Weber and Coulomb's MX-5

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Post by weber » Sun, 20 Jan 2013, 06:55

acmotor wrote: Perhaps check with Police Licensing or Mazda Australia for a GVM. One or the other MUST have it. I don't think any vehicle would get to be on Oz roads without Police Licensing having a GVM on their files. An approving engineer would need to find the number.

No less than Richard Larsen BEMech MIEAust of Loadsafe Australia Pty Ltd, and of MX-5 and RX-8 specialists Automotive Plus, and Project Consultant for the first edition of NCOP, and Queensland Transport "Approved Person", has told us that there was no requirement for manufacturers to establish GVMs for ordinary passenger vehicles when our model of MX-5 was made.

Web searches give the same answer on forums all over the world. It simply does not have a GVM, and Mazda will not supply one. Hence the need for tests and calculations to establish one.
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Post by jonescg » Sun, 20 Jan 2013, 07:01

All vehicles have a tare as a matter of record. GVM on the other hand isn't always easy to find. I like Richo's suggestion though - ~80 kg for every human in the car + 50 kg for luggage. Sounds about right.

It's tough, because there are different figures for different jurisdictions, despite the exact same equipment being used. I know the tare of the CRX is 895 kg. If you apply the 2 x 80 kg + 50 kg you get about 1100 kg give or take.

Yet the US manual I have here says the GVM of the CRX was 1300 kg! Then I met another CRX enthusiast who had a dual seat in the back, complete with lap-sash seatbelts. So make that 1260 kg...?

In any case, exceeding 1150 kg would be unwise... You guys never know your luck. If there is some literature somewhere that says you can get away with a higher GVM, then why not? You can always pull cells out to lighten the load afterwards.
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Post by acmotor » Sun, 20 Jan 2013, 07:23

Wow ! I didn't realise you had such a problem !
You might end up removing lots of cells !
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Post by weber » Sun, 20 Jan 2013, 18:39

acmotor wrote: Wow ! I didn't realise you had such a problem !
You might end up removing lots of cells !

What is it exactly, that you are referring to as "such a problem"?

[Edit: I was unsure if acmotor was referring to jonescg's post which immediately preceded his, or mine which was before that. And if mine, then the lack of a GVM or the weight? It often helps to quote the part you are responding to.]

Thanks for the suggestion BigMouse, but the most we could possibly have to remove is 27%. i.e. removing 60 cells, leaving 168 cells. And it may be as little as 6%. i.e. removing 14 cells, leaving 214 cells.
Last edited by weber on Sat, 26 Jan 2013, 11:08, edited 1 time in total.
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Post by Canberra32 » Tue, 22 Jan 2013, 01:17

Though I am lucky enough to know someone with a set of corner scales I do feel your pain.
In my hunt for perfect 50/50 weight balance plus compensating for my weight as driver assuming passengers (battery draining dead beats) are not in the car 90% of the time this makes for quite a cell shuffle :)
But having owned a mx5 with a wife that filled the boot with crap every trip I can say that with weight in the back the front gets floaty :(

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Post by weber » Fri, 25 Jan 2013, 04:56

I managed to track down a helpul guy called Garry who will bring his set of corner-weight scales to my place for a small fee. Hoorah!

To compensate for the slope on my concrete I have made four 400 x 400 mm pads out of 18 mm marine ply as suggested by Jeff Owen. Using Jeff's laser level I have roughly levelled them (within 3 mm). The car is hovering over them, up on its stands. Hopefully I can fine tune them tomorrow before Garry gets here at 10:30 am.

But I'm thinking it's the _car_ that should be level, i.e. the chassis rails under the cabin floor, not the wheels.

What do other people think?
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Post by celectric » Fri, 25 Jan 2013, 05:32

weber wrote: But I'm thinking it's the _car_ that should be level, i.e. the chassis rails under the cabin floor, not the wheels.

What do other people think?
If the car is sitting on level ground, the wheels will be level but the chassis rails may not be. So I would have thought that the most representative measurement (i.e. the weight distribution you would get with the car on a flat level surface) would be with the wheels level.

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Post by weber » Fri, 25 Jan 2013, 06:06

The way I look at it is this: What length the springs are now is irrelevant because ideally they will be replaced by new springs that make the chassis rails parallel to the ground when the ground is level. So what we need to measure is the force required on each corner to make the chasis level, except that it's over-constrained (only need 3 wheels). I'm guessing the ideal is to make the diagonals sum to the same value.

Edit:
Hmm. I was forgetting about the anti-roll bars (sway bars). But then I guess we just take the average of the two rear weights and make the two rear springs the same. Likewise with the two front springs. So I guess the only critical thing is making the chassis level front to back.
Last edited by weber on Thu, 24 Jan 2013, 19:53, edited 1 time in total.
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Post by BigMouse » Fri, 25 Jan 2013, 14:22

How much of a difference will even a few tens of mm make on the overall distribution of weight (F/R) for a car as low and short as yours? The shift in the centre of gravity would only be a fraction of the F/R height difference of the frame rails, so at most, maybe 5-10mm?

