Buikema

Okay, so I'm caving. In spite of my uncle's criticism ("Its not cycling if you have more than one gear. Too easy.") I have caved and ordered a Sturmey Archer S2 kickback hub w/o coaster brake.

Dilemma: what size rear cog do I need?

I've been to the Sheldon Brown gear calculator, but since this is my first foray into gear ratio selecting including more than one gear I frankly don't know what do with in information I've found.

This is for my very heavy, great than 40lbs, double bar DL-1-ish style rod brake roadster. Since it has a very nifty front sprocket, I can't change that.



So, its current gearing is 46/18 w/ 180 mm cranks and 28" wheels. Which works out to, I believe, 71.6 gear inches (whatever that means) or 5.0 gain ratio, Sheldon's preferred measurement.

Since the bike is quite heavy 46/18 is a bit tall for me. What I'd like is for the fast gear to be about the difficulty it is to pedal now, or even just a shade faster. Then have the low gear to be whatever it has be to make that work out.

Does that make any sense at all? As you may be aware, the SA S2 hub is geared at 100% and 138%. I'm sure this is pretty simple, but for whatever reason I can't quite wrap my head around it.

Help.

ColonelJLloyd 

Why did you choose the version without a coaster brake?

A 17t cog should give you somewhere around 76 gear inches for your main gear and 54 inches for the hills. With an 18t cog you'll retain the same cruising gear and your hill gear will be around 51 inches. Sounds like you've got your head around it. It's a bit hilly here and that's a heavy bike. If it were me I'd probably go with an 18t or 19t cog.

Post some more pics of your bike. Sounds cool.

Edit: Sheldon's gear calculator has an option for the SA S2, but it calculates gear 2 as direct drive, not gear 1.

In that case, I'd start with a 22-24t cog. A 24t would give approximately the following gears:

Gear 1 (direct drive): 54 gear inches
Gear 2: 74 gear inches

3alarmer 

Too much information has, I fear, confused us both.
From a purely logical standpoint, if you want your
top gear to equal what you have now, I believe you
need enough extra teeth in your rear cog to effectively
reduce the overall gear ratio by 38%.

In other words, approach the calculation by reducing
the ratio between front and rear by roughly
40%. When you kick up to the higher gear (138 %)
you should have about what you've got now.

Having said this, when I set the gears for the
Midnight Rider (my own prized practical transport)
I tried about four or five different cogs scrounged
from various places. It's a SA 3 speed, so various
cogs were readily available as scrap at that time.

Trial and error is so much more educational than
theory and application.

clubman 

I'd order a coaster too if you've got rod brakes.

If an 18 gives you ~72 inches now and it's too tall then you'll want a larger cog...the largest you can fit?

I run a two speed Sachs Duomatic on my roadster with a 46/22 combo and it works fine on the flats. Hills are a different story.

Buikema

Okay, that sounds like what I was guessing, just wanted to double check. I'll look for a 24t cog, but I understand they're not exactly standard, right?

RE: coaster brake vs. not, another poster was correct, since I have rod brakes I have elected to skip the coaster brake. This is also lighter, although at this point in time weight savings on this bike is a bit of a joke, and the idea of shifting every time I brake whether I want to or not doesn't sound that great to me.

The bike in question:


In the aftermath of acquiring this bike and lots of digging I feel like I've learned quite a bit about Asian roadsters, rod brakes and other oddities particular to this type of bike, at some point in time once I get it really dialed in and cleaned up I'd like to post something compiling this info so that I might be useful to others. Some day.

Thanks everyone for the help!

ColonelJLloyd 

They shouldn't be too difficult to find.

I think you are misunderstanding Clubman. I think he and I are in agreement that we would choose the coaster brake option because, well, rod brakes kinda suck.

FYI, the attachment didn't work for me.

Buikema

Oh, I see. Well I grant you rod brakes do offer somewhat questionable braking power, , but to me that is all part of the bike. If I need to go fast and/or stop quickly I take a different bike. I suppose one could then argue that why do I need a luxurious extra gear, but whatever. The difficulty in pedaling keeps me from wanting to take the bike out sometimes, yet the somewhat longer braking distance doesn't really bother me, since I never get going particularly fast in the first place.

Lets try this again...

ColonelJLloyd 

Nice bike!

Your reasons for choosing the rim brake version of the hub are good. Enjoy!

Velognome 

Until you get going down hill in the rain................just a thought

nlerner

I have the SC2 on a bike and am running a 45t chainring and a 22t rear cog. That gives me a 55" low (1:1 on the hub) and a 74.5" high gear (1.38:1 on the hub). As the Colonel posted above, that's about what you'll get with your 46t chainring and a 22t rear cog. Given that your current 71.6" gear is a bit too high, you likely want to go for a 24t rear cog.

For cog sources, BikeSmith Design sells them up to 26t:

https://bikesmithdesign.com/cogs/index.html

Neal

imabeliever1

I have the S2C in a Takara roadbike with 27" wheels. Because where I live it has some rolling terrain and lots of fairly long inclines, it is geared lower than what most here seem to prefer. Gearing is 38/21 . High is 67.4 with low being 48.8. My theory is to use high for normal most of the time and then have low for a bailout gear when I am tired or hills and wind are smacking me about. Seems to be pretty good so fair. I may drop it one tooth in the rear , time will tell.

tcs

That's because that S2 is the one from the mid-1960s, not the new one that has debuted since Mr. Brown's untimely death.

NormanF

The new one works fine. The low gear is for the headwinds and ascents and the high gear is for the tailwinds and descents. Once you get used to it, shifting is a matter of backpedaling to shift between gears.

Buikema

Thanks all for the input. I'm stoked to get it all put together. Now I just need to find a 36 hole Westwood...

alr

This is a very good point... and why I am considering a coaster brake in my Dl-1...

Also, 24 teeth sprockets are not that uncommon-- I found one in my LBS, though it was a little spendy, totally worth it for my hilly environs.

alr

yellow jersey has them from what I understand.

sailorbenjamin 

Is this one of those hubs where you can file some of the teeth out of a cassette sprocket?

Buikema

I have heard this is possible, but I haven't gotten together all the non conforming bits to see if than can be conformed.

As for the rain, mine still manage to stop me in reasonable time. It isn't particularly hilly around here though. Also my rims aren't in very good condition, so I expect my brake pads get a little extra grip out of that. Soon I'll have a shiny new rim to go with my shiny new hub, so I'll get back to you on the whole stopping thing.