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Jacking in the centre of solid axle
AlexXtreme - 21/11/15 at 07:31 PM

This is probably a simple question :-)!
I have always jacked cars on the chassis only or on my last car on the sierra type centre diff that was solidly bolted to the chassis
My "new" car has a solid rear axle, can I jack the car at the centre of the diff to the lift in one go to get in on axle stands? It is a G27 so weighs around 720kg ish.... I have a similar issue on the front but this is more model related
Cheers Alex

blakep82 - 21/11/15 at 07:38 PM

I don't see an issue? Don't it may times myself too. Just make sure the diff cover plate doesn't get damaged

twybrow - 21/11/15 at 07:39 PM

I have lifted my car on the axle casing quite a few times without problem. I try to always jack the rear before doing the front to minimise the load on the axle.

Sam_68 - 21/11/15 at 07:53 PM

Obviously, the weight of the sprung mass is fed into the axle at the spring/damper mounting brackets, at either end. So if you're jacking under the diff in the centre, you're forcing the axle to act as a beam in bending and the jacking point applies pretty much the maximum possible bending moment to an assembly that's not really designed to take it.

Having said all which, I've always done it and on light cars the axle tube seems to be strong enough to take it: I've never experienced any problems.

benchmark51 - 21/11/15 at 08:18 PM

Been doing that for 50 years without any problem.

mark chandler - 21/11/15 at 09:13 PM

If the axle cannot take the weight of the car jacking from the middle somethings wrong with it!

Oddified - 21/11/15 at 09:21 PM

Always done it, never been a problem

Ian

Sam_68 - 21/11/15 at 09:21 PM

quote:
Originally posted by mark chandler
If the axle cannot take the weight of the car jacking from the middle somethings wrong with it!


I tend to agree, and I've certainly never experienced any problems as a result.

But... (devil's advocate mode), remember that Lotus/Caterham found it necessary to welds a substantial brace to their (Morris Marina/Ital) axles to prevent issues from a similar but much smaller bending moment between the trailing arms and the A-frame attachment under the diff. That was from much more regularly applied braking and acceleration forces, though, not from jacking the car up once in a blue moon.

britishtrident - 21/11/15 at 09:52 PM

It was torsion that broke Lotus Seven and early Mk1 Lotus Cortina axles.

Jacking conventional live rear axles under the centre is normal they can easily take the bending moment, however jacking a De Dion by the centre of the tube is not a good idea.



[Edited on 21/11/15 by britishtrident]

craig1410 - 21/11/15 at 10:11 PM

Agreed, but with a de-dion you can just jack the diff and there is no bending load at all. I would recommend a rubber pad or piece of wood to minimise damage in all cases.

Sam_68 - 21/11/15 at 10:28 PM

quote:
Originally posted by britishtrident
It was torsion that broke Lotus Seven and early Mk1 Lotus Cortina axles.


We risk getting into semantics, but it was the fact that the articulation that would result in pure torsional loads on a 'normal' trailing arm set-up results in a combined bending and torsional load when you have the Lotus arrangement of a pair of trailing arms on the outer end of the axle reacting one 'side' of the torsion, with the A-frame that reacts the other 'side' being separated by almost half the track width.

But it was actually the bending element that caused the problem (and which the bracing plate that was added is designed to resist).

It's for the same reason that the latest Caterham de Dions are (optionally) fitted with longitudinal Watts links at their outer ends that act, effectively, like second trailing arms.

By comparison, conventional trailing arms place pure torsional loads on axles and never cause a problem. Indeed, the longitudinal Watts Links on Sylvas deliberately places quite substantial torsional loading on the axle tube and while the links have been known to fail, the axle tube seems quite happy with the arrangement!



All quite irrelevant to the OP's question of course; to which the answer is that everyone does it and it doesn't seem to cause problems ...but it's not good practice, so if (like the rest of us!) you're too lazy to jack and support either end of the axle under the spring mounts, independently, don't blame others for recommending/approving it if something goes wrong!

There's the side issue that the car will be less stable if the entire rear end is lifted off the ground by jacking under the centre of the diff (it is easy for the jack to fall over sideways, with a relatively gentle push against the side of the car), so again you need to use your own judgment on whether you're acting safely.

