Getting the correct gear ratio

Peter Diffey said:

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Every EV I have looked at seems to mount the motor on the old car gearbox, I am unsure why.

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That's typical DIY conversion practice, but when a serious manufacturer builds an EV - even on the platform of an existing gas-engined car - a conventional multi-speed transmission is not used. On the other hand, they're using PM AC (or in older designs, induction) motors with a high enough supply voltage that they have a very broad speed range is nearly constant power available.

Peter Diffey said:

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Every EV I have looked at seems to mount the motor on the old car gearbox, I am unsure why.

The car final drive ratio is 3.66:1 which gives top gear spread of 16.7 mph at 1000 revs. So 50mph is 3000 revs. The AC50/1239 torque curve is flat all the way to 4000 revs.

My understanding is that as the torque is pretty well constant over the urban speed range, there should be zero benefit in using high revs to pull away.

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Your calculation shows that top gear is too high if you only want to go 50 mph. A lower ratio would provide more torque to the wheels for a given road speed, and thus better performance, but a lower gear will also limit top speed... and so you see why a choice of ratios can be beneficial with a power band that looks like a pyramid.

Peter Diffey said:

I am wondering if I really need to keep the gear box, as it just seems to be a 80lb mounting bracket. It looks like it is easier to machine a motor flange that bolts direct to the prop shaft UJ. This would allow the motor to sit where the gear box went. This both reduces weight and frees up space for batteries. OK I will have to fab more mounting brackets and I may not be able to push the motor in exactly the old gear box position.

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That all makes sense, but it would be surprising if the AC-50 can be jammed anywhere close to the original gearbox position. Transmission tunnels can be modified, of course, but in the case of the Spitfire there are frame rails pretty close on each side. In a custom-built car, or one with a huge transmission tunnel (maybe it came with a big automatic transmission), there can be space to do this. A smaller-diameter motor can fit, too, but usually needs a reduction gear to operate at a suitable speed.

Also, think you'll want a little more than a shaft adapter. The "TransWarp" versions of Netgain's DC motors that are designed to be used this way have a housing (from the tail of a GM transmission).

Hi Brian, thanks for the response.

I am aware that AC50 may not fit all the way back so that I can use a standard length spitfire prop shaft, but it should allow me to push the motor back by 9in or so. Making a longer shaft is easy, there are plenty of old prop shafts around, so I can splice 2 together.

The AC50 has mounting holes on both end plates, so I can fab chassis mounts to fit the end plates. making a steel flange to attach to a lengthened prop shaft looks easier than messing around getting exact alignment between the motor and gearbox drive shafts. A couple of thou out of alignment can knacker the bearings.

I still get the sense that using a gearbox is "conventional wisdom", ie "that's the way everybody does it so it must be right". I really want some performance data for an AC motor. As my engineering instincts tell me that there will be little difference in bottom end performance.

I guess my aim is a car that gets off the line at the same speed as a cooking manual ICE car, I don't want to burn rubber. I want to be able to reach UK motorway speeds although I do not envisage going on the motorway - that's not what spits were ever about. I just want to create a cool little car for running out to local motor meets

Peter Diffey said:

I am aware that AC50 may not fit all the way back so that I can use a standard length spitfire prop shaft, but it should allow me to push the motor back by 9in or so.

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That sounds reasonable. It essentially means pushing the rear motor face back to about the front of the actual gearbox, so the rear of the motor occupies the space originally occupied by the bell housing (and enclosed flywheel and clutch) of the transmission. This is what Tom was able to do in his Suzuki Cappuccino Conversion. That still leaves a lot of motor in the engine space.

This approach to positioning would work for any motor up to the diameter of the bell housing, so perhaps a large-diameter but short "pancake" motor (most of which are axial-flux) would be a good fit? Unfortunately I don't know of any which are reasonably priced (e.g. a YASA P400 fits the size and power requirements, but costs more than most DIY converted cars are worth).

Any conversion which eliminates the transmission and doesn't occupy the original gearbox space, and would benefit from additional reduction gearing, could use a single-speed gearbox mounted on the output end of the motor. I have not found a reasonably priced commercial product for this, but if you can stand the US$3500 price - more than an entire typical Spitfire is worth - the ev-TorqueBox could work (and this is exactly what it is intended for).

Peter Diffey said:

I really want some performance data for an AC motor.

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Performance data for the motor, or for a car using the motor? HPEVS publishes motor performance graphs for their motors under various conditions.

Are you following Oliver's How to define the best motor/ gear box discussion? He is using the AC50 as one potential motor, with VW gearing (but any car could be similarly analyzed). No transmission simply corresponds to a 1:1 transmission ratio.

Running spitfire and motor numbers from the internet through my home brew calculator (seems to be within .3 of a second on 0-60's for petrol base vehicles nomally), I get the following comparison:
Spit1 by Nathan Gabbott, on Flickr

I'd see if an early Herald diff core (4.88) would go in a spitfire housing with the right output shafts as you can use more of the motors rpm and get a better launch:
Spit2 by Nathan Gabbott, on Flickr