Mountain Bike Reviews Forum banner
1 - 20 of 23 Posts

·
Registered
Joined
·
73 Posts
Discussion Starter · #1 ·
I just replaced my 10-year old, squared-taper XT cranks with the Hollowtech II XT. The q-factor is incredibly high. Will I eventually get used to the new ones? Did you folks experience the same thing when going to these new style cranks with the outboard bottom bracket?
 

·
Registered
Joined
·
73 Posts
Discussion Starter · #3 ·
I never bothered checking the q-factor before installation. I'd been so used to riding the old cranks for 10 years (replacing rings when necessary) that I could feel the larger q-factor with the new ones. The Hollowtech feels like I'm riding horseback. Over time, my legs will probably "unlearn" the old cranks and get used to the new ones. It just feels weird initially.
 

·
bang
Joined
·
1,290 Posts
welcome to the world of one-size-fits-all external cranksets. it would be nice if they made them for specific bottom bracket widths to get a better chainline / reduce q-factor / decrease weight. oh well.
 

·
Registered
Joined
·
434 Posts
You can reduce the Q-factor by modifying the pedal spindles and/or the splines on the crank arm. Mine are now down to 170mm which feels a lot nicer. A few people on here have done it as well. I think the version of the XT crankset (M761??) with the bigger rings (48T big ring) has an even bigger Q than the standard 44T big ring version.
 

·
the catalan connection
Joined
·
1,266 Posts
smithy said:
You can reduce the Q-factor by modifying the pedal spindles and/or the splines on the crank arm. Mine are now down to 170mm which feels a lot nicer. A few people on here have done it as well. I think the version of the XT crankset (M761??) with the bigger rings (48T big ring) has an even bigger Q than the standard 44T big ring version.
Do you have more detailed info on the spline/crankarm modification? Sounds not so easy... The pedal thing does though.
 

·
Registered
Joined
·
434 Posts
Basically it is a case of machining away 5mm from the splines on the crank arm. What happens now is the arm can sit much further in on the crank spindle. Now you can get rid of 1 of the drive side spacers, and the non drive side spacer. Obviously, this modification is only effective on a 68mm BB shell. You'll also have to shorten the plastic splash tube by 5mm as well. If you like, you can then trim the crank axle down so you can still use the preload adjuster. I've just left mine sticking out a couple of mm though so I still have the option of putting it back to standard configuration. Hope this helps!
 

·
the catalan connection
Joined
·
1,266 Posts
You really did take that much off from there? How is it holding? I mean, sounds scary, doesn´t it? I have a rocky mountain blizzard and has a 73mm bb shell. That means I only need to take off 2.5mm(only one spacer down there). But I´m afraid that little gain doesn´t justify the risk for me ...mmm...the crankarm runing short of splines, guarantee void...don´t know
Thanks for the fast reply !
 

·
Registered
Joined
·
9,589 Posts
that Q factor

I've tried to get a firm answer on this forum before about the relative difference in Q between XT 752 octalink and a new XT outboard bearing crankset but no one seemed to know. If I were to put a new XT crankset on a frame I would get "this" many millimeters from outside of crankarms at the pedals. Now if I put my XT 752 crankset on the same frame with the appropriate bb width for a proper chainline, can anyone tell me what the difference would be in Q factor (width at the pedals)? Thanks to the person who actually knows this.
 

·
Registered
Joined
·
9,589 Posts
Thanks

BipedZed said:
The XT M760 has one of the highest Q-factor measurements of all cranks. You definitely aren't imaging things.

Here's a handy q-factor chart for Shimano cranks
Finally the chart I've been looking for!

edit: If my eyes don't deceive me, my XT 752 octalink on my Turner Burner and Sugar + bikes have a Q of 175.8. If I put an XT 760 outboard bearing crankset on these bikes the Q would be LESS at 175. Don't know the number of XT crankset the guy who started all this with his square tapered XT, but in my case it seems as I've been wrongly paranoid about messing up my Q by going with outboard bearings. Yet so many people have complained about the radical increase of the Q factor from older style cranksets. Am I reading this chart correctly or are there other factors involving Q that I'm not considering?
 

·
recovering racer
Joined
·
172 Posts
xcguy said:
Finally the chart I've been looking for!

edit: If my eyes don't deceive me, my XT 752 octalink on my Turner Burner and Sugar + bikes have a Q of 175.8. If I put an XT 760 outboard bearing crankset on these bikes the Q would be LESS at 175. Don't know the number of XT crankset the guy who started all this with his square tapered XT, but in my case it seems as I've been wrongly paranoid about messing up my Q by going with outboard bearings. Yet so many people have complained about the radical increase of the Q factor from older style cranksets. Am I reading this chart correctly or are there other factors involving Q that I'm not considering?
Older frames were designed around a 48.5mm chainline. Some newer frames went to a 50mm chainline which required a longer BB spindle. If you have one of the newer frames then you won't notice much change in Q as the external cranks were designed around a 50mm chainline. Personally I have a Racer X with a 48.5 chainline and specifically run a M952 crank/112 BB for a 163mm Q.
 

·
Registered
Joined
·
9,589 Posts
You got a tiny Q there dude

BipedZed said:
Older frames were designed around a 48.5mm chainline. Some newer frames went to a 50mm chainline which required a longer BB spindle. If you have one of the newer frames then you won't notice much change in Q as the external cranks were designed around a 50mm chainline. Personally I have a Racer X with a 48.5 chainline and specifically run a M952 crank/112 BB for a 163mm Q.
OK, I guess coming from a 163 to a 175 would be a big jump. I'm kinda bowlegged so probably a wider Q is what I've always needed. Let's talk chainline now. I'm about to build up a 2007 Santa Cruz Superlight frame (my LBS is that is). SCs website says first use a 68 mm bb, then get the proper width bb to maintain a 47-50 mm chainline. This is where it gets confusing to me. Are there numbers on that Q chart that would help determine if that means a 113 or 118? (Chainline is measured from middle of bb to middle chainring, yeah?). Would I have to know the width of the SLs bb shell, then the distance to the middle chainring on an unmounted crankset to figure this out? Santa Cruz says most new outboard bearing cranks would work, then goes on about the myriad of proper bb lengths for older Shimano cranksets. To sum up, what numbers from what sources would you use to determine the proper length bottom bracket to mount my XT 752 on this frame?
 

