This is the first of a three part series from Chris Alston’s Chassisworks on tuning and calibrating for better drag race performance.
We all want to get the best performance out of our drag vehicle, and a properly tuned drag race suspension enables the vehicle to launch straight while transferring weight to the rear tires in an efficient, controlled manner. The launch is critical, and it makes sense to create an optimal environment for a solid start. So how can suspension tuning do that? The rest of this article will explain how suspension tuning works and how you can apply it to your own vehicle.
First, required settings for drag racing applications vary greatly depending upon vehicle weight, weight distribution, suspension geometry and travel, horsepower, and available traction. Figure out the results you want to achieve on the vehicle you have.
A few notes as we proceed: it is generally better to tune suspension according to improvements in ET’s (Elapsed Times) rather than for specific occurrences such as the amount of wheel stand. Due to differences in weight distribution, wheel base, tire size, and horsepower, not all vehicles leave the starting line in the same manner once their suspension has been optimized. Watch your ET’s and if your times start to get slower return to the prior adjustment. Once you have completed the following procedures, only fine adjustments may be needed to tune for specific track conditions.
What Happens During Launch?
You give it some gas, the front of the car comes up as weight is transferred to the rear tires to aid traction. That’s tied to suspension. The speed at which the front end rises is largely controlled by the spring rate and front shock force. As the rebound valving of the shocks is softened it will be easier for the front end to lift. If the car has a softer front spring, the front suspension also will lift more easily.
Why? Because a heavier rate front spring will take more force to lift the front end a fixed vertical distance than a lighter spring will. If you have 500-lb/in front springs and the acceleration force transfers 1000 pounds of front end weight to the rear (500 from each front spring) the front end lifts one inch. With 250-lb/in front springs, the same 1000 pound weight transfer will lift the front end a total of two inches. The lighter 250-lb/in rate benefits a drag car in two ways. The front end will move faster and farther because less force is required to initially extend the spring. And, it will rise higher, transferring more weight as the center of gravity rises, further assisting traction.
However, too much weight transfer can hurt your ET by causing excessive wheel stands and lost forward motion.
Tuning Front Suspension with Spring Rate
A drag race car should run the lightest front spring rate possible, without letting the shocks bottom out when making a pass. As a general guideline, lighter springs allow the car to easily transfer weight, and settle faster down track.
Changing spring rate affects ride height and the rate at which weight is transferred to the rear tires. A softer rate makes the front easier to rise during acceleration. A stiffer rate makes the front harder to rise during acceleration. If you are having trouble getting the front end to rise, you can soften shock rebound valving or change to a softer spring.
When using lighter rate springs, preload must be added by screwing the lower spring seat upward. Compressing the spring to achieve proper ride height will store energy in it. This is the very simple theory behind stored energy front drag-race springs. If you preload a spring, it is imperative that you verify that the spring has enough travel to not coil bind before the shock bottoms out. VariSprings features a very high strength steel that allows the spring to be wound more coarse then traditional springs, which makes the spring travel farther before coil bind.
In general terms, the worse a car hooks, the more shock extension travel it will need. If you need more extension travel, preload can be removed to lower ride height. This will cause the car to have less ground clearance and reduce the amount of compression travel. If you are going to operate the shock at a ride height shorter than recommended, the upper chassis mounts must be relocated to correct any major vehicle ride height issues. It may take some work with spring rates and upper mount relocation to get the correct combination of vehicle ride height and front suspension travel for your application.
Testing Your Suspension Tuning
Prior to test, make certain that wheelie bars are raised as high as possible while maintaining control and eliminating their influence as much as possible on suspension settings.
- Verify that the vehicle tracks straight before aggressively launching from the line. Begin with light acceleration and low speeds. If the vehicle tracks and drives acceptably at this level, make incremental increases in acceleration and top speed until the vehicle is safe at higher speed. Vehicles not tracking straight at speed should verify all chassis settings including but not limited to alignment, bump steer, tire pressures, etc.
- Once the vehicle drives in a safe manner at speed, move on to test launching. Test launches should consist of only the initial launch with no subsequent gear changes. Begin with low rpm launches and gradually increase rpm and severity if the car launches acceptably.
- The vehicle should leave in a straight line without extreme wheel standing or harsh bounces. Sudden, uncontrollable front end lift should be corrected by making suspension instant center adjustments, if possible. More gradual front end lift can be corrected by adjusting the shock valving, if possible. If the car gradually wheel stands or bounces violently, adjust front suspension first, then rear.
- If there is rear tire shake, wheel hop or excessive body separation, adjust rear suspension first, then front.
- After the car has been adjusted to launch straight, test launch and include the first gear change. Make any required adjustments and add the next gear change. Repeat until the car can be launched straight and driven at speed safely.
The car is now ready for fine tuning to optimize results.
Front Shock Adjustment
Pay close attention to what is happening to the front end during launch. Your goal is to eliminate all jerking or bouncing movements during launch and gear shifts. Ideally the front end should rise in a controlled manner, just enough to keep the rear tires loaded, then continue the pass with smooth transitions at all times. Front end rise without any appreciable traction gain is wasted energy that should be used to propel the vehicle forward instead of up. While testing, document your ET’s along with any changes made. If ET does not improve, return to previous settings. You can use the spreadsheet below to record this.
