102 replies to this topic

[MalcolmB]

First off – hello everyone – I've been lurking for a while hoovering up as much information as I can.
Still don't know much about minis, so I'm hoping for a bit of help with the mechanical side of my conversion.
The reason I first stumbled on this site was an interest in electric vehicles. I built an electric motorbike a couple of years ago, which was good fun and went surprisingly well – but I'm soft and don't like getting wet, so I started looking round for something with a roof that was light, fun and fairly cheap. The result was a Domino mini in half decent condition. It's currently got an asthmatic 1275 with discs up front.
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The plan is to mount a pair of series wound DC motors across the front subframe, shaft to shaft. Each motor will then drive the inboard ends of the drive shafts independently via a pair of toothed belts, so one motor will drive each front wheel. The motors will be wired in parallel to a single electronic controller, the theory being that they'll play nicely together and act something like a limited slip diff.
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I'm going to make up a set of bearing carriers that will support a pair of stub shafts in place of the existing diff. Each stub shaft will carry a timing pulley – as large as I can fit. The bit where things start getting hazy for me and where I'd appreciate some advice is making the connection between the stub shafts and the drive shafts. I'd like to stick with the pot joints I already have as they're easily available and pretty efficient from what I've read. What I've not been able to work out so far is whether there's supposed to be any lateral movement between the output shaft and the pot joint in normal use? Is the pot joint meant to be able to slide on the output shaft splines? If not, then the easiest approach I can see is to weld the pot joints to my stub shafts. Would that work, or can anyone suggest an alternative?

Cheers
Mal

[Ethel]

Interesting,

Pot joints are fixed solidly to the diff, the inner member of joint slides inside the 'pot' to accommodate suspension and steering travel. Hardy Spicers might be easier to use as they bolt on to the diff the same as a propshaft on a classic RWD setup.

[MalcolmB]

Hardy Spicers might be easier to use as they bolt on to the diff the same as a propshaft on a classic RWD setup.

Thanks. The Hardy Spicers definitely look easier to use. I've read that they sap more power though. Are there any figures on relative efficiencies or does anyone have a feel for the difference?

[Ethel]

I doubt there's a massive difference. Pots will be marginally smoother being a form of CV. Pots might not be too bad in other regards - find an old diff and you'll have a shaft that fits them and a suitable bearing, with a bevel gear on the other end to butcher to get the drive in. What rpm do the motors like, it may be using more of the diff would be handy.

[mrslaphead]

Check out www.plasmaboyracing.com Theres an article in last months retro cars magazine about a guy who has built
a datsun electric car that puts out 772 ft/lb of torque using a "custom siamese 8 dual armature series wound motor"
which I think is two 8" motors joined at the head.
The only info it mentions on drive shafts is custom but the website might be helpful.
Hope that helps
Chris

[MalcolmB]

What rpm do the motors like, it may be using more of the diff would be handy.

The motors have a continuous rating of 2500 rpm at 36V. I'm planning to use a 108V battery pack and limit them to around 5000 rpm. Reduction ratio will be around 3.75:1, close to the original diff ratio, although I'd like to have the option of a higher ratio if possible. The problem is the rear member of the subframe limits the size of the largest pulley I can use. I could cut an opening in the rear of subframe but the steering rack looks as if it sits right behind it. Minis aren't the simplest choice for a conversion, but I'm sure it will be worth it in the end!

Chris
Yes, I've pored over the details of plasmaboy's conversions and many more, I like his simple approach. Here's a little more inspiration for anyone who's interested: http://www.evalbum.com/

Edited by MalcolmB, 16 January 2009 - 05:47 PM.

[GTiRsalter]

Hi,

Interesting project.

Just something i picked up on....due to wiring the motors in parallel with one controller I would guess that both wheels will be turning at the same speed?
Would this cause problems at low speed turning, due to the inner wheel needing to rotate slower then the outer wheel hence the need for a diff in a normal car set up.

No trying to pick holes in your project as I wish you all the best in producing a class electric mini.

Ian.

[MalcolmB]

Ian
Pick away. I'd rather find holes now than later

In theory the motors should act like a limited slip diff when turning, but I've no first hand experience of how well it'll work in practice. There's only one way to find out. It's explained better here than I can: http://www.4qd.co.uk...bmnc2.html#diff

[Ethel]

Minis managed to fit slightly over 4:1 final drives inside the casing. The clearance issue is more likely to be between the drive shafts and motors I'd think. Min's also have unequal length shafts, so using 2 short ones may nicely give you some extra space

[MalcolmB]

Using two short drive shafts sounds like a good idea – I might end up doing that – I need to measure the space available between the inboard ends of the pot joints. Clearance between the pulleys and subframe is still a bit of a problem though. The pulleys I really want to use are 90-tooth 8mm pitch HTD timing pulleys. The diameter is 228mm, plus a bit for the belt itself means an overall diameter of say 240 mm. It looks as if I need an extra 20mm or so clearance at the back of the subframe. I'll have look and see if I can create that space without fouling the steering rack.

