Motion Ratio → Link Load Calculator

Convert a wheel-vertical load into the force actually carried by a suspension pickup point (spring/damper, pushrod, or link) using its local motion ratio. Feed the result into the Static Safety Factor, Tube Buckling, or Bolt Shear calculators as the operating load.

SUSPENSION KINEMATICS · VIRTUAL WORK PRINCIPLE

Motion Ratio → Link Load Estimator

Converts a wheel-vertical load into the force actually carried by a suspension pickup point (spring/damper, pushrod, or link) using its local motion ratio. Feed the result into the Static Safety Factor Calculator (Module 2) as the operating load.

Wheel Vertical Load
[lbf]

Estimated dynamic load at the tire contact patch. For a quick estimate, static corner weight × a dynamic multiplier (see Kd values in Module 2/5/6) is a common starting point.

Motion Ratio at This Pickup Point
Link Travel ÷ Wheel Travel

MR = (pickup point displacement) / (wheel vertical displacement), measured or read off a kinematic model at your target ride height. Typical double-A-arm spring MRs run roughly 0.5–0.9; pushrod/bellcrank systems vary much more widely.

Estimated Force at This Pickup Point

3,077 lbf

✔
Lower motion ratios multiply force — a link near the pivot sees higher load than the wheel itself.

Engineering Model (not an ISO/SAE formula)

Standard Reference
Motion-ratio force multiplication follows the classical virtual-work (energy conservation) principle for a rigid linkage, documented in vehicle-dynamics references such as Milliken & Milliken’s Race Car Vehicle Dynamics and Carroll Smith’s Tune to Win — not an ISO/SAE-issued formula.

Calculation Method
Wheel Force × Wheel Travel = Link Force × Link Travel ⟹ Link Force = Wheel Force / MR

Next Step
This is a static force multiplication only. For impact/dynamic scenarios, take the resulting Link Force and use it as the operating load (P) in the Static Safety Factor Calculator along with an appropriate Kd.

Assumptions & Limitations

Motion ratio is treated as constant, but in most real suspension geometries it actually changes through the travel range — this is a linearized estimate at one ride-height point.
Assumes a quasi-static, small-angle approximation with no friction losses in the linkage.
Does not itself include a dynamic shock multiplier — apply that separately when carrying the result into Module 2, 5, or 6.

STATIC REFERENCE DATA

Reference: Link Force by Wheel Load & Motion Ratio

Precomputed values from the exact formula used above — useful as a quick lookup without re-entering inputs.

Wheel Force	MR=0.40	MR=0.55	MR=0.65 (default)	MR=0.80	MR=1.00
1,500 lbf	3,750 lbf	2,727 lbf	2,308 lbf	1,875 lbf	1,500 lbf
2,000 lbf (default)	5,000 lbf	3,636 lbf	3,077 lbf	2,500 lbf	2,000 lbf
3,000 lbf	7,500 lbf	5,455 lbf	4,615 lbf	3,750 lbf	3,000 lbf

WORKED EXAMPLE

From Wheel Load to a Link Design Chain

A short-course truck’s rear corner sees an estimated 2,000 lbf dynamic wheel load. The lower 4-link bar’s motion ratio at this ride height is 0.65 — what does the bar itself need to carry, and how does that flow into the rest of the design?

Link Force = Wheel Force / MR = 2,000 / 0.65 ≈ 3,077 lbf.
Feed 3,077 lbf as the operating load (P) into the Tube & Bung Buckling Calculator to size the bar’s tube diameter/wall.
Feed the same 3,077 lbf into the Bolt Double-Shear Calculator to check the clevis bolt at each end.
If the link uses a rod end rather than a solid bushing, also run it through the Static Safety Factor Calculator against the rod end’s own rated load.
This is exactly the intended workflow across this tool suite: one motion-ratio load estimate, checked against three different failure modes (buckling, bolt shear, rod end rating) before committing to a design.

Related reading: Sway Bar Link Resource Hub →