Establishing the Ride Height
First, establish your ride height. If your car rides too low, it may bottom out. If any part of your car touches the ground, it may cause your car to slow down or cause specific wheels to lose traction and braking, acceleration, and cornering authority because it transfers weight from the tire onto the part of the car which has hit ground. Typically, a splitter strike will provide the car with infinite stiffness up front, causing a severe oversteer (plow) into the wall on an oval track. For this and other reasons, ride height is an important strategic setup problem to solve.
IMPORTANT: When making ride height changes, be sure to detach the front anti-roll bar (“ARB”) and reset the rear truck arm preload to as close to zero as possible. Keep resetting the rear truck arm preload to zero with each change you dial in. When you’re done changing settings, ensure that you re-attach the front ARB and reset the rear truck arm preload one last time after that. See “Setup Procedures” for details.
Part I: Rear Ride Height
Rear ride heights mainly affect corner exit, and is thus substantially more important than the front ride height. Without a good corner run-off, your entire lap time will suffer more than a sketchy corner entry.
Shorter tracks require more traction and straightaway speed is probably not that important. Superspeedways require very little cornering traction, and speed is a make-or-break priority. Because ride height strategically sets the stage for the effects from all setup items which are to follow, establish your general ride height before delving into anything else.
| Priority | Setup Action |
|---|---|
| Cornering Traction | Raise the rear ride height |
| Straightaway Speed | Lower the rear ride height |
Thoroughly understand what ride height means for the performance of your car, as this starting point will dictate all other setup parameters from here on out. Raising or lowering one end of the car or the other effects the angle between the car’s chassis and the flat surface below, is known as the “rake.” Lowering the rear reduces the rake and increases straightaway speed, but at some point, it’ll make the car unmanageable through the corners. Raising the rear increases the rake and makes the car stick to the track through corners. Already, you can see there are trade-offs; and playing with ride heights and testing help explore those trade-offs for the best overall result.
The takeaway is that you must be careful when making ad hoc adjustments to a setup, especially if you only mean to tweak your setup, as even a small change can have dramatic results. Try to maintain the ride heights established by your setup builders unless you truly intend to change your car’s driving characteristics.
Changing the Rear Ride Height
Before you decide to raise or lower the rear, test the car on the track at hand and decide if the car is generally “loose” (oversteers, slides, or tends to spin) or “tight” (understeers, plows, or resists turning). Most fixed setups (and some commercial setups) will be “tight” to make them more manageable by beginners and intermediate drivers. If you plan to reset the “truck arm mounts” to strategically address this tendency, know that this will have a rather substantial effect on ride height. Try not to change this later in the setup process unless it becomes absolutely necessary, as you’ll have to basically start over with your setup testing procedure if you do.
Only once you have determined which truck arm mounts you wish to use for their left and right sides do you then continue to change the rear ride height (using the spring perch offsets) for aerodynamic tuning.
Truck Arm Mounts
Note: The Next Gen (Gen-7) NASCAR Cup Car does not utilize truck arm suspensions.
Truck arms connect the rear axle to the frame and prevent longitudinal movement (from front-to-rear) beneath the body of the car. Each side can be set to “bottom”, “middle”, or “top”. Changing the truck arm mounts has a profound effect on ride height, so test your car to find out where you want your truck arms early in the setup process and leave them there. Be sure to reset your ride heights after changing the truck arm mounts!
Truck arms also affect your wheelbase (the distance between the centers of the front and rear wheels) on each side of the car.
| Truck Arm Mount | Wheelbase | Corner Entry | Corner Exit |
|---|---|---|---|
| Top | Longest | Tighter | Looser |
| Middle | Middle | Average | Average |
| Bottom | Shortest | Looser | Tighter |
As you can see, you can shorten the wheelbase by using the bottom truck arm mounts.
How high or low you mount the truck arms affects the general stability of the car:
- Higher-mounted truck arms will loosen the chassis.
- Lower-mounted truck arms will tighten the chassis.
Because you can stagger the truck arms (mount each end at a different height independently):
- Lower the left truck arm to tighten entry (braking).
- Raise the left truck arm to loosen entry (braking).
- Lower the right truck arm to tighten exit (acceleration).
- Raise the right truck arm to loosen exit (acceleration).
Truck Arm Stagger
How you mount the truck arms changes the wheelbase of the car. Higher-mounted truck arms loosen the chassis and slightly lengthens the wheelbase, while lower-mounted truck arms tighten the chassis and slightly shorten the wheelbase. Thus, if you were to mount the left truck arm at the top while mounting the right truck arm at the bottom, the left side of the car will have a longer wheelbase. This means the axle is pointing slightly left and will give you inward “rear steer.” Conversely, raising the right-side truck arm mount while the left left-side truck arm mount is at the “bottom” will give you outward “rear steer,” which is sometimes used to give the chassis “side force” aerodynamics. Think of a sideways airplane wing, pulling the car to the inside. The more the chassis yaws (with the outward rear steer bringing the rear end up the track), the more side force pulling the car to the inside. This side force helps the car stick through the corners at higher speeds. This added speed and looseness can be difficult to handle for less experienced drivers; but it can be an advantage for others.
