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Post by simonig10 on Feb 22, 2016 20:42:16 GMT
Ive never been very happy with my red Triban 3 gear change especially on the front chainwheel where there doesn't seem enough travel on the changer resulting in rattles and clicking (I hate this when riding). So, I decided on an upgrade and after much reading I fitted the following. Ive posted in case anyone else is considering it. The gear changing now is vastly improved and all very smooth.
Claris FC-2403 chainwheel 170mm BB-ES51 Octalink bottom bracket 118mm Sram PG850 cassette 12-26 Sram PC850 chain
On measuring the old bottom bracket end to end it measures about 123mm. This was fitted a few weeks after buying the bike by Decathlon as the bottom bracket was grinding. Did they fit the right one which has given me such a big offset? The 118mm version gives lots of adjustment at top and bottom of travel.
A question for you guys. The new chain came covered in what I can only describe as grease. Should this be removed? I have installed as it is with a blast of GT-85 all over.
Also, noticed in the spec that a 57inch frame should have 175 mm cranks. Will 170mm make much difference. I guess it make it slightly higher geared?
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Post by phred1812 on Feb 22, 2016 21:17:59 GMT
Crank length does not effect the gearing. The rotational speed of the front chainring remains the same in relation to the rear cassette ring regardless of the crank length. See attached. www.youtube.com/watch?v=eMAxH_Ud8YESorry, I still haven't worked out how to paste a youtube video but this from the GCN boys explains all.
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Post by Rocket on Feb 22, 2016 21:39:06 GMT
I can't agree with that. Gearing on bikes is mechanical advantage by rotary means using a chain to connect levers of different lengths. A longer crank is a bigger lever giving more mechanical advantage by virtue of travelling a bigger circle. Remember torque is a product of force and distance such as Kg X metres. So for the same force a longer crank will generate more torque and therefore be able to turn a certain gear easier but we are talking about very small changes in length. Longer cranks do lower your gearing.
To answer the OP. You may not even notice you now have shorter cranks. The right cranks are whatever you are happy with. Remember to raise your seat a little for shorter cranks and possibly a touch forward too. I tried 170mm but went back to 175mm. I'm 6ft with 33" inseam.
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Post by phred1812 on Feb 22, 2016 23:28:10 GMT
I can't agree with that. Gearing on bikes is mechanical advantage by rotary means using a chain to connect levers of different lengths. A longer crank is a bigger lever giving more mechanical advantage by virtue of travelling a bigger circle. Remember torque is a product of force and distance such as Kg X metres. So for the same force a longer crank will generate more torque and therefore be able to turn a certain gear easier but we are talking about very small changes in length. Longer cranks do lower your gearing. You are correct about the leverage Roy but not the gearing. The length of the crank does not affect the gear ratio ie the difference between the front and rear sprockets which was my point. Interestingly the GCN item came up with a couple of formulae for working out optimum crank length for a given leg length. The conclusion was that many people had cranks that are too long. Mine came out at 167mm but all of my bikes are at 170mm. I did a quick search but couldn't find any 167s available so am resigned to managing with what I have got.
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Post by Rocket on Feb 23, 2016 0:31:48 GMT
We are talking overall gearing here as that is what we pedal on. With a longer crank your foot will move further for the same distance travelled so the gearing is indeed lower which is what the OP was asking. Of course it has no effect on which gears are in use but those gears are only part of the full picture between pedal and tyre contact patch.
The same could be said of fitting smaller wheels. A smaller wheel would lower the gearing whilst not changing the gears in use.
Find a hill that you can barely manage to climb in your lowest gear standing up. Now try it with cranks that are only half as long and you will fail. Why? Your gearing has doubled.
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Post by Radchenister on Feb 23, 2016 8:07:46 GMT
You're right Rocket, although physiological range of motion issues are of course also important. Go too long and your body may protest. I usually ignore crank n wheel size, as assuming they're constants for the budget cautious but have tagged on comments like 'ignoring crank length' when talking ratios previously. Issues worth considering if you're upgrading or have bike fit discomfort issues. 32.5" inside leg n 5'10'', riding on 172.5mm here. Re chain 'grease', just ride it and it's going to wear off. Drive chain cleanliness is perhaps the major issue for keeping road bikes running smoothly. The cleaning and lubing regime should become routine, just like brushing your teeth - we likely all have our own methods. I use GT85 sprayed onto a backward spinning rear cassette (and chain), post soap and water wash at this time of year, mainly to try and neutralise road salt but let's be clear, I run the chain through a dry cloth after, as it's not chain lube - I always use chain lube to finish it off. I ride in the wet less these days, so don't soak chains in white spirit substitute so often. I do run them through a soapy sponge every outing in winter. I now have a best sponge, one for wheels / cranks, one especially for really oily parts. Pick up a budget sponge occasionally and downgrade them through the hierarchy.
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Post by erictherat on Feb 23, 2016 15:14:12 GMT
agree with all above, but dont think 5mm difference in crank will have an appreciable difference in gearing - it is more about fit from what i understand.
re chains - i clean the stuff off a new chain. Ive been told not to (apparently will last for ages as excellent deep lube). My rationale is that is will be cleaned off before the end of the week anyway (as im usually out in the wet), and i dont like the idea of something sticky on my chain attracting grime.
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Post by Radchenister on Feb 23, 2016 16:20:55 GMT
Really, you put on a new chain and then clean it? You wouldn't just ride it and then clean in on return? I'm a stickler for cleanliness but 1 ride with factory grease on it will make booger all difference in the great scheme of things.
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Post by Deleted on Feb 23, 2016 17:38:28 GMT
The old crank length debate. I noticed the difference when I switched from 175 to 170. I was able to spin up a little faster and my knees no longer banged the bars when climbing out of the saddle.
The Cannondale uses 172.5mm cranks but also different chainring sizes so I haven't tried to compare.
I would probably go to 170mm again if and when the cranks need replaced.
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Post by r0b1et on Feb 23, 2016 18:17:08 GMT
I tried 175s and quickly sold them... too long and hurt my knees. THough I have 172.5s on the TT bike with no issue.
OP - that longer BB could be an issue for sure. I suspect you have a much stiffer chainring/BB combo now though, which can only be a good thing.
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Post by simonig10 on Feb 23, 2016 20:04:59 GMT
Great advice guys, thanks very much for all the input. Has put mind at rest on couple of concerns.
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