My strategy is to leave the back half of the front piece of the gas tank support in place as it will reference and place the tank in place properly. We will see.

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John - I hope you haven't welded in the upper piece of the front crossmember sandwich. Looking at the picture from the trunk side of the lower piece, I see it is missing a reinforcement for the front A arm mount. There should be a piece which connects the two rear tubular nuts, sort of like a strap over the top which fixes their tops in place. Otherwise, it is only the spot welds on the exterior piece which takes up the forces. The factory used this extra interior reinforcement, but some of the aftermarket sheet metal makers do not include it. If you haven't welded yet, I can find a picture of the reinforcement if you'd like.

I had a front rip mount loose from an otherwise sound repro pan because this reinforcement wasn't included.

Hi Walt,
No, I havent welded them in place yet, they are just placed there for fit. I have the straps too. In fact, I am contemplating welding a plate on top of the front mounts and connecting that to the front struts with an x brace. Which of course means I get to cut into the new tank...

I know Porsche used X braces on some of their race cars. Sometimes complete with elaborate attachments so they could get their tank out and back in. Elephant will happily sell you a system for this.

Color me very dubious.

Think about what the forces might be on the front A arm attachments, which are the only places forward of the strut area where forces of any significance could possibly affect suspension geometry. Think of the A arm as a lever, and calculate what the lever arm fore and aft is from the spindle to the rear A arm attachment, and what it is from the spindle to the front attachment. For these purposes perhaps it is better called an L arm.

Under braking, all the braking force is taken up by the reinforcing cross member. With that long basically negative lever arm, could more than a few percent if the up and down forces be transferred to the front mounts rather than to the strut tower? Same with the side forces. So how is an X brace going to do anything of significance other than, as one of my shop owner friends believes, reduce the sheet metal damage from minor front end collisions on the race track?

What about the front mount which tore out of my car? Well, it is a track only car with coilovers and spherical bearings for A arm attachments. The guy who converted my A arm and chassis attachments to spherical bearings said that the rod through a spherical bearing should be under compression, so the bearing will rotate in its race, but the rod won't rotate against the ID of the bearing. So he put caps held on by bolts into the center rod so the spherical parts are basically fixed to the A arm. Which means that the braking forces are divided evenly between the front and rear mounts.

So it has always seemed to me that the torsional rigidity we want to keep the suspension forces we want to control and adjust independent of twisting of the body (by preventing that twisting) is not enhanced by reinforcing this area.

But perhaps you could devise a test and measure this? Right up your alley.

I clamped the pan in place, traced it and cut it out. I then tacked it in place and flipped the car over on the rotisserie and measured/tugged/pulled/clamped/tacked and repeated until I got the diagonal meaurements across the crossmember and the mounts to less than 1mm. I then adjusted the pan up/down with a digital camber level to less than 0.1 degrees difference between the crossmember and a level across the front spherical mounts.
I then fabricated a pair of gussets to weld on the top of the mounts, within the frunk. Those will be reinforced back to the front strut tops.

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more. Fabricating up some brackets for the front mounts and mocking up the placement of the front oil cooler to determine where to cut into the front pan for the ducting.

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You can see the laser I used to find the center of the car and align the front bumper and cooler.

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Walt, always read your posts with care....but I think I might actually disagree with you on this one.

I think the X brace has the effect of "putting the top on the box) formed by the trunk floor, inner fenders/upper strut mounts, front bulkhead and rear(dash) bulkhead.

It seems to me that it otherwise is an otherwise open box-like section that will have some tendency to warp unless the forces are directly lateral or longitudinal to the edges of the box section....which they are not from what I can see.

I think is why some vendors also put a diagonal brace from the top of one tower to the bottom of the other...to prevent the "box" from turning all parallelogram on you when forces are applied to the bottom and top of the vertical box sections.

I suspect the benefit is not large, the measurement idea is a really good one..hopefully it can be done as part of this thread!

Dennis

Well, on my GT car where the rules are no constraint, I have tied the shock towers together laterally with both a cross bar and two diagonal bars which attach on the body structure below the cross bar. Covers both bases. Some of this came with the car and was adjustable, so I kept that feature, though it would make more sense generally just to weld things in, seems to me - at least if you were satisfied that the tub was straight/symmetrical in that area.

There certainly are forces applied in that general area, though it seems to me they are mostly applied to the reinforcing cross member down below. The strut is going to send some of the forces to the top, but because of where the spindle is located (way closer to the bottom than the top), they should be substantially less.

My doubts about much bracing any forward of that is based on my notion that there can't be much force applied forward of the strut tower. If the lower control arm were more of an A arm, it would seem that the lateral forces would be applied roughly half and half to each attachment. But our A arm is an L arm. There have to be some forces on the front. Maybe under braking the diagonal rod tries to pull the front mount sideways some? But ultimately the rearward force on the chassis enters at the cross member, doesn't it?

Of course the X brace makes that segment of the chassis stronger - your top of the box- in the sense of more resistant to dimensional change. And connecting strut towers more strongly to the main chassis, adding diagonals and such if you can, should increase torsional rigidity vis a vis the rear of the car.

