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The following rhetoric shares my experiences during past work on my Seven's suspension. I make no claim as to its technical accuracy and, in fact, hope it stimulates some alternative views that I can then try out.
As is documented elsewhere in this site I run a Seven with Vegantune twincam engine. This is somewhat up from the 1600cc Kent with 110bhp as fitted to the original Eugene.
It was the hike in power that showed up the inadequacies of the original suspension. Like many people I had not thought much about the suspension, concentrating instead on the engine and how to get more power out of it. It became obvious that its no good having the power if it can't be put down on to the road. Therefore I decided to attempt to improve the cars handling and road holding.
In the same vain as the way the rest of the car was built (on the cheap) any improvements needed to be within a strict budget (Sarah has a lot to do with this) and without too drastic alterations to the basic '84 specification. This was pre the last re-build and I still had the original '84 chassis.
The obvious starting point seemed at first to be the springs and dampers, which at the back were the originals and at the front at least five years old. The idea of adjustable platform dampers also appealed, enabling corner weights to be set. Additionally, the original spring rates of 75-lbs. back, 105-lbs. front were on the low side, making the car run more on the dampers than the springs.
Before any decisions were made I decided to talk to those in the know, including Jim Whiting, Arrowstar, Red Line and Caterham. Initially, this was in as much an effort to find the cheapest components as anything else, but as usual there was no shortage of alternative ideas from all those I spoke to.
What their suggestions added up to was a three-stage approach to improving the cars handling and road holding.
The first stage, surprisingly to me, had nothing to do with springs or dampers. Unless the dampers are leaking badly, or the springs collapsed on to the bump stops, changing them would bring little improvement on their own. What would make a difference would be to improve the geometry of the front suspension to prevent it altering during cornering.
The second stage involved finding new tyres to suit the improved suspension and to replace the existing Goodyear NCT'S, that were close to the legal limit.
The third and final stage is where I intended to start and involved new dampers and springs with increased spring rates, but to compliment the other changes rather than to cure the initial problem.
Stage One - Top link & anti-roll bar.
The basic '84 spec front suspension harked back to the original Chapman concept of using the anti-roll bar as half of the upper wishbone. While fulfilling the Lotus ideal of keeping weight to a minimum the increasing engine power of today's Sevens and cornering forces of new tyres has increasingly shown up the limitations of the original design.
The old top link and anti-roll bar setup allowed the top link to move in it's mounts, allowing the upright to twist during cornering. This had obvious affect on the stability of the car. To overcome these problems Caterham evolved the Chapman design through a couple of stages to the present full double wishbone layout with independent anti-roll bar. This setup also includes the benefit of the top links being adjustable, enabling front wheel camber angles to be set.
The idea, therefore, was to incorporate the latest top link and anti-roll bar into the existing '84 lower wishbone and upright.
The first potential problem was that the taper of the new ball-joint was different to that of the original Triumph upright/hub carrier. Caterham have developed a new unit complete with lower rose-joint replacing the original trunnion, but replacing all this can get expensive and is money better spent elsewhere. In reality the tapers were close enough that ensuring a 'tight' nut on the ball-joint enabled the new and old to work together. A thought here is that Midas Metalcraft make traditional top links that are adjustable. I wonder if the thread is the same as the Caterham wishbone as this would give the correct taper?
Fitment of the new components was easy enough, except for the radiator fan which needed spacing to move it away from the larger diameter anti-roll bar and the recesses in the nose cone which needed enlarging, also to accommodate the bar. The only other part requiring some thought was the holes that have to be cut in the body panels to accept the new front wishbone mounts.
First impressions are important and the first was a crisper turn-in that appeared to respond quicker to initial movement of the steering wheel. Mid-corner control again seemed more precise with fewer tendencies to drift, either into or out of the bend, requiring less corrective input at the steering wheel. The overall stance of the car through the corner was also flatter with less body roll, inspiring additional confidence.
Negative aspects included the car being more 'twitchy' in a straight line due in part to the improved response of the steering and the fact I had set a smaller camber angle than previously. A second and somewhat less desirable result of the change of anti-roll bar was a less composed ride over rough or uneven surfaces. Its not so much a rough ride as would be caused by stiffer springs, but a rolling or jerking motion caused by an inverse reaction of the anti-roll bar to the wheels becoming uneven.
Anti-roll bars work well at increasing roll stiffness without increasing spring stiffness, preventing straight-line suspension from becoming too harsh. The problem occurs when hitting a bump or a pothole displaces just one wheel. Under these conditions the anti-roll bar attempts to keep the wheels level, which is obviously impossible, resulting in the car body rolling to compensate. Not a desirable aspect, especially if its mid corner. For racing cars, where track surfaces are smooth, this is not a problem and thick anti-roll bars can be used. Having said that, its worth remembering that when an anti-roll bar is twisted during cornering it attempts to untwist itself and the suspension with it. As this is impossible there is a resultant transfer of weight from the already lightly loaded inner wheel to the higher loaded outer, which causes understeer by reducing the available grip at the front compared to that at the rear.
Its interesting to note that the original Lotus Elan, one of the best handling cars, had neutral cornering and was designed without the inclusion of anti-roll bars.
For this reason, and at the suggestion of Jim Whiting, I've used the 5/8" bar, up on the original 1/2" but not as large as some of the alternatives. It seems to be the correct compromise although I would like to try a 1/2" if there was one available.
Its also worth thinking of wheel camber angles here as well. Caterham recommend 1 degree negative at the front for present road cars and use up to 2, possibly 3 degrees in race cars, all round, except for cars with crossply (bias belted) slicks which run 1/4 to 1/2 degree. The large camber angle is intended to increase grip by leaning the outer wheel into the corner.
This 1 degree or more camber may be correct but experimentation with my car suggested otherwise. Reducing the camber to virtually zero degrees gave an appreciable decrease in understeer which at first is difficult to understand. The reason, after discussion with Reg Price, seemed to be due to the NCT tyres in use at that time. They were of a fairly flat profile and consequently with less camber angle more of the tyre tread, of both the outer and inner wheels, was in positive contact with the road during cornering. Combined with a zero degree of toe this seemed to be the best setup for that car/wheel/tyre combination. When I moved to a more rounded profile tyre, as with most Yokohamas, an increase to 1 degree camber proved beneficial.
It cost around £200 for the parts and second to the money spent on the engine has been the best investment so far. Much like the enhancements to the engine it has had a measurable effect on reducing A to B times.
Stage Two - Tyres.
When it comes to tyres there are a bewildering array of possibilities. From race-bred rubber to those more at home on a touring car.
When I first bought my Seven it was shod with Goodyear G800 185/70/13, which seemed very capable as a first experience. At about the same time Caterham started using NCT'S, so when replacements were due I changed to them. The difference was amazing. More grip, better handling and they looked better too! But time moved on and now Caterham started using Michelin MXTs, which they said were better again (but they looked like saloon tyres!).
Tyres come equipped with a variety of information moulded on to them, which went through a change of description so the following breakdown may be of use but may now be dated.
Take a 185/70 R13 86H for example:
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