Bafang to Rohloff belt drive question.

Scambler, that is pretty consistent with what was happening. If I want about 18 mph on gear 11 at a cadence of 60, what changes do I need to make to the front and rear cogs?
 
Good question. If we are planning a 50 mile day before we get to the next town to overnight and recharge, I have found that I need to typically be riding the flats in level saving the current draining levels for the climbs or I run out of battery too quick and riding an 80lb bike with no assist is a pain in the neck.

maybe I need to start looking for a different bike?
 
Last edited:
OK, so problem is It does not look like there is a 57T front sprocket, it goes from 55 to 60.
So the closest to the 22+55 = 77 would be 19 +60 =79

at a pedaling cadence of 60, here are the result for 19 rear 60 front

1717923578628.png


For a cadence of 70

1717923554196.png



So speed wise you are good with 19/60, but the length of the belt will vary a bit.

To double check the implications, we can use gates belt calculator, but I would need the following from you

  1. What is your chainstay: distance between rear axle and center of bottom bracket?
  2. Can you check the Belt model reference on the belt so we can know how many teeth it has (count the teeth if you have to :) )
  3. Do you see any horizontal adjustment system on the rear axle, if so how much distance is there to move the wheel forward and backward from the current position?
  4. Last you need to check if a larger front sprocket could interfere with the rear arm (what distance between the existing 55 T sprocket and the arm before it touches it.
 

Attachments

  • 1717923093799.png
    1717923093799.png
    317.4 KB · Views: 15
  • 1717923135581.png
    1717923135581.png
    317.9 KB · Views: 19
Last edited:
I found a value for the chainstay on Zen site of 489mm (TO CONFIRM)

If that is correct, the gates calculator tells me that a 55/22 would use a belt with 128 teeth which leads to a chainstay of 489mm

Now if I use a 60/19 sprocket set with the same 128 belt, I get a needed chainstay of 481.5mm which is 7.5mm less, so we would need to be able to move the wheel 7.5 mm forward

If I use a 60/19 with a 130 tooth belt ( next one up, they only have belt for a discreet number of teeth), I get a chainstay of 492.6 or 3.6mm more than the current one, so if we can move the wheel 3.6mm backward (and have some more for tensioning) that would work too.

Finally we could use a 60/20 sprocket pair with the 130T belt, that leads to a Chainstay of 490mm only 1mm more than the current one, so I am sure that is workable, and the speed would be close to what you need.

So it comes down to confirming the values I asked about before so we can see what combination of sprocket and belt would best work, but I think you will have one solution among these, as long as a 60T front sprocket won’t touch the rear arm.

You can download the Gates excel sheet calculator below. It includes the list of available CDX belt sizes.

Below the speed with 60/20 for both 60 and 70 pedaling cadence

1717931654185.png


1717931673817.png
 
Last edited:
Just got up and had my coffee. This looks like the answer. Going out to check measurements. Only have inch info but can go to the local hardware store ia few hours to see if they have a cm tape.
 
1. Chainstay is 19 1/8”
2, Belt is 11m-128t-12ct. 284L-20-0
3. maybe between 1/4-3/8” space to adjust belt
4. chain guard space = 1/2”
Bike rear arm = 2”
 
128T confirms the belt size

19 1/8 is 486mm close to the 489 that is the nominal for that belt and 55/20 sprocket. Probably the imprecision of the measurement.

When you say 1/4 (6.35mm) or 3/8 (9.5mm), do you mean on each side of the axle (forward AND backward)?
May be take a picture of the axle with a horizontal ruler below the bolt.

When you say chain guard space 1/2", do you mean that if the sprocket radius is 1/2" bigger, it would touch the arm?
May be take a picture with a ruler flat behind the sprocket until it touches the arm. I need to find the diameter difference between the 55T and the 60T sprocket.

But so far the safest would be 20/60 with a 130T belt. If you do decide to do that you will need to confirm if the rear sprocket is a Rohloff "splined" or "threaded"

If you think you would really prefer 19/60, you would need to have very precise measurement of how much the wheel can be shifted back and forth from its position.
 
I found the radius of the 55T, it is 95.4mm = 3.76" and the radius of the 60T is 104mm = 4.09"
So the 60T will extend 8.6mm or 0.34" from the current one (a little under 3/8")
 
All i have here in terms of measuring tools are carpentry focused measuring tapes. I can go into my little town‘s hardware store to see uf they have anything ore accurate or in cm.

20/60 is still 3.0 which is better than I have as far as top soeed us concerned.

What do you need pictures of?
 
If you are happy with the speed table I gave you for the 20/60 gear ratio, I think you are fine with a 130T belt.

If you want to go with the 19/60 ratio, then using the same 130T belt, the chainstay is 493mm or 4mm more than what you have now.

Gates recommend to have 10mm (.39") available forward of the nominal position to be able to mount the belt, and 2mm (.08") behind the nominal position for tensioning.

To be safe, I would want at least 5mm behind the nominal position, added to the 4mm we need from above, this would mean you need to have room to shift the wheel 9mm (0.35" or about 3/8") back from its current position.

Unless you want to take the risk to buy a 19T then have to buy a 20T if it does not fit, you would want to mark the current axle position on the adjustment rail, then remove the belt, and see if the wheel can move back 3/8" from the marked position.

For the front sprocket, I would cut a piece of cardboard round with a 4" radius, cut a quarter or that circle, and hold it behind the existing one in the upper left quadrant (the one where the rear arm is), until it extend past the existing one by 3/8" and make sure it clears the arm comfortably at all shock position.

If you are unsure of what you are doing, a close up picture of these two areas when you are doing the measurement would help make sure we understand each other :)
 
While this certainly sounds like a great solution, I am a little worried that making this change will void my warrant. The trial period was only 10 miles and I have about 30 miles on the bike now. I will shoot a letter to the president of Zen bikes telling of my desire to go to a 20/60 set up and see how he responds. He is aware of the issue and said he needs to research a solution. I will suggest the solution you came up with.

i really appreciate all the time and trouble you have gone through to help me and I think this is the right way to go. Thanks a lot!
 
You should definitely do that.

Normally Rohloff sets a ratio limit of 2.5.

Unless you would be hammering the hub with max power in wrong gear, I am pretty sure that going to 3 (60/20) would be fine, but if you want to be safe warranty wise, it should be done with Zen's approval indeed....

EDIT: 2.5 is a minimum not a maximum, see below
 
Last edited:
You should definitely do that.

Normally Rohloff sets a ratio limit of 2.5.

Unless you would be hammering the hub with max power in wrong gear, I am pretty sure that going to 3 (60/20) would be fine, but if you want to be safe warranty wise, it should be done with Zen's approval indeed....
Also check the worst-case torque loads against Rohloff's stated torque capacity. This issue was discussed a month or so ago, I believe in a thread about the new 3x3 IGH.
 
You should definitely do that.

Normally Rohloff sets a ratio limit of 2.5.

Unless you would be hammering the hub with max power in wrong gear, I am pretty sure that going to 3 (60/20) would be fine, but if you want to be safe warranty wise, it should be done with Zen's approval indeed....
Definitely run it by Zen...
But if I'm understanding the specifications correctly they recommend limiting the torque into the hub.. so the recommendations are Minimum Gear Ratio and none on the maximum allowable ratio.
Screenshot_20240609_112309_Chrome~3.jpg



edit: In my loiter I see this has been answered.... Nevermind.
 
Back