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Weber and Coulomb's MX-5

Posted: Thu, 01 Aug 2013, 22:26
by BigMouse
Johny wrote:Where does one purchase "black automotive exterior-grade double-sided tape"?
I've purchased that grade of tape from blackwoods. 3M's VHB and UHB tapes are suitable.

Weber and Coulomb's MX-5

Posted: Fri, 02 Aug 2013, 04:31
by weber
Johny wrote:Where does one purchase "black automotive exterior-grade double-sided tape"?

I found it at my local hardware store. It was this product: ... p-146.html. Also available in 24 mm wide. The 3M products may be more reliable.

Weber and Coulomb's MX-5

Posted: Fri, 02 Aug 2013, 15:40
by Johny
Thanks BigMouse, thanks weber. Yes, I've seen that stuff. I thought you may have been onto some other stuff that might be stronger. They have an enormous reel of white double sided stuff here at work that is almost impossible to get back off glass when you even touch it on. I'm after something similar in black.

Weber and Coulomb's MX-5

Posted: Tue, 06 Aug 2013, 00:31
by weber
Hi guys. Last Thursday my father and I leveled up some pads under the MX-5's wheels using Jeff Owen's laser level, jacked the car up to a known road clearance front and rear, and measured the static lengths of the suspension springs. Then we removed the four coilover units (coil springs over shock absorbers) in preparation for sending them off to have new springs made and installed, to deal with the extra weight of the converted MX-5.

But how to be sure of getting the right springs made? I've learnt an awful lot about car springs in the last 24 hours and thought I'd share it. Most of the information was in pounds and inches. I have converted it all to real units.

The two most important parameters for a spring are free length (mm) and spring rate (kg/mm or N/mm). These are available off the net for an NA MX-5 as
Front 282.4 mm, 2.94 kg/mm (or 28.8 N/mm)
Rear 339.6 mm, 1.73 kg/mm (or 17.0 N/mm)

One major advance in my understanding was learning about the difference between "spring rate" versus "wheel rate" and the ratio between them, which is the square of a number called the "motion ratio". The motion ratio is effectively the lever ratio due to the wheel's road-contact patch being further out on the wishbone than the coilover unit is. Various people on the net have measured the motion ratios for an NA MX-5 and most seem to agree on
Front 0.66 +- 0.01
Rear 0.73 +- 0.01

Also the unsprung weights for an NA MX-5 with our standard 12.5 kg wheels and tyres are given as
Front 28.0 kg each
Rear 26.6 kg each

Being able to find these numbers is one advantage of converting a vehicle that many people have modified in the past, not for electric operation but for racing.

My father and I measured the static spring lengths at Mazda's specified 115 mm of road clearance. We got
Front 162 mm
Rear 169 mm

Hooke's law of springs plus the law of levers tells us that for an ordinary linear spring like the MX-5's (as opposed to "progressive" springs) we have the formula:

static_length = free_length - (corner_weight - unsprung_weight) / spring_rate / motion_ratio

Given all the above measurements, we can run this backwards and calculate the corner weights that would give 115 mm road clearance.

corner_weight = unsprung_weight + (free_length - static _length) * spring _rate * motion_ratio

We get corner weights
Front 262 kg each
Rear 242 kg each

Before starting the conversion we weighed the MX-5 with all ICE gear including spare tyre and a full fuel tank (but no driver, passenger or luggage) with corner weights:
Front 258.5 kg each
Rear 233 kg each

These are only a few kg lighter than the figures calculated above, so it's clear that the Mazda 115 mm road clearance was only for the unloaded vehicle.

In Queensland we currently have a minimum road clearance of 100 mm when the vehicle is fully loaded. The motion ratios tell us that a 15 mm reduction in road clearance translates to the following reductions in static spring length.
Front 15 * 0.66 = 9.9 mm and the new static spring length is 162 - 9.9 = 152.1 mm
Rear 15 * 0.73 = 10.95 mm and the new static spring length is 169 - 10.95 = 158.05 mm

From these we can calculate the corner weights that would have resulted in minimum road clearance.
Front 281 kg each
Rear 256 kg each

This is only an increase of 66 kg in total vehicle weight over the 115 mm case, which suggests the addition of only a light weight driver. So with a passenger or luggage it must have been well below the Qld legal minimum, particularly in the rear. I calculate that with 80 kg driver and 80 kg passenger, road clearances would have been 93 mm front and 72 mm rear. No wonder the top-hat rails spot-welded under the cabin floor are so badly scraped and dented.

