The more rigid your chassis,...
The more rigid your chassis, the faster it will be. However, a more rigid car will ask more and forgive less from your setup. A less rigid car will likely not be as fast, but it will forgive any setup mistakes. Where do you draw the line?
How rigid should your cars chassis be? Generally speaking, you would want the chassis to be as rigid as it can be for handling reasons. The more rigid a chassis is, the more it works the suspension, and in a perfect world, you want the chassis to be stiff with the suspension doing all the work. But thats in a perfect world, and, unfortunately, we dont live in one. If your car flexes a little bit, depending on which area is flexing, it cant be such a bad thing. Your race car will go faster if you have a rigid chassis and your setup is spot-on. But buddy, it better be spot-on, because a rigid chassis is more unforgiving. A chassis that flexes a little bit is more forgiving, so you can afford to be a little off on your setup.
You can hardly get a chassis rigid enough, but there are cases where certain parts of the chassis are too stiff. Telltale signs your chassis is too rigid are cracks and wrinkles on the frame and sheetmetal splitting on the body and other structures. If things are cracking, chances are they are moving. If you see wrinkles in your firewall or sheetmetal splitting, then something is moving and that might be a good place for a brace. Without having a computer analysis done on your chassis by a structural engineer, which a lot of people might not have the budget for, youre going to have to let your chassis talk to you. If youve installed a rear firewall in your car and you see it wants to rip one corner out, that means that panel is working back and forth. If a weld sitting on top of a framerail has a split in it or some rust or a crack, that tube is pumping up and down on that flat framerail. That might be a good area for another brace. When stiffening your car, always try to make a triangular brace because a triangle is the strongest shape we have.
It is very important for the front suspension to be rigid. As your car goes down through travel and pulls vertical and lateral g-forces in the corners, you dont want the toe on the car to change radically because of chassis flex. If that happens, the car is driving you, not the other way around. If your car or front snout is flexing so the spindle travels and the frame moves, then it pulls the drag link that tugs your tie rod that tugs on the spindle ... youre going to put steering input into the car and the chassis will add its own input as well. You want all your steering assemblydrag links, idler arms and steering boxesto be rigid. You want your ball joints and rod ends with no slop in them. Thats going to make for a better-driving, better-handling car. If drag links or steering boxes are flexing, theyre steering your car, not you.
On our cars at Hendrick Motorsports, we have bars just about everywhere we need them. If we added a few more, theres a good chance the car would be too rigid. The car would go faster for qualifying as long as the setup was perfect, and we all know how hard it is to hit the perfect setup. Dont put in any more bars or braces than you think you have to, because bars and braces carry weight. If you have the means to go test, build up some pieces you can bolt on. We use an X-brace on our cars, and so do a lot of Winston Cup teams. You might X-brace the front-frame horns up to the shock towers, for example. You can make the brace so it bolts on and off, but make sure you bolt it on in a very positive way so it doesnt flex.
One last thing: dont work on one end of your car a whole lot more than the other, because if you do, youre going to have problems. If you had the perfect-driving car and you wanted to stiffen the front of the car up, then you need to go to the back and stiffen it the same amount. If you make the front end very positiveyou turn the wheel and the car turns immediatelyand the back of the car is flexing, then theres going to be a delayed reaction. If your car feels wormy in the backhunting around, so to speakyou might need some more structure. That will help the car all over, because maybe the front end is good. If the tail clip on the car is wagging around, put a couple more braces in and you could fix your problem.
Just remember, the more rigid the car is the faster it will go, but it will also be less forgiving to your setup. As long as it isnt moving too much, youll be OK. Eddie Dickerson is head of the chassis department at Hendrick Motorsports in Concord, N.C., where he builds cars for Winston Cup champions Jeff Gordon and Terry Labonte as well as Jerry Nadeau.