BMW i3 aluminum chassis frame: Drive Module characteristics

A rational frame construction featuring a highly rigid chassis

2017/10/27

Summary

The body-on-frame construction of the BMW i3 EV is comprised of two independent modules. The Drive Module integrates the drive system, battery, and chassis components with the chassis frame. This structure serves as the foundation of the Life Module, which is essentially the cabin or passenger cell, which is made from CFRP (carbon-fiber reinforced plastic) and equipped with all of the controls for operating the vehicle. The rear of the frame is made from aluminum die cast, on which the five-link rear suspension system is mounted as well as all of the electric drivetrain components such as the electric motor, gearbox, and inverter. The design of the frame functions to support the front suspension to absorb impact energy in the event of a frontal collision by using a combination of extruded aluminum, aluminum die-casting, and welded aluminum reinforcement plates. Also, the strength of the frame is increased through reinforcements welded with aluminum piping. In addition, the Drive Module is constructed so that the battery is located in a sturdy aluminum housing mounted in the underbody section of the frame, below the cabin at the center of the vehicle.

BMW i3のドライブモジュールとライフモジュール
BMW i3’s Drive Module and Life Module

 

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Related Reports:

BMW i3: Battery System (April 2017)
BMW i3: CFRP Life Module Vehicle Structure (April 2017)



Basic construction of the Drive Module

ドライブモジュール:アルミニウム製シャシーフレーム
Drive Module: aluminum chassis frame

In this aluminum chassis frame, the rear suspension system mounting area and the front strut housing area, both made of die cast aluminum, are joined together by ladder-shaped aluminum beams. A total of about 160 aluminum components are joined by over 19 meters of welded seams. The front and rear aluminum parts are die cast in BMW’s Landshut plant. By using aluminum die-casting, the number of parts can be reduced and the dimensional accuracy of those parts improved compared with conventional steel plate materials. The aluminum chassis frame is blackened by electrophoretic deposition.

BMW i3のドライブモジュールとライフモジュール
Drive Module: aluminum chassis frame


Construction of the rear mounting area for the power unit and rear suspension

ドライブモジュール車両後部
ドライブモジュール車両後部
The rear area of the Drive Module

In the rear area of the Drive Module frame, two large aluminum rear frame castings support the power unit from each side. The frame also provides mounting points for the rear suspension.

ドライブモジュール車両後部
ドライブモジュール車両後部

Drive Module: electric motor mounted area (Source: BMW)

Beneath the trunk, which is located behind the rear seat, power unit components including an integrated electric motor and gearbox are mounted to the aluminum frame with a rubber mount insulator. To reduce range anxiety, the BMW i3 can be equipped with an optional range extender (series hybrid), which includes a two-cylinder gasoline engine dedicated to generating power to the battery and an electric motor and gearbox, all integrated as a single power unit mounted to the frame with a rubber mount insulator.

 

ドライブモジュール リヤサスペンション取り付け部 ドライブモジュール リヤサスペンション取り付け部
ドライブモジュール リヤサスペンション取り付け部
ドライブモジュール リヤサスペンション取り付け部
Drive Module: rear suspension mounting area


The multi-link rear suspension

マルチリンクリヤサスペンション リヤショックアブソーバー リヤストラットアッパーマウント
Multi-link rear suspension Rear suspension’s shock absorber Rear strut upper mount

The rear suspension’s shock absorber and the upper mount of the coil spring are both made of die cast aluminum. The insulation function of the mount insulator is enhanced by using a cylindrical rubber bushing that is press-fitted into the upper mount.

マルチリンクリヤサスペンション リヤショックアブソーバー リヤストラットアッパーマウント リヤストラットアッパーマウント
Five-link rear suspension system (made of steel)

The links of the five-link rear suspension system are all made of steel. Forged aluminum links were not used for two main reasons: to avoid expected deformation of the link bushing; and because the shortness of the link rod means there are minimal weight saving benefits.

