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Discussion Starter · #1 ·
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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Cool project.

As you've observed with roll centers, vehicle dynamics is more complex than simple kinematics (or static forces). I think with rear wheel torque and cg shift/torque, you should be able to plant more than 660 lbs of driving force to the rear wheels. Which would be good, because I think an equivalent WarP 11 motor with a Zilla will be able to put out more than 135 ft lbs of torque at the motor.

Can't say the spreadsheet in my signature has much experience to draw on, but it's fun to play around with the numbers. Looks like you would have decent performance.
 

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I think theoretical roll centres matter as it sets a 'ball park' for you to design and fine tune from. It also catches the plain silly designs early on.

Some aspects of the calcs causes problems like parallel wishbones having intersects at infinity and others having roll centres outside of the vehicle footprint during dynamic cornering.
So as a good starting point I would use them, from there on it has to be 'seat of the pants' for me.
 

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Roll centers theory can be used, among other things, to explain why swing axles are a Very Bad Idea. Also, the relative height of the roll centers between the front and rear axles can give you an idea of the expected understeer/oversteer characteristics that you might encounter. So don't throw those ideas in the waste bucket... since you're designing things from scratch you at least have the opportunity to ensure that the geometry you're designing moves things in the right way at the right time. As part of that, you should at least check that the theoretical roll centers and the roll couple are reasonably close in height, at the front and rear axles.

It's good you're working out all the numbers ahead of time... saves you from encountering a major roadblock in the middle of the build.
 

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Discussion Starter · #5 ·
Hi Tiger
I totally disagree about roll centers -
No use to man or beast

If you want to know what it will do you need
Position of center of mass
Roll stiffness of each end

What your suspension does to your tire on road angle

If you know these you can predict roughly what it will do

If you know your 'roll centers" and you don't know the other things then you can predict nothing!

Anyway mine is nice and simple as I am using struts all round!

Incidentally Subaru use their struts exactly as I would have predicted with a greater inwards incline (and hence angle recovery on roll) on the back.

I developed the suspension I used on my Lancia mini in the 80s - and I made a number of learning experiences (mistakes) in the process
The final car went very well indeed.

Roll centers - Ba Humbug!
 

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Hi Duncan,
I don't think that anywhere in what I wrote, I suggested that roll center analysis gives you all you need to know about the suspension dynamics. What I intended to say is that, once you get your camber gains (front and rear) and caster angles (for the front) sorted, then running a quick check to see that the roll centers are in a reasonable place, throughout the suspension range of motion, is probably a good thing to do.

The nice thing about strut suspension is that if you take care of the camber gain, you almost automatically end up with the roll center in a reasonable range. It helps to make the lower control arms as long as possible, and if you can, make the inboard chassis pivots adjustable for height... that will also allow you to adjust anti-squat on the rear suspension, at least to the maximal extent allowed by the struts.

The books I found helpful for suspension analysis and design are Fred Puhn's "How to make your car handle", and anything written by Carroll Smith on the subject. His books "Engineer to Win" and "Tune to Win" are especially enlightening.
 

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Discussion Starter · #7 ·
Hi Tiger
I like the books you mention,
the Puhn book led to lots of rude remarks about door handles!

I also like the Alan Staniforth books although "High Speed Low Cost"has been out of print for decades.

I remember these books from when I used to play in the 80's

Is there anything more recent??

The car
I am cheating by using the Subaru sub-frames and control arms
This limits what I can change but makes things like rack location trivial

This will also help with certification
 

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Discussion Starter · #8 ·
Hi
It is past time for me to update this thread.

I have completed the rear of the chassis,
I am using the right hand rear hub on the left hand side and vice versa,
On the Subaru the rear hubs are arranged with the strut in front of the wheel center line with a forward facing control arm and the handbrake cable going forwards
By reversing the hubs the strut moves back clear of the passenger area, the hand brake cable goes backwards where it can sweep round in a single curve to the transmission tunnel
The forwards arm on the Subaru would have to go under the seat area,

A rearwards arm can be allowed for much easier

I did make a major mistake
I had intended to lower the motor into its location where the gearbox would normally go,
This space is quite tight as the motor is eating space for the pedal box

I had brazed the top and bottom rails in place so that the motor would fit between the tunnel sides which are to be attached to the outside of my tubes.
This meant that the motor would not fit between the inside faces of the tubes!!
PANIC!!
Luckily I had plenty of height between the top and bottom tubes so I will have to install the motor by lowering it into the passengers foot-well and then moving it sideways into its home

A lot of the chassis is not yet triangulated this is because I intend brazing a floor-pan, front and rear bulkhead and battery boxes from steel sheet
(this removes the need for the diagonal bracing)
Several holes cannot have complete sheets but I intend to gusset them

I have made up the frame for the front battery box and the mountings for the rear of the front wishbones, the next stage is to make up the mounts for the front sub-frame and front strut tops

I must ration my time on this forum as it is slowing down my brazing!!
 

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I did make a major mistake
I had intended to lower the motor into its location where the gearbox would normally go,
This space is quite tight as the motor is eating space for the pedal box

I had brazed the top and bottom rails in place so that the motor would fit between the tunnel sides which are to be attached to the outside of my tubes.
This meant that the motor would not fit between the inside faces of the tubes!!
What about re-arranging or rebuilding the control arm so that it effectively becomes a Chapman strut, as used on Lotus rear suspensions and also the Datsun 240Z?

