tjc4golf

The background: Picked up a used bike a month or so ago. Bike but the cassette was ready to be replaced, one of the original shifters had broken and replaced with a functional but mismatching unit and the chain was worn. I decided it I was going to fix this gear I might as well update if from a 7 speed to a more modern 9 speed. A little more work/parts required than I originally realized but it's been a great learning experience and I'm almost there.

I've addressed the shifters, cassette, chain, wheels/freehubs, rear derailleur pulleys and now I'm focused on what I believe to be the last piece of the equation.... the crankset.

My questions are:

(1) Do I need a new crankset?
At first I didn't think I needed one. Then I did. Now I'm not sure. I realize the 7 speed chain is skinnier than 9 speed one... this is why I thought I'd need a new crankset. But then I read that the skinnier chain is achieved by slimming down the outside diameter of the chain and that the inside, the part that comes in contact with the chainrings, is the same size on both 7 and 9 speeds. So I draped a 9 speed chain over my current crankset and it seems to fit perfectly. So I'm wondering, do I need a new chain. If so why?

(2) If I do need a new crankset, what size bottom bracket/crankset do I need?
I realize there are a ton of threads on this... I've done some searches. My confusion is that most of them are lacking pictures so I'm not sure if I'm supposed to measure the bottom bracket itself or the section of the frame that houses the bottom bracket. I measured the section of the frame that houses the bb and it looks to be 68mm. Also, I've read that most triple mountain bikes take 68mm, 113mm so I think that's what I need. If it helps, bike is a 1990 Trek 790 with original Deore DX crankset... assume bb is original too. Here is a picture of the measurement I took (pic should say mm not cm... wasn't focused when doing my photoshop work).

thomamueller

You should wait for mechanic to reply. I have a 89 Trek and replaced cassette, chain ring, and cranks. That lead to a new BB because chain line issues. I thought the chain got skinner as the number of gears went up. I also believe the chain rings could simply be replaced with 9 speed. In any event, while waiting for a mechanic to respond, read this:
https://www.sheldonbrown.com/k7.html

Replace that cone and cup BB with a cartridge type regardless.

tjc4golf

Thanks for the reply. Don't think it answers my questions but you do raise a good point. I realize that even if a change is necessary I wouldn't need a entire new bb/crankset... just new chainrings. However, as I'm trying to do this on a budget, I've looked around a little bit and it seems that buying a quality, relatively new bb/crankset that's been pulled off a bike is much cheaper than buying the chainrings individually so I'd probably go that route.

tjc4golf

I found the info below courtesy of Sheldon Brown. So I think the answer to my question is new chainrings are optional. However, I'd still be interested in hearing what others think. Also, SB refers to a problem with skating on the "inner chainring". However I think as I've got a triple, the potential for "skating" exists anytime I downshift using the front derailleur (i.e. with both the middle and inner chainrings)... correct?

https://www.sheldonbrown.com/dura-ace.html
"9-speed" Chainrings

When the move from 8- to 9-speed took place, starting with 1997 Dura-Ace, a very slight change in chainring design occurred. Shimano was concerned about the potential for the chain to "skate" over the teeth of the small chainring. "Skating" occurs when a narrow chain is used on a crankset intended for a somewhat wider chain. In downshifting, instead of the chain meshing properly with the teeth of the inner chainring, the side-plates of the chain can ride along the tips of the teeth of the small ring. This causes a momentary "freewheeling" forward. If you are foolish enough to downshift the front while standing up, this skating could conceivably cause you to crash.
The difference between "9-speed" cranksets and older models is only in the inner chainring. The teeth on a "9-speed" inner ring are slightly displaced to the right to better accommodate the slightly narrower chain.
Shimano will tell you you need to replace the inner ring when converting an older bike to 9-speed. Their lawyers say that covers them if your clumsy enough to hurt yourself due to "skating" and, besides, they make a nice profit selling the chainrings.
My advice is to not worry about this, and not to routinely replace the inner ring when upgrading to 9-speed. This type of "skating" is very rare, and mostly only happens if you're in top gear front and rear, then decide to downshift the front before downshifting the rear. There's no situation where this is a rational shifting sequence anyway.

I_bRAD

As noted above, you won't need a new crankset.

For your future reference, you are measuring the BB cup correctly. Also, your crankset most likely takes a 122mm BB.

Also also, that is most likely NOT the original BB as it's a cartridge style and that bike almost definitely came with a cup and cone BB