Lowering gears and changing crank length on 1994 Bridgestone MB-5

Question

There are a few things going on here with my relationship to my free barn-find 1994 Bridgestone MB-5... centered around my height and the interplay between my physical capacity and the geography in the areas I ride.

First off: "It's not worth it to dump money in an entry-level obsolete bike." I understand this. I'm here to find out whether the things that I want to change on this bike are possible, not cost-effective... Assume it's a fun hobby for me instead of the easiest solution, please!

Let me introduce both of us: I'm a short-torsoed 5'2". The Bridgestone is an 18.5" frame, which sports a 24-36-46T 175mm Shimano STX crankset with five-leg (arms?) 94mm BCD spiders, and a 12-28T rear (12, 14, 16, 18, 21, 24, 28). (Also Shimano) The rear derailleur is an STX, the front derailleur I replaced, still a Shimano from a different 3x7 mountain bike. Given my measurements, 18.5" is fine. I've swapped out the handlebars and quill so that my short torso is not reaching quite as far. I'm very happy with the bike up front.

Here are my goals...

1) I want 165mm cranks

2) I want this bike geared lower because the steep hills around here kick my ass. My partner has a lower geared (3x7 mountain) bike and after finally riding mine, understands why I struggle a bit. 22T in front and 34T in back would be awesome.

Here are my problems...

3x7 drive train systems seem so obsolete that you are just out of luck unless you want slim pickins. Yes, I've found 7 speed triple cranksets. But... they are either geared at least as high as mine, or they only come with 175mm cranks. I cannot find any 165mm 94mm BCD crank arms, they all seem to be 110mm BCD, or if it's the correct BCD then they are again 175mm. Whole cranksets? there are a few 22t, 165mm out there, but.... for 9 speed.

I'm getting lots of contradicting info from the internet about whether 9 and 7 speed triple cranksets are compatible due to chain width, spacing... I'm a hands-on person, so this internet-research is super frustrating for me. I did find the Shimano FC-TX801 Tourney TX square crankset with 165mm cranks and a 22-32-42T set, but it's not available anywhere. It's exactly what I want, but that is a moot point, because I can't get it.

Can I use a 9 speed triple crankset? Do I have to buy a 7 speed crankset with the wrong sized gears but right length cranks, then swap out for a granny gear? or would I buy a crankset with the right gears and swap crank arms? What's interchangeable, what's riveted together? (it looks like a lot of the arms are not removable from the whole shebang now) Advice? Wisdom? pleeeeeaaaaaase?

Not worried about the back, I think I've found simple options for back there.

If I've missed anything or been not clear, I apologize. I hate sitting at a computer, and to boot, my computer is broken and as slow as molassas on a chilly day. I'll be happy to specify or clarify anything....

Answer 1

There are some differences in chainring spacing between crank speed generations. But even within a speed generation, the spacing numbers are not truly standardized. That's one reason for the ambiguity you've found. Another is that with front shifting compatibility considerations on bikes in general, there are a lot of things one can do that fall under the category of setups that can be made to work but possibly with some level of compromised performance. Some voices you're likely encountering would think little of giving all those setups a stamp of approval. Others, such as those of us who do this for a living, tend to operate more in the realm of asking what will perform optimally or as-designed and disregarding everything else, because if we don't we're going to let people down and/or waste a bunch of time.

All that said, dropping in a 9 speed triple crank on a seven speed bike with a 7/8 speed chain and front derailleur is generally fine. So if 165 with a 22t small ring is what you want and you can find it in 9-speed, go for it. If it's another square taper crank, make sure to get a new BB of the correct spindle length unless it happens to use the same as what you've got. I recommend having some .6mm Wheels Mfg chainring spacers to tune things if needed, i.e. enough to put them under both the large and small ring to slightly increase the spacing if you find that your existing shifter and FD are producing too much total throw.

Also, in case it's helpful, 104/64 is the predominant modern MTB triple BCD. The smallest ring size the 64mm can take is 22t. One spendy but not all that difficult way you could go would be getting one of the various fancy interchangeable spider cranks, most of which come in 165 and have or have had 104/64 spiders available.