[bigmotherwhale]

hi, i do not wish to pick fault, but wiring the motors in parallell to make a differential will not work, and will proably blow your speed controller. to make an eDiff you need to connect the motors in series and power them off a battery pack with double the voltage. i know this for a fact becuse i have made remote controll cars of the same principle, try getting two motors and wireing them in parrallel hold one shaft and the other motor will reduce speed, now wire them in series and hold one shaft, the other motor will speed up, - exactly what a diff is supposed to do. if you dont belive me try it, it works!

i have been wanting to do something similar for years but i would want a 3 phase motor and controller and nimh or ni cad battery packs at the least - very expensive, dont bother with lead acid unless your making a quater mile beast. there far too heavy and you wont get much more than 20mile range with a boot and floorplan full. , litium phosphate would be my first choice of course, but im not a millionaire. where are you planning to get your battery packs and what type? im sure u know about the dangers of 100v battery packs so i wont go into that. a very interesting project by the way i will be watching closely. i recon all youl need for your pot joint problem is 2 cartrige bearings on each pot joint and a steel tube to put them in then weld that to your drive asembly. hope this has given you some ideas - chris

I just read that link about battery motors and controller and the bit about the series current doubling is wrong it halves on the same voltage and is the same amp with double voltage the bit about brush wear might hold true becuse of back emf arcing the brushes.

Edited by bigmotherwhale, 05 February 2009 - 08:54 PM.

[MalcolmB]

wiring the motors in parallell to make a differential will not work, and will proably blow your speed controller. to make an eDiff you need to connect the motors in series and power them off a battery pack with double the voltage.

You're right, the differential action won't be as good as it would be with the motors wired in series, but it should be good enough. It's more like a limited slip diff. The tyres are likely to scrub a little on hard lock, but I'll only be using light throttle then so there's little risk of overloading a motor. Electric golf carts have been using parallel motors for years, so it must work fairly well.

The reason for wiring the motors in parallel is to get decent top speed with a relatively low voltage pack. I plan to use lead acid to test everything out, but like you say, it's not really a practical proposition for a mini if you want more than 20 miles range. I've got the beginnings of a lithium phosphate pack (LifeBatt), and I'm hoping prices will drop significantly over the next year or two.

A brushless or AC setup would be very nice, but again you need deep pockets. I picked up the series motors cheap on ebay, which is what kicked off this project. At the moment I'm collecting drive components so I can make a start on fabricating the bearing mountings and modifying the subframe. I'll post some photos - and more questions - soon.

Cheers
Malcolm

[bigmotherwhale]

It wont be like a limited slip diff it will be the oposite to a limited slip diff pusing you out of the corners, if you do this your esc will blow and if its powerfull enough not too then your batteries will be severly depleted after a few turns, golf carts are designed with rear wheel drive where the difference is not so great in wheel revolution even the link said that, and they also are on grass so the wheels slip, if your basing your desicion on that web page link you listed only, i sugest you do more reasearch and as i said before try it out on a small model, all you need is two small motors and a battery, you will be beter off using 100 volts driving one motor or even two in series at half the voltage, than trying to do 2 in parallell, it simply will not work i guarentee it. and at best extreemly dangerous.

more simply put the outer wheel will be going faster than the inner acting as a generator and tryin to spin up the innner wheel steering you out of the corners, can you immagine the current that will flow between the motors - its going to be huge!
It wont pass road worthy test because its almost exactly the same as joining the two driveshafts together in the middle - like for drift cars and is highly illigal and once you crash which in my opinion will be very soon with a set up like this, there will be 1 hell of a lawsuit on your hands if they know what youve done. SERIOUSLY RECONSIDER!
there is a way to know what will happen in practice! i just told you ;-P

[MalcolmB]

Thanks for the input. I'll definitely take a closer look at this as I'm not planning to build a dragster

I'm no electrical engineer, but I have read around a fair bit and checked the idea out with a few experienced EV builders. No one else has raised any concerns over the motor arrangement. Here's another link, see about half way down the page: http://www.canev.com...ansmissions.htm

Perhaps you're thinking about brushless or permanent magnet motors, which have quite different characteristics from series wound motors?

[bigmotherwhale]

hey no problem its what forums are made for. im thinking of series wound motors, you cannot connect 3 phase in this arangement as far as i know not becuse of the motors, they would do exactly the same as with dc ones if you conect the same phases, but becuse of the controller which is sensitive to the tiny electrical changes in the motor to tell where it is in the cycle - for sensorless config, and for sensored arangement its simply not possible unless you want it to go in an absolute stait line or jerk as one of the motors slips a phase turning a corner.
I have over 15 years of electronics experience, and have tried in practice that exact set up and found it to do exactly what i described. parallel for offroad, series for on road. if you were building a 4x4 i might have told you to go for it. if you have any motors lying around have a play, its how i designed my ediff sytem - that and a bit of lataral thinking.
however i wouldnt go any further with the project untill you can get large quatities of nice batteries, it would be a shame to hack up a mini and later find it only does 20mph in a strait line! nicads are resonably cheap and can be given a good ragging and they love it. run out a set of lead acids once too much and they are as good as scrap. with nicads they like to be taken down all the way so are ideal. nimh are similar in chemistry but expensive unless you are lucky enough to find a prius battery pack (or 3) which are around 200v and ideal for this arangement. look on ebay america they are the cheapest high power batteries on the planet that i can find - but shipping will be a tad pricey.