Rover SD1 Suspension – the Macpherson the SD1 Has
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Rover SD1 Suspension – The MacPherson The SD1 has independent front suspension. Each front wheel moves individually from the other and the following types are common. • Leading/trailing link systems • Double wishbones • Multi link systems • MacPherson Multi link systems can provide the best comfort and handling. Next best is the double wishbone if properly set up. For instance most American cars use it but the geometry often leaves room for improvement. And then there is the MacPherson, giving good comfort and reasonable handling. Why then, did Rover choose a slightly inferior system? They preferred the double wishbone approach but it would not leave room to locate the catalytic converters needed for the U.S. market, plus the cost was higher. So the simple MacPherson was chosen which consists basically of a coil spring and shock absorber built into the spring leg. The leg pivots on a ball joint on the lower control arm. This can be either an ordinary A- arm or a narrow lower control arm which locates the lower end of the strut in the transverse direction and a separate member called a radius rod locating the assembly in the longitudinal direction. However on the SD1 the anti-roll bar serves a double function as the longitudinal link taking the drive and brake forces and thereby eliminating the separate radius rod. At the top, the SD1 has a roller bearing (not shown here) to allow the spring leg to turn without the spring winding up. The strut itself is the load-bearing member in the assembly. ie: - The spring and shock absorber hold the car up. The steering is connected directly to the lower spring leg or to an arm from the front or (as on the SD1) back of the spindle. A small camber change during bounce and rebound is characteristic of this design. The vehicle roll centre is controlled by raising or lowering the inboard anchor point of the lower control arm, and by varying the steering axis inclination (KPI). Roll centre height. The determination of roll centre height is as follows. • Draw a line perpendicular to the spring axis (1) • Draw a line from the lower control arm of the leg to the opposite side (2) • 1 and 2 intersect at Y • Do the same for the other side and find X • Draw a line from Y to the centre of the opposite tyre footprint (3), and the same from X. • The roll centre height is at the intersection of these two lines. At the design level, what can be learned from this? • Moving the leg more upright without changing lower control arm lowers the roll centre. • Greater inclination raises the roll centre. • Changing spring leg angle has no big effects on roll centre height. • Raising or lowering the lower control arm pick up point has far more effect but will give extreme positive camber on rebound and cause large track variations. To be avoided. • Choice of roll centre can be made only within strict limits. • High roll centre isn't possible with MacPherson. To compensate anti-roll bar is standard. • Lengthening lower control arm can reduce camber variations but it impacts engine space. • Roll centre isn't necessarily always in the middle of the car, when for instance the right wheel hits a bump the roll centre is slightly offset to the left. • The roll centre moves inwards when cornering. So MacPherson handling is somewhat less than perfect but Spen King (designer of the suspension) created an appropriate set-up for the SD1 within the system limits. The roll centre is relatively high and the lower control arm as long as possible. Camber changes are not that big on the SD1. The KPI angle (see basic suspension) is also not very high so the car is not unduly influenced by lateral forces, and the SD1 is equipped with a sturdy anti roll bar. MacPherson tends to be very sensitive to poor wheel balance and tyre uniformity because the mounting points are so far apart. The wheel forces caused by imbalance are magnified at the suspension pick-up points due to the leverage effects. It also has low “unsprung” weight and although the spring leg is heavier than an upper control arm, the upper part with spring and shock absorber is “sprung” weight. Another advantage is that shock loads from the suspension are distributed in a widespread area of the body enhancing the possibility of a lighter unitary body. So MacPherson has its Pro's and Con's. It's not that bad, but not that good either. Even so it can be better than a mediocre double wishbone set up, and the Rover SD1 set-up is a good one. By Comparison to the Rover P6. Out of interest, comparing the SD1 front suspension with the more complex system on its predecessor, the P6, which has a horizontal spring mounted against the bulkhead. The long lower control arm of the P6 gives only small camber changes. By using a leading upper arm the caster angle also varies on the P6 (Not so with our SD1). To minimise caster variations Rover used a very long upper leading arm. The advantage of this design was a very high roll centre, 7 inches above the ground. No anti-roll bars needed here. As roll was minimal due to the high roll centre, soft springs could be used and because the set-up gave enough room for generous wheel travel the P6's ride was very smooth. The unsprung weight is comparable with the SD1 (at least for the front suspension – the rear suspension is another story). So the P6 system worked better by a good margin but it required detachable front wings to reach the upper suspension components. This was out of the question for the SD1 (due to cost) so the excellent P6 suspension set up was never carried over to its successor. At the user level, is there room to improve the SD1 suspension? Yes! Of course! Moving the suspension pick up points to optimise them even further for specific driving conditions is possible but beyond the capacity of an average enthusiast, but here are some other points to consider. • Fit Vitesse wheels to reduce the unsprung weight (They also look better). • Use quality tyres. (Expensive Michelins tend to work good for the SD1). • Carefully balance the wheels to eliminate MacPherson’s sensitivity to this. • Fit the best shock absorbers to give superior damping and long life. Koni's apparently have the edge over Monroe or Spax. (It’s a Dutch thing!!) • Use polyurethane bushes to provide better horizontal location of the suspension and give better feel in the steering. They last longer than the rubber ones, which ideally should be replaced every 5 years. They definitely stiffen things up but generate a harsher ride. • Use stiffer/lower front springs to make the car more stable (more understeer). • Alternatively use a heavier front anti-roll bar for better road holding because it only stiffens the suspension a bit when one wheel is lifting. As a learning exercise, I distilled and anglicised the above information from the Dutch Rover SD1 web-site and I thank them for access to the material used. I made some alterations to expressions I found unfamiliar and I trust it bears examination by MacPherson Freaks. As companions to this essay, additional material is prepared on The Basic Suspension and The Live Axle, also fitted to the Rover SD1. Please let me have any errors and omissions. Ramon Alban Tel 01234 838770 [email protected] .