2 or 3.....what's the difference

[benseddon]

As usual, I'm just thinking aloud here.

I know that a double consists of 2 chainrings and a triple is 3 but is there a difference in distance between the chainrings on a double and a triple??

For example, if I remove the granny ring have I turned it into a double?? Is it that simple???

 

[ishaw]

It is indeed that simple on older stuff, I believe anyway.

 

[benseddon]

Thanks ishaw. Simples is good....we like good.

 

[legrandefromage]

simple.

and you can add spacers if you want.

 

[velomaniac]

If you dont use a shorter BB axle or space both rings inward though you'll not be at optimal chain alighnment for a proper double.

 

[benseddon]

Are spacers placed on the chainring bolts??

How do you space them inwards??

 

[drystonepaul]

If you dont use a shorter BB axle or space both rings inward though you'll not be at optimal chain alighnment for a proper double.

Indeed. I run a double on my 1993 Univega Alpina Comp. Deore MT60 chainset would normally run 24-36-46 chainrings and a 122.5mm bottom bracket.
I use a 26-38 set up with a 118mm bottom bracket. Chainline is perfect.

Initially I used Deore MT62 thumbshifters. Obviously with front friction shifting everything worked fine.

I changed to Deore DX M075 Rapidfire Plus shifters which also work perfectly.

 

[cyclotoine]

Re:

It is that simple, but as above you would need to change your spindle length for ideal chainline.

Randonneurs are into "super compact" doubles now with the resurrection of old T "pro-5-vis" style cranksets and the white industries cranks. It is popular to run something like a 28/42. I honestly don't know how they get decent shifting performance out of those old Nuovo Record derailleurs. However I am going to try it on a newer 110/74 triple with a 42 on the middle ring position and an XT 9 speed front derailleur on my "monster cross" which is more like a retro MTB/ cross style geometry that will clear a 29x2.1 tire (built the frame at a frame school).

 

[benseddon]

But if you had gone from a 24-36-46 to a 46/38 would you still have to alter the BB?

 

[cyclotoine]

But if you had gone from a 24-36-46 to a 46/38 would you still have to alter the BB?

You don't have to, but the middle ring is centered to the cassette (or should be anyway). So, ideally you should now be shortening your BB so it is centered on the tab between the 46/38.