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      BMW Z3 Subtly Stiffening a Z3
      Article reproduced with the kind permission of Mike Fishwick

      Part one – Subtle Stiffening

      The Z3 must be the most-maligned sports car ever built, most of the motoring press and television presenters pouring scorn on it, particularly for its use of ‘old fashioned’ semi-trailing arm rear suspension from the E36 Compact. The same people, of course, are full of hysterical praise for the E30 M3, which has the same suspension, but which is set up for a more specialised task.

      The detractors would do well to remember that some of the most successful sports cars were based upon major assemblies from ‘ordinary’ cars, such as the early Porsches (not to mention their 924 and 944 series) and of course a succession of designs from MG, Triumph, Morgan, and Lotus etc.

      Of course, such economies have a price, in that compromises which are acceptable in an ‘ordinary’ car may not satisfy some sports car owners. Depending upon how and where the car is driven, some owners will seek to modify their cars into a form which suits their own requirements.

      Front Wishbone Bushes

      A popular, and well-justified, criticism of the Z3 is its habit of following undulations in the road – particularly when fitted with the optional seventeen-inch wheels. This ‘tramlining’ is accepted by some owners as being ‘character’ yet it is virtually absent from the M Roadster, and is therefore capable of being eliminated.

      The M Roadster is fitted with different bushes at the rear ends of the front wishbones, as are used on the M3. These are of almost solid construction, rather than the ‘gapped’ design used by the other models, which permit the rear ends of the wishbones to move slightly in response to cornering loads, this misalignment generating the understeer which is deemed necessary to desensitise the steering and so protect uninterested drivers from themselves!

      [​IMG]
      Non-M wishbone bush – a recipe for poor handling on rough roads


      On a poor surface, particularly when fitted with seventeen-inch wheels, this can generate unwanted steering inputs. The car then tends to leap across the road in small but violent movements, with associated reactions in the steering wheel. As the rubber softens with age the situation becomes steadily worse.

      This effect is particularly bad on the Z3 due to the reduced mechanical advantage (i.e. leverage) provided by its high-geared steering rack – the highest-geared of all E36 variants. This means that the steering, while sensitive, cannot always provide sufficient torque at the front hubs to stabilise them against these sudden changes.

      The steering therefore reacts in response to suspension loads. On a car which will be driven with enthusiasm this situation is less than ideal.

      While wheels of a different offset may reduce the problem, an economic first step in the battle against tramlining is the fitting of stiffer wishbone bushes, using either the M Roadster or polyurethane type – my choice fell on Powerflex polyurethane items, obtained from Proven Products of Tavistock. These bushes are a two-part design, with a stiff PVC carrier holding the polyurethane pivot bush, which is lubricated with a coating of silicone grease on final assembly.

      Fitting is not difficult, but it helps if a pair of trolley jacks can be used to safely lift the car onto some wooden or concrete blocks for increased safety.

      Using plenty of leverage, unfasten the two bolts securing the housing of the rear wishbone bush, and swivel it to a more comfortable working position. Unless you have access to a large bearing puller with ‘knife edge’ plates to pull the housing and bush off the wishbone pin in one operation, it will be preferable to carry out the task in two stages.

      Assemble a junior hacksaw with the blade passing through the bush, and using plenty of lubricant (WD-40 or paraffin) cut through the sold portions of the bush, and remove the outer section with the housing. With the housing in a vice, cut through its thin outer steel shell, so that it may easily be pushed out of the housing.

      Now fit a small two-legged puller around the central tubular portion of the bush, and draw it off the wishbone pin – little effort is required. You are now ready to fit the new parts – but make sure you are aware of their correct locations!

      Heat the semi-rigid PVC bush carrier in boiling water, and using the vice press it into the metal housing, making sure that its larger diameter bore faces forwards.

      At this stage I found a problem – the inner bushes were too long, as there must be a subtle difference between the wishbones used on the Z3 and those of the E30, for which my bushes were originally intended. I therefore found it necessary to cut a recess (20 mm diameter, and 10 mm deep) at the front end of their bores, which was not difficult, and preferable to sending the originals back to Tavistock from the Dordogne. This problem should not now be experienced, as a special Z3 kit has been developed.

      [​IMG]
      Powerflex replacements


      After pushing the bush fully along the wishbone pin, fit the carrier over the bush, and check that the housing bolts line up with the body – the Z3 uses tapered dowels to securely lock the housings in position.

      When you are satisfied with the fit, coat the bush with the supplied silicone grease, and make the final assembly. Coat the housing bolts with Loctite and tighten to a torque of 34 lb-ft.

      The result, even on a rough road, is steering action devoid of tramlining or reaction through the steering wheel – just like a normal car! As a bonus, now that the front wheels are being held in their correct positions, toe-out under braking, and the associated wear on the inner area of the tread which is common on a Z3, is totally absent.

      [​IMG]
      Note the recess in the front edge of the bore – essential for E36 models


      The stiffer bushes do not transmit any noise or harshness, are easily fitted, and at £20 per side represent excellent value. It does, however, make one wonder why BMW did not fit the M Roadster type of bush in the first place, as they must have been aware of this unsettling problem which has discouraged many a potential Z3 owner. Curiously enough, I never heard any of the TV presenters who used to regularly criticise the Z3 ever mention this problem – it makes you wonder if they ever drove the car!

      Rear Suspension Cross-Member Bush Inserts

      Another typical compromise concerns the rear suspension cross-member bushes, which incorporate spaces designed to reduce road noise. As the bushes soften with age, these spaces permit the entire rear suspension to twist, so misaligning the wheels. This promotes instability and tyre wear, particularly under hard cornering, or in straight lines on rough roads.

