Replacing my cranks & BB: what derailleur adjustments should I expect?

[secstate]

I am on the cusp of buying a new bike. Once I do, I plan to gradually overhaul my current bike, a mid-1990s Bianchi Advantage hybrid. That bike has served me very well and I will continue to use it.

I’ve ridden it hard and it is due for some work, so one of the bike’s next tasks will be helping me improve my maintenance skills! URL=”http://www.bikepedia.com/QuickBike/BikeSpecs.aspx?year=1998&brand=Bianchi&model=Advantage”%5DSpecs at BikePedia[/URL I took a bike overhaul course at a co-op in Minneapolis, so I have a little hands-on experience with the relevant tools & techniques, but I am still very much a novice.

The first order of business will be replacing the bottom bracket. Before I order parts, I’d like to run my thinking by the forum to see if it’s on target.

I believe the BB is a 68mm square tapered design. It was last serviced by a LBS, and I’m not sure what they put in there, but I plan to replace it with a Shimano UN55. I also anticipate replacing the cranks (Shimano AceraX) and chainring, both of which are original. The bike has three rings up front and 7 cogs in back, so the best match for the cranks & chainring set appears to be the Shimano M361 URL=”http://www.chainreactioncycles.com/us/en/shimano-acera-m361-triple-chainset-square-taper/rp-prod106885″%5Dlink%5B/URL for 7/8 speed bikes. According to BikePedia, the current BB likely has a 110mm spindle; the specs for the M361 call for a 122.5mm spindle. Is this difference in spindle length, or any other possible differences in the cranksets, something I need to worry about for my derailleur alignment? I anticipate needing to adjust the low/high screws on the derailleurs, but anything beyond that is likely to exceed my current comfort zone.

Any thoughts or advice would be most appreciated.

Topic

[TwoWheelsDC]

Looks like the big rings have the same number of teeth, so you may be fine leaving the derailleur at the same vertical position. But the outward move of the cranks may require more than just a high/low limit adjustment and you *might* have to start from square one with your derailleur cable and give it more tension than the barrel adjuster allows. You also may have to adjust the angle of the derailleur since your pushing it outward slightly. If you are savvy enough to replace the crank, I think an FD will be easy enough. Once it kind of clicked in my head how the thing works, it became relatively painless for me to work on.

[peterw_diy]

Why would you replace the cranks? What’s wrong with the old ones?

[secstate]

@peterw_diy 151613 wrote:

Why would you replace the cranks? What’s wrong with the old ones?

Ha, I have asked myself the same question. In truth, I probably don’t need to. I got it in my head because a mechanic once mentioned it was possible that the chainrings couldn’t be replaced without also replacing the cranks. Apparently some cheaper bikes were like that once. I doubt that’s the case for the Shimano components on this bike.

[anomad]

I suggest removing the existing bottom bracket and see if the bearings are serviceable. If it does require replacement you can measure the spindle and get exactly the same width. Unless you are changing chainring sizes or have major chain line issues you probably want exactly the same size.

[vvill]

If I was going to replace a square tapered crankset + BB I’d just spring for a Hollowtech MTB crankset (you’ll want spacers for the 73mm MTB cranks in a 68mm BB) or the SRAM GXP equivalent. Otherwise yeah as others said just see if the bearings can be serviced – if it can’t and you want to stay with square tapered then just get the same size BB that’s in there.

[DismalScientist]

If your chainrings are shot, its probably cheaper to just buy a new crank rather than multiple chainrings.

Do you have an integrated bottom bracket? If you have cup and cone, it’s just easier to replace it with an integrated BB than to try to adjust it.

[secstate]

Thanks very much for the suggestions, everyone!

@DismalScientist 151636 wrote:

If your chainrings are shot, its probably cheaper to just buy a new crank rather than multiple chainrings.

Do you have an integrated bottom bracket? If you have cup and cone, it’s just easier to replace it with an integrated BB than to try to adjust it.

I’m not sure, because I’ve never been in there, myself. But the research I’ve done suggests that it’s probably integrated. Good to know that I can install an integrated BB even if it’s currently cup-and-cone style.

In the end, I’ll probably install new cranks and purchase a BB with the spindle length suggested by the specs for the cranks. If that means more involved derailleur adjustments than I’ve done previously, so be it!

[peterw_diy]

@vvill 151623 wrote:

If I was going to replace a square tapered crankset + BB I’d just spring for a Hollowtech MTB crankset (you’ll want spacers for the 73mm MTB cranks in a 68mm BB) or the SRAM GXP equivalent

But he’s starting with a 7 speed setup. Most Hollowtech II are 9+ speed, which means the chainrings are closer together, which means he might need a narrower chain to avoid rub, and that’s how you call down the upgrade rabbit hole…

I haven’t run Hollowtech II long enough to have an informed opinion, but I’ve sure seen enough complaints about outboard bearings like HT II uses, and had great experience with standard Shimano square taper cartridge BBs. 0

[vvill]

@peterw_diy 151657 wrote:

But he’s starting with a 7 speed setup. Most Hollowtech II are 9+ speed, which means the chainrings are closer together, which means he might need a narrower chain to avoid rub, and that’s how you call down the upgrade rabbit hole…

Ah, true. I switched out all my square tapered but went to 10 speed from 7/8 speed.