We were previously considering allowing for an 80 kg driver, 80 kg passenger and 40 kg of luggage in our new total weight. However, if we have the new springs made to give us 100 mm clearance under those conditions, we will end up with the rear end sitting 30 mm higher in driver-only conditions, which can't be good for handling, particularly given all the extra weight due to the batteries.

So now I plan to go for a total weight that includes only the NCOP14 minimum load for a two-seater, of 68 kg driver, 68 kg passenger and 27.2 kg of luggage. When this is distributed (assuming all luggage is in the boot) we get corner weights of
Front 369 kg each
Rear 409 kg each

Now we solve for the spring_rate and substitute the above corner weights and the static lengths corresponding to 100 mm road clearance (152 mm and 158 mm).

spring_rate = (corner_weight - unsprung_weight) / (free_length - static _length) / motion_ratio

And we obtain new spring rates of
Front 3.96 kg/mm (or 38.8 N/mm)
Rear 2.84 kg/mm (or 27.8 N/mm)

If we make a small concession in the direction of the previous plan and also allow for the case of 80 kg driver and 80 kg passenger (but with no luggage), then we need to increase the Front spring rate to
Front 4.03 kg/mm (or 39.5 N/mm)
Rear 2.84 kg/mm (or 27.8 N/mm)

Compare these to the original spring rates of
Front 2.94 kg/mm (or 28.8 N/mm)
Rear 1.73 kg/mm (or 17.0 N/mm)

That's a 37% increase in front spring rate and a 64% increase in rear spring rate. That 64% increase in the rear isn't because we increased the rear weight by 64% (heaven forbid) but because the original car was illegally dragging its rear around 63 mm off the road when fully loaded. In fact we have "only" increased the rear weight by 39% (and the front weight by 30%).

But wait! Why didn't I consider changing the free length of the springs instead of their rate? The answer is that the "feel" or stiffness of the suspension is given by its natural bounce frequency, which is around 1 Hz for typical passenger cars including the MX-5, and around 2 Hz for racing cars. It turns out that the bounce frequency is given by

bounce_frequency = sqrt[ 9.8N/kg / (free_length - static_length) / 1000mm / motion_ratio ) ] / (2*pi)

So, if we change the free length we change the bounce frequency and therefore the feel of the suspension. Another way of looking at it is that we need to increase the spring rate at least proportional to the mass so that if it took a 1 gee bump to bottom out the suspension originally it will still take at least a 1 gee bump to bottom it out with the increased mass.

But why not reduce the free length, increase the spring rate and go to more racing-like suspension? The answer here is money. I'm going to try to get away with using the same shock-absorbers. While the bounce frequency is proportional to the square root of spring rate divided by mass, the amount of damping required from the shock absorber is proportional to the square root of spring rate multiplied by mass. So I don't want to increase the spring rate any more than is required to maintain the bounce frequency and the bottom-out gees. So the final spec:

Front 282.4 mm, 4.03 kg/mm (or 39.5 N/mm)
Rear 339.6 mm, 2.84 kg/mm (or 27.8 N/mm)

I'd love to hear anyone's thoughts on this before I take the old coilovers in to King Springs with these specs.

Weber and Coulomb's MX-5

Posted: Tue, 06 Aug 2013, 18:11
by weber
Coulomb has pointed out, and I agree, that there must be something wrong with either my calculations or the measurements I have used, since there is no way that the original MX-5 was "illegally dragging its rear around 63 mm off the road when fully loaded".

As Coulomb reminded me, the day I bought it, we threw some toolboxes in the boot and drove from Brisbane to Gympie to do some modifications to a battery-backed-up solar power system I had installed. Sure it scraped on my driveway, but it wasn't that low.

Further research suggests that the 115 mm road clearance figure was when loaded. This history claims 140 mm (presumably unloaded). The current model MX-5 specs clearly state "118 mm laden". The history also states that the much vaunted 50:50 weight distribution occurs "with an average weight driver and a 2/3 full gas tank".

