All the gear and all the ideas….

In a new series of articles, the technical boffins at Electrogenic will be providing us with a series of expert pieces covering off some of the more common areas of Retro-EV conversions. 

Each week Steve, Ian and the team will be delivering some behind the scenes explanations around key parts of the conversion process, we’re kicking off with one of the most commonly misunderstood areas.

In the world of Electric Vehicles, gearboxes are an interesting subject, and one of much ongoing debate and development at Electrogenic. Because all new Electric Vehicles on sale are automatics, those new to the world of electrifying classics would be forgiven for assuming your conversion should also be automatic, but this could not be further from the truth.

Production EVs don’t have a gearbox, instead they utilise a reduction gear. There are 3 main reasons for this. Firstly, a gearbox is complicated. Secondly, its more expensive and thirdly, an electric motor doesn’t need one due to the linear power delivery and the high operational speed, with some motors spinning past 20,000 rpm.

But with your Retro-EV you already have a gearbox and you need to connect the motor to the wheels.

Your original ICE needs a gearbox because the torque required to accelerate, or the power needed to keep you going as you labour uphill, is only available within a narrow band of engine rpm.  Try driving away from standstill in your ICE powered vehicle in a high gear and the engine will stall as it does not have enough torque to pull away. However, do the same in a Retro-EV and the car will simply go.  

This is because electric motors have all of their torque available from zero rpm- so you don’t need a gearbox to steam away from the lights. Don’t confuse a flat torque delivery with a flat power delivery though. The power provided actually climbs throughout the motors rpm range, just like an ICE (power = torque x RPM), but that’s an explanation for another day.

Your car needs torque at low revs and power at high revs, so an electric motor is a great fit. It is also worth pointing out that gearing can multiply the torque seen at the wheels. Remember the old racing adage of “bhp sells cars but torque wins prizes”. 

Keeping the gearbox contributes to the “torque grin” you experience when driving your Retro-EV, and of course it makes your classic still drive like it used to (only better!). But its also a cheaper and less complicated solution, as removing it entirity means you don’t have to substitute it with an alternative reduction gear (electric motors typically run at 8,000rpm +) or find complicated drive train fabrication solutions.

Using the original gearbox with an electric motor is not straightforward, however.  When you take your foot off the accelerator in your ICE vehicle the engine reduces its rpm very quickly, which provides a natural engine braking and helps ensure that the clutch and transmission speeds are matched when shifting gear. 

An electric motor doesn’t reduce its rpm, so you have to reduce the motor speed electronically.  It also requires a certain presence of mind if you accidently hit the accelerator and the clutch at the same time,.

Most Retro-EV drivers only use one or two of the original gears, often being 3rd and 4th ( or 5th if you are lucky enough to have a 5 speed box) this is because the torque of an electric motor can be too much for the lower gears, its also noisy and inefficient to keep all that unnecessary metalwork constantly turning.  

Many people initially assume that they will need an automatic gearbox for their conversion, however conventional wisdom states that “automatic gearboxes don’t work well with electric motors”. Primarily, an older auto boxes needs the engine at idle to maintain hydraulic pressure which can be very difficult ( and expensive) to replicate.

Newer auto boxes are essentially manual boxes with electro-mechanical control.  On the face of it this could be mapped to the motor controller, so theoretically it should be possible.  To date though, for large automatic cars we have taken the easier route of simply replacing the automatic box with a fixed reduction gear.  

So what does this mean?  To minimise cost, start with a manual box (but remember it doesn’t need to be the one that your car was born with) and understand that there are pitfalls as well as benefits to this, which can require complex solutions.  If cost is less of a factor, install a bespoke reduction gear and go “automatic”.

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