Thursday, December 11, 2008

What is the best length shock for my 4WD suspension lift kit? Long-travel shocks

Daily, we get calls and emails from people that go like this:

"I've got a (- insert your vehicle here -) with a 3" lift kit, and I'm chasing shocks that extend out 3 inches more than original, but compress down to the same amount as the factory shocks..."

It's not an unreasonable question. I can see the logic here. Unfortunately, the logic is wrong.

Firstly, let's assume that the vehicle with a 3" suspension lift kit, will benefit from 3" longer shocks. (more about this in a minute).You can't have a shock absorber that is 3 inches greater in the extended length compared to original, yet compresses down to the same length (body length) of the original shock. This is because the shock body (which is your compressed length) must be greater in order to allow for the greater extended length (rod length).

Memorise this.

The body length of the shock must also be greater in order to provide the greater extended length.

The body length of the shock must also be greater in order to provide the greater extended length.

The body length of the shock must also be greater in order to provide the greater extended length.

The extended length of the shock, is determined by the length of the chromed piston rod, combined with the length of the shock body itself. On full compression of the shock, the chromed piston rod is contained within the shock body (almost halving the length of the unit compared to when fully extended). If we made say, just the chrome piston rod longer, and left the shock body length the same, on full compression of the suspension, the shocks chrome rod would simply smash through the base of the shock body. Game over.

Back to the bit about needing shocks 3 inches longer for a vehicle with a 3" lift.

Perhaps your 4WD does need the shocks that are 3" longer to suit the 3 inch suspension lift. However, often this is not the case.

Firstly, even with a 3" lift suspension kit fitted, the potential amount of extra movement or extra travel between the shock mounting points may be minimal. This may be due to the way that the suspension is configured relative the positioning and angle of the shocks to the axle and the wheel. It may be that the sway-bar mechanics on the vehicle are holding the travel of the suspension back, rather than the shocks. It could be that (in the case heavy-duty springs) that the extra spring rate creates less flexibility in the suspension, negating the benefit of fitting a longer shock.

If we go ahead and fit the 3" longer shock, in the case of a coil-spring vehicle, the coils may not remain captive on extended travel (the coil falls out). That's a big deal.

On compression travel (because the shock body is longer) you might crunch (destroy) the shocks as the vehicles suspension pushes down past what the body length of the shocks specification will allow. Ouch!

If the longer shocks do allow for more suspension travel than original, your brake-lines may be too short (stretch and tear). You might put a lot of extra load or tension on your sway-bar. In some cases, your drive-shaft or tail-shaft may pull apart.

I'm not a drama queen (really), so this is how it works...

On most popular Australian-sold vehicles with a suspension lift up of to 50mm of lift. The shock absorber length supplied (in almost any brand in the marketplace) is about the same as the original unit. This is because in most cases there is sufficient travel in the shocks stroke to allow for this type of lift. Particularly as the amount of extra stretch between the shock mounting points is usually less that the amount of vehicle lift height.

I'm not saying you could not get extra travel by fitting a longer or slightly longer shock, but the modifications and checklist involved are usually more involving that what the average family-orientated 4wd wagon owner wants to be involved with.

"But I want maximum travel!"

OK. I hear you.

If your vehicle can benefit from fitting longer shocks than original and you are happy to do what you have to do to maximise the travel - here are some guidelines to start with.

First up, determine the travel your vehicle is capable of with no shocks fitted (none). You usually need to drive your vehicle up onto a dedicated travel ramp for each corner of the vehicle. If you don't have access to a dedicated travel ramp, you can achieve the same thing by jacking up each corner of the car with a forklift (one corner at a time), or finding some off-road terrain (or adapting some) that allows you to fully work the suspension.

Do this with great care. Regardless of the method that you use, you don't want to roll the vehicle over or it to fall on you and crush you. If it is a coil-sprung vehicle (as the shocks are removed) you don't want the coils to drop out.

Record the distance between the shock mounts on full travel each direction. Also for reference, measure the distance between the shock mounting points while the vehicle is parked on flat ground. This measurement is often called the "static" or "rest-height" position.

Then work with your supplier of shocks for the closest match to both the open and closed measurements. The closed (upward travel) is just as import as and the extended (downward travel). This is because in the case of a solid-axle type suspension system, one side of the vehicle must be leveraged up in order for the opposite side to be pushed down.

A technically perfect application sees a shock absorber opened to the the half-way position while the vehicle is parked on flat ground. So, if a shock had 10 inches of travel, the shock, as fitted to the vehicle at normal ride height, would have the ability to extend out 5 inches before coming to the end of its travel, as well as compress 5 inches from the normal rest height position before crunching the shock.

A technically perfect application is not always possible, due to shock length availability, or the travel characteristics of the vehicle (may have more travel in one direction than the other). Essentially, as long as you have optimum travel in both directions, you have a good long-travel shock application.

I mentioned earlier about modifications required to fit genuinely longer shock absorbers than original.

In most cases, extending the axle bump-stops is required. This is to protect the shock from "crunching" and becoming damaged on full compression of the suspension as ...the body length of the shock must also be greater in order to provide the greater extended length.

Extending the axle bump-stops means you are reducing the bump-stop gap, so the rubber bump-stop makes contact with the new metal strike plate position before the shock crunches into itself. Extending the axle bump-stops (reducing the bump-stop gap) sounds counter-productive to the intention of maximising wheel travel. While it does include some inefficiency, the benefit of doing so together with the genuinely longer travel shocks, still outweighs the performance of a high-lift suspension kit fitted with factory-original length shocks, particularly on the bigger-type lifts.

A way of eliminating the need of extending the axle bump-stops and maximising the travel of the suspension is to relocate the 4x4's shock mounting points, so that they are spread much further apart. Weld-on shock hoops are one example of how to do this. It's means that you can fit a shock with a massive amount of travel (more travel than what the vehicle is capable of in any direction). So here, no need for extended bump-stops or inefficiencies in the bump-stop gap travel. No need to worry about crunching the shock on full compression of the suspension.

Next check you brake lines. Purchasing extended-length ones is often part and parcel of custom suspension because the suspension can now articulate further than factory intention. Check (in the case of a coil-sprung vehicle) that the coils remain captive (can't fall out) under the travel of the suspension. Check the stress on other related components on the vehicle such as sway bar and link pins. If you fit longer link pins or extend the original ones, this might alleviate some problems. It might also cause other problems too.
The extra suspension travel achieved may also mean that parts of the suspension or axle may make contact with other parts and accessories of the vehicle. You will need to counteract and compensate for this. Lots of extra travel can even allow your CV's, drive-shaft and tail-shaft to be damaged, slip out or fall apart. Again, you need to compensate for this.

All the things you need to do and check for are beyond the scope of this post. Also, (and this is where I have to do the legal stuff) you need to make sure that any modification you make to a vehicles suspension will not affect the safe and reliable operation of the vehicle, or contravene any applicable regulations, rules, laws or insurance requirements. This is your responsibility. Don't shoot the mail-man.

An alternative we could look at to eliminate a lot of hassles associated with long-travel shocks, is if we started to make the shocks out of rubber. This way, a customer could simply stretch the rubber-made shock out and in to whatever length they want. They could get the exact open length they want, together with the ideal closed length they want also.

It would be good business sense for us too. We would only need to stock one part number on the shelf, rather than the hundreds of different part numbers we currently do.

I'd probably get a raise!!!

So, if you are interested in purchasing the new rubber shocks for your next suspension application, please click here

Posted by Kirk Barker.

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