Reply With QuoteHi Jordan. This is a really simple question which opens a whole can of worms.Originally Posted by JordanS3
My short answer is don't. Just stick on a set of Rocky Mountain parabolic springs and matched shocks. These are not fantastically cheap but will give all the improvement to handling, comfort and articulation which you will need. The kit comes complete and is bolt on, well designed and won't require engineering to keep DoT happy.
The longer answer is really complicated and probably going to be contentious. Anyway...
Is yours a SWB or a LWB?
Do you currently have your axle check straps fitted or have you already removed them to marginally improve articulation (or have they just rotted out?!).
Taking the issue from the top.
The axle check straps are there to protect the shock absorber when your wheel hits maximum droop off road. The maximum droop allowed by the check strap is slightly less than the full extension of the shock absorber. That way, if the wheel drops hard, the force is taken by the check strap rather than the shock absorber hitting full extension. If this happened it would pretty quickly cause the welds on the shock absorber to fail and pull the thing apart. That is why the check straps should be in good condition and left well alone.
With that in mind you can run the existing shocks in the sense that they will still work on road, but you won't get any technical benefit off road from your lift because the check straps will still limit the axle motion to exactly what it was before. Basically the car will sit cosmetically higher (and will be more unstable because of the raised CoG), but there will be no practical benefit. So actually unless you are after a pure cosmetic effect, don't bother.
What you are going to have to do to set this up right is do your lift (and replace the springs with parabolics I would suggest) and then, on flat level ground take the shock absorbers and check straps off. Get a high lift jack and jack up (say) the rear left corner of your car until the wheel is just about to leave the floor. At this articulation put a large axle stand under the rear cross member and make sure that everything is safe and stable. This will be the maximum achievable droop of your wheel on one side. Then get a ratchet strap and, on the compressed side, compress the spring (by putting the strap around the spring and chassis and winding down on it) until the axle hits the bump stop.
On the extended side make up a new check strap to constrain the wheel's motion just before the axle reaches this point. This gets the required length of the check strap right.
On the extended side, the new shock will have to be slightly (say 2cm) longer than this when fully extended. This will ensure that it is the right length to allow full travel and that the axle check strap functions comfortably before the shock comes to full extension.
Next problem. Because the new shock will also be longer than original when fully compressed, the chances are that when the wheel is compressed up, the shock will hit full compression before the axle hits the bump stop. This will knacker your shock even more quickly than using it as an unofficial check strap as it will just bend. Also the shock on the rest of the car will be very harsh as essentially you have replaced a cheap rubber bump stop with an expensive tubular metal one...
On the compressed side, what you will need to do is raise (repeat - raise) the position of your upper shock mount to allow for the use of the longer shock. This will need to be raised (not dropped as some people would mistakenly contend) by about the same amount as your new shock is longer. This will ensure that the axle hits the bump stop before the shock hits full compression. Once you have worked out what length shock you need to achieve the full extension, you can take the compressed length of the shock, apply the measurement to the compressed side and see where the new shock mount needs to be. Then make it a couple of cm higher to achieve a bit of extra clearance. What this will also do of course on the extended side is raise the point at which the shock reaches full extension by a few cm. Experiment here to pick a shock of the right length and finally adjust the check strap to protect it at full extension.
Repeat at the front.
Complicated. Does that make sense?
Be aware that all of this definitely constitutes a modification which will require DoT approval and engineering sign off. If you don't do this and have an accident, you can absolutely guarantee that some shyster lawyer and/or insurance inspector will contend that the accident was caused by the handling changes caused by the illegal modification (irrespective of whether true or not) and you will get screwed, even if the accident was transparently caused by some drunken hoon running a red light and taking you out as you bimbled along minding your own business.
Also be aware that this amount of lift will put significantly more strain on the UJs of your front prop shaft, particularly if you have a SWB where the angles will start to become ridiculous, particularly at full extension. You will certainly be looking at a bespoke front prop shaft with increased slip and preferably double cardan joints. If you don't do this you will end up with serious driveline vibration, rapid wear of the joints and possibly even pull the two halves of your front prop apart on full axle droop at the front or at the very least start to put real strain on the splines when the prop starts to be over-extended. The mess which will be caused if the prop shaft gets pulled apart doesn't even bear thinking about. If you are doing it properly, you could modify your front axle to rotate the diff housing forward, reducing the angle the the cone makes with the front prop shaft.
When I put Rocky Mountain shocks and parabolics on my SWB I got about an inch of lift, far better articulation and didn't mess too much with stock geometries although there is a bit more front driveline noise on road.
Thoughts?
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