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Drag racing suspension setup basics...well, maybe a little more than basics...

Discussion in 'Racing Forum' started by Ashentep, Apr 21, 2009.

  1. Ashentep

    Ashentep Nitrous Junkie

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    Before beginning, please note that all suspension geometry changes should be made at race weight with the driver in the car, helmet on, nitrous bottles mounted and full, tires aired to race pressure, gas in the tank/cell, etc.

    All measurements for suspension height should be referenced from the ground to the front mounting bolt for the lower control arm.

    Also, everything in this article assumes you have good drag radials or preferably a bias-ply slick.

    Suspension terms

    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.

    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. A car with the anti-squat line below the neutral line, there is less than 100% anti-squat.

    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. This is the reason its SO important to have the ability to alter ride height, especially on a straight gear.

    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 to make the suspension work for you since they don't shock the drivetrain very much (except with a transbrake), so they will generally want a PR higher than 50%. Manuals already shock the drivetrain 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, well under a second, and just a few feet out), tightening the rear shocks can help, but the real fix would be to go to a less aggressive PR by moving the IC (which, remember, is done by altering ride height). Make sense?

    Now, if the IC is in front of the CoG, the car will tend to lift the front of the car more so 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, which is not the correct way to fix it, but *might* get you by in a pinch, but will likely open a new can of worms. 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, giving more grip out of the gate.

    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, fight with it until you blindly pin the tail on the donkey, or find someone to help you out.

    Picture example (gotta love that car): [​IMG]

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    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 belief, 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 one end of a long straight board from the opposite end. Have someone help you lift it from one end and you’ll be supporting less weight on your end (step on a scale to confirm). 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.

    Long, soft springs store more energy than a stiffer, shorter spring, and as such will help lift the front more, but not necessarily give more weight transfer. Also, once the car has reached full droop in the front, the car will either lift the front tires, (which is ok to a certain point, but any more than a few inches off the ground and its a complete waste of energy, hurting 60' times and ET), or begin to settle back down, reducing weight transfer.

    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 about 5-10', 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. A shorter, stiffer spring will also help. Also, increasing droop travel may help 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 do quite a bit. You can decrease rebound dampening to allow the rear to react stronger, which can increase initial bite. Too little rebound dampening can result in a loss of control over the rear suspension, and causing the rear to unload.

    Increasing rebound dampening can control wheel hop problems, bring an out of control axle back in control by softening the hit, and could help get you by with too aggressive of an IC setting without diving back deeper in suspension settings.

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    Pinion angle, centering the rear, and preload.

    Pinion angle is often blamed for bad 60' times, but all correcting the pinion angle will do for you is give the driveshaft a more efficient path to the rear axle.

    Setting pinion angle is simple, but it needs to checked each and every time any other rear suspension setting is changed. Every adjustment you make to the rear suspension (other than shock setting) can alter pinion angle.

    Its worth noting that pinion angle can only be properly adjusted with adjustable upper control arms. To properly adjust pinion angle (remember to be at race weight), disconnect the passenger side upper control arm and be sure the axle does not move side to side. Next, adjust the driver's side upper arm to get the desired angle, and reconnect the passenger side arm so that the bolt slides in easily. This will give you zero preload.

    Finding desired angle:
    With the car on level ground and at race weight, place an angle finder on the engine damper or pulley (water pump pulley also works if its flat and unbent) and take note of the reading. Now disconnect the driveshaft and place the angle finder on the pinion flange.

    For all solid bushings, you want the angles to match.
    For poly bushings, the pinion angle should be 1-3 degrees negative.
    for rubber bushings, the pinion angle should be 2-5 degrees negative.

    Centering the axle should be done both front to rear and side to side. This shouldn't be effected much by any other adjustments, but should be checked from time to time to double check. Center the axle front to back with adjustable lower control arms, if you have them, if not, it is what it is.

    Centering side to side is done (poorly) with adjustable upper control arms or a panhard bar. Move it towards the driver's side by lengthening the passenger side and shortening the driver's. Going the opposite way requires the opposite adjustments. Simple enough, but keep in mind that if you are using the upper control arms the move the axle, and have one arm significantly longer than the other, your car will have preload and tend to dance across the track and not launch straight.

    Preload

    Preload is basically forcing the axle to not point straight in an attempt to make the car go straight. This is a band aid attempt, and IMO, should be corrected in other manners. Preload causes bind and as such, doesn't allow the rear to react "correctly". Avoid it if at all possible. I already covered achieving zero preload under pinion angle.

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    All this sounds more complicated than it really is, and once you get the car to pull good 60' times at one track in certain conditions/weather, it rarely takes much adjustment to hook at other tracks and in other conditions/weather.

    For example, I rarely alter my ride height more than 1/2" and keep my rear shocks between 5-8 (of 1-10).

    Also, for what its worth my full suspension setup is:

    Front:
    -----
    struts: stock
    springs: '99 or '00 v6 stock springs with no isolators
    K-member, a-arms, etc: stock
    No swaybar

    Rear:
    -----
    shocks: Strange 10 way adjustables
    springs: '99 or '00 v6 stock with no isolators
    LCAs: Pro3i with solid bushing on one end, poly at the other, and adjustable spring perches for ride height changes
    UCAs: Baseline Suspensions ProLaunch kit (which alters the axle mounting point, correcting stock geometry)
    Panhard bar for side to side adjustments and stability on the top end
    No swaybar

    My car has pulled a best of 1.42 60' on 26x8.5" ET Drags, yet never pulls the driver's side tire more than 1-2", and hasn't ever pulled the passenger side tire.
     
  2. tinaflux

    tinaflux New Member

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    Nice info, very informative.