Some thoughts on modding the LR suspension

• Hybrid LR

Some thoughts on modifying the Land Rover coilsprung suspension setup for best off-road performance …

Back in the early Seventies, the Range Rovers suspension setup was a revolution. Without traction aids like locking differentials, the key to off-road performance is keeping the wheels in contact with the ground. The Range Rover acheives this by using soft, long travel suspension front and rear. At the back, a ‘Self Levelling Device’ makes it possible to carry or tow heavy loads without the rear sagging to the ground. At the front, that famous lightweight V8 engine allows the springs to be as soft as possible. No anti-roll bars are employed, making certain that maximum ground contact is maintained.

The rear axle has coilsprings, with shocks mounted one leading, one trailing (presumably to help combat axle tramp?) The axle is located by trailing arms mounted from the bottom of the axle tube at each side, and at the top by an A-frame structure. The SLD is mounted on the A frame, and operates by employing the bouncing of the vehicle as it drives along to pump itself up to a predetermined height. The effect is to take the main weight off the road springs, allowing softer
springs at the axle ends than would normally be necessary.

At the front, the axle has coilsprings with centrally located shock absorbers. The axle is located by trailing (leading?) arms which located towards the rear at each side of the chassis. Twin mounting points under the axle tube prevent axle roll, while sideways location is provided by a panhard rod. There are many modified suspension setups available on the market, from a simple
upgrade to gas shocks, to the amazing 3-Link suspension setup from Safari Gard. However, with any vehicle as diverse as a Land Rover, there cannot be a ‘one size fits all’ solution for suspension.

There are many hybrids and modified Land Rovers around, with different bodies, winches, tyres, engines etc., and many seem to use either standard suspension components or at best a “+2 inch” kit from one vendor or another. It’s not uncommon to see a modified vehicle lifting a wheel off-road where a standard one may go through easily with all wheels on the ground.
Land Rovers’ typical customer probably doesn’t require the ultimate in off-road suspension articulation, he (or she) needs something which is very good off-road, but won’t put up with a compromise on towing ability, general road manners, or handling when fully loaded.

When I began to study the operation of the hybrids’ suspension, I found one suprising thing. When one corner was lifted or dropped (such as when crossing a ditch correctly), the rear suspension did the majority of the work. There was several inches of travel left in the front suspension before hitting bump stops, or the end of the shock absorbers, but the rear end was lifting a wheel. This says that the rear spring are too soft - or does it? It could also mean
that the front springs are too hard, or that the rear shocks are too short, or too long, or mounted too high, or too low… and so on. In short, what appears at first to be a simple task of choosing a new set of rear springs becomes a much more complex issue.

Since I don’t posess a large enough ramp to perform RTI tests, I use a pair of 16 inch high wheel ramps, and cross-axle the vehicle over them, or try to(!).
While it doesn’t sound too difficult to drive onto a pair of 16 inch high ramps, these photos show what happened when we tried with a standard Defender 90 with 235/85R16 tyres. This is absolutely as far as we could get a standard vehicle, with or without centre diff-lock.

As it turns out this is actually quite useful, as if the front and rear suspensions work equally, the body will remain level. In fact the front suspension has less available travel, so the body will always tilt up at the front, but it is a useful visual cue as to what is happening. While the above
standard vehicle does this pretty well, my hybrid initially certainly didn’t!

In these pictures you can clearly see that the rear suspension does much more work than the front

I reasoned that in a perfect world, this situation would result in the front and rear suspension reaching maximum travel at the same time. If (in that same perfect world) axle travel is not limited by shock absorbers or bushes, then it should be a simple matter to set the relative spring rates front and rear to give this effect. Of course, changing the spring rate means changing the free
length also, otherwise the ride height of the vehicle will change, and so the available axle travel will probably change also, making your spring choice wrong.

One thing which I have not had time to do, but would have made life much easier, would be to visit a friendly public weighbridge, and find out the corner weights of the vehicle - how much weight is present on each wheel. This would have made it easier to choose spring combinations, though with careful measurement and a new set of springs it should be a simple matter to estimate it. While soft springs are good for ground contact, springs soft enough to allow the
bumpstops to hit when the axle moves up level are not ideal. This is dependant on the mass of the vehicle, and to some extent the shock absorbers. My take was that the mass of the vehicle should be just enough to fully articulate the suspension in static testing. Any stiffer suspension and the full articulation could never be used. Any softer and the suspension will ‘bottom’ too easily at any speed over rough ground.

