🌟 Honda’s FWD Marvel: ATTS System 🌟

🌟 Honda’s FWD Marvel: ATTS System 🌟
Did you know the fifth-gen Prelude Type S pioneered the Active Torque Transfer System (ATTS)? 🚗

Here’s what it did:
✅ Enhanced handling with near RWD-like performance 🎯
✅ Reduced inner-wheel torque and boosted outer-wheel speed by 15% ⚙️
✅ Brought apex-hitting precision to FWD vehicles 🚀

❓ Why did ATTS disappear after 2001? Find out in this deep dive into Honda’s engineering history! 💡

✨ Bonus Content: Discover how ATTS inspired the SH-AWD system 🌐

Original Author: Honda Magazine
Date: November 20, 2024
Featured Topic: #Innovative Japanese Car Technology

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Honda's FWD Handling Marvel: ATTS System
The full name of the ATTS system is Active Torque Transfer System, which can be directly translated as "Active Torque Distribution System" or "Active Torque Distributor."

The system was designed by Koji Shibata, who joined Honda in 1986 after working at Nissan. At Nissan, he was the mastermind behind the famous HICAS four-wheel steering system, which powered the rear wheels of the legendary NISSAN GT-R. While the ATTS concept was proposed early, Honda hesitated to adopt it due to their existing rear-wheel steering system. Although a prototype version of ATTS appeared on the rear wheels of the FS-X AWD concept car at the 1991 Tokyo Motor Show, it wasn’t until November 1996 that it debuted in production on the fifth-generation Prelude Type S (BB6), where it was applied to the front wheels to achieve near RWD-like responsive handling by adjusting torque and axle speeds between the left and right wheels.

Interestingly, both RHD Type S and LHD Type SH versions were equipped with ATTS. However, since the H22A engine in the LHD model is mounted on the left side and rotates clockwise—opposite to the RHD’s counterclockwise rotation—there's no way to transplant the system across these platforms.

How the ATTS System Works
According to official data, the ATTS system was designed to "more accurately translate the driver's intentions into vehicle behavior, enhancing driving enjoyment." It remains inactive during straight-line driving but activates during cornering or when accelerating out of a turn. In such situations, the system reduces driving force on the inner wheel and appropriately transfers more torque to the outer wheel based on conditions like speed and steering angle.

When activated, ATTS alters the vehicle's cornering trajectory:

Standard cars have a larger turning radius (R value).
Cars with ATTS maximize tire grip, making it easier to hit the apex point.
Components of the ATTS System
The ATTS system consists of two main mechanical modules:

MCU (Motor Control Unit): Includes a planetary gear differential with built-in hydraulic clutches.
Differential (Diff): Connects the transmission to the ATTS unit.
The MCU, composed of three sets of planetary gears and two hydraulic clutches, redistributes torque (up to 15%) to the outer wheels during cornering. Sensors detect vehicle states—such as steering angle, throttle input, speed, and lateral G-force—and determine when to enhance turning performance by increasing the torque difference between the left and right front wheels.

In straight-line driving, torque is evenly distributed 50:50 between the front wheels. During cornering, the torque ratio shifts up to a maximum of 85:15, favoring the outer wheel for both left and right turns.

Operational Scenarios
Straight Driving:
Multi-plate clutches are disengaged. Planetary gears and sun gears rotate as a single unit, ensuring equal torque distribution (50:50) between the front wheels.
Left Turn:
The left multi-plate clutch disengages, locking the left sun gear to match the left wheel's speed. Meanwhile, planetary gears adjust, increasing the right wheel's speed by approximately 15%.
Right Turn:
The right multi-plate clutch disengages, locking the right sun gear. Planetary gears adjust to increase the left wheel's speed by about 15%.
Why ATTS Was Discontinued in FWD Models
Despite its effectiveness, ATTS required significant suspension redesigns for optimal performance. For example, during cornering, the torque transfer caused additional steering torque around the kingpin, necessitating a unique double-jointed suspension arm. This arm shifts the virtual kingpin location and reduces the steering torque difference by modifying the wheel offset from 43.7mm to 25mm, improving operational stability.

While this double-wishbone suspension design was feasible for the fifth-gen Prelude, it became incompatible with the MacPherson strut suspensions used in later high-performance Honda models (like the Integra and Civic). As a result, ATTS was phased out.

Legacy and Evolution
The ATTS system evolved post-Prelude into the SH-AWD (Super Handling All-Wheel-Drive) system, first appearing in the fourth-gen Legend (KB1) in October 2004. SH-AWD isn't just another differential-based AWD system—it was the world's first system capable of varying torque between the left and right wheels of both axles. Stay tuned for a deep dive into SH-AWD in an upcoming article!