Suspension bridge

With the ever-increasing popularity of race track days, the launch of the Lotus Exige S claims to close the gap between road comfort and circuit performance. Charles Clarke reports.

Lurking beneath the surface of most mechanical engineers you’ll find a latent boy (or girl) racer, but until fairly recently this is where it had to stay.

This thirst for speed can be slaked by the increasing popularity of track days, where you drive your car to the track and jostle with other weekend warriors for your 15 minutes of fame.

At best this is an unhappy compromise. Road cars don’t go round tracks very well without modification. And if you’ve modified your car for the circuit then the road experience is very uncomfortable. Family and friends rarely appreciate the nuances of superior ‘ride and handling’ that stiffened suspension can provide.

So to do the track day experience properly you had to have a track car as well — until now. Lotus has introduced the Exige S, ‘the fastest accelerating product car in the world’ (with the exception of the Ferrari F430 that beats it by a whisker — but then it is nearly four times the price).

The engineering challenge was complex. On the one hand, Lotus set out to design a car that performed equally well on both the road and the track. On the other, the company wanted user friendly adjustable suspension to cater for dyed-in-the-wool Lotus fans who liked to ‘get out and get under’.

This meant refining the geometry and the damping characteristics, for which the company used the Lotus-developed suspension kinematics and compliance software Shark and Raven, which is commercially available as LSA Lotus Suspension Analysis software.

The company also made extensive use of its SKCMS (Suspension Kinematics and Compliance Measurement System) test rig. This is a unique tool and allows every development stage to be quantified to ensure that the changes and the adjustments you are making actually comply with the kinematics analysis.

Very often you can make changes or substitute components — but if these aren’t accurately within specification they can produce undesirable results on the test track.

With the SKCMS test rig, however, you can measure and isolate every component to verify that the physical build is exactly the same as the virtual build.

As test drivers cannot isolate single components when evaluating road or track performance the rig is invaluable. For Lotus, any new car or suspension system developed first goes to the rig to check that the basics are right, providing a verified, quantified platform from which to start the development.

The Exige S is the latest evolution of the resurgent Lotus Elise/Exige brand and it is arguably the best yet. The marque began in the mid 1990s. Early versions were fragile and spartan and you had to be a battle-hardened Lotus afficionado to live with the idiosyncrasies.

The Series 2 was much better and every successive model has introduced more refinement and amenity to where we are today with the Exige S. Air-conditioning, airbags, sports packs and even carpets are all available — getting in still means bending double, but that is the only downside.

The fundamental suspension geometry is the same as previous models. The only change is the substitution of one-way adjustable aluminium dampers all round. This allows a little more comfort on the road.

When you reach the track you crawl underneath and adjust the dampers — you don’t need any tools — which are very accessible and adjusted using a numbered knurled nut at the base of the damper (outboard, bottom end). This can be turned with your fingers between 10 click stops. It is a one-way adjuster that adjusts compression and rebound simultaneously. Anticlockwise increases the damping force and clockwise decreases it.

The difference it makes is startling — if you attempt to take to the track with the road settings it doesn’t take long to feel totally frustrated. This doesn’t happen on the road because you are usually not pushing so hard and so continuously. The adjustment increases the damping force by about 30-40 per cent.

Adjustable dampers only work within specific ranges. The Exige is a fairly extreme sportscar already and it is amenable to this kind of damping regime. It is not possible using adjustable dampers, to achieve ultimate road comfort and ultimate track performance — no conventional adjustable damper yet invented can cope with this wide a range.

The front anti-roll bar is also adjustable using a spanner. There are basically five positions for adjusting the anti-roll bar lever ratio.

You can adjust the dampers with the car on the ground, while the anti-roll bar adjustment needs ramps or a trolley jack. There is also a facility that enables you to lower the ride height by 10mm — this is done using adjustable spring platforms — again an easy spanner job.

Lotus doesn’t recommend lowering the car more than 10mm because if it’s any lower it won’t cope with standard speed bumps or race track kerbs.

The Ultimate in Exige performance is the recently-announced Lotus GT3 race car. The company has an amazing capacity to crank out new cars or new variants of existing cars very quickly. the GT3 took two months and should be racing this year.

The main software tools used to develop the car allowed engineers to refine the geometry and the damping characteristics