Personally, I have no intention of modifying my stock suspension. I never have anything helpful to add to any of the "rodstoration" threads and I couldn't tell a Heidts from a Hide's. On this one, however; I think I can take up some board space along with the rest of you "non-stock-restoration" boys and girls.

I don’t do custom auto suspensions but I am directly involved every day in failure analysis, crack initiation and propagation mechanisms, design, and material science for the Metals Behavior Branch at the Air Force Research Laboratory. From the available data I doubt very much that any NTSB official would venture to pronounce a definite cause of failure; however, after studying the available photos for a couple of days there are some interesting features.

1. The cracks are through-cracks in all three webs. They were NOT initiated or propagated by bending in the individual steel web sheets.
2. All three cracks initiated at stress concentrations and not in the bulk material. This makes it less likely that there was a material quality issue involved and more likely that either the stress risers in this particular cross-member were worse than normal or the loads were higher than normal. I'll agree to differ from the two auto engineers on this one.
3. Both cracks on the rear side of the cross member are larger (one failed the member) than the one on the front side. In these materials, compression rarely causes a crack. These rear cracks were caused by tension loads on the lower edge of the web on each end of the cross member. When braking, the rear of the cross member is put in compression, not tension. Therefore, it is extremely unlikely that these cracks were initiated or propagated by braking loads.
4. The only mechanism available to put significant tension loads on the cross member where the cracks initiated is the vertical loading of the spring on the upper spring pan.
5. The overall weight of the vehicle and the sprung weight on the front wheels will have a large effect on the static tension load on the lower side of the cross member. It will, however; have a much smaller effect on the higher-frequency cyclic loads applied to the upper spring pan since those are controlled solely by the characteristics of the moving mass (tires, springs, control arms, shocks, etc. – everything outboard of the control arm pivot). Therefore, the amplitude of the cyclic loads on the upper spring pan (and thus, the tension on the bottom side of the cross member) is mainly determined by the moving mass. Since (even from just these photos) it is clear that these cracks are fatigue cracks, the amplitude of the cyclic loads will drive crack initiation and propagation rather than the magnitude of the static load.
6. For this type of material, periodic large magnitude overloads on the cross member (like hitting a big pothole) actually tend to delay the onset of crack initiation and tend to stop a propagating crack. Therefore it’s far more likely that the cross member saw a typical loading profile during service but all the loads were increased in magnitude for some reason. (On the other hand, hitting potholes continuously will generally accelerate the crack initiation and growth processes.)
7. For those of you still reading this, the first places to look for the failure cause(s) are: 1) the magnitude of the stress concentrations on this particular cross member (bad welds, machining marks, ???) and 2) the characteristics of the moving mass and springs on the car it was installed in.

If I were a betting man my money would be on an unusual characteristic of the moving mass. An unusual outboard geometry, shock absorber or spring setup, tire pressure, or a combination of these or a hundred other things caused those springs to hammer that upper spring pan harder than normal on every bump. Eventually, the cracks initiated and one grew to final failure.

OK, I’ll go back to sleep now …

George

 

Guys,

Please read my post on the HAMB (From ElSwede). I've been on a business trip so I'm quite sorry that I didn't get a chanse to look into this earlier, especially considering all the bad press the M-II suspension gets. Too bad! The Heidts/TCI M-II's are good suspensions. The Fat Man one is bad. The M-II suspension give a significant improvement in handling over the original front suspensions on our trucks. Don't hesitate to use them. Just my $0.02...
289merc: It's up to you to trust the Carps person on the HAMB. Beeing in the car industry I know that Toyota R&D is not in Australia. I have a pretty higth position within R&D at one of the european brands if that helps you trust me
Later,

 

Earl

Thanks for your explanation. It was much more comprehendable for me than most of the engineere-ese I have read. Sounds like a plausible theory to me. I am done clogging this board with my opinions but would like to email you for definitions of a couple terms you used if that would be OK.

'fenders

 

lars, what makes the heits good and the fat man bad?
arnt all the m2's the same basically?
which kits do you rate good and why?
george, excellent explanation, makes me feel better.

