Wednesday, 14 February 2018

A Small Repair and Memories of a Near-Death Experience

Just before the New Year I adjusting the chain on my CBF250 and I found that rear sprocket was rattling about. The cause was not hard to find. The sprocket carrier bearing was shot. It's a simple and inexpensive job to replace the bearing.

The sprocket and bearing, seal & spacer

I was keen to fix this because I have a memory from twenty or so years ago when this kind of fault had dramatic consequences.

I was commuting on another Honda 250, a CB250RS. This was a great little bike but the rear sprocket arrangement was prone to a problem. The bike didn't have a carrier, the sprocket was connected to the wheel by steel pins that fitted into rubber units to provide cushion drive. The sprocket was on a hub secured by a large circlip. Then this wore the sprocket because loose. This is a problem because, under power, the chain pulls the sprocket out of alignment. I did a makeshift repair but things were about to go very bad. 
CB250RS rear wheel - pins(16), rubbers(4), circlip(20)
One winter's morning I was riding to work. I left town, opened the throttle and got up to about 50mph when there was a bang and the rear wheel locked. (my adventure last summer on my SV650 was therefore a case of déjà vu) Luckily on this occasion I kept the bike upright until it stopped. Well done to the following motorists who avoided running into me.

The chain had jumped off the sprocket and jammed against the swing arm. The force was such that the arm was considerably bent. 
One for the bin
The moral of this story is to keep your chain in good condition because if anything goes wrong the consequences can be serious. 
Gwen riding the CB250RS on the beach at Lindisfarne Island in 1984. The little bike carried the pair of us for a weekend away, I think we must have been a bit slimmer then!


  1. Sounds like you have had your fair share or rear wheel lock ups!! I remember having to change the cush drive rubbers on the CB250RS too. That bike had alloy rims, it always bothers me that to this day, Triumph insist on putting crappy chrome rims on the Bonneville's, even my old R60/2 has alloy rims! Think my MZ had alloys too.

    1. I've had two such incidents in 40-odd years - but that's still two too many!

      All my MZs had alloy rims - so much cooler than chrome.

  2. Damn chains and sprockets causing troubles. Good thing you caught it early this time so no near death experiences.

    1. The bike let me know something was wrong by "clunking" when I wheeled it out of the garage.