From the November 2022 issue of Car and Driver.
Maybe you’ve seen a photo of a one-wheeled SUV propped high in the air and thought, “That’s cool.” Except it isn’t, at least not if you’re serious about off-roading. A locking differential might keep you moving, but a vehicle with so much daylight under one tire can suddenly wobble as the terrain changes, with the suspension unable to fully cushion the landing. The transition can be rough and potentially dangerous. For good traction and stability, you want all four wheels on terra firma at all times.
Getting through the tough stuff requires ground clearance along with good approach, departure and breakout angles, but what’s often overlooked is suspension articulation.
To measure articulation, we drive a test vehicle’s driver-side front tire up a 20-degree ramp to generate a Ramp Travel Index (RTI) score. The test stops when the driver’s rear tire just starts to lift off the ground. This is the point of maximum flex, where the driver’s front tire is at maximum compression while the passenger’s front tire is at maximum sag, with the opposite true in the rear.
We then measure how high the driver’s front tire rose off the ground and use a little high school trigger to convert that to a distance driven on the ramp (remember SOH-CAH-TOA?) . We also take wheelbase into account. The final RTI score is distance driven up the ramp divided by wheelbase, then multiplied by 1000 (to eliminate decimals). A perfect score of 1000, which we’ve never seen from an unmodified vehicle, occurs when the driver’s rear tire hits the ramp before it ever lifts off the ground.
Why wheelbase is important
Imagine two vehicles that can both climb the ramp until their driver’s front tire is three feet off the ground. Having a 60-foot wheelbase isn’t an impressive feat, in part because the vehicle will drag its chassis over any obstacle it tries to overcome. Say there is a six foot wheelbase on the other vehicle. It will be much better able to adapt to the terrain and take on just about anything.
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