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Replacing triple crank with double(13 posts)

Replacing triple crank with doublenancer
Aug 5, 2003 12:28 AM
I have a 105 triple crankset and I want to move to a 105 double crankset. Can I make this change by changing only the crankset? The Shimano site lists different shifters and deraileurs for the double and triple sets. The bottom bracket is also a different length.
re: Replacing triple crank with doubleRusty Coggs
Aug 5, 2003 5:01 AM
If 9 speed,the shifters swing both ways.If you just stick a double crank on the triple spindle,the chainline will not be optimum,but everything will work.You can also get the same result by removing the granny from your crank and blocking the shift to the granny with the low limit screw...To get to a factory double setup,you need crankset,BB, front and rear derailers.
I've never heard the explanation for the RD being swapped...theweasonator
Aug 5, 2003 6:18 AM
Can someone tell me the technical reason behind changing the RD when going to and from triple/double. Is it because a longer cage is needed to accomodate the "pivot point " for the chain, the RD act as due to the longer distance the chain travels on the front?

In essence if I used a short cage RD for a triple set up the chain would "sway" laterally too much and wear faster and not perform as precisely/smoothly?

Did I answer my own question?
I've never heard the explanation for the RD being swapped...divve
Aug 5, 2003 7:03 AM
A medium cage derailleur can take up more slack when using the smallest chain ring.
the eplanation......Rusty Coggs
Aug 5, 2003 7:24 AM
Manufacturers match cage length to the ammount of chain needed to be wrapped,which goes by the basic formula: difference in front chainring teeth + difference in large and small cogs in the rear.Nothing catastrophic happens if one does not follow the rule, and knows what they are doing when they set one up,and use it acordingly. The longer cage wraps more chain. You can use a long cage with a double and just have extra wrap capacity.You can also use a short cage with a triple,but depending on the tooth spread some of the small cogs may not be usable in the granny ring.Not really a big issue since one shouldn't be using them anyway and for many the granny is just a seldom used bailout anyway......Another point not often understood is that large cog capacity is independent of cage length. An ultegra triple RD has the same large cog cap as the short cage double. An XT short cage has the same large cog capability as the long cage. ....Wrap is in the cage length, and large cog is in the parallelogram design.
A couple different waysMR_GRUMPY
Aug 5, 2003 7:23 AM
1) Take off inside chainring = $0
2) Take off inside chainring and buy double BB = $
3) Buy double crank and double BB = $$
4) See #3 and buy front and rear changers = $$$
5) See #4 and buy new STI's = $$$$

Stick with #2 or even #1 (If you are really cheap)
Thanks for the explanation....theweasonator
Aug 5, 2003 7:46 AM
So as long as one understands with a triple there is more of a chance of duplicating/overlapping your gear inches with different gear combos and, don't cross chain or shift like a moron. You should be okay?

I'm asking because I am thinking of making my current bike a triple and retiring it to be a burley mule to haul the kids in or a rain bike and then getting a newer racer.

And I want to keep my short cage RD.

Also, Grump, would I you really need new STIs? I thought the 105 and Ultegra STIs (6501) were interchangeable between double/triple. I thought the DAs were the only ones that were not interchangeable.
Yes, you are correct.MR_GRUMPY
Aug 5, 2003 8:28 AM
I was thing of the old 9 speed STI's where there were double and triple STI's.(6500)
You are still incorrectRusty Coggs
Aug 5, 2003 8:54 AM
All the 6500 fronts would shift a double or triple, as would all the other shimano 9 speed levers other than pre 2002 DA. The 'old' ones were 8 speed 6400 series and did not even offer a triple option.
You are rightMR_GRUMPY
Aug 5, 2003 10:52 AM
I was thinking that 6500's didn't work with triples because the first year that they were out, triples weren't offered. I believe 6501's came out the first year that triples were available. Didn't 6500's have both buttons on the right side, and 6501's have one on each.
Please don't tell me I'm wrong about triples not being available the first year. I'll have to start taking some of those "memory" pills.
You are rightRusty Coggs
Aug 5, 2003 11:05 AM
Acording to old QBP, ST-6501 IS compatible with SC-6500 flight deck computer. ST-6500 IS NOT compatible with SC-6500 flight deck computer. Only difference I am aware of.
#5 ...new shifters???Rusty Coggs
Aug 5, 2003 8:47 AM
Really? if its 9 speed other than pre 2002 DA they all shift either a double or triple. 7 speed rsx sti shifted either a double or triple. 8 speed RSX,rx-100, and 105 were double or triple specific front shifters,but the triples would shift a double ok, and there was no triple option for 8 speed ultegra/600 or DA.
what middle ring will fit a triple?komatiite
Aug 8, 2003 6:36 AM
i'd like to convert my triple from a 42 to a 39. my friend converted his and i cant remember what exactly he used. the BCM has to be correct. i guess i could just measure that. I know that it was a Salsa. would i see any improvement/changes going from a 42 to a 39 middle ring?