yeah without a coupon it's twenty bucks more alright..
BUT, just think of an additional belt and two additional pulleys to buy
not to mention the two additional pillow blocks, the jackshaft itself, the keys... that quickly add up to way above 20 bucks, right?
get the 212 at least
the ratio thing is quite easy actually..
your engines turns 3800 rpms at max, and the ratio reduces that rotation to the rear axle;
by the given factor (6:1 for example means the axle and thus the wheels rotate 633.33 times per minute)
The circumference of the wheels now let you determine the maximum speed;
which is exactly what most "cheap" [as in quickly thrown together] calculators will give let you know.
Two more things need to be considered though, to get a good estimate:
1) the overall weight!
engines produce a rather low amount of torque (compared to your legs for example)
that's why you NEED gear reduction since the torque increases by the same amount the rpm decreases (six times the engine torque in our 6:1 ratio example)
and you need torque to move mass.
2) wheel diameter is part of the ratio
Most people forget that the whole torque dilemma doesn't stop at the axle, instead it stops where the rubber meets the road; everything rotating is basically part of your torque equation.
Putting it all together, it looks quite complicated at first (but is in fact easy)
All you need to know is that you need a certain amount of force to move a mass, multiply that by the wheel radius to know the minimum amount of torque needed;
and then divide by the engines max torque (in the documents) to know the minimum gear ratio you could get away with.
and having that ratio, putting it in the cheapo calc I talked about earlier will tell you the speed you'll achieve.
Or, if you're lazy.. get a decent calculator
(mine for example; at kartcalc.net [free to use, no ads!])
that's basically what I used to get the numbers above (that's quicker than doing it by hand, even for me
)
'sid
