How Do I Know when It's Bottoming?

Since GPL gives no audible warning, I had to train myself to recognize more subtle signs of the suspension bottoming on the bump rubbers. To effectively develop good setups in GPL, you need to do this too. (See Spring Rates and Ride Height for an explanation of why this is important.)

What's Bottoming? When I say "bottoming the suspension" I am not referring to bottoming the chassis on the track, which does give an audible signal in GPL - and a shower of sparks as well. I'm referring to the state in which the suspension has deflected to the limit of its sprung travel and is compressing the bump rubbers, but has not compressed the bump rubbers sufficiently to permit the chassis to touch the track surface. In this state, the springs, anti-roll bars, and dampers become essentially irrelevant because the spring rate of the bump rubbers is so much higher than that of the actual springs. Transition to this state is a crucial one, but it's a transition about which GPL is silent.

To help you learn how to recognize when the suspension bottoms, I've provided several setups which will almost certainly cause the rear suspension to contact the bump stops. These setups have the prefix BOTTOM, and are adapted from my baseline setups with a few small modifications to promote bottoming.

I suggest you take one of my baseline setups and try it for a few laps. Then take the BOTTOM setup for the same circuit and chassis and run a few laps. The sensations resulting from bottoming should be quite distinct.

Triggers

There are several different conditions which tend to bottom the suspension.

Power squat occurs when the outside rear suspension bottoms under acceleration in an essentially flat corner. A good example is when exiting Parabolica at Monza. The resulting transfer away from the inside rear wheel causes snap oversteer, which can show up as a "greasy" feeling from the rear as it starts to slide.

Inside chassis lift occurs in high speed banked corners such as the first turn at Rouen, the left-hander in the Masta kink at Spa, and Fast Bend at the Glen. In corners like these, the chassis rolls over until both front and rear outside suspensions contact the bump rubbers. Then the inside edge of the chassis begins to lift as lateral loads continue to rise. Once I recognized what was happening, I found this to be a slightly sickening feeling.

Bumps can bottom the suspension, when the rear suspension is soft and the chassis is running low, especially when the suspension is already loaded by a banked turn and/or an increase in gradient. A good example is the bump near the exit of Abbey at Silverstone. The suspension is compressed by the banked corner, and further compressed at the rear as the driver applies power. Then the suspension contacts the bump, which snaps the tail out, and then goes light after the peak of the bump. This is one of the big reasons why spins at the exits of Abbey are so common.

Dropoffs, or negative bumps, have an effect similar to that of bumps. In the original GPL tracks, these are found at the entrance of many corners. Examples include Copse, Stowe, Club and Woodcote at Silverstone, and Crowthorn and the entrance to the left-hander in the Esses at Kyalami.

When a car goes over a dropoff, the suspension goes light as the track goes away (allowing the tail to get out), and then loads up sharply as the car slams down on the other side of the "hole". On cars sprung too softly at the rear, the rear suspension can bottom even under braking.

When this occurs at the entrance to a corner, when the car is already somewhat destabilized by transfer of weight away from the rear wheels under braking, the result can be a sudden, unrecoverable slide. At the very least, the driver becomes very busy with the wheel and pedals reacting to the car and trying to keep it from going around, and also trying to keep it on the correct path toward the apex.

I think the numerous corner entry dropoffs are one of the reasons Silverstone is disliked by many GPL enthusiasts; people find it very difficult to point the car as they began to push the limits, and going in just a bit deeper than the lap before often results in a spin. Proper setup, however, can completely eliminate this as an issue. Silverstone becomes a much more enjoyable circuit as a result.

High vertical load can easily bottom the suspension. When coupled with the need to turn and apply power, snap oversteer can result. The right-hander in the Esses at Kyalami and the exit of the Loop at the Glen are good examples.

Outside Views

In addition to being alert for snap oversteer, inside chassis lift, and other symptoms, watching rear suspension from various views can help identify points where the suspension bottoms. In left-hand turns (such as on the ovals) watching a replay from the rear suspension view can provide a very enlightening experience as you watch the suspension work up and down. Travels to extremes immediately become very obvious. It's a shame there's no view from the left rear suspension, since most of the corners in GPL's road courses are to the right!

When I'm developing a setups, I sometimes use the dreaded F10 arcade view to watch what's happening at the rear. This gives a way to see what the rear suspension is doing in real time and also in replays. I watch the angle of the halfshafts. When they are level or inclined slightly downward, all is well. But when they reach a certain more acute angle (depending on the chassis and the length of the bump stops), especially if this is followed immediately by snap oversteer, the suspension is bottoming. Some careful observation should enable you to identify this angle.