Bottom Bracket Blues

[iangreenhalgh]

Hi folks

I have a quandary with mating my crankset to my bottom bracket. My frame has a traditional square taper BB screwed into it. However, the FSA Comet crankset I have has what looks to me, like an ISIS splined fitting so won't mate with the traditional square BB.

So, what to do? I'm guessing these are my options:

A. Replace BB with ISIS splined one - drawback is I don't have a BB removal tool and I'd prefer not to have to do this as it's a bit more complex a task than the other options.

B. Buy a replacement crankset with the correct tapered square fittings.

Just wanted to canvas opinions and explore options before I spend any money on more parts.

 

[dogman dan]

I'd sell off the crank, and get a square one. for me the best option because most of my bike frame junkyard is full of square taper type.

You'd have to be really in love with that crank to pay to change the bb. It's an e bike right? not like you pedal it as hard as a racer.

 

[iangreenhalgh]

Yeah, I agree Dan. My 28 year old Raleigh MTB is still going string on it's original crankset so nothing wrong with square taper.

I'm debating what size crank to buy. I've put a 16t freewheel on the rear and am foregoing gears and derailleurs, although I might end up fitting them in the end. I don't know whether to go with a medium 40t crank or a large 52t one. I keep reading that to keep up with a motor you always end up using your highest gear and most ebike users only ever use top gear to pedal. Aiming for a top speed between 15 and 20mph, what do folks recommend for the crank size to go with the 15t freewheel?

 

[nutnspecial]

Assuming you're not referring to crank length (such as 150- 170mm), you mean 'chainring'. Check out this calculator to determine speed-to-cadence for potential considered gearing, as cadence will be your determining factor of preference imo. http://www.machars.net/bikecalc.htm

I like up to maybe 120rpm @ full speed, but if you're ever planning on pedaling faster than initial target speed, or just prefer less leg movement, perhaps go with a lower cadence, such as 40-60@ 15mph. Obviously the lower you go, the more difficult it is to input human power to get rolling.
http://triathlon.competitor.com/2015/08/training/what-is-the-ideal-bike-cadence-for-you_120914
http://www.active.com/cycling/articles/cycling-cadence-in-training-and-racing

Now, personally if I have a complete crankset I like, and didn't have many/any unmatching 'spares', I'd price the matching splined BB as it might be far less than a brand new similar quality crankset. Also consider mounting patterns (104, 110mm) because some cranksets don't fit the larger 52t chainrings. Yes, you'd have to borrow or factor in the 20-30$ for the two different BB sockets. But BB's are a cakewalk, as are crankset removal/installs. Jump on goog or utube for howto vids, get a tool, and you're set!

 

[iangreenhalgh]

Cheers nut, that's very useful.

Using that calculator, it looks like a 52T chainring is what I need - 15mph is 63rpm, 30mph is 127rpm.

Or maybe I should go for a 48t to make it that bit easier to get rolling.

Hmm, decisions, decisions.....

I just discovered the retaining cup on my BB is not screwed in all the way and is cross threaded. Grrrrr. So I might have to use force to get it out, destroying the threads in the process and thus necessitating a replacement BB.

If I end up needing a new BB, I'll just have to get an ISIS one and use this FSA set of chainrings and cranks I already have.

If I can get the cup out without dmaage and screw it in properly, I'll just buy a replacement chainring/crank set, there are tons of cheap secondhand ones on ebay, a massive variety to chose from in fact.

 

[iangreenhalgh]

I decided to buy this:

http://www.ebay.co.uk/itm/VINTAGE-SR-SILSTAR-CHAINSET-52-42-STEEL-RINGS-CHAINGUARD-170MM-CRANK-ARMS-/182016468448

Maybe I'll fit a front derailleur so I can drop down to the 42t ring when needed.

 

[iangreenhalgh]

Well, the crankset arrived and I fitted it to the bike, only to discover it won't turn because the cogs touch the lower rear frame. Grrrr.

So what to do now - buy yet another, smaller crankset or replace the BB with one that spaces the cogs wider so they don't touch the frame?