Front Rebound (Extension) Adjustment Overview
Too light of a spring rate or shock rebound (extension) setting allows excessive front end chassis separation and may result in the front wheels jerking violently off the ground during launch. Also, during gear change, too light a spring rate or shock setting allows the car to bounce off its front rebound travel limiter and then bottom out in an oscillating manner. Too firm a spring rate or shock setting will prevent the front end from rising sufficiently, limiting the amount of weight transferred to the rear tires. Spring rates should only be changed if the shock valving range is not great enough to correct the issue.
While testing, document your ET’s along with any changes made. If ET does not improve, return to previous settings.
|Front Wheels Lose Contact with Ground||Increase Rebound Stiffness||Violent chassis separation and may result in jerking the front wheels off the ground. Increase spring or shock stiffness, then test again.|
|Rear Tires Hook Then Lose Traction||Increase Rebound Stiffness||If weight transfer occurs too quickly the rear tires may hook then lose traction as the front end begins to travel downward. Slowing the rate at which the front end rises prevents the shocks from topping out too quickly and increases the duration of time that the rear tires benefit from the weight transfer. Increase spring or shock stiffness, then test again.|
|No Front End Rise||Decrease Rebound Stiffness||Too firm of a shock setting limits the amount of weight transferred to the rear tires, resulting in poor traction. Decrease spring or shock, then test again.|
Front Bump (Compression) Adjustment Overview
After the launch or during a gear change, a firm spring or shock setting will cause the chassis to bounce off the front tire as the chassis settles down. Too light of a spring or bump setting allows the shock to bottom out and bounce off the stop travel bumper. Spring rates should only be changed if the shock valving range is not great enough to correct the issue.
While testing, document your ET’s along with any changes made. If ET does not improve, return to previous settings.
|Front “Bottoms Out“ After Launch||Increase Stiffness||If front suspension settles too fast after launch or gear change it may cause the front suspension to bottom out at the end of its downward travel. If the suspension bottoms out hard enough, rear traction may be lost. Increase spring or shock stiffness, then test again. If increasing shock stiffness cannot extend weight transfer duration long enough, a higher rate spring should be installed.|
|Hard Front End Bounce (After Launch or Gear Change)||Decrease Bump Stiffness||If the tires cause the front end to bounce upon landing, the shocks are too stiff. The front end should settle in a single, smooth motion. Decrease spring or shock stiffness, then test again. This can be a very subtle problem. Watch the front tire sidewall as it contacts the ground.|
High Rate Rear Springs
It is common in drag race rear springs to use a spring rate higher than the baseline spring rate. There are two primary reasons. One is to overcome some of the additional weight loading due to weight transfer. The second is not very well understood, and has to do with the way the rear suspension affects how the suspension moves.
As the car is initially launched, many suspensions will actually compress at launch for a fraction of a second, slightly reducing traction. Higher rate springs will resist harder against compression and help prevent the loss of traction. VariShock also manufactures a unique line of specially valved shocks that more precisely remedies this situation.
As always, there are practical limits to how stiff of a spring can be used. If the spring gets too stiff, a very dangerous condition develops. The suspension essentially prevents any compression travel, making the tire sidewalls the effective springs, and eliminating much needed damping that helps to stabilize the vehicle. This can potentially be a catastrophic problem if the vehicle gets out of shape at high speed. As weight shifts to the outside of the vehicle, the tire side wall compresses then rapidly unloads throwing the vehicle in the opposite direction. In a properly setup vehicle, shock damping slows the dramatic side-to-side weight shift, allowing the driver more time to react.
Rear Shock Adjustment (Double Adjustable)
The goal is to maintain traction by controlling the rate at which torque and weight is transferred to the rear tires. Ideally the rear suspension should be as firm as possible before a loss of traction occurs. Changes to the vehicle such as ride height, tire size, weight distribution, or suspension link adjustments will alter the instant center location in relation to the vehicle’s center of gravity. Any shift of either the instant center or center of gravity will usually require a shock setting adjustment to optimize traction. While testing, document your ET’s along with any changes made. If ET does not improve, return to previous settings.
|Rear End Squats||Increase Bump Stiffness||Some vehicles will squat during launches instead of pushing the vehicle forward. To assist in planting the tires, increase shock bump stiffness by one, then test again. Spring rates should only be increased if the shock valving range is not great enough to correct the issue.|
|Vehicle Separates from Rear End||Increase Rebound Stiffness||Some suspension geometries plant the tires so forcefully that the rear end of the vehicle rises away from the housing too rapidly. The vehicle may hook initially, then spin the tires once the shocks are topped out. Slowing the rate at which the rear end rises increases the duration of time that the rear tires benefit from the improved traction. Increase shock rebound stiffness by one, then test again. Spring rates should only be increased if the shock valving range is not great enough to correct the issue.|
|Loss of Traction with Minimal Chassis Movement||Decrease Bump/Rebound Stiffness||A suspension system that is too stiff can hit the tires too hard, causing a loss of traction. Softening the suspension slows the transfer of weight and reduces the initial tire shock. Minimal chassis movement makes if very difficult to visually tell if the bump or rebound needs to be decreased. We suggest adjusting bump first and watch for a gain or loss in the ET. If ET does not improve, return to previous setting, then adjust rebound instead and test again. Spring rates should only be decreased if the shock valving range is not great enough to correct the issue./td>|
Completion of Testing
When all adjustments have been completed, reset your wheelie bars as low as possible without affecting your ET.