Overdrive - why not?

[Ferretusmaximus]

When I was a nippa many cars were fitted with an overdrive gear activated by a switch on the gear stick or a stalk. It gave you a higher cruising gear. Has anyone seen anything like this in an EV? With a reduction gearbox and buckets of torque available in every EV it would seem an efficient way to gain extra range without the need to add more heavy batteries.

 

[siteguru]

Very few EVs have more than a single speed gearbox ... I think maybe the Porsche Taycan has a 2-speed box.

I guess the equation is one of range vs reliability ... using a multi-gear box adds a failure point.

 

[okohll]

Yes I think electric motors don't reduce in efficiency half as much as 4 stroke engines at high revs. I've not looked into the details but you can imagine for example the physical mass of a piston having to change direction many times per minute has a greater effect, plus heat losses from the explosions, friction etc.

Whereas in an electric motor with the electromagnetic force acting at the speed of light anyway, however fast the thing is rotating, you just have to adjust the timing of the pulses to pull on the magnets, you don't necessarily need more energy. Of course there will be more losses at some points (eddies? inverter? heat due to rotation?) but probably not as much

 

[Gomev]

When I was a nippa many cars were fitted with an overdrive gear activated by a switch on the gear stick or a stalk. It gave you a higher cruising gear. Has anyone seen anything like this in an EV? With a reduction gearbox and buckets of torque available in every EV it would seem an efficient way to gain extra range without the need to add more heavy batteries.

I remember overdrive well, but how many modern ICE/gearbox cars have overdrive, let alone an EV.

 

[decrep]

I remember overdrive well, but how many modern ICE/gearbox cars have overdrive, let alone an EV.

My Commodore has 9 gears.
At 100km/hr it's doing just over 100RPM
And doesn't get into top gear until you're doing more than 90km/hr.

That sounds a lot like overdrive to me.

Most modern cars are looking for economy, and minimum pollution, a high final gear is a must.

 

[m4rmite]

My Commodore has 9 gears.
At 100km/hr it's doing just over 100RPM
And doesn't get into top gear until you're doing more than 90km/hr.

That sounds a lot like overdrive to me.

Most modern cars are looking for economy, and minimum pollution, a high final gear is a must.

100rpm? Doesn't it stall?
Or do you mean 1000rpm?

 

[decrep]

of course, you have to allow a bit for this silly old fart.

 

[Ferretusmaximus]

My Commodore has 9 gears.
At 100km/hr it's doing just over 100RPM
And doesn't get into top gear until you're doing more than 90km/hr.

That sounds a lot like overdrive to me.

Most modern cars are looking for economy, and minimum pollution, a high final gear is a must.

Overdrive was usually delivered by means of a separate gearbox after the main gearbox in ICE cars. This meant it could be sold as a purchase option or as an after-market accessory. It also allowed it to work on 3rd and 4th gears back in the days of only 4 gears. I was mainly thinking of overdrive as a range extender by means of a tweak to the reduction box, most ICE cars have complicated gearboxes these days which seem to be quite reliable.

 

[Rolfe]

I always thought of fifth gear (or indeed sixth in my Peugeot and my Golf) as "overdrive" but I was corrected on that. I don't know the difference though.

 

[Archev]

I always thought of fifth gear (or indeed sixth in my Peugeot and my Golf) as "overdrive" but I was corrected on that. I don't know the difference though.

Overdrive always sounded “ high tech” and wonderful but basically was an admission by motor manufacturers that for the price they couldn’t build gearboxes with enough gears to suit the ice engine. This latter had a fairly limited power band so you had to go in steps through the gearbox.
They then used the same gearbox in perhaps a lighter car or a more highly tuned version.
They found the vehicle was over-revving or screaming when in top gear so fitted another gearbox after the main gearbox. They called it overdrive so it sounded good and they could charge extra.
It was a dream to be able to build an engine with such power and torque that such things were unnecessary.
Don’t ya just love driving an ev and realising you don’t ever need to move to a higher (or lower) gear?
Enjoy ?

 

[Rolfe]

Yes. Even though my previous car had a DSG which worked almost uncannily well.

 

[tsedge]

Gears? What next, starting handles?!

As has been pointed out, EVs don't significantly lose efficiency at higher RPMs so the losses of having a second gearbox to provide the overdrive would outweigh any benefits.

 

[johnb80]

I always thought of fifth gear (or indeed sixth in my Peugeot and my Golf) as "overdrive" but I was corrected on that. I don't know the difference though.

Overdrive means the ratio in the gearbox is better than 1:1. In a non overdrive gearbox the highest gear would be 1:1, 1 revolution on the input shaft = 1 revolution on the output shaft. Modern cars instead of adding an overdrive unit on the output part of the gearbox simply add another ratio or two, once you exceed 1:1 you then have an overdrive box.

 

[Rolfe]

Thanks, I see.

 

[decrep]

I guess it's easier to adjust the diff ratio to get the final drive ratio to the wheels, than using an overdrive in the gearbox.

 

[johnb80]

I guess it's easier to adjust the diff ratio to get the final drive ratio to the wheels, than using an overdrive in the gearbox.

That would compromise the performance 0 to 60 etc

 

[fnegroni]

Gears do help both acceleration and top speed of an EV.
Formula E has multi-speed boxes.
In terms of efficiency though, not so much.

And considering for the public road both acceleration and top speed are good enough with the lowest power EV on the market…

 

[shpub]

According to In-depth: the Formula E powertrain explained
"Formula E cars has two speed boxes - a lower speed gear for slow hairpins and starts but as of 2020 no Formula E car has a multispeed box."

They certainly has 5 speed boxes initially but the option of single speed was allowed with the gen 2 cars. The gen 3 cars have four wheel drive but with no gearbox connecting the transmission.

Having gears can improve acceleration - providing the rest of the system ie tyres can cope and it can allow higher top speed providing there is enough torque available. As for efficiency it all depends on whether the motor revs keep the engine in its efficiency sweet spot. Lower revs will cause the motor to be less efficient in most cases. This increase the power needed and reduces the range.

 

[dsimpkins]

I'd be concerned about the torque of an electric motor and the longevity of an overdrive. They are usually used in higher gears on ICE cars once most of the acceleration is done. My old Range Rover has an overdrive, and a big warning sticker on the dash that states the overdrive can be used in third and fourth gears only. The torque of the V8 is too much for the overdrive in the lower gears.