Ask The Editors: Semi-Automatics 101
Because even the mere risk of stalling sucks.
In the hierarchy of ATV drivetrain options, the method you refer to has come to be known as a semi-automatic transmission. The alternatives are the fully automatic CVT drive (which mimics that of a snowmobile) and the fully manual transmission (which uses a bar-mounted clutch just like a motorcycle).
In answer to your first question- indeed we have the centrifugal clutch to thank for the semi-automatic’s success. These operate, as the name suggests, by using the outward pull of anything in rotation. As engine revolutions per minute (RPM) increase, weighted arms in the clutch swing outward and force the clutch to engage automatically. However, it operates on almost the opposite principle of what we usually associate with clutch engagement.
With a standard clutch, when engaged, it would separate the engine and driveshaft (chain) so applying throttle at that point would simply rev the engine out but result in no forward motion.
With an automatic clutch, the engagement that happens due to centrifugal force turns the center shaft as well once those weights are extended to make contact with an outer drum. In other words – the RPMS must increase to get the machine into forward motion. Drop the RPMs- either slowly or abruptly, so that the weights retract and the shaft is automatically disconnected once more. That’s why it’s impossible to stall the machine.
In answer to your final question, yes they can idle in gear so hopping on and giving it some throttle means you’re off and running. However, when it comes to pushing this concept to the extremes, not necessarily. While it is true you could pop the gear select shifter at your left foot into a much higher gear than first or second and engaging the throttle wouldn’t immediately stall the machine, the strain of pulling too high a starting gear on the engine would be felt just the same (bucking, stuttering and possibly finding false neutral if it were unable to build the RPMs necessary for full clutch engagement). You may be able to force it into happening were you to keep steady throttle against the protest of the engine but clicking it down a gear or two would be the far more sensible option.
Lastly a lot of older scooters and even modern minis use another type of automatic transmission that makes get-up and go possible with no risk of stalling – an automatic clutch coupled to a single, extremely tall gear. This is likely what you experienced back in the day. A single speed that was equivalent to roughly 3rd gear in a standard gearbox that doesn’t stall on the bottom end due to the centrifugal clutch discussed above but then pulls until it starts to rev out. Despite feeling like it needed it, these couldn’t be upshifted into the next gear.
