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Duncan's Dubious Device

188422 Views 402 Replies 57 Participants Last post by  remy_martian
I thought I should start a build thread now I have started brazing
The plan
Two seater lotus 7 Locost type vehicle
Appropriate performance for the style – not too slow
And I would like to enter some speed events – sprints, hill-climbs, 1/8th mile drags

I live in the wrong part of the country for electric vehicles, there are only one million people in the whole of South Island and Southland has a low population density even for South Island,
This means that the limited range is more of an issue as you have to drive further to get anywhere, my commute is only 4 Km , I should use my bike!
My car will come into the “toy” category
The nearest “Big City” is Invercargill (50,000 people) so I would like enough range to get to Invercargill and back – 160Km may have to recharge in Invercargill

Budget (New Zealand Dollars)
As a Scot I intend to spend as little as possible
Motor -------------$100
Donor Car----------already owned
Chassis + Bits--------$2000?

Controller Expensive -----$2,500??
Batteries Very Expensive---- 11,000??

I will replace any parts that need to be replaced – but if it’s still good I will clean, paint and re-use

Subaru front and rear suspension – same make front/rear so the wheels fit
Subaru have four wheel drive, the front hubs can be used by removing the driveshafts
The rear has a nice diff and suspension
Common in NZ,
I had an old Subaru Legacy that died so I now have all of the bits

Hitachi 48v 10Kw motor from forklift
motor is 11 inches in diameter and weighs 102Kg

Those are the bits I HAVE,
I intend to buy:
Zilla Controler
48 off Thunder-Sky 160 (or 200) Ah cells

Pieces I have weighed (Kg)
Rear Sub-frame and diff----------44.5
Rear corners (2)------------------44
Front Sub-frame -----------------20
Front corners (2) ----------------50
Wheels (4)----------------------64

Estimates of weight
Batteries ------------------270 or 290 (from spec sheet)
Rack, -------------------------5
Driveshafts ------------------10
Body ------------------------50


Grand Total ---------------744--------764

Bit disappointing I was hoping for 650Kg!

I am a heretic in that I am very suspicious of “roll centres”
There is a method of logic called “Reductio Ad Absurdum”
You take an ASSUMPTION OR RULE to its extremes
If it produces an absurd or silly answer then the ASSUMPTION OR RULE is absurd or silly

The idea of a roll centre (found by extending lines from suspension components) is that the vehicle rotates around that centre and the distance between the height of the centre of mass and the “roll centre determines the roll couple which is then resisted by the roll stiffness.
It is quite easy to produce suspension with “roll centres” from under the ground to above the centre of mass.
Those will according to the “roll centre rule” produce massively different roll couples

Simple physics says that a force is required move the car around the bend (centripetal force)
This force acts at the centre of mass
For a car the force has to re-act through the tire contact patch
This causes a couple (torque)
(The centripetal force times the distance between the ground and the centre of mass)
The roll couple which is then resisted by the roll stiffness

This does not change with the suspension geometry -
If you maintain the same roll stiffness and centre of mass you roll the same

Using the roll centre you get a result that is contrary to the laws of physics -Jim
(A Silly Result)

Throw away the roll centres!

What do we have left to worry about?
The suspension's job is to cope with bumps without rattling my teeth
To keep the tires vertical
Not to move the tires sideways (scrub)

Without active suspension this is not possible so we go to
To keep the tires as vertical as possible
To move the tires sideways (scrub) as little as possible

Avoid Camber change in Roll or Acceleration/braking
Avoid Scrub in Roll or Acceleration/braking

The standard Subaru Legacy probably rolls over 5 degrees when being hooooned

The strut suspension only recovers 1 degree at the front and about 2 degrees at the back
Which means the tyres lean by 4 degrees at the front and 3 degrees at the back.
Not good for keeping the tread square to the road

It is easy to get more correction in roll – just move the strut tops inwards towards each other
(You then need to modify the base of the strut where it bolts onto the upright to achieve the correct wheel camber)
The standard setup has an effective strut angle of 76 degrees at the front and 65 degrees at the rear
If you tilt the struts inwards you get a situation where you lift the front of the car when you steer.
Many years ago I had a mini with a Lancia engine and heavily inclined struts
That handled really well and the front lifting effect was not a problem

Anyway the PLAN is to have all of the heavy things (except me) right down on the floor so I calculate the centre of mass will be between 400 and 500 mm from the ground

I intend using springs with the same rate as the Subaru,
With a lower mass this will make the vehicle sporty without rattling my teeth too much

The combination of a lower centre of mass and a lower weight along with the same roll stiffness will reduce the amount of roll
I calculate a 1g corner would result in less than 2 degrees of roll

With this amount of roll the standard suspension set-up should be fine
If I do decide to move the strut tops it will be because of aesthetics.

