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Recreational Cyclocross and Gravelbiking This has to be the most physically intense sport ever invented. It's high speed bicycle racing on a short off road course or riding the off pavement rides on gravel like :The Dirty Kanza". We also have a dedicated Racing forum for the Cyclocross Hard Core Racers.

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Old 03-17-09, 07:50 PM   #1
mikerhymeswith
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mountain or trekking chainrings for CX

i'm building a cx bike that i want to race this fall; i'll be a c4; cx is a secondary thing for me so i'm on a tight budget

problem is with the frame i picked up; the chainstays are wide so the shimano hollowtech 175 mm arms practically touch on the non-drive side; choice of 3-piece cranksets is diminishing but i found a cheap mountain 44/32/22, a trekking 48/38/28, and a cheap 50/34 all using octalink which should allow me to get more crankarm to chainstay clearance with proper selection of BB length

reference point: i can spin out a 48/16 to 32 mph on a downhill - is that much speed necessary in cx? cx i watched last fall didn't appear to be going that fast

i've got a 26/12 cassette

i'm leaning to the mountain 44/32/22 and removing the 22, but i'd appreciate hearing more input; i've seen some similar questions in other threads but none with the 3-piece crank limitation
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Old 03-18-09, 10:35 AM   #2
flargle
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You'll be so pinned during your races that if there happens to be some smooth downhill section where you can spin out a 44/12, just enjoy the chance to catch your breath.
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Old 03-19-09, 03:28 PM   #3
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you will not need 32 mph in a cx race!

(I think my 28 38 48 + MTN cassette is the perfect all around, everyday riding setup btw)
~Steve
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Old 03-23-09, 11:19 PM   #4
thirdin77
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Originally Posted by mikerhymeswith View Post
the chainstays are wide so the shimano hollowtech 175 mm arms practically touch on the non-drive side
I'm just curious as to if this is a double or a triple as the triple actually requires a 4mm spacer on the non-drive side.

If you've got a double, you could leave your hollowtech II bb cups in place and just add the triple and its spacers to get that extra 4mm on the NDS. Leave the small ring or remove it, your call. I guess you would need a triple front derailleur just because they are set up for the 45mm chainlines that triples run and I don't know if the double front derailleurs, which are intened for 43.5mm chainlines, would work.

I would just suggest to add the 4mm spacer on the NDS of a Hollowtech II double but I don't know if that would give the NDS a different q-factor than the DS.

So, OK, which crankset do you have on there now?
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Old 03-25-09, 12:20 AM   #5
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I had a 42 x 12 top gear last season and never spun out. None of our courses had long descents though. The only descents were short and rough enough that you were coasting anyways.
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Old 03-25-09, 02:04 AM   #6
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used 42 x 11 several seasons.......no problems.
that's about 50kmh (30 mph) @ 90-95 rpm
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Old 03-26-09, 08:26 AM   #7
mikerhymeswith
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well....the mtb crankset option is out because the chainline is 50mm and 45mm is more appropriate; last night i built up the bike to test ride capability (using 105 hollowtech, 175mm, 50/34) and even though there is only a flat screwdriver blade sliver of clearance between the left crankarm and chainstay there was no touching when i pedaled hard; i did not do a 50/12 uphill sprint effort to torque the frame hard however, so testing at the limits was not performed

then i got the idea that i could grind and/or file away some material from the inside tip of the left crankarm to increase the clearance by a few mm; i don't think this will compromise the integrity of the crank and i accept that this will null any warranty on it;

another idea was to deform the left chainstay a bit but i decided to not try this just yet

i thought about using a 170mm crankset, but that would lift my costs and i really want 175s

the alternative is to scrap the frame - which only cost me usd110 - no big loss, but this bike is supposed to be cheap and serviceable for a CX4 rider, so scrapping is not good

if left arm tip grinding idea works then i'll replace the 50/34 rings with 46/36 (hmmm, maybe 46/34 or 44/34) ) and be ready for CX season

i'll take some before/after pics - unless someone scares me off of this latest idea

Last edited by mikerhymeswith; 03-26-09 at 08:33 AM.
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Old 03-26-09, 08:44 AM   #8
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Originally Posted by thirdin77 View Post
I would just suggest to add the 4mm spacer on the NDS of a Hollowtech II double but I don't know if that would give the NDS a different q-factor than the DS.

So, OK, which crankset do you have on there now?
105 Hollowtech 50/34 175mm on now

i looked at possibility of spacers but it looked to me like I'd be using too many and have too little spline interface between the spindle and crankarm

SRAM told me all their road cranksets have a 120mm Q factor and mtb have 140mm; my 105 crankset has 120mm also.
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