Bicycles gear ratios...?

Question

if u went to a shop that specializes in bikes and told him that you wanted a set of gears and that you had a list of gears would he be able to build a custom set using the gear ratios you gave him....?

Additional Details

and if he could would he be able to use the following...

2.5:1

2.07:1

1.69:1

1.38:1

1.18:1

1.04:1

the top 6 gears would be for the back whereas the final would be 5:1...

if it would be too short what final gear ratio could i use...?

Answer 1

Like the first answer states, you need to talk bike lingo. Use GI or teeth on the cogs and chain rings. You must have a freewheel if you only have 6 cogs. You can change out individual cogs on a freewheel. You can't completely do this on a cassette.

While you're having fun doing this, keep in mind the manufacturers have been there - done that. The gear setups have been looked at and optimized many decades ago.

Go ahead if you like but you may have some difficulty finding seperate cogs available for purchase. No harm it trying it out.

I have a couple cassettes for my 9 speed road bike. The one I use most a 12-23 has cogs as follows:

12, 13, 14, 15, 16, 17, 19, 21, 23 as you can see there isn't really any room to play anyway. With 53x39 chainrings this provides the best ratios in that range.

Answer 2

I'm not sure how you are defining your ratios. The normal way is to describe gears in "gear inches", where:

GI = (number of teeth on the chainwheel) x (wheel diameter in inches) / (number of teeth on the rear cog)

So for a 9-speed road bike (nominal 27" wheels) freewheel / cassette with 11, 13, 15, 17, 19, 21, 23, 25, 29 teeth and a 49 tooth chainring the gears would be: 120, 102, 88, 78, 70, 63, 58, 53, and 46.

While you can use the ratios direct the advantage of gear inches is that it allows gears to be compared between different types of bicycles (e,g, road, mountain, hybrid, BMX).

Now to the heart of your question. If your bike takes a cassette you will be limited to the gear ratios defined by the several cassettes that fit your hub. On the other hand if you use a freewheel (see link for difference) it can be disassembled and different sized cogs used to allow more customization. There are some limitations on the smallest and largest (and the teeth differential between adjacent cogs) to ensure good shifting, but by playing around you can get pretty much what you want.

Talk to your local bike shop if you aren't sure. Otherwise you can order parts on line -- once you define what you need.