The ride height of the vehicle (at each end) is to some extent a free variable. It is easily raised by adding a spring platform, though that space could more usefully be occupied by a longer spring! Given that custom wound springs is going beyond the pockets of most of us, there is no shame in it!

I think the ideal ride height allow the axle to reach maximum extent on one end and maximum compression at the other end simultaneously. This may be affected by tyre size, body interference etc., but we are aiming for maximum articulation here!

A point about extended shocks - if the shock extends longer, it will almost certainly have a longer compressed length too. If the ride height isn’t changed at the same time, or bump stops extended, there is a serious rick of damaging the shock under full compression. Some shocks have integral bump stops, but dropping one corner of a two ton truck on them hard is just asking for trouble. In principle, if you fitted a shock with two inches more travel, you should move the mounting point up an abouth inch so that the shock ‘bottoms’ at the same point in the axle travel . (For all that metrication stuff, everyone with a spanner in the hand has a much better feeling for inches and thou than centimetres and microns). Another limitation to axle travel is bushes. On the rear axle, the trailing arm to chassis bush can be put under some serious stress by high articulation. Due to the ride height, the bush is always under some tension , and at full axle droop the bush looks like it is about to self destruct. ‘Cranked’ radius arms can help to solve this problem, the bend serves to return the bush to its ‘rest’ position at normal ride height, maximising the amount of travel available. I expect to change these bushes regularly on my vehicle.

On the front axle, the setup is slightly different.There are twin picks from the radius arm to the axle tube, which prevent axle roll, and locates the axle fore-and-aft. However, this also limits the articulation, as it prevents the axle tube from twisting as one side drops. The bushes take up the tension very quickly, preventing further movement. One of the best solutions I have seen to this
problem (without the major redesign of the Safari Gard setup) is to hinge one front radius arm. This allows the axle to twist, taking tension out of the bushes. Of course, this setup should only be used off-road - usually provision is made to lock the arm solid for road use, returning the suspension to completely standard operation.

The assumption has been made throughout that the normal ride height of the vehicle represents mid-travel for the suspension components. This is almost certainly not true for most vehicles, certainly those with any suspension modifications. Maximum upward travel on my hybrid is limited by the axle bump stops, at the point where the tyre begins to hit the top of the wheelbox. The
shock is close to, but not at full compression. Maximum droop occurs when the limit of shock absorber lenght is reached.

In the case of the rear axle, at this point the rear axle springs are sometimes unseated, the gap between spring seats being longer than the uncompressed spring. The usual solution is to tether the spring, or to fit ‘cones’ which will re-seat the spring when the axle levels out. However my current feeling is that if the spring is not seated it cannot provide any downforce on the wheel, so cannot provide any traction. The weight of the vehicle sits mainly on the bumpstop or fully ompressed spring on the high side. Some weight is transferred by the A frame in the case of a vehicle with self levelling still fitted. Otherwise the only weight on the drooping wheel is due
to the axle itself. Longer, softer springs seem to be the correct answer, though this may not be
possible for several reasons, not least the increased compressed length of the spring which may be problematic. The various components of the suspension must all work together to obtain the
best performance from the whole setup. Changes in one part of the setup may be rendered useless if limited by another aspect - eg taller springs with standard dampers, or fitting longer dampers without changing chassis bump stops or ride height.

I have been through three sets of springs and two sets of dampers on the hybrid, just to arrive at a setup which I consider functions correctly. Articulation is not in any way ‘extreme’, just balanced and making full use of the parts chosen. After a period of evaluation I will consider if further development is required and in what direction that should be.

Luckily I have sold or re-used almost all the ‘leftover’ bits!

I have a list of Land Rover coil springs, their lengths and rates, and fitments
where known This data was culled from various sites on the net, magazines, LR manuals etc.

Further Reading

Suspension Revisited

Teri-Ann Wakeman has an excellent set of articles on the tradeoffs in suspension
on her website here

The Tech section of the Gon2Far website has some useful links and notes.

Book - “Race Car Engineering and Mechanics” by Paul Van Valkenburgh

“Physics of Racing” by Brian Beckman. A superb set of articles which I suggest you print out and keep on the bookshelf next to the Bosch Blue Book.

Some discussion comparing suspension setups here

Comparison of aftermarket LR suspension kits at D-90.com

Rules for Twist-Off competitions at YellowDefender.com

Useful Spring calculator tool for suspension setup here written by Trevor at LR90.com

Disclaimer
Changing your suspension operation off-road will also affect on-road handling. If you turn your truck into an evil handling swine and then stuff it into a tree - tough! You bend it - you mend it.