I learned elsewhere that the lowest points in the ICE NA were the drain plug in the engine oil-pan and some part of the exhaust system. I had mistakenly assumed that what is lowest now was lowest then -- namely the chassis rails under the cabin floor. But this just makes my calculations even more off, as does the fact that the tyres are currently inflated to an excessive 260 kPa. The wheels and tyres are stock standard 185/60R14.

Coulomb suggested an SI conversion error, but I had already done them all twice. Then he suggested putting all the original pound and inch numbers into the spreadsheet and converting my own measurements to pounds and inches, and then only converting the final results back to SI. That sounded very wicked, but I was desperate enough to try it. It made no difference beyond some tiny rounding errors. Now I have to dig a deep hole to bury that spreadsheet in, and take a shower. Image

I don't actually have to give the spring guys a free length and a rate. Since I am giving them the original springs I can just tell them I want the same free length and give them a static length and tell them how many additional kilograms of weight I need the new springs to support at that length. They can measure the rest off the old springs.

But in that case, the two sets of numbers I need to be certain of, and that the failure of these calculations makes me uncertain of, are
(a) the static spring lengths that corresponds to 115 mm ground clearance, and
(b) the motion ratios between wheel and spring.

I can still make my own measurements of the motion ratios, although they will not include the effect of the compression and expansion of the rubber bushes in the wishbone mounts and coilover mounts. But I'd prefer not to have to put all the coilovers back in to repeat the static length measurements. I'll tell you how we did them so you might suggest a possible flaw.

With the car on stands we sat Jeff's laser level on the concrete at the front of the vehicle and put under the wheels the same 19 mm hardwood ply pads that we used for the recent corner weighing. We packed them up with other pieces of wood until they were all 30+-0.5 mm below the laser, after the car was lowered onto them. The car had only half its complement of batteries. We bounced the car up and down several times front and rear to settle it and eliminate any wheel-spreading effect that might have been holding it up.

We found the lowest points (the aforementioned chassis rails) front and rear by measuring up from the laser. They were around 80 mm up. We placed a jack under the diff and under the front cross-member and gently jacked until the rails were 85 mm above the laser for a total of 30 + 85 = 115 mm above the wheel pads. Then I crawled around with a tape measure and a head-torch and called out measurements to my dad, which he wrote on the whiteboard. The measurements were made in 3 places 90 degrees apart around the spring in the case of the rear springs and two places in the case of the front springs. The lower spring seat was flat but the upper one was cupped over the spring in some places. I mostly measured inside to inside between the spring seats, but where it was cupped I measured to the outside and subtracted the 3 mm seat thickness. The measurements on any one spring only ever difered by 1 mm. And the average measurements differed between same-axle springs by at most 1 mm. We averaged all rear measurements and round to the nearest mm and got 162 mm, and for the front 169 mm.

Then we rechecked the ground clearance measurements and found that they had crept up by a few mm. Groan. We readjusted the jacks and repeated all the measurements. A few changed by +-1 mm but the overall result was the same. Then we removed all the coilover units.

Weber and Coulomb's MX-5

Posted: Wed, 07 Aug 2013, 05:05
by weber
I got the coilovers into the laundry tubs and cleaned them up. I counted the active coils and measured the wire diameter. These, and the coloured paint dots (red for front, blue for rear) agree with the specs in this table. And plugging the numbers into this spring rate calculator gives pretty close agreement with the spring rates I've been using.

Then I measured the installed lengths of the springs and the available travel to the bump stop, and subtracted them to obtain the spring length when on the bump stop.
      Spring     Spring     Calculated
      installed  travel to  length at
      length     bump stop  bump stop
Front 218 mm     79 mm      139 mm
Rear  232 mm     89 mm      143 mm
Dividing the spring travel by the motion ratio gives the corresponding wheel travel.
      Motion  travel to
      ratio   bump stop
Front 0.66    120 mm
Rear  0.73    122 mm
Given the spring lengths that we measured as corresponding to 115 mm ground clearance (Front 162 mm, Rear 169 mm) we can work out how much wheel travel remains from there to the bump stops. It comes to: Front 35 mm, Rear 36 mm. That means that the bump stops are contacted when the chassis rails are 80 mm off the ground. And at full spring droop the car will be 200 mm off the ground. The unloaded 140 mm clearance figure is exacly halfway between these extremes. So this all makes perfect sense.

What doesn't make sense is the low weight that the formula says is required to get to those ground clearances.

Here are some diagrams showing the suspension geometry of the MX-5.



