[jjc839]

Hi all,

New member here, also new to the EV world. I have a '78 Mini Clubman that I am contemplating an EV conversion. It seems as though the classic mini isn't ideal because of the original integral engine/gearbox and lack of room to fit a different gearbox. Which has led me to believe that a direct drive single speed reduction AC motor may be the way to go.

My initial thought are:

- electricmotorsports.com (or similar) AC-34 AC Motor Drive kit in either 72 or 96V giving 37 or 55hp (max).

- reduction drive in ~6 or 7:1, rated motor rpm of 7500, giving a top speed of approx 60-74mph if using the stock 10" wheels.

- reduction drive mated directly to only one wheel via one CV joint, or possibly both wheels via a dual output 90* gearbox.


I am a decent fabricator/mechanic with basic electrical knowledge, but like I said, totally new to EV's, so please let me know if there are any fundamental flaws with this plan. It will be a city commuter only, so 80km range would suffice, and I don't need a race car. Stock-ish performance would be ok. The battery sizing seems straightforward I just want to know if the AC direct drive is doable.

I did read a thread about a guy using an AC motor in a classic mini, but he went through a pile of trouble to still use a gearbox from a different car and it seems like it would've been easier to go direct drive.

Thanks for any advice.

Jeff

 

[Kevin Sharpe]

It seems as though the classic mini isn't ideal because of the original integral engine/gearbox and lack of room to fit a different gearbox.

What are the overall dimensions of the mini engine and gearbox?

The Tesla front motor/inverter/gearbox/differential from the all wheel drive Model S is surprisingly small at just ~500mm long

At ~85kg the Tesla is also around half the weight of the mini engine/gearbox according to a quote I found - "a mini engine and gearbox is about 330lbs according to my (very old) Haynes book so about 150kg".

Here are some photos showing the front motor/inverter/gearbox/differential from a Tesla Model S 85D... >200HP would also make this a rather interesting conversion

 

[jjc839]

Thanks for that. I think the Tesla motor is quite a bit more powerful so alot bigger. The AC-34motor bolted up to the reduction gearbox should be about 110lbs and 18x9x15"

 

[Kevin Sharpe]

I think the Tesla motor is quite a bit more powerful so alot bigger.

I've just checked a 'small' Tesla rear motor which is almost identical in size/weight to the 'small' Tesla front motor. It weighs 190lbs and is 24" long, 24" wide, and 12" high including the motor mounts. Remember this package includes the motor, gearbox, inverter, and differential. If it would fit in the space occupied by the current ICE, gearbox, clutch, diff, then you'd be saving ~140lbs that you could 'spend' on battery weight.

 

[jjc839]

Thanks, I've also just found an 8.75:1 gearbox from an electric Smart ForTwo, which may also work and already have an appropriate output for CV joints.

 

[Kevin Sharpe]

For future reference it looks as if a Tesla 'small' motor will fit into the space occupied by the ICE and gearbox in a 'classic' mini

Here are some rough dimensions for a Honda B16 engine according to the 16V Mini Club Forum;

"crankshaft pulley to end of gearbox is 850mm

top of cam cover to bottom of sump is 550mm

from the front exhaust heat shield to rear of diff is 600mm"

http://www.16vminiclub.com/showpost.php?p=54701&postcount=4

 

[brian_]

The complete motor and transaxle (transmission with differential) from any common modern production EV is a possibility: all or nearly all of them are configured similarly to the Tesla unit, with a transverse motor, single-speed reduction gearbox, and differential. Of course you want a small one... the Smart ForTwo ED has already been mentioned, and the Mitsubishi i-MiEVis also smaller than most.

Any of these setups will require custom half-shafts, with Mini outer joints and the inner joints which work with the transaxle.

If the original motor is used, a suitable AC inverter/controller will be needed; if a different motor is used, mounting the motor to the transaxle might be a challenge.

 

[brian_]

- reduction drive mated directly to only one wheel via one CV joint, or possibly both wheels via a dual output 90* gearbox.

I strongly suggest not trying either of these. One-wheel drive would be bad at the rear, and nearly undriveable at the front. Driving both wheels without a differential to allow them to turn at different speeds will also make the car almost undriveable, since it will be unable to turn without substantial scrubbing.

You need either a differential (which is included in any transaxle), or separate left and right motors (which also means separate left and right transmissions).

 

[jjc839]

What are the implications of only driving one wheel at the front? Torque steer?

Aside from physical fitment, are there other major factors to consider for running 2 small motors/reduction drives, one for each wheel? Can you run two ac motors from a single controller?

