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I am involved in concept stage designing of a chassis/frame structure for an SUV vehicle. What are the typical attributes required in the initial stage of desgn. My understanding is that the basic chassis/frame structure should have enough static(torsional and bending) stiffness and dynamic stiffness to avoid any primary ride problems.Subframes/xbeams are generally for the secondary ride issues which are included in the developement exercise.what should be the criteria for design from nvh point of view.

Do we consider durability/fatigue or crashworthiness in the concept stage??

RE: Chasssis/frame


You are quite right about the Bending & Torsional stiffness part of the frame. Thats the reason cross-sections like C, I, Box(Tubular) are more common for the frame.
Sub-frames carry aggregates like engine/gearbox/suspension/etc & help minimize transmission of forces & moments from road jerks through the frame onto theses aggregates & vice-versa.

I do not think crashworthiness should form a part of the concept stage, but it may vary with the requirements for whom you are making the frame. But, consideration for fatigue, various loadings, etc definitely play an imp role in the frame design.

NVH comes into picture when the frame is considered with all aggregates & body loaded onto the frame & bolted at the mounts. & the dampening property of the body/powertrain mounts plays a great role in NVH of the vehicle.

Some other points you could consider:
* Location of the bolting points of Body & Frame from the   possible loadings due to various field uses of the SUV.
*  Finite Analysis for simulating vehicle use & thus the requirements from the chassis.


RE: Chasssis/frame


The statics are set for handling (the car will not be tunable if itis too soft) and squeaks and rattles.

Make sure your whole vehicle torsion and bending modes are well clear of other excitations, and that they are pure torsion and pure bending.

Make sure the mobility of each mounting point is appropriate:

(a) the dynamic stiffness of the mounting point needs to be 5x that of the bush, in that direction


(b) the mobility needs to be less than a certain limit (0.1 mm/s/N from memory). This work was done at MIRA I think, in about 1980

Crash - definitely.

Fatigue - not really. If you meet the stiffness targets then usually you can solve fatigue by detail design and selective up-gauging.

Packaging will actually dominate your design.


How can you say that crash should not be considered? It is the dominant requirement for the most of the front end longitudinal structure - the position and detailing of crush zones for all the crash events largely controls the size and shape of the rails, and the cross members.


Greg Locock

RE: Chasssis/frame

Hey Greg,

Now that u mention this, the point did slip by past me.....
Yes, the position & details will play a role in the design & the need for crash will need to be incorporated.

Tks for pointing it out.


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