Setting Up Your Mustang's Suspension for Drag Racing

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[edit] Setting your Mustang's Suspension up to Drag Race

To start with we’ll just touch on a few terms, and the basic meaning. There are more detailed articles on these subjects out there, and if you wish to further your knowledge, I’d highly recommend reading more about them.

Center of Gravity (CoG): The center of gravity is the location where if you could balance your car on a pinpoint, the exact pinpoint would be. The location of which for drag racing purposes needs to be estimated vertically and front to rear.

Instant Center (IC): For drag purposes, we only concern ourselves with the rear suspension IC. This is the point at which the upper and lower control arms would criss-cross if an imaginary line was drawn through them.

Percentage of Rise (PR): Found by drawing an imaginary line from the contact patch of the rear tires through the Instant Center mark and intersecting a vertical line through the center of gravity. Lets just say for simplicity, the line you drew through the center of gravity intersects halfway up to the actual center of gravity. This is a 50% percentage of rise. If the line intersected exactly on the center of gravity it would be 100%, ¾ of the way up 75%, etc, etc. PR will determine how hard the car hits the tires. Adjusting PR will adjust how hard the car hits at the track. More later on this.

Neutral line: Used to find Anti-Squat. A line found by drawing a vertical line through the center of the front wheel, and then a horizontal line from the center of gravity intersecting the vertical line on the front wheel, THEN drawing a line from the contact patch of the rear tires through the intersection of the horizontal and vertical line on the front tires. Got all that?

Anti-Squat (AS): The amount of anti-squat is found with the neutral line, by comparing it to the instant center. If the IC is located above the neutral line, there is more than 100% anti-squat, and will raise the rear of the car on launch. A car with the anti-squat line below the neutral line, there is less than 100% anti-squat and the rear of the car will tend to squat when launched.

Terms out of the way, lets talk.

With the design of the Mustang’s rear suspension (short upper arms, longer lowers), the geometry changes somewhat with ride height. The percentage of rise, instant center, and amount of anti-squat will all be changing as you alter ride height. The exact effect will be dependant on what upper and lower control arms you have on the car. Some lower control arms have a lower rear mounting point, altering the geometry, and likewise, some upper control arms have rear mounting points higher than stock.

Instant center is what it’s really all about when determining how the car is going to react at launch since most everything is based off of it, such as percentage of rise. If you’ve got a really aggressive PR, the car is going to hit the tires really hard. If you’ve got a less aggressive PR, the car will hit the tires softer. Automatics need a harder hit since they don’t hit very hard, so they will generally want a PR higher than 50%. Manuals already hit very hard off the line, so they will generally want a PR lower than 50%. Naturally, this will be car/driver dependant, and you’ll need to play with this to find what works best for you.

Instant center location affects the PR, but depending on where the IC is located front to rear will effect how the suspension wants to react to launching. If the IC is located behind the CoG, the car will tend to lift the rear of the car, planting the rear axle down on launch. If the car hits too hard off the line in this situation (remember, this is effected by PR), the car will initially hook really hard, then as the springs compress back in, the axle can unload the tires and spin. If this happens (and it will happen quickly), tightening the rear shocks can help, but the real fix would be to go to a less aggressive PR by moving the IC. Make sense?

Now, if the IC is in front of the CoG, the car will tend to lift the front of the car moreso than the rear. So long as you have more than 100% AS, the rear will lift some, but less aggressively. If the IC is too far forward, the car will spin right off the line, and to correct this you *may* be able to loosen the front struts, but probably not. If the car isn’t planting the rear hard enough to initially hook, it probably won’t have enough traction to lift the front, regardless of front strut settings. The correct way to correct his problem is to relocate the IC further back.

You’ll likely need to read back over this section 10 times before it starts to click with you (or it may never click), but without basic understanding of this, you’ll either get lucky and land a good setup, or you’ll fight with it until you find someone to help you out.

Picture example (gotta love that car):

Now that I’ve loosely covered these topics, lets move on to springs, shocks and struts.

Front springs are simple. They will either help the front rise, or not. Contrary to popular believe, a stiffer front spring that doesn’t help lift the front of the car will actually place more weight on the rear axle, so long as the front of the car is rising. A softer spring will allow a lower hp car to lift the front of the car, but will also transfer less weight while rising. I know this sounds somewhat backwards, but think about lifting something long and straight. Have someone help you lift it from one end and you’ll be supporting less weight on your end. If you lift that same object and have the other person help less, you’ll be supporting more weight on your end. Naturally, in this scenario you’re the rear tires and your helper would be the front springs.

Now, to put this into what to use when…If you’ve got a car that hooks initially (IC is working for you), but tends to spin after rolling out, but the rear axle isn’t causing the spin by unloading, you’re going to need more weight transfer. You can get this in several ways. If you’re already getting transfer, but the front struts are maxing out in extension, then the car spins, you need to try stiffening the front struts to slow down the front end rise. What this will do is keep the front end rising longer, keeping weight on the rear longer as well.

If you’re not getting much of any front end lift, you can try softening the struts, softer front springs, or an IC relocated further forward (so long as it doesn’t interfere with the initial hook, causing you to spin immediately). It’s all about balance.

The only other thing to keep in mind with front springs/struts is that the less front end rise you can get away with and still keep traction, the better your car will launch. Lifting the front tires looks good, but if you can keep traction without doing that, you’ll waste less energy on lifting the front of the car, and more keeping you going down the track. This is why it’s important to get adjustable front struts instead of going with “90/10s” like everybody tends to recommend.

Rear springs aren’t too important, so long as they’re stiff enough to control any squat. Squatting is generally bad in a drag car, unless its needed to absorb the initial hit on a really powerful car.

Rear shocks on the other hand really help control the axle’s reactions, and can either allow the axle to plant harder by decreasing rebound dampening, or to help control wheel hop by tightening dampening. You’ll want to play with shock settings to really get the best launch, so a set of adjustable shocks is really important as well.

That’s all the suspension issues I really want to cover in this, but again, there are much more detailed articles available on these topics, so if you want a further understanding, starting reading.

Ashentep