Alright guys. Currently running a 42/16 and at roughly 70 gear inches it seems a fairly decent set up.

I want to change my rear wheel to run double fixed and looking to keep one of the sprockets as a 16t. Does anyone know what would I be limited to in terms of sprocket sizes without having to change my chain? I also don't want to change the chainring at the moment, if possible.

thanks

Same chain = 4t difference usually (actually 5t span eg 16-20t).

If dropouts allow it.

Each tooth added/subtracted moves axle 1/8th inch forward/backward

yep if the dropouts are long enough. On my roadrat hybrid I can go up and down 15 - 19 with ease but my track bike can only go from 15 - 18

Thought it was 1/4", half the pitch? (willing to be wrong have not really thought about this much).

with a small front chainring, you can move quite a few either direction

Me too cos I was convinced I'd never drop 4T for riding Herne Hill but then I saw the 1/8th inch quoted and worked it out.

Chain total length 100 links = 50 inches
Chainring 48t circumference= 24 inches
Cog 18t circumference= 9 inches

Chain wraps around half of chainring and half of cog = 12 + 4.5 =16.5 inches

So you have two straight runs of chain top and bottom of 16.75 inches.

Drop the cog to 16T and chain wrap is 12 + 4 = 16 inches

Straight runs of chain = 17 inches so 0.25 inch increase for 2T = 0.125 inch per T

(the super exact answer is fractionally different because the straight bits of the chainrun aren't parallel as the wrap around cogs and chainring is just over 50% )

It would be good if I had the 15-19 range. i'll take a closer look at my dropouts later and get the steel ruler out.

thanks for your help

It's definitly 1/4".

My pista will run a 19t down to a 14t on the same chain, 48 front, pretty long ends.

Edit: Although I wouldn't for precisely the reason below

Need to decide what's practical in terms of wheelbase too ie. do you want to run it at its shortest for one purpose? It's for this reason that I end up removing a link for track use, cos otherwise I'd have the smaller cog/longer wheelbase combo which is a bit inappropriate.

^^ Why do you say its inappropriate to have a smaller cog and longer whellbase BMMF? I never thought of that. It is true that the wheel base is longer when I have a 15 tooth cog on, but what difference will that make on the track?

It could be the difference between success and failure in the 'devil'! ;-)

Nope definitely 1/8 ;)

http://www.63xc.com/toddp/halflink.htm

@ BMMF - thanks for the reponse. i've been reading the below thread
http://www.londonfgss.com/thread6151.html

i'm not trying to acheive anything specific i.e. track specific / trick specific bike.
I know I am limited by the fact I'd rather not change the chainring at the moment so my range of gear inches is limited using the 42t.

Maybe ditch the 16t for a 15t and maybe go up to 18t on the other side. Lot of "maybes" there, in truth I really don't know yet, need to do more research on different ratios.

Hmmm. Wrong. I don't like being wrong. You'd better watch your back mister ;]

What andyp said, and just for sharper twitchier handling. The difference is pretty minimal though. I've never really noticed much difference between mid-point of track ends, and shortest 'setting'.

I've deliberately added a link in the past for century rides to go for the longer extreme, and appreciated the slightly more neutral handling, especially at high rpms going downhill - short back end gets skittish.

WOW that 63xc article was interesting! is the whole site like that?....whoopee!