Chassis stiffening--full frame car

[jaybee]

Searching and reading I see very extensive discussion of how to stiffen a unibody car, and it's very interesting stuff. What is the best approach if you're working with a full-frame car? I'd like to specify a budget that doesn't allow for full replacement frames like the amazing pieces from Art Morrison and no roll cage. My thoughts are to start with a welded X frame to brace the frame rails and a shock crossmember in the axle kick up area to prevent spreading. Thoughts on these and other modifications?

[bbcc]

An x brace welded at or around the mid point of the chassis will help with torsional rigidity. Aswell, if the frame is an open C chanel, boxing it will help with torsional and longitudinal (ie from front to rear) stiffness. As far as adding extra cross members, you should look at where the loads are being applied to the frame.

The most obvious are the suspension mounting points. Where the springs and effective control arms, be it front or rear, are areas of high loads, obvioulsy.

Another area of significant loading is the engine and trans mounting points. The torque of the engine will, to a degree, twist the frame. If you can increase the strength of the frame in these locations, aswell as spread the load out along a greater area of the frame, the frame will be able to better handle this load.

To effectivly add overall stiffness to the chassis think about how the loads are being applied at these high stress areas. If the load is mostly verticle, brace for verticle. The same goes for horizontal loads, either longitudinal or latteral. The main goal is to spread the loads from these points into a larger area (stress=force/area, increase the area=less stress). Although this may sound very simple the exicution is a bit tougher. Packaging is always a limiting factor.

A good way to cope the loads place on a chassis is to distribute it through the body. A role cage, that ties into both front and rear of the chassis will counteract the bending moment longitudinaly, aswell as increase torsional stiffness. You mentioned you would like to stay away from a 'cage. So, as you stated, an x brace will work great. If you can manage to build a backbone structure going along the transmition tunnel, that would be great aswell (but if you don't want a cage, its likely this isn't an option either). Also, running square tubing along the inside of the rocker panels and tieing into the frame for. and aft. the passenger compartment will aid in longitudinal rigidity. Solid body mounts and reinforced body mounts also help to spread the load into the body. If you want to get really carried away, you can reinforce the body around the mounting points aswell.

This is a really novice analysis of chassis stress. I'm sure many other members can more eligently explain (with fewer spelling mistakes to boot) what is really happening in a full frame car. I hope this gives you an idea of what needs to be done to increase the strenght of your frame.

cheers bbcc

[DeltaT]

What type of car? I like the X-frame idea but would probably go bolt-in unless you figured out some clever way to keep it in place while you dropped the exhaust and driveshaft. Boxing the frame is useful, and I've added gussets or boxing to key suspension pickup points. Some of the other G-body guys swear that a rear seatback stiffener really makes a difference, and we all have some braces from other G-bodies for front suspension, radiator and (in my case) radiator support to drip tray reinforcement bars. Ralph (Buick GN) showed up at my house last weekend with some rear bumper mount tie bar, and you could do the same on the front.

Lots of options - need to weigh the alternatives (literally).

Jim

[jackfrost]

on a g-body, the front and rear bumper bars are a great first step. I was amazed at the difference this simple mod makes.

currently, I have my frame separated from the body, so I'll be doing some stiffening as well, i'm interested to see what folks have to say. so far I'll be boxing in the c-channel. does anybody have photos of an x-brace on a frame? preferably home-made? :D

[jaybee]

In my case I'm referring to a '57 Chevy frame although I purposely kept my question vague in hopes that the discussion could be applied widely. For familiarity this http://www.trifive.com/garage/55%20C...anual/2-15.gif would be it. As you can see it has a really massive front crossmember and boxed rails but little else to give it strength. I was considering a crossmember with X legs that would hit the frame rails at the front spring hangar and transmission bellhousing mounts, which will be removed. Trans would be mounted either to the crossmember or to a drop out that would bolt between the front X legs. Shocks will be relocated to a crossmember in the axle kick to reduce flexing. That can be fairly severe and cars that have had air shocks very frequently have cracks in the trunk pan. This will be a street car that I want to be able to drive hard and maybe do an occasional autocross or open track day for fun as opposed to a hard core track car so I don't want the intrusion of a roll cage. That said I'm not a restorer and have no problem with modifications including to the floor pan. Would it help to run rectangular tube from the front spring hangar, between the outside body mounts, to the front body mount that's located in the narrowed part of the frame behind the front crossmember?

[86Cutlass383SR]

Dave we seem to be heading down about the same path as of now. I, too, am getting ready to pull the body off the frame for new poly mounts and while off box the frame along with cleaning/painting the underbody. If you happen to find any outside info on this, I'd be interested in it too.

I also used the bars that tie the frame rails together and was amazed at how such a simple 5 min mod could stiffen so much!

