Rules for the 1999 D-90 Twist-Off
Richard Hills
Department of Mechanical Engineering
New Mexico State University
Las Cruces">
Rules for the 1999 D-90 Twist-Off
Richard Hills
Department of Mechanical Engineering
New Mexico State University
Las Cruces, New Mexico
With contributions from
Jim Allen
Grand Junction, Colorado
Chris Hinkle
D90 Suspension Developer
Dallas, Texas
Purpose
The primary purpose of the March Twist-Off is to evaluate several after-market modifications to the Defender-90 suspension. Most of these modifications are changes in the suspension springs and shock-absorbers which are designed to increase articulation. Some of the modifications include increases in caster angle of the front axle to increase on-road stability, minor changes in the rear trailing arm lengths to modify pinion angle, or the use of high performance off-road shocks to increase vehicle control.
The secondary purpose is to provide a forum for Land Rover enthusiast and after-market developers to meet and to share ideas about future developments in suspension and other components for our Rovers.
Background
Rock-crawling technology is in the early stages of development. Over the past decade, there has been an increase in the usage of 35 inch or larger diameter tires, lower gears, stronger axles, and a movement toward long travel suspension designed to increase articulation.
The development of long travel suspension is not new. In the 70’s, a major focus of the motocross motorcycle designers was to increase the travel of suspensions. As shock absorber technology improved, longer travel suspension became possible. Developers of competition motocross bikes eventually found that more travel was good, up to a point. Beyond this point, additional travel decreased lap times due to inconsistency in the handling behavior. Presently, most motocross manufactures use front suspension travel in the 11.5-12 inch range and rear travel in the 12-13 inch range.
Off-road race trucks have gone through a similar suspension travel cycle during the past decade. It was not unusual for Baja type race trucks to run 30 or more inches of front suspension travel and 36 inches or more of rear wheel travel. These trucks had high centers of gravity (cg) and were unwieldy in the corners. More recently, the trend is toward lower cg designs. Part of this is due to the continued development of the by-pass shock which allows impacts to be absorbed in a controlled fashion with less wheel travel.
It is not clear where rock crawling suspension design will go in the future. Despite the differences between race speed and rock crawling speed, many of the handling problems are similar. For example, a truck which is loose on the road (the rear end wants to come around when cornering at the limit) will tend to be loose on a steep slide slope (the rear end will slip sideways first) even though the vehicle is moving much slower. In both cases, there are lateral forces tugging sideways on the truck. In one case, these forces are inertia forces. In the other case, they are gravitational forces. While the sources of the forces are different, forces are forces and the results are the same. The vehicle will be loose in both cases.
We all know that vehicles which tend to tip over sideways on the road will also tend to tip over on the trail. However, we have one condition that is fairly unique to rock crawling, that of extreme pitch. Since we like to climb the steepest rock possible, and occasionally travel (or slide) down steep pitches, we also must worry about tail standing or nose wheeling. These conditions can all be aggravated by lots of wheel travel. When climbing, the weight transfer toward the rear axle combined with the torque exerted by the rear axle will lighten the front end. If soft springs are used up front with lots of shock travel, the front can rise significantly on its springs. The rising front transfers additional weight backward, which can further aggravate front-end lift. Likewise, when descending steep rocks, the forward weight transfer coupled with the brake torque on the front axle can lead to a nose wheelie. Fun to watch, but not fun to pay for.
Not only can softly sprung, overly articulated suspension be less stable, but if taken to the extreme, they can provide less performance in real world conditions. A front end that lifts on extremely soft springs can increase the cg height of the entire vehicle unless the rear drops by a similar amount. This causes more weight transfer to the back, which makes it harder for the rear tires to climb off sandy bottoms onto rock faces or to climb over rock lips.
Presently, there is no consensus on how soft a suspension can be without being too soft, or how much articulation is enough for a given vehicle. It is not clear whether the best systems articulate mostly in the front, the back, or evenly front and back. Because of these issues, a suspension should not be judged on only its ability to articulate. It should be tested under real-world conditions. A suite of such tests are proposed below.
Basic Suspension Configuration for the D-90.
The basic front suspension of the Land Rover is comprised of two leading torque arms which provide the dual functions of locating each end of the beam axle fore and aft, and transmitting the torque generated by the axle (braking and traction torque) to the chassis. Lateral axle location is controlled by a Panhard rod. Because the front axle is over-constrained in pitch by the two torque arms, larger than normal flexible rubber bushing are incorporated into the leading arms at the axle to provide compliance. This compliance allows some axle articulation, but not as much as obtainable with 3 and 4 link designs and upper A-arm designs.
The rear suspension uses two lower trailing links to locate each end of the beam axle for-aft, and an upper A-arm to locate the axle laterally. The lower trailing links and the upper A-arm work together to transmit axle torque (including brake torque) to the chassis. Because of its design, the rear suspension is capable of very large articulation with the appropriate choice of spring systems and shock absorbers.