When I built my tube frame race car, I ended up with a few ill-located tubes that required some irritating workarounds. In retrospect it would have been better to cut out the offending tubes and relocate them so that everything around them wasn't a compromise. What if you relocate the lower tubes for the motor tunnel so that you can jack the motor up into place from below? You could then attach the trans/motor tunnel sheetmetal to the inside of these tubes, and have very little impact on the footwell space.
 

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Discussion Starter · #10 ·
Hi TigerNut

I decided that as I didn't have all of the drippy things in an IC engine I could have a complete flat floor pan
So I intend brazing a sheet of (22 gauge 0.7mm) steel under my chassis

This would make it difficult! to raise the motor up to fit it!!

I don't expect any problems in lowering it into the passengers foot-well and then moving it sideways into its home
It weights 102Kg but I will use a strap to take most of the weight when I am slipping it sideways

The rear suspension is what I would have called Chapman strut
It consists of a strut with a rubber mounting up high, two horizontal rods that stop the rear hub from steering and a forwards (or rearwards) arm to keep the hub in place

There is a lot of rubber involved at all of the joints - I suspect it would not move at all if each joint was perfectly constrained

The Subaru handled OK, I expect with less weight (and lower) I will be OK,
If it does feel squidgy there are lots of after-market plastic replacements for the rubber bushes
 

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Discussion Starter · #11 ·
Update on my (small) progress
Rear of chassis complete,
I have swapped the RHS rear unit for the LHS so that the forward facing reaction rods face rearwards and do not go under my butt
The handbrake cables now come out facing rearwards which is good as they can sweep around to their mounts – facing forwards with me sitting so far back would have required an S curve

The front of the chassis was complete BUT I have sourced some different struts and springs so I am going to remove the front strut top mounts and associated tubing – two steps forward one back!

The motor now has its brushes advanced – four M10 x 1.5 holes drilled and tapped,
I stripped the rack to see how to convert to manual – know how to do it now but now I need another rack!

I’ve fought my way through the OpenRevolt thread and I’m getting a kit for Christmas
I am now thinking about 48 TS 60AH cells rather than 200AH cells
Much less money and much lighter – lose a lot of range but I’m not building a GT car!

I am using front wheel drive front hubs – but I don’t need the front driveshafts (and the CV joints get in the way) so I have cut the driveshafts down – hacksawed through four inches of steel! Twice!
Good for the biceps!

The front of the chassis is now complete – still need to braze in the steel panels for the floor bulkheads and battery boxes
 

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Discussion Starter · #12 ·
Modified the steering column to tilt upwards to help for getting in and out
(The Subaru column normally only tilts downwards a small amount)

Questions!
I have this reversing contactor pair – no numbers on it
I am not worried about using it in reverse – but what about the current through these terminals when I play at silly buggers in forwards (500Amps +) ??

I have this contactor – again no numbers on it – be OK for 150V and 500 Amps ??

Are these bits any use or should I try and find a museum
 

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Hi
I had brazed the top and bottom rails in place so that the motor would fit between the tunnel sides which are to be attached to the outside of my tubes.

I must ration my time on this forum as it is slowing down my brazing!!
Have you checked that brazing is an acceptable method of jointing an automotive frame in your country?

I know it wouldn't be acceptable in the UK for any structural part as the base metal isn't fused together and would only be relying on the strength of the brazing filler.
Even bicycle frames have a mecanical 'tube into a tight socket' connection before brazing. Yours looks like butt joints.

Just a thought.
 

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Discussion Starter · #15 ·
Hi Woody

Just to be pedantic - true brazing involves a close fitting overlapped joint and the metal flows into the joint'
I am using bronze welding
End of pedantry back to calling it brazing

This is stronger that fusion welding as fusion welding always has a weak zone where the melt pool stops - often a small loss in section as well
Bronze welding was used on all of the famous racing cars when they were "space frames" Lotus, Maserati, Cooper.., - I believe it still is when maximum strength is required
The reason it went out of general use is it's slow and expensive!!
Those rods are bloody expensive I should have spent the capital and bought a MIG!

The filler metal is stronger than mild steel and is used as a fillet so the joint is stronger than the structure

I used this years ago on replacement front wishbones for a Mallock - testing by Armco destroyed the wishbones but the brazed joints survived

Here in NZ I can use it for my structure - BUT I will have to get my roll bar welded as the sporting association specify the weaker process

Hi Karlos
I now have an OpenRevolt controller kit - it is a superb kit I hope that I can solder it up successfully

I have a complete chassis and I am brazing in the floor pans and the bulkheads
I will do a proper update soon

My certifier has required me to do a full design and build "story" so this will be uploaded to this site when I get the famous round tuit.
 

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Hi Woody
The filler metal is stronger than mild steel and is used as a fillet so the joint is stronger than the structure

I used this years ago on replacement front wishbones for a Mallock - testing by Armco destroyed the wishbones but the brazed joints survived

Here in NZ I can use it for my structure - BUT I will have to get my roll bar welded as the sporting association specify the weaker process
OK, that sounds good.

Actually, I'm sure, now, that you may have said this before (I may have asked before:confused:). I recognise your explanation.

I hope it passes inspection.:)
 

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Get a Soliton 1 or jr. Zilla's are hard to find, require LOTS of parts to do half of their features, require contactors, precharge....

Solitons have internal precharge, water cooling(optional!) and frankly, just seem to be a better controller from what i can see. Zilla's continuous current is 300 amps, Soliton is 1000amps. All day. Every day. None of that 10 second crap.:D

-my $0.02
 

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Discussion Starter · #18 ·
Hi Alex

I have purchased an OpenRevolt controller kit - I would love a Soliton but I am a cheapskate - and building the controller should be fun in the winter
 
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