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Duncan's Dubious Device

188427 Views 402 Replies 57 Participants Last post by  remy_martian
Hi
I thought I should start a build thread now I have started brazing
The plan
Two seater lotus 7 Locost type vehicle
Performance
Appropriate performance for the style – not too slow
And I would like to enter some speed events – sprints, hill-climbs, 1/8th mile drags

Range
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)
Motor---------------------------102
Rear Sub-frame and diff----------44.5
Rear corners (2)------------------44
Front Sub-frame -----------------20
Front corners (2) ----------------50
Wheels (4)----------------------64
Total----------------------------324


Estimates of weight
Batteries ------------------270 or 290 (from spec sheet)
Zilla-------------------------15
Chassis----------------------60
Rack, -------------------------5
Driveshafts ------------------10
Interior------------------------10
Body ------------------------50

Total-----------------------420--------440

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

Bit disappointing I was hoping for 650Kg!

Suspension
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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Discussion Starter · #282 ·
If it was just the left side glowing like a pool of molten lava, it's probably a jammed-up caliper or caliper piston(s).
I didn't notice it at the time - brakes pulled me down nice and square from 150 kph

I didn't notice anything until I saw the photo - done a couple of brake downs from 70 kph - no sign of anything - both discs too hot to touch but not instant frazzle
 

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What are the chances that you are seeing a reflection of the safety cone in the nice shiny brake disk? It seems a bit of a stretch, but maybe..?


Bill


I think you are on the money.


I don't know about the cone part but I am convinced it is reflection. There are a couple of reasons.
1. The colour is consistant with the reflection on the other components like the wheel rim.
2. Take a closer look at the bottom right of the disc, just where it goes behind the spoke of the wheel. there is a portion there that is not orange. If it were glowing hot it would be consistant across the whole disc.


I think Duncan is safe......well safe from melting discs anyway.
 

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What are the chances that you are seeing a reflection of the safety cone in the nice shiny brake disk? It seems a bit of a stretch, but maybe..?
[Edited to add - Damnit, galderdi beat me to the same observations by 30 seconds while I was typing]

At first I laughed it off, as of course that's not happening... but...



1 - Why is the outside of the black rim orange/red? That's not reflected light from inside the wheel well.

2 - Why is the rightmost edge of the discbrake not orange, but grey?

3 - Why is the top of the disc brake only slightly warm, but the bottom glowing bright?

...

Was there bychance also an orange sandfence or safety fence nearby?

Alternatively, cameras pick up infrared much easier than our eyes do. Something that appears to emit zero heat light yet, will often show up as pink/purple/orange on a camera that has weak infrared filtering (nearly all cameras, if you point a TV remote at your camera in video mode, and hit some buttons, note that it lights up white hot on your camera screen).
 

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Discussion Starter · #287 ·
What are the chances that you are seeing a reflection of the safety cone in the nice shiny brake disk? It seems a bit of a stretch, but maybe..?


Bill
I have never heard of that before - BUT there is orange on the rim of the wheel and on the polished black "spoke"

And none of the smell and smoke I would associate with a disc that was actually orange hot

None of the other cars showed anything

I think that you may be right!
 

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Thanks guys for your comments - I do feel much better about that now

- I'm down to 7.90 seconds - how do I get down even further?

Aero? I am not talking about downforce. I mean streamlining. At those speeds don't underestimate how much power it takes to overcome every bit of wind resistance.


The balancing act is making some improvements that are either compatible with your normal use of the car or able to be applied on the morning of your events.


Your nose is good.
The front wheels are a problem area but I'm not sure what you could do there short of a whole new front clip.
The passenger area is a fairly easy fix. Get some 0.4 mm polycarbonate and make some covers to extend the bonnet and sides to bridge the gap where the passenger seat is.
Temporarily remove the passenger side headrest. It doesn't sound like much but try puting your palm against the wind at 130kmph and you'll see how much power it is sucking.
Remove the passenger side windscreen for these events.
I would make an alternate driver side windscreen which would extend up as high as you can cope with. Try and get it high enough to divert the air over your head.
The rear of the car possibly has some potential too but would take significant effort like making a streamlined tail to reduce the eddies behind the boot.


Please don't take this as criticism. I think the car is awesome just as it is. But we are always seeking those next few %
 

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Discussion Starter · #293 ·
Excellent suggestions

I started to take the passengers side head rest off - but I stopped when it became clear that the nuts inside would come loose - Have to strip it down and re-make it

I do have tonneau covers for it - with a centre zip so that I can drive and help streamline the passengers side - I should have done that

Taking the passengers side aero screen off is a bit of a *** - I may try just putting it flat

Not sure if I need a taller drivers side screen - it already boosts the air up above my head (I think)

The back has a tonneau cover on it already

I'm stuck looking after my wife at the moment - so don't know when the next event will be
 

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Tuft it and get someone to take pictures. Bet you have lots of backwards facing ones.
5280 is close to your motor rpm limits? Also means lots of back emf which limits output power. Slightly taller tires, perhaps?

Chin spoiler / splitter. Underbody covering. Ask the experts over at ecomodder about aero.
 

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Excellent suggestions

I started to take the passengers side head rest off - but I stopped when it became clear that the nuts inside would come loose - Have to strip it down and re-make it

I do have tonneau covers for it - with a centre zip so that I can drive and help streamline the passengers side - I should have done that

Taking the passengers side aero screen off is a bit of a *** - I may try just putting it flat

Not sure if I need a taller drivers side screen - it already boosts the air up above my head (I think)

The back has a tonneau cover on it already

I'm stuck looking after my wife at the moment - so don't know when the next event will be

Good news then, sounds like you are on the right track. I also concur with the taller tyres comment. That would put your motor back into it's torque band. Maybe try adding some string or tape hanging off the top of your screen, drive the car and see where the string is hitting your helmet. If it is hitting your chest or chin then it's not high enough. If it's hitting the top of your visor then all is well. I would be surprised if it were up to your chin.
 

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Discussion Starter · #296 ·
Major seems to think I would be OK at 6000 rpm - and I'm using 340v to keep the power at high revs
But I will look for taller tyres once these are worn out! - currently 225/45/17 and labeled "Recommended for competition use only"
Not until then! - these were expensive $300 each

I like the tape idea on the screen - definitely try that
 

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Discussion Starter · #297 ·
It looks as if I did do a little damage on my four full power runs

Yesterday my reversing contactor stuck! - I had reverse but no forwards until I poked the contactor with a screwdriver to release one of the contactors

The contacts look a bit second hand - I've given them a quick clean and they seem to be OK now
 
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