The Proposed Tests
The proposed tests are designed to test vehicle articulation of the modified vehicles and to evaluate the effect of these modifications on both on and off-road performance while minimizing driver influence.
Each manufacturer/supplier of suspension systems may field up to two vehicles if the configuration of the second vehicle is significantly different from the first. Two configurations are considered significantly different if they are clearly different in their intent, design, or cost. For example, a vehicle with 2 inches of lift (perhaps through spring spacers) and no sway bars would clearly have a different focus (toward the trail) than a lower vehicle with sway bars installed and street oriented tires. Significant design differences include the use of variable rate vs. fixed rate springs with significant differences in shock travel. Customer vehicles may be used to represent the supplier. If a supplier chooses to not participate, the committee may ask members of the D90 group who are equipped with the corresponding components to participate in their place.
The components of the suspension systems on the vehicles tested must be commercially available through the supplier or through other after-market sources. For example, the supplier may build and sell suspension shock mounts and specify after-market shock absorbers and springs. The use of prototype or earlier versions of the suspension components will be allowed if such components do not significantly affect the performance of the suspension system relative to the commercially available components.
The tests consist of a road test, a modified ramp test, and an off-road trail test. The total maximum number of points a participant can earn will be 1000 points. The results of the road, ramp, and trail tests will be weighted 40%, 20%, and 40%, respectively, in the overall score.
The Measurement and Ramp Tests, and Ride/Drive Tests will be performed during the morning. The Trail Tests will be performed in the afternoon. There will be two test stations for the morning tests – the Ride/Drive station, and the Measurement/Ramp station. The participants may go to either of these stations first. The order of testing will be on a first come/first serve bases. After completion of tests at one station, the participants must proceed directly to the other test station.
Hold Harmless Waiver
All participants in the formal tests will be required to sign a Hold-Harmless Waiver before the test.
Basic Vehicle Configuration for the Tests
To maintain consistency in testing, all test vehicles should possess lockers front and rear. The vehicles should also be in soft-top configuration (the top may be removed) as a hard top will raise the cg height and affect on-trail performance.
A vehicle with a stock suspension but with the anti-sway bars disconnected will be provided for reference measurements and reference performance. A stock vehicle with lockers will be preferable, but such a vehicle may not be available.
The supplier of the vehicle will provide an itemized description of the suspension components, modifications, and the associated cost. Sufficient detail must be provided so that the consumer can purchase these components through the supplier or through other identified, after-market sources. Tire/wheel configurations for each test vehicle will be provided. Additional information may be provided at the discretion of suspension component supplier.
No changes will be allowed to the vehicle spring rates or suspension geometry anytime during the twist-off. Changes in shock damping, if provided by external adjusters on the shock or by in-cab controls, will be allowed. Anti-sway bars may be connected only during the Ride and Drive tests if rapid connectors are installed on the vehicle (i.e., participants may not remove or install the anti-sway bars between tests). Different tire pressures will be allowed front to back but not side to side. The tire pressures must remain unchanged between the road tests and the ramp test.
The tire pressures may be changed prior to the trail run but not changed during the trail run.
With the exception of the ride and drive test, the vehicle supplier will provide a driver for all dynamic and static tests. Drivers and riders must wear seat belts during the ramp test and during all dynamic tests. Stock D90 roll cages must be present around the driver (the front section and the section just behind the drivers seat). The section over the rear tail gate may be removed if diagonal members from the section just behind the driver to the rear rails are in place.
Reference Measurements
The following measurements will be made:
Ramp Tests
In preparation for the ramp test, the ramp will be placed on level ground and the side of the ramp will be marked at the equivalent 20-degree ramp RTIs of 750 and 1000. This will require simple height calculations since a non 20-degree ramp will be used. The tires pressures must be the same as used for the Ride and Drive test. The ramp test will be performed with the left front tire on the ramp.
Lockers will be turned-on and anti-sway bars disconnected (if installed). The vehicle pitch and roll angle will be measured with a simple bubble gage on level ground and at a 20 degree RTI of 750, 1000, and the maximum RTI obtained. The measurements will be made with the gage on the front right fender. The distance between the reference point on the rim and the fender flare for the 4 wheels will be measured at a 20 degree RTI of 750, 1000, and max. These measurements will be used with the reference measurements made on level ground to determine the % of articulation that is provided by each axle. The maximum RTI obtained will be recorded.
The ramp test will be scored as follows:
RT score = max RTI obtained by the vehicle * 200 / max RTI obtained by any of the vehicles.
The vehicle with the max RTI will thus receive 200 points.
Road Test
The road tests will be a ride and drive test to evaluate ride comfort and stability of the vehicle over pavement and over dirt/gravel roads typical of those to the trail head. This test will occur with the tires and wheels provided by the vehicle supplier and will be run concurrently with the ramp tests. The course for the ride and drive test will be chosen by Jim Allen. This course should be of sufficient length to subjectively test the vehicle under normal driving conditions.
