SRAM Automatix 2 speed

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I have the one that comes with the hub, 19 teeth. If I need lower gearing, it's easy to get and change it out. Just have to pop it on and add a link to the chain. My old bike has cottered cranks, meaning the drive sprocket is a part of the right crank and the whole thing would need changed out. But I'm hoping 2 less teeth will do the trick.
 
Wildcat said:
I have the one that comes with the hub, 19 teeth. If I need lower gearing, it's easy to get and change it out. Just have to pop it on and add a link to the chain. My old bike has cottered cranks, meaning the drive sprocket is a part of the right crank and the whole thing would need changed out. But I'm hoping 2 less teeth will do the trick.
Click to expand...
Here is the dope on your setup and possible changes.

CR=Chainring
C=Cog
GI=Gear Inches

48CR / 19C = 50.5GI
46CR / 19C = 48.42GI

48CR / 20C = 48GI
48CR / 21C = 45.7GI
48CR / 22C = 43.64GI
 
Check your math; that folder is rolling on 20s. With a 48t ring and a 19t cog, he'll be sitting on 47.2" in direct and 64.3" in overdrive. Swapping to a 46t chainring, with the same 19t cog, it'll be 45.2" direct and 61.6" overdrive. Considering that traditional BMX gearing is about 51", I think going any lower would be a mistake on a commuting bike.
 
Bicycle808 said:
Check your math; that folder is rolling on 20s. With a 48t ring and a 19t cog, he'll be sitting on 47.2" in direct and 64.3" in overdrive. Swapping to a 46t chainring, with the same 19t cog, it'll be 45.2" direct and 61.6" overdrive. Considering that traditional BMX gearing is about 51", I think going any lower would be a mistake on a commuting bike.
Click to expand...
The math is correct:

48T / 19T = 2.5263 x 20 = 50.5263 gear inches

@ 125% = 63.1578 gear inches
 
I just used Sheldon's calculator, which takes tire width into consideration, etc. That Michelin looks to be a 406x45mm to me. Even still, Sheldon's calculator doesn't always jive with the standard formula.... I get 49.3, which is right between your figure and the one spit out by Sheldon's calculator...

Beyond that, the overdrive on the Automatix is +37%, not +25%...

In any case, I can't see how 46tx19t on 20s would be overgeared.
 
My tires are 20 x 1.75. That's all the math I can handle. Correction on the Dahon, the chainring is 52 teeth. I've been out riding my DNB, 46/19. It's just right for me. I think the hub gearing is 1-1 and 1-124. It seems to be slightly loweer than 1st and 2nd gear on my old 3 speeds, shimano and SA. That's with the 46/19. It rides just a little below an old 3 speed's 1st and 2nd gear. I didn't open it up to check the lube, it's too dusty here and I didn't want to contaminate it.

2euliq8.jpg
 
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Wildcat said:
My tires are 20 x 1.75. That's all the math I can handle. Correction on the Dahon, the chainring is 52 teeth. I've been out riding my DNB, 46/19. It's just right for me. I think the hub gearing is 1-1 and 1-124. It seems to be slightly loweer than 1st and 2nd gear on my old 3 speeds, shimano and SA. That's with the 46/19. It rides just a little below an old 3 speed's 1st and 2nd gear. I didn't open it up to check the lube, it's too dusty here and I didn't want to contaminate it.

2euliq8.jpg
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A Shimano 3 speed is set up stock with a 19T cog and the gearing is 1st 75%, 2nd 100% and 3rd 133%.

So if your Sram is 1st 100% and 2nd 124% then with a 52T chainring and 19T cog you will have around 54.74 gear inches in first. Then in second gear it will be around 67.87 gear inches.

