As the spring rate has not changed it will take the same torque to twist the torsion bar to allow the spring plate back onto the lower stop, weather the angle iron bar is 10 meters long or 1 meter long, all that has changed is the force required at the end of the bar to generate the torque required, the longer the angle the easier it is, it will still try to lift the car of the stand exactly the same as if you applied the force directly to the end of the spring plate as the only thing that has changed is the lenght of the lever. if you check out the factory tool all the force is contained in the car with no chance of it slipping of the stands, this is similar to the chain and trolley jack method
you are 100% correct with everything you've said until...
vwo60 wrote:it will still try to lift the car of the stand exactly the same as the only thing that has changed is the lenght of the lever
Torque itself won't lift the car. Its the vertical force that lifts the car. This vertical force can be reduced by using a longer bar to produce the same amount of torque.
The reason why the factory tool works is that the force is being applied between the car and the spring plate. Using the jack method you are applying the force between the ground and car (spring plate). Its the point of application of the force which is key in this comparison
if i scrib a line on the torsion bar housing at the point were the vehicle begins to lift of the stand using only the spring plate and a trolley jack to lift it onto the stop then attach the angle bar and try to lift it onto the stop again it will still lift of the stand at the same scribed line.
you guys are funny.
that means most likely your extension bar isnt long enough as the vertical force is still greater than the car's weight
Just saw this thread and not going to get into the debate over lifting cars or not. But looking at the springplate clamp reminds me of sringclamps for coil spring suspension. It looks like they would do the job as well and if you have them sitting in your workshop from working on other cars (as I do) maybe they would do the came job.
BTW when lifting the baja using a floor jack and chain didn''t seem all that safe due to the baja lifting approx 4-5 inches of the jack stand.
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vwo60 wrote:if i scrib a line on the torsion bar housing at the point were the vehicle begins to lift of the stand using only the spring plate and a trolley jack to lift it onto the stop then attach the angle bar and try to lift it onto the stop again it will still lift of the stand at the same scribed line.
100% correct, it's the jack that sees less effort, the car will lift at the same torsional load.
Matt Berry Motorsports...air cooled advice, repairs and mods Ph 0408 704 662
OFF-ROAD,CIRCUIT,DRAG,STREET,ENDURANCE
Last post from me on this....
The physics involved is simple. With all due respect but there is some over complication and misunderstanding on terminology on this matter.
Torque in itself can't lift the car. Think about it like this... imagine that there is no spring plate at all and a pure torque is applied to the torsion bar. What happens? The "spring" will twist and twist and twist until either it shears or the splines strip. The car wont go anywhere at all. No lifting will occur.
Also think of just a coil spring being wound up between your hands. You let it go and what happens? It will probably try to spring against your hands (outwards) but it wont jump upwards or downwards.
Coming back to the spring plate and torsion bar. So using some MADE UP numbers to illustrate the point...
So lets say your car weighs 1000kgs (ie 10,000N) or 250kg per corner (2500N). Lets also say the torsion bar needs 1500Nm to be applied to get it over the stop and the spring plate is 0.5m long.
I need to apply 3000N to the end of the spring plate to raise the torsion bar onto its stop (ie 3000N x 0.5m = 1500Nm). But the 3000N applied to the spring plate is greater than the weight of the car on that corner (ie 3000 > 2500) therefore I raise the car up before I can fully apply the 1500Nm torque.
So I add an extension to my spring plate to make it say 2m long. Now I only need to apply 750N to the end of the lever (750N x 2m = 1500Nm). And the applied 750N is less than the weight of the car in that corner (2500N).
In other words by using the extension bar I:
a) achieve the required torque (1500Nm)
b) reduce the effort required to do so (3000N vs 750N)
This is all on the assumption that the car is well supported and that the car is essentially "ground". If this isn't the case then all bets are off and you've got a Darwin Award waiting to happen.
Agreed that the factory tool does the job well , but not everyone has one and a simple compromise can be made by using the extension bar method.
mmm, an engineer that's WRONG, never seen that before...
Maybe torque isn't the correct terminology for the load transferred to the chassis by the torsion bar, but it is a force that has a twisting moment at the spring plate that is transfered to the chassis.
I am still convinced (I've done probable 300+ springplate adjustments and done this exact experiment) that no matter the length of the lever, the car will lift off the jack stand at the same amount of springplate twist (060's scribed line). I'm not going to post on this anymore....... :/ unless I have to 🙂
And Ash, most of us do have a factory tool, it's a std VW jack with a small modification.
Matt Berry Motorsports...air cooled advice, repairs and mods Ph 0408 704 662
OFF-ROAD,CIRCUIT,DRAG,STREET,ENDURANCE
Thank godness for that, the spring plate does not know that it has a piece of angle bolted to it
matberry wrote:And Ash, most of us do have a factory tool, it's a std VW jack with a small modification.
So wheres the pics? make this thread useful again....
I Have been caught up with stuff, but ill try get pics up soon.
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Here is another way to think of it. It comes down to different centres of rotation.
To rotate the spring plate into position you need to apply enough torque to overcome the preload. The torque doing this is is lever length x force.
To lift the back of the car off the ground you need to apply enough torque to rotate it around the front axle line.
The torque trying to do this is (lever length + distance from front axle to rear torsion bar) x force.
If you add a metre or 2 to the lever you get a much greater increase in torque at the bar, with a small increase in torque trying to lift the car.
But after you increase the lever much more than this you will find that the torque about the torsion bar gets closer to the torque about the front axle. Consequently the lever method is not so good for high preload applications like off roaders.
Lauzboy's theory is correct, but Mat is right in practice.
Still I like the lever method because you can stand on a kitchen scale and lift the lever just enough to get the spring plate off the stop to measure the preload. Matching the preload on the other side is then easy.
I am also a Mechanical Engineer, please don't hold it against me!
