Teardown of Toyota's Flagship Sedan (Part 2)

Chassis technology for high-end rear-wheel drive cars common to Toyota and Lexus vehicles

2016/06/08

Summary

Front double-wishbone suspension in place

Front double-wishbone suspension in place

Rear multi-link suspension

Rear multi-link suspension

 This is a continuation of a series reporting on a teardown analysis performed by the Hiroshima Industrial Promotion Organization of the Toyota Crown Royal, a V6 engine-powered rear-wheel drive model launched in Japan in December 2012.

 Part 2 of this series will focus on the chassis components and related technologies found in the 14th-generation Crown, which inherits the basic platform originally developed for the 12th-generation Crown that was released for sale in 2003. Even so, the original platform already features well-thought out technical considerations, from the suspension and steering layouts to the technologies for small parts, that are desirable on high-end front-engine rear-wheel drive vehicles. Spanning three generations, the Crown platform features a bundle of technologies that make it worthy of a teardown analysis today. The next report (Part 3) will focus on the Crown’s body and sound insulation technologies.

Previous teardown reports:
Teardown of Toyota's Flagship Sedan (Part 1)(May 2016)

4th-Generation Toyota Prius
4th-Generation Toyota Prius Teardown (Part 1)(Feb. 2016)
4th-Generation Toyota Prius Teardown (Part 2) (Mar. 2016)
4th-Generation Toyota Prius Teardown (Part 3) (Mar. 2016)
4th-Generation Toyota Prius Teardown: Photo gallery (132 parts) (Apr.2016年)

Daihatsu Move (Feb./Mar. 2015)
  Daihatsu Move Teardown (Part 1): Equipment comparable to B-segment cars
  Daihatsu Move Teardown (Part 2): High fuel economy and improved performance
  Daihatsu Move Teardown (Part 3): Linear body structure optimizes space

VW  Polo (Nov./Dec. 2014)
  VW Polo Teardown (Part 1)
  VW Polo Teardown (Part 2)

Nissan Note (Sep. 2014)
  Nissan Note (Versa Note) Teardown (Part 1)
  Nissan Note (Versa Note) Teardown (Part 2)

Honda Accord Hybrid (Feb. 2014)
  Honda Accord Hybrid teardown (Part 1)
  Honda Accord Hybrid teardown (Part 2)
  Honda Accord Hybrid teardown (Part 3)

Honda Fit Hybrid (Dec. 2013)
  Honda Fit Hybrid teardown (Part 1): Battery components & brake system
  Honda Fit Hybrid teardown (Part 2): Engine and transmission

Toyota Aqua (Nov. 2012)
  Toyota Aqua (Prius c) teardown: Part 1
  Toyota Aqua (Prius c) teardown: Part 2

Nissan Leaf
  Nissan Leaf teardown (Part 1) (Feb. 2012)
  Nissan Leaf teardown (Part 2): main components disassembled (Sep. 2012)
  Nissan Leaf teardown (Part 3): body cutaway (Nov. 2012)



Front suspension components

Basic layout common to rear-wheel drive Benz and BMW vehicles

Front suspension on Toyota Crown(Photo: Courtesy of Toyota)

Front suspension on Toyota Crown
(Photo: Courtesy of Toyota)

Front suspension on Toyota Crown

Front suspension on Toyota Crown

Front suspension on Mercedes-Benz C-Class car(Photo: Courtesy of Mercedes-Benz)

Front suspension on Mercedes-Benz C-Class car
(Photo: Courtesy of Mercedes-Benz)

Front suspension on BMW 5 Series car(Photo: Courtesy of BMW)

Front suspension on BMW 5 Series car
(Photo: Courtesy of BMW)

 The Toyota Crown has a front double-wishbone suspension system with virtually the same layout as that of high-end rear-wheel drive cars like those produced by Mercedes-Benz and BMW. In comparison with the suspension system in the Mercedes-Benz C-Class model (which is nearly the same as that of the W213 E-Class launched in 2015) and BMW 5 Series shown above, the Toyota Crown has two layout features in common; the high upper link position and the steering gear positioned before the front axle. This suspension layout offers two advantages:

