Weber and Coulomb's MX-5

The tie rod end IS a ball joint.

How do you weigh a car, and the various pieces that come out of it? Ross Pink suggested this idea: various planks of wood and a set (about 8) of bathroom scales. We adapted his idea a little by only measuring one axle at a time and adding the result. We considered just one wheel at a time, but that would compress the spring on the wheel being measured more than usual, and add more force that the scales would interpret as extra weight.

Here is the setup we used:


A closeup:

Those are six identical bathroom scales purchased at the local hypermarket for $11 each. We considered pooling our various existing scales, but they were slightly different heights, so they would not share the load evenly. As it was, the unevenness of the concrete floor meant that some scales registered twice that of the others, but it doesn't matter; the sum of the weights should be correct.

Edit: The tyres rest on some square section steel fence posts I had lying about, cut to the length of three scales side by side. They are held together with some steel strap, so they don't come apart and let the weight of the wheel come down on the middle scale by itself.

Edit: We compensated for the weight of the posts (about 3kg) by simply adjusting the zero to actually read zero after the posts were placed on the scales. The actual readings with only the posts changed a bit, but it all cancelled out as the readings still added to zero.

We ended up with 466kg on the rear, and 517kg on the front, for a total of 983kg. It happens that the 1993 MX-5, which has the same 1.6L engine and power steering as standard, has a published kerb weight of 983kg. Amazing. For the record, this is with a stock 1990 MX-5 but with power steering, air conditioning, full petrol tank, spare tyre (taking it out registered 8.5kg less on the rear), front tie hooks removed, rear tie hooks present.

Edited by coulomb - 12 March 2009 at 10:38pm

Finally, the tank is out:

At the top you can see some of the pipes that came out (and ran 3/4 of the length of the vehicle), and also the "jiggle siphon" that we used to drain most of the fuel from the tank before starting. I tried to buy an ordinary squeeze siphon, and couldn't find one. Big W had these, and they work well enough. Jiggling the hose in conjunction with the ball valve weakly pumps fuel to get the siphon started (no sucking required ).

We decided to lower the rear sub-frame (carrying the differential), rather than taking it out altogether. That meant we had to manoeuvre the tank between the top of the tank retaining metalwork and the top of the subframe. It was a huge effort to get it through this small gap.

The biggest mistake we made was to worry about the subframe sagging down too far, or falling on us. So we built stacks of bricks and wooden planks to rest the bottom of the subframe on. However, this proved unnecessary and also prevented the subframe from going low enough to get the tank out. We had to rotate it so much to get it out that it ended up upside down.

The resultant space looks like it will provide good battery space for us and make our planned conversion possible. Zeva found that he could not get his larger batteries in there, but with our much larger motor controller, we'll need to use most of that space (near where the fuel tank came from) for batteries.

Originally posted by weber I think we'll just replace the spare tyre with a can of emergency inflater-sealer. Is that legal?

I've heard that it is not a registration requirement to have a spare but if you do have a spare it must have legal tread. This may have been NSW, not sure about Qld.

Originally posted by weber The MX-5 fuel tank is the most ridiculous and asymmetrical and complicated shape you ever saw. I've seen toads that look nicer.

This image may convey a little more of the toady ugliness:

Originally posted by Electrocycle wow! worst fuel tank ever!

Yeah. But I have to say, the vehicle that was wrapped around it so tightly is the most gorgeous thing ever. And soooo much fun to drive. Next best thing to a motorcycle for throwing around corners on winding mountain roads, with the top down. I'm having withdrawal symptoms.

But gentlemen, we can rebuild her. We have the technology. We have the capability to build the world's first AC MX-5. Better than she was before. Better, stronger, faster.

And she won't cost six million dollars.

Forget the Tesla, here comes the Coulomb and Weber Roadster.

Sorry. I got a bit carried away there.

Edited by weber - 25 March 2009 at 9:34am

Looking at that tank photo, I think some of that 6 million dollars will need to be spend on air bladder support before that leaps anything

often it's easiest to remove the engine and gearbox together, but I haven't done it on an MX5 :)

Oooh, fan-ceeeee!       Welded, painted, shiny bolts, very nice.

