Tuesday, April 26, 2011

Cross frame crank compatibility...

Being out of the bike industry insider club for several years, I missed the regular technical updates from Shimano and SRAM. The pages that make up these manuals are a bike designer's bible. When I designed the chainstays of the cross frame, I used what I had and thought I had all my bases covers so that a road compact crank would be compatible with the cross frame.

Well, I got it almost right. I physically checked the frame by installing compact cranks with external bearings from both SRAM and Shimano. External bearing cranks have no left/right adjustability with regards to chainline, arm clearance, ring clearance. Bearings go in. Cranks go on. Done. So I installed cranks from both Shimano and SRAM and thought - good, they fit.

Doh! I didn't check the new Shimano 105, Ultegra, and Dura Ace compact cranks. It seems their arm stance is narrower than their Tiagra compact or R600 compact cranks by enough that these cranks are not recommended on the Black Mountain Cycles cross frame.

Check out the chart below and notice the dimension W5. Here are some cranks and their respective W5 dimensions for reference.

SRAM Red, Force, Rival, Apex, S900, S500 compact doubles: 60.0 to 61.0mm (okay)
Shimano Dura Ace FC-7950 - 57.2mm (too narrow)
Shimano Ultegra FC-6750 and 105 FC-5750 - 57.6mm (too narrow)
Shimano Tiagra 4550 - 59.5mm (okay)
Shimano R600 - 61.0 (okay)

Crank

In summary, Dura Ace, Ultergra, 105 compact cranks - not compatible with my cross frames. Any others are good to go. Sometimes compromises need to be made to fit those fat tires. I'm not sure about the compatibility of the previous generation of Dura Ace, Ultegra, 105 cranks, but will try to squeeze the info out of Shimano.

(What's playing: KWMR Musical Variete and some Albert King)

24 comments:

Tim Joe Comstock said...

If you think YOU find the byzantine workings of the Shimano mindset a sometimes slippery slope, what do you think it is like for normal humans? In my painful struggle to attain enlightenment, however, I notice that SRAM seems to be more "user friendly" than their nefarious counterpart, leading me to wonder why they do not have a larger market share.

Anonymous said...

By new Dura Ace, Ultegra and 105, do you mean all outboard bearing models or just the newest versions of those cranks?

blackmountaincycles said...

Good question. Those Shimano numbers are for the current generation. I don't have the numbers for the previous (Dura Ace 7800, Ultegra 6700, or 105 5700) series cranks.

Ari said...

no issues with a sugino xd600 triple with 26-36-46 chainrings and shimano un54 110 mm. spindle.
ari

blackmountaincycles said...

Yep, no issues with just about any 3-piece type crankset. I set up an XD500 triple with a 107 UN54 earlier this week.

Anonymous said...

@Tim Joe Cornstock: Methiks it would help a lot if SRAM focussed more on the user friendliness of the actual front shifting.

Exhausted_Auk said...

Hey Dude,

What about Campy cranks?

blackmountaincycles said...

Good question and one I can't answer since I can't get access to dimensions from Campy. If the W5 dimension is more than 59mm and the inner chainring is 34 or smaller, they should work.

Anonymous said...

Mike - I just picked up a 2005 vintage Ritchey WCS compact (110 bc) crank, and will run 48/34t rings. It uses an Octalint BB.

Can you suggest an Octalink spindle length for those cranks?

--Scooder

blackmountaincycles said...

2005 is vintage? Not sure, Scooder. Those Ritchey crank arms have a fairly narrow q-factor and may have the same problem as the Shimano Ultegra/Dura Ace with chainstay clearance at the end of the arm.

You'll need to verify if the Octalink version is V1 or V2 (I think it's V1). There are only two available widths of V! today: 110 or 118. You might need to experiment.

Anonymous said...

Ha! OK, here's where a well placed hyphen would have clarified it as a "2005-vintage" crank, meaning from 2005. Not sure which version or how they're distinguished, but these are post-recall versions.

blackmountaincycles said...

Or just leave the word "vintage" out altogether and call it a Ritchey Octalink crank....

Post or pre-recall, I believe that they all have the same q-factor / bb size requirement.

Anonymous said...

Have you ever installed a mountain double crank (28/40) on a cross frame with a road derailleur? The only reason I thought it may not work is the tail of the front derailleur may hit the chainstay because it is positioned so low.

blackmountaincycles said...

The mountain double is something I'm working on. I've mocked it up but need to test it better. It's promising, but I'm doing it with a 42/28. The 40 puts the derailleur tail too low.

Anonymous said...

Would a Shimano MTB 2x10 gearing work? I wonder if the Shimano FC-M785 will fit and without chainline or the FD being not low enough?

blackmountaincycles said...

Anon - that's something I'm going to look at soon. I have a SRAM 42/28 mtn crank that I want to check. Any mtn double will physically fit. The potential problem is if the bike is run with drop bars and brifters, you have to use a road front derailleur and I'm not 100% certain there is enough throw in the road front derailleur to get to to shift to the outer ring. It could be done using the mountain double front derailleur with bar-end shifters. I'm planning on testing some things and will report back when I can.

Rob Jones said...

Just verifying that the 34t is the largest small ring possible? So no 46/36 cx setup? Thanks.

blackmountaincycles said...

Rob, 46/36 combos fit no problem. SRAM Apex/Rival/Force are okay. The Shimano CX50 and CX70 also fit no problem.

Anonymous said...

any update on the mt double (28/40) setup?

blackmountaincycles said...

Regarding mtn doubles (26/38 or 28/40), the problem is that if you are running integrated road brake/shift levers, you need to run a road front derailleur (although, I have heard of adapters that allow using mtn f/d with road shifters). A road front derailleur won't make the throw to the much wider chainline of a mtn double.

If you run bar end shifters, there *should* be no problem using a mtn double specific derailleur. I say should because there may be an issue with the tail of the f/d cage hitting the chainstay as the bb drop is lower than on mtn bikes and the angle of the seat tube / chainstay is more acute.

Phil B said...

I had a XTR 952 crank with 44/32 on the bike and worked fine.

However, I want something taller. I tried a Middleburn 48/36 square taper with a 113 and it is too wide for the Fder to reach. Guess I'm going to have to try a 108.

Thoughts?

blackmountaincycles said...

Phil B: Middleburn with a 113 axle will give you a 50mm chainline - which is okay on a mountain bike with 135mm spacing and a mountain front derailleur that has more throw. However, road front derailleurs won't shift that far out - as you've found. A 108 will bring it in a bit and it might be just enough. I have found that Shimano's FD-R440 has a good amount of throw that will reach a 47.5mm chainline, but not enough for 50mm.

If you are using bar-end shifters, you could also use a mountain front derailleur that will work with either the 113mm or 108mm bottom bracket. The mountain front derailleur won't, however, work if you are using integrated brake/shift levers.

dc designer dad said...

Anyone running campy (double road) on the cross frame? I have a Centaur and some Chorus parts sitting around and would love to do a campy build on this platform.

blackmountaincycles said...

There are a couple Campy equipped builds out there. I've done up a couple of the US frames and one Taiwan frame w/ Campy. They've turned out great.