RoxMDO

My (pre-loved) Bike Friday Silk came with a NuVinci 360 hub and Gates Center Track CDX Drive. I'd like to modify the system for lower gearing to help in the hills.

The 60 tooth front sprocket matched with the 24 tooth rear is as high a gearing as they come. I'd like to replace the front with either a 55, 50 or 46 tooth chainring while retaining the 24 rear sprocket. It has 20 inch tires.

The chain stay length is 400mm. According to the Gates Calculator, the three sprockets mentioned will fit chain stay lengths 400.57, 404.44 and 399.64 mm in that order with belt lengths of 113, 111 and 108. Gear ratios would be either 2.292, 2.083 or 1.917.

I'm accustomed to working out gearing on 27 and 24 speed derailleur 26 and 29 inch wheel bikes.

I'm trying to get a handle on how what I'm accustomed to would translate to the Gates system on 20 inch wheels. Which chain ring would I be happiest with on this bike in the hills?

Any suggestions on gaining insight on equivalency would be very helpful.

BruceMetras

Right now, you're at approximately 23 to 83 g/i..

55/24 nets 21 to 76 g/i

50/24 nets 19 to 69 g/i

46/24 nets 18 to 64 g/i

So I would take an existing bike that you have that you are comfortable with, see what the g/i lowest gear is that you like, and then match it to one of the scenarios above..

RoxMDO

Bruce. I'm afraid that don't understand the significance of g/i. Could you please clarify? Thanks.

tds101 

Please read: https://www.sheldonbrown.com/gain.html

BruceMetras

g/i = gear inches .. makes it easy to figure out what you have and what you want.. if you use a calculator like Sheldon's , you can input the gears that you are familiar with to get your gear range and then factor in the gear range that you want for your new bike.. you're lucky you have so many choices with the Gates/cogs/belts that will work with your bike.. that's usually a stumbling block to convert a bike to belt drive..

fietsbob

At its simplest, It's Ratio Math X wheel diameter in inches.

RoxMDO

Thanks everyone. I'm starting to get a handle on this.

My Zootr Swift with 20 x 1.75 tires, a 170mm crank and 46 tooth chainring with a 7speed 13 to 34 cassette is giving a range of 66.1 to 25.3 gear inches.

The NuVinci with the maximum allowed 46 tooth front and the existing 24 rear with the same tires and crank would give a range of 64.4 to 17.9.

I do suspect that the continuous NuVinci 360 by it's nature is creating a more effective low range.

Rick Imby

The drag in the Nuvinci is significant also. I would be uncomfortable on a bike with lower than an 83 high gear... Smooth flat road roll out...

Are you carrying a load or pulling a trailer?

I had the same problem with my Shimano 7 IGH. If the high gear was high enough the low gear was higher than I wanted.

Some systems you have to choose between optimal high gear or optimal low gear. Where I ride I use low gear occasionally and I use my over 80 gear inch gears a lot.

Good luck with your adaptation, at least after the change you will be able to easily go back to your previous gearing.

fietsbob

the external gear ratio, falls in the center..

with overdrive ratios created turning the grip-shift one way, the inverse turning it the other..

its a fractional inversion , like the classic AW3, whose high is 4/3, low is 3/4 of 2nd.




...

fietsbob

Gates belt drive, here is the website Gates Carbon Drive? System for Bicycles | Gates Carbon Drive?

Florian Schlumpf also made belt drive crank cogs, He has produces an internally geare 2 speed crankset. 1.6x overdrive..
so like a 34t that acts like a 54t, as the crank arms are not connected directly, but thru the gearbox..


[I have their reduction geared one on my Brompton]






...

Rick Imby

A gear box in your crank sounds like a more mechanical resistance and expensive complication. At least you aren't changing the direction of the energy (which usually costs more energy). However this sounds like a great option for the 16" wheel Bromie.

The chain and derailleur setup have so little mechanical drag... too bad they have to be out In the Weather...

RoxMDO

I appreciate your attention but I'm afraid you've lost me.

RoxMDO

My wife wants gearing that is optimized for the hills and gravel/dirt not necessarily always for asphalt.

fietsbob

Read the Manual , they state the % of gear difference.. range. + & -

Example: sticking with the distinct gears of the 100 year old epicyclic hub..

1:1 is the cog turning the hub-shell at the same rate

1:0.75 is the cog turning faster than the hub-shell, low,

1.33:1 is the hub-shell turning faster than the cog, high..

RoxMDO

Thanks. I need to get ahold of the manual. Bought the bike used. The Gates site has been helpful.

downtube

How much did you pay for the bike? Just curious.

I was interested in bringing in NuVinci hub bikes three years ago, however the NuVinci rep lied about everything. I love the product, but I am not a fan of the representation.

Thanks
Yan