Can I fit a 34T ring on a regular chainset?

[swee'pea99]

I've never really understood 'compact' v regular chainsets...I have a 52/39 setup (I swapped a 39T for the original 42T to get a lower gear) - would a 34T ring would fit on the (Ultegra) chainset? Or would I need to change the whole thing? One other thing, if I could get a 34T ring to fit, are they all standard, or do, eg, Shimano & Campagnolo have different fittings? Thanks.

 

[accountantpete]

With compacts the arms shrink slightly to keep everything in proportion - so the space between the bolt holes is a bit shorter.

Specifically, the BCD for a standard Shimano double is 130mm (measured from the centre of one bolt hole to the next with a ruler and multiplied by a fraction as per Sheldon http://www.sheldonbrown.com/gloss_bo-z.html).

The BCD for a Shimano compact is 110mm. So a normal compact chainring won't fit on the standaDrd cranks.

Campagnolo have a different fitting (BCD = 135mm) so they won't fit.

As Adrian says - a 34t won't fit.

 

[PpPete]

You need to look around for a 130BCD 34t chainring .

Sadly - such a thing is impossible. The bolts would be outside the teeth!

130 BCD - 38 teeth is the smallest possible as Adrian says
135 BCD - 39 teeth is the smallest possible.

Another way of getting lower gears (if your mechs and shifters will handle it) is a converter ring to make your double into a triple.

 

[accountantpete]

Sadly - such a thing is impossible. The bolts would be outside the teeth!

I amended that after a quick google!

 

[swee'pea99]

I'll take that as a 'no' then! I kind of suspected as much, but thanks for the confirmation. Back to the drawing board...

 

[al78]

If you want lower gears, could you change the cassette to one that has a bigger sprocket as its bottom gear?

 

[swee'pea99]

Fair point, but as it happens it's only the bottom end I'm interested in right now. I'm doing the etape in a month, and I need some *low* gears! As it happens, I spent today filing down the jockey wheel teeth on my rear mech, to stop it interfering with the 32 tooth on the MTB cassette I just fitted. It worked too! I think that'll do me. Any lower and I could climb trees.

 

[swee'pea99]

Well, yes & no. Mostly no! And I do appreciate help. Even if it didn't directly sort my problem this time, now I know about B screws. It all comes in handy in the end...