Doug

[aosborn]

This is a photo of a chassis I built back in the early ninetys. It is for a 55 Chev and features C4 suspension front and rear with composite springs. The diagonal bracing on the trans crossmember bolted in, and the crossmember and diagonals between the trans and rear axle were welded. Those simple diagonals made a surprising difference in the torsional stiffness of the chassis. I did torsional rigidity testing on the chassis at the time, but alas I don't recall the figures. It stiffened the chassis to a factor of at least two if I remember correctly. Keep in mind these chassis were Jello in torsion when in stock trim.

Andy

[silver69camaro]

It stiffened the chassis to a factor of at least two if I remember correctly. Keep in mind these chassis were Jello in torsion when in stock trim.

Andy

I have a different experience. My thoughts were the Tri-5 chassis weren't too bad when stock due to the 4x4 rail and decent overall width. Chevelles are a different story.

I have to wonder how you tested your chassis. Doubling the stiffness is a extreme amount, something I would expect by adding a 6 point cage. A well designed aftermarket chassis should be at least 30% stiffer than stock (torsionally), which is a huge improvement if it still fits under the stock floors.

[aosborn]

I have a different experience. My thoughts were the Tri-5 chassis weren't too bad when stock due to the 4x4 rail and decent overall width. Chevelles are a different story.

I have to wonder how you tested your chassis. Doubling the stiffness is a extreme amount, something I would expect by adding a 6 point cage. A well designed aftermarket chassis should be at least 30% stiffer than stock (torsionally), which is a huge improvement if it still fits under the stock floors.

I would support the front suspension crossmember off the floor, and then anchor it down. Then put a support under the frame on one side typically at the rear axle centerline or suspension pick up point. Using a lever would twist the frame say 1 degree and measure the force required. Add triangulation and retest. The boxed tri-5 (California) frames weren't as bad as the unboxed units, and the convertable frames with the X-bracing were the best. Unless it was a convertable frame I was using, X-bracing made a huge difference. With all crossmembers perpindicular to the frame rails there is very little torsional rigidity, just like a typical pickup or Model A frame. The chassis pictured was one of my very early units. In later chassis I greatly increased the X bracing and improved the stiffness even more. This chassis is a good example of how a few added diagonals that are easily added will get you going the right direction.

The doubling factor in stiffness may be a product of how much load was used in testing. To go from 200lb/degree to 400lb/degree is not that tough to achieve for example. I don't recall the figures I achieved with this chassis, or the testing load.

What I liked to do back then was model the chassis in balsa wood and add crossmembers in different configuations to see how the chassis responded.

Andy

[BillyShope]

More than half of the torsional stiffness of a fifties era car came from the body.
http://home.earthlink.net/~whshope
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[fishtail8]

Good point Bill. I was suprised when I got the "body mount kit" for my 50. It was only 1/4" thick rubber chunks to go between the body and frame mounts. With only a few extra braces, and most of the stock top hat frame intact, the car seems quite rigid.

[Bjkadron]

Solid body mounts(at least stiff poly mounts, possibly aluminum or other metal) will help a LOT. They are used on stuff that has removable subframes (1st gen Camaros for example) and work amazingly well. So I would imagine that it would also work for the full frame cars. It really wont change the ride qualities as that is the suspensions job.

Another thing you'll definitely want to try is the balsa wood models. They can show you where to best position the x-braces and such. they are very usefull. that's how I'm designing the frame structure on my Duster.

[Norm Peterson]

More than half of the torsional stiffness of a fifties era car came from the body.

I'd bet that the same can be said for GM's separate body/frame cars all the way into the nineties, with the sole exception of the Corvette.

The frame on my original 1979 Malibu (F41 with all of Chevy's extra bracing) was still flexible enough that you could prybar readily visible amounts of frame deflection by hand without exerting huge amounts of effort (and I'm not all that big). That loading was applying both bending and torsion to an individual frame rail, during a body bushing upgrade mod.


Norm

[jackfrost]

so is there stiffening you can do to the body to help torsion?

[Norm Peterson]

Sure.

Just what depends on how much you want to sacrifice from daily "liveability", what any sanctioned competition you might be involved in permits, and the extent of your own creativity. A little aptitude for structural engineering wouldn't hurt.

Ever climb over the door sills of a 60's E-type Jaguar? Seen Herb Adams' "Silverbird"? Looked at any recently fabricated cages or pictures to see how they tie in to body structural elements?


Norm

[Norm Peterson]

There was a rumor about a couple of folks on another forum investigating another approach entirely, but I don't know if anything ever came out of it.


Norm

[jaybee]

In terms of added X members, how much difference does it make whether these are simple X's across the bottom of the frame as opposed to structures that are full height of the frame? I ask that because the latter obviously makes exhaust routing and the like more difficult.

[Norm Peterson]

I'd guess half-height tubes would be about 25% as effective as full height tubes of the same width.

If you can compensate locally for hole cuts, you'd probably lose less general stiffness.


Norm