But what is going to make the area forward of the strut towers (which also take up about all the vertical forces, especially with coilovers) distort? That has been what I don't see (perhaps through ignorance - always a possibility). Making something stiffer if it doesn't need to be isn't going to improve handling.

When a trained engineer does one of those force/reaction arrow diagrams to see where load paths are, what is it going to look like when done for a 3 dimensional 911 front end? Maybe we can lure an engineer into doing this.

I agree with your thinking. The weight of the front of the car is born by bending in the crossmember ultimately (with torsion bars) and the front mounts should really only be loaded in braking but they still have a pretty good leverage ratio. And even then they'd put the front suspension pan "crossmember" into a horizontal tension. Strut top definitely takes less load due to the leverage over the spindle, still has some load, especially with coilovers.

I'd think that for torsional rigidity, a bulkhead (or welded in X brace) between the strut towers as on a 914 or a Boxster would be the most bang for the buck when combined with a top to that box between that bulkhead and the cowling (via welded in X brace or sheet metal). Front of the crossmember and strut towers would yield diminished returns for torsional rigidity. It would give you more crush structure or fuel tank protection, not to mention you could build a tube frame and move the suspension pivots around or other things.

Hi guys,

Firstly let will cover myself under the pseudoscience disclaimer in the thread topic!

I quickly hacked together a few sketches that try to analyse the various force vectors that wil (to my mind) be at play during various conditions of motion:

http://forums.pelicanparts.com/uploa...1421840078.jpg

The direction of the force is indicated by the colour of any given arrow, and the magnitude (serious guesswork here) depicted by the length of the arrow. I too feel that there is little benefit from any bracing forward of the strut towers, and would also agree that most a diagonal brace across the towers should be more than sufficient.

Please tell me how far I'm off then I'll happily update!

I think your magnitudes are a bit off on the strut top, at least the lateral ones (should be smaller than the A-arm laterals). Vertical too unless you are doing a coil spring car. Since there is no torque shown on the A-arm I assume it is a coil spring car. Bump should look more similar to braking.

I don't fully agree with all of your vertical arrows for the front of the A-arm. Since you can't transmit a torque directly from the wheel to the A-arm (due to the ball joint not supporting a torque other than frictionally) the reaction force at the front will be dependent on the angle of the arm and also the front to back position of the ball joint relative to the rear A-arm bushing. I have a feeling that all of your arrows should be upwards at the front. Magnitude (small) is good I'd say.

The calculations wouldn't be difficult, the hard part is getting every measurement needed and I don't have a full list at the moment. This would include the tie rod pivot points. I just don't have enough interest to devote the time at the moment. I could make some good old engineering guesses and probably get pretty close though.

Thanks Flieger - very valid points! Magnitudes were not calculated but more "guesstimated". Agreed on the inability to transmit torque via the ball-joint too. Thank heavens for my pseudoscience disclaimer then heh! :D

I could take measurements if needed....everything is apart!

You forgot cornering...

JR

I took a holiday today and made a bunch of progress on the suspension pan, front mounts and cooler shroud.

It took longer than it might have mostly because I was doing some 'gineering on the fly. As I welded on the front mount braces, I changed the plan few times to try and get the best result. I ended up with something I think is pretty trick, for a banker.

Not only do the front mount tabs brace to the pan, but are also boxed to the tank support crossmember. Furthermore, the 2 bolts for the front mounts and the front of that triangle, the bolt for the guard all extend up through the gas tank support, through a thick plate that is welded to the top of the support. That thick plate is going to get a pair of brackets supporting half-moon-shaped bracket. I can then connect the front mount brackets directly back to the same-side strut tower. Or I can connect it diagonally to the other strut tower. Or I can X them across to the opposite strut tower. I could check if there is any difference in twist or bending.

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The tanks support all tacked in, the top plates go on next...

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Something like this for the mount...
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Arg..upsidedownsie trick going on with the uploads again...

With some better pics and the tank test fitted.

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You could add to the collective knowledge by seeing what differences adding any forward bracing might have by using your proven techniques, I think. Bolt the long lever arm to the front suspension mounts you have reinforced when you are done with that part of things, and see how much movement there is - at least as to vertical force - without adding any bracing. Then add bracing (perhaps using temporary attachments rather than your usual carefully designed stuff) and do it again.

That ought to deal with any torsional movement. But how to check for lateral distortion, as when cornering? Bergermeister used a hydraulic jack (and a load cell?) against the 2x4 framing of his garage to see how much the strut tower tops moved (not much, as it turned out). There must be some way which is as accurate (or innacurate) as the testing you did at the reinforcing crossmember location before you started cutting and welding.

Maybe you could anchor at the front and rear A arm mounts without the arms in and use a porta-power to see what trying to spread them would do, and also to see what drawing them together would do.

Another interesting thing might be to figure out what line of welds you could most easily subject to distortion. Measure it with what you have, then double or triple or fully seam weld, and see what difference there was.

I made some more progress today, between sledding sessions and plowing off the pond for hockey tomorrow...

I got the front cooler mounted, and I fabricated up a duct behind it and got it all tacked in. I put the bumper, hood and fender on to check fit, it all looks great!

Back onto the rotisserie tomorrow, finish welding up the underside, and then backwards to the rust repair near the rear of the doors.

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