I currently have a 68-122.5mm square taper bottom bracket on my hybrid. I'm replacing the stock SR Suntour chainset with an SRAM S600. I read that the SRAM S600 chainset is for 113mm bottom brackets. The technical specs of the SRAM S600 chainset appear to be the same as my SR Suntour in all other respects.

Am I correct in assuming that I will have problems? I thought square taper was square taper so am confused as to why the BB spindle length actually matters.

The taper is the same, the shape of the crankset and chainring mounting offset isn't.

https://www.parktool.com/blog/repair...nline-concepts

Is there a reason for not wanting to use the correct bottom bracket?

Me too... Or rather I have never thought about such measurements and their meaning. I just buy something off Ebay that matches the one I want to replace. 68mm turns out to be the width of the bit on the frame that you put the spindle in, I've just gone and measured one. The spindle itself will/should be such that when you have fitted it, added balls and adjusted the cups you end up with something that spins 'freely' with no slop...

Spindle length will matter but, making it up as I go along, what might/will matter more is the length ring crank side to length other side. I am having some word problems here so have a picture. Top is from a bike which had a triple ring crank. Bottom is from a bike that had a double ring crank. Left Side is more or less the same length but Right Side Triple is longer. This bit affects your ultimate chain line.

Mumble mumble. I am sure someone else can explain things better.

...

That's an assymmetric spindle. Common among cup & cone BBs. Never seen one among cartridge BBs.
BB spindle length together with crank design is what sets the chainline.
Replacing one but not the other at random has an element of gambling to it.
Might work. Might not.

https://janheine.wordpress.com/2013/...s-demystified/

I purchased the SRAM crankset off Amazon based on it looking to be an identical replacement for my current SR Suntour. It has the same crank arm length and teeth on the three chainrings.

Unfortunately in the specs it made no mention of bottom bracket requirements. It was only when I started to Google more information about the SRAM S600 that I found some a couple mentions of it requiring a 113mm BB spindle.

I may just return it to Amazon now as I haven't opened it.

What I've done in the meantime is taken the pedals off the hybrid and thoroughly cleaned out the pedal hole and the pedals threads, put some fresh anti-seize on and fitted the pedals back on. The slight cross-thread didn't feel as bad as before so I suspect there was a lot of grunge in there.

Anyhow, I now understand that cranksets are built for specific BB spindle lengths! it's a lesson learned.

FWIW, bottom brackets are one of the least expensive components. No reason not to have the size you need.

Crank set designers vary the profile , some design them to come in to meet a shorter spindle.

others , a flatter profile, then the steel spindle must be longer. get the set that goes with it's mate..

the newest hollowtech bond a tube spindle to the crankarm eliminating choice and confusion over 'right length'.





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Buy the specified BB. To do otherwise is letting the tail wag the dog.

As dsbrantjr sais the industry see's it the other way around. Cranks determine the BB. But it's really a bit chicken and egg.


Two people weigh the same, are right handed and have blond hair. Are their feet the same size? Andy

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...FWIW, here are three links on Q. Q is what you are redefining (and changing) when you go with a longer spindle (in most cases).

What is 'Q Factor', and does it make a difference? - Cycling Weekly

https://en.wikipedia.org/wiki/Q_factor_(bicycles)

Q Factor | Why Pedal Stance Width is Important | iSSi Pedals

Also here is a 20 minute video by someone I don't know that I've never watched who purports to explain it:

If you are moderately sophisticated in this endeavor, it's not considered a big deal to extend out your Q factor from 113 to 122.5.
The difference of 9.5 mm is split in half so maybe 4.75 mm (call it 5 mm) on each side is not huge, but it's probably enough to offset your chainline to the point where you need to adjust for that in some other way. So generally people who are fortunate enough to know the required length of the spindle for your crankset as designed by the manufacturer just buy one for 20 bucks as part of the new installation.


It's more of an adventure when you're working with older stuff and can't get the original specs anywhere.