The GTM Libra has a conventional double wishbone suspension at the front and double trailing arm suspension at the rear. The mid-engined layout gives a near-perfect weight distribution of 45% front / 55% rear and its low polar moment of inertia makes it inherently nimble. In 2004 GTM embarked on a significant ride and handling programme with MG-Rover's X Power Chassis Development team. This programme resulted in some significant changes to both the hardware and wheel geometry.
Front Suspension
The front double wishbone suspension has long and wide based wishbones. Long wishbones give small camber change, good track control and less roll centre movement. Wide based wishbones minimise toe and castor change by reducing the shock and braking loads on the inner bushes. This reduces unwanted deflection and movement of the outer joints, giving better stability and a more consistent steering feel. |
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Anti-dive suspension geometry imposes castor change with suspension movement, so the front suspension has very little anti-dive. This is enough to allow the wheel to move back slightly under a bump condition, reducing the shock transmitted into the car and improving the ride.
The steering is unassisted rack and pinion with 2.7 turns lock to lock. The rack is bespoke to GTM with pre-load tuned to the weight of the car and responsiveness necessary to support its sports nature. To make the steering communicative without too much kickback, the scrub radius (ground offset) is 11mm positive. Self-centring is provided by 7 degrees of castor angle, and bump steer is virtually non-existent with near-perfect suspension geometry.
Rear Suspension
The rear double trailing arm suspension concept is similar to a double wishbone suspension turned around 90 degrees. This allows the chassis to be shortened to reduce weight, give better torsional stiffness and improve packaging around the engine.
In 2004 GTM concentrated on making further improvements to the rear trailing arms by introducing new "Pillow Ball Joints" with innovative cam bolts for track and camber adjustment. The arms are now made from high strength steel with a space frame design. |
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These changes increase the stiffness of the rear arms and as such improve lateral control.
In addition, the hub lower ball joint is now offset to introduce better lateral load input to the wheel and with slight negative static camber there is a natural toe in of the wheel under bump.
Both anti-lift and anti-squat geometry are applied at the rear to compensate for the lack of anti-dive at the front and reduce the amount of vehicle pitch during heavy braking or hard acceleration. |
Handling
The front static camber is 0 degrees to give maximum adhesion during heavy braking. A low front roll camber compensation of 20% gives consistent grip and feel on bumpy roads at the expense of some ultimate cornering grip on the track.
The rear static camber of -2 degrees and 30 minutes combined toe-in is designed to give the maximum possible adhesion from the outside rear tyre when accelerating out of a corner. The greater rear roll camber compensation of 29%, and the small amount of toe-in behaviour into bump help to ensure a stable understeering balance.
The roll centres are positioned as close as possible to ground level to minimise suspension jacking, give good camber control and improve feel. Careful design work on Computer Aided Design ensures any roll centre movement when the car rolls is kept to an absolute minimum to give the car a consistent feel whilst cornering.
Body roll control can be further improved by fitment of GTM's anti roll bars. These are for both the front and rear suspension and use a 18mm bar, which has been tuned to fit with the springs and shock absorbers.
Wheels and Tyres
The wheel and tyre combination front and rear have been optimised to ensure correct grip balance for the weight distribution of the Libra. This set up has resulted in 16" alloy wheels with bespoke Goodyear 195/45 front tyres and 215/45 rear tyres.
Tuning
The front static camber of 0 degrees can easily be adjusted +/- 2 degrees by shimming the wishbone mounts in and out from the monocoque chassis. The rear static camber can be adjusted by the trailing arms' cam bolts to increase cornering grip or give the car a more neutral oversteering balance.
Custom made coil-over dampers are used on each corner. These have adjustable spring seats to adjust ride height and corner weights and have 22 position simultaneous bump and rebound damping adjustment in-situ on the car. Springs can easily be changed so you can tune the vehicle to your own liking, and a little progressive rate springing is built in, due to the suspension linkage geometry.
Please see the Suspension Settings page for more information on optimum settings. |
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