Changing from 39T to 34T on CXbike

[iiinge]

Howdy!
I decided to switch from a 39T to a 34T chainring. It is a Shimano, 5hole setup on a Viscious crossbike. Where can I get a 34T ring and will I need to adjust anything else? How do I know if I need a 110mm or a 104mm? Where are these measured? Thanks so much for your help. These are probably really elementary, but obviously I usually don't work on my own bikes.

Thanks again,
Inge

 

[daveryanwyoming]

Howdy!
I decided to switch from a 39T to a 34T chainring. It is a Shimano, 5hole setup on a Viscious crossbike. Where can I get a 34T ring and will I need to adjust anything else? How do I know if I need a 110mm or a 104mm? Where are these measured? Thanks so much for your help. These are probably really elementary, but obviously I usually don't work on my own bikes.

Thanks again,
Inge

Bring your bike to your Local Bike Shop (LBS). It's unlikely you'll need a chainring drilled for a 104mm bolt pattern, but it's quite likely that you have a standard (as opposed to compact) road crankset with 130mm bolt spacing in which case you won't be able to mount a 34 tooth chainring.

You can measure the center to center distance between two mounting bolts(compact 5 bolt cranks measure 64.7mm between two adjacent bolt centers) but if your inner ring is currently a 39 then it's very likely you don't have a 110mm compact crank. Your LBS will know at a glance what crankset you have and should have the chainring you're after if it will indeed fit.

-Dave

 

[iiinge]

Thanks for the quick response, Dave!
I went out and looked at the chainrings again. The bolt to bolt distance is 78mm.
The current ring says SG X 105 39B. Does the B mean anything?
The big ring says 130mm 48T, if that helps?

I called the LBS and they don't have any 34T rings.

 

[alienator]

Thanks for the quick response, Dave!
I went out and looked at the chainrings again. The bolt to bolt distance is 78mm.
The current ring says SG X 105 39B. Does the B mean anything?
The big ring says 130mm 48T, if that helps?

I called the LBS and they don't have any 34T rings.

That means you can't run a 34T chainring on that crank. The bolt circle on the crank is too big. You'll need a compact crank to run a 34T.

 

[iiinge]

Can I downsize to something smaller? Or should I look into a different cassette?

 

[alienator]

Can I downsize to something smaller? Or should I look into a different cassette?

The smallest chainring you can use is a 38T. You'll likely have to get a different cassette to achieve the gearing you're looking for.

 

[iiinge]

Sheeeeeeeeeeeeet! Can I get a different crank?

 

[alienator]

Sure you can change cranks. It's just more expensive to do things that way.

 

[iiinge]

Yes, that makes sense. So what is the equivalent cassette?

Thanks so much for your help!!

 

[alienator]

Yes, that makes sense. So what is the equivalent cassette?

Thanks so much for your help!!

Well, the equivalent cassette depends on what gearing is on your cassette now. What are the smallest and largest cogs on your cassette?

 

[iiinge]

12x26

 

[alienator]

12x26

A 39T chainring with a 12x26 cassette has about the same gear range as a 34T with a 11x23 (or 12x23....it's close in the high gear).

 

[iiinge]

So can I get a 13x28 or 32 to be equivalent to having a 34T w/ a 12x26? If you have a 34, what do most people ride in the back for 'cross?

 

[PeterF]

The first time I tried a cross race, I ran a 34/46t compact crank with a 13-26t cassette (yes campy, sorta rare in cross around here). Well just imagine "spinning" up a muddy hill in a farmer's field in a 34/26 combo, and then try and imagine any scenario where putting the bike over the shoulder and jogging wouldn't have been faster. Plus the 34t is pretty much useless in any other part of the race. I switched to a 36t chainring, but honestly I could have stayed with the standard crank 46/39t that I originally had before wasting the money on the compact.

 

[iiinge]

Thanks PeterF!
That is exactly what the guy that built up my bike told me and that is the reason I have a 39/48. I recently have been riding some trails that would be nice to ride instead of run, since it isn't a race situation, and thought that I'd like some more rideable gearing. But now that I think about it I wasn't really off the bike much more than the other riders w/ the 34s and just a bigger cassette would be an easy switch for those days.

So I will conclude by saying I'm thrilled w/ all the feedback this site provided and I'm just going to get a bigger cassette and BE A HAPPY CX rider!!