4x4Wire's TrailTalk Forums: Gen 3 Flex-ability

I recently conducted some experiments to improve my understanding of the way the Gen 3 suspension works and to measure some baseline performance parameters of the suspension to allow objective comparison if any part of the suspension is modified.

My investigation focussed on measuring what happens when the suspension is forced to "flex" or articulate in the sense that it would when the vehicle is cross-axled. I believe cross axle flex to be the suspension parameter most relevant to a vehicle's off-road ability.

My experiments were carried out by lifting an individual wheel (using a trolley jack) until any other wheel just lifted off the ground (this was verified by sliding a sheet of paper under the tyre). The height that the lifted wheel was off the ground was measured at this point and also at several intermediate points. The following pictures show the method:

Oops, I should have cleaned/tidied the garage up a bit

.

WARNING: Please note that there is an element of danger to this experiment. The car is inherently unstable on the trolley jack when lifting individual wheels, so if you try something similar, please be safe and remember that you are responsible for your own safety and for the safety of anyone that might come close. To minimize the risk of something going wrong and someone getting hurt, chock the wheels that are on the ground, apply the park brake, activate the steering wheel lock and engage 4WD. Don't get any part of your body under the car while it is lifted and don't allow anybody else to do so either.

The most universally adopted measure of a suspension's "flex" is the Ramp Travel Index (RTI). I think this measure was devised Four Wheeler Magazine in the early 90's in order to compare the suspension flex of different vehicles.

The way RTI is measured is simple. The vehicle is lined up with a ramp and one wheel of the vehicle is driven straight up the ramp until any wheel just starts to lift off the ground. The distance from the base of the ramp to the point where the tyre on the ramp comes in contact with the ramp, directly below the center of the wheel, is now measured and divided by wheelbase. Finally, this figure is multiplied by 1000. It is important to specify the ramp angle as well. The original ramp angle was 20°, but other (steeper) angles have been used to accommodate highly modified vehicles with extreme amounts of suspension flex. If a vehicle with a wheelbase of 2500mm is able to travel 1500mm up a 20° ramp, then the vehicle's RTI is 1500/2500*1000=600. Conventionally, the RTI is measured travelling forward up the ramp as well as in reverse and the two figures are then averaged.

The RTI figure is simple and easy to measure, but there are several disadvantages to the method. The calculation does not take the track width or tyre pressure into account, for instance. Still, the RTI method is the most common method and most published articulation figures are RTI values measured according to this method. If the height that a wheel is off the ground is known, then the RTI value can be calculated for the vehicle on any ramp angle. I will not bore you with the mathematics (unless you want me to, of course

), but this is what I did to compare my car's flex with some published figures for other vehicles (more on that later).

Observations:
The front anti-roll bar is very stiff and very restrictive in terms of front axle flex. A rear wheel is lifted long before the front suspension travel is halted by the bump stops:

Pardon the muddy debris on the lower suspension arm. This shot was taken after some off-road playing before I had time to clean it...

I could fit my forefinger into the gap between the bumpstop on the lower suspension arm and the stop pad on the chassis.

At the back, the anti-roll bar is sufficiently flexible to allow the bumpstops on the upper suspension arms to contact the pads on the chassis (the bump stop is at the top of the picture in between the shock and the coil):

The following picture shows how much the mounting bush of the rear anti-roll bar deforms at maximum flex:

The average RTI score for my Gen 3 LWB DiD was 482 with the tyres at 2.4 bar and calculated for a ramp angle of 20°. To place that figure into perspective, have a look at the list of RTI values (from here: http://sliprockmedia.com/info/tech-corner/rti.htm, but credited to Fourwheeler magazine) below:

Let me also be the first to say that it is probably dodgy to compare measurements taken using different, but supposedly equivalent methods

. It is interesting to note that the standard Gen 3 does not seem to have less flex than the Gen 2 in the above list ('92 Mitsubishi Montero 4-dr. in the list) and that it seems to hold its own among other vehicles with solid rear axles and even some with solid front and rear axles in terms of flex

.

Whatever the measurement method, as long as one goes about things consistently, it should be valid to make comparisons before and after modifications on the same vehicle, so I then disconnected the rear anti-roll bar and remeasured the RTI.

Although the rear suspension was reaching the bumpstops with both anti-roll bars attached, disconnecting the rear anti-roll bar improved the RTI rating slightly (about 1%).

Disconnecting the front anti-roll bar as well, improved the average RTI rating to more than 550 (more than 15% increase).

As can be seen from the photo below, the front suspension flex is limited by the bumpstops when the anti-roll bar is disconnected.

With the anti-roll bars connected, about 70% of the flex comes from the rear suspension and only 30% from the front. When both anti-roll bars are disconnected, the rear axle contributes 57% of the flex and the front 43%.

Disconnecting the rear anti-roll bar does not affect on-road handling greatly. It would probably make the vehicle slightly more prone to understeer at the limit. Disconnecting both anti-roll bars has a marked effect on the vehicle handling; the roll upon entering corners is increased greatly.

The vehicle feels more stable off-road in cross-axle situations with the front anti-roll bar disconnected, because the flex is more equally split between the front and rear axles and the body roll angles are less.

If/when I finally get around to upgrading the suspension, I intend to redo this exercise and determine the effects of the modifications on the flex-ability.

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2003 Pajero GLS DiD A/T LWB...
Factory Rear Lockable LSD

Edited by pampaskat (11/03/07 11:15 AM)

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