Post by Dave Plowman (News)Post by Dave Plowman (News)I remember one CVT equipped car (rubber band drive) who used software to
make it work like fixed gears.
I‘m curious, which car was that?
Sorry, can't remember. Not really of much interest to me. Although I've
heard some complain about the way a CVT can stay at near constant engine
revs when accelerating. But whether this was after buying one an living
with it for a while, I dunno.
They all do it and as far as I know none of them have a ludicrous mode
that Coulthard tested in Williams F1 car and instantly knocked several
seconds off his lap times.
Maximum acceleration is at maximum power. Peak torque corresponds to
peak acceleration in a fixed gear but the acceleration at peak power in
the gear below that is far higher.
So when you floor the accelerator pedal what should a CVT do? You have
sent a signal that you want everything it's got. It should give you
every thing it's got. That means winding the engine speed up to peak
power rpm and then continuously adjusting the gear ratio to hold it at
that speed.
A normal manual gearbox just gives you what the engine can deliver at
the current engine speed and you have to sit and wait for revs to rise
as the car accelerates and flatly farts. If you do this at 45 mph in top
gear you can be waiting a very long time for the car to get a move on.
If you want more you have to predict the need and change gear. Rule of
thumb if in doubt change down 2.
So there were 2 objections to CVT gearboxes.
First there was lag while the engine wound up to speed. The extra power
the engine made is initially used to speed up the engine and not the
car. The fact that with a normal gearbox you should be changing gear
during this time just isn't allowed for.
Second. Once wound up, the peak engine power is delivered to the wheels,
like all the hounds of hell let lose. It didn't change down 2 gears, it
went down 3.3. "Well I just wanted to go a little faster, I didn't want
Mach 2!". The only way to ask for "a little faster" is a smaller input
on the throttle, instead of mash it and lift off when you have enough.
So all CVT on the market pretend to be fixed ratio gear boxes. That
means the belt/chain/disc is always running on the same parts of the
cones/toroid. That makes tracks and the CVT fails. There's not supposed
to be any contact as the motion is transferred by a non Newtonian fluid
that becomes solid like silly putty when under shear loads.