Replacing the cranks/chainrings.

Questions about purchasing bicycles and parts

Postby mikesbytes » Fri Feb 23, 2007 8:19 am

LuckyPierre wrote:
mikesbytes wrote:With all the trouble you seem to be having with getting the right chain ring, have you considered changing the cassette ?

Given that your daughter's effectivley using a 12-13-14-15-16-18-21-24 cassette it would be worth experimenting with Mike's idea. It shouldn't be too hard to find an 8-speed mtb cassette to play with. I realise that the rear derailleur will have a 'maximum number of teeth' it can handle, but I don't know what the impact of 'locking out' some ratios will be. Any HG cassette is a potential candidate for experimenting. It could be interesting to see whether a 7-speed would work, given that not all of the cassette is being used - I can send you a couple of them to play with. Extending this line of thinking, a 12-27 9-speed is also a candidate (because most 8-speed hubs are 7/8/9-speed compatible), but I need my granny gear more than your daughter does! :)


A 7 speed would fit a shimano hub, with the addition of a spacer, but it may not work with the shifters and chain due to different spacing.

14 - something cassettes are quite common.

Possibly you may need to fit a long cage rear derailier, but then you will have a spare one and derailiers arn't 8/9/10 specific.

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by BNA » Fri Feb 23, 2007 10:52 am

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Postby tuco » Fri Feb 23, 2007 10:52 am

Reading these threads has been very helpful. I had a good talk to the lbs guy today and he asked all sorts to technical questions and I was able to answer all of them without hesitation and without looking like a fool which was good because there were 3 other club members in the shop while I was there.
He has a 38T chain ring which will give her 5.97m rollout which is better than 39T which she currently has with 5.69m. An extra 18cm for every pedal circuit will make a big difference over a long race.

Sadly they couldn't help with a used 165mm triple crank set. If I could get one then the smallest chain ring could be a 30T or a 34T and the middle a 38T which would make a good combination for flat and hills.

I was trying to get out the shop (he was almost too helpful) so I could get back to work on time to open my shop so I forgot to ask if I'd have to change the bottom bracket if I went from a double to a triple. I assume it would need to be longer but I could be wrong. Anyone know for certain?

So the shopping list so far :
38T chain ring
30T or 34T chain ring
triple compact crank set
maybe a bottom bracket to accommodate the triple
tools - hopefully club supplied.
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Postby tuco » Sat Feb 24, 2007 1:06 pm

Has anyone got a chart for the roll out for different gears combinations for 650 wheels?

This is frustrating. I've got a couple but they seem to give different roll out values even with the correct tyre size. None seem to match the actual measured roll out in real life.
I'm trying to decide if the 38T will be a good replacement for the 39T.
According to the first roll out spread sheet I used the 38 gave and extra 18cms taking the roll out to 5.97m but then I double checked with another web site and it was over 6m. Triple checking on another came up with a totally different value.
I'm beginning to think some of these calculators don't take into account the tyre height.

This may all be redundant if she doesn't stop growing. Last night I noticed Megan was taller than her mother and we jumped on the bikes for a ride today and was said the seat was too low!
The bike may be too small sooner than I expected.
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Postby mikesbytes » Sat Feb 24, 2007 2:05 pm

30 or 34T chain ring. Gut feeling is that 34 may be a better choice, but what cogs will she be running?

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Postby tuco » Sat Feb 24, 2007 2:16 pm

mikesbytes wrote:30 or 34T chain ring. Gut feeling is that 34 may be a better choice, but what cogs will she be running?

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12-13-14-15-16-18-21-24
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Postby mikesbytes » Sat Feb 24, 2007 2:58 pm

Have you seen this page ? Ie, how to get even closer to the optimum rollout.

If 34t is low enough, then you will be able to keep that lower gearing for quite some time and simply change the big chain ring with each age change.

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Postby tuco » Mon Feb 26, 2007 10:47 am

mikesbytes wrote:Have you seen this page ? Ie, how to get even closer to the optimum rollout.

If 34t is low enough, then you will be able to keep that lower gearing for quite some time and simply change the big chain ring with each age change.

Have a nice day


Useful but not useful. I'm after something for 650 x 23c wheels.