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Post by weber » Fri, 25 Jan 2013, 15:21

BigMouse wrote: How much of a difference will even a few tens of mm make on the overall distribution of weight (F/R) for a car as low and short as yours? The shift in the centre of gravity would only be a fraction of the F/R height difference of the frame rails, so at most, maybe 5-10mm?

Right. So I'll stop worrying and just go with what's easiest to set up. Level wheels. Thanks guys.
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Post by weber » Sun, 27 Jan 2013, 01:43

The way the weather is going here in Brisbane we need to convert the MX-5 to one of these:

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Post by weber » Tue, 29 Jan 2013, 02:21

On Friday, the very helpful Garry turned up with his corner scales and we weighed a number of different combinations. With those as baselines, Coulomb, Newton (Jeff Owen) and I spent the rest of the day calculating front and rear weights for various equal-half-pack battery options and how hard their rewiring would be.

We also looked in more detail at how many cells we can fit in each of the front mudguards. I never imagined we'd be designing new battery boxes at this late stage. Here's the design. [Edit: We later changed this to fit 7 cells and achieve 34 mm clearance from the tyre.]

Image

On Saturday I reviewed the 25 MX-5s (Miatas) in EV Album and emailed a questionnaire to the converters/owners of all those which are heavier than we are considering. There were 6. Most significant was the reply from Ken Watkins, owner and converter of the heaviest one, with 1497 kg kerb weight.

http://evalbum.com/2969

He was able to tell us that he had 835 kg of that kerb weight on the rear and 662 kg on the front. He claims not to have had any failures, accidents or near misses (that might be associated with excessive loading), after 24,000 km.

We have presented this info to our engineer and have requested a 1560 kg GVM with no more than 830 kg of that on the rear -- a 53:47 distribution. That's with a payload of 180 kg where 120 kg is on the rear and 60 kg on the front. So kerb weight will be 1380 kg with 710 kg on the rear and 670 kg on the front. That's 125 kg less on the rear than Ken Watkins MX-5 has had for the past 24,000 km. This will allow 218 x 40 Ah cells, down from our original 228.

Now we wait for the verdict. And of course we will still have to pass the braking tests.

[Edit: Added "That's 125 kg less on the rear than Ken Watkins MX-5 has had for the past 24,000 km."]
[Edit2: Updated drawing]
Last edited by weber on Thu, 30 May 2013, 09:45, edited 1 time in total.
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Post by PlanB » Tue, 29 Jan 2013, 21:03

Lead acids! No wonder he's heavy. Ken's DC controller does regen too I see.

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Post by coulomb » Wed, 30 Jan 2013, 00:28

When the scales were here, we were conscious of paying by the hour, and forgot to take a photo. This one is similar to Garry's:

Image

The sample is in pounds, but I assure you we were working in kilograms.

Our first measurement was without the yet-to-be made box of 14 cells; we found out later that day that 14 won't quite fit, and we can only readily squeeze 10 in there. The latest thoughts have 12 there, as per the post two above this one.

When looking for the above image, I found this photo of an MX-5 on scales:

Image

Note the cute little ramps. We needed some packing to compensate for the slope of the garage floor. Weber put the car on stands the night before, so when Garry arrived we jacked up the rear, slid in the two rear load cells, and lowered the rear onto the load cells. We then jacked up the front, slid in the two front load cells, and lowered the front. It was a little disconcerting relying on the handbrake and the friction of the rear tyres on the load cells and the load cells on the wooden block.

Weights; 34 cells under boot in all cases.

No driver or passenger, nothing stacked on front
LF 329 326   RF 655
LR 344 349.5 RR 693.5
               1348.5

No driver or passenger, 28 kg stacked on front (representing 14 cells in front mudguards)
LF 346 343 RF 689
LR 341 346 RR 687
             1376

Driver and passenger (161.5 kg), 28 kg stacked on front
377.5 374 751.5
387   399 786
        1537.5

No driver or passenger, chargers removed from boot (14.2 kg), 16 kg stacked on front (representing 8 cells in front mudguards)
340 335 675
333 342 675
       1350


It seems that our bathroom scales were reading about 5% low. I wonder if they do that to make the typical customer feel better (in a weird attempt to boost sales). Every 10 kg is important, so at least in our case, it was well worth it to get accurate weights.
Last edited by coulomb on Wed, 30 Jan 2013, 17:45, edited 1 time in total.
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Post by PlanB » Wed, 30 Jan 2013, 22:21

What about a remote control foot that quietly extends from the car's c of g with a discrete press of the remote in your pocket at weigh in time?

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Post by coulomb » Thu, 31 Jan 2013, 04:52

PlanB wrote: What about a remote control foot ...

I realise that this is tongue in cheek, but we actually want to pass legitimately. We want not only to pass legally but also the safety, peace of mind, and performance that optimal cell placement and count will provide.

One scary thought about a heavy conversion that relies on stronger brakes is how it will perform, and how safe it will be, in the wet.