 

[brian_]

What are the implications of only driving one wheel at the front? Torque steer?

Yes - torque steer is the direction of the car changing as drive torque is increased and decreased, and you would have lots of that.

Also, it will pull to one side all of the time, so it will go down the road slightly sideways (called "dog tracking", because dogs often run twisted to one side so their front and rear paws don't hit), with compensating steering needed all the time, and extra drag.

Motorcycles with sidecars do drive a driven wheel on only one side, but it's hard on tires and the car would be worse.

 

[brian_]

Aside from physical fitment, are there other major factors to consider for running 2 small motors/reduction drives, one for each wheel? Can you run two ac motors from a single controller?

The major issue would be cost. It saves a differential, but requires two of everything else (controllers, motors, transmissions), and two small ones is always more expensive than one big one.

Control complexity is the other. One controller won't work for AC, but I noticed a feature in the Curtis controllers that is designed for coordinating two controllers, one for each motor... exactly for this purpose. From the Dual Drive feature description of the Curtis Model 1239E web page:

Most Curtis AC controllers have Dual-Drive functionality built into the operating system as a standard feature. This feature is required for vehicles with dual fixed axle drive motors and a steered wheel or axle, such a 3-wheel counterbalance trucks. The traction controllers must provide an electronic 'differential' effect which prevents excess tire wear and allows the trucks to smoothly execute tight turns. The Dual-Drive function allows the traction controllers (one master, one slave) to proportionally reduce the relative speed of the inside wheel and increase the speed of the outside wheel as the truck corners. As the steering angle increases, the direction of the inside wheel is reversed, giving the truck the smallest possible turning circle. Curtis Dual-Drive provides smooth and efficient control of vehicle speed, acceleration and motor current while turning, and ensures safe operation in the event of a fault with either of the motors, controllers, or the steering angle feedback sensor.

You won't turn anywhere near tight enough for tightest turn feature (it's not a forklift), but otherwise this looks like the required functionality.

Separate left and drive motors is quite unusual in road vehicles, but there are some production examples... so far in exotics and in hybrids.

 

[MalcolmB]

Hi Jeff,
I did a Mini conversion a few years ago using a pair of small series-wound DC motors to drive the front wheels independently through a chain drive reduction to each drive shaft. The motors were powered by a single controller. This worked well with the motors wired in series at the low power level I was limited to with an Alltrax 7245 controller.

I didn't get round to trying higher power levels as I felt the chain drive solution was a bit "agricultural" and was likely to require regular maintenance, which I'd like to eliminate. I'm now looking for a neater solution, so I've taken a more conventional approach this time round and bought a small transverse gearbox from a VW. The issue then is finding a motor that's short enough to fit without requiring major surgery on the subframe.

Kevin's suggestion of using the Tesla drive unit would be great, and it's crossed my mind a couple of times, but the critical dimension for fitting a different gearbox/drive unit to the Mini is the distance from the drive output centreline to the back of the diff casing. This is very small in the Mini since it's limited by the steering rack, which is right behind the diff. The maximum dimension is about 100 mm. If you increase this significantly the drive shafts will be at an excessive angle. Looking at the photo of the Tesla unit I estimate this distance is around 120 mm. Any chance you could measure this Kevin?

Malcolm

 

[jjc839]

Great info guys, thanks. I'm a few hours away from the car right now but once I get under the hood I'll have a much better idea of what kind of space there is to work with. Also going to investigate FWD diffs and see what the smallest is out there.

Jeff

 

[jjc839]

Hmm...just browsing around...anyone consider an ATV diff? Very compact, with gear ratios in the mid 3's. I would think a diff from a newer 700+cc ATV would be plenty strong for what I have in mind.

So, AC motor at 7500rpm with reduction drive of 2:1, driving a 3.50:1 diff, 19.5" tires, top speed of 62mph.

Still need to take some measurements.

 

[Kevin Sharpe]

Any chance you could measure this Kevin?

I'm not sure exactly which dimension you are looking for... any chance you have a picture showing the critical dimensions in the mini drivetrain?

I'll see if I can find someone with a 3D scanner so that we can easily record the critical dimensions from the Tesla motors

 

[brian_]

Two motors (for separate drive to each wheel) on on controller works with DC motors, but not with AC. AC synchronous motors must be driven in synch with the rotor position, AC induction motors are driven at a frequency closely related to the motor speed, and the point of separate motors is to allow them to run a different speeds for turning corners.

