Not all 2021 Ford F-150 Raptors will be the same underneath.

The third generation Ford F-150 Raptor promises more capability, more technology, and a better sound than the outgoing Raptor. It also has two big headline-grabbing features: a five-link rear suspension with coil springs and optional 37-inch tires. Choosing the latter will be a more important decision than consumers may realize at the time of purchase.

Raptor program manager Tony Greco told Road & Track that Raptors with 35-inch and 37-inch tires will have different frames.

While the overall structure of the 2021 Raptor is the same regardless of tires, specific details of the frame and suspension set them apart. Jack Cooper, architecture engineer for the 2021 Raptor, told Motor Authority that Raptors with 37-inch tires get rear jounce bumpers mounted in different spots on the rear axle. In addition, the spare tire winch mounts and a plate that moves the winch rearward are mounted to a rear frame crossmember that is rotated so they sit on the bottom of the crossmember instead of the top.

This is critical information because a 2021 Raptor has to be ordered from the factory with the optional 37-inch tires to get the frame that will allow a 37-inch spare to fit under the bed. Order a truck with 35s and upgrade to 37s later and the 37-inch spare won't fit under the bed.

2021 Ford F-150 Raptor

2021 Ford F-150 Raptor

Trucks with the 37s also get front Fox internal bypass shocks upgraded with a larger 1-inch rod (up from 7/8 inch). This increases the force displacing the fluid in the shock to make up for its shorter stroke and allows the shock to better absorb energy with the larger, heavier tire.

Trucks with the 35-inch tires don't get the updated frame as it would move the tire and change the departure angle and ride height.

Cooper noted the change to rear coil springs was key in making room for the 37-inch tires. Had the Raptor kept the leaf springs it would have been "very difficult" to make the 37-inch tires fit and meet all Ford's corporate requirements for packaging, Cooper said.

Despite losing an inch of suspension travel both front and rear with the 37-inch tires, the Raptor is more capable with the larger tires. "The 37s are top dog," Cooper said. Opting for the larger tires increases ground clearance by 1.2 inches, increases the approach angle by 2.3 degrees to 33.1 degrees, and improves the departure angle by 2.2 degrees to 25.1 degrees.

Still thinking of buying a Raptor with 35s and upgrading to 37s afterward? Cooper advises against it. "We would not condone switching them at this point in time," he said.