dishing wheel for single speed set freewheel why

[biggs682]

took an old 27" rear wheel with screw on hub in to lbs for them to remove the screw on block and replace with a single speed bmx stlye rear gear to finish of latest project only to be told >>> it wont work you will need to re dish the wheel to get the new gear to line up with crank so the chain does not jump off all the time.

anybody able to explain in plain english why please

 

[montage]

You understand the chainline being important bit? Basically the front chainring and the cog at the back must be lined up perfectly.

Put your wheel in the frame and imagine you have screwed your BMX freewheel on (don't do this unless you have the tools to remove the bmx freewheel). The freewheel will be way out of line with the chainring at the front.

So to rectify this you can respace the hub on the wheel's axel, to move it along to the right so that the freewheel is now in line with the chainring.

But this creats another problem, the wheel which was sitting nice and equally between the seat stays will now be offset to the side, and so by adjusting the tension in the spokes, it must be re-centralised to the frame.


This help?

 

[bikefettler]

the existing wheel was dished to accept a multiple 5-6-7 speed block and straight chainline would be prob gear 3 or so if you fit a single you need to respace the hub to achieve a straight chainline and then redish the wheel to centralise the rim in the frame , hope that helps ?

 

[PpPete]

or leave the wheel as is, and fit a shorter spindle BB (assuming frame geometry allow)

 

[biggs682]

ok guys have had local bike shop put gear on and going to try it and see what happens if no good wil get them to dish wheel to suit

 

[tyred]

Re-dishing the wheel isn't a huge job. It will need to be re-spaced first. If you look, there will be a spacer between the drive side cone and lock nut on the axle. This will probably be moving the hub too far to the left for a good chain line. Either make calculations carefully and cut the spacer or else replace the spacer with washers to give the same distance as the spacer currently does and then move the washers, one at a time to the other side until the chainline is correct.