CR=Chainring
C=Cog
GI=Gear Inches

52CR / 19C = 1st 54.74GI 2nd 67.87GI
50CR / 19C = 1st 52.63GI 2nd 65.26GI
48CR / 19C = 1st 50.5GI 2nd 62.62GI
46CR / 19C = 1st 48.42GI 2nd 60.04GI

52CR / 20C = 1st 52GI 2nd 64.48GI
52CR / 21C = 1st 49.52GI 2nd 61.40GI
52CR / 22C = 1st 47.27GI 2nd 58.62GI

The 21T or 22T rear cog looks to be your best and cheapest change. with a 52T chainring, 22T cog, 20" tires and the 2 speed hub it will do great. Of course the slight differences in the real tire diameter will change the real world gearing of the bike. Of course if you want even lower gearing you will need to change out the chainring and maybe even the cog.

NOTE: See how close these are for the price of a new rear cog and the chainring that is on there now.
52CR / 22C = 1st 47.27GI 2nd 58.62GI
46CR / 19C = 1st 48.42GI 2nd 60.04GI
 
It's not a 24% jump; aparently, it's a 36% jump. (I'd read 37% elsewhere, but I'm getting this info from the manufacturer.)

From the SRAM site:http://www.sram.com/sram/urban/products/automatix

WeightCoaster brake 1420 g, Freewheel 780 g, Disc brake 720 g, SH Rollerbrake comp. 780 g
Gears2
Transition136%
Technology Highlight(s)Mechanical centrifugal clutch
ColorsSilver, Grey, Silver, Ivory
OptionsCoaster brake, Freewheel, Disc brake, Aluminum hub shell, SH Rollerbrake comp., Gates beltdrive
Details
2-speed automatic shifting
 
us56456712 said:
I don't know about the Sram, but the Bendix Blue Band overdrive calls for 36 tooth sproket with 20 inch wheel. I think mine has 18 teeth.
Click to expand...

I would think it is talking about the Chainring being a 36T not the cog. Both can be called a sprocket but the front one is called a chainring and the back one (on the hub) is a cog.
 
The reviews I read on the Sram all wondered if it would be geared too low for a 20" wheel. With a 52 tooth chainwheel it's probably good for most people. I can do 20mph easily in high gear. With the 46 chainwheel I use, It's just right but I like it a little lower. The Dahon is setup with a single speed freewheel cog with 22 teeth, and it would suit me better with one less tooth, it's a little to low for me. My niece likes that gear so it'll stay that way. My bike will remain 49/19 with the 2 speed.
 
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Being that the Automatix has an OD gear, I don't see how it could be geared too low. If the primary is a little low, the jump to OD should make it a little high, yes? That 37% jump, of course, becomes more significant as the primary ratio goes up.

My worry with the auto on a 20", or any bike, would not be that the gearing is too low (that can be addressed with the set-up), but rather, too narrow. As a guy who mostly rides singlespeeds, that seems like a petty concern, though. But you can't beat a kickback coaster and the lack of cables on something like a folder, can you? (I think the real worry with an auto an a 20" is that, due to smaller diameter wheel, it will spin faster and upshift more quickly, at a lower actual speed b/c a 20" wheel spins faster at a given speed than a 26".... it's an rpm vs mph thing.)
 
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If I understand how these hubs work it is possible to change the shift point some if you open it up and modify the spring that controls this. Now of course this will void any waranty that you have on a new Sram hub.

Here is a link talking about doing this mod to the hub. https://respectcycles.com/automatix-build/
 
I read a how-to on that... might even be the same one. Seems pretty straightforward. I kina wish SRAM would make a manual kickback, though as I've heard a lot of bad things about the SA kickback.
 
When I bought them on amazon, I read all the reviews, many said a 20" will shift at 7mph, and for a 26", the shift is at 11mph. A lot of riders want it at a higher speed and have done the mod to the spring.
 
Had frame problems, so I put it back on the Dahon with 52 tooth chainwheel, only I changed the cog to a 22 tooth. It's a little higher geared than the 46/19, but it's still a good ratio for me. Brakes are great.

5lbas4.jpg
 

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