If you’ve been reading or listening to any discussions about the Ford Twin Traction Beam (TTB) suspension, you may have heard that you have to factor in 1.5 for the leverage caused by the axle beam when calculating or considering a spring rate. What is this Leverage Factor?
NOTE: You may also be interested in How To Calculate Motion Ratio & Wheel Rate For A Ford TTB Suspension
Simplified Explanation:
With the Ford TTB suspension, Leverage Factor refers to how much a given force (such as the vertical load from the vehicle’s weight) is amplified or reduced as it is transferred through the suspension components. It’s a function of the relative lengths and pivot points of the components involved — in this case, the axle beam, the radius arm, and the pivoting axis.
Components:
- Axle Beam: This is the independent beam that connects the wheel(s) to the rest of the suspension and pivots at one end (usually near the frame). It moves up and down in response to the terrain or road surface.
- Radius Arm: The radius arm connects the axle beam to the vehicle’s chassis. It usually pivots at one end (the chassis side) and helps locate the axle beam during movement.
- Leverage in Suspension: In this system, the leverage factor is influenced by the lengths of the axle beam and radius arm, as well as where the forces act along the length of these parts. When a vertical load (like the weight of the vehicle) acts on the axle, the system “amplifies” this force depending on the relative lengths and pivot locations.
- Longer Lever Arm = More Leverage: If the radius arm or the axle beam is long, it provides greater leverage, meaning a smaller input force at the axle beam could result in a larger force applied at the pivot point or vice versa.
- Geometry and Force Distribution: The angles and distances between the pivots and the attachment points will determine how much force is transferred through the system. For example, if the axle beam is angled relative to the radius arm, the leverage factor increases at certain points of suspension travel, altering the force seen by different parts of the system (such as the spring or shock absorbers).
Leverage Factor in Action:
- When the suspension is compressed, the vertical load on the axle is transferred through the beam and radius arm.
- The length of the radius arm and the angle at which it pivots influence the amount of force that is applied to the axle and to the rest of the suspension components (like the springs and shocks).
- A longer radius arm or a greater angle between the components can result in a higher leverage factor, amplifying the force that needs to be handled by the suspension.
Example (in Simple Terms):
Imagine you have a seesaw. If you apply a force at the end of the seesaw, the force at the pivot (center) will be much smaller for the same input. However, if you apply the same force closer to the pivot, the force at the pivot will be higher.
Similarly, in the TTB suspension, the leverage factor works by taking the force applied at the axle and amplifying it based on the geometry of the system — how long the radius arm and axle beam are, and how they interact at the pivots. The result is that certain components (such as the springs or shocks) may experience a higher or lower force than what is directly applied at the axle.
Calculating Leverage:
In the diagram above you will see that there is a Pivoting Axis. The Pivoting Axis is a straight line drawn through the axle beam mounting (pivoting) point at the frame (crossmember) and where the radius arm mounts to the frame. You will need to take two measurements:
- The distance (inches) from the center of the wheel to the Pivoting Axis (D1).
- The distance (inches) from the center of the coil spring mount to the Pivoting Axis (D2).
To determine the Leverage Ratio, you will divide the distance from the center of the wheel to the Pivot Spring (D1) by the distance from the center of the coil spring mount to the Pivoting Axis (D2).
Example:
As an example, we’ll say:
- D1 = 45-inches
- D2 = 30-inches
- LF = Leverage Factor
Leverage Factor = D1 ÷ D2
45 ÷ 30 = 1.50 (Leverage Factor)
In this example, the suspension has a leverage factor of 1.50:1 meaning that the suspension is applying 1.5 times the force (weight of the vehicle) onto the suspension.
Measuring The Weight:
The weight used in your suspension calculation is going to be the sprung weight of the vehicle multiplied by the Leverage Factor. The sprung weight is the weight that sits above the springs, or more simply, the weight of the front of the vehicle minus the weight of the axle, tires, and wheels.
Example:
- The front of your truck weights 2400 lbs.
- The axles, wheels, and tires weigh 300 lbs (unsprung weight).
Sprung Weight = Weight – Unsprung Weight
2400 lbs – 300 lbs = 2100 lbs
The sprung weight in this example would be 2100 lbs (2400 lbs – 300 lbs). Now we need to multiply it by the Leverage Factor and divide it in half to get the amount of force (weight) being applied to each of the front springs.
2100 lbs x 1.5 (Leverage Factor) ÷ 2 (each front spring) = 1575 lbs applied to each spring
2100 lbs x 1.5 (Leverage Factor) = 3150 lbs.
3150 lbs ÷ 2 = 1575 lbs of force (weight) at each of the front springs.
Applying This to Calculate Compressed Spring Height:
Spring Rate is a measurement of how far a spring compresses pre unit of force. A coil spring with a 430 lb/in rating will compress 1-inch for every 430 lbs applied to it. If we take the amount of force (1575 lbs) and divide it by the spring rate (430 lb/in), we’ll get a measurement of how far the spring will compress.
Example:
- Force (weight) on spring = 1575 lbs
- Spring Rate = 430 lb/in
- Spring Free Length = 20-inches
Compressed Spring Height = Force (Weight) ÷ Spring Rate
1575 lbs ÷ 430 lb/in = 3.66 inches
If you were to forget to multiply in the Leverage Factor of 1.5 and simply use the weight on the wheel (2100 ÷ 2 = 1050), you would get a much different result:
1050 ÷ 430 = 2.44 inches
3.66 – 2.44 = 1.22 inches
As you can see, that made a pretty significant difference. So, you can see how important the Leverage Factor is.
If this spring had a free height (no weight on it) of 20-inches, then it’s compressed height would be 16.34 inches. Having this information allows you to calculate what you’re ride height is going to be. The compressed height of a 1983-1997 Ford Ranger coil spring is 10.50 inches. 16.34 – 10.50 = 5.84 inches. So, in this example, the coil spring would provide 5.84 inches of lift.
Why The Leverage Factor Matters:
If you’re trying to calculate how much a coil spring will compress, you’ll need to know the spring rate of the coil and the weight (force) being applied to it. In order to calculate the correct amount of weight, you’ll need to multiply the Leverage Factor into it. If you didn’t know how to calculate the Leverage Factor, you do now.
Conclusion:
In your TTB suspension, the Leverage Factor helps explain how forces are transferred and magnified between the axle and the vehicle’s frame. By understanding the lengths of the axle beam and radius arm, as well as their pivot points, you can predict how different parts of the suspension will respond to various forces, like bumps or the vehicle’s weight.