Hi All, A friend of mine owns a 1958 tiger cub converted for Trials and my question is how important is it that the wheels are central to the frame and wheels in line as to centre? There is very little clearance for the chain as regards the rear tyre. He is a little obsessed to get the wheels central and in line and has started to cut and weld (bodge)the frame. I think he is slowly destroying the bike. The swingarm has been widened a long time ago and the engine was over in the frame as far as it would practically go but the wheel rim was off centre so as to clear the chain. I suspect they mostly all suffer this problem.Is there A way to get this right without cutting and shutting the front frame loop? Any info appreciated.I suppose all the new replacement frames for Cubs and Bantams are symmetrical?

Thanks.

the wheels need to be inline as near as damn it.. when the bike rolls it will be fighting to run straight all the time if they are offset.. a wise old fella said ride the sections not the bike..

 

the engine needs offsetting by relocating the the front and rear near side mounts and the wheel needs 'dishing' appropriately.. do it right

You want the wheels in line if possible (though many Bultos in the day were not once they had been used a bit☺), but all central is not always achievable, though one would hope that the front certainly was in line with the frame.

With modern purpose made, slimmer, trials motors things are better but when using a road going motor compromise is necessary to get a chain line, as your friend has discovered.

I recall an interview with Sammy Miller in the Sixties where this topic came up and he said that not having the wheels on a common centreline helped balance at very low speed. I'm not sure if I entirely agree with that but I certainly would not be bothered about a bit of "off centre" for trials use.

No doubt someone with practical experience of adapting frames will be on soon with a view.

Edited January 1, 201610 yr by 2stroke4stroke

The Cub is always a compromise simply because of where the final drive sprocket is positioned in the engine. The best compromise in my opinion is to widen the swinging arm in order to move the tire away from the chain. Offsetting the engine slightly also improves this and to some extent these two modifications cancel out the bad effects of the other.

Edited January 1, 201610 yr by blocky

My C15 is quite a way out, I spent ages when rebuilding it to get it right but it was never going I happen. For trials use you'll never notice it, I don't even notice it on the road to be honest but I bet looks a bit wierd from behind!

the inside face of the chain as it goes over the engine sprocket is close to 2" from the centre line of the engine.

so,

if you have a 4" tyre on the centre line of the frame , the edge of the tyre is 2" from the centre line or pretty much in line with the inside face of the chain, leaving no space for a chain guard, a bit of clearance or any allowance for tyre defection.

There are various options, all of which have some degree of compromise,

pack out the engine.. but the exhaust and oil pump manifold will hit the frame before you get perfect alignment

pack out the rear sprocket... the chain will miss the tyre but will be running out of line. You put more strain on the tapped holes in the hub and you may need to shave the outside diameter of the brake plate as it will rub on the overhanging sprocket if you go too far

build more offset into the wheel.. but the wheels are then no longer perfectly in line. (bearing in mind that the frames were never that good that the wheels were perectly in line in the first place, and it would take a rider with the skills of valentino Rossi/Dougie Lampkin to have handling issues with a cub). The spokes on the RHS of the hub will be almost vertical, compromising the strength of the wheel. (hence the desire of wider hubs)

so.. there is usually a compromise between any combination of all three which still usually leaves the chain and wheel running slightly out of line.