Above I said 'to each his own', but before I edited it out, it read 'to each his own, but to me a sidecar has absolutely positively 0 appeal'.
I removed that because I thought it was a bit abrasive, but I really see no point in fitting a sidecar to what might otherwise be a decent bike. I never rode (drove?) a sidecar combination, and IMO that appendix removes everything that makes bike riding worthwhile. You cannot lean the bike. There are historic reasons for it, but nowadays, if you want to haul more, just take a car. Anyway, that's just my personal view, I know it's not shared by all, and up until not too long ago people were enthusiastically racing sidecars combinations (both on road & MX). I hated them because they ruined the track, making deep ruts & gullies.
Anyway, back to the technical. I do not know the answer to your question. The ESs had a swingarm, and that could easily be made adjustable (like at the rear), but I don't think it was, at least not standard:
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But that looks quite different from the pic I showed earlier (which also has a nice & non-standard double leading shoe), so modification
is possible.
I believe the BK had telescopic forks; upside-down avant la lettre.
For sidecar aficionados it would be useful if you could please explain in more detail what you did, why you wanted that, and how trail influences a bike's behaviour.
David, I understand earlier 60's models all had the swingarm front end (aka Earles forks), which was very suitable also for sidecar duty, but the '69 US model indeed got telescopic forks:
That looks like a pretty decent bike to me, and I bow to your BMW knowledge, recognising that frame from the earlier pic! I believe that was advertised as an early 60's R50, but I cannot find it back.