Depends what actual width you mean. Nearly all Thorn tandems and triplets use 559ISO (26") wheels. We have used tyres as narrow as 35mm on our tandems with this wheel size but now prefer wider tyres with 42mm Marathon Supremes being our present preference for reasonable roads. They run very easily and are comfortable and long lasting. Last winter we used 47mm wide basic Marathon on the front and 40mm on the back of our Dawes for local short rides on muddy back lanes and the tread probably helped reduce rear wheel spin on muddy climbs.I have kept a very close eye for side wall failure as the basic Marathons have let us down several times before with the rim hook sawing into the carcass threads. The more expensive Supreme has a lighter carcass but has a good chaffer strip against the hook and has not failed in the same way in our experience.

The Schwalbe site usefully gives maximum loadings per tyre (at maximum pressure) in the detailed specifications. The figures are probably quite cautious but they indicate the way to got on width for good load carrying.

thank you for your response

We have had a Thorn Trident for many years. The tyres we have used are the Schwalbe Marathon Plus 47 which were the type fitted when we had it. With our all up weight of about 180 kg + triplet(large!) + luggage we use a pressure of around 75 PSI. The tyres being quite wide roll pretty well considering their tread and type at this PSI. They are quite heavy, but robust and the bike seems sure footed and stable even at speed and good on rougher surfaces. Although they do look a bit wide for the rims, we haven't had any noticable damage against the rim's hook at this PSI.

thank you i got new tyres for it today its a really good skinny tyre not sure what make though.