Adjusting gear ratio

Question

I currently ride a single-speed converted bike (vertical drop-outs). I like the ride but feel that when I'm going for high speed on the road, I'm pedaling too much. Currently running a 18T cog on the rear and 42T on the crank, with a 2.333' ratio.

I'd like to increase the ratio and have a cassette on the rear hub, with a 16T cog. I'm wondering what's the best way of switching the chain to the 16T and keeping the chain tensioned.

Would it be as simple as removing one link from the chain or would the tension not be there?

As the 16T is one level down in the cassette from the 18T would the chain line be OK?

Thanks in advance for any advice.

EDIT: Here's a picture of the gearing setup to make it clearer:

Answer 1

If you poke around on the web enough, you'll find a "magical" gear ratio calculator somewhere. You input the length from BB to dropout and it spits out the front/back combos that will work w/o a tensioner.

Try this one.

http://eehouse.org/fixin/fixmeup.php

It would be much simpler to change the front chainring, if you can find a magical combo that matches what you want for a gear. There are also "half links" available that allow you to tweak the chain by less than a full 1 inch link pair.

FWIW, a single speed bike means your almost always in the "wrong" gear. Any change you make will only change when you're in the wrong gear, not the amount of time you spend in the wrong gear. If you're not happy being in the wrong gear, just buy a derailler and a shifter.

Answer 2

I found a site a while ago called Velosolo, the guy who runs it seems to be quite knowledgeable about conversions, and he has a couple of kits on there - you might be able to get away with ripping your cassette off and replacing it with a cog and a bunch of spacers.

Because of the vertical dropouts you'd still need to tension the chain somehow (the obvious way being a tensioner).

That approach would certainly get you to a "fixed" state (an would allow you to change cog size on a whim), you might need to look a little more closely if you want to mix a hub in there instead, but it is definitely worth asking them the question - I've contacted this chap before and he's very helpful.

You might have to settle for less-than-perfect chain line, but at the end of the day the chain you're running right now is designed to traverse a cassette, so that's not going to be a showstopper. As regards chains, 1/8" is certainly the convention going back in time, but most fixed cogs & chainrings will also come in 3/32" variants, you should have no problems there.

If you do find something on the web, you'll first need to remove your existing cassette. To do this, you'll need an appropriate lockring nut and either a chain whip or a vice. Park Tools have a selection of nuts that fit just about everything that moves. If the cassette is either Shimano or Sram, you'll need one nut (FR-5, but double-check), if it is Campy you'll need a different one (FR-11, but double-check). In my experience, its a lot, lot easier to stick the cassette into a vice than to use a chain whip. As a tip, a Black and Decker workmate makes a great vice!

Your other alternative for removal is to take the wheel to your LBS and ask nicely. For someone with the right tools it really is just a 30-second job, you might find if you ask them nicely they'll loosen everything there and then, and not charge you... But if you did this, convention is just to buy something (anything) from them as a token of thanks.

Answer 3

Sounds like you have multi-speed cassette, no derailleur, and no chain tensioner. On the 18T you have good chain tension. Even in this set up as the chain stretches you will get slack on the 18T.

Moving to the 16T is not full chain link. A horizontal drop out is typically a little longer then one chain link. If you get all the way back and have slack then you take out link and you will be up in the horizontal drop out.

I assume chain line would be OK moving one gear as you probably still have a multi-speed chain on it. But you are going to have slack.

If you want to ride single-speed then I think you need to go to true single-speed set up with a tensioner. Another option is to put a derailleur on that bike.

Single-speed conversion is not that difficult. Nashbar has a single-speed conversion kit with spacers, three cogs, and tensioner with shipping is like $30. I have seen the cogs in sizes 14T, 15T, and 17T and sizes 16T, 18T, and 20T. Yes you can pay more for higher quality but I have one and it works. Most single speed cogs are 3/32 and I have seen 1/8 (White). Your front chainring looks OK so I would not replace it. When you do replace the front chainring then go single-speed. At then you could go 3/32 or 1/8. If 1/8 then need to go 1/8 chain. 1/8 is more metal and longer life. 3/32 or 1/8 there are single speed specific chains - they are cheap just get one when you order you kit. I run 3/32 cogs with 1/8 chain and chainring - I don't think they have to match. You should replace your chain when you replace the chainring. Not a given your bike would even take an eccentric hub and they are more expensive.

Answer 4

You can simply add a cheap chain tensioner and be done with it. The chain line will most likely not be ideal anyway if you left the entire cassette on there, so that shouldn't be a concern.

Answer 5

So this bike at one time had a derailleur, right? But you only want to ride with one of two rear cogs now? And whichever you choose, you'd stick with that one for the whole ride? If so, I'd be inclined to pop an inexpensive derailleur on there to provide the correct chain tension in either cog. Then I'd just use the derailleur limit adjustment screws to shift from one cog to the other as need be.