Ghettobuilt transmission jack

[3gress]

Morlock you are right in calling attention to safety. There was a tragic garage accident down this way not long ago taking the lives of father and son. As Keith knows i have care of a child to consider and very little in the way of a support network so safety is a major consideration.
The trolley/jack device was tested extensively before being wheeled under the car so we could be certain it wouldnt shift while being manipulated about. However it did inflict an injury or two...or three, as i had a tendency to continually bang my head against the exposed jack screws. One modificafion I'd make would be to place a rubber bung over the end of the screws to prevent such pain inducing incidents!

[97 MXV]

Looking at this video also suggests There must be a better way. But only if it is a better way and that is using a Ghettobuilt transmission jack with mandatory protected threads.

...i had a tendency to continually bang my head against the exposed jack screws. One modificafion I'd make would be to place a rubber bung over the end of the screws to prevent such pain inducing incidents!

Some say 3gress is a legend to volunteer so willingly for prototype safety testing ie forseeable misuse/abuse of the jacking threads head butted time after time.



Now believe it or not, Risk was calculated as Probability times Consequence.
While 97 MXV was laying under the car busy calculating the Probability of another occurrence each time as zero, 3gress's was also under the car preparing to bang away again to determine the Consequence to threads without the extra cost of having high visibility rubber bung thingos fitted to protect them.

[shirtz]

out of intrest, was the car on standard supercheap type stands? I ask because I need to remove the box soon and wondered if, even at their max height the transmission and that dolly would be able to slide out from under the car?

[97 MXV]

The car was on 4 cheapish stands under the normal jack points when we were under the car. Car ends up fairly low on such stands. 3gress might have the dimensions.

With the PPF, RHS rear wheel and rear NB8b body bracing removed we were able to easily slide the box back outwards down the tunnel and slide it out sideways with bellhousing projecting into the wheel arch in front of rear hub and slipping under handbrake cable. The new box went in the same way in reverse order. (I am not sure if there is bellhousing clearance if PPF still connected to diff ?)

Accordingly we did lift the box off and on the Ghettobuilt Transmission Jack under the car. That turned out easier than expected with two people. 3gress is thinking about how that could be made easier again rather than lifting the car further or redesigning the GTJ lower as those changes no longer satisfy the cheap Ghettobuilt improvisation demonstrated in the first place. (Good mates will always lend their MX5 scissor jacks and Supercheap stands for the cause of cheap improvisation)

[shirtz]

thanks keith. am preparing to tackle the job soon. any words of wisdom for a first timer on his own?

[slomo]

Don't rely on the 4 stands alone.

Make sure either rear or better still the front wheels are on blocks say half timber sleepers. (With slight weight on the wheels)

This will ensure the jack stands remain stable. When raised in final position, give the car a good shake to ensure stability.

Need to raise the car about 430mm (17inches) to give good clearance and room to move under the car. 18 inches is better, which is max height of most quality stands.

[greenMachine]

I bought 2 pairs of 3 or 4t jackstands, I think they go up about 5-600mm, cost $140 for the two pairs. Magic! Plenty of clearance, quite stable because wide bases. So good I went and bought another set of 4.

Do yourself a favour, get a set of these - you will thank me

[shirtz]

jack maxes out at 470mm unfortunately :/

[97 MXV]

Stands must conform to
AS/NZS 2538:2004 Australian/New Zealand Standard™ Vehicle Support Stands
Originated as AS 2538—1982.
Previous edition AS/NZS 2538:1995.
Fourth edition 2004.

CONTENTS show AS2538 stands are well designed so using just 4 of them is fine if floor is hard. In fact on a concrete floor, Sparkley was so torsionally stiff that it was hard to put load on more than 3 stands. AS2538 should have a strength and stability factor of safety to allow for load to be taken on just two stands diagonally opposite each other....it does happen.

1 SCOPE............................................................................
2 REFERENCED DOCUMENTS....................................................
3 DEFINITIONS....................................................................
4 DESIGN AND CONSTRUCTION................................................
5 STRUCTURAL INTEGRITY AND OVERLOAD CAPACITY.....................
6 INSTRUCTIONS FOR ASSEMBLY..............................................
7 INSTRUCTIONS FOR USE.....................................................
8 MARKING.......................................................................
9 PACKAGING....................................................................
APPENDICES
A METHOD FOR THE DETERMINATION OF STABILITY FACTOR............
B TEST FOR RELEASE UNDER LOAD..........................................
C TEST FOR STRUCTURAL INTEGRITY........................................
D OVERLOAD TEST..............................................................

The issue I have found is how well stands are located to the car body....can it slip/tip off sideways with just metal to metal contact..slippery steel with oil and grease. Proper locating means is key.
The idea of say front tyres sitting on large properly rated timber blocks or on properly rated ramps is a very stable and safe as houses for doing the job...even on a windy day.
The idea of shaking is a good idea too. If the car does not feel as solid as a rock with a good hard shake it needs to be solidly stabilized sideways with interference or friction or mechanical location.

As a young man I caught my arm between the tyre and fender when a car slipped sideways off a dodgy located jack and was lucky to get away with bruising.

[97 MXV]

Sparkley was so torsionally stiff that it was hard to put load on more than 3 stands. AS2538 should have a strength and stability factor of safety to allow for load to be taken on just two stands diagonally opposite each other....it does happen.

In the old days two stands under the live axle at the back and two stands under the body/frame at the front was safe as houses. The live axle sat on leaf springs which were stiff enough to be stabilize the car but compliant enough to share the load. But in our case we are not supporting under an axle with its inherent springiness, we are supporting a fully rigid body.

