89_RS

I've been toying with getting a thread going on the basic relationships between the Suspension & Chassis since we lost a pair of the finest threads on the subject. My aim in this thread is to give solid technical information and overviews of the many subjects involved in suspension & chassis function. But there are areas where I don't have enough technical understanding and experience to intelligibly explain the subject as I am not an engineer. About time I put to paper the 2yrs of knowledge & experience I gained on FSAE & in the machine shop.

Topics I hope to explain (red indicates my limited knowledge & experience):

Torsional Stiffness
Instant Center
Roll Center
Center of Gravity
Weight Distribution
Sprung Weight
Unsprung Weight
Bind
Spring Rate
Wheel Rate
Motion Ratio
Ride Height
Tires
Bushings, Rod Ends, Ball & Socket Joints
Structural Integrity
And any other topics that any readers come up with that I haven't thought of.

Hopefully, we can get a great body of knowledge together for Suspension & Chassis Mechanics.

89_RS

First up, Torsional Stiffness & Structural Integrity. For the sake of simplicity, lets assume we aren't gas axing & welding the uni-body to suite our needs and are working with a from the factory 3rd gen uni-body.

As in construction, every car begins with a good foundation. The chassis is the foundation we have to work with. As it comes from the factory, the 3rd gen is composed of a Front Sub-Frame, Floor Pan, Rear Sub-frame, and Roof section depending on whether you have a Coupe, T-Top, or Convertible. These pieces aren't much more than laminated sheets of stamped steel held together with spot welds & industrial adhesive. Doesn't sound very strong, but it is and it gives a very good strength to weight ratio as well. The new Dodge Durango this year is the first uni-body Durango and exemplifies this well. Chrysler engineers were able to maintain the stiffness of the original Durango's body on frame while cutting nearly 2000lbs from the weight of the vehicle.

The uni-body as it comes from the factory is well suited for getting groceries and what not, but start ratcheting up the power & getting into the twisties and pretty soon chassis flex becomes a serious issue. Some of you may have noticed the car flexing going up the curb into your driveway, now imagine that at 40mph in a turn at the track. Not desirable to say the least. Thankfully, there is a solution to this problem: Sub-Frame Connectors (SFC's). SFC's triangulate and tie together the front sub-frame, floor pan, and rear sub-frame to substantially increase torsional stiffness and virtually eliminate chassis flex.

Torsional stiffness is how well the chassis can resist twisting along its longitudinal axis (centerline of the chassis from front to back). Torsional stiffness is the amount of torque per unit of deflection and can be expressed as lbft/degree or Nm/Rad. Higher torsional stiffness means less chassis flex in the turns. A way to think of our cars from the factory is as two blocks (the front & rear sub-frames) held together by a plywood sheet (the floor pan). With nothing but the sheet to resist torque, its rather easy to get that sheet to flex with your hands. Now, if you put a couple blocks length wise (SFC's) between the 2 blocks and nail those blocks to both the sheet & other blocks, you'll have a setup that is so stiff you'll need a crowbar to get it to flex. This is how SFC's increase torsional stiffness.

As for the chassis structural integrity, they are sound from the factory with the addition of SFC's and no coil-overs. As they are designed, 3rd gens were never designed with coil-overs in mind. 3rd gens distribute the vehicle weight through 8 points on the chassis: 4 Spring Pockets & 4 Shock/Strut Mounts. Coil-overs reduce this to 4 points: the 2 Strut Towers in the front and the 2 Spring pockets in the rear. These points were never intended to carry this amount of load and while some coil-over kits do reinforce these areas, they are just distributing that load over an area that already wasn't meant for that kind of loading. There are plenty of cost effective options that utilize stock suspension points for Drag, AX, & RR already.

More to come

89_RS

Quick note on roll cages. They do contribute the torsional stiffness of the vehicle, but not as much as SFC's. They are primarily meant to keep you in one piece should you wreck.

Now that the chassis is stiffened up, what are we going to do with it. Do we want to want to do daily driving, drag racing, autocross/road racing, or some combination of the above? If were aiming more at drag racing, then stiff suspension bushings and a shock & spring package that help promote weight transfer are preferable, for autocross/road racing were looking for spherical bearings/ball & socket joints for the suspension items and the best tires one can buy. Depending on how aggressive we want the car to be in either case, daily driving characteristics would be affected. Since I have far more experience in racing FSAE cars and their suspension setups for autocross & road racing, I'll stick to them. Anyone with more experience in drag racing setups, please chime in and contribute.

First thing that needs to be looked at, besides the budget, is the stock weight distribution of the vehicle. Stock thirdgens, IIRC, near a 57% front/43% rear weight distribution. Ideally, we want a 50/50 setup for autocross & road racing. This means we need to cut weight off the front end in the form of aluminum engine parts, fiberglass hood & clip or relocate weight to the rear such as the battery. Once a weight reduction plan is laid out, plan out the suspension mods as these will reduce the unsprung weight of the suspension.

Unsprung weight is any weight in the vehicle that isn't carried by a spring. This means the axle, rear LCA's, panhard bar, front A-Arms, torque arm, driveshaft, brakes, spindles, and wheels & tires are all unsprung weight. Reducing unsprung weight increases the responsiveness of the suspension, or simply put the suspension moves easier & faster when it weighs less. The chassis, K-Member, engine, transmission, body, and anything else are part of the sprung weight. Sprung weight, not surprisingly, is weight that is carried by the springs. Once suspension mods are hammered out, we need to look at the affect of sprung weight reduction and unsprung weight reduction on the center of gravity on the vehicle.

The center of gravity is the point where the mean location of mass can be found. On thirdgen's, that point is inside the vehicle cabin. My best guess as to its general location would be the area of the center console and maybe a couple inches above or below it. In any case, it needs to be checked to make sure that were maintaining a desirable CG in the vehicle. Cut to much weight off the top of the vehicle, and the CG will sink too low in the vehicle. Cut too much weight off the bottom of the vehicle, and it will actually rise up in the vehicle. Raising the CG of the vehicle is never desirable, whereas (while being theoretically possible) having the CG a little too low wouldn't be a deal breaker. Point is, if weight is cut from the bottom of the vehicle make balance it out with a weight cut on top of the vehicle. One thing to keep in mind is that the relationship isn't linear. Weight cuts off the bottom of the vehicle need to be offset by a 2 to 4 time weight cut off the top as far as I've seen.

More to come