We condidered just doing one wheel, but decided it would be inaccurate because of the different compression of springs. I hope your "chocking up" works well to counter that.

The MX-5 is ready for Nerd's Night:


The idea is that many eyes and hands can watch for and correct hoses and cables that are about to be caught etc. Plus, it's a nice, hands on nerdy thing pulling out an engine (and gearbox this time; we decided it will be easier to separate the engine and gearbox out on the garage floor).

There is still one last hard to get to bolt that has to come out before the last hydraulic pipe (clutch) is free.

The engine is out!

Weber decided that it was easier to take the gearbox with it, and separate the two on the garage floor.

The engine bay is moderately clear; there are plenty of cables and pipes to clean up.


When attempting to lift the engine at first, we found that we were just lifting the front of the car. One engine mount was coming loose, but the other seemed stuck. We tried various raisings and lowerings of the engine and gearbox, to no avail.

Finally, I decided it might be stuck against the firewall, so we reconfigured the chain to pull only on the engine lift hook nearest the firewall. Almost immediately there was a great crack and a jolt; I'd say the engine mount had 19 years of sticktion that needed force from a different angle. We reconfigured the chains on both bolts, and it came out. It was a very tight fit, partly because we couldn't readily adjust the angle of the engine with the chain in tension. In fact, the top of the gearbox pulled a bit on part of the firewall at the top of the transmission tunnel, distorting it slightly. No biggie. We were much more concerned about damaging the air conditioner radiator.

Pulling the engine and gearbox really was easier with four people. Two handling the engine lifter, two making sure that things weren't getting stuck or damaged.

The engine and gearbox combined weighed in at 170 kg, including alternator but no steering pump or air conditioning compressor.

Edit: extra picture

Edited by coulomb - 02 April 2009 at 5:41pm

The digital scales on the boom lift say 500kg

(just above the safety footware !)

Question. Do you have a mechanical engineering signatory lined up ?
What is their comment re cutting of bodywork in a monocoque vehicle ?
What suggestions do they have re structural integrity ?

This would be good info for followers of this thread.

That's our friend Eddie with the camera and the bare feet. You'll note he was well back. Although he had crawled under the vehicle earlier, to free the gearbox from something it was catching on. It really helped to have 4 competent people there when removing the engine and gearbox.

We were very concerned ourselves about what we could and couldn't cut on the MX-5, so we talked to the best person imaginable, in the whole of Australia, for advice on this topic. He just happens to be here in Brisbane.

*****************************
* Richard Larsen, BEMech, MIEAust *
*****************************

He not only wrote the first edition of NCOP 14, as you can see here: NCOP 14 Preface
but he also happens to be an expert on MX-5s (and a really friendly bloke).

On weekdays he runs Loadsafe Australia and on Saturdays he runs MX5Plus.

So last Saturday we traded a bunch of MX5 parts that we won't be needing any more, for an hour and a half of his time. We prepared a list of questions in advance. And, being at MX5Plus, we had a stripped shell of an MX-5 in front of us to walk around and point at.

Even though it is monocoque, there are fairly well defined chassis rails and cross members. They are recognisable as box beams despite being made of folded and spot-welded sheet steel of the same thickness as the outer skin. But sometimes it's not obvious. Sometimes you have to measure things inside and outside the vehicle to be sure it's a single skin you're looking at and not a boxed in section.

Basically, we could cut any single layer of steel sheet between those rails and cross members, on the assumption that the battery racks, trays or boxes put into those spaces will restore the lost stiffness. They will typically more than do so.

Richard wasn't able to be our engineering signatory, but recommended another engineer, David Blythe, who we will be contacting soon.

[Edit: Changed "did crawl under the vehicle later" to "had crawled under the vehicle earlier" and fixed up NCOP 14 Preface URL]

Edited by weber - 03 April 2009 at 9:04am