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Suspension Geometry - Part 2 (Roll Center, Double Wishbone, MacPherson Strut)
XF Motorsports
Overview
This video explains the concepts of roll center and pitch center in vehicle suspension geometry, detailing how they influence a car's handling during cornering and braking. It then contrasts two common front suspension designs: the double wishbone and the MacPherson strut. The explanation covers how the geometry of these suspensions affects factors like body roll, jacking, camber gain, and structural integrity, highlighting their respective advantages and disadvantages for both racing and production vehicles.
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Chapters
- Roll center is an imaginary pivot point around which a car's body rotates during cornering.
- Pitch center is a similar imaginary point around which the car's body rotates forward or backward during braking or acceleration.
- The height of the roll center relative to the center of gravity affects the amount of body roll; a lower roll center generally leads to more roll.
- Moving the roll center closer to the center of gravity reduces body roll, but raising it too high can cause 'jacking,' where the car lifts during cornering, increasing weight transfer to outer tires.
- Pitch center geometry can be adjusted to counteract diving under braking (anti-dive) or squatting under acceleration (anti-squat).
Understanding roll and pitch centers is crucial for tuning a car's handling characteristics, allowing engineers to balance body roll reduction with avoiding undesirable effects like jacking or excessive weight transfer.
If the roll center is set at the road surface, cornering forces create a large torque around this point, causing significant body roll. Moving the roll center higher, closer to the car's center of gravity, reduces this torque and thus reduces body roll.
- In a double wishbone suspension, roll center is estimated by extending lines through the upper and lower control arms until they intersect, then drawing a line from that intersection to the tire's contact patch.
- Lowering a car can significantly lower the roll center, leading to increased body roll, which can be corrected with parts like ball joint spacers or custom fabrication.
- Adjustable suspension mounting points are common in racing to allow precise control over the roll center for different tracks.
- Pitch center is measured similarly but by extending lines through control arms viewed from the side, with its position determined by the fore-aft angle of the control arms.
Knowing how to measure and adjust roll center allows for targeted modifications to improve a car's stability and responsiveness during dynamic driving conditions.
When a car is lowered, the angles of the control arms change, causing the calculated roll center to drop significantly, resulting in excessive body roll. This can be addressed by adjusting the control arm mounting points or using specific components.
- A double wishbone suspension uses two A-shaped control arms (wishbones), an upper and a lower, connecting the chassis to the steering knuckle.
- The lengths and angles of these arms determine key suspension characteristics like camber gain, roll center, and pitch center.
- Shortening the upper control arm relative to the lower one increases 'camber gain,' helping the tire maintain better contact with the road during body roll.
- Excessive camber gain from overly short upper arms can cause significant camber changes during braking or acceleration, reducing grip.
- Structurally, double wishbone arms are often designed as straight, cylindrical members carrying only compression or tension, allowing for lightweight yet strong construction (e.g., carbon fiber in racing).
The double wishbone design offers excellent control over suspension geometry, enabling engineers to optimize tire contact and handling, making it a preferred choice for performance applications.
In racing, upper control arms are often made much shorter than lower ones to maximize camber gain during cornering, ensuring the tire stays flatter against the road surface.
- A MacPherson strut replaces the upper control arm with the strut itself, which is bolted directly to the steering knuckle and the strut tower.
- This design is cost-effective and space-saving, using only a lower control arm and the strut assembly.
- The steering axis is defined by the strut's top mount and the lower ball joint.
- Camber gain is achieved by angling the top of the strut inward, but it's generally less effective and more limited than in a double wishbone system.
- Adjusting caster or camber in a MacPherson strut often affects other geometry points simultaneously, limiting tuning flexibility compared to double wishbone.
The MacPherson strut is a popular choice for production cars due to its simplicity and cost-effectiveness, though it offers less precise geometric control than a double wishbone setup.
In a MacPherson strut, the strut itself acts as the upper locating link. Angling the top of the strut towards the car's center helps achieve some camber gain as the suspension compresses.
- Double wishbone suspensions offer superior control over suspension geometry, allowing for optimized camber gain and independent adjustment of roll and pitch centers.
- MacPherson struts are simpler and cheaper but provide less control over camber gain and geometric adjustments, often leading to compromises in handling.
- The inherent design of MacPherson struts limits their use in high-level racing where precise geometry control is paramount.
- Production cars often use MacPherson struts to save space and cost, while performance or racing vehicles favor double wishbone for its superior handling capabilities.
Understanding the trade-offs between these suspension types helps explain why different vehicles are designed with specific systems based on their intended use and cost considerations.
While a MacPherson strut can be angled to provide caster, this adjustment also affects the steering axis and potentially the pitch center, unlike the more independent adjustments possible with a double wishbone setup.
Key takeaways
- Roll center and pitch center are critical imaginary points that dictate how a vehicle's body behaves during cornering and braking.
- The height of the roll center relative to the center of gravity is a primary factor in controlling body roll, but excessively high roll centers induce undesirable jacking effects.
- Double wishbone suspensions provide greater geometric control, allowing for optimized camber gain and independent adjustment of suspension parameters, making them ideal for performance.
- MacPherson struts are a cost-effective and space-saving alternative, widely used in production cars, but offer less precise geometric control and limited tuning flexibility.
- Adjusting suspension geometry, such as control arm angles and lengths, directly impacts handling characteristics like stability, grip, and responsiveness.
- The design of suspension components, like the relative lengths of upper and lower control arms in a double wishbone system, directly influences camber changes during suspension travel.
- Structural efficiency in racing suspensions, like the straight, load-bearing members of a double wishbone, allows for lightweight yet strong components.
Key terms
Roll CenterPitch CenterCenter of Gravity (CG)Body RollJackingDouble Wishbone SuspensionMacPherson StrutControl ArmsWishbonesCamber GainSteering KnuckleStrut TowerKingpin InclinationCaster AngleBump Steer
Test your understanding
- How does the position of the roll center relative to the center of gravity influence a car's body roll during cornering?
- What is the 'jacking' effect, and why is it considered undesirable in vehicle dynamics?
- Explain the primary geometric differences between a double wishbone and a MacPherson strut suspension.
- How does the relative length of the upper and lower control arms in a double wishbone suspension affect camber gain?
- What are the main advantages and disadvantages of MacPherson strut suspensions compared to double wishbone suspensions, particularly in the context of racing versus production vehicles?