Bafang to Rohloff belt drive question.

[Cybersnow]

It appears that my Zen Photon Ultra current belt drive set up is geared to low. Gear 1 is ridiculously low and I find myself riding in gear 14 just to barely maintain a touring speed of 16mph. I would like to be cruising in gear 11 but the pedal rotation speed is beyond my capabilities (must be in the 90s). I am told by Ravi that the ratio between the front sprocket and rear sprocket is 2.5. Is there an upper limit on this? Can I jump to 3, 3.5 or 4? What makes sense?

 

[Jeremy McCreary]

It appears that my Zen Photon Ultra current belt drive set up is geared to low. Gear 1 is ridiculously low and I find myself riding in gear 14 just to barely maintain a touring speed of 16mph. I would like to be cruising in gear 11 but the pedal rotation speed is beyond my capabilities (must be in the 90s). I am told by Ravi that the ratio between the front sprocket and rear sprocket is 2.5. Is there an upper limit on this? Can I jump to 3, 3.5 or 4? What makes sense?

In another thread, I wrote,

Not so easy to find, but I eventually dug up this technical page on their E-14 Speedhub.

The internal ratios for gears 1 and 14 are 0.279 and 1.467, resp. With the Ultra's external ratio of 2.5 and an assumed 27.5 inch wheel diameter, that gives 19 and 101 gear-inches, resp.

With that information and answers to the questions below, you could figure out your ideal external ratio and see how close you can get with available front and rear belt sprockets.

Q1. The Photon's Gear #14 is 101 gear-inches. How many gear-inches would be ideal for cruising at your target 16 mph? Call it X.

Q2. What gear number on the Rohloff would you like X to be?

Q3. Your preferred cruising cadence (at 16 mph)?

Q4. How many gear inches do you need for Gear #1?

Happy to help with the math.

In principle, you could use the same belt if the sum of front and rear sprocket teeth stays the same. For example...

Stock Photon Ultra: 55 + 22 = 77 total teeth, 55/22 = 2.5 external ratio, gear 14 = 101 gear-inches with 27.5" tires.

Modified: 58 + 19 = same 77 total teeth, but 58/19 = 3.05, and gear 14 = 123 gear-inches.

 

[Cybersnow]

Great questions, thanks!

Ideally I would like to be cruising in Rohloff gear 11 @ a cadence of about 70 and going about 16 mph. My legs are relatively strong but not so fast. I can maintain a 60-70 cadence without much trouble. My previous e bike was very light Salsa touring bike with a 750watt bafang motor and battery kit and we did a number of long trips where we averaged about 50 miles a day.

Sorry, not so smart on gear inches. By my simple math of 27.5x3.14x3,05=263.4 which is the distance traveled in inches for 1 pedal rotation. 263.4/12=21.9 is the distance in feet. 21.9x60x60/5280= mph 14.9 at a cadence of 60 and about 17.4 mph at a cadence of 70. This is in gear 11 which I understand is 1 to 1. I compare that to current speed of 12.3 with the same cadence of 60. Not sure what the speed would be in gear 14.

Not sure I did this right but totally open to your input. Also Ravi was asking if I could hit a speed of 20 on a flat but I managed to get to about 18.x in gear 14.

I was very surprised at the overall performance of the bike, not at all that I expected.

 

[scrambler]

I have an excel sheet that calculates speed in each gear with sprocket cadence and wheel size as variables. It has sheet tabs for each IGH, just click on the Rohloff tab
Speed and gear ratio.xlsx

You may have to download to change the variables

 

[Cybersnow]

Just checked the bike it is a 55/22 set up like Jeremy suggested. Oh and my wife‘s Aventon bike with a 750watt rear hub motor and only 50Nm of torque flat ran away from me.

 

[Cybersnow]

I have an excel sheet that calculates speed in each gear with sprocket cadence and wheel size as variables. It has sheet tabs for each IGH, just click on the Rohloff tab
Speed and gear ratio.xlsx

You may have to download to change the variables

I am not sure how to download the file to my ipad. I will try later to download to a Windows laptop

 

[Jeremy McCreary]

Great questions, thanks!

Ideally I would like to be cruising in Rohloff gear 11 @ a cadence of about 70 and going about 16 mph. My legs are relatively strong but not so fast. I can maintain a 60-70 cadence without much trouble. My previous e bike was very light Salsa touring bike with a 750watt bafang motor and battery kit and we did a number of long trips where we averaged about 50 miles a day.

Sorry, not so smart on gear inches. By my simple math of 27.5x3.14x3,05=263.4 which is the distance traveled in inches for 1 pedal rotation. 263.4/12=21.9 is the distance in feet. 21.9x60x60/5280= mph 14.9 at a cadence of 60 and about 17.4 mph at a cadence of 70. This is in gear 11 which I understand is 1 to 1. I compare that to current speed of 12.3 with the same cadence of 60. Not sure what the speed would be in gear 14.

Not sure I did this right but totally open to your input. Also Ravi was asking if I could hit a speed of 20 on a flat but I managed to get to about 18.x in gear 14.