      Fitting polyurethane bushes or the solid rubber type used on the E30 M3 is the real answer, but this is not a simple task. The alternative is to fill the spaces with PVC stiffeners, purchased from America via Ebay, from ‘Skunkworks27′ and at $24 plus postage they must be the most cost-effective handling modification available.

      [​IMG]
      Bush inserts in position


      Fitting is easy, but there are a some points which the unwary should consider before starting work.

      As you will be removing the nuts which hold the rear cross-member to the body, there is a risk that it may drop to such an extent that it strains the flexible brake pipes or wiring between the body and the cross-member. Before you undo anything therefore, jack the car and support the centre of the cross-member on some wooden blocks.

      Initial fitting of the inserts – to about the half-way mark – is easy enough, but to fully insert them requires more force than can easily be applied. Hammering them into the bushes is virtually impossible – just like trying to hit a chisel with a rubber hammer! The answer is to press them into the slots, a process which takes a considerable pressure.

      I found that the best technique was to screw a suitable nut and washers along the stud to force the inserts into place, after lightly lubricating them with WD-40, which does not harm rubber when used in small quantities.

      I then finished the job by pressing the inserts into position with a piece of suitably thick-walled tube supported on the platform of a trolley jack. (I would not even attempt the job without at least one trolley jack)

      Make sure that the insert is pressed into the bush sufficiently to clear the bracket which braces the stud to the door sill, then slightly raise the car to lower the cross-member’s position, in order to ensure that the insert does not protrude above the bush far enough to make contact with the upper plate. Failure to do this will provide a transmission path for road noise.

      When replacing the brackets between the lower ends of the studs and the door sills, carefully examine their M8 socket screws, and replace them if the hexagon sockets are at all worn. I prefer to replace them with stainless steel items, lightly coating their threads with Copa-Slip before tightening fully by hand, using about a foot of leverage.

      On the road the difference is not vast, but for the amount of money involved it is considerable! These little pieces of PVC are a work of genius, and even though my bushes appeared to be in excellent condition, they provided a small but definite improvement in steering response, while stability on poor road surfaces was certainly improved. Road noise does not appear to be any different – at least in a roadster – although some coupe owners consider it to be slightly higher. In the longer term they will doubtless help to reduce wear on the edges of the rear tyres.

      For those who wish to modify the suspension of a Z3, E30, or E36 Compact, this subtle but effective measure represents excellent value for money, and is well within the capabilities of any reasonably practical owner.

      Anti-Roll Bars:

      There are a number of different front and rear anti-roll bars and associated mounting bushes available from BMW dealers in larger (and therefore stiffer) diameters. Basically, increased roll stiffness at the front will promote understeer, which we do not want. Increased rear stiffness, however, will have the opposite effect, a small increase reducing the basic understeer characteristic to provide neutral handling and sharper steering.

      Most models of Z3 have different anti-roll bars, in addition to those available in the M-Tech suspension kit. For example, a 2.8 litre model such as mine uses a 24 mm front bar combined with a 15 mm rear.

      [​IMG]
      Rear ARB bushes are sold in different sizes – check before ordering


      If roll stiffness is proportionately increased at both ends, body roll will be reduced without changing the basic steering characteristic. More importantly, increased roll resistance does not change the effect of the springs and dampers, so reducing body roll without affecting ride quality.

      As the Z3 is not cursed with excessive body roll, overall increases in roll stiffness can usually be limited to that of the M-Technic suspension kit, which uses a 25.5 mm front bar and a 16 mm rear for the 2.8 litre model. If, however, you wish to also change the basic handling characteristic – which generally means a reduction in understeer – the front bar can be reduced to 23 mm, and the rear bar be increased in several increments up to 19 mm.

      Mounting bushes to suit all diameters of anti-roll bar are available in polyurethane from Powerflex, but it should be remembered that a bush for an E30 front bar does not necessarily suit a later car such as a Z3. Powerflex now manufacture bushes specifically for these later cars.

      This is a good opportunity to check the condition of the front anti-roll bar drop links, as their rubber bushes can often distort and twist out of position. New bushes are not available separately, new links costing about £20 each.

      [​IMG]
      Worn front ARB bushes


      While specialist manufacturers produce even stiffer bars than BMW provide, the use of such additional stiffness at the rear can bring its own problems, in terms of the mounting clamps being torn out of the body, as they are each secured by only a single bolt, and a tongue which fits into a slot.

      My preference is to retain the original 24 mm front bar, while increasing the rear from 15 mm to 17 mm, mounted in Powerflex polyurethane bushes. This makes a small but definite reduction in understeer, sharpening the steering response without introducing any untoward characteristics or placing excessive loads on the mounting straps.

      [​IMG]
      Fitting new drop links to the rear ARB (see seperate article on this subject)


      When fitting a new rear bar, it is preferable to use new drop links, particularly on a pre-April 2001 Z3, as these were fitted with links which often worked off the ends of the bar. Position the link at 76 degrees to the bar, using a cardboard template, and press them into place after lubrication with rubbing alcohol, otherwise known as surgical spirit, which provides the necessary short-term lubrication. Leave the car jacked up for at least half an hour after fitting, to permit the lubricant to evaporate.

      General:

      These modifications will make a notable difference to your car’s handling, in a manner which will not be visually obvious. This factor can be quite important if you are a follower of the ‘Strict originality’ fashion, or are worried that your insurance company may object should you have a claim.
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