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Old 06-09-09, 09:35 PM   #1
pdrizzle
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triple chainring isis -> single square taper

i abandoned geared bicycles some years ago because of complications that i didn't feel like dealing with then (derailleurs, shifters, etc). now i'm rethinking things and want to convert a roadbike with 3 rings up front and 9 cogs out back to be a 1-9 setup. i know i'm going to be labeled a noob and get flamed or whatever you internet forum fools do, but i've done a bit of searching on here and haven't found the exact answers i'm looking for. so here goes:

current setup:
truvativ isis triple cranks
truvativ (?) isis bottom bracket
shimano 105 derailleurs
9 speed shimano cassette

i wish to get rid of the front derailleur and 2 extra chainrings and replace the crankset/bottom bracket with a square tapered single speed crank/chainring setup. so...

what size square tapered bottom bracket should i use to replace the current isis bb? (i realize this depends on the size of the current isis bb, which i have yet to remove/measure, but i'm sure someone's done this, right?)

i plan on using a bmx crankset/chainring, so will there be any issues with dropping the chain? is there any way (the perfect chainline spacing) or a product (bash guard?) i can install to keep problems from arising?

i know people love to call others out when they ask stupid/naive questions, so if that's you, please, reply and i'll get a kick out of your hater speak. but...

if anyone on here has done this or has read a thread where this is answered, please help me!

thanks in advance.
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Old 06-09-09, 09:53 PM   #2
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To determine ideal bb spindle length you should measure the chainline - the distance from the centre-line of the bike to the chainring and the distance from the centre-line of the bike and the middle of your cassette. If the chainring is further out than the middle of the cassette then you need a bb spindle shorter by two times the difference. Also make sure the crankarm and chainring and the frame have enough clearance to spare.

I'm not too sure, but I fear BMX chainrings might be wider than 9sp cogs and not work well with a 9sp chain. You may be better off with a basic mtb chainring without profiled teeth or ramps.

You should also mount some sort of chain-stopper on the front (like a front derailleur) because rear derailleurs introduce a lot of slack you might not be used to on your SS bikes.
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Old 06-09-09, 09:59 PM   #3
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Why don't you just remove the inner and outer chainrings off your triple? You'd need a set of singlespeed chainring bolts, and if your current middle chainring isn't of the proper size, you'd need to replace it as well, but that'd be considerably cheaper than replacing your crankset and bottom brackeet.

Well, maybe you have plans for that triple crankset I don't know about... But it seems like you're going to a lot of trouble for nothing when all you could do is remove the extraneous chainrings on the crankset you've already got.
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Old 06-09-09, 11:49 PM   #4
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Originally Posted by pdrizzle View Post
i abandoned geared bicycles some years ago because of complications that i didn't feel like dealing with then (derailleurs, shifters, etc). now i'm rethinking things and want to convert a roadbike with 3 rings up front and 9 cogs out back to be a 1-9 setup. i know i'm going to be labeled a noob and get flamed or whatever you internet forum fools do, but i've done a bit of searching on here and haven't found the exact answers i'm looking for. so here goes:

current setup:
truvativ isis triple cranks
truvativ (?) isis bottom bracket
shimano 105 derailleurs
9 speed shimano cassette

i wish to get rid of the front derailleur and 2 extra chainrings and replace the crankset/bottom bracket with a square tapered single speed crank/chainring setup. so...

what size square tapered bottom bracket should i use to replace the current isis bb? (i realize this depends on the size of the current isis bb, which i have yet to remove/measure, but i'm sure someone's done this, right?)

i plan on using a bmx crankset/chainring, so will there be any issues with dropping the chain? is there any way (the perfect chainline spacing) or a product (bash guard?) i can install to keep problems from arising?

i know people love to call others out when they ask stupid/naive questions, so if that's you, please, reply and i'll get a kick out of your hater speak. but...


if anyone on here has done this or has read a thread where this is answered, please help me!

thanks in advance.

Since you already have the front der, you could just lock it in place with the limit screws to serve as front chain keeper-that is a common practice with time trial bikes running a single ring.
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Old 06-09-09, 11:51 PM   #5
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Originally Posted by LarDasse74 View Post
To determine ideal bb spindle length you should measure the chainline - the distance from the centre-line of the bike to the chainring and the distance from the centre-line of the bike and the middle of your cassette. If the chainring is further out than the middle of the cassette then you need a bb spindle shorter by two times the difference. Also make sure the crankarm and chainring and the frame have enough clearance to spare.

I'm not too sure, but I fear BMX chainrings might be wider than 9sp cogs and not work well with a 9sp chain. You may be better off with a basic mtb chainring without profiled teeth or ramps.

You should also mount some sort of chain-stopper on the front (like a front derailleur) because rear derailleurs introduce a lot of slack you might not be used to on your SS bikes.
Could even get mtb bash guard to go with the crankset. Then cut and redrill the guard to the larger bcd.
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Old 06-10-09, 12:28 AM   #6
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www.bbgbashguard.com is your friend.
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Old 06-10-09, 12:32 AM   #7
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Old 06-10-09, 09:27 AM   #8
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thanks for all the great feedback!

i'm going to use this bike for delivering in the french quarter (new orleans), which means lots of turning, so i've been experiencing pedal strike with the current 175 length cranks. i already have shorter 110 bcd bmx cranks and a 3/32 110 bcd 43t chainring just laying around. also, my big chainring on the triple is bent pretty bad. i guess leaving the front derailleur would solve the chain jumping issue, but i hate hearing the chain rub against it on the outer-most gears, which it does now. i was hoping there would be a way to completely eliminate it, but if there isn't, i'd rather be safe than drop a chain while accelerating in traffic.

good info about the bb.

thanks again!
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