Weber and Coulomb's MX-5

Posted: Wed, 07 Aug 2013, 13:57
by coulomb
The spring geometry looks like it would be easy to get the motion ratio wrong. With the coils removed, is should be pretty easy to move the "wheel" from top to bottom, and measure what the spring travel would be if it was installed. Just to sanity check those numbers.

Weber and Coulomb's MX-5

Posted: Wed, 07 Aug 2013, 22:51
by weber
coulomb wrote: The spring geometry looks like it would be easy to get the motion ratio wrong. With the coils removed, is should be pretty easy to move the "wheel" from top to bottom, and measure what the spring travel would be if it was installed. Just to sanity check those numbers.

I did as you suggest, for the rear only, which is where the biggest discrepancy is. I've been using 0.73. It is very non-linear. The best I can determine, after measuring 7 positions, is that it is somewhere between 0.71 and 0.75 in the region of interest. Changing it to 0.75 in the spreadsheet made a slight improvement but nowhere near enough. I'd have to change it to 0.9 to get sensible answers.

Weber and Coulomb's MX-5

Posted: Sun, 11 Aug 2013, 03:41
by weber
Last EV day I made a crude measurement of the preload and rate of a rear spring, using Jeff Owen's carpenter's clamp, two bathroom scales, some planks and packers and a tape measure. It only served to confirm the crazy low number obtained elsewhere.


I took my spring puzzle, in a simplified form, to the US MX-5/Miata forum, which is where I got most of the numbers from. You may be reassured that it made their brains hurt too. Unfortunately, it remains unsolved [Edit: or maybe not]. ... ost6488594

Coulomb wryly observed, that in that thread, I am "speaking imperial [units] like Emperor Nero".

One suggestion, which I have taken up, was that we should use adjustable coilovers so it isn't necessary to calculate everything precisely. After much deliberation I chose these Yellowspeed DPS from MX5 Mania, partly because the other brands were too long at the top and would have poked into various contactor boxes.


Coulomb completed this contactor box inside the under-boot battery box, which breaks it into two ELV segments.




The two of us also tied off a number of software loose-ends, in cell management software and charging software.

All-in-all a satisfying and productive day.

Weber and Coulomb's MX-5

Posted: Sat, 17 Aug 2013, 05:48
by coulomb
Where does the time go in an EV conversion? I've asked this before. I'm starting to believe that, if you choose to develop your own battery management software, then most of it will go to weird software issues.

Today for example, a string of BMUs installed in the car seemed to not to download a new version of software. This is particularly irritating, because we pride ourselves on the ability to download new software at any time, and taking the battery boxes out of the car to JTAG program each battery management unit (BMU) is a fearsome amount of work.

So we went to repeat what we thought we did on the rollbar box, because if needed, we can access those cells easily. We loaded in our TestICal software, because it has a few options that the regular monitor software doesn't have. That downloaded successfully, but when we tested it, it behaved extremely strangely. For example, most of the BMUs thought that they had identity number 49, when the last ID in the box was definitely 45. Some reported their voltages in hex, others in decimal as standard. There were other weirdnesses that I don't recall now.

We eventually traced this to the software not receiving its initialisation call, so it was operating on uninitialised RAM. For a PC program, this will guarantee spectacular failure, but these programs have battery backed up RAM Image It was only because Weber had recently moved locations in RAM that this showed up. The reason for the lack of initialisation was traced to a discrepancy in version between two pieces of software. For reasons I don't want to go into here, we have three pieces of software running at once; BSL1 (bootstrap loader number 1), BSL2, and either the monitor or the TestICal software. There is an easy way to ensure that this won't happen again in future, and we'll fix that soon.

But now we realised that the reason the original download failed was another incompatibility between these three pieces of software. Worse, this incompatibility meant that when the "password" sequence for a download was detected, the program would jump off to one of the other pieces using the wrong address. In fact, it ended up in the middle of an instruction. This meant that we could not download new software, and would have to JTAG each BMU separately, which necessitated pulling out all the battery boxes again. Surely there was another way!

