Wednesday, December 15, 2010

Shock absorbers Part 1 - a cautionary tale

A question for the class... Hands up all those who can tell me what happens when a shock absorber is used repeatedly as a bump stop?

That's right - it breaks. However, if it is a well made, high quality shock absorber such as are built by our friends at Koni, it only breaks internally, and the rod and shell remain intact. When it comes to actual damping though, well it doesn't really work any more.

That is what the Koni representative told Hugh today about my shocks. Unbelievably, despite the sorry state of the things (and I just know my shocks are going to become one of those legendary tales told by greybeard shock techs about the Unbelievable Things People Have Done To Their Shock Absorbers), Koni can rebuild them (presumably in slow motion and to the faint strains of the "6 million dollar man" theme music). The interesting questions now are why did this happen, and how do we ensure it doesn't happen again...

On hearing the news from Koni, Hugh thoughtfully took a tape measure to the Ricciardi, and discovered an interesting factoid. Currently the rear shock travel is 40mm less than it would be in a 1750 Berlina, and 30mm less than the absolute minimum required to ensure that the suspension bottoms out on the real bump stops before the shock hits the limit of its compression range.

So, the shock was indeed acting as a bump stop, and in all likelihood the actual bump stops in the Ricciardi have never been used. Every time the car hit a big bump, or even a small bump while one or both rear wheels was already in compression, the shock bottomed out. Interestingly modern Konis have an internal rubber stop to ensure the shock doesn't get damaged if it is compressed too far, but mine are the older design and didn't have it. In any case, it's not designed to be the suspension bump stop, just a safety feature in case the unexpected happens.

This does go some way toward explaining why the transition between "cornering on rails" and "facing the wrong way in a cloud of tyre smoke" has always been very sudden, especially on a bumpy corner. There's nothing like suddenly going from normal suspension compliance to infinite roll rate to make things interesting...

It also explains why the rear shocks are shot but the fronts are fine, but we still come back to the questions of "why?" and "what next?". The car could certainly stand to come up a little in ride height, but 40mm is a fair whack. There would definitely be 105 coupes running around that are lower than that. We could go up 30mm and it would probably be ok, but wouldn't be lowerable again if required in future.

Hugh's view is that the upper spring mount/shock tower assembly is just too short, and at the moment I can't see any other explanation. The rest of the geometry is correct, the ride height isn't ridiculously low, and it's not a mounting problem, because the spring mount and shock mount are the same part.

It's just conceivable that there's supposed to be an extra part between the shock tower and the spring, but it looks correct as-is (the bottom of the tower has the spring pan built in to it). Possibly it's not an original Ricciardi part and was substituted by the first owner during the build - some of the other cars were raced very hard in the 90s, and their owners would certainly have discovered this problem had it existed. When I spoke to David Williams last week he was a bit surprised that I'd had problems with the car bottoming out, and I think that in the context he'd have mentioned this if it was a known issue.

The logical move seems to be to extend the shock tower, as Hugh suggests. That way we can run at any ride height if required. He already had the welding gear warmed up when he rang to give me the glad tidings, and it should still all be done by the weekend.

The good news is that when I get it back the car should handle not just better than it has in my time of ownership, but better than it ever has.

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