I will still have the option of changing all of the rubber bits for aftermarket bushes if necessary

Motor and driveline
The motor I got second hand, it had just been rebuilt when its forklift was scrapped so everything looks new. The brushes were not bedded in so I shaped them using sandpaper.
The motor had a splined output shaft and a brake on the commutator end.
Inside the brake was a female spline adapter with a spigot and four studs.

The original plan was to use a ford gearbox but I calculated
Motor torque
Assume it’s like a Warp 11 then 135 Ft-lb’s is available
Diff ratio 4.1:1
Torque at wheels 135 x 4.1 = 553 Ft-lb’s
Wheel + Tire radius (185/70/14) = 1 ft
Force at tire contact = 553 lbs-force

Using my planned wt
Vehicle wt ,650 kg = 1430 lbs ------------(750 Kg = 1650 lbs)
50% on rear = 715 lbs --------------------(825 lbs)
Tire grip (std road tires) 80%
Tire grip = 80% of 715 lbs is 572 lbs--------(825 lbs = 660 lbs)

It did not seem to be worth the extra complexity of a gearbox,
the extra weight makes it less definitive but by then I had found that the gearbox would have made the drive train too long

The plan then became
Use the Motor driving the diff through a prop-shaft,
The Subaru used a two piece shaft
I laid out the motor and diff and the rear of the Subaru shaft –
RATS too long I will have to get it shortened
The drive-train was too long as I was planning on using the “engine bay” as my main battery box

The female spline adapter from the brake will be used with a simple adapter to match it to the prop-shaft.
The spline adapter will be left free to move along the motor splined shaft
(Like the output from a gearbox)


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221 - 240 of 403 Posts

· Administrator
6,613 Posts
Discussion Starter · #223 ·
Hi Guys
I now have the new software in my controller - I don't know when the next event that hammers it enough for me to check that it's working but it seems to work fine so far

Annual Cobra 1/8th mile Drags on Sunday the 4th March - see if I can beat some more of the supercharged V8's

I have gone slightly backwards in one way - the laptop power supply that I have been using as a DC-DC to keep my 12v battery topped up has died
So I have gone back to simply charging the 12v battery at the same time as the main pack
It's made of eight "16Ah" Headway cells - so that should be plenty of capacity

· Administrator
6,613 Posts
Discussion Starter · #224 ·
First update from the Drags

Not good!
The new tyres still spun on take off - so I should have done a warm-up spin

More of a problem at the end of my FIRST run there was a bang - and while I had power if I used more than a tiny bit of throttle there was a horrible noise
One of the guys said that sparks flew out from under my car

It did 8.69 seconds at 83 mph - not a bad time!

I limped home (not very far) and it died completely on my drive

I did win an award - the broken piston "Bad Luck" trophy

So it's take the motor out and see what I have done to the com and brushes!

· Registered
1,629 Posts
Oh shit! sorry to know that.
Well this is probably not abnormal. How much Amps and Volt did you push during this race?
Can't wait to see the picture of the torn armature... and the solution you will find if you need to replace this motor.

· Administrator
6,613 Posts
Discussion Starter · #227 ·
Hi Guys

Motor is out - brushes look chipped but not too bad

I have started adding some pictures - Armature first - it's only letting me do 10 pictures so the next post will be of the brushes

Sequence of events
Took off - spun rear tyres - backed off and they gripped - back to full throttle

Just before the end of the 1/8th there was a "Bang" and power went away
I lifted and then gently pressed the accelerator
I could get a small amount of throttle - then a rattling sort of noise - seemed to be from the front (brush end) of the motor

Rolled to the end of the parking area and turned around

I was second to last car of that group so when Owen in his electric pickup joined use we drove back to the start

I found that I could drive slowly so I drove past the start and waited to be let back onto the main road

There is a steep bank just there - managed to get up it but lots of rattling noise

Then it was about 500 meters to my house - two Stop signs which I treated as Give Way and it managed to roll almost to my drive

Then I got out and pushed it the last little bit


· Administrator
6,613 Posts
Discussion Starter · #228 ·
These are the four pairs of brushes - each is photo'd twice - good side and bad side


· Administrator
6,613 Posts
Discussion Starter · #229 ·
The Brush holders

One of the springs is obviously bad - and the other spring on that brush pair is also a little weak

None of this looks like enough damage to be causing the "Bang" and the rattle

I could see the bit that is broken off causing the bang - but not the rattle and lack of power

Unless the brushes were simply sticking up and not contacting at all

I'm hoping that Major or one of the other experts comments

As far as revs when it happened are concerned my top speed was 2 mph slower than last year but my time was 0.6 seconds faster - more grip from the tyres which were fractionally larger

225/45/17 rather than 215/45/70

I calculate 4580 rpm - I don't think that was relevant

I believe that my controller is on 100% before than so it would see 340v (full charge) minus the sag - maybe 300v ??