[secstate]

@vvill 151663 wrote:

Ah, true. I switched out all my square tapered but went to 10 speed from 7/8 speed.

Yes, I expect this bicycle will eventually make its way to my folks’ house in Minnesota for use during my winter and summer visits. Drivetrains with fewer cogs tend to be more reliable in snowy & icy conditions, so that’s my excuse for not diving into a 10 or, heaven forbid, 11-speed conversion. Plus, 7-speed components are much less expensive!

[vvill]

@secstate 151682 wrote:

Yes, I expect this bicycle will eventually make its way to my folks’ house in Minnesota for use during my winter and summer visits. Drivetrains with fewer cogs tend to be more reliable in snowy & icy conditions, so that’s my excuse for not diving into a 10 or, heaven forbid, 11-speed conversion. Plus, 7-speed components are much less expensive!

Agreed. But my 7/8 spd shifters were rusted and I also wanted to have to only stock 10 speed chains and cassettes. For a time last year I had all my geared bikes on 10 speed! (Then I went and bought a new gravel bike…)

[Harry Meatmotor]

@secstate 151590 wrote:

It was last serviced by a LBS, and I’m not sure what they put in there, but I plan to replace it with a Shimano UN55.

@anomad 151622 wrote:

I suggest removing the existing bottom bracket and see if the bearings are serviceable. If it does require replacement you can measure the spindle and get exactly the same width. Unless you are changing chainring sizes or have major chain line issues you probably want exactly the same size.

The stock BB isn’t serviceable. A UN55 isn’t serviceable either.

@secstate 151637 wrote:

But the research I’ve done suggests that it’s probably integrated. Good to know that I can install an integrated BB even if it’s currently cup-and-cone style.

In the end, I’ll probably install new cranks and purchase a BB with the spindle length suggested by the specs for the cranks. If that means more involved derailleur adjustments than I’ve done previously, so be it!

As long as it’s a 68mm wide english threaded BB shell, you’ll have no problem finding a cartridge BB. Getting the spindle length correct is going to depend on the crankset you choose. Here’s what I’d recommend:

• Purchase a square taper crankset for 7/8 speed chains with the same or similar tooth counts
• Remove the existing cranks & BB
• Clean the BB shell and BB threads with some degreaser & steel wool
• Take the bike and new crankset to an LBS and have them size a new BB
• Either have the shop install the new BB, or purchase the BB with a good spindle length and install it yourself

Getting the chainline set will depend on a couple things: you want the chainrings to be as close to the chainstay as possible (within 5mm) without either the chainrings or the crank arm rubbing the chainstay. This takes some trial and error if you’re using a new crankset, or a different crankset from stock. The other thing to keep in mind is that finding the proper spindle length depends on how well the crankset is made. When you tighten a square-taper crank arm onto the spindle, the tolerances and quality of material will affect how far the crank arm seats onto the spindle when tightened to spec. The whole point of a square-taper spindle-to-crank interface is that it will tighten and stay tight with parts that are wildly out-of-spec/tolerance. IOW, it’s one of the sloppiest fits on bike, mechanically speaking.

If it were me, I’d also run a tap through the threads if they’re looking a little crummy. No need to face the BB on a bike like your Bianchi, no offense.

A “good” mechanic will know that taking a brand new inexpensive crank arm (read: cast aluminum not forged, and sloppy tolerances) will slide pretty far up the taper on initial install. So, you’ll usually want to start by dry-fitting a BB that looks too wide. Where a previously installed crank arm will slide up the taper 1-3mm when tightened to spec, a brand new crank arm might slide up the taper by a full cm or more.

Now, once you’ve get the crank dry fitted to the partially installed BB, the other issue that can come up is FDer swing. Sometimes there are frame clearance issues on the seat tube that can keep the FDer from swinging far enough inboard even with the lower limit screw backed out to allow the chain to shift into the inner chainring. Also, all your typical chainline issues will need to be checked (rubbing on middle chainring in little-little combo).

TL;DR – this isn’t difficult, it just takes some trial and error, and having access to a handful of BBs to try different spindle lengths is something only (usually!!) an LBS will be able to help with.

[secstate]

Thank you — this is very useful advice, and good to have on record in the forum. The point about inexpensive crank arms sliding farther up the taper when new is especially interesting. Definitely not something one would guess.

[Author]

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