I was very surprised at the overall performance of the bike

Made a quick online spreadsheet in Google Sheets to explore the problem. It's meant to be viewed in dark theme (white on black). I think I have it set up so that anyone with the link below can play with it without changing the master. Let me know if you can't edit.

Adjust only the cells with blue backgrounds.

Rohloff

docs.google.com

At 70 RPM cadence with the stock external ratio of 55/22 = 2.50, gear #12 gives 16.2 mph, #1 gives 4.0 mph, and #14 gives 21.0 mph.

No idea what external sprockets and belts are available or workable on your bike. But suppose you could go to 57/20. The external ratio would rise to 2.85, but total external teeth would still be 77. At 70 RPM cadence, gear #11 would give 16.3 mph, #1 would give 4.6 mph, and #14 would give 24.0 mph.

Sounds like what you're after — provided the top and bottom gears are acceptable.

 

[scrambler]

Similar here, by the calculation in my excel sheet, with a 584mm rim (the standard rim for a 27.5" Wheel), a 2.4" tire, a 22/55 sprocket, at a cadence of 70rmp, the bike should be going 21mph.

Can you measure the outside diameter or circumference of the rear wheel. a 584mm rim with a 2.4" tire should be about 27.8" diameter or 87.31 circumference
a 70rpm cadence with a 55 front and 22 rear give a rear sprocket rpm of 70 x 55 / 22 = 175rpm
In gear 14 the gear ratio of the hub is 1.467 so the wheel rotation is 175 x 1.467 = 257 rpm

This mean 87.31" x 257 = 22438.7 inch per minute, or 21.2 mile per hour

This does not seem to be what you say you are experiencing, so we need more precision on your circumstances.

But in all cases, you can definitely explore changing the front rear ratio to increase your top speed within the belt size constraints.....
It would be interesting to know if there is any horizontal adjustment available for the rear axle position

 

[scrambler]

Looks like gates sprockets for rohloff are available in 19, 20 and 22

Rear Sprockets

Rear Sprockets

www.gatescarbondrive.com

Depending on the type of mount for the front sprocket, you can find below the available sizes

Front Sprockets

Front Sprockets

www.gatescarbondrive.com

 

[Tom@WashDC]

It appears that my Zen Photon Ultra current belt drive set up is geared to low. Gear 1 is ridiculously low and I find myself riding in gear 14 just to barely maintain a touring speed of 16mph. I would like to be cruising in gear 11 but the pedal rotation speed is beyond my capabilities (must be in the 90s). I am told by Ravi that the ratio between the front sprocket and rear sprocket is 2.5. Is there an upper limit on this? Can I jump to 3, 3.5 or 4? What makes sense?

I am running a ratio of 3.075 (chain setup 13T rear, 40T front) which seems ideal to me. At cadence of 70 I'm doing about 22mph in 11th gear.

 

[Tom@WashDC]

I have an excel sheet that calculates speed in each gear with sprocket cadence and wheel size as variables. It has sheet tabs for each IGH, just click on the Rohloff tab
Speed and gear ratio.xlsx

You may have to download to change the variables

Nice spreadsheet!

 

[Jeremy McCreary]

Just checked the bike it is a 55/22 set up like Jeremy suggested. Oh and my wife‘s Aventon bike with a 750watt rear hub motor and only 50Nm of torque flat ran away from me.

Hope she comes back.

 

[Jeremy McCreary]

Help! I originally posted a link to the wrong Google sheet — one called "ebike". Now I can't edit it, even as the owner.

If you have "ebike" open, please close it, ditch that link, and use the link above to a copy called "Rohloff".

Thanks!

 

[Cybersnow]

I am really appreciating this great advice. I just connected up an older bike computer and took the bike for a 15 mile ride. While I did a cadence of 70 several times, my sustained cadence was closer to 60. That comes as no big surprise to me as my 78 year old legs ain’t what they used to be (or was that a song about an old grey mare?). Sorry to ask this but my Ipad is not liking the initial spreadsheet. If we change the cadence to 60, where do we end up?

 

[Cybersnow]

Hope she comes back.

She did and rubbed it in!

 

[Jeremy McCreary]

If we change the cadence to 60, where do we end up?

At 60 RPM cadence with the stock external ratio, gear #11 gives 14.0 mph, #12 gives 15.9 mph, #1 gives 3.9 mph, and #14 gives 20.6 mph.

CORRECTION:

At 60 RPM cadence with the stock 55/22 external ratio, gear #11 gives 12.3 mph, #13 gives 15.9 mph, #1 gives 3.4 mph, and #14 gives 18.0 mph.

This agrees with the result @scrambler posted. (Sorry, above was with 57/20 gearing.)

 

[Cybersnow]

One thing about e-bike rides up here is that they run into hours vs just a short ride.

 

[scrambler]

 

[scrambler]

18mph on gear 14

 

[Cybersnow]

I wasnt counting my cadence on that first ride but found myself quickly being fatigued and working hard to maintain 17 mph in gear 14 and assist level 1.