Hello all

Im getting near (I hope) the end of my 1947 M20 restoration. I have been refitting rear wheel and and trying to alighn it in the frame. This apears a bit tricky and not as obvious as i would expect. The alighnmnet and chain tension being set by the snail cam on one side and the bolt and lock nut on the other side that goes though the frame and presumaby pushes on axle. i made some measuremenst from frame 'chain stays' on each side to the rim of the wheel and they are very different on each side. I was expectng them to be the same with the wheel being in the centre of the chain stays. Is this to be expected?

Looking at service sheet 710x on the diagram for the M20,M21 and M23 rigid, the plan view shows the frame centre line to be asymmetric about the chain stays. one side is 4" and other 3 13/32" about centre. Would this be the reason?

In an added complexity, my bike has a somewwhat non standard back end and apparently i have a 'crinkle hub' wheel and also a 3/8" chain rather than the standard 1/4". Could all this upset the alignment?

I read on here somewhere that wheel alighnment is best done with a couple of long strips of wood to place against the wheels to make contact i guess on each wheel at 2 points at same time? I tried this with a legth of 15mm copper pipe which is pretty straight but seems hard to achieve.

any advice on how to aligh the rear wheel and check all is correct would be great.

cheers

Neil

email (option): nlhclarke@gmail.com

Hi Neil
I can't help with the measurements, but I find the best way to check wheel alignment is to "stringline" them.
Run lengths of good quality string or similar down each side of the wheels under slight tension & sight down them.
Depending on one's age & general athleticism this may best be done with the bike on a bench but it is feasible to do it on the ground.
I use housebricks to tension the string.

Neil the reason the chain stays are different widths from centre is because of the brake drum on the left. The wheel is then built with an offset to put the rim in the centre of the frame.

The different size chain won't have anything to do with alignment but not sure about running it if you still have the 1/4" gearbox sprocket?

I have a straight piece of ally channel that I use for wheel alignment. Actually the snail cam should push the wheel back straight, but only if your rear frame is dead true? The bolt on the right side just keeps it there, but in reality you often have to use it to alter the alignment. Not BSA's best idea and just requires trial, error and perseverance. Ron

email (option): ronpier@talk21.com

Hello Ron /Bob

many thanks for the replies. I will try the string idea soon to check that out. I also noted in looking over that method that my bike was not properly linear in the stand with the front wheel at an ever so slight angle which threw out any hope of 4 contact points on 2 wheels.

Ron your comment about the wheel being 'dished' to accommodate the drum makes sense; I guess just like the rear wheels on a push iron to compensate for the presence of the rear cassette. However as you say this should put the wheel in the centre of the rear stays. This is not what I am seeing as in the pics:


non drive side is about 44 mm (sorry callipers upside down)




Drive side about 69 mm




The chain looks nice and straight in this position and the axle looks in the same position on each side in how far it is pushed home into the chain stay slots.

any thoughts?

cheers Neil

Ps i have a 3/8" gear box sprocket on so fits chain perfectly

email (option): nlhclarke@gmail.com

Naturally the distance from the chain stays to the rim will be different by 19/32" (15mm). The centre of the rim should be in line with the centre line of the frame. Ron

email (option): ronpier@talk21.com

As Ron,s drawing you can see the frame offset to accommodate the brake drum. Since you have a later wheel I would start from scratch. You need the tank and the rear mudguard removed. Set the brake drum sprocket so that the chain line is in line with the gearbox sprocket, then look at the centre of the rim, this should be in the centre of the top tube. If not you need to pull the rim over with the spokes so rim is central to the frame top tube. I always check this when building any bike, same with the front wheel it should sit central to the frame top tube. Re the wider chain just watch out for the chain rubbing on the inside of the chain case.

Hi Tim. Without being there to look. It seems as though Neil's wheel is correct, from his Vernier readings showing about 15mm bigger gap on the left. Ron

email (option): ronpier@talk21.com

neil can you post some more pictures of the hub assembly/spacers in the frame. i thought the wheel sat more to the right side? is the gearbox straight, have you got the correct rear engine plate spacers with brake anchor arm

This might help.



Mark

email (option): pes.sales@btconnect.com

Mark you're in the wrong thread mate:smile: Ron

email (option): ronpier@talk21.com

It was supposed to be a picture of the later M20 Frame.
For some reason I'm having nothing but trouble posting anything.

Mark

email (option): pes.sales@btconnect.com

The handy picture is there Mark. But it needs to be in the thread on frames differences.


Ron

email (option): ronpier@talk21.com