· Administrator
6,613 Posts
Discussion Starter · #230 ·
The spring that was "Bad" - it looks as if the post that it attaches to was bent

I measured the com runout - I get a total of 0.05 mm of runout

The more I look at the damage the less persuaded I become that this was the problem but while it's all apart I should make sure that it goes together BETTER than it was

· Administrator
6,613 Posts
Discussion Starter · #231 ·
Still looking for advice

Is that damage likely to have cause the symptoms? - possible?

Where can I get "better" brushes?
Where can I get new springs? - or new springs and brush holders

· Registered
7,793 Posts
Hi Dunc,

Been hoping to view your photos on larger screen. But this tablet will do. Guess you're lucky not to have big damage in the motor from the sounds of it. I don't see anything obvious which would cause symptoms of shutting down or losing power as you described.

Comm looks a bit raw like high current/arcing has stripped the film. I'd definitely check out the comm for t.i.r. Needs to be 30 microns.

When you lost traction it may have oversped the motor causing a bar to lift. A high bar, or segment could damage/chip those brushes. I think you need the bar-to-bar deviation at 5 microns.

Replacement brushes/springs. Maybe forklift repair shops. Motor rebuilder.

If comm checks out or is serviced, I'd be tempted to install current brushes and do a lengthy break-in.

I found an excellent read on Mersen website.



· Administrator
6,613 Posts
Discussion Starter · #233 ·
Hi Major

Thanks for that - interesting read

I have checked runout and I get 0.05mm - 50 microns - so a bit more than you suggest

I have "fixed" the worst spring - the bar it mounts on was bent

My "plan"

Put it back together out of the car and run it on a 12v battery - just to see if it goes

Assuming that it spins OK - I will try and get it to a forklift motor shop - either that or I will try and "machine" the edges of the com blocks
Just using a stone??
To get shot of the burnt looking area and the to get the chamfer looking like the chamfer in the Mersen doc you referenced

Then re-measure the runout and go around carefully segment by segment looking for steps

See what I find

I have had a good look at the rest of the car - can't see anything else wrong but I can't test the controller until the motor is back in

· Registered
575 Posts
Hey Duncan,

I am no expert but I wonder if one of the Comm bars may have raised at higher revs, chipped the brushes but then were pushed back down as the revs reduced. In other words I would be gently checking if any of the comm bars move at all.

But I am no expert

Also I wonder if you have overloaded a winding/conductor. Maybe check for resistance across your terminals.

But I am no expert

You can try "Find it parts" for brush springs

Did I mention I am no expert?

· Administrator
6,613 Posts
Discussion Starter · #235 ·
Something that I should have done FIRST

I just measured the resistance from the Com segments to the armature shaft - should be infinity or at least very high
Instead I get 0.7 Ohms - NOT GOOD!

I think the armature is stuffed

· Administrator
6,613 Posts
Discussion Starter · #236 ·
This armature wire has blown completely through - and it has sprayed copper all across the inside

I should have taken it all apart on Monday - but I was sure that the problem was to do with the com and brushes

Time to look for another motor!
But I will ask about getting this armature re-built


· Administrator
6,613 Posts
Discussion Starter · #237 ·
Hi Guys

Motor failure
It looks as if that wire simply cracked or otherwise nearly failed then arced

I wonder if something got into the motor - damaged the insulation on one wire then did some more damage on the one that failed

Looking for another motor!

As far as I can see a 9 inch motor will have about 25% less torque at a given current but will enable me to rev much higher

I can still spin my lovely sticky "Nitto" tyres so giving away 25% of torque may not be too disasterous

I will just have to see what I can find in the way of second hand forklift motors

· Registered
995 Posts
Too late now, but I try to give these older, used at higher voltage motors, a good dose of spray-on, good quality insulating paint. Apply at different positions so it runs down the slots, lower windings, and, if possible, apply multiple coats while the arm. is slowly rotating on something like a BBQ rotisserie.

Motor shops can also do a commercial dip, rotate, bake, and re-balance. Sometimes they use a varnish, sometimes an epoxy for the severest duty. One trade-off is that the extra insulation may trap more heat in the windings - something to watch out for.

· Administrator
6,613 Posts
Discussion Starter · #240 ·
Hi Molten

I got five years of abuse out of a $100 motor - so I don't think I'm entitled to complain!
although I wish it had held on for another couple of runs - 8.6 seconds is good for an 1/8th miles but I think that when the track stickied up a bit I could have got into the 7's

I have found a forklift breakers in Dunedin with a "pile of motors" - so I'm going there soon to see what I can find

I'm thinking about going down to a 9 inch motor - give away some torque for higher speeds - or maybe not
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