Bicycle Mechanics - 2 Chain rings to 3, help please
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I have a Tiagra groupset and am looking to upgrade my 2 ring chainset to a 3 ring one. Will I be able to use the same front mech or will I have to buy a new one. Other than the appropriate new tension arm and chainset itself, is there anything else I will need to get.
01-18-02, 12:43 PM
Going from double to triple requires replacing several parts. First you will absolutely have to replace your bottom bracket with one having a longer spindle, usually 118.5 mm on Shimanos. Next, assuming you have STI or other indexed shifters you will almost certainly need a new front der, but they are not expensive. You might even want to take the opportunity to do a little upgrade and get an Ultegra or even DA triple. You MAY wind up needing a new rear der also. There are specific models for use with triples. I would try to do without it initially. You can always add that later. The rub may come with the front shifter itself. I know Ultegra and 105 STI front shifters will handle double or triple. You need to find out if Tiagra does also. Anyone else know this?
Another route that may get you to the same place for less money would be to replace your rear der with a Deore XT model. This will allow you to go up to 34T cog on the back. Assuming your small chainring is 39, this will get you down to 31 gear inches. By way of comparison, a typical low gear for a triple might be 30/25 or 32 inches, 30/28 would be 29. The XT rear der works fine with STI. I am using one with 105 9-speed STI on my commuter/tourer. Shifting is not quite as snappy as my DA STI/rear der on my other bike, but it is pretty close. Shifts seem to take a split second longer to happen, presumably because of the longer arm. Just another approach to achieve similar end. You might even be able to go up as high as 32T cog with your current rear der, giving you 33 inches. As an experiment, I have used a 32 with my DA rear just fine. You give up a couple of links of chain wrap, but I am not sure how much of a problem this might be. I am getting a 34T cog for extreme use on my commuter and will certainly try it on all my bikes just as an experiment. I am always trying odd combinations just to see if they are workable. If you do put a cassette with a large cog on, you may have to adjust the "B" tension adjustment which controls the vertical distance between the jocky (top) pulley and the cog. If you have the tools, chain whip and cassette lockring remover, you can try this for the cost of a single 32t or 34T to see what your current setup will handle.
01-18-02, 01:06 PM
Ouch! have been studying this myself lately trying to update the wifes sora to 105. Check out shimano.com. Some of what i've learned may apply to tiagra check it out first.
shifters - 105 is different for 2 and 3 chainring
front derailler - 105 is different for 2 and 3 chainring
rear derailler - 105 is different for 2 and 3 chainring
cranks\bb - different for 2 and 3 chainring
my suggestion would be to keep your 2 chainring and change the sprocket sizes to match your gearing needs then upgrade to ultegra deraillers front and rear if you want. that could all be done for about $150(rough estimate).
01-18-02, 01:37 PM
Most doubles are happy working with chainrings smaller than 39.
If you can do without 52/12, then you should think about swapping both chainrings. I have used a 36/48. If you drop the front mech a few mm it should work OK. Combine that with a wider range rear mech and a cog set from maybe 12/13 to 32/34 and your 18 gears will become much more useful. Just make sure the rear mech is compatable with your STIs.
If you have trouble finding smaller chainrings from Shimano, have a look at Specialities TA. It is used by the US Postal team and very good quality stuff.
I like Michael's recommendation to substitute smaller chainrings, except that the standard 130mm BCD Shimano road double can't go below 38T, so you're still looking at a new 110mm BCD crankset (cranks and rings). However, if you keep your current 14-tooth(?) chainring drop with something like a 48-34 or 46-32, you may be able to use your existing rear derailleur, cogset, shifters, and BB. I run top gears of 48/13 = 52/14 on two of my road bikes and have no need for anything higher.
01-18-02, 05:47 PM
You are correct, Sirs! I have a 110/74 BCD crankset on one of my bikes. It is a triple, but I am using it as a double for the time being and even have a 34T small ring I can put on for extreme situations. I have the appropriate bb for the triple, I just haven't felt like putting it on. But John E is right, 38 is as small as you can go on 130 BCD cranksets.
Niall, the short story is, avoid going to the triple if you can because it can run into money. See if you can accomplish the same thing with larger gears in back. My favorite 9-speed cassette is 14-25. I can't even drive 52/14, much less anything higher.
01-19-02, 01:33 AM
I once found a used bike with a triple set-up for less than what it would have cost to "switch" any bike I already had!! Look around, maybe you could do the same!
Ride little gears
Of course, you could always look for a Stronglight 100 double, which with a longer bb axle and different c/r bolts converts to a triple!
I used to run the type of setup Chewa describes -- using a long spindle, long stackbolts, and spacers, one can build a single-BCD triple out of any crankset. For years, I had a Sugino Mighty Compe set up as 49-46-43, with a 13-16-19-23-26 freewheel, which gave me a 14-speed (avoiding large-large cross-chaining) third-step variation on the old half-step system. A 3-pin TA Professional crank, with its 116mm BCD, could support a 48-45-35 half-step-plus grannie on 3 long stack bolts!
Thanks for the plethora of advice. I hadn't realised it would be so complex.
I have a few thoughts to throw into the pot. In my own research at Shimano.com, especially the exploded views and parts lists, I got the impression that the Tiagra (and 105) left-hand shifters served for both double and triple chainrings. This was given strength by the fact that when changing from one chainring to the other there is an intermediate stage between the two (I didn't notice similar effects when I was trying out bikes with a Tiagra triple chainring at the time of purchase).
Also, looking at the Shimano parts lists the only difference in rear mechs is the tension arm cage plates and appropriate spring.
Is this correct or am I being a prat and missing something obvious?
ps. the bike shop I bought it from, at the time said it would be a relatively simple process. They neglected to mention that it would be simple for them (not me) and the cost involved!
01-21-02, 10:46 AM
What you will need is the long cage rear der, the front triple der, ( the capacity of the the triple front is 22 rather than 15) a 118 bottom bracket and a triple crank. The front shifter has the third chainring position already, and the chain will be long enough if you don't change the rear cog set.
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