Many cyclists think I am crazy when I started a project of installing a quadruple crankset. They believe that riding a chainring of 20 teeth in the front is slower than walking so it should be better off pushing the bike than riding with such a low gear.
Is a chainring of 20T as low as most people think? Let us look at it.
The overall mechanical advantage for the transmission of the pedaling force to the motion of wheels in contact with the ground is calculated as follows:-
Overall mechanical advantage = no. of teeth of rear cog / no. of teeth of front chainring x crank arm length / diameter of wheel x 2
So suppose the bike has a smallest front chain ring of 20T, a largest rear cog of 26T, a crank arm length of 172.5mm and a wheel diameter of 700mm, the mechanical advantage is 26 / 20 x 172.5 / 700 x 2 = 0.64, which is significantly lower than 1, i.e. still 1.56 times ( = 1 / 0.64 ) faster than walking but require 1.56 times more force to ride up a hill than walking.
And this is already a super low gearing. How about the more common bicycle gearing? Let us look at the following table.
Configuration
Mechanical advantage at the lowest gear
Factor of required force compared to walking
Force required to climb up a 45 degree slope for an overall weight of 170lb
Ordinary road bike with 53 x 39T chainring, 12 x 21T cogs, 172.5mm cranks, 700C wheel
0.27
3.77
320 pound force
Ordinary road bike with 53 x 39T chainring, 12 x 26T cogs, 172.5mm cranks, 700C wheel
0.33
3.04
258 pound force
Ordinary Sports / Touring bike with 53 x 42 x 30T chainring, 12 x 26T cogs, 172.5mm cranks, 700C wheel
0.43
2.34
199 pound force
Special Sports / Touring bike with 53 x 42 x 30 x 20T chainring, 12 x 26T cogs, 172.5mm cranks, 700C wheel
0.64
1.56
133 pound force
Walking (for comparison)
1
1
85 pound force
Now it should be obvious who is crazier, a cyclist who want to ride up a hill with a minimum force equivalent to 320 pounds to be exerted per foot, or a cyclist who want to exert a pedaling force of 133 pounds only which is closer to the force required for walking?
This is how I have constructed my quadruple crankset system:-
obtained the adaptor for fixing the 4th chain ring (I use Avid's microadaptor which is no longer made). This adaptor use a BCD (bolt circle diameter) of 58mm so the smallest chainring is 20T.
obtained longer chainring bolts for fixing the adaptor (Campy's chainring bolts for its triple crankset seem to be shorter than normal, so ordinary bolts will be long enough for the extra thickness of the microadaptor).
obtained an extra wide range front derailleur that could handle chainrings from 53T to 20T (this item is no longer produced so the stock is very limited).
added a spacer to the 111mm long bottom bracket. This length of Campy's BB for triple crankset is actually similar to other brand's BB for double. Campy's own double is 102mm only. I have taken a spacer ring from the cogs and installed it on the right hand side. It fits perfectly and adds only about 2mm to the righthand side. So the quadruple crankset almost perform as efficiently as ordinary double crankset.
made necessary modification to the front derailleur to enable it to make downward shifts to chainrings smaller than 30T (sounds complicated but actually just glue a piece of nylon cut off from zip ties to the inner side of the outer cage).
So the project is almost completed. The outstanding task is to either find a 30T chainring with 74mm BCD that has pick up pins, or to fit some rivets to the 30T chainring as pick up pins myself. Currently the chain will shift from the 4th chainring (20T) to the 2nd chainring (42T) in the upshift, skipping the 3rd chainring (30T) which does not have pick up pins because it is originally designed as the smallest (aka innermost) chainring. Downshift has no problem though because downshift does not rely on these pins.
While I have been thinking about riveting pick-up pins on the 30T 3rd chainring to improve upshift from the 20T 4th chainring, an expert on quadruple chainring has sent me an email today suggesting me to chisel off certain teeth for better upshift. I think this practice is also adopted by Shimano and should work. I will try it out some time later.