 

[brian_]

Kevin's suggestion of using the Tesla drive unit would be great, and it's crossed my mind a couple of times, but the critical dimension for fitting a different gearbox/drive unit to the Mini is the distance from the drive output centreline to the back of the diff casing. This is very small in the Mini since it's limited by the steering rack, which is right behind the diff.

This will typically be a challenge in any front-wheel-drive car with a transverse engine (the layout which the Mini popularized and is now the most common), because except in rare cases they have the engine just ahead of the axle line, so the steering rack ends up just behind it.

The Tesla Model S drive assemblies (motor and transmission) for both front and rear place the motor behind the axle line. Most recent EVs emulate the layout of a gas-engined model and place the motor at or ahead of the axle line; examples include the Nissan Leaf, Mitsubishi i-MiEV, and presumably the VW eGolf, Ford Focus EV, etc.

The maximum dimension is about 100 mm. If you increase this significantly the drive shafts will be at an excessive angle. Looking at the photo of the Tesla unit I estimate this distance is around 120 mm.

That sounds like the front of the Tesla differential housing. The motor on the back will extend much further from the axle line than this... to the point that interference with the firewall (not just the steering rack) is a concern.

If only the steering rack is an issue, it may be feasible to convert the Mini to front-steer (rack ahead of the axle line). The steering column may be able to get past the Tesla hardware on the right-hand side (driver's side in the UK), because the motor is offset to the left and the smaller inverter is on the right; however, with the narrow track and narrow engine space of the Mini I suspect that it will need a shorter right-side shaft, bringing the motor and inverter to the centre and blocking steering shaft access.

For illustration, I have attached a helpful photo from Kevin showing the Tesla Model S drive unit, mocked up with a VW van (aftermarket) rear suspension... seen from the front (from Kevin's VW van thread), plus an overhead view of the dual-motor Tesla Model S motor/transaxle units in position relative to the axles (from Tesla).

The Tesla unit in Kevin's photo has the motor to the vehicle's left side, which means it is a rear motor - the front drive unit puts the motor on the right-hand side of the car.

 

[brian_]

Also going to investigate FWD diffs and see what the smallest is out there.

Keep in mind that in a normal front wheel drive (or rear-engine rear-drive) configuration, the differential is integrated with the transmission (so the combination is a transaxle) so you can't reasonably use just the diff - you would use the transmission gearing as well.

Hmm...just browsing around...anyone consider an ATV diff? Very compact, with gear ratios in the mid 3's. I would think a diff from a newer 700+cc ATV would be plenty strong for what I have in mind.

So, AC motor at 7500rpm with reduction drive of 2:1, driving a 3.50:1 diff, 19.5" tires, top speed of 62mph.

This makes sense to me. You could use a chain or belt drive from the motor to the final drive (diff) input, which would place the motor longitudinally (assuming a longitudinal shaft drive in the ATV). The motor could be above the diff, and to one side to keep it lower. The rear diff from an ATV with independent suspension would put the input shaft and chain or belt on the front side, which you would need to avoid steering rack (and firewall) interference.

The rear final drive (diff) of any front-engined vehicle with independent rear suspension - and rear-drive or all-wheel-drive - would be similar, but of course most of them would be a heavy. They can also be surprisingly long. The Mazda Miata/MX5 has one of the shorter and lighter ones, because of the way the front of it is mounted and the low weight of the car. You would want to avoid electronically-controlled clutch-based units used in some AWD cars.

 

[jjc839]

This makes sense to me. You could use a chain or belt drive from the motor to the final drive (diff) input, which would place the motor longitudinally (assuming a longitudinal shaft drive in the ATV). The motor could be above the diff, and to one side to keep it lower.

This is just what I was thinking, motor on top, longitudinally, except using a front diff so your belt/chain drive would be on the firewall side, hopefully with enough clearance from the steering components.

 

[brian_]

This is just what I was thinking, motor on top, longitudinally, except using a front diff so your belt/chain drive would be on the firewall side, hopefully with enough clearance from the steering components.

A front final drive from an ATV might be small enough for this work; however, some of them do not contain a differential - they just have a bevel gear drive. This works on dirt (especially since these things can usually be disengaged to make it a rear-wheel-drive vehicle), but not on a hard surface. Early ATVs didn't even have a diff in the rear axle, and some still don't!

While the rear diff from an ATV seems suited to the load, I would wonder about a front diff handling all of the torque needed to move the Mini; an ATV front end is pretty light.

The front diff from any front-wheel-drive car will be integrated with the transaxle, so there's no potential there. The front diff from a more traditionally laid out four wheel drive (truck, SUV, or car) is a separate unit, but the input shaft is probably too long to avoid conflict with the steering rack.