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CX gearing question(17 posts)

CX gearing questionD_man
May 18, 2003 5:01 PM
Took my new Gunnar Crosshairs out this weekend for a couple of singletrack rides. I'm realizing that the gearing (48 -38 in front, 12 - 27 rear) just ain't cuttin' it for the trails. I'm considering buying a MTB crank, removing the granny gear, and using that. That would give me 44 - 32 front. How many have gone this route? Did you find that you then top out on paved road rides? Since things are pretty flat here in Denver, I'm thinking that the 44T front ring would be just fine for around town and the 32 front would make the trails a lot better. I'd prefer not to go to a triple if I can avoid it. Alternatively, I could just buy some MTB crank arms and then choose my two front gears.

Any suggestions?
re: CX gearing questionmsmootsiemartin
May 18, 2003 6:24 PM
I should have my new cross bike this weekend and can give you more info then. I decided to have the bike built with TA cyclocross cranks...48-34 (Schwab sells them) and an xt 12-34 cassette. I thought this would be a great range of gears while staying with a double chain ring.

re: CX gearing questionD_man
May 18, 2003 6:44 PM
Does the 12 - 34 cassette require you to use a long rear derailleur?
re: CX gearing questionThe Walrus
May 18, 2003 7:53 PM
It requires an MTB derailleur (XTR, XT, LX). A long-cage road mech is still only intended for a max 27T cog, although with some diddling you can get away with somewhat bigger.
44T should be finerockbender
May 19, 2003 7:55 AM
I ride a 44T big ring on my CX rig and it ha been fine (on a 44-38-30 triple). If you need another gear on the top end, you could always replace the 12T with an 11T on the cassette for an 'overdrive' gear... as long as you wouldn't miss the 12 too much - I suspect not.
May 19, 2003 9:20 AM
What are you running for a crank right now?

1) you might run into compatibility issues with newer MTB cranks. For instance, an Ultegra splined BB (bb-6500) won't work with an XT or LX MTB crankset (spline pattern is deeper). ISIS and square taper stuff should be fine crossing over from road to MTB.

2) Chainline issues. Usually triple cranks require a different length BB to get the desired chainline. Thus, even if your existing bb works with an MTB crankset, you might have some some chainline or even chainstay issues...

Up until 3 weeks ago I was running an 11-32t cassette with an 22-32-44T XT crankset. This gave me all the trailriding gears I needed but was a little lacking on the high gears on the road (spinning like a madman on downhills). I recently swapped to a 38-48t Ritchey crank and I am really happy with the results. the MTB cassette still provides enough low end gearing to handle most trails while the 48t front ring gives me more capacity to go fast on the road.

The catch with this setup is that I am running an XT rear derailleur. Still the cost of a new derailleur and cassette might be cheaper then a new crankset and BB...
May 19, 2003 10:17 AM
Right now I'm running Truvativ Elita CX cranks w/ an ISIS bb. Regarding the chainline issues, can these concerns (possible need for longer BB) be removed by simply removing the granny gear and running the MTB cranks as doubles?

In any event, based on people's comments, I'm leaning towards just swapping out the rear derailleur from Ultegra to XT (I hate the look of the long cage though) and going to an XT cassette (I suspect I'll need a longer chain too). In that case, my best climbing gear would be 38 (front) 34 (rear), which would be lower than my best climbing gear with new MTB cranks and my current Ultegra cassette: 34 (front) 27 (rear).
I don't think so...Steve_O
May 19, 2003 11:12 AM
Regarding the BB width some funky things can happen... If you choose the MTB crankset I would mount them up and see how it shifts before going to a longer/shorter bb. Removing the granny gear probably won't help as it won't make the crankset "closer" to the centerline of your BB shell (IE - The crankset won't mount any different with the grany gone or not).

As a rule of thumb triple cranks are wider thus require a wider BB. Going from MTB to road confuses the issue. For example. I had a 113mm BB with my XT cranks. My new Ritchey double ring cranks required an Ultegra BB. Ultegra BB's come in 109.5mm and 118.5mm sizes. I figured that a double ring crank like the Ritchey would let me get away with a BB that is 3.5mm shorter (Hey it's only 1.75mm difference on the crankarm side right!) so I bought the 109.5mm. Unfortunately the 48T rings are larger diameter and would contact my chainstay if I went with the 109.5mm BB so I had to get Shimano's "triple" BB with 118.5mm spacing to make it all work!

