de Dion Or IRS

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deDion Or IRS

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1. Time:
   we didn't know how long we had to develop the system before we'd run out of live axles, and the dD system enabled us to use most of the existing structure and geometry of the live axle as a starting point, shortening development and reducing the risk.
   
   
2. Cost:
   the dD system required a few jigs and tools for a few new fabricated parts, but no major investment for castings or exotic parts. (Remember that we were in a period of dubious economics and runaway vehicle legislation, a difficult time to pour huge amounts of money into weird little sports cars. As I remember, any spare money was being used to keep the Twin Cam engine alive until we found a substitute).
   
   
3. Technical:
   I can't agree with your statement that:
   IRS improves the dynamics of a vehicle in every aspect.
   Then or now, a GOOD IRS may be better than a good solid axle, in some, repeat some, circumstances. If it's good, it may give better wheel control on bumpy corners, and may offer better traction in difficult conditions. Even a good IRS, though, is unlikely to give better outright dry road grip than a dD, because the IRS can't maintain the wheel upright in a corner without sacrifices in the geometry that will give up performance elsewhere.
   
   I suppose it comes down to whether a manufacturer wants to use IRS as a genuine technical benefit or a cynical marketing ploy - I doubt that anyone would cite the Triumph Spitfire as an argument for adoption of IRS. Or for that matter, the miserable semi-trailing system that BMW persists in using on its low-end models..
   
   In contrast, the term live axle usually conjures images of Cortinas and cart springs, although the Seven system is actually very effective: giving good location, good geometry and low mass - it's a difficult act to follow ! To get back to the point, we felt that we had a very good live axle suspension, and weren't prepared to give up that level of performance just to have a new acronym on the car. Which leads to the next point...
   
   
4. Capability:
   Neither Reg Price nor I felt that we had the knowledge or resources to design and develop a really good IRS system, considering the stages of concept, design, development and validation (including durability testing). In the early 80's, without today's PCs and modelling software, it needed either a vast amount of experience to lay down the right geometry from scratch, or a lot of time (and money) to get there by trial and error testing. (Around that time, Mercedes claimed to have taken six months to work through the complex kinematics of their multi-link rear suspension ). That's without considering the loads, stresses and compliances....
   We didn't feel confident of getting it all right with little time or money, so went for the system that gave us a head start. We already understood the kinematics of the live axle pretty well, and separating the differential from the axle tube enabled us to improve that geometry in some areas. By carrying over much of the live axle concept, we were able to reduce the amount of durability testing, and concentrate stress work on the new parts...
   
   
5. Weight:
   I can't agree with your statement that IRS doesn't carry any weight disadvantage. I don't think this is generally true, and it was certainly not true in the case of the Seven. We did a detailed study that showed that either dD or IRS would carry a weight penalty over the live axle, because of the sheer mass of the Sierra diff. (Even if a smaller diff existed, I believe the overall result would have been similar).
   An IRS system would have been heavier than dD, when we took into account the structure required to support the links and give the necessary stiffness and integrity. This brings us to....
   
   
6. Structure:
   the Seven structure is nicely laid out to accept a solid axle in a very simple way, so anything else requires surgery. Not impossible, but it all adds cost and mass - just attaching the differential to the tunnel adds both of these; any additional links need to be attached to something, and that something will also need to be bought and weighed.
   
   
7. History:
   Caterham wasn't as well established as a marque in those days, and there was always a reaction from the traditionalists to any change (eg, some customers were heartbroken when we couldn't get chrome bezels on the Smiths instruments any more). We felt that there was a good historical link to the Lotus 11, which would foster acceptance.