The Honda S600 came stock with a very neat rear suspension arrangement were it used a straight axle connected by chains to independently sprung wheels at the rear. Rather ingenious as it does away with camber changes that can cause issues with other independent designs most notably swing axles. The later S800 (except for some early ones) did away with this complication and went with a live rear axle.
My S600 is of course missing the whole rear suspension. Being rather hacked about and the parts themselves being rare it is not worth tracking down the missing pieces. With that said I need some sort of suspension so it seems to make sense to follow the S800 path. So I’ve got diagrams of both the S600 and the later S800 frame design.
On the S800 chassis the rear is quite different. The biggest issue I see right off the bat is a notch in the frame to allow enough travel for the live axle. The notch is of course not needed on the S600 since the differential is fixed in place. I suppose one could go with a jacked up rear end instead but outside of perhaps a 70s Nova I’m not a fan of that look. So it looks like I will have to notch and strengthen my frame.
The type of suspension to be used is the next question to answer. I’ve already ruled out independent rear suspension because I think I am better to do a simpler suspension well rather than have a more complex suspension but (likely) poorly engineered. From what I can tell the S800 uses a four link with panard rod.
Happily there are hundreds of home made and kit derived Locosts (Lotus Seven style roadster usually built on a budget) running around with this exact suspension style so it is well understood and within reach of the home builder. I also built a Locost a couple years back but the suspension bits were mostly off the shelf.
The other option would be to go with leaf springs. This would surely piss off just about every Honda purist but would be quite simple to do. It would consist of a leaf for each side, a couple shocks and handful of brackets. Well … maybe not that easy but about as simple as suspension gets. Of course short wheel base cars with leaf springs aren’t particularly known for the their smooth ride but they can be made to handle ok. I’ll keep that as an option as there is less fabrication.
I think motorcycle coil over shocks have some promise as they have seen some use on Locosts and my S600 will be in the same weight ballpark. Some more research on LocostUSA is needed. The top mount for the stock S600 shocks is still there on my car and should be usable if I can find some short shocks.
Here are some bonus frame blueprint of the S600 pictures