Introduction

For the more technically inclined amongst you, this document outlines the detailed physics and car dynamics modeling strategy employed in the SBDT�s GT RACING 2002. In all, about 1200 hours of work has gone into these aspects of the GT RACING 2002 since I got the first EA Sports F1-2002 betas back in April 2002.

1. Suspension

All cars have unique (to that car) suspension geometry. The GT cars in the game feature traditional double A-arm suspension modeled, except, of course, the Porsches. The Porsches have a McPherson front strut, and the rear is the Weissac 5-link (both accurately modeled). ISI's Physics engine is unique in that it calculates real-time suspension geometry whilst the car is in motion. The roll centers, camber gains, anti-dive, bump-steer, caster are all done in real-time. So to have the car behave realistically, the suspension geometry must be realistic. I spent much time designing each car�s suspension as a real car designer would � adding individual characteristics along the way. For instance, you may notice the Porsches have a bit of bump-steer, which is accurate, where as most of the double a-arm cars don't. Also because of the McPherson front strut, the Porsches require a bit more front static negative camber to compensate for their lack of camber control. And, in case you�re wondering � no, GPL doesn�t do this.

1.1 Suspension Travel

The cars have approximately 40-50mm bump travel at the default ride height until you hit the bumpstop. You will hit it even without packers. The packers just space the bumpstop close to the shock body, limiting travel. Default ride height is 60mm front and 70mm rear. The bumpstop is modeled on a progressive Koni bumpstop from my own race shop. It starts around 150lbs/inch and after 20mm is around 1200lbs/inch.

1.2 Springs

All Spring values are wheel rate. This means it�s the spring rate that the wheel �sees� after itworks its way through the lever arms and installation angles of the shocks (the motion ratio). For instance, the Porsche might use, in real life, a 150nm front spring � but after it works its way through the motion ratio, the wheel only sees it as 110nm working rate. That�s the number you are adjusting in the garage. So don�t get worried if you try to plug in some real world spring values and the car doesn�t handle how you�d expect it to.

1.3 Shocks

All Shock values are wheel rate too, like the springs. The dampening values are in newtons/meter/second - which means if you have a 1000 bump newtons setting in the garage, the shock will provide 1000 newtons of resistance when the shock is traveling at 1 meter/second (1000mm/sec). Most race cars of the GT type operate the shocks from 0-100mm/second at low speed (driver inputs) and up to 500mm/second at high-speed (bumps and curbs). Typical bumpy sections will be in the 200-400mm/sec range. Now, the telemetry graphs need to be setup to display this properly or you won�t get much use out of them. As shipped they seriously under-graphed and shouldn�t be filtered (except for histograms). Also the sampling rate should be set to 40 in the .PLR file (from the default 10). Use the graphs I've included (should have been installed for you), which sets the shock velocities from 0-500mm/sec and takes out the filtering � you need to know the peaks with shocks. Anything under 100mm per second is considered �low-speed� and is mostly your driver inputs, and over that is considered �high-speed� - bumps and curbs. So if you have a problem at a certain part of the track, or a persistent problem with something like corner entry understeer, look at the shock velocity graph at that point of the track and pick out the shock speeds. If it�s all under 100mm/sec then you want to change the low-speed dampening, if you see higher speeds going on (maybe bumps you don�t feel) than adjust the high-speed � or sometimes both. High-speed rebound dampening is typically at least double the fast bump because it is damping the main sprung mass in addition to the energy built up in the spring. On the other hand, the bump is only damping the unsprung mass (wheel, tire, spindle, brakes, and suspension arms) and the spring oscillations.

Slow speed dampening is not as cut and dry for the initial setting. I usually start slightly higher than the low-speed numbers and go from there. Here�s a chart I did way back for GP2, but it�s still relevant and is based entirely on real world settings and effects. I�ve included the intro tests, but the most useful is the corner phase descriptions and the chart
for adjusting them � (please ignore the specific GP2 references):

1.3.1 General

At all times cornering balance is affected by the weight distribution on the four tires. Springs, sway bars and wings give constant resistance or affect weight distribution through the ENTIRE length of a turn. Dampers however, and their amount of resistance, can affect the balance at different _parts_ of a turn. This occurs because at different parts (or what are
called "phases") of a corner, different dampers and their travel are dominant at that point. This makes for a excellent way to adjust the corner entry and exit independent of each other, or to take a corner that is unbalanced from entry to exit, to one that is balanced (ie: understeer on the way in - oversteer on the way out).

1.3.2 Fast-Damping

Fast damping is what the tires see and feel i.e.: reactions over bumps or kerbs. Its job is to keep the rubber on the ground over the various surface undulations. Traveling over a bump at speed causes a relatively large and �fast� movement of the damper shaft, and hence it's name. If the front of your car is �overdamped� in the fast bump direction, then you will experience UNDERSTEER on the bumpy sections of turns. If the rear is overdamped you will experience OVERSTEER.

For fast speed adjustments, pick a bumpy turn at the particular track you're working on. Start with bump at 0 and rebound at 2 and work your way up until the front UNDERSTEERS over the bumps, then back off 1 or 2 clicks. Then do the same for the rear until it OVERSTEERS over bumps, again back off 1 or 2 clicks. Always keep the fast rebound higher than the bump - 1.5 to 3 times so. The stiffer the spring the stiffer the rebound setting. It is the fast rebound's job to resist spring pressure and unsprung weight (wheel, tire, hubs, brakes etc) when the suspension oscillates. Usually a setting of 2 times the fast bump works well in GP2. Make sure the car likes "usable" kerbs, too. This may require softer settings than done in your bumpy turn test - everything is a compromise.