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Old 10-06-07, 08:18 PM   #1
yamcha
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Need help will return love.

As most of you might know, I have modified a Dahon single speed Boardwalk with a 3-speed Shimano Nexus internal hub. The bike is actually my wife's and she is a small girl and not a strong or fast rider. Our city bike paths are mostly flat except when they go under freeways and roads. Thats when you have a nice ride down and a hell of a ride up! There are about 5 or 6 of these under paths and 3 or 4 moderately steep bridges, so as you can see that actually a decent amount of uphill riding.

Now I am wanting to switch out the rear cog on this bike to make things a bit easier for my wife but can't decide on what size rear cog.

This is how the ratio is set up right now with a 44t chainring and 14t rear cog:
1st (-36.4): 46.1 inches
2nd (1:1): 62.9
3rd (+36%): 85.5

With a 16t rear cog:
1st: 40.3
2nd: 55.0
3rd: 74.8

With 17t rear cog:
1st: 37.9
2nd: 51.8
3rd: 70.4

And finally with an 18t rear cog:
1st: 35.8
2nd: 48.9
3rd: 66.5

Though 17 or 18t rear cog might sound nutty, please remember that we are just leisurely type riders and we never need to go that fast or more than 8 or 10 miles at furthest. So having the 3rd gear as the main gear isn't a big problem. I would just like to have your opinion and advice please.

Last edited by yamcha; 10-06-07 at 09:28 PM.
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Old 10-06-07, 08:46 PM   #2
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These cogs are cheap. Buy the 18T one and ride it. As you & wife get fitter and faster, buy a 16T cog.
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Old 10-06-07, 09:06 PM   #3
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These cogs are cheap. Buy the 18T one and ride it. As you & wife get fitter and faster, buy a 16T cog.
That's pretty good advice. Thanks. Though these cogs are cheap and fast to pop on at the LBS, I really don't want to do it all the time but would rather just do it just once. I am thinking about the 18t cog too but feel kind of indecisive since the 3rd gear might be just a tad too easy while with a 17t it might be just a tad too hard. Hmm, maybe I should have got the SRAM 5-spd instead but then that would have cost me much more and the bike only cost me $200.

I myself am a strong rider and when I ride without her I go the speed of light. I also do wheelies, jump off curbs and bunny hop back on them!
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Old 10-07-07, 02:53 AM   #4
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An 18 tooth will still get ~15mph at 80 rpm and ~20mph at 100 rpm in top gear.
So for cruising it should be fine, most smaller riders I know tend to be spinners which helps.
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Old 10-07-07, 05:14 AM   #5
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+1 on the 18 toothed cog if it will fit. Some frames won't take a big cog - the brompton for example, in which case, another way is to get a smaller front chain ring.

The ratios you quote for the 18 toothed sprocket are just about the same as I get on the Merc with the small front ring engaged. I find I can climb steep hills easily with that bottom ratio of around 39 inches so she'll spin up most hills I expect. Certainly, form what you say your wife will be overgeared with a 14 toothed sprocket. I doubt she'd want to use the 85" very often for sure.
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Old 10-07-07, 10:20 AM   #6
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You can also change the chainring too.

Considering the type of riding your wife does, I would err on the low side. If you are lucky, she might learn to spin if she wants to go faster. Then she will be ready for a performance bike.
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Old 10-07-07, 10:31 AM   #7
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What is spinning?
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Old 10-07-07, 10:40 AM   #8
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What is spinning?
pedaling with a fast cadence
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Old 10-07-07, 11:15 AM   #9
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The 18 sounds like a good choice as the 36-66 range gives a great gear for climbing and a nice top gear that most folks can still spin nicely.

My vintage Twenty runs similar gearing and compared to my other triples the gearing is low but I set things up so the bike would be suitable for towing my trailer and for taking in some long steep climbs.

The 66 gear inches at the top still allows for a decent cruising speed at a higher cadence / spin.
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Old 10-07-07, 03:06 PM   #10
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All right, I want to thank all you guys for your help. I just changed out the 14t with an 18t. Total was $25 including cog. It would have been only $15 but they had to add a few chain links. If I were to go with a 17t then no links would have been added.

It rides just perfectly now!
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Old 10-08-07, 04:11 PM   #11
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Update.

I changed out the 18t with a 19t. That's what I should have gone with from the start because now the gear inch on the highest gear is 63", same as how it was when it was a single speed.
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