And BTW, given its 50/50 weight distribution, an MX5 is more likely than most cars to find itself supported on just two stands assuming the 4 stands are symmetrically placed relative to wheelbase.
This diagram shows a diagrammatical plan view looking down on a rigid body and its centre of mass in the middle supported on rigid floor with 4 rigid stands.



In this scenario given stands are not infinitely adjustable in height, it is most likely that 2 diagonally opposed stands support the load, much less likely that 3 stands support the load and practically impossible for 4 stands to share the load evenly. On the other hand If the body was torsionally flexible or the floor deflected down enough under the car's weight, or the stands were springy up or down, the 4 stands could more evenly support the load.

Practically speaking the system is not compliant enough to adjust for the the fact that the adjustment intervals in some stands cannot account for the different stand heights needed to take up the dimensional differences in height at each point of support.

Therefore the stands need to have a healthy safety factor just to start with, to account for this uneven load distribution possibility.

Also the eventuality that an unloaded stand is inadvertently bumped and moved is more likely. In this respect a resilient and grippy support pad such as thick insertion rubber between the stand and body can be a really good thing for making the stands more springy up and down and so loading the stands more evenly and therefore friction gripping the body to each stand better.

Also if possible get stands with a lot of adjustability positions. The stand on the left in this pic below is pretty cool in this respect.



Have not tested them, but these 4t Stanfred ones look awesome with their infinite position thread adjustment.





Not tested these 1.5t Stanfred ones either with their infinite position thread adjustment but they are a slender post attached by a weld to the middle of a thin wobble plate base so when fully extended obviously not likely to inspire confidence during the shake test. So likely not the thing if one is looking for extra height.

[97 MXV]

Imagine this scenario....a classic unlikely inevidability

The car seems to be sitting happily on 4 of these types of stands



These have a quick drop handle sticking out which is easily brushed against under the car. Hey presto if there is no load on the jack, the handle easily releases the ratchet mechanism and the stand collapses in the blink of an eye. The only thing stopping its rush to the bottom is the weight of the car pushing down and preventing the brushing force applied to the handle from lifting the ratchet mechanism a couple of mm out of its safety position and over the edge into its Kamikaze dive.

There is no positive retention keeping the ratchet safe or threaded adjustment to get rid of clearance for the ratchet to disengage, it simply relies on the balance of forces and the shape of the ratchet teeth. These are not positive means of protection against disengagement.
So if the brushing force magnified by the handle is greater than a threshold weight pushing down over a couple of mm needed to lift the ratchet out of its safety, then that stand is gone, collapsed in an instant, with nothing to stop it happening.

Now in the above post an argument is made that the car could be supported by 2 stands diagonally opposite each other. The other two stands are just placebos. They are taking hardly any load and there may be a couple of mm clearance between the two of them.
Lets say one is lightly loaded below a threshold force needed to secure the ratchet and the other is unloaded. Someone brushes heavily against the handle of the lightly loaded one and lifts it a couple of mm before the other stand takes up the slack.
But those couple of mm are just enough to lift the ratchet over the edge into a free fall.

Without the stabilizing support of that lightly loaded stand, the car goes into a death roll about the other two fully loaded stands, creating a tilted bearing surface and lateral thrust in the process, kicking them out of the way as it slides off them onto the victims trapped underneath.

Your Honour, there is no positive safety catch on the handle preventing release nor is there a threaded adjustment to remove the dangerous slack allowing the ratchet to lift out of its engagement. Lack of such basic safety mechanisms will lead to the inevitable catastrophic collapse of that stand foreseen in this scenario, a scenario where some of us could find ourselves brushing up against that fateful handle, fingers crossed its not one of us just Seconds from Disaster.

[97 MXV]

Don't rely on the 4 stands alone.

The problem that I see is many stands cannot be infinitely adjusted or spring loaded to remove slack to preload them so they can't be inadvertently shifted or disengaged by people working under the car.

My previous post identified the death roll about two opposing load supporting stands causing them to be ejected and the car coming down on top of its victims.
Adding extra unloaded backup arresting stands to restrain the car before it rolls appreciably is an option provided we are totally confident they will not fail on demand, that is they must not fail to arrest the car at the critical time needed when they transition from unloaded to loaded while people are under the car.

At the end of the day, some just slide a couple or inflated tyres and rims under there. To me that is admitting defeat. There must be a better way and I think that involves threaded adjustment and/or resilient support pads.

[manga_blue]

Firstly, 3gress, I just want to say thank you so much for posting this. I've got to do another gearbox install (the 4th?) in the near future and I've always struggled physically to shift the box in and out from under the car using just a trolley jack. As it happens I already have two of those dollies and 3 MX5 jacks, so I'm already set!

I have the same stands that 97 MXV just posted and to be honest I still don't like to have the car rocking a bit while I'm heaving on bits underneath. If I'm going to be doing something serious like engine or gearbox out I stand 4 custom made logs underneath instead, about 50cm diameter and 50-60cm tall. Easy for me since I have a chainsaw collection, live on the edge of State Forest and have annual permits to take firewood. I'd post a pic except I burnt my set of stands last winter.

[ianoz]

stand 4 custom made logs underneath instead, about 50cm diameter and 50-60cm tall.

not quite as good, but I always backup the jackstands with lengths of 5" cypress post stacked under the sills. If something does happen it may dent the sills but at least I'll be alive...

[greenMachine]

... or throw a couple of spare wheels under it. After all, the wheels are probably coming off anyway, or can come off if you don't already have a collection of spare/redundant wheels handy. Plenty of options lying around most garages, certainly mine!