Which Type of Roll Test Should I Run?
One of the finer points in K&C testing that is often overlooked is the type of roll test to run. For those unfamiliar, Morse Measurements offers two styles of roll test: a fixed axis test and a natural axis test. In the fixed axis test, the chassis is rolled about an axis in the ground plane. In the natural axis test, the chassis is allowed to pitch and heave as it rolls to maintain constant load across each axle. This results in roll about what we call a “natural” roll axis, so named because it more closely replicates the roll motion of the car in real life. After all, in real life the vertical load on an axle of a car does not increase as a result of pure roll motion.
There are numerous practical differences between the fixed axis and natural axis tests. For asymmetric cars with significant spring split side-to-side, roll stiffness will measure very differently in these two tests. Similarly, for cars on bump stops or with coil bind setups, roll stiffness will again measure very differently. The reason they measure differently is because when the car rolls onto the stiff side (because of stiff springs, bump stops, or other causes) in the natural axis test, wheel travel is less on that side and the car tends to pivot off of the stiff side. The pivot motion is the result of a combination of roll and vertical heave and most often cars will have a lower wheel rate in vertical bounce as compared to roll, so the roll stiffness measured in the natural axis test will be a combination of roll and heave rates. Further, the very small wheel travel on the stiff side may make it difficult to accurately curve fit the wheel rate on that side at all. The fixed axis test results in more wheel travel on the stiff side, making it easier to curve fit, and with no bounce motion it more accurately reflects the true roll stiffness rate.
The above discussion applies to asymmetric cars and cars with bump stops or coil bind. But what about symmetric road race cars without any such stops? Are the fixed axis and natural axis tests comparable in this case? The answer is yes and no. Yes, in the sense that if you make no changes to the car and run both styles of roll test, you will get very similar roll stiffness rates. However, if you are rating anti-roll bars, as you adjust bar stiffness at one end of the car it will change the chassis motion in the natural axis test and this change in motion will result in a measured change in stiffness at the other end of the car where you didn’t change anything. To avoid this confusion, we recommend doing bar rating with a fixed axis test and running natural axis tests only on sensible combinations of front and rear bars. In the fixed axis test the roll stiffness at one end of the car has a minimal impact on the stiffness at the other end.
In summary, both the fixed axis and natural axis roll tests tell you useful information about the car. By using the correct test for your purpose you will be sure to get the data you need. When practical, Morse Measurements recommends you run both styles of roll test while on the rig. This way you have the answers you need and aren’t left wishing you ran the other test too. Happy testing!