Thunder Horse

Question: Is it easy to have an Ultegra 10 speed cassette and change the sprockets to suit the race terrain?

I have heard that some sprockets cannot be bought singularly because they are fixed with other sprockets.

What I want to be able to do is change the sprocket set up. For instance:
11, 12, 13, 14, 15, 16, 17, 18, 19, 20 (no gaps)
OR
11, 13, 15, 17, 19, 12, 23, 15, 27, 29 (every second sprocket)

Is this possible?

dsaul

Just buy different cassettes and change them to suit the terrain. I have an 11-23 for the area where I live and and 11-34 for climbing.

I doubt you will be able to find a cassette with no gaps. The closest you will probably get is the 12-23. Miche sells individual cogs for 1st and second position, middle positions and final position and you can build your own cassette using those to get 1 tooth jumps from 12 to 21.

cny-bikeman

It does not make any sense to have equal cog spacing all the way through, especially on a wider range cassette. The percentage change from the beginning to the end is almost triple in your 2nd example. Typically it's harder to tolerate large percentage changes on the higher gears where you have less torque, but in your examples the lower percentage changes are the low gears. Your 2nd example results in an 18% increase in gear ratio from the 13 to the 11, and the 15-13 is 15%. That's not sensible when one can move those extra jumps to the low end of the cogset. a 3 tooth jump from 26 to 29 is only 11%, for example.

HillRider

I agree with cny-bikeman, the "straight block" has been the "ideal" but makes little mechanical or mathematical sense. An 11-12 jump is 9% while a 19-20 jump is only 5%. The readily available 11-23 makes much more sense as it's a straight block from 11 to 17 and only two teeth each after that.

Thunder Horse

Thank you for clarifying that for me.
My goal is to build my own combinations of chainring and sprockets according to selected gear ratios.

Kontact 

You may be able to do this - usually the cheaper cassettes are easier to separate into individual sprockets because they are all steel plates rather than hollow sprockets pinned to aluminum assemblies.

But as CNY explained, you gear combinations are less ideal than the factory offerings. In the past it was desirable to customize 6 or 7 speed freewheels, but when there's 10 or 11 sprockets there is less likelihood of having gaps in the wrong spots.

cpach

Modern cassettes are shaped for shifting performance--mixing and matching adversely effects shift performance negating any other possible performance advantages. Some custom cassettes can be acceptable by keeping the number of transitions between disparate cog sets low, but it's still not ideal.

Kapusta

Thunder Horse

Thank you all for sharing your information. suggestions and advice. Much appreciated.

noglider 

You might be thinking about it wrong. It's not that you need to find the best ratios and put them in. What matters is not what gear you're in at the moment. What matters is how far the next gear up or down is, and the meaningful answer is expressed as a percentage of the change. Some people need an 8% change, and some want a 15% change, just as two examples. It depends on your bike, conditioning, terrain, etc. As we get more experience and strength, we tend to want our gears closer together. So pick the bottom ratio you want, the top, and then figure out how to get evenly spaced gears in the middle. You should also consider the shifting pattern. Some bikes have shifting patterns that are dumb enough that you don't end up getting nice spaces between the gears because to do so is too inconvenient.

JohnnyCyclist

Agreed. I won't bore anyone with details of what I've changed ... just say that I've changed the % between gears to suit me (via a "franken-cassette").