I read on the DVLA website that you are allowed to add crossmembers and braces between chassis rails, and you are allowed to cut off mounts you don't need. You are also allowed to "box in" a 3 sided chassis rail.
But you can't shorten or lengthen the chassis, unless you are bolting the lengthening bit to existing boltholes.
But I can't see anything specifically mentioning springs.
A while ago I was thinking of making it bolt on, without removing the existing leaf mounts - so I can swap between leafs and 3 link.
I wasn't thinking of a panhard (at least, not on the rear) but the idea is there - to make it bolt on like the top link in that panhard setup pic. Plan was to make a spring mount and shock mount that bolts on/around the chassis, then the mount for the wishbone can be welded between the chassis rails. Then the leaf spring mounts can be used for Vitara trailing arms (they're just exactly the right dimensions, funnily enough). Et voila, chassis is unchanged except for a new crossmember, which is perfectly legal.
That way if they wanted to have a whinge, just swap back to the leafs.
It's just that SJ leafs... Ugggh, every time I go over a speedbump, it's like being punched in the arse by Evander Holyfield.
Last time I offloaded it, me and my brother both ended up with back injuries!
If I did something like this, I'd like to take it to be inspected under the "8 point system", so I know no-one can do me for it! I know I can swap in YJ's, but coils appeal to me more. Easier to get the ride height I want, don't have to worry about leafs inverting (so I can have more upward travel), no shackles digging into the mud...
I'm not sure if I should make something like this, drive to VOSA on leafs, swap to coils in the carpark, get it inspected...
Or buy a Jimny. Which costs more in insurance and tax...
