changing a chainring

[johnnyh]

Hi folks,

Just looking for a steer really, mt lad has outgrown his Triban 3 (put on 10cm since May... I never gave him permission to grow), and as such his beloved self funded Triban is 6 months old and too small

This eve we have ordered a new bike for him, a Felt F95, but for his youth racing and the gear restrictions, I kinda need to put a 39T chainring in place of the 50T (it is a compact).

If I get a 130PCD 39T chainring, should I be able to swap out the 50T that is coming on the bike?

Cheers for any pointers - the other option is a 52/39T double, and lock out the 52 - but then I would rather leave him the 34T for the local hills

Johnny

 

[Dusty Bin]

If the Felt comes with a compact (110bcd) chainset, then a 39t 130bcd will not fit.

 

[johnnyh]

ah, hard to tell, I presumed 130, but I guess it could indeed be 110...

providing I get the correct PCD will it work?

 

[HLaB]

Ive never changed rings but you'll have to get the same 110mm BCD not a double and I think they are also inner an outer rings to improve shifting.

Edit: The FSA rings are labelled Inner or Outer.

 

[Dusty Bin]

ah, hard to tell, I presumed 130, but I guess it could indeed be 110...

providing I get the correct PCD will it work?

Yes, it will work no problem assuming the ring is the same dia as the crank. Depending on what his roll-out restriction is, you may or may not need to lock out any of the rear sprockets. Plenty of reasonably priced rings to be had on ebay.

 

[johnnyh]

yep, I have a 12-27T cassette for it, so providing I can sort the 39T chainring all will be good.

From scouring the web it does look like the BCD is 110, so good shout there

 

[Dusty Bin]

Don't worry about the rings being inner or outer, any will do and they will shift ok. If he's riding circuit races, (which as a youth is pretty much all he can do) then he would probably only use the 39 anyway.

 

[johnnyh]

yep, he pretty much only uses the 39 on his previous/current bike, and the racing is circuits. But on Sunday rides he does sometimes use the inner chainring on the triple for the steeper climbs, so just want him to be able to drop to the 34. (the triple he has I have locked out the 50T so he effectively has a 39/30T)

 

[Dusty Bin]

that's fine - the jump from 34 - 39 will be so small that ring profile will not really come into it. My lad uses a 34-42 setup and his 42 outer is a actually a single speed ring designed for DH MTB racing - no shift pins, no profiled teeth, and it still shifts as smooth as anything.

 

[johnnyh]

that's good to know, thanks for that. I kinda guessed that a middle ring 39 would shift up and down fine, but good to know

 

[fossyant]

I wouldn't go using an inner on the outer edge. The offset of the teeth may be slightly out so you'll be having to fiddle with the front mech. A double pain if you plan on regularly switching back to the 50t.

I think you will struggle getting any lower than a 46t outer ring. Other options would be to build a cassette with larger sprockets and restrict the mech so the 12 can't be shifted to. Your bottom sprocket ideally needs to be the one with the serrated teeth for the locknut, but there is nothing stopping you buying an additional cassette and splitting the current one, and swapping and changing the sprockets so you have a good range.

 

[Dusty Bin]

I wouldn't go using an inner on the outer edge. The offset of the teeth may be slightly out so you'll be having to fiddle with the front mech. A double pain if you plan on regularly switching back to the 50t.

Genuinely, it's not an issue when the jump is so small. Typically, the shift pins and teeth profiling on the 'outer' rings is there to help the chain climb a fairly large step onto a significantly larger outer with 10-16 extra teeth. I can't see any circumstances where a rider on youth gearing would need to switch back to a 50.

I think you will struggle getting any lower than a 46t outer ring. Other options would be to build a cassette with larger sprockets and restrict the mech so the 12 can't be shifted to. Your bottom sprocket ideally needs to be the one with the serrated teeth for the locknut, but there is nothing stopping you buying an additional cassette and splitting the current one, and swapping and changing the sprockets so you have a good range.

Building a cassette is not necessary - there are plenty of 14-25 or 16-25 cassettes out there for this purpose.

 

[fossyant]

In my opinion, changing the cassette is the best option, not fiddling with chain rings.

 

[Dusty Bin]

In my opinion, changing the cassette is the best option, not fiddling with chain rings.

10sp cassettes are around £40 - but individual chainrings can be picked up on ebay for less than £20. 'Fiddling with chainrings' can be done by anyone with just a couple of allen keys, while changing cassettes requires a chain whip and the appropriate lock ring tool, so it's a bit of a no-brainer as to which is more straightforward. In any case, you do understand we are talking about BC's youth gear restrictions here, right?

 

[fossyant]

Dusty yes I do and once raced with those restrictions myself. Don't forget the lad will most likely have to use the bike at weekends too, and changing a cassette also means not having to readjust the front mech.

Personally I wouldn't use an inner ring for an outer, I don't do bodging kit.

It's quicker to swap a cassette then a chain ring then adjust the front mech for the correct chainring gap. Better still a spare wheel.