16:64 or 20:80

[Deleted member 21174]

I know this is an old thread but it provides support for the OP's hypothesis that bigger sprockets are faster (at least at speeds < 70 mph). (and that a ratio is not a ratio).

Optimisation of the chain drive system on sports motorcycles

Also suggests that 219 chain would be faster than 35 chain and that proper chain tension is critical.

(The full text is out there if one googles enough).

 

[Bob Evans]

This is just a gut feeling, but it might be worthwhile to look at a 18 -72 for a 4 to 1 ratio.
I say this because a 15 -60 was always my go-to for a 4.00 ratio, that was with #35 chain. With 219 chain, a 18 driver has the same pitch diameter as a 15 tooth #35 driver.
Fwiw.

 

[Messick Racing 320]

This is only my opion!!!!! This subject is inconclusive because there is too much missing data. The dyno info is merely a hypothesis of data to real word racing. What your looking for is called torque to turn ratio. When the testing was done with the dyno between 16t and 20t the reason it picked up .05 is because of torque to turn ratio. Your changing the curve of torque. Smaller driver equals more low end torque, larger driver equals more top end speed, that's why you seen the .05 difference on the dyno. There was never any info of lap time difference between the two drivers, which we all know at some point one Is better than the other. The only way the two would be totally identical would be on track conditions which would be near impossible. If the track your racing at has long straight a ways and sweeping turns the 20t will prevail on the long run. If you have tight turns and short straight a ways then 16t will prevail due to torque off the corner. Any driver in between can win due the fact of air pressure changing ratio and track conditions.

 

[knighty]

for me the data is conclusive.......and your hypothesis is slightly wrong, but not completely.......bigger driver equals more bottom end torque due to friction reduction........since conducting the tests several years ago I have concluded its a simple case of frictional gains, whereby spreading the same load across less teeth increases friction and causes more drag........these engines are so low powered they are affected by such frictional losses.

If you imaging picking up a heavy grocery bay with just 1 finger really starts to hurt your finger due to excessive contact pressure.......then pick it up with 3 fingers and you have spread the same load across more area and therefore reduced the contact pressure on your fingers and it hurts a lot less.

Typically in the UK we run on 19 and 20 tooth drivers........I now have a handful of customers running drivers with even more teeth on fast circuits and they are very fast........all the logic stacks up for me that bigger drivers are faster, the only down-side is that on tight and twisty circuits going too big on the rear axle sprocket does start to hold the kart back.......so then you start to use 19 tooth drivers, some have been 18 tooth with great results on tight and twisty tracks.