マルチリンクリヤサスペンション リヤショックアブソーバー
The rear area of the Drive Module (Source: BMW) Vehicle rear brace

The rear of the power unit mounting area consists of reinforcement plates welded with aluminum extruded material and aluminum plates, mounted to each side of the aluminum die cast frame. In addition, the undercover is also mounted in a similar manner to the lower surface of the power unit mounting area, thereby significantly strengthening the rigidity of the vehicle’s rear area.

 

車両後部アンダーカバー 車両後部アンダーカバー
The rear area: undercover

The undercover located on the underside of the power unit is constructed using an aluminum panel riveted to a steel frame with a spacer inserted between the two structures.



Construction of the front of the vehicle to mount the front suspension and steering systems and to absorb crash energy in the event of a frontal collision

ドライブモジュール車両前部 ドライブモジュール車両前部
The front area of the Drive Module

At the front of the vehicle, the front suspension and the steering systems are mounted to the aluminum frame. The strut housing is made of die cast aluminum welded to aluminum extrusions on the frame.

ライフモジュールとドライブモジュール結合ブラケット
ライフモジュールとドライブモジュール結合ブラケット
ライフモジュールとドライブモジュール結合ブラケット
ライフモジュールとドライブモジュール結合ブラケット
Bracket used to join the Life Module and Drive Module

The upper part of the strut housing is bolted to an aluminum die cast front mount bracket, which is bonded with adhesive to the A-pillar of the CFRP Life Module frame. The bond is made using a mixed material structural adhesive. The lower right photo above shows that when the bonded part is peeled off from the vehicle the surface of the CFRP material peels off, rather than the adhesive, which remains affixed to the bracket. The upper right photo above shows the mount bracket bonded to the Life Module, where you can see the crimping of the mount bracket and the protrusion of the generously applied adhesive.



Construction of the front of the vehicle to absorb crash energy in the event of a frontal collision

The front area: crumple zone designed to absorb crash energy (Source: BMW)

The crumple zone of the aluminum frame of the Drive Module has been designed to absorb crash energy in the event of a frontal collision. The front rail directly mounted to the front bumper has the largest cross-sectional area and absorbs most of the energy. The front grille reinforcements are mounted to the upper side of the front rail, and the reinforcements that connect the lower end of the bumper and the two front strut tower castings are mounted to the lower side of the front rail, giving the frame a three-tier structure. This three-tier crumple zone structure is designed to absorb crash energy from the front while releasing it downwards to the frame under the floor to minimize deformation of the CFRP Life Module passenger cell in the event of a frontal collision.


Drive Module (Source: BMW)
Front grille support reinforcement
Front bumper reinforcement

The front bumper reinforcements are made of extruded aluminum welded to the front rail at the same height as the extension members, which are also made of extruded aluminum. The front grille support reinforcements are located one level higher and are welded to the front strut tower castings at the rear of the extruded aluminum extension members, parallel to the side rail.

 

The front area: reinforcement beam

On the inside of the left and right front side rails, made of aluminum extrusion material, the rigidity of the front of the Drive Module is improved by incorporating an X-shaped beam reinforcement welded with aluminum piping. This X-shaped beam is designed to prevent the cabin from deforming during collisions and ensure suspension stiffness to improve stability. The increased rigidity of the beam is directly responsible for improving steering responsiveness.

Front suspension member

Suspension members are created by welding each of the pieces of the aluminum extrusions to the basic skeleton. In parallel with the upper front side rail, the front suspension member has been designed to absorb crash energy in the event of a collision.

 



The strut type front suspension

Front strut Front knuckle

The front suspension is a MacPherson strut type suspension system. The knuckle is made of die cast aluminum and connected to the steering tie rod attachment area. The thickness from the strut attachment area downward is shaped to enhance camber stiffness.

Transverse link Front strut upper mount

Forging is commonly used for transverse links made of aluminum, but an aluminum die cast transverse link is used in the BMW i3. The structure of the front strut upper mount is almost the same as that of the rear strut, receiving the load of the spring through the spring seat, and the load of the shock absorber via the central cylindrical rubber bushing (shown at the center of the front strut upper mount (rear view) in the photo on the right).

 

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Keywords:
Teardown, Munro, BMW, i3, Drive Module, Life Module, aluminum, suspension

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