Question on frame construction

[jaybee]

I've noticed the current trend in frame construction is to put a pair of "auxiliary frame rails" flanking the drive shaft, tie them firmly to the main frame, and make some effort to cross the middle.  Look here to see what I'm talking about:

http://www.artmorrison.com/maxg/profile/index.cfm

Let me state up front this isn't meant to trash Art Morrison Enterprises, As far as I've heard they build a top-notch product. I'm having trouble visualizing how this results in a big increase in chassis stiffness, however.  Maybe in beam, but torsionally it looks to me like the smaller members that connect across the center mostly just serve as a hinge.  On a strength to weight basis an X member to prevent the frame from "diamonding" as it twists appears to me to be superior.  I would think the whole industry wouldn't be going in this new direction unless it offers some sort of advantage, can anyone explain it to me?

[PeterR]

I've noticed the current trend in frame construction is to put a pair of "auxiliary frame rails" flanking the drive shaft, tie them firmly to the main frame, and make some effort to cross the middle.

Any open section structural member, (C U or I) has very low torsion stiffness compared to a closed section (round or rectangular).

If a chassis constructed from an open section is to have appreciable torsional strength then it will require some form of diagonal bracing.

The ideal arrangement is for the braces to run from the front suspension pickup point to the rear suspension pick up point on the opposite side.   This means the chassis side rails are exposed only to the vertical loads, and torsional loads are resisted by pure bending in the two X braces.

However the space required for engine and exhaust headers prevents the X members running any further forward than about the position of the bell housing.   Also, if a one-piece drive shaft is used, the intersection point of the diagonals has to be close to the end of the gearbox extension to allow vertical movement of the drive shaft in response to suspension travel.    This means practical constraints prevent straight diagonals from meeting the side rails at the ideal position close to the front and back axle, and instead would meet the side rails much closer together.

An arrangement of the form >=< gets around some of these restrictions as it provides space for the drive shaft and allows for different length gearbox extension housings.  However it is worth investigating whether this affects the function of the cross member.

Remember, if the X braces are straight, then they will carry pure bending loads so they can be a simple open section.   The largest bending moment is at the intersection point, however it is not too difficult to provide a hole for the drive shaft and still maintain adequate strength.

In any case other than two straight members arranged as a simple X, there will be large torsional loads applied to some elements of the cross member.   This does not mean the simple X is the only satisfactory form of providing torsional stiffness, but it does mean any other form requires some quite complex engineering calculations to be effective.

Over the years vehicles have been constructed with the chassis as a single center spine splitting into a fork at the front and rear to connect suspension components.   In some cases the center member has simply been a large diameter tube, some have been a rectangular truss, and others a quite complex "torque box" which forms part of the floor pan.   In all of these the distinguishing feature is that the portion running down the middle of the vehicle is carrying large torsional loads and must be designed accordingly.

From my observations of the chassis you describe as having "auxiliary frame rails", I would conclude they do not have sufficient material tying the two inner rails together to allow them to behave as a single member with high torsional strength.    Under these circumstances the inner rails provide transmission and suspension mounting points, but contribute very little to the torsional rigidity of the chassis.

The commentary above relates to aftermarket chassis intended for use with bodies originally designed for chassis mounting. However, the particular item pictured is a perimeter frame intended to be fitted under a unitary body which will provide torsional stiffness.  Further it is constructed from 4x2 box section that does have some inherent torsional strength, so will likely be better than the original arrangement.

[jaybee]

Thanks, I appreciate the feedback.  That pretty well confirms and expands my understanding of how frames work.

On the most basic level, of course, is boxing a frame.  It adds a lot of stiffness with minimal weight because your turning an open section into a closed section.

My feeling about an X member has also been that should be as close as possible to the pickup points because that's where the stress is loaded into the frame.  With parallel leaves that's going to be front spring hangers which is easy.  I'm not quite sure how that translates into a buggy spring suspension.  It looks to me lke the weight is held by the spring, braking and acceleration lift or press down at the wishbones, and there shouldn't be much twist because there's very little roll stiffness.  So in that case suspension would put very little torsional stress into the frame and most twisting forces would come from the motor mounts and spring crossmember through the shackles due to driveline torque, right?

In a four bar or link-type suspension, should the X join the frame where the bars mount, or as close as possible to the wheels, where the forces actually originate at the road?  Most suspension forces are applied to the frame at the link mounts, so that should allow a very satisfactory X member.  Of course when you start adding coil overs, sway bars and the like that clear picture starts to get pretty muddy.

[PeterR]

The discussion above related to preventing the frame warping from unequal vertical loads, not preventing it distorting to a diamond shape.  

The text "suspension pick up points" would more correctly been written as "effective point of application of vertical suspension forces" but that is a bit clumsy –even for a pedant like me.

For parallel springs the vertical loads are carried by the shackle end as well as the fixed end.   Not surprisingly the effective point of application is directly above the axle.   Except for some unusual torsion bar arrangements the force can generally be assumed to be applied to the chassis directly above the axle.

The transmission driveline does induce some twist, but not as much as you might imagine.   Compare the amount the body tilts under full engine output to the body roll around a fast curve.

Where anti-roll bars are fitted, the point of effective load application will move towards the bar pivots, but not much as few vehicles derive more than half their roll stiffness from the anti-roll bars.