Newbie, cafe racer "big" tyres question

[conkers]

Hello all...

I am helping a mate with his Cb650 '82) cafe racer build and it's time for new tyres. Does anybody know or can suggest what the biggest (widest) tyres he might be able to fit between the swingarm/forks?
He doesn't ever ride quick and is aware that most likely the standard sizes will give the best ride and comfort, but he is really more after the looks, he will be using the original wheels.

Any help or suggestions appreciated.

Conk.

[Bryanj]

I think from memory about 425/85 x 18 is the biggest without rubbing

[pae]

Just measured up the Avon Roadrider on the rear of mine (120/90-18). Avon says this tyre is 129mm wide and there is 27 mm total clearance so you can (in theory) get a 155mm tyre in there, but that's with no clearance and assumes my dimensions are spot on. I've not got a chain hooked up at the moment so the wheel might need to come forward and that will narrow down the space available as the swingarm tapers in.

On the front I've got a 100/90-19 (109mm wide ) and this has 34mm clearance total, so you could get something up to 143mm wide

http://www.avon-tyres.co.uk/motorcycle/roadrider

Avon list a 140/70-18 which is 142 wide and almost the same circumference. Can't see anything else for the front, although other makes might do something.

This is measured on my 1979 650Z (similar to the one in the picture)