bfloyd6969

Hello all. I'm wanting to turn my current 1x8 road bike into a 1x4. I am wanting to remove the outer and inner 4 cogs and leave the ones I want (23t, 21t, 18t, 16t) spaced in the center. Will the derailleur limits be able to be set to not allow it to go above and below the centered 4 cogs? Will I just need longer limit screws? I'm thinking I'll most likely run the shifter (downtube style) in friction mode, so there will be some slop at the high and low end. Also, will I be able to get by with a short cage derailleur with these rear cogs? Thanks for the help.

Retro Grouch 

An interesting project.

1. If you take away the unused cogs you'll need to replace them with some kind of equal thickness spacer.
2. I've never tried something like that so I don't know if the derailleur limit screws will reach that far or not. If I were doing it the I'd try it first before doing anything else simply because adjusting the limit screws inward is a zero cost experiment.
3. It you can find an 8-speed index shifter and keep the same spacers between the cogs, it'll still work.
4. Short cage rear derailleur will work for sure.

fietsbob

You cannot re dish a cassette hub to actually strengthen it,
but clustering the 4 cogs
close to the single chainring best chainline is cool ..

folding bike thread 2 speed crank , 2 cog hub, is another 4 speed set.

bfloyd6969

Yes, I did intend to use spacers for the 4 removed cogs to fill the 2 low and 2 high cogs in their relative positions.

Monster Pete

I'd just leave the cassette intact. A gear that you hardly ever use is better than having a spacer when you need the gear. I don't see what you're gaining from this.

strock 

Yeah, this was my first reaction, too. But sometimes I like to tinker just for the sake of tinkering. It will make the drivetrain look a little cleaner and save a few grams. And it will be unusual, if not unique. To the OP: I'll just reiterate what RG has already said. Short-cage RD will be fine. You can still use indexing, if you want. Finding out about the limit screws will be a matter of trial and error. It is possible, as you suggest, that if the existing screws don't work, longer ones might. Good luck, and when you're done, post photos.

dsbrantjr

I don't quite see the point either, BUT if you decide to go ahead I'd suggest that you make certain that you can get the derailleur llimit adjustments where you need them before you go ahead and start pulling things apart. Then ride the bike to see how you like it. You might find yourself trying to grab that lower gear on occasion. If you decide to remove the larger cogs you will probably want to shorten the chain a bit as well. You will probably also want to slacken the shift wire enough that your lever hits its stop when you are in your new low cog to avoid trying to "push" past the limit stop to a cog which no longer exists. This seems like a lot of trouble to shave a few grams but, hey, it's your bike and your time. Enjoy it.

JanMM

Appearances are important!
When I converted from 8 to 1 for a singlespeed conversion, I left two cogs, just in case - making it easier to try the other gearing. Never did use it before selling the bike.
It did look a little funny.

HillRider

AFAIK, most derailleur limit screws are M4 so you should be able to find longer ones if you need to. You could off-set the cluster of four cogs to center them on the single chainring as fietsbob suggested and perhaps only have to change one limit screw.

fietsbob

NB I think the last cog and lockring are a paired mechanism, so you have to retain those on the outside .

Pulling the cable over-tight will keep the RD from moving over to it.

bfloyd6969

Thanks for the replies and advice everyone. Yes, I was just thinking for a cleaner look. I did find that the limit screws as is are just a bit short - but, I work in a machine shop and we had plenty different size lengths of M4's so all is good there.

Next, I plan to order a short cage derailleur (probably Ultegra) and then shorten the chain some. Next question - when shortening the chain, would the top pulley to lower pulley be vertically lined up with each other when I was in the middle gear? If not, what is the best way to judge chain length? Thanks for all the help.

Retro Grouch 

You'll have plenty of slack take up capability so the chain length doesn't have to be spot-on. Lining up the pulleys with the chain in a middle gear will work fine.

Retro Grouch 

You have a point but it doesn't matter. There is only one person in the whole world that bike is required to make happy - that's bfloyd.

hueyhoolihan

a few months ago i did just this. i cracked open an inexpensive hyperglide 7-speed cassette and am now using only five of the cogs. 14-15-17-19-21. if your deraillerur is anything like mine (short cage dura-ace 6800?) the limit screws will be near their er... limits. am using shimano 600 clamp-on firction shifter. good idea about longer llmit screws.

BTW, i added spacers on the inside of the freehub so that i could put my single chainring on the outside of the drive-side crank and have a good chainline.

bfloyd6969

Thanks guys.

hueyhoolihan - it is the short cage Ultegra 6700 that I am looking at buying. Currently, there is a long cage Sora on there. Can I ask what spacers you used for your build?

HillRider

You really don't have to have a top position cog to tighten the lockring against. In the past I've installed 7-speed cassettes on 8/9/10-speed freehubs and placed the required aluminum 4.5 mm spacer outboard of the smallest cog so the lockring tightened against it. It worked out fine so the OP could tighten his lockring against any spacer outboard of the smallest cog he keeps.

hueyhoolihan

good question as i had to stop at the LBS the other day to address what i thought was a loose lockring. turned out the spacers compressed a bit after the first few hundred miles and the lockring had bottomed out. quick fix by the LBS mech who wouldnt' take any money!

so ya, i used a few standard 7 speed shimano plastic cog spacers and the very very narrow shim that i have found that is often used between, i think, the smallest two cogs, or maybe it's between the second smallest cog and the third. i don't know for sure. i think i might have a few fancy semi-custom ones i originally bought for a single speed setup on a hyperglide hub on there too. i found it is not too critical. i (now) just make sure i can get a few threads on the lockring and that it doesn't bottom out.

if i have a choice i like metal ones...

bfloyd6969

Thanks guys!

hueyhoolihan - Thanks for the info. - nice mechanic! I'll pick up some metal spacers. I believe I seen alot of single speed conversion kits that come with spacers in various sizes. I'll take some meaurements and get what I need.

HillRider - I believe the smallest cog on my cassette is the lockring, from what I recall. I think I'll pickup a non-cog type lock ring for a better look