A racecar has multiple, complex systems that all must work together to achieve a common goal: get down the track as quickly as possible. Things like the engine, transmission, and electronics are important, but they’re nothing without the chassis. If your cars’ chassis isn’t doing its job, everything else will fail behind it.
A very important area that many racers overlook with their chassis is ride height. The ride height of a racecar is a key brick in the foundation that defines the car and sets the table for success at the track. If the car isn’t using the correct ride height for its purpose, the chassis will never work correctly, be a source of tuning headaches, and cost the driver wins at the track. Plus, it might look a little funny, and as we can all attest, looks matter.
The Basics Of Chassis Ride Height
Having the accurate chassis ride height on a racecar for its intended use is critical, as it ensures the car will work correctly and respond to changes based on track conditions. The chassis ride height is something that’s set at conception when the car is being constructed, and rarely can ever be changed without cutting the car apart. Having a plan of what you want to do with the car up front is paramount in making sure it will work for your intended use, and keep the “new car blues” time period much shorter when you’re at the track.
Generally speaking, chassis ride height is the height the car as it sits after you have the suspension at its best possible location. There are many different variables that will factor into this, including whether or not the car is a tube chassis build, a back-half conversion, the type of suspension the car has, and the driveline. The factor that trumps all of those, however, is the wheel and tire choice, which will dictate exactly where your chassis ride height will or should be.
When the wheels and tires have been chosen and diameter established, the radius of that tire diameter can be used to find the distance needed from the ground for chassis ride height. For the front, you would measure from the center of the mounting point of the tire, like the spindle, while in the rear you’d use the axle as your center measuring point.
These basic measurements are how you set the baseline for chassis ride height; if you don’t base it on the wheels and tires being used, the ride height equation will never be correct. By establishing the correct ride height, things like driveline angle and height, along with the rear suspension mounting points, can be mapped out.
Shocks, Springs, And Chassis Ride Height
The shocks and springs on a car are important because they will help it maintain the desired ride height of the chassis and make sure everything is working correctly in the suspension for the track conditions. Curt Perry from Chassis Engineering has been in the shock and chassis business since 1995 with different manufacturers, and offers some insight into spring rates for drag racing shocks. “For the rear, you’ll see everything from 65 pound to 200 pound springs on a 12-inch long shock up to a 14-inch long shock. The norm for most cars is from 85 to 130 pound springs for the rear of the car,” Perry explains.
Most people hear a spring rate and aren’t sure exactly what it means for their car, but Perry sums it up nicely, sharing that “the spring rate is the number of pounds it takes to compress the spring one-inch … that is, unless it’s a progressive spring that compresses at different rates. Most springs for drag racing are working at a linear rate.”
Jerry Bickel has decades of experience building race cars and adds to the concept of the correct spring rate for ride height.
“You need a spring rate that will hold the shock at the designed distance apart for the weight of your car. So, the heavier the car, the stiffer the spring you will need to hold that shock in its designed position. Racers really need to look at the spring that will hold the shock in the correct position.
“If you had a car with a 100 pound spring on it, and the shock is in the perfect position, and you switched to a 150 pound spring, you wouldn’t be able to get the car low enough and get the shock where it needs to be to work properly for the designed ride height,” Bickel goes on to explain.
Shock Ride Height 101
The spring rate is part of the dampening equation when it comes to chassis ride height. The shock itself plays a role. If you don’t have the correct shock for your chassis ride height, there will be issues on how the shock operates. As Jerry Bickel shares: “A shock has a full open dimension and a full closed dimension. You can’t run it at ride height at full open or full closed — you want it in the middle of its travel, because you can’t have the shock bottoming out or reaching full extension during a run. Let’s say you have a shock that has measurements of 19-inches open and 12-inches closed — you would want that shock somewhere around 15.5 to 16-inches at ride height and then it would be in the middle of its travel.”
“When it comes to how shocks and springs play into this, there are three things you need to consider. First, the ride height you want your car and body to sit at; second, the ride height the shock is supposed to be sitting it; and third, the ride height the spring is supposed to be working in.
“You can have an over-sprung or under-sprung car and it looks exactly how you want the body ride height wise, but the car just won’t work,” Perry continues. “But when you have the proper shock and spring ride height, the suspension will work correctly. It’s very important to have the shock and spring ride height correct in relation to the desired overall ride height of the car.”
How The Rear Suspension Ties In
The rear suspension of a racecar plays an important role in how the car is setup and in its final chassis ride height. Most modern high-horsepower bracket, Top Sportsman, Pro Stock, and Pro Modified cars use a 4-link-type suspension. These caliber of cars strictly use 4-links because it provides them with numerous instant center (the invisible intersect point of your upper and lower 4-link bars) possibilities, which is key when you’re trying to plant a lot of horsepower. The 4-link also provides these cars with the ability to control the amount of rotation the rear end housing has inside the car, and adjust the amount of traction based on the track conditions.
