Question about rear sprocket with a Sturmy Archer hub

[BrianK]

A local archery shop has a steeply discounted left over 2018 Rambo R750cG3. I took it for a good test drive this weekend, really enjoyed it and put money down on it.

However my riding will be 50/50 road and rails to trails, and very little off road riding. I really like the idea of the internal hub gearing, but I found the gearing with their Sturmy Archer 3 speed rear to be way too low, such that I can hardly pedal fast enough to keep up at road speeds.

I assume I’ll need to change the rear cog to a different tooth count so I don’t have to pedal so fast while riding roads. It appears Rambo uses a 32T front chain ring with a 26 T rear sprocket according to their current offerings.

How far down should I go on the rear sprocket Tooth count to make it more of a road bike and less for crawling along hunting trails and hills?

20T, 16T, 14T, 12T?

I have no idea how to figure what rear tooth count would be appropriate, nor which type of sprocket to buy for this particular Sturmy Archer 3 speed.

Eventually I’ll also be changing from the knobby 26”x4” tires a road type tire. Any suggestions?

 

[drewberz]

Sheldon Brown has all things bikes:

Sheldon Brown Gear Calculator

I did a quick analysis for you with provided numbers. Note I left the crank arm length stock (it's usually stamped on the inside of the arm), used 2.35" instead of 3" because that's what was available, and assumed 80RPM. You can play around with all of these settings on the site.

 

[BrianK]

Sheldon Brown has all things bikes:

Sheldon Brown Gear Calculator

I did a quick analysis for you with provided numbers. Note I left the crank arm length stock (it's usually stamped on the inside of the arm), used 2.35" instead of 3" because that's what was available, and assumed 80RPM. You can play around with all of these settings on the site.

View attachment 54926

Wow, that’s awesome, thanks!

At the risk of showing my ignorance I assume 80rpm pedaling is a medium pace?

Judging by this I think I’ll try a 14T rear sprocket.

 

[BrianK]

I talked to support at Rambo bikes. They recommended changing the front sprocket, not the rear cog. They have a 46T front sprocket in stock specifically for this purpose so I’ll order that, and he said the front is easier to swap out than the rear.

 

[drewberz]

Wow, that’s awesome, thanks!

At the risk of showing my ignorance I assume 80rpm pedaling is a medium pace?

Judging by this I think I’ll try a 14T rear sprocket.

90RPM is known as the 'most efficient' cadence for road cycling. Personally, I like low 80's. 80 is probably a medium to high cadence for most, but best thing to do is to measure your own. Does your bike have an instantaneous cadence setting on the display?

 

[drewberz]

I talked to support at Rambo bikes. They recommended changing the front sprocket, not the rear cog. They have a 46T front sprocket in stock specifically for this purpose so I’ll order that, and he said the front is easier to swap out than the rear.

Makes sense, your drivetrain will last longer tooth with a higher tooth count (not sure if appreciably or not in your case).

 

[BrianK]

90RPM is known as the 'most efficient' cadence for road cycling. Personally, I like low 80's. 80 is probably a medium to high cadence for most, but best thing to do is to measure your own. Does your bike have an instantaneous cadence setting on the display?

Honestly I don’t know yet. The bike was missing the magnet for the speed/mileage sensor from the rear wheel so the display wasn’t working correctly. They’re ordering a replacement and putting mad flaps on it while we’re waiting for that to come in.