[ Edit: p.s. the weight results have been added to the post two above this one. ]
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Post by Canberra32 » Thu, 31 Jan 2013, 11:09

An mx5 is set up so perfectly that when you change anything it is a radical difference to handling.
With our one it took much tuning suspension and brakes before we got it right and in the end bit the bullet and put the 300mm disks in the engineer asked for and it was the right thing to do.
When we lightened it it took weight off the back wheels and shifted CG more to the front and was super noticeable.
Took some billet aftermarket parts to drop under bonnet weight and moving of some items to balance out again.
Mxs are temprimental but awesome handling when set up right :)

How much did they charge per hour for the scales?

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Post by weber » Thu, 31 Jan 2013, 22:26

Canberra32 wrote: An mx5 is set up so perfectly that when you change anything it is a radical difference to handling.
With our one it took much tuning suspension and brakes before we got it right and in the end bit the bullet and put the 300mm disks in the engineer asked for and it was the right thing to do.
What did you do to your MX-5 that made 300 mm disks advisable? And where does one get 300 mm disks for an MX-5?
When we lightened it it took weight off the back wheels and shifted CG more to the front and was super noticeable.
Took some billet aftermarket parts to drop under bonnet weight and moving of some items to balance out again.
Mxs are temprimental but awesome handling when set up right :)
Thanks Canberra32. We really appreciate your experienced input into our MX-5 weight deliberations. Would you say that 50:50 front:rear was very important, say plus or minus 1%? Or is total weight more important, or perhaps C of G height?
How much did they charge per hour for the scales?
Garry has a couple of race-cars (see http://www.youtube.com/user/ghz28) and bought the scales for his own use. But he is happy to help out other DIYers. He insists on going along with his scales, as we fully expected. We offered him $50 per hour, and some tea and cake, and he was happy.
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Post by zeva » Thu, 31 Jan 2013, 22:53

weber wrote:What did you do to your MX-5 that made 300 mm disks advisable? And where does one get 300 mm disks for an MX-5?
I came across this supplier when I was looking into the same thing a few years back for my MX5-EV:
http://www.mx5mania.com/brakes.htm
weber wrote:Would you say that 50:50 front:rear was very important, say plus or minus 1%? Or is total weight more important, or perhaps C of G height?

FWIW I seem to remember my MX5 ended up significantly more tail-heavy, something around 45:55. I don't think it's something one would notice on public roads because generally we drive well below the limits of traction, but it was certainly noticeable on the racetrack (tended to oversteer more readily).
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Post by Canberra32 » Fri, 01 Feb 2013, 00:38

Yup zeva mx5mania is where I got most of the stuff I diddnt make myself :)
1% not noticeable :)
Higher CG maybe but depends how you drive.

And bigger brakes coz it did 0-100kph in a flat 3seconds :)
Was a little beast :)

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Post by klaasdc » Sun, 03 Feb 2013, 00:50

Hello Weber,

A quick question; did you completely remove the ECU and its wiring from the car, or is it still needed for something?

Thanks!

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Post by weber » Sun, 03 Feb 2013, 01:35

klaasdc wrote: Hello Weber,

A quick question; did you completely remove the ECU and its wiring from the car, or is it still needed for something?

Thanks!

The ECU is not needed, as far as we can tell so far. But the wiring often proves useful for other purposes. We are putting two Tritium Driver-Controls units (DCUs) in the location where the ECU used to be. For example one DCU receives the signals from the accellerator pedal unit via wires that used to carry the throttle position sensor signal to the ECU. Power for the DCUs is also taken from those that used to power the ECU.
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Post by klaasdc » Sun, 03 Feb 2013, 08:11

weber wrote:The ECU is not needed, as far as we can tell so far. But the wiring often proves useful for other purposes. We are putting two Tritium Driver-Controls units (DCUs) in the location where the ECU used to be. For example one DCU receives the signals from the accellerator pedal unit via wires that used to carry the throttle position sensor signal to the ECU. Power for the DCUs is also taken from those that used to power the ECU.
Good to hear that :) My fear was a dash full of warning lights, disabled ABS, airbags etc.

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Post by weber » Sun, 03 Feb 2013, 16:59

klaasdc wrote:Good to hear that :) My fear was a dash full of warning lights, disabled ABS, airbags etc.

Ours does not have ABS or airbags, so I'm afraid you can't take any comfort from our example. But many thanks to Tom Madracki for putting MX-5 wiring diagrams up on his website http://www.madracki.com/miata/wiring.html where we can see that the ABS control module is separate from the ECU (1999 diagram page 3). However that doesn't guarantee that the ABS module doesn't expect some signal from the ECU. A closer examination should reveal any signals going between the two units, but not their direction.
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Post by weber » Sun, 03 Feb 2013, 20:49

Here's another piece of cubist sculpture from the workshop of Coulomb, Weber, Newton and Joule. It's the first of a mirror-image pair of battery boxes that will hold 7 x 40 Ah cells in each front mudguard of the MX-5. It's only tack-welded at this stage, and it's upside-down.

The photos are a stereo pair. i.e. they were taken from two slightly different camera positions which were about as far apart as your eyes. They are arranged here for cross-eyed 3D viewing.

Instructions on how to view cross-eyed 3D images

Image Image
Last edited by weber on Mon, 04 Feb 2013, 04:16, edited 1 time in total.
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