Superformance wheels ... 245-45-16 rear .. 225-50-16 front. Thanks for the input everyone. Lots of excellent suggestions that are so much easier than wittling from billet. There's an offroad place locally here that has all kinds of goodies as suggested. I think first I need to send this to my buddy just to make sure it is just production steel not chromoly. I've also come up with a pretty good welding/reinforcement plan ... I might do that also just to see how it works. cheers
Basic design, stress and fatigue problem: (part is called a "clevis") 1. Design: parts was made by bending very thick plate, aspect ratio in corners looks very bad. 2. Stress: tensile stress in outside corner very high due to forming of original part 3. Fatigue: old car many miles, many 1000's of stress cycles on the part. fix: a) machine clevis (same dimensions, big fillets) from 2" plate A36 mild steel, drill for stud, tack weld as req'd, will last for 100 years. or b) machine clevis from 4130 alloy steel, drill, heat treat at 600 F for 8 hrs, install stud, will last 1000 years. Another option would be to machine from 7075-T6 aluminum, it cuts much faster, so it's cheaper to machine, make the part thicker to eliminate increased deflection, so the car would feel the same. Reminds me of parts on my 69 Toyota, suspension parts were made of a very hard but fatigue prone steel. Make the parts out of a softer but slightly thicker mild steel, and they will last almost forever.
...which are way bigger than the originals. With the extended performance of modern tyres the loads will have gone up exponentially. I wonder if there is a connection with this?
um yeah and the fact I track my car with slicks. It's probably nothing most people need to worry about. For the 2 or 3 people that do what I do it's an important issue maybe . cheers
Other side isn't cracked but to add another variable to the life expectancy I replaced 2 of these with used ones when I first put poly bushings in the car in '96. Back then I didn't document anything so I don't know which side the replacements went on. Here's a pic of the other failure I've had related to suspension and very spirited driving ... rear lower control arm cracks. I repaired the silver arm back in 2007 and have put probably 30,000 miles on them since then. When I bought my second 308 it had the same cracks. I'm just getting around to rebuilding those arms and putting solid bushings in them with grease nipples. Since the other 308 isn't going to be on the road for a while I decided to put these arms on my driver. The shock from the solid bushings won't help the fork longevity. cheers Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Here's how I'd beef up one of these if I didn't want to start from scratch. Anyone have finite element analysis kicking around? Be nice to have a real numerical comparison between the stock piece with no gusset vs. my mod. A simple gusset like that "should" significantly strengthen the flange in the direction of motion that I'm thinking caused the failure. cheers Image Unavailable, Please Login Image Unavailable, Please Login
And if I went to that trouble I'd probably grind it pretty smooth and paint it a color which makes any cracking highly visible .... Image Unavailable, Please Login
What is your opinion about using U shaped construction beam, I think that is made not by bending, not by welding but by extrusion at high temperature so there should be no stress in the metal at all? It is mild steel so the appropiate thickness must be chosen to avoid fatigue. A track car with constant acceleration and braking stresses the part more than a road car of course.
Sean, I assume you gas welded the gussets as MIG or TIG tends to leave thermal induced stresses at the weld borders. I once made an air powered chisel for taking off some old bricks by MIG welding a 3" cold chisel's shaft onto the shaft cut off of an air tool. It kept breaking just past the weld after about 5-6 hour use. Finally a friend suggest gas welding to broaden & diffuse the heated area rathar than have a sharp thermal induced stress line. Worked like a charm, never broke again after 20 or so hours use.
Verell, Thanks for the input. Stress risers from welding always bother me because of what you are talking about AND where you put the weld! I thought just about everything critical was TIG no? MIG is harder to control. I don't think I've wanted to gas weld anything in a while. cheers
how crap! That is about as close to catastrophic failure as you can get. Tires/wheels/suspension/usage? Definitely a great idea to at least visually inspect these once a year.
Everything stock except for +1 sized tires. This was by far the worst fork I had. I suspect it was an acute event that caused that damage, though it must have happened before I had the car.
