# Planning an old 70's Mini conversion



## TX_Dj (Jul 25, 2008)

Behn,

At least over here stateside the reason why no-one (or not many) have done a conversion on a classic mini is due to the fact they're just about non-existent here.  Since a lot of the conversion craze (but not all) started in the states, this probably has something to do with why you're not seeing any.

I think a classic mini would make a great donor, it's light, relatively aerodynamic if only due to it's small stature, BUT... it's not going to fit a big motor or big batteries and make lots of range... 

You could probably get away with the stock transmission, 6.7" motor, and 72v or less and still have an in-town cruiser with a 20-30 mile range at best. 

If you want to get anything more than that, you're certainly going to be looking at lithium batteries ($$$$$$) and/or a custom chassis to carry all the batteries and weight.

It may be advisable to keep your ambitions in proportion with the vehicle being converted. Having said that though- nothing is impossible. Make it happen and prove me wrong. I'd love to drive a MinEV


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## madderscience (Jun 28, 2008)

The biggest challenge with the Mini (based on what litttle I know about them) is going to be hooking up the motor. A typical DC conversion loses the engine but keeps the transmission unit. In a Mini, the engine and transmission are a single unit: The transmission is in the engine sump. A fellow in seattle here has a honda 600 sedan which is a mini clone that is converted and it has the same design. He had to do some clunky and inefficient things to get a 6.7" motor hooked up to the transaxle like using a belt drive from the motorl to the engine crankshaft (which drives the clutch via a gear).

Your best bet would be to try to find a transaxle out of a different vehicle that will easily lend itself to being attached to an electric motor, but still fit in the mini.

The other, possibly better solution would be a direct drive AC motor with a single speed gearbox. With the small mini wheels you'd probably want a 6:1 gear ratio if you can get something like that but most are a higher ratio. But going AC means more $, and you will probably want want to design for a higher voltage battery, meaning more, smaller cells. 

If you can keep the weight down, you might also be able to do a homebuilt dual motor direct drive using two perm-132 pancake motors. Use a motorcycle chain gear reduction of about 6:1 ratio to each one, and each one drives one wheel so you don't even need a differential. You'll need to either use dual controllers or research if it is possible to hook two brushed DC motors in series, but its worth a thought. Somebody with an NEV-class car in Germany has done this but I can't find the link right now.

Size is definitely on your side. If you can achieve about a 40% ratio of battery weight to overall weight, you should be able to hit a maximum range of about 70km, or 45 miles or so.


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## xrotaryguy (Jul 26, 2007)

The Suzuki Swift tranny sounds like it should be a slick upgrade for this car. You may want to take some measurements and make sure that the motor will not hit the fender. that car is pretty darn small, so you never can be too careful when sizing parts.

Going with Li ion batteries in a super small car like this is a good way to go since you will not need very many of them compared to a larger, more conventional conversion. 

I am not a big fan of British cars, but because this one has a roof, because you are changing the trans-axle and because the car actually has a back seat, I think you are making a very good decision.


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## MalcolmB (Jun 10, 2008)

Hi Behn
Sounds like an excellent choice for a conversion, but then I'm biased as I'm planning to do the same 
Here are a couple of links you might like:
http://homepage.ntlworld.com/electric.mini/
http://www.theminiforum.co.uk/forums/index.php?showtopic=55272&st=90&start=90
The interesting bit in the second link is the photo near the bottom of the page, which shows what looks like a Warp 9 and Swift gearbox mounted on a mini front subframe. It's supposedly located in your neck of the woods, but I've not been able to track down any more information.

My plan so far is to use a pair of Prestolite 7" motors to drive each front wheel indpendently, with duplex chain drives to sprockets mounted on the inboard ends of the drive shafts. I was tempted to go the Suzuki Swift gearbox route myself, so I look forward to hearing how you get on!


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## skullbearer (Jul 9, 2008)

If I were to take a shot at a MiniEV using the older models (non-BMW models) I'd look into yanking some similar wheels off something that had RWD, cutting some holes and welding some boxes for two motors in the boot. I figure I would have been using the boot for the batteries otherwise, and the original engine compartment gives me a huge battery space, and potentially if I pack it together and vent it right, a good fiberglass flat surface would give me a bigger boot in the front as well!