 

[Ykick]

BB’s can be such a PITA. There seems to be many options and “offset” dimensions too? Maybe worth paying a good mechanic to best sort it out? They've seen a lot more of 'em than we have....

 

[iangreenhalgh]

Sadly, there isn't a bike shop within 35 miles of here and I don't have a car. So I have to sort it out myself.

 

[Deafcat]

Yea I don't see a reason to use anything other than square taper, unless you buy a full bike that comes configured with ISIS/etc. easier to make/modify stuff for square splined too, simple manual operations

to get additional strength over original square tapered, all that would really be necessary is to bump up the size to like 3/4". but I guess that wasn't proprietary enough for the folks at Shimano etc

 

[ecycler]

Well, the crankset arrived and I fitted it to the bike, only to discover it won't turn because the cogs touch the lower rear frame. Grrrr.

So what to do now - buy yet another, smaller crankset or replace the BB with one that spaces the cogs wider so they don't touch the frame?

Definitely been there before. Replacing the BB really is not that difficult. Your 3 options as you might suspect:

1. New bottom bracket that will displace the chainring further out (have you measured your chainline yet?!)
2. Smaller chainring that won't rub the frame.
3. If you have a steel frame dent the poop out of it where the chainring wants to rub if that will give you the clearance and you don't mind 'modifying' your frame.

 

[Chalo]

The old Sakae Ringyo crank was intended for use with a 122mm or wider bottom bracket spindle-- on a road bike frame with narrow width across the stays.

You'll need to get a more modern crank, or else a much wider bottom bracket. In my shop, the bottom bracket costs $16 and a similar amount in labor. Cranks start at $30 plus labor. Both require special tools to replace.

 

[iangreenhalgh]

Cheers guys.

My frame is 7005 alloy, so some 'hand fitting' with a lump hammer won't end well, if it was steel, I'd have bent it already.

Yup, silly me tried to put a unit for a road bike with narrow stays on a mountain bike.

I still have an FSA Comet chainset and cranks.

So I have to either replace the BB or buy yet another chainset.

I'm leaning towards getting a new BB that the FSA chainset fits.

Yes, I'll have to get a tool to remove the BB already in there.

 

[ecycler]

Just make sure your chain line isn't doesn't get too crazy or shifting is going to suck.

 

[iangreenhalgh]

It's a fixed single speed, no gears, so the chainline shouldn't be an issue I hope.

Thanks for the tip, something I shall bear in mind.

I checked my FSA chainset/cranks and they are BB30, so I guess I need a BB30 BB. I measured the shell on my frame and it's 68mm.

Watching this FSA instructional video makes me think however that my frame just isn't suitable:

http://www.fullspeedahead.com/install-maintain-bb30-bottom-bracket-mountain-bike-fsa-mtb-2/

I'm pretty confused over this whole thing to be honest.

 

[Chalo]

BB30 won't fit into a normal threaded BB shell. If yours is a two piece crank (spindle permanently attached to one arm), then there's no way it will work with your frame.

If it's a three piece crank, then it's not specifically a BB30 item. Any ISIS BB should work.

It's a fixed single speed, no gears, so the chainline shouldn't be an issue I hope.

Single speeds are where chainline matters the most. Derailleur bikes can tolerate fairly awful chainline because the derailleur guides the chain back onto the rear sprocket even when it comes in at a bad angle. Take the derailleur out of the equation, and the sprockets must line up much more closely. Push it too far and the chain will skip and stutter as it tries to unship from the sprocket. Short of that point, bad chainline exaggerates wear and takes a toll on efficiency.

 

[iangreenhalgh]

Hmm, the FSA crankset I have, the spindle is indeed attached to one side.

So I need a different crankset that will fit the BB I already have in my frame.

I just hope I can avoid buying another one that fouls against the stays.

 

[Chalo]

http://www.sheldonbrown.com/cranks.html#tread

The section on "tread" and the subsequent section on "profile" explain the issue you're dealing with. You have a short BB intended for a low profile crank, so you'll need one of those. Most of the cranks made in the last 20 years will do.

 

[iangreenhalgh]

Cheers Chalo, most helpful.