Paul_P

I'm looking at this FC-T4010 Alivio Crankset. In the specs, it specifies a Q-factor of 187.8 mm and an BB axle length of 126 mm.
I don't understand where these values come from since the Q-factor is dependant on the BB-ES300 spindle length, and that can be one of a bunch of values.

Is it that a BB spindle length of 126mm will produce a Q-factor of 187.8mm, and any other spindle length would cause a different Q-factor ?

And am I correct in believing that I can adjust the Q-factor for my bike fit just by selecting a BB with an appropriate spindle length (my bb shell is 68mm) ? This would save me installing pedal spacers which I was thinking of doing.

choddo

Yes, the BB axle is really a BB spindle so 126mm locks that Q factor down to 187 in that example (cranks involved matter to Q factor too), the other available spindle lengths aren’t relevant

but you can’t just pick any spindle length. The chainring position will be dependent on it too.

Kontact

It's an Octalink crank. There aren't multiple Octalink spindle lengths to choose from.

More generally, cranks are designed with a specific spindle length in mind to produce correct chain line and Q. So Q really isn't a variable if you are speccing the bike as designed

choddo

That Shimano page has spindle lengths from 113 to 126?

maddog34

"It's an Octalink crank. There aren't multiple Octalink spindle lengths to choose from."

wanna bet on that?

https://www.ebay.com/itm/23316988159...3ABFBM_K2lstxi

https://www.ebay.com/itm/12142878780...Bk9SR_6tpbLcYg

Etc....

and there are TWO "octalink" spline patterns too.. V1 and V2.. they do not play well together.

https://www.sheldonbrown.com/images/bbsx4-labeled.jpg

Kontact

For a given crank, it is correct that there is just one length.

It isn't like square taper where you could use a 113 or a 115. The steps between even compatible spindle types is huge

Kontact

Do you think a crank specd with a 126mm axle is going to work with a 121 or a 118? It will hit the frame.

dedhed

The 126mm BB gives the specified 50mm chainline that the crank is designed for so gearing like the FD works and stuff clears the frame
Most likely that uses a V2 octalink
https://www.sheldonbrown.com/bbsize.html#shimano​​​​​​

maddog34

unless the bike is a touring bike with 700c wheels, or an old steel MTB frame with longer chainstays,. etc.

and then there's personal sizing/fit to consider... "i was considering adding PEDAL SPACERS..."

lots of variables to consider.
an MTB crank mated to a Road/touring bike could negate the need for pedal spacers.

"68mm BB width"............ old MTB or road/touring.

Paul_P

Thanks for the clarifications. This is for a 3x8 MTB (2005 Specialized Hard Rock Sport). Right after writing my post, I realized that everything probably revolves around the chainline, so should that be chosen first then everything else ? Above Shimano specs are for 50mm.

The current BB is a TruVativ with a 113mm spindle according to the above specs (68mm shell). I don't know that the current chainline is, and I guess I have to figure out what I want it to be ? Like which gear combination is the most used and then chose the rest so the chain is the straightest for that combination ? Current Q-factor seems to be about 180mm and I'm pretty sure I'd be more comfortable with a bit more.

Kontact

Chainline isn't something you select, it is a specification of the frame. It is based on the rear axle spacing and the number of chainrings. Period.

choddo

That’s not the same as there only being one spindle length for Octalink.
I didn’t spot that his crank was already using the longest one though (and also agreed he can’t just swap it out anyway )

choddo

Front mech wouldn’t reach. As per Kontact’s comment, although sticking a longer spindle on seems intuitive a way to push the pedals out, think you need as to have a different crank arm shape or like you said, pedal spacers. Never seen those, assume they shorten the amount of pedal axle that goes into the crank arm?

Paul_P

Thanks Kontact, this is not the first time I appreciate your cutting through some vagueness with a single precise sentence or two that forces me to study further to understand what you mean.

I just measured my chainline and it's 49mm which is apparently standard for SRAM (47.5-50mm for Shimano).

So it seems that this FC-T4010 crankset and 68/126 BB is a good fit for my bike, with possibly just a bit more Q-factor which will be welcome.

dedhed

Sounds like the OP is replacing the existing crank so the current 113mm BB length is immaterial. Proposed 4010 crank uses 126mm and should be fine.

choddo

Having just done exactly this, and had to install a shorter spindle to maintain the chainline, of course you’re right.
D’oh!