Gear ratio on Bosch CX

#1
The gear ratio on my Bosch CX / Nuvinci with a Gates carbon belt is a bit too low for me (I believe it is 26 front to 24 back). How difficult would it be to change out the front / rear sprockets? If I am not mistaken I need something larger in the front to gain more top end speed? The bike in question is R&M Tinker. Will I need a longer belt as well? From the Gates site I see that 28 front is available. How do I calculate the teeth number for the belt? And will going up 2 teeth in the front make that much of a difference? Any help is appreciated.
 
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Trail Cruiser

Well-Known Member
#2
This is a very tricky situation since you have a fixed length carbon belt. A change in diameter for the front sprocket should have a equal but opposite change in diameter for the rear so that the carbon belt will remain tight.

In my best estimation, you can increase 2 teeth at the front sprocket and lose 2 teeth at the rear (from 26/24 to 28/22).

With that change, you gain 16% at the top end and lose 16% at the low end. Example, if your cadence at 20 mph was 80 rpm, it will go down to a more comfortable 67 rpm.

You can find the 28 teeth front sprocket here.
https://www.bikeonlineshop.net/gates-carbondrive/cdx-front-sprocket-bosch

Unfortunately, the 22 teeth rear sprocket is hard to find. However, there is a universal CDX 22 teeth sprocket that fits hub geared drive and nuvinci (?).
https://www.boodabike.com/gates-surefit-3-lobe-rear-sprocket
 
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Trail Cruiser

Well-Known Member
#4
Somebody else may have ideas on tools needed.

The other advantage for the change in the sprockets is that you will spend more time on the in-between ratio (more efficient) when you are already rolling, and not on the highest ratio where the nuvinci ball is at extreme angle and less efficient.
 
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#5
Ok, just spoke to the dealer and he mentioned something about Bosch being at 2.5 ratio on the front cog (one spin of the crank is 2.5 spins of the cog, if I understood correctly). That might change the maths quite a bit. He is going to check with his mechanics and get back to me. I'll keep the thread updated in case others are considering something similar.

A big shout out to Propel Bikes of Brooklyn for being so responsive to their community.
 

Trail Cruiser

Well-Known Member
#6
The typical Bosch front sprocket turns 2.5 times faster than a non Bosch sprocket.
A 26 teeth Bosch front sprocket is equivalent to a 65 teeth non-Bosch front sprocket.

However, It doesn't change anything since the front sprocket diameter is also reduced by 2.5 so the overall length of the belt pulled per pedal rotation is the same as the non-Bosch front sprocket.

It is intentionally made like that so that it will be compatible/interchangeable with non-Bosch rear sprockets or geared hubs.
 
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#7
Ok, just spoke to the dealer and he mentioned something about Bosch being at 2.5 ratio on the front cog (one spin of the crank is 2.5 spins of the cog, if I understood correctly). That might change the maths quite a bit. He is going to check with his mechanics and get back to me. I'll keep the thread updated in case others are considering something similar.

A big shout out to Propel Bikes of Brooklyn for being so responsive to their community.
Propel Bikes with Chris and the entire 'gang' are a great bunch of folks.
 
#8
The typical Bosch front sprocket turns 2.5 times faster than a non Bosch sprocket.
A 26 teeth Bosch front sprocket is equivalent to a 65 teeth non-Bosch front sprocket.

However, It doesn't change anything since the front sprocket diameter is also reduced by 2.5 so the overall length of the belt pulled per pedal rotation is the same as the non-Bosch front sprocket.

It is intentionally made like that so that it will be compatible/interchangeable with non-Bosch rear sprockets or geared hubs.
I meant this changes the gear ratio calculation. Would going two teeth up in the front and two down in the back be too much with the multiplier? So instead of 65/24 (2.7) we'd be moving to 70/22 (3.2) direct drive equivalent. Are these the right numbers?

I don't have a good feel for what they mean. As it stands I'm in the highest gear 95% of the time because everything else spins way too fast. I'd like to gain top speed and move the majority of my riding on the flats to the middle of the gearbox.
 

Trail Cruiser

Well-Known Member
#9
I meant this changes the gear ratio calculation. Would going two teeth up in the front and two down in the back be too much with the multiplier? So instead of 65/24 (2.7) we'd be moving to 70/22 (3.2) direct drive equivalent. Are these the right numbers?

I don't have a good feel for what they mean. As it stands I'm in the highest gear 95% of the time because everything else spins way too fast. I'd like to gain top speed and move the majority of my riding on the flats to the middle of the gearbox.
The OEM is 26/24 (1.08333), planned change is 28/22 (1.27273).

The difference between the 2 ratios (1.27273/1.08333 - 1 ) equals 1.1748 - 1 = 0.17 or 17%.

Seventeen percent is roughly about the same as the ratio gap between gear to gear on rear gear clusters or on hub gears.