·
recovering racer
Joined
·
172 Posts
xcguy said:
To sum up, what numbers from what sources would you use to determine the proper length bottom bracket to mount my XT 752 on this frame?
According to Santa Cruz go with the 113 for your M752 cranks. The elevated chainstays allow for a 68mm BB shell with a 113mm BB spindle.

Santa Cruz said:
It really depends a lot on the crank manufacturer. The SL (like 95% of the bikes out there) requires a 47.5-50mm chainline. That is a measurement from the center of the BB shell to the middle ring. All of the new external bearing cranksets should work- just follow the manufacturer instructions for set-up on a 68mm BB shell. Old XTR cranks (before the external bearings) should use a 112.5mm. Most ISIS cranks should use a 113. Older XT cranks (2002-2003) use a 113. LX cranks from'03- '04 use a 121mm. As always, call your crank manufacturer if you have doubts- just tell them you need a 50mm chainline. ALWAYS buy for a 68mm shell.
 

·
Registered
Joined
·
9,589 Posts
Thanks for your help

BipedZed said:
According to Santa Cruz go with the 113 for your M752 cranks. The elevated chainstays allow for a 68mm BB shell with a 113mm BB spindle.
I saw that on the SC website but wasn't sure if they were specifically referring to the M752. You say you have a 48.5 mm chainline on your Racer-X. Did you crunch some numbers before you mounted that crankset to specifically come up with a 48.5 or did you just measure it after it was installed? What I'm getting at here is if the SC website had just said 47-50 chainline is what is required, how would I know that the M752 plus a 113 (or 118 or whatever) would be what I would want to install? Am I asking the right questions? I still don't know how to independently choose components to achieve this mythical proper chainline. Is it pretty much up to the bike manufacturers to tell us what size spindle or do mechanics come up with these numbers on their own?
 

·
recovering racer
Joined
·
172 Posts
xcguy said:
I saw that on the SC website but wasn't sure if they were specifically referring to the M752. You say you have a 48.5 mm chainline on your Racer-X. Did you crunch some numbers before you mounted that crankset to specifically come up with a 48.5 or did you just measure it after it was installed? What I'm getting at here is if the SC website had just said 47-50 chainline is what is required, how would I know that the M752 plus a 113 (or 118 or whatever) would be what I would want to install? Am I asking the right questions? I still don't know how to independently choose components to achieve this mythical proper chainline. Is it pretty much up to the bike manufacturers to tell us what size spindle or do mechanics come up with these numbers on their own?
Whoops, I meant 47.5 chainline on my RX. It's basically a Shimano spec for MTB, using their BB/cranks you will either have a 47.5 or 50mm chainline. The 47.5 is preferred. With the XT Octalink BB you have the choice of a 113 which will yield a 47.5 or a 118 for the 50 chainline. With the external BBs there is only a choice of 50mm chainline. I don't really care about the chainline number, as both work fine for shifting and lateral chain load, it's the resulting q factor that concerns me. If your frame allows the choice, go for the 47.5 chainline.

Sheldon Brown explains chainline well.
 

·
Old man on a bike
Joined
·
12,395 Posts
The Shimano specs for the M752 of 113 or 118 (or pretty much any of their non-external bb cranks when two spindle lengths are given) are for a 47.5 or 50mm chainline...and your determination of your M752 having a wider qfactor than the M760 was only if you currently run a 118 bb (there are two lines, one for a 113, one for a 118).
 

·
Registered
Joined
·
9,589 Posts
Ah, a fellow Front Range rider

BipedZed said:
Whoops, I meant 47.5 chainline on my RX. It's basically a Shimano spec for MTB, using their BB/cranks you will either have a 47.5 or 50mm chainline. The 47.5 is preferred. With the XT Octalink BB you have the choice of a 113 which will yield a 47.5 or a 118 for the 50 chainline. With the external BBs there is only a choice of 50mm chainline. I don't really care about the chainline number, as both work fine for shifting and lateral chain load, it's the resulting q factor that concerns me. If your frame allows the choice, go for the 47.5 chainline.

Sheldon Brown explains chainline well.
Thanks for your input. Sheldon Brown's website is pretty comprehensive. I think I'll sit a spell and read his stuff, since (I'm also a Front Range rider) there isn't any riding to be had at the moment. Some weather pattern, huh?
 

·
Registered
Joined
·
9,589 Posts
more "older" Shimano cranks questions

OK, I'm just going to accept my LBS is going repeat from I've been told on this thread and tell me to probably go with a 68 x 113 bb with my M752 (this is currently mounted to another bike with a 73 x 118 so a new bb is required for the Superlight). Currently on cambria they have a LX FC-M572 and a XT FC-M752K (notice the K on this one). The ad states that BOTH REQUIRE a 121 mm spindle so buy the appropriate bottom bracket. Please, someone, explain to me why these would require a specific length spindle and what frames would they actually work on? Not on my Superlight build? Not on my current Sugar 2+ or Turner Burner builds, unless I'd be going for a wider Q. I mean, structurally, what would differentiate these two cranks that would absolutely REQUIRE a 121 mm spindle? Thanks.
 
1 - 20 of 23 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top