The oldest bootstrap loader, BSL1, was still in these BMUs, but would immediately hand over control to the monitor program. The monitor program only listened for the password that is handled by BSL2. We had intended for it to handle the password for BSL1 as well, "just in case", but hadn't gotten around to it. Well, it would have been really good if we did! If we could find a way to cause the BMUs to activate their watchdog timers, then they would not allow the monitor to take control, and BSL1 would be there to accept new downloads. But the only way the watchdog timer takes effect is if something goes badly wrong, like a crash. I joked about getting one of those "zapper" machines that unscrupulous people use for crashing poker machines, sometimes causing them to eject all their coins. We also considered whether we could engineer a buffer overflow, but we'd fixed that problem years ago.

The astute reader will have realised by now that the answer lay with the monitor program jumping into the middle of an instruction when it detected the download password. But we almost missed this, and had resigned ourselves to pulling out the battery boxes "one last time". But sure enough, when the monitor jumped into the middle of an instruction, it crashed, and the watchdog timer reset the processor, and BSL2 wisely refuses to allow the monitor to run, so BSL1 is available to download new software. However, when the first BMU crashes, it doesn't pass the complete password to the next BMU, so only one BMU is thereby able to download again. But we found that by downloading a second time, the second BMU became able to download. So we could just attempt 109 downloads, and all would be good! On the 110th download, we could load a new BSL2, which would match the monitor, and so it would be able to download a new monitor or TestICal from then on.

In fact, we found we didn't have to wait the ~ 2 minutes for a complete download; we just needed to send the password (4 control characters). It only took a few seconds to send this by hand using a terminal program, and just repeat until all the BMUs were ready for downloads.

While this is a great theory, when we tried it, it didn't work. We tried it on a second set of battery boxes (nothing to loose, right?), and it also failed. One of the battery boxes has its lid readily able to be removed, so we did that, and used JTAG to figure out what was going on. Alas, this caused more confusion. It turns out the clock calibration on the first BMU was off a few percent, which corrupted only BX bytes into 9X, and didn't seem to affect other bit patterns at all. How weird is that? Well, par for the course today, it seems.

After this was fixed, the software still would not propagate past the third BMU. I suspect its speed calibration was also off a little, but by now it was after 10pm and I had to go home. But these BMUs have had many successful downloads, so it seems ridiculous that they would corrupt downloads just today.

Stay tuned to find out if Weber and Coulomb have to take out the battery boxes after all, or can they do it all with just one easy cover removed!

Weber and Coulomb's MX-5

Posted: Sat, 17 Aug 2013, 15:49
by PlanB
TestIcal diagnostics! I'd expect nothing less from the MX team. Should I cough once?

Weber and Coulomb's MX-5

Posted: Sat, 17 Aug 2013, 16:02
by weber
More plot twists than an Agatha Christie novel, but with suspects and weapons whose nature is hard to follow? As usual, Coulomb was nice enough not to spell out that it was all my fault.

You would think that since bootstrap-loading new software into our celltop monitoring units is so obviously fraught with danger at this point in development, we would be testing each download with a string of them on the bench before risking it on those buried in the car. The fact is, we have a strict policy of doing that, and I did it in this case too, and it worked on the bench! Twice! But the gods have ways of disrupting the best laid plans of mice and maintenance engineers. It turns out that I did something in a different order when I came to do it in the car, which normally wouldn't have mattered, except for the mismatched software-components bug Coulomb mentioned, which didn't cause a problem with the order used in the bench test. Very clever, Loki. Or is it Lilith? (the demoness who had a computer named after her)

Anyhow, while I was busy stuffing things up with the software, there was some real work getting done. Mechanical work. Work where you can actually see the results with the naked eye.

Coulomb successfully modified the steel-angle lid of the under-boot battery box with cutting disk, angle-grinder, paint and foam sealing strips, to allow a new cable exit now that it has a reduced number of cells.

Newton (Jeff Owen) sucessfully installed the four new coilover units (spring over shock absorber), after first adjusting the preload on all their springs, by numbers of millimetres calculated to deal with the additional weight, and thereby restore them to the middle of their travel. The actual ground clearance is a separate adjustment which will be done when the car is fully loaded and back on the ground.

Weber and Coulomb's MX-5

Posted: Sat, 17 Aug 2013, 16:25
by weber
PlanB wrote: TestIcal diagnostics! I'd expect nothing less from the MX team. Should I cough once?

I get it. Very good. I'd never noticed that before. I just thought it stood for Test, ID and Calibrate. Image

But then Mark Aylott did keep saying "This thing's got cohones!" when he had a ride in it.