  1. Locating the upper link in a higher position with a longer vertical span increases the tire support rigidityrigidity and improves alignment characteristics.
  2. Positioning the steering gear before the front axle increases the steering system rigidityrigidity. It also balances the rigidity between suspension and steering, which contributes to increasing the steering response without compromising high speed stability.
Front suspension on Toyota Crown with the front under-cover and suspension member reinforcement brace removed

Front suspension on Toyota Crown with the front under-cover and suspension member reinforcement brace removed

Front suspension on Toyota Crown with the upper link removed

Front suspension on Toyota Crown with the upper link removed

 The front suspension system on the Toyota Crown differs from that of the Mercedes-Benz C-Class or BMW 5 Series in that both the C-Class and the 5 Series have the lower link separated into two parts in a double joint design that uses two suspension ball joints. The Toyota Crown has a classic triangular transverse link that is connected to the knuckle by a single suspension ball joint.

 The double-joint structure on the C-Class and the 5 Series allows the tire rotational axis after the ball joint to move to the outside of the vehicle when the steering wheel is turned in order to increase vehicle stability and reduce vibrations by minimizing the offset between the rotational axis and the center of the tire.

Special characteristics from situating the steering gear before the front axle

 

Positioning the steering gear before the front axle serves to improve cornering stability. Shown below are two top views of a right front wheel with cornering force (lateral force) acting on the tire when negotiating a left curve.

 The graphic on the right shows a steering gear located behind the front axle. When cornering force is acting on the tire, compression force acts on the transverse link and the tie-rod end. The compression force causes the transverse link bushing and the steering gear mount insulator to sag and a difference between the two sags causes a slight change in the angle of tire travel. Normally, the steering system is engineered in such a way that a change in the angle of tire travel causes the vehicle to travel toward the outside of the curve, which is known as toe-out.

 This stabilizes the vehicle’s motion when cornering. Because of this, a toe-out angle must be chosen by introducing slightly different rubber hardness (spring constant) for the transverse link bushing and the steering gear mount insulator respectively. However, the freedom of choice is limited as the transverse link bushing is subject to restriction not only in terms of stability, but also of quietness and durability.

Steering gear located before the front axle on the Toyota Crown

Steering gear located before the front axle on the Toyota Crown

Steering gear located behind the front axle

Steering gear located behind the front axle

 The graphic to the left above shows the suspension layout on the Toyota Crown, in which the steering gear is positioned in front of the front axle.

 When cornering force (lateral force) acts on a tire while cornering, compression force acts on the transverse link bushing, but not to the steering tie-rod. Since the cornering force acts slightly behind the ball joint which serves as the fulcrum point of its counterforce, a rotation moment is generated in the tire, which, in turn generates pull force on the tie-rod end.

 Since force always acts in a toe-out direction in this case, the hardness (spring constant) of the transverse link bushing and the steering gear mount insulator can be chosen by selecting a bushing in consideration of steering rigidity and quietness. This layout is preferable especially when a hard steering mount insulator is needed for precise steering and response.

 Front-wheel drive and many other production cars often have the steering gear located behind the front axle. In contrast, suspension layouts with the steering gear located before the front axle are limited to high-end rear-wheel drive vehicles and sports cars.

“Parrying” suspension

 The front suspension system on the Toyota Crown is unique in that it optimizes alignment by using the “parrying” movement of the link. An offsetting shape is added to the tie-rod end to reduce its rigidity to an optimal level. When lateral force acts on the outer tire during cornering, rotational torque is generated in the toe-out direction as shown in the graphic to the right below. When the tie-rod end rigidity is reduced in this condition, an effect is generated that will bring the tire slightly in toe-out direction. This slight toe-out tire movement prevents the vehicle from being overly sensitive to road surface irregularities and stabilizes the vehicle when cornering.