 

Gwilkrrs,
Try to find a supplier that has FatMan and TCI/Heidts and look at the X-members. You will easily find that the FatMan X-member is quite cheap. What I dislike the most is the way it meets the frame of our trucks. The welding area is smaller and it doesn't have any reinforcements to prevent the frame from twisting. I believe that Heidts/TCI will keep the wheel angles a lot better when you drive the truck and it will handle better. I also like the craftmanship better on Heidts/TCI and there is always the durability issue.
There are other X-members similar in design to Heidts/TCI but I haven't studied them.
Later,

 

George,

there was a nice long detailed post on the H.A.M.B. board by Elpolacko which detailed the major differences and drawbacks of the Fatman crossmember... but mainly it was as Lars stated... lighter material and lesser workmanship. If you're interested in building your own there is an excellent article there also by the same author on that subject... it's under the Tech-o-matic section... it's a good read even if you're not of a mind to tackle the fabrication.

I looked at Fatman, Heidts and TCI. I bought Heidts on a personal recommendation but would have also bought TCI. The comments and experiences of others that I trusted steered me away from Fatmans.

John

 

the only thing that bugs me about the heits kit runs 9" rotors and other kits run 11" rotors.zigs has a kit with 11" for the same price as heits, and tci has the same 11" for the same price. i dont know abouit the differences in the crossmembers so should i just run the proven heits and deal with the smaller rotors? all help appriciated

 

[updated:LAST EDITED ON 14-Nov-02 AT 05:12 PM (EST)]George,
I bought the economy Heidts crossmember only... ground the welds myself.. I then bought stock Mustang II parts and ended up using only the spindles..

then bought a set of Granada 11" rotors, Chevelle Calipers and pads.. I used upper and lower control arms from Jim Weimer...much cheaper than Heidts and really nice quality...

point being.. you don't have to buy a complete set up from heidts. you can piece it together .. But I would buy Heidt's crossmember for the beginning...

hope it helps

john

 

yes that does help. i didnt know what i could piece together so i waass looking for complete kits. i was scared thar iwould have too much money involved by buying the wrong stuff trying to make a kit instaed of buying a kit. guess i have more homework

 

Was thinking about it some more today and was wondering - what is the general (not detailed) procedure for welding the cross member to the frame?

I'm imagining you have a frame with some exact width between the outside of the rails. Do you buy a one-off, custom cross member to fit your exact frame width? Do you put spacers between the inside of the upright posts on the cross member and the outside of the frame rails? Do you just put 'er on and clamp the rails to the uprights before welding? That last one would cause HUGE problems. Hope that's not the general practice!?!

As usual, more questions than answers. But then, you guys and gals seem to like it that way!

George

 

[updated:LAST EDITED ON 14-Nov-02 AT 08:40 PM (EST)]George,

the '53-56 crossmember is made to fit that specific frame width..the crossmember slides nicely into the framerails and is the proper width inside including the 3/16" boxing plates welded onto the rails not inside the rails... then the top hat is measured and welded to the top and outside of the frame rail...

it's a very straightforward installation and as long as you snug everythng up the frame rail and have the proper distance to set the wheelbase... you can't go wrong (considering plumb and square are done first)

if you call Heidts they will send you a copy of the install insructions for your particular model before you buy. I can fax you a copy for the '53-56 if you like

john

 

how much drop is associated with these frontend kits?
are all the m2 kits the same drops?
i want the kits but also my present stance or close to it.

 

Thanks John. My concern was this - if the cross member was a little wider than the frame and you squeezed the cross member so you could fit it to the rails, you'd put a heck of a tensile load on the bottom of the cross member. Sounds like that's not a problem unless somebody tries to use a cross member on a frame that's narrower than it was designed for. Or unless the frame had been modified and wasn't the standard width anymore? :-X23

George

 

The post on HAMB started out good but went bad after the started making fun of Gary and talking trash. I wouldn't put much thought in the last three pages.

Nathan