Sturmey Archer 3 speed gear ratios ?

[BADGER.BRAD]

Hello all,

I've posted this in this section because I am exploring the possibility of using an old folder for some low speed forest gravel track riding with my working dogs, I am restricted in both space at home and in my small car so I thought cheap folder may get me on the bike again. My question is what sort of difference is there between gears on a 3 speed hub compared to derailleur gears ? I don't intend to go fast but will need enough gear range to get up hills but not be peddling like mad on the flat.

Thanks all

Brad

 

[N0bodyOfTheGoat]

My goto for bike gearing has been the old Sheldon Brown site for years...

https://sheldonbrown.com/gear-calc.html

 

[TheDoctor]

Working out hub gear ratios isn't too bad.
Assuming 3 speed, middle gear is 1:1 - so divide chainring teeth by sprocket teeth, and multiply by wheel size. It'll be around 60" or so.
Bottom gear is 3/4 of that, top gear is 4/3. 45", 60" and 80" or something like that.
You can raise or lower the range by changing the chainring or sprocket.

 

[mjr]

Working out hub gear ratios isn't too bad.
Assuming 3 speed, middle gear is 1:1 - so divide chainring teeth by sprocket teeth, and multiply by wheel size. It'll be around 60" or so.

Maybe, but mine's set to 42" in line with Sheldon Brown's advice.

 

[TheDoctor]

Maybe, but mine's set to 42" in line with Sheldon Brown's advice.

To give top as your cruising gear? Not a bad shout.

 

[Chris S]

Most old Raleigh folders have 69, 52 and 39 inch gears. You can do most urban hills in a 39 inch gear. A 52 inch gear is ideal for start/stop city cycling and a 69 inch gear will give you a cruising speed in the low teens. You will find yourself freewheeling down hill though.

They'll probably be OK on a gravel track as well.

 

[BADGER.BRAD]

Thank you everyone much appreciated. It seems they give a fair range ! I will be free wheeling down hill anyway at a relatively slow speed and not going any major pace on the flat so the dogs can keep up easily.

 

[rogerzilla]

I know it's an old thread, but the maths are quite neat. With a single-stage epicyclic (planetary) geartrain like the AW, the ratio is given by (gear ring teeth + sun teeth)/gear ring teeth. The planet teeth don't come into it. The gear ring being connected to the sun effectively makes it roll further each rotation, by the number of teeth on the sun.

An AW has sensibly-sized gears, 60 on the gear ring and 20 on the sun, so the ratio is 1.333. Depending on which way round the gears are used, you multiply or divide by 1.333. Gear ring driving planet cage (low gear) divide by 1.333. Planet cage driving gear ring (high gear): multiply by 1.333. In normal gear, the driver drives the gear ring but it's locked directly to the shell, so the planetary bits just idle around.