I was thinking about this small bike, but it has very short crank arms to spin fast insted of brute force cranking.


Disclaimer: I do not under stand this, it may be wrong.
https://sheldonbrown.com/gain.html

the bikergo people told me to abou tthis formula:
https://bikergo.com

I have used short cranks on my recumbent bikes to help spin the crank faster for easier hill climbing, but I was not able to compensate for shorter crank arms, it takes a smaller gear (gain ratio) than you think, because there is less momentum in the cranking.

So I had to think about it:

The formula for gear gain ratio is: wheel radius / crank length x front sprocket / rear sprocket x hub ratio (if using a geared hub). The gain ratio represents your foot speed relative to ground speed. 1.22 is about as low as you get on a mountain bicycle. I must pedal at 60 rpm to maintain my slowest balancing speed of 3 mph when climbing a steep hill with too much cargo. (350lbs total combined weight)

3 mph = 80467.2 millimeters per minute
4 mph = 107289.6


My trike’s lowest gear:
(254 tire radius /170mm cranks) x (18 / 34) = .79 gain ratio
(80467.2 &divide .79) / (170 x x2 x3.14) = cranking 95.4 rpm to get 3 mph

My MTB’s lowest gear:
(330.2mm tire / 175mm cranks) x (22/34) = 1.22 gain ratio
(80467.2 / 1.22) / (175 x 2 x 3.14)= 60 rpm cranking to move at 3mph

My recumbent bike’s lowest gear:
(323.85/170) x (24/34) = 1.344 ratio
(80467.2 / 1.344) / (170 x2 x 3.14)= 56 rpm at 3 mph
and at 4mph =74.77 rpm

Bikergo’s gears:
In 1st gear with a gain ratio of 2; the crank rpm =
(80467.2 mm / 2 gain ratio) / (125mm crank arm length x2π) = 51 rpm
(80467.2/1.344) / (125mm cranks x2 x pi) = 76.26 rpm (for short arm cranking) at 3 mph

At least I do under stand this: if the crank arm is 125mm : 73.5% of 170mm so a 26.5% lower gear is needed Aprox a gear gain ratio of '1 ' is needed?

I don’t know just how to get that low of a gear, perhaps a mountain tamer fourth chain ring? the schlumpf mt crank set is a 2.5 to 1 reduction.

https://www.abundantadventures.com/quads.html

200lb man + about 50lb for bike and trailer + 100lb cargo max = 350lbs it really adds up fast.

I think that I need an even lower gear , does any one know about just how low of gear is needed with short cranks. , percentage wise.??

this is what is needed for this bike, an infinatly variable gear hub

https://www.youtube.com/watch?v=cd2-vsTzd9E&NR=1

https://www.youtube.com/watch?v=UmptP...eature=related

The shorter the cranks the lower the gear and the faster the sin is needed. So 140mm cranks at 86rpm are about my limit .