Fixed Gear Ratio of Choice?

[Dirt]

On the tweed I’m running 48×17 with 32mm tires. a little over 76 gear inches. The bike weighs a freaking ton.
On Big Pink I’m running 48×16 with 23mm tires. A little short of 79 gear inches. The bike is light and aerodynamic and fast. That’s what I’ve been riding for endurance rides.

You can get a 48t chainring, and bolt it on and go from there. Depending on the bike, usually the outside position is the one that must be used for a ring that big. There is a bit of spacing that you can do to adjust the chain line with the hub, but it is rarely more than a mm.

With the chain ring you need to make sure the bolt circle (BCD) matches the crank. I’d have to look up what the BCD is for the Sora crank. I’d guess it is 110, though a quick search on the interwebs told me it is 130mm. 5-bolt is the pattern. Make sure the ring that you buy is not ramped. Salsa makes nice ones that work well. Sugino makes nice 130bcd rings too.

I hope that helps a little. Ask more questions if you’ve got them.

[DismalScientist]

Have you thought about just sticking with the 52 outer chain ring and considering cogs with one addition tooth?
per Sheldon Brown:
52-16 ~ 85
52-17 ~ 80
52-18 ~ 76
52-19 ~ 72
52-20 ~ 68
52-21 ~ 65

[OneEighth]

The only reason I am usually using a 48-tooth chainring is because it comes with my preferred crankset (SRAM Omnium).
I switch to Sugino 75 chainrings with fewer teeth when I put on fatter tires. Going to drop to 44 x 17 next time I run the studded 35s. Gotta use different chainring bolts, though, because the SRAM chainring is thicker than the Sugino.
Sorry, vvill, this probably doesn’t help you right now, but maybe it’ll be useful later and maybe it’ll help other folks, too.

[vvill]

Nice! Not sure I would dare lock up my bike where I do with an Omnium crankset though.

Dismal – yes I have thought of that too. But the 52 outer chain ring will be ramped I assume and also my concern would be with the chainline when the outer ring of a triple is being used (not sure about how this works though).

@Dirt 43309 wrote:

You can get a 48t chainring, and bolt it on and go from there. Depending on the bike, usually the outside position is the one that must be used for a ring that big. There is a bit of spacing that you can do to adjust the chain line with the hub, but it is rarely more than a mm.

With the chain ring you need to make sure the bolt circle (BCD) matches the crank. I’d have to look up what the BCD is for the Sora crank. I’d guess it is 110, though a quick search on the interwebs told me it is 130mm. 5-bolt is the pattern. Make sure the ring that you buy is not ramped. Salsa makes nice ones that work well. Sugino makes nice 130bcd rings too.

Thanks, I do know about BCDs but that’s about it as far as cranksets are concerned With chainline, if you can only adjust the rear cog by a mm in/outboard at most, how can you tell if you’ll get a good chainline if there’s only 3 already-defined places to put the chainring? I assumed people adjusted/carefully spec their BBs/crankset on their builds to do this. I’m wondering if there is a standard or simple “go-to” crankset for this situation. (What do all those cheap ubercolorful heavy-steel fixie manufacturers do?) I don’t mind buying a new crankset for this purpose – better than buying a chainring and then finding the chainline is too wonky. I imagine a SS/GF specific crankset would be lighter than the stock one too (don’t really care about stiffer – I’ve never pedaled hard enough to notice any flex there).

I never realised having one gear could be so complicated!

Also, I have been reading this: http://www.sheldonbrown.com/chainline.html
or perhaps I should say digesting. There’s a lot in there.

[dcv]

hey will, the eno hub should work with a road double crankset. from sheldon brown:

“The ENO hub is designed for a 47.5 mm chainline, which matches the middle position of a typical MTNB triple crank. A typical “road” double gives about a 46 mm chainline with the standard bottom bracket, which is a decent match”

if you buy a track crankset, you will have to change the bb width to match the 47.5 mm chainline – track cranksets are usually set up for 42 mm chainline. I had to do this when i mixed an eno hub with a sugino RD track crankset, i had to calculate the difference between 47.5 and 42, then size the bb (i forgot what i ended up with, but it took a lot of google foo).

i’ve used SRAM and Blackspire SS chainrings (no ramps). I’m using a standard 53t ring from a double (with ramps) on my SS and it’s been fine, no dropped chains. I don’t think I’d feel confident with a ramped chainring on fixed though, backward pedaling / skidding may be problematic.

btw – bigger ring / cog combos are more efficient and make less noise.

edit: i’m using road double cranksets (SRAM Force) on both my FG bikes with ENO hub and chainline is fine (ENO is 130mm wide hub). On my bike with SRAM Omnium crankset i have a track hub (120mm wide) with 42mm chainline.

[vvill]

Thanks dcv.

I think the easiest thing to do will be to mount the wheel on my bike, measure the chainline, flip the wheel, measure the opposite side to make sure it’s the same, write down the chainline (expecting 47.5mm for both I’d guess), and then see what I can do with my existing crankset (also measured the BCD last night to confirm it’s a 130mm). If the chainline is too far off I can look into a road double crankset or whatever else might work (1/2/3mm spacers are also suggested by sheldon brown).

[Author]

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