Weber and Coulomb's MX-5

Posted: Sat, 17 Aug 2013, 16:31
by PlanB
Meanwhile, the missus is off cruising around in her Leaf? I envy you both. Nice day here in Sydney, I might take the EV scooter for a run. Hey, it's humble but it's gearless hub motor sustains me while I dream of other things, like a Model S.

Weber and Coulomb's MX-5

Posted: Thu, 22 Aug 2013, 19:47
by coulomb
coulomb wrote: Stay tuned to find out if Weber and Coulomb have to take out the battery boxes after all, or can they do it all with just one easy cover removed!

Weber has done it! No battery boxes had to be removed, although he did JTAG all the easy-to-get-to BMUs in the front fuel tank area.

Why our recovery procedure didn't work first time is still a bit of a mystery. But it probably involves a blunder we made where we JTAGed the monitor software into the first accessible BMU. When downloading BSL2 via BSL1 (yes, our heads hurt too), the monitor was not passing on the download image bytes unchanged as we were imagining, but was doing subtle transformations to what it regarded as status bytes (any with the high bit set). In particular, it was doing the transforming of Bx to 9x, not slight clock speed variations as we had thought. That's why it was so selective. We were lucky that there were carriage returns ($0D bytes) in the BSL2 image, so the monitor didn't try to interpret the image bytes as commands and insert its responses into the download stream.

We've saved "core dumps" of the BMUs that were JTAGed, in case we can distill more spirit messages from those tea-leaves.

Weber and Coulomb's MX-5

Posted: Sat, 24 Aug 2013, 18:57
by weber
Yesterday was an historic EV day for the MX-lightning. Many thanks to Coulomb and Newton (Jeff Owen). (And all those who have helped at earlier times). We drove it for the first time with its full mass. All 10 (!) battery boxes were bolted in for the first time and it was lowered onto its new suspension for the first time. Rear ground clearance was barely the legal 100 mm minimum so we'll be adjusting that up a bit, but ride and handling seemed just fine on the brief test drive (power steering working well, but noisily).

We are still only driving on the first half-pack. There are some more contactor boxes to be completed and wired in. But we can start making compliance-checking noises to our engineer now.

Here's a delightful photo sequence captured by Jeff.

Image Image Image Image Image Image Image Image Image

Weber and Coulomb's MX-5

Posted: Sat, 24 Aug 2013, 20:05
by gholm
A hearty congrats to you two. What an epic journey and to finally have it all installed and rolling is truly a major milestone.
Well done.

I wish you well along the ever-shortening trip to the Engineer's elusive big green tick.

Weber and Coulomb's MX-5

Posted: Sat, 24 Aug 2013, 20:36
by weber
When I see that big slab of cells being lowered into the front of the MX-5 and I realise that's only a quarter of the cells in the MX-5, and we still have both seats and much the same boot space, I appreciate what a miracle has been performed by this team in getting 28 kWh of LiFePO4 cells into this vehicle. That's more than a Nissan Leaf.

Yes, that makes it 33% heavier than the original when both are fully loaded, but at only a 4C discharge rate that's 28 * 4 = 112 kW available, which is about 33% more power than the original petrol engine.

We often joke about the tight clearances involved in doing this. The standard line is "What are you worried about? It's got the standard 0.5 mm clearance." Yesterday Jeff had to invent "Special Tool number 53" which unfortunately we didn't photograph before it was disassembled, in order to do up some battery box mounts which had seemed readily accessible before cables and contactor boxes and optic fibre bundles got in the way.

But at one stage we thought we were really up against that silly rule about no two solid objects being allowed to occupy the same space at the same time.

When I installed the PWM module for the power steering pump, I thought I had plenty of clearance to the under-bonnet battery box, but I forgot the DC-DC converters bolted to the side of it. Fortunately we had to move the under-bonnet box by a metric milli-smidgen in a favourable direction anyway, to get its mounting holes lined up. Whereupon Jeff announced that yes, there was now clearance. However this was not the aforementioned 0.5 mm. I said I guessed light might get through. But Coulomb suggested possibly only blue light. I said what about an air molecule? Coulomb and Newton both said, "Only if it happened to be rotated the right way at the time".

Ah, the company of a few wise men engaged in a common pursuit. There's nothing like it. Image

Weber and Coulomb's MX-5

Posted: Sun, 25 Aug 2013, 16:34
by Johny
Great work guys. I'm reading along with the progress.