Several judges will drive each vehicle with the vehicle supplier (or their representative) riding in the passenger seat. The ride and drive test will be performed under normal driving conditions. If at any time the vehicle supplier feels that a judge is operating the vehicle in an unsafe manner, the supplier may chose to terminate the test and ask for a substitute judge.
A score sheet will be provided to each judge containing score boxes for several categories. A score will be assigned by each judge on a 1-10 scale (poor - excellent) for each of the following two categories.
The scores for these categories are meant to provide guidance to the judges for the final overall rating of the vehicle handling characteristics. Each judge can weight the items as he wishes, and can include any other handling characteristics that he desires in the overall rating for drive quality. Based on the results of the above categories and any other handling characteristic observed during the test, the judge will assign a single overall score of 1 to 10 to the handling characteristics of the vehicle. Any unsafe handling characteristics should be noted on the score sheet.
The ride test will be based on ride quality. A score will be assigned by each judge on a 1-10 scale (10 - excellent) for each of the following categories.
Again, the above categories are meant to provide guidance to the judges for their final overall rating of ride quality. The judge will assign an overall rating to ride quality (1-10) based on the response to the above conditions and any other ride quality condition which the judge deems important. Any unusual ride characteristics should be noted on the score sheet.
A check box will be provided on the Ride and Drive score sheets to record whether front and rear anti-sways bars were used during the tests.
The score for the road test will be as follows:
unweighted test score = average of drive test + average of ride test
Final ride/drive score = unweighted score * 400 / max unweighted score obtained by any vehicle in the test
At least one participant will thus receive 400 points.
Trail Tests
The trail tests are designed to evaluate suspension performance under rock-crawling conditions. The intent is to test performance under near static conditions (i.e., almost zero momentum conditions). While it is recognized that momentum (even a small amount of momentum) is a very important component of driving technique under rock crawling conditions, the optimum application of momentum and the corresponding throttle control is very driver dependent. It would be hard to separate the effect of driver skill from suspension performance under such conditions.
Various trials-like sections will be used. Vehicles will attempt to complete these sections with minimum wheel-spin at near zero crawling speeds. Jim Allen will identify these sections on 21 Road prior to the event. The trail organizational committee will preview these sections on the day prior to the event to make minor adjustments to the sections, if needed. The committee will also assign a total number of points to each section based on the number of obstacles and the difficulty of each obstacle. Judges, if available, are encouraged to preview these sections as well. The trail organizational committee reserves the right to make changes or additions to the sections at any time during the event for sections that are deemed to be too easy, too difficult, or too risky.
The sections will be chosen to test the vehicles under a variety of conditions. In the case of steep sections, the sections should be near the limits of vehicle traction in climb and descent (i.e., 35-45 degrees slopes) and below the limits of vehicle tip over stability on slide slopes (30 degrees). The sections should be chosen such that if a vehicle slips off a section, there is minimal safety hazard to the driver or damage potential to the vehicle. It is desirable to choose sections that contain one or more of the following features
The line and speed at which a participant must pass through a section will be defined by the trail organizational committee on the previous day and controlled by the spotter. The spotter for each section will be one of the judges. The spotter will spot all vehicles through that section. The vehicles may be asked to come to a stop, with the front or rear tires resting against an obstacle. The driver will then be asked to climb the obstacle without backing up. This will test the ability to climb this obstacle with little momentum. The vehicle driver, owner, or any judge may stop the test at any time for safety or other considerations.
All participants will complete a section before the participants and judges move to the next section. This will allow the same judges to be used for all sections, maintaining consistency in judging from section to section. The vehicle order may be changed from section to section if sufficient trail space is available.
The performance on each section will be judged by the following criteria:
The overall scoring for the trail tests will be as follows:
Trail score = number of points received * 400 / maximum number of points obtained by any vehicle
At least one participant will thus receive 400 points.
Final Scoring and Data Distribution
The maximum RTI obtained and the overall scores for the Ride and Drive and for the Trail Tests will be tabulated for each vehicle at the close of Saturday. We will share these results with the D90 group during the trail run on the following day.
A detailed summary of the results will be posted to the D90 group by Richard Hills shortly after his return to Las Cruces. This will include all of the measurements made at different RTIs on the ramp and a tabulation of vehicle performance on the trail sections. The intent of this posting is to provide members of the D90 group with sufficient information to weigh the advantages and disadvantages of each suspension type from their perspective.
The total score will be based on the following formula:
Total Score = Ride and Drive score + Ramp score + Trail score.
The maximum score possible is 1000 points.
Publication of the Results
The event coverage is intended for two publications, Four Wheeler magazine and LRM in Britain. The results of the competition will be posted with my observations of the equipment and tests. For magazine purposes, this is more a "comparison" than a "competition," though the competitive aspects will no doubt be very present at the event itself! Each manufacturer and product will be discussed individually, with the good points of each system highlighted. The goal is to provide readers with enough information to make an informed choice when it comes to purchasing a suspension system with no "buyer's remorse" later. Since we will have a wide variety of systems in the test, from mild to wild, it should make an interesting comparison.