Fender stress cracks return

[Dave K.]

[Kent D.]

Re: Fender stress cracks return

I would think these cracks are caused by 1. a knee against the fender as you are working in the engine compartment or 2. a tire hitting the inside of the fender when you are turning.

I have had my '66 Sting Ray on lifts and yes the body will flex but not enough to crack.

Kent

[Mark S. Lovejoy]

Re: Fender stress cracks return

These cracks suggest the nose is too low at the radiator support. C1's suffer from this when the lower radiator support cushions begin to deteriorate and the nose droops.

Check the radiator support to inner fender attachments and the shims between the radiator support and the front frame horns.

Hope this helps.

Mark

[Chuck R.]

Re: Fender stress cracks return

Hi Dave,

Obviously, the wheel wells are the weak links on a mounted nose.

It sounds like there is too much upward force being placed on the nose forward of the wheel well to me. This could stem from either excessive shimming or even improper lifting with floor jacks.

Were these cracks also matted on the back side as well?

These could just be poor quality repairs that have unfortunately re-surfaced.

Chuck

[Chuck S.]

Re: Fender stress cracks return

I agree with you, Chuckster (What am I saying?. What do we know?...We're both C3 guys! )

I suspect the nose IS shimmed too high, creating a bending moment (tension in the bottom of the wheel well flange). That bending moment is being relieved at the weakest point, or the top of the wheel well. Improper repairs may also fail to provide the strength needed to constantly resist this stress. Mat and resin should be laminated to both the inside and the outside surfaces. Knowing where to set the nose so as to generate no residual stress in the panel is a problem, particularly on C3s...they have an even LONGER nose. Ideally, we could install some strain gages on the wheel well flange to know when the stress passed from compression, through neutral, to tension.

Check the 67 AIM for a page in UPC 0 (Yep, in the very front) called Trim Heights. Compare the dimension to the top of the wheel well with the height given to the rocker panel. If the nose is set high enough to cause the flange to crack, you may be able to see it by comparing your dimensions to those chart dimensions.

[Chuck R.]

Re: Yup, what you said *NM*

[Michael M.]

Re: Fender stress cracks return

Look for seams on the underside. It was common for a half front end to be used in repairs. It no sign of prior repirs I agree with the others look for a shimming problem. Good luck!

[Dick W.]

Re: Fender stress cracks return

What Chuckster??? Fiberglass has no RBM??

[Chuck S.]

Re: Fender stress cracks return

RBM??!! You got me there, Dickie, what's "RBM"? They didn't teach us about "RBM" down at Harvard On The Brazos.

[William B.]

Replace fender, is only perminent solution.

Hi Dave, If the fender was repaired using proper techniques with reinfored mat on top and bottom sides. I would replace the fender from the factory seams. I have seen lots of corvettes that have cracks above the wheel, or someone has cliped on the front end and made repairs, that ALWAYS come back. I started a procedure years ago, in that after I make all body repairs, haul the body shell down to a body shop, ask and pay them to bake the body at 140 or 180. After that is done you can catch any repairs that are going to give you problems. Or you could leave it out in the summer sun for a couple of years and get the same afect. I like the paint oven it gives fast results.

[Tracy C.]

RBM = Runny Bowel Movements *NM*

[Dick W.]

Re: Fender stress cracks return

Resist Bending Moment! Look it up. I learned the meaning of this the hard way when several trucks that I had ordered started breaking in half. Expensive lession

[Chuck S.]

Re: Fender stress cracks return

OK...I get it, "RBM" is the internal bending moment supplied by the material to resist the applied bending moment, which in the this case is primarily the front end's own weight. (Serves you right for buying them old Fords. )

Fiberglass has bending strength, but in the case of body panels, fiberglass is mostly a resilient pretty face. I don't know if it's bending strength is enough to resist constant loading for a long period, but I doubt it...if there are no road shocks involved, then I believe failure would be a gradual relaxation of the material with permanent altering of the panel shape. This would be particularly true when, say, a long Shark front end is completely unsupported.

Usually, cracks indicate where the highest stresses were...cracks in the wheelwell flange indicate to me that the tensile strength of the material was exceeded at that point due to bending. About the only way you could get tension in the wheel well flange is to raise the nose such that the fenders are acting as simple beams. If the "box structure" of the fender and inner fender is still too weak to supply the necessary "RBM", then the weight of the fender combined with inertia loads created by driving may cause it to crack. A poor repair might cut the available "RBM" by half.

[Chuck S.]

Edit ...

"About the only way you could get tension in the wheel well flange is to raise the nose such that the fenders are acting as simple beams."

Actually, I said that wrong...the problem is the fenders would NOT be acting as simple beams, but as cantilevered beams anchored at the firewall by bonding adhesive and with point loads applied upward at the radiator support.

The weight of the fenders also applies as a more or less uniform load downward across the length of the beam. Then, if you hit a pot hole, the existing tensile stress in the wheel well flange increases significantly...maybe enough to cause a crack to start.

[Dick W.]

Re: Fender stress cracks return

Not Fords Chuckie, Cornbinders of years past.