This probably would not have happened on a standard road bike but the chainstays on CX bikes tend to flare out more to clear larger diameter tires. In my case, the combination of wider chainstays and a larger big ring forced me to get a wider bigger bottom bracket...

Geeze.... After reading this I think your on the right track. Get an MTB cassette and derailleur and be done with it!
I don't think so...D_man
May 19, 2003 11:45 AM
Yeah, looks like a derailleur/cassette swap is the better way to go. Now the question is whether I could use a mid-cage XT derailleur, or if I would have to go long-cage. I think the mid-cage XT can still handle up to 34t. And I think it has a chain-wrap capacity of 32. Since my front gears are 48 and 38 and the rear cassette would be 34 -- 12, I think this gives me a chainwrap of 32:

(48 - 38) + (34 - 12) = 32.

Does that sound right?
Your good!Steve_O
May 19, 2003 12:04 PM
Both GS and SGS version of the XT derailleur can handle a total capacity of 33T... Fill us in when you get it built up!
May 19, 2003 2:11 PM
run MTB cranks on both of my cxers, no trouble. both set up 48x34. run one 8 sp 11-30 XT, the other 12-27 Ult. 9 both work great. Prefer the 8 as I love the 11-30 spread. may go to 11-32 9 on the other. in all honesty I rarely used the 48x11 or 48x 12. a 44 or 46 tooth big ring is fine for most apps. wish Shimano made an 11-30 9. I'd shy away from the 34 as you will be more prone to chain derailment. Invest in some type of chainwatcher regardless for this kind of set up. (especially if you plan to race)
re: Got Granny...jrm
May 20, 2003 8:22 AM
It is very seldom that i use my 22T chainring on my LX MTB crankset but when i do im kinda glad i have it. Im using a 12-27 cassette.
My thoughts on MTB Cranks w/ 130 rear spacing..Mitch2
May 20, 2003 9:05 AM
I used MTB cranks for years on my cross bike and the biggest issue I ever had is the chain wearing out too fast. Chain would last only around 2-3 months max for my type of riding. I blame this on the 130 rear spacing and the further outboard postion of my mtb cranks(107 bb spindle) I think I put too much pressure on the chain on long offroad climbs when I use 32 front and 34 rear. I don't have a granny ring so when I use this mode on my climbs the chain line really looks extreme and could be the cause of my early chain wear. Now I am about to replace it w/ a double Ultegra 46/39 and see how that works.
re: CX gearing questionD_man
May 20, 2003 10:50 AM
I just ordered an XT cassette (11 - 32) and XT rear derailleur. That should be a big improvement over my best climbing gear with my current ultegra cassette. If I still can't handle the climbs, I'll consider going the more expensive route of MTB cranks + new BB.
Maybe give the gearing and yourself a chance ...Duncann
May 20, 2003 8:27 PM
you may just need a little time to grow into your new ride. If you're not killing your knees, you could think of it as your trial-by-fire to becoming a stronger rider. See if you can improve your riding with time and practise. After all, if mtb-like gearing is what you want, you might as well be taking your singletrack spins on a mountain bike. Not intending to be disrespectful, but just my 2 cents. Enjoy (just bought a a Kona Jake, 48-39/12-25 ... and after some apprehension, I'm really diggin' the gearing). D.
Maybe give the gearing and yourself a chance ...D_man
May 21, 2003 6:13 AM
I hear what you are saying. Running CX gearing on singletrack does increase fitness rapidly! However, part of the problem is that I do have knee problems, and low cadence grinding really sets them off. But I think by going to the bigger rear cassette, I'll be able to keep my cadence up on some of these climbs. Actually, I'll probably just save the Ultegra cassette I'm running now and put it on a second set of wheels I plan to order for road use. So I'll have one set of wheels with slicks and an Ultegra cassette, and another with fat CX tires and an XT cassette.
Right ... so save your knees then. (nm)Duncann
May 21, 2003 11:18 AM