Replacing chain ring

[pubrunner]

Hi,

I own an Ellis Briggs (circa 1973/4) which has a double chainset with an outer ring of 52 and an inner ring of 48. (Steep hills can be a bit difficult). I'm not sure how to measure the pcd, but there are only 3 bolts holding the chain rings together - each about 9cm apart.

I'd like to replace the chainset with something perhaps in the region of 50/38 - a little variation either way would be fine, I'm looking at getting a double, not a triple.

Are there any issues that I should be aware of when making such a change, especially with regard to fitting ? Can anyone suggest a chainset which might be suitable for an older bike ?

Thank you very much

 

[02GF74]

without googling, PCD is something like pitch circle diameter.

I am assuming the bolts are same distacne from centre of bottom bracket - so you need to measure the diameter of the cirle they are on.

since you have odd number of bolts, that I guess have no two diameterically opposite, you would measure distance of centre of bolt from centre of spindle, then double it.

The only chainsets I am familair is with are shimano and they have 5 bolts - maybe newer ones have 4? I've never seen any 3 bolt ones.

If you cannot find smaller toothed chain ring, then you need to replace chainset. If you are lucky, the chain set will use the same spindle (bottom bracket) fitting i.e. a square taper - I have seen cotter pins used but never seen a Shimano one using them.

I am sure there are companies that can make chainrings according to your spec. - that may not be economically sensible.

Also you would need to have the chain rings on the new chainset be in about the same place as the old ones - if they are offset, the rings may be contacting the chainstays.

You may need a new bottom bracket, hopefully you can find with with the correct threads and spaing.

Finally you may need a different front gear changer as the larger difference in chain ring teeth may be more than what your current shifter can handle.

 

[pubrunner]

Thank you for your comprehensive reply; it is fitting a new chainset to the existing bottom bracket that concerns me. All I know is that currently, it has a cotterless system fitted; but do sizes vary much ?

Thank you again for such a prompt reply