help to maintain the designed steering geometry.

      Without any other modifications to the suspension of my 1998 2.8 model, I certainly noticed a small improvement in initial steering response, and a slight reduction in tramlining, from dreadful to just awful!

      [​IMG]

      Strut brace designed to fit around the inlet manifold of a four-cylinder car – a straight type is always better

      I am however using a tubular Hamann brace of the ‘bent’ type, this being designed to fit around the intake manifold of the 1.9 litre M44 engine. While this design is widely used, particularly on the E30, it can never be as effective as one using a straight tube or beam, but is certainly better than nothing. My next birthday present will be a ‘straight’ brace, manufactured in thin steel at £205 by Strong-Strut of the USA, which as the ultimate strut brace will make an interesting comparison.

      To sum up, a strut brace is easy to fit, and is certainly an effective modification, but the more expensive types (Dinan, AC Schnitzer etc) are no more effective than cheaper equivalents, if of a similar ‘straight’ design. The weight saving of an aluminium brace (‘Aircraft quality,’ naturally!) compared to steel represents such a small percentage in terms of the total weight of the car and passengers that it is not worth mentioning. A steel brace will, however, resist an offset load better than one of aluminium, and most designs introduce a slightly offset loading between the strut tower and the brace.

      Butt Strut:

      If, like me, you spend many happy hours lying under your Z3 contemplating its well-polished final drive cover and the glittering black paintwork of the rear suspension, you may have noticed that the suspension is mounted on a large cross-member, via bushes secured to the bodyshell by a pair of 13 mm studs about 200 mm long.

      These studs are braced to the insides of the door sills by thin triangular steel plates about 2 mm thick, which are not an ideal solution to absorb the considerable cornering loads which can be generated by the 245-40 section rear tyres.

      This is a design which has been passed down by generations of BMW cars, and while satisfactory back in the ‘seventies on a 2002 with relatively thin tyres, it now begins to show its limitations on the Z3. What we basically have is a long thin ‘box,’ with the base and rear side removed. One end of each cross-member stud is secured to the top of this box, the other ends being braced to the narrow sides by their triangular plates, which make a valiant – and partially successful – attempt to substitute for the missing base and side.

      [​IMG]

      Butt Strut – it makes a far larger improvement than a strut brace

      This obviously permits the lower ends of the studs to move slightly in response to cornering loads, giving a ‘squirm’ to the rear tyres which introduces an unwanted steering action. If the open base of the box could be closed, the situation would be somewhat improved, as the bracing plates could then share the loads, which are mostly carried by one or the other. Stiffer bracing plates would also help.

      The answer to this problem lies in the ‘Butt Strut,’ another product of Strong-Strut, which is also applicable to the E30 and E36 Compact, as well as the Z3. It takes the form of a thick aluminium strip, bent where necessary to clear the exhaust pipes and final drive. This strip bolts to the cross-member studs by means of special sleeve nuts and spacers, so making them share the lateral suspension loadings and reducing their effects.

      Fitting is easy, and can be made with the car on its wheels, or jacked up and supported under the cross-member. Several large spacing washers are supplied with the kit, and it is simply a matter of selecting those which locate the strut as high as possible without touching the dished sides of the bush housings or the exhaust pipe. All that is then required is to secure it with the large sleeve nuts which are supplied.

      On the road I found an appreciable reduction in understeer, providing sharper steering response and making the car more ‘chuckable,’ with a definite feeling of increased solidarity. These improvements are of course a sign that both bracing plates were now sharing the load, so that the rear wheels were staying more in line with the car, and are no longer fighting the actions of the front wheels.

      As one would expect, the rear wheels are still susceptible to ‘pattering’ when under power while cornering on the rougher road surfaces, such as on the exit from a roundabout, but the gains are large, and without any problems. The ground clearance is of course reduced by some 20 mm, but with standard springs this is not a problem, even when fully laden for a camping trip.

      To sum up, the Butt Brace is very effective, at £168 it represents excellent value for money, and does not introduce any problems of its own. If fitted by itself it makes definite improvements, but of course its limiting factor is that the longitudinal location of the cross-member studs is still dependent on the bracing plates. As part of a system to reinforce the rear suspension mountings, however, it can do more.

      Body Brace:

      While the Butt Strut makes a definite improvement, longitudinal location of the rear cross-member studs remains dependant upon the small bracing plates, even though they are now sharing the lateral load. The Body Brace consists of two strips of steel angle, with shaped plates welded to their ends which bolt between pre-drilled holes in the Butt Strut and the bolts securing the rear end of the ‘X’ brace below the engine.

      The Butt Strut – and therefore the rear suspension studs – are now effectively part of a bottom to the ‘box,’ bracing the studs against longitudinal movement, and restoring the rear suspension geometry to its designed range.

      Fitting is easy, but does require the car to be jacked up on at least one side at a time. If the Butt Strut has been fitted correctly it will not have to be disturbed to align the Body Brace with the floor of the car – mine fitted to it perfectly. I had to slightly file the bolt holes at the front to suit the angle of the ‘X’ brace bolts, after which it fitted as if BMW had made it.

      [​IMG]

      The rear ends of the Body Brace runners attach to pre-drilled holes in the Butt Strut

      Although when on the road the Body Brace makes itself felt in terms of reduced understeer and improved steering response, its largest benefit is the reduction of rear wheel ‘patter’ during tight turns under power on rough roads, making the Z3 feel as if its suspension is almost modern. The usual Z3 habit of wearing the inner edges of the rear tyres has been eliminated, the tyres now wearing evenly across the entire tread area, a sign that they are now following the direction of travel, instead of deciding their own path. This will obviously increase the useful life of the tyres by a substantial amount.

      The only problem brought by the Body Brace is that of reduced ground clearance, in terms of its 30 mm thickness, including that of the Butt Strut. I have had no problems at the rear, where I use standard springs, but with 30 mm lower Eibach front springs short duration ‘safety’ humps of the plastic type favoured by UK supermarkets etc. need to be approached with caution. In such conditions, however, the Body Brace provides excellent protection to the car’s underside, and particularly the vulnerable fuel pipes.

      [​IMG]

      Fully braced – the Butt Strut runners connect the Butt Strut to the ‘X’ brace below the engine.

      While some owners swear that their Body Brace has stiffened the entire car, and made the doors close with precision etc, I would say that its real benefits are in the area of handling, making the car feel more agile yet also solid, while giving a feeling of greater refinement. At £197 it represents good value, and I would be loathe to lose it.

      (Strong-Strut, Scottdale, Arizona. See www.strongstrut.com)
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