An offset shape in the tie-rod end improves cornering stability by parrying effect(Photo courtesy of Toyota)
An offset shape in the tie-rod end improves cornering stability by parrying effect
(Translated by MarkLines based on Toyota materials)


Reinforcement brace increases suspension support rigidity

Front suspension on Toyota Crown with the front under-cover removed
Front suspension on Toyota Crown with the front under-cover removed

 The suspension member is mounted to the body at the rigid parts of the front side member. A triangular reinforcement brace is used to further increase the support rigidity. The brace is intended to increase the suspension support rigidity by increasing the body rigidity to a force from the suspension which, in turn, increases the steering response and high speed stability.











Front suspension components

 The upper link is a pressed steel component with an open cross-section. It is mounted to the upper part of the engine hood ridge via a pair of rubber bushings. The upper link on the Lexus GS is a forged aluminum component.

 The transverse link is a steel component with a closed cross-section. It is joined to a closed cross-section extending from the suspension ball joint mounting point to the rear transverse link bushing mounting point by welding the front transverse link bushing mounting point. The rear transverse link bushing is press-fit into an aluminum mounting bracket and bolted to the transverse link. The transverse link on the Lexus GS is a forged aluminum component.

 The front knuckle is an aluminum component, and is fastened to the suspension ball joint of the upper link at the upper end, and to the knuckle arm at the lower end. The front hub and front brake caliper are mounted to the knuckle center.

 The shock absorber is supplied by Hitachi Automotive Systems and has a co-axially assembled coil spring.

Upper link

Upper link

Transverse link

Transverse link

Front knuckle

Front knuckle

Knuckle arm・Knuckle arm ball joint・Suspension ball joint

Knuckle arm
Knuckle arm ball joint
Suspension ball joint

Shock absorber with a co-axial spring

Shock absorber with a co-axial spring

Suspension member reinforcement brace

Suspension member reinforcement brace

Front knuckle

Front suspension member (bottom view)

Front suspension member (top view)

Front suspension member (top view)

 The front suspension member is a die-cast aluminum component, with the transverse link joined to the bottom surface, and the engine mounts to the top surface. An elongated space is provided in the center to leave enough capacity for the engine oil sump. (Reference: Teardown of Toyota's Flagship Sedan (Part 1)



Steering components

 The steering gear is rack-coaxial electric power steering made by JTEKT Corporation. A hollow-shaft motor and a ball-joint reduction gear are located coaxially on the rack to directly assist it. This design provides the driver with direct steering feel. Toyota started using rack-coaxial electric power steering on the 12th Crown, which was launched in 2003, and the same system has been used for three generations in a row.

Steering gear supplied by JTEKT Steering gear supplied by JTEKT
Steering gear supplied by JTEKT
Steering column, combination switch

Steering column, combination switch

Intermediate shaft

Intermediate shaft



Front axle, front brake, engine mounts

 The front axle has a built-in bearing. A rotation angle sensor is also included with a signal output terminal located on the back. The front brake is a fist type disc brake supplied by Advics. The right and left engine mounts are fluidic mounts that are joined to the engine via die-cast aluminum bracket.

Front hub unit

Front hub unit

Brake rotor, front hub unit

Brake rotor, front hub unit

Front brake caliper

Front brake caliper

エンジンマウント

Fluidic engine mounts



Rear multi-link suspension overview

 The rear suspension is a 5-link suspension with a common layout for front-engine rear-wheel drive vehicles. Five independent arms are used in place of the two A-shaped arms of a double-wishbone suspension. This provides a steering geometry that optimizes lateral and longitudinal force compliance steer while minimizing tire vibration caused by road irregularities. Using an I-shaped arm instead of the A-shaped arm reduces its weight as it is only subject to axial force.

 The Lexus IS/GS also has a multi-link rear suspension. The suspension members are similar in basic structure to those of the rear suspension on the Toyota Crown but have a few differences. The position of the toe control arm on the Lexus is moved from the front to behind the rear axle out of necessity. Four-wheel steering is available on some models in the Lexus series, which requires locating actuators that control the toe control arms in such a way the right and left parts are coupled to each other. The coil spring is independent rather than being positioned co-axially with the shock absorber. The rear upper arm on the Lexus is a forged aluminum part unlike the Crown, which has a steel part.