AlexXtreme - 22/11/15 at 12:15 AM

Thanks for the answers! The axle has trails arms at the outboard ends and as you all say I was worried about the bending moment on the axle... as an engineer I should do the BM calcs however with the diff and not a std shape it would not be a normal calc. That said it is only less than 200kg at each end and the section is pretty big so what can possible go wrong ;-O (perhaps I've had a beer or two!))

JeffHs - 22/11/15 at 10:31 AM

(it is easy for the jack to fall over sideways, with a relatively gentle push against the side of the car)
Indeed and it came to my rescue recently. I made a cock of parking in my very tight garage and ended up with the rear arch jammed against the side support for the roller shutter. It was obvious that going in or out would do more damage. I was on my own and unable to lift the rear of the car, so jacked up under the diff and pushed it sideways off the jack.
End result - no visible damage to the car, slight ripple in garage upright, no paint damage at all.

rusty nuts - 22/11/15 at 11:38 AM

Jacking up under the diff on live axle vehicles has been going on as long as live axles have been around . As with everything in life a bit of common sense makes thing safer.

02GF74 - 22/11/15 at 12:50 PM

.... but really how much extra effort is it to jack under thr sill/chassis, put axle on stand and repeat on other side?

Lifting jack are called that as that is their purpose, they are not meant to be used as supports.

5 minutes of laziness can end up with a lifetime of regret should there be an accident.

rusty nuts - 22/11/15 at 02:50 PM

In nearly 50 years in the motor trade I've never seen any damage caused by jacking under a live axle whereas I've seen loads of damaged caused by jacking up under sills even when the correct location has been used. As for using axle stands , in my previous post I mentioned common sense

AlexXtreme - 22/11/15 at 07:12 PM

It is not being lazy, I can only see one area on the chassis that I can safely lift on and so it is not possible to lift and put an stand on the same side for my car.
Thanks for the comments and I have just brought a low height new jack which is really stable so will lift under the diff.
Cheers
Alex

steve m - 22/11/15 at 07:35 PM

When jacking up the rear of my 7, I have always done it on the axle, as the original car weigh 4 times what mine does now

And with out the rear axle and brakes etc fitted, I can lift the back end of the car very easily

steve

SJ - 23/11/15 at 08:12 AM

Anyone who had old RWD cars in the UK (Vivas, Cortinas, etc) usually had little choice to jack under the diff as trying to do it on the sills would result in a nasty crunch as the rust, filler and underseal gave way!

britishtrident - 23/11/15 at 10:13 AM

quote:
Originally posted by 02GF74
.... but really how much extra effort is it to jack under thr sill/chassis, put axle on stand and repeat on other side?

Lifting jack are called that as that is their purpose, they are not meant to be used as supports.

5 minutes of laziness can end up with a lifetime of regret should there be an accident.


Doing it that way the jack is much more likely to cause the oposite side stand to topple jacking under the diff then putting two stands under is by far the safest way. A lot of cars with IRS have a substitute centerline jacking point at the rear for this purpose. Jacking under the sills is always an option but should not be an automatic first choice.

Proper pro trolley jacks never topple even at very high lift heights, those little super market trolley jacks are a different matter.

[Edited on 23/11/15 by britishtrident]

britishtrident - 23/11/15 at 10:21 AM

quote:
Originally posted by rusty nuts
In nearly 50 years in the motor trade I've never seen any damage caused by jacking under a live axle whereas I've seen loads of damaged caused by jacking up under sills even when the correct location has been used. As for using axle stands , in my previous post I mentioned common sense


Ditto

We started using icec hockey pucks as jacking pads, they are made of hard rubber/plastic and cost buttons off the internet.

AlexXtreme - 24/11/15 at 01:14 PM

I like the idea of the hockey puck.... I use bits of soft wood at the moment from pallets but now have ordered a couple of pucks to see if they are better!

Any suggestion for jacking at the front of the car and where to put the axle stand? Other than the chassis obviously! There are no real obvious point for jacking on the tubular chassis (Ginetta G27)