K&C vs. Pull-Down

K&C and Pull-Down: What's the difference?

The AB Dynamics K&C test machine at Morse Measurements is not a "Pull-Down" rig. The capabilities go far beyond simply find vertical suspension rates and travel limits. Hiighly accurate suspension geometry measurements are made and all appropriate tire loads are introduced while testing.

"Pull-down" rigs are great for making quick suspension checks, but beware: Ride rate measurements on a pull-down rig can easily be 10% off (or more!) because the tires are not allowed to float and relieve their scrub forces. Yes, tire scrub forces influence vertical suspension rates. As illustrated below (using K&C test data from an actual vehicle), when cornering and braking/traction loads are correctly introduced, key suspension geometry and stiffness characteristics change significantly.

The first figure shows lateral weight transfer measurements with (blue) and without (red) the introduction of tire cornering forces. The red traces could be obtained from a pull-down rig (or even a 7-poster), but for a given chassis roll angle, the measurements do not reflect what's really happening. The blue traces, measured on the Morse Measurements K&C rig and including proper tire cornering forces, tell a very different story:

As shown above, front and rear lateral weight transfer is not measured correctly unless lateral tire loads are applied. It is particularly noticeable in the rear of this vehicle, where some 200 lb of additional weight transfer comes from suspension jacking forces. Since total lateral weight transfer is dictated only by the cornering G's and C.G. Height, what we are seeing here is an effective change in roll stiffness when jacking loads are taken into acount. The rear suspension is also shown to have significantly more load variation due to friction when properly loaded to simulate cornering. Only a properly set up K&C test will get after this kind of information.

Jacking force influence on weight transfer becomes even more pronounced when suspension rates become non-linear (due to bump-stop or coil-bind situations) and when a vehicle is subjected to large aerodynamic down-force loads. And although this example only shows the lateral weight transfer characteristics, jacking will similarly influence longitudinal weight transfer during both throttle-on and braking.

Cornering loads will also have a pronounced effect upon suspension geometry and vehicle trim. The next figure shows the steer angles at each corner of the vehicle, again with (blue) and without (red) the introduction of tire cornering forces. Once again, the red measurements do not reflect what's really happening. The blue traces, measured on the Morse Measurements K&C rig and including proper tire cornering forces, provide a more accurate representation of vehicle steer trim while cornering :

As shown above, a seemingly nice zero roll-steer set-up in the front suspension has degraded into roll-understeer when tire cornering loads are taken into account. This clearly demonstrates the importance of the "C" in K&C testing. It also raises questions about what else is happening to the suspension under load. Again, only a properly set up K&C test will get after this kind of information.

back to "What is K&C Testing?"

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Phone: 704.638.6515 or 877.270.3230