I missed an auction yesterday for a crank set which would have almost done the trick only the cranks were 170mm. It only went for $46 :cry:
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Postby mikesbytes » Mon Feb 26, 2007 12:24 pm

tuco wrote:
mikesbytes wrote:Have you seen this page ? Ie, how to get even closer to the optimum rollout.

If 34t is low enough, then you will be able to keep that lower gearing for quite some time and simply change the big chain ring with each age change.

Have a nice day


Useful but not useful. I'm after something for 650 x 23c wheels.

I missed an auction yesterday for a crank set which would have almost done the trick only the cranks were 170mm. It only went for $46 :cry:


Yeh, I haven't seen anything equivilant for 650c. Assume you have surfed Selborune brown.

170mm is the lenght I run, your daughter has long legs ?

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Postby tuco » Mon Feb 26, 2007 12:38 pm

mikesbytes wrote:
tuco wrote:
mikesbytes wrote:Have you seen this page ? Ie, how to get even closer to the optimum rollout.

If 34t is low enough, then you will be able to keep that lower gearing for quite some time and simply change the big chain ring with each age change.

Have a nice day


Useful but not useful. I'm after something for 650 x 23c wheels.

I missed an auction yesterday for a crank set which would have almost done the trick only the cranks were 170mm. It only went for $46 :cry:


Yeh, I haven't seen anything equivilant for 650c. Assume you have surfed Selborune brown.

170mm is the lenght I run, your daughter has long legs ?

Have a nice day


Being a junior she has 165mm at the moment. She's working hard on the long legs. She's shot up quite fast in the last two weeks. Just passed her mother and I had to put her seat up on Saturday.

You mean Sheldon Brown? I had a good look last week but nothing found.

I didn't get a chance to get to the club on Saturday and see what tools and spare parts and knowledge they had because it was raining and being an out doors track, track training was canceled.

The guy at the lbs said he had numerous charts then I mentioned 650 and his face went blank. It's ridiculous not being able to find roll out data for 650s considering these roll outs are for juniors and juniors are more likely to have 650s.
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Postby europa » Mon Feb 26, 2007 1:56 pm

I've got to rush now mate so I can't look up the link, but Sheldon Brown's gear calculator allows you to specify the wheel, so you'll be right with his.

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Postby tuco » Tue Feb 27, 2007 4:38 pm

europa wrote:I've got to rush now mate so I can't look up the link, but Sheldon Brown's gear calculator allows you to specify the wheel, so you'll be right with his.

Richard


Found it http://www.sheldonbrown.com/gears/ but it doesn't match real life either.

As a technically minded and trained person I don't see how the crank length can effect the roll out yet it does in his calculator.
The crank could be 50mm or 200mm but when it comes back to the starting point the chain ring has still moved the distance. It must be included for one of the other 'Gear Units' but it shouldn't effect the roll out when 'gear ratios' is selected.
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Postby europa » Tue Feb 27, 2007 4:54 pm

I also wonder about crank length. They say that youngsters should use a shorter crank, but doesn't that mean that they have to apply more force to the pedals within a given gear?

Richard
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Postby Bnej » Tue Feb 27, 2007 5:21 pm

Tuco with Sheldon brown's calculator, use "Metres development" not "Gain ratios". Think you misread gain ratios as gear ratios.

Gain ratios accounts for longer cranks providing more leverage, and hence lower gears.

Metres Development is the number of metres the wheel rolls with one revolution of the cranks.
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Postby tuco » Wed Feb 28, 2007 9:48 am

Bnej wrote:Tuco with Sheldon brown's calculator, use "Metres development" not "Gain ratios". Think you misread gain ratios as gear ratios.

Gain ratios accounts for longer cranks providing more leverage, and hence lower gears.

Metres Development is the number of metres the wheel rolls with one revolution of the cranks.


Thanks Bnej. So many names for the same things.
I realised they were taking leverage into account but didn't know why.

I did find a site with a good calculator but the bookmark is on the home computer.
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Postby tuco » Wed Feb 28, 2007 6:20 pm

Back at home - here it is :
http://www.juniorvelo.com/?p=409
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