Weber and Coulomb's MX-5

Posted: Sun, 25 Aug 2013, 17:20
by Renard
Terrific commitment and stamina -- leaves me feeling a bit faint.
And it's just as well that even when it's on the road and legal, it will never be completely finished. Image

Weber and Coulomb's MX-5

Posted: Mon, 26 Aug 2013, 16:20
by woody
Good work guys, your "warts and all" documenting of the process is a great read :-)

Weber and Coulomb's MX-5

Posted: Mon, 16 Sep 2013, 07:58
by weber
Thanks all, for the kind words. And now, for all those reading along with the progress, here's the next warts-and-all episode from those terrifically committed* guys, along the ever-shortening path to the Engineer's elusive big green tick. *[They should have been committed long ago]

We finally got around to building and installing the heater. You can see the PTC ceramic elements being removed from their original 240 V fan heater in this earlier post.

We cut out two sections of the original wet heater core, using a jigsaw, giving 10 mm clearance all around the elements. We also cut away the inside of the two tanks, and later drilled some holes in their outsides, to feed the fibreglass-insulated wires through.

Matchboxes (remember those?) just happened to be the right height to hold the elements centred while they were tacked with some blobs of silicone. When that had cured overnight the rest of the space was filled in. Some fibreglass-reinforced epoxy (printed-circuit-board material) was used to fill in the large sections near the terminals.

Image Image Image Image

Then we decided to see what would happen if the worst came to pass and the heaters were operated without airflow. Smoke bad.

A little while after Jeff stopped taking this video, the elements got to about 215 degC and one of them made a popping noise and spat out something, with a small but visible flash. Apparently the breakdown voltage decreases with increasing temperature and it arced over at the weakest point. There's a great paper that Johny found, on the properties of PTC ceramics, here.

Here you can see a closeup of the damage, and the damaged element being removed to be replaced with a new one.

Image Image

Here's where it connects to the two 360 V half-packs. The brass tubes have clear PVC tubing pushed onto them for insulation. The next photo shows it with the cover on, and the 12 V microswitch that senses the Cold-Hot flap position and the relay that drives the coils of the two EV200 contactors that switch the 360 V. The microswitch is in series with a contact in the fan-speed switch that is switched to chassis-earth only when the fan is set to speed 2 or above. And the other side of the relay coil is powered from the same fuse that the fan is on. These three things make it very unlikely that the heater can operate without air blowing thru it.

Image Image

Here are the uncomfortable and undignified positions one has to adopt to install this heater, and take the above photos. My wife walked past at one stage and offered the following helpful advice, "I'm sorry to have to tell you this, David, but that's not the correct position for driving a car. It's the right posture, but the wrong orientation."

Image Image

Copy cat.


I think Eric von Dunnycan got it wrong. Based on my experience above, I think we now know what this guy was doing.


But we still have absolutely no idea what she's doing. But hey, I'm not complaining.


[Edit: Uploaded the image that was originally at, which is no longer accessible. Turns out it was a Mayan inspired art installation entitled "Chariot of the Goddess". The joke is that the ancient Mayan image above depicts, not a ancient astronaut, but an ancient DJ.]

Weber and Coulomb's MX-5

Posted: Mon, 16 Sep 2013, 13:58
by coulomb
weber wrote: My wife walked past at one stage and offered the following helpful advice, "I'm sorry to have to tell you this, David, but that's not the correct position for driving a car. It's the right posture, but the wrong orientation."

I don't know. With suitable mirrors and a lot of high quality padding, it might be ideal for maximising that "EV rush" Image
Copy cat.


Heh. Cute kitty, in this photo. What you didn't mention is this cat's psychology. It seems to be able to lead Jeff Owen into some sort of feline trance every EV day, whereby Jeff is put into a false sense of security. Then inevitably the cat sinks the teeth in at a moment of vulnerability. Still, it's now one of the EV day rituals, like the announcement of first blood for the day. Image

Weber and Coulomb's MX-5

Posted: Mon, 16 Sep 2013, 18:13
by PlanB
You should be careful of unusal positions when working on cars

Weber and Coulomb's MX-5

Posted: Mon, 16 Sep 2013, 19:16
by Johny
Image I nearly feel off my chair at the punchline.