How we test SUVs and trucks for the right amount of flexibility

From the November 2022 issue of Vehicle and Driver.

Perhaps you’ve seen a photo of a single-wheeled SUV going through the air and thought, “That’s cool.” Except it’s not, at least not if you’re serious about off-roading. A locking differential can keep you on the go, but a car with a lot of daylight under one tire can suddenly wobble as the terrain changes, with the suspension failing to fully cushion during landing. The transition can be difficult and potentially dangerous. For good traction and stability, you always want all four wheels to run on solid terrain. [Note: Our steel ramp is homemade. For scale drawings and instructions, send a request to The New Yankee Workshop.]

Overcoming difficult roads requires good ground clearance along with good approach, departure and overtaking angles, but often overlooked is the articulated suspension.

To measure responsiveness, we drove the test car’s driver-side front tire up a 20-degree ramp to generate a Ramp Ride Index (RTI) score. The test stopped when the driver’s rear tire was barely lifted off the ground. This is the point of maximum bend, where the front tire of the driver’s side is at maximum compression while the front tire of the passenger side is at maximum deflection, and at the rear it is the opposite.

We then measure the height of the driver’s front tire above the ground and use a bit of premium wheeling to convert that to a distance when driving uphill (remember SOH-CAH-TOA?). We also take into account the wheelbase. The final RTI score is the distance going up the ramp divided by the wheelbase, then multiplied by 1000 (to remove the decimals). A perfect score of 1000, which we’ve never seen from an unrepaired vehicle, occurs if the driver’s rear tire hits the ramp before lifting off the ground.

Why wheelbase matters

Imagine two cars can climb uphill until their driver-side front tires are three feet off the ground. If one has a wheelbase of 60 feet, that’s not an impressive feat, in part because the car will pull its underbody over any obstacle it tries to overcome. Let’s say there’s a 6-foot wheelbase on the other car. It would be much better to be able to match the terrain and tackle anything.