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## behen (Aug 23, 2008)

Thanks for all the great advice guys!
You've given me a lot to read and think about 

There's a few things I wanted to ask:

* I was wondering whats the point of a gear box?
As I've been reading its to help the IC engine produce torque at low speeds and stay in reasonable rev limits at high speeds.

Why exactly then do I need a gear box when the electric engines will produce enough torque at low speeds, and wont be spinning fast enough to rev out at high speeds?

Can I just hook the engine straight into the diff/trans axle instead? Or connect the engine straight to the axle using a chain & sprocket? That would even let me set a ratio of 6:1 with different sized sprockets, yes?

* If I remove the back seat and put lithium batteries there, above the engine in the engine bay, and in the boot, I should have enough space for more than my needs with batteries. This will obviously mean I can only take 1 other passenger, but I think I can cope with that!



xrotaryguy said:


> and because the car actually has a back seat


Sorry xrotaryguy, I guess I've lost your support now 

* That of course brings me to my next more pressing question. How do I go about working out how many and of what specification lithium batteries I need.
I've read a few forum posts about working this out, and done a fair few calculations, but nothing has worked out for me yet. 

Any tips on that one? 

* I was also thinking I can possibly drop the top speed from 110km/h to about 90km/h. I never use the highways anyway. Will this make a huge difference in battery requirements or is it more use having the highway capability there just in case?

Oh and I'm definitely going to be using lithium batteries 



MalcolmB said:


> Sounds like an excellent choice for a conversion, but then I'm biased as I'm planning to do the same


Malcom how far into the planning process are you? May I ask for some more information on what you've come up with so far? Got any other good info links that have been helpful to you?

Also the swift-gearboxed mini your referring to on the miniforum is being built by a guy here in Australia. His build thread is over at Ausmini: http://www.ausmini.com/forums/viewtopic.php?t=42150



TX_Dj said:


> I'd love to drive a MinEV


Genius! Mind if I borrow that? I've been trying to think of a name for a while!


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## MalcolmB (Jun 10, 2008)

Thanks for the link to the Aussie site! Will be following that guy's progress closely.

I'm still relatively new to this myself, so don't take anything I say as gospel. So far I have a fibreglass mini lookalike (Domino mini), which doesn't actually weigh a lot less than a real mini, but does have the advantage that it doesn't turn into ferrous oxide in front of your eyes in the British climate. I've also got a pair of army surplus 7.25 inch series motors that weigh 40 kilos each

I intend to go lithium as well. I'd rather go light and fast than have long range, so I'm planning on a 108V 60Ah LifeBatt pack. The cells are expensive but they're good for 10C+ and the pack should only weigh 80kg.
This will be split in two and hopefully fit either side of the boot, leaving a small space for storage in the middle.

I'm going for a single ratio transmission, probably 4.5:1, driving the front wheels independently. The aim is to have a top speed of 70 mph (110 kph) combined with decent mid-range acceleration. Higher voltage would naturally give me a better top speed but I'm limited by the battery chargers that I've already bought. A mini isn't exactly designed to slice through the air anyway, so battery range would be pathetic above 60 or so. I could probably get away with a single motor if I used a gearbox, as a single ratio means you need a bigger motor (or two motors) to get sufficient torque at the upper and lower ends of the rpm range. At the moment I'm pretty much set on a 1000A logisystems controller. 

Haven't worked out the mechanical details yet, but basically I need to make some bearing plates to support the inboard ends of the drive shafts, fit a sprocket on each end and drive these with chains that will run in enclosed guards. The motors will sit in front of and above the drive shafts. The drive system might seem pretty agricultural, and people have told me it's likely to be noisy, but I like the KISS approach – sort of fits me 

An alternative way to get direct drive with a single motor would be to use a chain driven diff, something like this: http://www.westhouse.plus.com/chaindrive1.htm


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## TX_Dj (Jul 25, 2008)

behen said:


> * I was wondering whats the point of a gear box?
> As I've been reading its to help the IC engine produce torque at low speeds and stay in reasonable rev limits at high speeds.
> 
> Why exactly then do I need a gear box when the electric engines will produce enough torque at low speeds, and wont be spinning fast enough to rev out at high speeds?
> ...


