My Spyder has a tendency to bottom out at the rear on some kind of bumps. The low point is the bracket that holds the gear change cable outer and when that hits the road it tends to bend, lose the spring clip that holds the cable in place and then you lose the ability to change gear. Its happened to me several times now so I've got quite good at reaching under the back to manually put it into third at the gear box bell crank then driving home in just third gear. I've also wired the spring clip so when it pings off it stays attached to the car as they are becoming rare now. The bracket straightens up okay in the vice but sooner or later its going to give up and break all together. The exhaust is only another 5mm or so higher and that has taken a few knocks too.
I was thinking of ways to avoid this happening so today slackened off the springs and raised/lowered the car on jacks and took the following measurements:
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IIRC the GTM recommended ride height is 150mm measured at the rear of the sill. At this height there is only 20mm spare throw on the shocks before they fully extend and I've heard them knock when the car goes light indicating I've run out of droop so I'd dropped the ride height to 140 as then I never get the knock. This does exacerbate the grounding problem though. I only measured down to 90 under the sill as that's as low as my jacks go. At that height the gear bracket is only 30mm off the ground. When its grounded it been on country lanes where you typically straddle the crown of the road. It seems road camber is typically 1:30 so if the wheels are 1600 apart the road crown is 800/30 = 27mm above the wheels so too close for comfort and its quite feasible for the shocks to compress more as there is still 35mm compression left in the shocks stroke.
There is currently a microcell foam bump stop on the coilovers. This has a free length of 70mm but seems quite soft - I was able to compress it to 35mm with just my fingers but it does get stiffer from then on but it seems the gear bracket will have touched down by the time it has squashed to 35mm. I decided to add additional bump stop height and had some polybushes from my Locost with a 13mm inner bore (shock shaft is 12mm). The top hats were 18mm tall so by putting one between the top of the shock and the microcell foam I've started the foam compressing 18mm sooner. I'm hoping this will now mean that the bump stops will become firm enough to prevent the gear bracket bottoming out. Time will tell!