Suspension: Coilly Thingies & Math
This month we have suspension. I'll be bringing in our Suspension Team Leader and Lead Engineer Steven Stanfield to help with the explanation on this one, as it involves a lot of math and complicated events that I know very little about. Before we get to all of the juicy number crunching, however, here's the basic rundown of how the suspension system as a whole works.
How it works
The suspension on any car - whether it's your daily driver or an F1 car - helps absorb the bumps the car encounters when it's on the road. Most systems are composed of springs, shocks, and A-arms (or some sort of mount that allows the wheels to move up and down). For racing purposes, the tires can also be put in with the suspension system and I'll explain why later. Each component has its purpose and work together to give the best ride possible.
On street cars, like your Civics and Corollas and whathaveyou, the suspension is designed to give the passengers and driver a smooth ride; the bumps are barely felt and the car feels very loose when taking a fast corner. These softer setups allow the car to roll more, which is why they crash 99% of the time when ricers (not to be confused with racers) try to drift them.
Your bigger SUVs and vehicles that will be carrying heavier loads tend to have a stiffer and raised rear with a relatively soft front setup. This allows the truck to sit level to the ground when towing a boat, lawn mowers, or a gaggle of rednecks in the parade. When these vehicles are under load, they are relatively stable. You wouldn't want to go to Hockenheim, but your long haul across the state shouldn't be nerve-racking. However, when they are unloaded, oversteer is a huge problem, especially in pickups. Pickups have very little weight over the rear wheels without a load in the bed or towing something. You can see on the news during the winter months video clips of pickups sliding around in the snow. This mainly comes from poor weight distribution and can be countered by filling the bed with snow, but the stiff suspension can still cause problems.
Racecars run very stiff setups compared to your luxurious Rolls Royce Phantom. Tune in to any WEC event like the 12 Hours of Sebring and watch the GT and LMP cars go around the corners. The body hardly rolls. A stiffer suspension leads to a more predictable car and more maneuverable. I'll explain why in the next few sections. While these stiff setups are great for on the track, that is about all they're good for: racing on a groomed surface (a well-maintained paved track) with little bumps, no potholes, and no old ladies complaining about their backs as they attempt to get out of the car.
Series of Events
The car is in a state of rest, whether at speed or stopped.
A change is made to the system, whether it be the application of the brakes, throttle, or steering wheel.
Weight is transferred in the opposite direction of the force being applied, following Newton’s 2nd law of motion. So if you turn to the left, the weight of the car is going to the right.
The shocks and spring on the weighted side of the car begin to compress while the shocks and springs on the opposite side expand. When turning left, the right side of the car will compress while the left expands.
Once the weight is finished moving around the car, the car returns to a steady state, which also returns it to a state of stability (more on that later).
Components & Their Purposes
The Shocks
The shocks are not the springs. Those are the coilly bits that you take out of pens and flick at people. Shocks and springs do work together, though. The shock controls how fast the spring moves. These are called bump and rebound rates, and can even be adjusted farther in high-speed and low-speed ranges for both the bump and rebound. The bump stiffness is how fast the shock allows the spring to compress, usually caused by a bump in the road (hence, bump stiffness). Rebound is the same, but opposite: how fast can the spring expand. When adjusted properly, the shocks can make the car feel amazing and locked in.
The Springs
Springs are the coil things that rest on the outside of the shock. These work in tandem with the shocks to control the bounce and roll of the car. When the car is stationary or not moving vertically, the springs are what the car actually sits on; without the springs and just the shocks present, the car would bottom out the shocks. The springs also affect the transitional behavior of the car. The stiffer the spring, the sooner the weight transfer stops. However, if springs that are too stiff are used, the car can be skittish on bumpier tracks.
Tires
While it wouldn’t appear at first that these would be a part of the suspension system, they actually contribute a lot to how the car behaves over bumps. Air is a compressible fluid, so when tires are filled with it, thrown on a car, and run over a bump, the tire compresses and absorbs some of the energy caused by the sudden rise of the suspension assembly. With thinner tires, the ride is rougher, but there is less error when tuning the suspension. The tire itself has its own spring rate that can be controlled to a degree, but it’s not dialed in because the spring rate will change as the tire warms and cools. That’s why the LMP prototypes and your wannabe track cars run low-profile tires. Jeeps, trucks, and more daily driver-type cars run larger tires because it’s more comfortable for the driver and passengers.
Uprights
Uprights hold the hubs the hold the tires. The design of these is purely structural; there are no spring rates here or adjustability of the part itself. Make sure they’re strong enough for the event you’re doing and you’re good to go.
A-Arms
A-arms hold the uprights to the car. They also allow the wheel to move up and down in relation to the car. The design of the a-arms and location of the mounts on both the car and upright side determines a bunch of other things that will be covered in a later post. While we use them on our racecar, your typical street car wouldn’t have these. The tire can be mounted several different ways besides a hub.
Pushrods
Pushrods connect the a-arms to the bell cranks or go directly to the shocks. These transfer the energy of the a-arm’s movement to the shock. Pretty simple stuff.
Bell Crank
The bell cranks redirect the energy directed into it by the pushrod and sends it to the shock. As you can see in this picture here, the pushrod goes up and into the car. It attaches to the bell crank, which pivots around a bolt, and redirects the energy to the shock which lies in-line with the shock here. The shape of the bell crank determines the motion ratio – how far the shock moves for every inch the tire moves vertically. Make sure these are strong and you stay on the track, or stuff like this happens.
Whew… That was a lot. So we’ve covered what occurs when the suspension moves, what the system is made of, and the overall theory. In the next post about suspension, and as part of my tuning series, I’ll go over what to change when certain things occur.
- K