Indeed you can... what a gearbox really does though is multiply the torque from the input to the output. Ignoring inefficiencies from friction and such, if you have a 3:1 ratio in a gearbox and put 100 ft-lbs torque on the input, you get 300 ft-lbs torque out the output. It is advantageous to have a gear reduction for this effect.

Because electric motors are essentially a "short circuit" at 0 rpms, the less time you remain at low rpms, the less overall current is flowing thru the motor, and the longer your motor will last. Drag racers us a direct drive setup often, because they are not looking for maximum longevity of the motor, but rather the maximum speed and minimum time to get from the start to the finish. If they can eliminate gear changes and still meet this goal by pumping 1000-2000 amps into the motor to get the vehicle rolling, then this is to their advantage. Even still, they typically direct drive the rear axle, which is a gearbox. 

In the context of on-road driving, it is advantageous to have additional gear reduction and a few ratios to choose for the type of driving and speed you are operating at. Whereas 1st gear may only get you 15 mph, it takes less effort on the motor's part to move the vehicle from a stop in 1st gear, and that translates into less amps drawn from the battery and overall greater range. Even still, most owners find that 2nd gear is an acceptable ratio to roll out from a dead stop.

In the case of series-DC motors, they are far more efficient at higher RPMs, and most of the reputable motor manufacturers will show you charts with the max efficiency provided. In the case of some of our more common motors like the ADC 8" and 9", this RPM is around 4000-5000 rpm. The motor *wants* you to operate it at that speed, which is why so many people tend to "only" use 2nd gear, because in many vehicles 2nd gear is low enough that the motor doesn't struggle too hard to get you rolling, but around 4000-5000 rpm is enough to push you down the road at 40 mph. Switching to 3rd will take most EVs onto the freeway at 60 mph very near the peak efficient RPM. 

The faster one of these motors turns, the higher the voltage is at the motor, thus the amperage is reduced, which also turns Peukert's law towards your advantage vs. turning slower and drawing more amps.



> * If I remove the back seat and put lithium batteries there, above the engine in the engine bay, and in the boot, I should have enough space for more than my needs with batteries. This will obviously mean I can only take 1 other passenger, but I think I can cope with that!


You'd be surprised. Lithium packs designed for EV use are very small. Each 3.2v nominal cell is only the size of a large paperback book, and they're designed to strap to each other. Also, they are orientation-agnostic, so you can lay them on their sides, turn them upside down, whatever is necessary for them to fit. If you have a couple cubic feet of space under the rear seat, you could easily store [email protected]+ Ah worth of lithium cells, even more in the boot or under the bonnet.



> * That of course brings me to my next more pressing question. How do I go about working out how many and of what specification lithium batteries I need.
> I've read a few forum posts about working this out, and done a fair few calculations, but nothing has worked out for me yet.


Well, it all depends on how much physical volume you have to store the batteries, and how much weight you can safely carry while still having enough gross weight available to carry yourself and desired # of passengers. Typical sizes for the "good" lithium cells are 3.2v nominal, and between 80-160 Ah capacity per cell. The Ah rating is going to set your range. If you can fit 500 lbs of batteries at maximum, that is going to have a direct effect on the Ah capacity at your desired voltage.



> * I was also thinking I can possibly drop the top speed from 110km/h to about 90km/h. I never use the highways anyway. Will this make a huge difference in battery requirements or is it more use having the highway capability there just in case?


With lead acid it will make a bigger difference than lithium, but lithium will also be affected by the speed you drive... just to a lesser extent. You can always design with a higher maximum speed than you will use (it's handy to have a little extra power sometimes) but then only use the speed you require for your day-to-day needs.



> Oh and I'm definitely going to be using lithium batteries


More power to you! With as slow as my project is currently progressing, I may just be able to save up enough to go Lithium by the time I buy my batteries. 



> TX_Dj said:
> 
> 
> > MinEV
> ...


I don't think I came up with it, but sure. I just want my cut if you start marketing a vehicle with the name unless someone else can prove they came up with it first.


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