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gfr 10-26-07 01:29 AM

Gears Ratios
 
Hello,
I've got a Merc bike (like Brompton bike) with 16'' wheel and a Sturmey Archer 3 gears
In some "hills" its very hard to use it, and now i want to change the ratios of gears.
Now I've got a 52 tooth front chain wheel and a 14 tooth rear drive cog, and i have this ratios:
82'' / 61'' / 46''.
And i have two solution:

1. Reduce 12%. Put 46 tooth front chain wheel and a 14 tooth rear drive cog. The ratios are: 72''/54''/41'' (similar ratios than Dahon Curve D3 or similar than brompton with 44T chain wheel and 13T rear drive cog)

2. Reduce 25%. Put a 39 tooth front chain wheel and a 14 tooth rear drive cog. The ratios are: 61''/46''/34''

What do you think its better ?
Someone has similar ratios ?
I think the best solution is to reduce a 12% buy i hope you con help me.

Thanks

Leigh_caines 10-26-07 04:22 AM

I'd go with 2
I have a 35 low on my folder and in hills I wouldn't want it any harder.
But you could be stronger then me :)

jnb-rare 10-26-07 08:47 AM

Advice from an older, admittedly-less-fit-than-I'd-like-to-be, rider.

The last option would give you a much better ratio for hills. The highest gear (61") would give adequate flat-land speed at 80 rpm or so (around 24km/hr?). You'd have to get used to coasting down hills, however.

If your fitness level improves, you can always convert to the 46-tooth later and even back to the 52 over time. If you're truly having difficulty now, I'm not sure the 12% reduction will be enough initially. Could you not also split the difference between first and second options with a 42-tooth chainring?

EvilV 10-26-07 10:03 AM

Quote:

Originally Posted by jnb-rare (Post 5525149)
Advice from an older, admittedly-less-fit-than-I'd-like-to-be, rider.

The last option would give you a much better ratio for hills. The highest gear (61") would give adequate flat-land speed at 80 rpm or so (around 24km/hr?). You'd have to get used to coasting down hills, however.

If your fitness level improves, you can always convert to the 46-tooth later and even back to the 52 over time. If you're truly having difficulty now, I'm not sure the 12% reduction will be enough initially. Could you not also split the difference between first and second options with a 42-tooth chainring?

I agree with this advice. The Merc chainwheels are very cheap at 4.50. Get them both and try each to see which you like best.

yamcha 10-26-07 10:08 AM

Why don't you just pop off the 14t rear cog and pop on a19t? That's what I did. See here:

http://www.bikeforums.net/folding-bikes/350929-need-help-will-return-love.html

EvilV 10-26-07 10:59 AM

Quote:

Originally Posted by yamcha (Post 5525666)
Why don't you just pop off the 14t rear cog and pop on a19t? That's what I did. See here:

http://www.bikeforums.net/showthread.php?t=350929

Neat idea but not in this case. His bike has a chain tensioner that runs a jockey wheel right next to the 14 tooth rear sprocket. There is no room for a larger sprocket. It would foul the tensioner.

In any case, the Merc chainrings are amazingly cheap. They are also rather nicely made so he has the best of both worlds - far easier to change the chainring than the rear sprocket, and cheap.

invisiblehand 10-26-07 03:26 PM

Quote:

Originally Posted by yamcha (Post 5525666)
Why don't you just pop off the 14t rear cog and pop on a19t? That's what I did. See here:

http://www.bikeforums.net/showthread.php?t=350929

Supposedly--I have not personally tested it--the biggest cog you can fit on the Merc/Brompton is a 17t cog.

yamcha 10-26-07 06:54 PM

Can you find a 40T chainwheel?

SesameCrunch 10-26-07 08:55 PM

You could install the 39 in addition to the 52. Just put the 52t ring on the outside of the crank spider and the 39t inside. Then you can switch chainring manually as you need it.

EvilV has pictures of his setup. Check out the Merc thread.

gfr 10-28-07 02:24 PM

Finally I put a 39T, i think it's better to my city, if i put a 39T my hard gear it's the same to the second gear now (with 52T).

San Rensho 10-28-07 02:28 PM

Good choice, thats about the same gearing I have on my 3spd. I ride exclusively on the flats and the low gear is small enough to easily accelerate from a dead stop, big gear is just right for cruising.

EvilV 10-29-07 07:19 AM

Quote:

Originally Posted by invisiblehand (Post 5527686)
Supposedly--I have not personally tested it--the biggest cog you can fit on the Merc/Brompton is a 17t cog.

I'm amazed that you can fit a bigger cog. The clearance between the 14 tooth and the inner jockey wheel is not great. You have to fiddle quite carefully to get the chain in that gap even with the original cog. I'm not denying what you say, but just surprised.

EvilV 10-29-07 07:21 AM

Quote:

Originally Posted by gfr (Post 5536193)
Finally I put a 39T, i think it's better to my city, if i put a 39T my hard gear it's the same to the second gear now (with 52T).

Oh - you've done it then. That's good. Hope it works well for you.

invisiblehand 10-29-07 09:30 AM

Quote:

Originally Posted by EvilV (Post 5539336)
I'm amazed that you can fit a bigger cog. The clearance between the 14 tooth and the inner jockey wheel is not great. You have to fiddle quite carefully to get the chain in that gap even with the original cog. I'm not denying what you say, but just surprised.

Check out BromptonTalk. You will find a person that swapped his 13/15 cog combination with a 12/17.

-G


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