How We Test SUVs and Trucks for the Right Amount of Flex

From the November 2022 issue of Car and Driver.

Perhaps you’ve seen a photo of an SUV with one wheel hiked high into 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 that much daylight under one tire could suddenly teeter as 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. [Note: Our steel ramp is homemade. For scale drawings and instructions, send a request to The New Yankee Workshop.]

Getting through the tough stuff requires ground clearance along with good approach, departure, and break-over angles, but oft overlooked is suspension articulation.

To measure articulation, we drive a test vehicle’s driver’s-side front tire up a 20-degree ramp to generate a Ramp Travel Index (RTI) score. The test stops when the driver’s-side rear tire just barely begins to lift off the ground. This is the point of maximum flex, where the driver’s-side front tire is at maximum compression while the passenger’s-side front tire is at maximum droop, with the opposite true at the rear.

We then measure how high the driver’s-side front tire rose off the ground and use a bit of high-school trig to convert that to a distance driven up 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 if the driver’s-side rear tire touches the ramp before ever lifting off the ground.

Why Wheelbase Matters

Imagine two vehicles that both can climb the ramp until their driver’s-side front tire is three feet off the ground. If one has a wheelbase of 60 feet, that’s not an impressive accomplishment, in part because the vehicle will drag its undercarriage over any obstacle it tries to surmount. Say there’s a six-foot wheelbase on the other vehicle. It’ll be far better able to conform to terrain and tackle just about anything.