Meaning, if you had a hub gear such as the alfine, the sprocket change makes you gain one shift up (better for top end) but lost 1 shift down (your first gear ratio has changed to the second gear).

I hope that makes sense to you.
 
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David Berry

Well-Known Member
#10
Increasing the front sprocket by the same number of teeth as the rear sprocket is decreased is surely the simplest way to go. No new belt and a pleasantly higher range - but check your warranty conditions.
 
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Trail Cruiser

Well-Known Member
#11
The nuvinci hub does not operate as efficiently at the extreme angle of the ball (either the highest gear or the lowest gear).
Since you will be spending less time at the highest gear, you will also gain in efficiency.
 
#12
The OEM is 26/24 (1.08333), planned change is 28/22 (1.27273).

The difference between the 2 ratios (1.27273/1.08333 - 1 ) equals 1.1748 - 1 = 0.17 or 17%.

Seventeen percent is roughly about the same as the ratio gap between gear to gear on rear gear clusters or on hub gears.

Meaning, if you had a hub gear such as the alfine, the sprocket change makes you gain one shift up (better for top end) but lost 1 shift down (your first gear ratio has changed to the second gear).

I hope that makes sense to you.
That's what I thought as well, but the front gear also has a 2.5 multiplier on the Bosch (one revolution of the crank is 2.5 revolutions of the gear). Would this not mean that the factory ratio is 26*2.5/24?
 

Trail Cruiser

Well-Known Member
#13
That's what I thought as well, but the front gear also has a 2.5 multiplier on the Bosch (one revolution of the crank is 2.5 revolutions of the gear). Would this not mean that the factory ratio is 26*2.5/24?
It has 2.5 multiplier but the front sprocket diameter is also reduced by 2.5 so it balances out to 1.000.


One pedal stroke of Bosch pulls the same lenght of belt as one pedal stroke of non Bosch/regular pedal crank. That's why a bosch motor can also use the regular/standard rear gear cluster without any gearing issues and has the same overall ratio.

If you factor in the 2.5 multiplier it would be like this:

The OEM is 65/24 , planned change is 70/22.

The difference between the 2 ratios ( 2.921667 / 2.708333 - 1 ) equals 1.1748 - 1 = 0.17 or 17%.

The same, 17% difference.

I am sure people who have a hard time getting a handle in understanding about gear ratio will outright say that it cannot be done.
 
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#14
Thanks to all in this thread. I am going ahead with the change and will report back on the results.

Notes:

  • Both cranks were available at https://www.bike-components.de and much cheaper shipping than directly from Gates.
  • Instructions found at youtube.com/watch?v=Urtjc4-LKcQ
  • Tools needed:
    • 9mm alan wrench.
    • Crankarm extractor tool ($10 on Amazon).
    • Ring extractor tool is too expensive, using lock ring pliers instead. Not sure what size yet.
  • Stay tuned.
 
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Trail Cruiser

Well-Known Member
#17
I searched all over the internet but could not find a 22 teeth nuvinci compatible CDX rear sprocket. It only comes with 24, 26, and 28 teeth rear sprocket.

The other possible option is to leave the rear sprocket alone and change the the front sprocket from 26 teeth to 30 teeth. However, there is also a road block here, I could not find a 30 teeth front cdx Bosch sprocket.

If we just leave the rear sprocket alone and change the front from 26 teeth to 28 teeth, you only get 8% of gearing change which may not be enough. In terms of a regular gear cluster, that would roughly be equal to half a gear shift.

The other option is a risky proposition that involves machining the inner spline of an existing 22 teeth sprocket so it would fit the nuvinci spline.

1537585288729.png

https://www.ebay.com/p/Gates-Carbon-Drive-CDX-CenterTrack-Rear-Cog/24012750574?iid=362129681297

I say it's risky since there may not be enough material at the inner spline to be machined away. So have the supplier take an eyeball and compare the inner spline between the picture above and the nuvinci sprocket in their store.

I thought this would be easy but I was wrong.
 
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#18
Thank you so much for the help TC, I've learned a lot. I'll start working on the front---I think there is enough movement on the rear wheel belt tension adjuster to move the hub forward without changing the belt. Meanwhile, I've contacted Gates for advice. 8% is better than nothing for now.

Another thought: I see that chained Nuvinci setups are sold with regular Shimano rear cogs. The 9 spline CDX you linked is also listed as "fits any Shimano style freehub body." That sounds like it would fit to me.
 
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#19
Another update, Gates got back to me to say that they are just finalizing the production of 22t Nuvinci. It will be available in the spring of 2019.

Hawt dawg! Gates sent me the 22t Nuvinci rear. A huge thanks and a shout out to Gates.
 
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#20
Any update how it went? I’m considering a similar change on a r&m charger but got the response from gates that the 22t is build from ground up for the newer enviolo nuvinci transmissions and wont fit on the ‘classic’ 380 series. As such I would need to increase size on the front sprocket instead.