When it comes to the chassis ride height, that part of the calculation is already baked into the car, and the racer should not look to the 4-link as a way to adjust this. Bickel explains how the way the 4-link plays into how you adjust the car.
“The ride height is meant to be in one spot — you should look to move weight, adjust the shocks, or move the 4-link but not adjust the ride height too much. The 4-link itself, when it comes to adjustments for traction, don’t effect the ride height at all. Sometimes you have to adjust the ride height of the car to get the intersect points where you want them. Depending on what kinds of brackets you have, the intersect point of the 4-link does not change the ride height,” he says.
Selecting the ride height for the rear still goes back to the wheel and tire choice and how it ties in to where you mount the 4-link. Perry goes into some detail on how the choice of ride height is made for the rear.
“The ride height is dependent on the wheel and tire diameter you’re using, and all of that goes into consideration when you’re building a chassis, where your 4-link brackets are going, and so on. You have the ride height of the car set before you start building a chassis car, or doing a back-half job. The 4-link is a fixed factor with the chassis and has the ride height locked in … you move the 4-link around to adjust what you want to do traction-wise, whether its for wheel speed or crushing the track with the tire,” he explains.
Why Chassis Ride Height Is Important
As the concept of a car moves along, it becomes important to understand how ride height will play into the final product.
“The ride height all really starts at the back of the car when a customer contacts us for work,” Perry says. “For example, if a customer comes in with a car looking for a back-half, and they like the current height you lock it in. Everything you do after that needs to make sure and match that from the shocks to the position of the rearend and new frame rails.”
There are other factors to consider when you’re building a car from scratch in regards ride height. You must pay close attention to where you’re positioning all of the parts in the car, besides the driver. “When you build a chassis, you build it with the driveline alignment so the engine and transmission is aimed at the pinion. If you don’t have the chassis at the correct ride height, you won’t have the correct driveline alignment,” Bickel says.
So you must have everything within spec of the designed ride height for the car or there will be problems getting the chassis to work correctly – Jerry Bickel
Being able to get the ride height correct requires the chassis builder to be very precise in what they do. Perry goes over the process at Chassis Engineering that they use to make sure everything is perfect, and the ride height is correct.
“When we’re building a new car, whether it’s a back half or a full tube chassis car, we’re trying to get the chassis to sit where it really needs to be. Usually, there’s a crankshaft centerline that most people build their cars around based on how high they want it off the ground. When we get a car in the shop everything is being jigged up to make sure it’s all in the correct position before we start to weld anything.
“We have a jig that sets the rear end in place based on the customer’s wheel and tire package, and then another jig goes in that sets the proper ride height of the shocks where they need to set up. Then, the framerails and everything else are built around that. So, the proper ride height is built into the car in these first steps based on what the car will be used for,” Perry outlines.
Not only do you have the chassis ride height for the build to consider, you have to make sure the body ride height will work with the chassis ride height that’s been established.
“There are limits as to what you can have your body ride height off the ground, with aerodynamics playing an important part, along with the material of the body,” Perry explains. “You will have the chassis ride height, and then you place the body on that and have that body ride height based on what the manufacturer dictates, what the rules state for a class, and you have to factor in what the customer wants.
“We set the bodies on the jig table at the heights that everything is supposed to be set, and the chassis is then set in at the ride height its supposed to be set at. From there, everything is made to match based on the final height needed,” Perry goes on to explain.
Chassis Ride Height At The Track
What really drives the point home about the importance of chassis ride height, is what the car will do at the track when power is applied.
“The chassis ride height is going to affect how the car reacts, handles, and leaves the line. It’s very important with weight transfer, and comes back to the crankshaft centerline; essentially, the higher the engine is in the chassis, the easier it is to transfer the weight to the rear. But, the bigger the horsepower you make, the more you can drop that motor down into the chassis.
The chassis ride height is going to affect how the car reacts, handles, and leaves the line.- Curt Perry
With the chassis ride height being set at the start, there are ways racers can work with it, but they have to have the correct height to start with. After the ride height is set, you can then use weight, shocks, springs, and suspension to use it correctly to get the car to do what you need it to do. You’re not changing the initial built-in ride height, but rather, just working with what was built to get your desired outcome.
Chassis ride height plays a critical role in the setup and how the car will react when at the track. The ride height is built in from the start and will not change unless you cut the car apart. So determining the correct or proper ride height based on what the car is being used for is key when a chassis car is being designed, or a back-half project is under consideration.
No matter what type of racing you do, making sure you have a chassis that works properly is imperative to getting the quickest elapsed times and at the track. Ride height plays a vital role in the grand scheme of the car and how it will behave in all phases of the race.
Whether you’re looking to build a brand new race car or just update the one you have, the experts agree that chassis ride height must be taken into careful consideration and matched to your goals.