What Everyone is Missing About Suspension

Summary

The video explains how suspension geometry affects tire grip and vehicle balance. Its central point is that suspension design involves trade-offs, not a search for one ideal set of numbers.

Tire Load and Load Transfer

A tire’s coefficient of friction generally decreases as its vertical load increases. When load shifts between tires, the vehicle’s total available grip tends to decrease. Braking transfers load toward the front axle, while cornering transfers it to the outside tires.

The amount of load transfer depends on factors including center-of-gravity height, track width, wheelbase, and acceleration. A wider track generally reduces lateral load transfer, while a longer wheelbase generally reduces longitudinal transfer. Wheelbase also affects turn-in and stability under braking.

Suspension geometry cannot eliminate the total load transfer for a given vehicle and maneuver. It can, however, affect how quickly the transfer occurs and how it is distributed between the front and rear axles.

Roll Centers and Load-Transfer Timing

The creator distinguishes between two types of load transfer:

• Geometric transfer acts through the suspension links. It happens quickly, and higher roll centers generally increase its share.
• Elastic transfer acts through the springs and anti-roll bars. It develops more gradually and can be influenced by damping. Lower roll centers, stiffer springs, and stiffer anti-roll bars generally increase its share.

Higher roll centers can reduce body roll without requiring very stiff springs, but they increase jacking forces and may make load transfer abrupt. This can reduce grip or make the car more sensitive to driver inputs. Lower roll centers reduce jacking, but may require stiffer springs or anti-roll bars, potentially compromising suspension independence over bumps. Body roll can also affect tire alignment.

Handling Balance

Increasing roll stiffness at one end of the car tends to make that axle take a larger share of lateral load transfer, generally reducing its grip relative to the other axle. This can be used to tune understeer and oversteer.

The video also distinguishes between initial turn-in and mid-corner behavior. Fast geometric transfer can influence the car early in a turn, while slower elastic transfer becomes more influential as the body rolls. The right setup depends on the car, tires, track, and driver.

Jacking-Force Example

The creator estimates jacking force using the angle from the tire contact patch to the roll center, suggesting that roughly 15% is a practical upper limit—not a universal rule. In the video’s example, a 60-inch track and a 9-inch roll center would imply about 30% jacking force, which the creator considers excessive.

The Creator’s Project

For a motorcycle-powered car with a 60-inch track, an estimated 15-inch center-of-gravity height, and a 42% front / 58% rear weight distribution, the creator’s current design uses:

• Front roll center: 1.8 inches
• Rear roll center: 3.1 inches
• Estimated front jacking force: about 6%
• Estimated rear jacking force: about 10.3%
• Estimated geometric load-transfer share: about 12% front and 20.7% rear

These figures are provisional choices for the creator’s project, not general recommendations.

Design Tools

The creator says CAD makes it easier to model suspension geometry and adjust pivot points. They designed the suspension in Onshape, noting that its free version is usable and that the video description offers a six-month Onshape Professional trial. They disclose that they receive no commission, though the mention could help with future sponsorship. Optimum G is also mentioned as a specialized alternative that was outside their budget.

Tutorial Topics

The guide covers:

1. How tire grip changes with vertical load.
2. How wheelbase, track width, center-of-gravity location, and acceleration affect load transfer.
3. How roll centers, springs, and anti-roll bars influence the timing and distribution of load transfer.
4. How jacking force constrains roll-center height.
5. How front-to-rear suspension stiffness can be used to tune handling balance.

Source: The solo creator of Crusty Creations discusses research and design choices for their own motorcycle-based car project. Onshape and Optimum G are mentioned as CAD tools; the video does not present a product review or independent test.