Hi M, I have tried two types, one with sealed bearings and one without. In addition, I tried units with separate axles and integral axles. Ironically, I found that the integral axle with sealed bearings do well. Having said that, the sealed bearings are a real pain to maintain with their special use once nut. It is also a pain (and costly) to change the bearings. It is not a bearing that is easily changed at the side of the road.
I have a trailer with a single axle for a 1.5 tonne boat (for sale incidentally, if anyone is interested) and that is great, but I do not use it in sea water and I use low profile tyres.
Suppliers are the usual Indespension and they will do a little price matching due to the pressures now. Alternatively try Banbury Trailers:
http://www.banburytrailers.co.uk/The MD has been involved in trailers for years and builds his own axles, which can be adjusted for ride height. Mention OSB as he is a friendly dealer and manufacturer, who may do a deal, you never know. Cheers BN