Rear multi-link suspension with rear differential in place

Rear multi-link suspension with rear differential in place

Rear multi-link suspension
(Photo courtesy of Toyota)

Rear multi-link suspension
(Photo courtesy of Toyota)

 Toyota’s multi-link suspension system for front-engine rear-wheel drive vehicles uses the “parrying” movement of the arm in the same way as the front suspension system.

 An offsetting shape is introduced in the toe control arm so that the arm sags when it is subjected to cornering force (lateral force) while turning. This allows the toe change characteristics to be tuned to the toe-in side so as to improve vehicle stability.

 The upper arm has an open cross-section that optimizes the torsional rigidity of the arm and reduces the prying action of the arm bushing. This leads to the suspension having a smoother vertical stroke, and better ride comfort and stability.

Toe change characteristics optimized by the toe control arm
Toe change characteristics optimized by the toe control arm  (Translated by MarkLines based on Toyota materials)
“Parrying” suspension with an open cross-sectional upper arm designed for smoother vertical stroke
“Parrying” suspension with an open cross-sectional upper arm designed for smoother vertical stroke  (Photo courtesy of Toyota)

 Increased suspension support rigidity and body rigidity is an important factor in increasing steering response and high-speed stability. Reinforcement braces are used effectively around the rear suspension members and the under-floor of the Toyota Crown. A suspension member reinforcement brace is used that connects the front lower arm mounting point to the rear lower arm mounting point of the rear suspension member. The reinforcement braces are used to suppress small displacements that cannot be eliminated by the rugged structure of the rear suspension member itself.

 A longitudinal floor reinforcement brace is used to connect the front mounting point of the rear suspension member to the rear edge of the body side member. A lateral brace is used to connect the right and left of the rear edge of the body side member for added reinforcement.

Floor reinforcement braces in place

Floor reinforcement braces in place

Suspension member reinforcement braces

Suspension member reinforcement braces



Rear multi-link suspension components

 The five arms have different cross-sections due to varying contingent load distribution. The rear lower arm has the thickest cross-section. The rear upper arm and the knuckle side of the toe control arm have ball joints so there is no prying force on the arm. The front lower arm and the rear upper arm have rubber bushings in both ends. To reduce prying force around the rubber bushing and ensure smoother vertical stroke, the arms have open cross-sections so that proper twist is applied. The rear knuckle is a die-cast aluminum part. The rear shock absorbers are supplied by Hitachi Automotive Systems.

Rear lower arm

Rear lower arm

Front lower arm

Front lower arm

Rear upper arm

Rear upper arm

Front upper arm

Front upper arm

Toe control arm

Toe control arm

Rear knuckle

Rear knuckle

Rear shock absorber parts

Rear shock absorber parts

Rear shock absorber, rear spring

Rear shock absorber, rear spring

 The rear suspension member is made of steel. The longitudinal frame is presumed to be a hydroformed steel pipe. The two rear differential mount insulators each bear vertical load and lateral load and are made of rubber with suitable hardness.

Rear suspension member (top view)

Rear suspension member (top view)

Rear suspension member (bottom view)

Rear suspension member (bottom view)

Suspension member support bracket

Suspension member support bracket

Rear floor reinforcement brace (side view)

Rear floor reinforcement brace (side view)

Rear floor reinforcement brace (longitudinal view)

Rear floor reinforcement brace (longitudinal view)

Suspension member reinforcement brace

Suspension member reinforcement brace



Rear axle, brake, rear final drive

 The rear hub has a built-in bearing.

 The rear brake is a fist type disc brake. The parking drum brake is built into the brake rotor. The rear brake is supplied by Advics.

 The Toyota Crown is fitted with Michelin’s 215/60R16 Primacy LC tire.

 The rear final drive is mounted to the rear suspension member at four points via rubber mount insulators. The front mount insulators are built into the final drive and the rear mount insulators are built into the suspension member.

Rear hub unit

Rear hub unit

Rear brake caliper

Rear brake caliper

Rear brake rotor, parking brake shoe

Rear brake rotor, parking brake shoe

Tire and aluminum disc wheel

Tire and aluminum disc wheel

Rear final drive

Rear final drive

Propeller shaft

Propeller shaft

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