Ron Cooper, Time VXSR, rock lobster, rock lobster, serotta, ritchey, kestrel, paramount
another child stoker question
I just bought a child stoker crank off ebay and it mounts well to my Davidson tandem. It came with a 28 chainring. My crank that syncs with the captain's crank has a 42. I want to mount a 39 chainring to connect with the child crank, should I change out the 28 with a matching 39 or can I leave the 28? I can't mount a 28 because its bolt circle is much smaller than what my crank will support...it looks to be a 110.
should I change out the 28 with a matching 39 or can I leave the 28?
The stokid crank's chainring needs to have either the same number of teeth as the chainring you attach to the backside of your rear timing ring OR a higher tooth count, but never a lower tooth count.
If you use 39 for the stokid cranks and the backside of your rear timing ring you end up with a 1:1 ratio. Therefore, your child's cadence will match your cadence, noting that the short 130mm stokid crank arms make it a lot easier for those short legs to keep up even when adults are spinning a cadance above 100 rpm. This is the preferred and normal set-up for most.
However, if you leave the 28 on the stokid cranks and put the 39 on the backside of your rear timing ring you end up with a 1.4:1 ratio. That means for every turn of your crank your little one will have to turn their cranks 1.4 times; thus, if you were cranking along with a cadance of 80 rpm your child would be spinning their cranks at a cadance of 112 rpm: this would be bad.
Therefore, for very young children, some parents will intentionally fit a chainring to their kiddie cranks that is larger than the one on the backside of the rear timing ring to reduce their child's cadance, i.e., the opposite of the example above where you end up with 0.7:1 ratio such that if you are spinning a cadence of 100 rpm your child is cruising along at a cadance of 70 rpm. Again, this would only be used for a very young child who's legs are merely "going through the motions" of pedalling. Once they actually start to apply power you want to have chainrings with a 1:1 ratio.