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Doing the twist Q

Improving a production live axle rear suspension

Over here in Australia we have a race class called Improved Production. A popular car is the Gen 1 Mazda RX7 with

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. Here Mark answers your chassis set up and handling queries. If you have a question to put to him Email: markortizauto@ Tel: +1 704-933-8876 Write: Mark Ortiz 155 Wankel Drive , Kannapolis NC 28083-8200, USA

a live rear axle. The rules say the suspension must retain all the original links and that all bushes must be elastomeric, so no spherical bearings are allowed. Additional longitudinal links are permitted and lateral location devices are free. Conventional wisdom says that the original links are prone to binding (they consist of parallel lower trailing arms and shorter upper arms about 10in long and angled inwards towards the front). Most


or readers unfamiliar with this suspension, it has four trailing links for longitudinal

location and torque reaction, and a Watts linkage for lateral location. The upper trailing links do splay out a bit at their trailing ends, mainly just to clear the springs. This is not, however, a triangulated four link, where the link angles in plan view are sufficient to allow the four links to provide lateral location without any other mechanism.

“the system produces

capricious variations in roll resistance distribution”

people use very soft bushes in the top arms and add another set of parallel top trailing arms, but my question is this: the track is about 60in so, if the outside suspension is compressed, say, 3in in a corner and the inside extends 3in the total difference is 6in, giving an axle angle of about six degrees compared to the chassis. I accept that this would twist the suspension arms and perhaps cause binding, but would the degree of twist not be the same in the new parallel top arms as well? And does the fact that the original top arms are angled make the situation any worse?

The upper links are much shorter than the lowers. This is partly for packaging, but also it makes the side view geometry (and, accordingly, the anti-squat and anti-lift) roughly in accordance with Olley’s Rule: that is that link lengths should be inversely proportional to their height above ground. This makes the longitudinal antis roughly consistent, regardless of ride height. The problem is the side view swing arm length fluctuates with suspension displacement. Depending on where the suspension is in its travel range, the longitudinal links try to rotate the axle housing forward with compression, or rearward, or neither, at a varying rate. In ride, no problem, but in combinations of ride and roll, the longitudinal links on each side of the car try to rotate the axle housing different amounts, and / or in different directions. Any time the links try to create differing rotational displacements of the housing, they effectively turn the housing into an anti-roll bar. This might be lived with if

Gen 1 RX-7 has a beam rear axle. Adding a further pair of parallel links won’t improve it much, but a single, central third link makes it more consistent

the effect were consistent (the car has a rear anti-roll bar, so eliminating roll resistance is not the point), but the problem is that this component of roll resistance varies erratically in different combinations of ride and roll displacement. In a straight line,

the system behaves well, even if the surface is undulating. And in cornering, it also behaves well, as long as the surface is smooth. However, when presented with hard cornering and a bumpy or undulating surface at the same time, the system produces capricious variations in roll resistance distribution. It is true that any cylindrical bushing only allows torsional movement by deforming, and that is inescapable if sphericals are prohibited, but that is not the main problem with the stock layout. In the US, at least under SCCA rules, bushing material or construction is entirely free, but the links still have to be there. It is, however, permitted to add ‘traction control devices’. The most common approach therefore is to use either urethane bushings or spherical bushings in the lower links, replace the bushings in the upper links with foam rubber, and add a single new central upper link, bent to clear the driveshaft tunnel. This effectively converts the suspension to a three-link design with a Watts linkage for lateral location. That does at least make the

system consistent. It doesn’t eliminate torque roll, which should be an objective in any live axle suspension design, but it does solves the most urgent problem.

January 2012 • 47

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