Honda developed the world’s first Variable Gear Ratio Steering (VGS) system, which is equipped on the S2000 type V that was launched on July 14, 2000. The genuine open FR S2000 sports car was first launched on April 15, 1999, making the S2000 type V a new variant.
S2000 type V
The VGS system varies the steering gear ratio continuously according to vehicle speed and steering angle. At low speeds, the front end of the vehicle turns sharply in precise response on winding roads as a less amount of steering input is needed. The amount of steering angle is reduced such as when parking at low speeds, making parking in a garage easier compared to conventional steering systems. When lightly changing the steering wheel while changing lanes or turning at high speeds on expressways, VGS’ steering gear ratio is equivalent to conventional systems, providing the same smooth and reliable driving feel.
VGS characteristics
Taking rack-and-pinion steering as an example, the gear ratio of conventional steering systems is determined by the number of teeth on the pinion gear compared to the rack gear, and is generally constant regardless of vehicle speed. If the steering gear ratio is optimized for high-speed cruising, the steering angle input when entering a garage, for example, is higher, and the driver's burden increases. Conversely, if the steering gear ratio is optimized for low speeds, the vehicle's yaw rate gain becomes higher at high speeds (the vehicle reacts excessively to a small steering angle), making accurate driving difficult.
With a fixed gear ratio steering system, the yaw rate gain varies depending on the vehicle speed and the magnitude of lateral acceleration, even if the steering angle input is the same. Yaw rate gain lowers when the vehicle speed is low, and increases when it is high, changing greatly depending on the vehicle speed and lateral Gs. Since this varies depending on the drive system and suspension characteristics, the driver needs to learn and compensate for each vehicle.
Driving can be made easier by preventing yaw rate from changing in response to steering angle input despite vehicle speed, by increasing the steering gear ratio as vehicle speed increases, and by decreasing the ratio as steering angle increases. This hypothesis is the starting point for realizing VGS.
Analysis of the driver's gazing point in field tests showed that on average, the driver gazes about 1.2 seconds ahead when entering a curve, independent of vehicle speed. This means that the driver draws a mental route of where the vehicle will be 1.2 seconds later, and controls the steering angle while compensating for yaw rate gain, which varies with vehicle speed, lateral acceleration, and other factors, based on their experience.
By eliminating yaw rate change from steering angle input, the driver no longer needs to compensate steering for yaw rate gain based on their experience, making driving easier. In search for the ideal steering gear ratio from this perspective, Honda found that it was necessary to select a gear ratio characteristic that varies with vehicle speed and steering angle. Honda’s solution was a base characteristic of increasing gear ratio (making response slower) as speed increases, corrected by reducing gear ratio (making response quicker) as steering angle increases.
Simulation of lane changes at three different vehicle speeds (20 km/h, 80 km/h, and 120 km/h) confirmed that the variable gear ratio reduced the amount of overshoot at low speeds and was more stable at high speeds without divergence compared to the fixed gear ratio. In addition, simulations of the response time from when entering a spin on a slippery surface to commencing steering confirmed that the variable gear ratio allows a larger steering angle input with the same reaction time as the fixed gear ratio, leading to an easier correction.
At low speeds, the steering gear ratio is increased, and at high speeds, the gear ratio is decreased. A correction is made to increase the gear ratio at large steering angles. This is what VGS achieves. VGS integrates with a rack-and-pinion steering system a variable gear ratio system into the upper section of the pinion gear, and an electric power assist unit is mounted in the rack section.
Variable system
Principle concept
The gear ratio is changed by changing the amount of offset between the steering wheel input shaft and the pinion gear rotational shaft. When the variable gear ratio mechanism’s DC motor rotates the worm gear, the amount of offset between the steering wheel input shaft and the pinion gear rotational shaft is changed. When the offset is reduced (minimum 1.25 mm between the shafts), the transmission ratio of the input shaft is increased, the pinion moves more in response to turning the steering wheel, and the rack stroke becomes larger (quicker response). Conversely, when the offset amount is increased (maximum 5.0 mm between shafts), the transmission ratio of the input shaft is decreased, the pinion gear moves less, and the rack stroke becomes smaller (slower response).
When a double lane change test at 100 km/h was conducted with a vehicle equipped with a prototype VGS, it required less corrective steering than a vehicle with a fixed gear ratio. On a parking test, it was also confirmed that the amount of steering required was less than half of a conventional vehicle. When the vehicle fell into a spin on a slippery surface, the time required to recover contraction, the maximum steering speed, and steering angle indicated that the vehicle with VGS had a high ability to recover from disturbances in the vehicle’s behavior.
VGS has higher steering gain at low speeds. It was found that a driver experienced with conventional steering systems can adapt to driving a VGS vehicle with high yaw rate gain at low speeds for the first time, as long as the yaw rate gain’s phase delay is small.
Based on these in-depth studies and test results, it was confirmed that a steering system with gear ratio characteristics that change according to vehicle speed and steering angle dramatically improves the ease of driving, which led to the system being applied to the S2000.
