A vehicle’s RTI or ramp travel index is a measurement of a vehicles suspension (and frame) flexibility. In many situations off-road a vehicles flexibility may be the determining factor of whether or not it can get over an obstacle. If a vehicle’s suspension is too stiff this may cause a tire or two to be lifted, loosing contact and traction, especially apparent in vehicles with open differentials. While locking differentials will help to maintain forward momentum when a wheel is off the ground, good flexibility will help to keep the wheels on the ground and more able to conform to the terrain.
You may have seen at many off road events 4×4’s taking a shot at the ramp. The ramp tells a great deal about a vehicle’s capabilities on the trail. It’s also very useful for evaluating suspension systems, examining shock length, finding interference points and other things that may only be apparent on the trail while under the extremes of wheel travel. There are several factors that make up the RTI score. First the ramp. In most cases the ramp is on a 20 degree angle. Some events measure the extremely flexible vehicles with a ramp of greater angle like the 23 degree ramp. The vehicle attempts to climb the ramp as high as possible without lifting a wheel off the ground. The distance the vehicle can travel up the ramp without losing contact with the ground is measured from the leading edge of the ramp to the center of the hub on the ramp. That number is then divided by the vehicles wheelbase (the distance between the vehicles centerline of the front axle and the centerline of the rear axle) and then multiplied by 1000 to calculate the average. For instance, if a vehicle with a wheelbase of 94 inches travels 62 inches up the 20 degree ramp then the RTI would be calculated as: 62″ divided by 94″ x 1000 resulting in a score of 659. This method of multiplying by the vehicles wheelbase allows one vehicle to compete with another vehicle like a CJ-5 to an F-250 Crew Cab on the same ramp test.
While you can’t “cheat” the ramp, you can get the most out of your shot at the ramp and in the process improve your vehicles ability to articulate the terrain. One the the easiest things that can be done to improve your RTI slightly and improve trail performance is to “air down” the tires. Airing down the tires to an air pressure of about half of the recommended tire pressure for normal driving will slightly improve your RTI score but more noticeably it will improve your the vehicles trail performance by smoothing out the bumps and allowing your meats to conform to the terrain resulting in better traction. Some people air down to as low as 5 PSI on the trail however pressure this low can result in the tire bead separating from the rim. As a result bead locks are used to hold the tire to the rim. Always be sure to bring your air pressure back up to normal on-road driving pressure when you hit the road home again. Another way to improve the score slightly and smooth out the ride involves only to grease between the leaf springs of the leaf spring pack. Some folks say that this helps more than they would have expected when the hit the ramp. Other ways of improving the RTI score such as sway bar disconnects, compound shackles such as Revolver Shackles, better suspension systems all help to improve not only the RTI score but the performance off-road.
How to measure your RTI score without a ramp.
So you don’t have access to a ramp and you don’t feel like building one. How can you find out what your RTI score would be if it were tested on the ramp? Since the RTI score is simply mathematics it can be measure and calculated without a ramp. The RTI is a measurement of how high a tire can travel vertically you will need to safely raise one of the tires until one of the other tires almost comes off the ground. This can be done by using a forklift with the tire resting on a pallet or by using a floor jack and the tire resting on a flat surface underneath which is the floor jack. Be sure to do this safely. The main objective is to raise one wheel as high as possible before lifting a tire off the ground. Once raised measure the distance between the bottom of the tire and the ground.
If using a fork lift you would measure where the tire meets the pallet to the floor. Since a 20 degree ramp rises at .345 inches per inch divide the height of the lifted tire by .345 (for 20 degrees). Next divide that number by your vehicles wheelbase. Multiple that number by 1000 to acquire an average and you have your score. While the ramp is a true measurement of your RTI this calculation is pretty close.