In a recent test session, staggering the truck arms (mounted “low” on the left, “high” on the right) gained over 0.2s (two tenths), but added nearly 30-degrees of dynamic tire temperature through the turn entry – a byproduct of the faster speeds, plus the increased slip angles from the chassis yaw (about 7-degrees counterclockwise). Needless to say, this might be handy for a qualifying lap, but wouldn’t be the brightest idea if your intention is to build out a long green flag run race car. The steeper the turn banking, the less the effects. The yaw and temperature penalties from truck arm stagger are greater on flat, chassis-tight turns; but so are the side force benefits. It’s a trade-off.
| Stagger | Rear Steer | Side Force | Entry | Exit |
|---|---|---|---|---|
| Higher left truck arm + lower right truck arm | Inwards (to the left) | reduces | loosens | tightens |
| Lower left truck arm + higher right truck arm | Outwards (to the right) | increases | tightens | loosens |
IMPORTANT: Focus on optimizing your car’s corner exit behavior. This is the most important setup task to nail down (along with ride height), and cannot be compromised by any other factors – especially during this early stage of chassis setup.
Raising the Rear
A higher rear end helps on shorter tracks, and hurts longer (faster) track performance.
- Reduces top speed on long straightaways.
- Makes the rear end more stable in tight turns.
- Decreases rear end traction and bite.
- Trends towards decreasing looseness/increasing tightness.
- Lowers the front (see below).
Lowering the Rear
A lower rear end helps on longer tracks, and hurts shorter track performance.
- Increases top speed on long straightaways.
- Makes the rear end less stable in tight turns.
- Increases rear end traction and bite.
- Trends towards increasing looseness/decreasing tightness.
- Raises the front (see below).
Track Bar
The track bar (sometimes called a “Panhard bar,” after its inventor) prevents the rear axle from shifting laterally (from side to side) beneath the body of the car, since one end of it (on the driver’s side) is bolted to the axle assembly while the other end (on the passenger side) mounts to the frame. The height from the ground of each end can be set, in a range generally from 7.000" to 14.000". This is the final stage of the rear ride height adjustment process as it has a minimal effect on ride height while allowing you to fine-tune the overall balance of the car, as well as the chassis response to braking (corner entry) and acceleration (corner exit).
How high or low you connect the bar affects the stability of the car:
- A higher-mounted track bar raises the roll center and will loosen the chassis.
- A lower-mounted track bar lowers the roll center and will tighten the chassis.
| Adjustment | Entry Effect | Exit Effect | |
|---|---|---|---|
| Raise the left-side track bar | Loosens Entry | Tightens Exit | |
| Lower the left-side track bar | Tightens Entry | Loosens Exit | |
| Raise the right-side track bar | Loosens Exit | Tightens Entry | |
| Lower the right-side track bar | Tightens Exit | Loosens Entry |
As with the truck arms, the effects of adjusting the track bar on one side of the chassis has the opposite effect on the other side; e.g., raising the left track bar loosens entry, but tightens the exit.
IMPORTANT: Track bar adjustments are convenient as a quick means of making a chassis setup adjustment to accommodate differences in track temperature. However, this adjustment will have a stronger effect on the handling of the car at the end of a long green flag run. That means if your chassis gets tight long run, raise the track bar. If the chassis gets loose long run, lower the track bar. It also means if the chassis needs an adjustment “early run,” you must be warned that using the track bar to make that adjustment may have late run consequences on handling as well.
REMEMBER: You’re trying to optimize your car’s corner exit behavior, so focus on that when setting your track bar height.
Part II: Front Ride Height
Front ride height mainly affects straightaway speed and corner entry. High power, low downforce cars aren’t as strongly affected by this; nor are cars set up for low-banked (low speed) turns. Lower powered cars, cars racing on steeply banked (high speed) turns, or cars with high downforce aerodynamics will be more sensitive to a properly configured front ride height setup.
The main idea is to set the front ride height as low as possible without scraping the track with the splitter (see below). Note that lowering the front will raise the back, and vice versa, so be prepared to keep “resetting” or “adjusting” your rear ride heights as you now work with the front. Of chief concern is that any established rake is maintained. If you have not established the fastest rake (i.e., the relationship between the height of the rear end of the car with the front end), you can wait to correct the rear ride heights until rake testing takes place.
The splitter rides parallel to the track just like the wing on an airplane. It is designed to prevent air from going under the vehicle (which creates drag, and slows the car down) and helps create a high pressure air pocket in front of the car. That high air pressure creates down force on the front end of the car, and that increases traction at the front tires for steering and control of the car. Lowering the ride height at the front of the car, therefore, is of critical importance as it also determines how high off the surface of the track the all-important splitter rides.
Start by lowering the front to within about 1/2-inch of the lowest level allowed. Test the car, and see if the splitter is hitting using telemetry (more on that in another article), or by examining your car using the replay view (more on that in another article, too). Recheck the rear ride heights each time an adjustment is made before testing the car. Then, you can raise the front end a small amount (perhaps just 1/10th of an inch at a time) and re-test to see the effect it has on your lap times. You can also try lowering it. If you find that your change made your lap times worse, you’re probably hitting the track with the splitter; raise the front back up. If you find your track times are increasing, keep raising the front and re-testing until it begins to decrease again; this is the point at which your aerodynamics are beginning to suffer and you need to go back to a lower front ride height.
Try to achieve a “flat” ride height through the high-gravity turns. This means the ride heights in the front (left and right sides) are equal; and the ride heights in the rear (left and right sides) are equal. Telemetry is the only means to measure these quantities, so you’ll need to install telemetry software and learn how to use it.