Bicycle Mechanics - Crank/Sprocket spacing

I have an '85 Trek 460 that I've been riding about 75-100mi weekly. I havent owned it but for a month. After becoming familiar with the "personality" of the bike (it's a keeper) I've noticed that the smaller chainwheel (of the double) is very slightly making contact with the chain stay. When sitting you can see it's a close fit and under flexing it's making contact. I cant hear it, and seemingly cant feel it. The crank is tight, the bearings and adjustment feels good. The chainwheel is straight, the frame isnt bent or warped from what I can tell. Is there a fix I can perform on this...perhaps a shim to space the crank out another .030-.040?


thanx!


Olds

On a bike that old I think that's what I would try. 2mm bottom bracket spacers are easily available and fairly economical. It takes a few special tools to remove and replace the crank and bottom bracket. Take a close look at the LEFT side of your crank to make sure that you will have room to move it inward 2mm before you commit yourself to this idea.

Gotcha!


Thanx,

Olds

Another question.......why is it rubbing? Nothing is obviously wrong, I'd think if the BB needed R&R or service it would explain the rubbing. The spacing is really close and everything looks stock at this point. I just dont want a hole in the chainstay.


Olds

Another question.......why is it rubbing? Nothing is obviously wrong, I'd think if the BB needed R&R or service it would explain the rubbing. The spacing is really close and everything looks stock at this point. I just dont want a hole in the chainstay.
Olds

A couple of possible causes come immediately to mind:

One is that the bottom bracket may have been replaced sometime in the last 18 years. If that's the case, whoever did the replacing may have acquired a spindle that was slightly shorter than original.

The crank may have been replaced. Cranks have different amounts of offset and different bottom bracket spindle lengths are recommended for each crank. If the crank has been replaced, whoever did the replacing may have thought the existing bottom bracket was close enough.

I suspect the most likely possibility is that the crankarms have been periodically tightened over the years and have gradually drawn farther onto the bottom bracket spindle.

What ever the cause, the "A" grade repair would probably be to replace both the crankset and bottom bracket. On a bike this old, a 2mm bottom bracket spacer is lots cheaper. If it was my bike, that's what I would try first.

I doubt this is the case, but it might be worth mentioning:

I had a similar problem, and I thought I had bent the chainring as I had used this bike for some time without any problems. As it turns out, I had reinstalled the smaller chainring backwards after repacking the bottom bracket. The ramped design (for shifting) also prevented it from rubbing. When I flipped the ring, the ring rubbed ever so slightly.