Outer Limits 4x4 Board

Just wondering why people don’t use 3/4 elliptic springs on front ends? Or even both end’s I was thinking it might be because it has no sideways stability but then neither does a coil spring. I can’t think of any reason why a ¾ spring would cycle any different to a coil spring.

Complexity of the link system for control.

I built my Sierra with a shackle reverse and 3/4 eliptic in the front about 7 years ago. It can be done, But it will only be workable with a shackle reversal, or else you will have a massive reduction in approach angle on the drooped wheel, and odd effects as the steering tries to squirm the 3/4 about.

There is very little efect on stability with a well designed 3/4. We have a number of cars in the club with 3/4 onthe rear, and it gives no trouble at all. No extra links are required to maintain acceptable lateral stablity (on the rear), although this could be an issue with power steer in the front, although it should be better than inferior shackles and the like.

In the front, the arch of the chassis and other attachments tend to limit the length of the 3/4 leaf and prevent it really opening up so the travel gains are small.

If 3/4 eliptic was done at both ends of the spring, the result would be "full eliptic", and this will have very very little lateral stability. Full eliptic leaves were very popular on horse drawn buggies, but were pretty much abandoned when the axles had to transmit power You are right, with a full eliptic leaf, exactly the same locating links etc would be needed as for a coil, except in comparison to a coil, a full eliptic leaf will be much bulkier, have more internal friction, and possibly have a smaller working travel range.

I have thought of using a full eliptic leaf transversely though, but it have very poor roll stiffness characteristics, so will require heavy antisway assistance, adding complexity.

PS just realised your post refers to using 3/4 eliptic at both ends of the same spring. In that case, skip through to the last couple of paragraphs.

steve

wouldn't the front pinion angle be shot to buggery as the suspension cycles if you have 3/4 at the front end with a shackle reversal (fixed mount at the front and the 1/4 leaf and shackle at the rear of the spring)?

In a word, yes.

However, with stock OME front springs and a short 3/4 leaf, I was only getting 9.5" of vertical travel (and starting from not very much lift) As you are aware, in a stock height Sierra the front driveshaft climbs to the diff, so there is plenty of angle to play with. I never saw this being a problem.

There would have been ore problems with slip yoke length, If I had ever run it with proper length shocks.

The 3/4 up front experiment was a bad one for a number of reasons - 1) I wasn't much up to fabbing stuff back then so it was all overengineered but really rough looking, with some dodgy desgn. 2) It was NT so If it did travel I was going to run out uf tyre/inner guard clearance 3)With the SR I lost approach angle 4) I ended up with heaps of caster, so I had to go back to stock length shackles 5) the 3/4 leaf was too short to do much anyway. once the caster was sorted, Id drove REALLY well at speed though - heaps better than shackled up front. No real gain anaywhere else though.

I think if you are going to the trouble you might as well just fit coils and a decent link suspension - it will (if properly designed) be much better as you remove the interleaf friction you have with leaves and travel is more controlled through the full cycle of flex.

A lot of US rigs used to run the setup, but just about all of them have changed to links and coils.

If it is a street rig I doubt an engineer would EVER approve a 3/4 elliptic setup.

Maybe this is a silly question but....... is it possible to have an interchangable rear suspension??? On road, use a normal spring setup... and for comps change to 3/4 eliptic. Just curious if the mounts have to be relocated or if they use the same ones. would solve the engineers prob of on road use.

brighty wrote:Maybe this is a silly question but....... is it possible to have an interchangable rear suspension??? On road, use a normal spring setup... and for comps change to 3/4 eliptic. Just curious if the mounts have to be relocated or if they use the same ones. would solve the engineers prob of on road use.

It would be possible to lock the 1/4 elliptic top section in place when on the road using a pin. But I have never heard of an engineer approving a "lockable" suspension like this. There is no guarantee you won't drive on the road unlocked.

ISUZUROVER wrote:

brighty wrote:Maybe this is a silly question but....... is it possible to have an interchangable rear suspension??? On road, use a normal spring setup... and for comps change to 3/4 eliptic. Just curious if the mounts have to be relocated or if they use the same ones. would solve the engineers prob of on road use.

It would be possible to lock the 1/4 elliptic top section in place when on the road using a pin. But I have never heard of an engineer approving a "lockable" suspension like this. There is no guarantee you won't drive on the road unlocked.

I think that one of the ORI boys ahd pinnable 3/4 on his lux but i dont know if it was engineered

i've seen an interisting set up on a horse drawn buggie that conected the rear mounings of the longatudal leaves to a transverse leaf, that pivited in the middle

Matt

twinnie wrote:i've seen an interisting set up on a horse drawn buggie that conected the rear mounings of the longatudal leaves to a transverse leaf, that pivited in the middle

Matt

can really uderstand what you mean but do you mean full elliptic?

No I think he means normal semi elliptic springs each side, except that instead of a shackle at the back there was a third semi-elliptic spring (I assume upside down) running across the vehicle that could pivot in the middle. Sounds like an interesting idea...

Heaps of horse drawn buggies had full elliptic springs (two semi-elliptics mounted eye-eye though.

ISUZUROVER wrote:No I think he means normal semi elliptic springs each side, except that instead of a shackle at the back there was a third semi-elliptic spring (I assume upside down) running across the vehicle that could pivot in the middle. Sounds like an interesting idea...

Heaps of horse drawn buggies had full elliptic springs (two semi-elliptics mounted eye-eye though.

yeah thats it if i could post pics i'd do a drawing (my useal 3d goodness) and show ppl

Matt

I have seen the type of leaf set up you are describing. with a large range of travel though, think that the shackles from the transverse leaf to the normal leaves might try and do some strange things, esp when power is added to the equation.

locked out 3/4 eliptic has and can be engineered in Vic. Check out the "SR20 finds a new home" thread in the Zuk forum.

I cannot see the point in changing set up for comps/road. If it is set up correctly for the bush, It will be fine on the raod. Many people build stuff with too much travel and too soft/too little roll stiffness to be road driven IMHO, this stuff is junk to drive in the bush too.

Most of the US rockcrawlers (which are pushing suspension design in so far as traction @ the wheels) run designs that would be very streetable - low, quite stiff and very stable.

I have run a 3/4 eliptic setup on the rear of my Zuk for about 6-7 years now. never had a problem with it on road. You don't even notice it, even now when combined with 34" swampers and an auto locker in the rear. If you are getting any sort of adverse reactions from it then it isn't setup correctly