Original 427 Street Cobra Suspension Rebuild

[CobraV8]

Heat treating the parts will make them grow a very small amount in volume, because the crystals with the metalurgical structure of hardened steel (martensite) take up more volume than that of non- hardened steel.

Maybe the hubs were out of specs?

On the other hand, 900 degree fahrenheit is more in the range of heat treating aluminium, than hardening steel. I can offer this as a second option.

The last thing that comes to my mind is that rust free chrome- moly steel will begin to rust when exposed to high temperatures, such as brake system parts are. That is because of the chrome will begin forming carbids with the carbon, and so a layer of chrome oxide will not protect the surface of the parts from oxygen any longer (that's what actually makes them rust free).

In order to re-solve the carbides and get the chrome back into the steel, solution annealing of the material is necessary.

That's all I can think of. No further ideas so far...


Simon

[Lowell W]

I'm gonna guess one of the 400-series stainless steels...maybe 416? Heat treating will make it tougher and increase its rust resistance. Can't recall the draw temp for maximum toughness, or the hardness at that point, but I'd guess you'd want it somewhere in the 40's RC.

I wouldn't think there are any dimensionally matched parts here, as any matching would be done by grinding after heat treating and stubs and hubs would be interchangeable. They're all in the same batch because they're all the same material...OR because they're all drawn at the same temp.

Just a guess...

Lowell

[David Kirkham]

Lowell,

I will answer you shortly. Good ideas, however.

Back to the suspension:

Well, there really isn't much to say here except that it is time to bolt the rear suspension back on again. Notice how there is NO hub cap on the rear suspension! That is original for S/C cars. This car is made like an S/C with the roll bar, side pipes and hood scoop. Original street 427 Cobras did have hub caps on all four corners.

[David Kirkham]

Here is a picture of an original front suspension on a 427 Street Cobra. Notice the front caliper is cast iron--just like the rear. It is also a very strange 3 piston unit. Notice the rotor (hard to see in this photo) is solid--just like the rear rotors.

Finally, notice the hub cap is missing. If you are REALLY good you will also notice the funny golden color of the spindle in the photo. That is because Sandwich is faster than senor dumb fingers here at the keyboard. (Funny, I can play Chopin Nocturne 27.2 but keep hitting wrong keys as I type!) Anyway, Sandwich has already changed this side out. Oops! I just gave away the plot...

He just hasn't put the hub cap back on yet.

[David Kirkham]

Notice the cool Koni shock! (Cool, but HEAVY!)

[David Kirkham]

Here is a picture of the other side. This is a really good shot of an original front upright, front steering arm, front upper ball joint, and steering rack. Notice how it is missing the spindle (front axle), front hub, and front rotor. Speedy Gonzales got to that before I could take a shot.

So, why did we take the front spindle out???

[mickmate]

Good stuff, I'm thinking the temperature might be a bit misleading. Dark straw is less than 500 deg (approx 475) as a temper color? Maybe a trick question David?? I would think 4140 would be a good material for those components but you mention corrosion which is why the stainless was mentioned. Corrosion could be from no seals on originals which you are obviously adding to yours..........

[RedBarchetta]

Great, David! Thanks a lot! Now we are all going to go home and stare at our lowly FFRs, Hurricanes, BDRs, SPFs, first gen Kirkhams....heck, even the 'real McCoy' original CSX cars that some here own, and immediately start losing sleep because of all the creepiness and unknowns that lurk deep within the very steel that keeps our cars going straight and true (or fast around corners while staying intact).

-Dean

P.S. AWESOME thread...thank you for the education (I think!)

[David Kirkham]

Here is a picture of the spindle going back into the front upright. Sorry for the blur! It looks like there aren't any threads on the end of the spindle, but I assure you they are there!

[David Kirkham]

Here is a picture of the front hub assembly. The bearings have been checked and the outer bearings have been replaced to the larger bearings.

[David Kirkham]

Here is the outboard view of the hub and rotor assembly with the new Set 12 bearing ready to go on.

Notice the synthetic red Mobil 1 grease we use. It has a very high working temperature. If you have ever raced on those solid rotors, then you know why you need the high temperature grease.

[David Kirkham]

Here you can see the new 5/8 "D" washer we made to go with the new upsized outer bearing. The original spindle only had a 9/16 "D" washer and nut.

[David Kirkham]

I just get into this posting daze and don't realize other people are posting and asking questions. I will try to answer.

FIA-ERA,

Please PM me and I will get you out your prize!

rbray,

For heating to the temperatures I am talking about to do you any good, you will have to use a precipitation hardening alloy.

CobraV8,

Chromoly steel is notoriously bad for corrosion resistance. You are correct that a standard heat treatment on heat treatable steels actually makes the steel grow in size. But, that only works on steels that are "heat treated" by quenching to keep the martensite. For example, 4340 changes from austenite to martensite at 1525-1550 degrees F. You are correct martensite takes up more volume.

Here, however, we are working with precipitation hardening stainless steels. In our case, we "only" heat it up to 900 degrees and the process actually makes our parts SMALLER--but it is actually very, very slight. We do not quench and temper with 17-4 ph. This touches on another VERY important reason to use 17-4. It is stable during heat treat. There is no quenching to cause cracks or to warp the part. Our parts come out perfectly concentric and through hardened when we are done. You pay for it initially as it is extremely expensive--the billet alone for our hubs is $235!

You gain it back when you can sleep at night knowing your parts are the best you could have possibly made.

mickmate,

If you look in the picture, the worst corrosion is actually right by the junction of the spindle to the upright. You can see a darker band there. The bearings are sealed, but that little part of the spindle is NOT. Water gets in there and the rust monster rears his ugly butt. (But not as ugly as Jamo's)

As for the color--the straw color is the color the hubs come out of the oven when they are cooled. They darken as they cool. Notice in the above picture the blue is just starting to appear on the hubs.

mylesdw,

On an original Cobra, the hub and spindle never touch as they are separated by the bearings. It is VERY true, however, that parts that touch and rub LIKE THAT DUMB BEARING SPACER must be made out of hard steel.

Dean,

I spend a lot of nights awake trying to figure out how to do this better. One ride with a professional driver on a track and your perspective changes. WOW, these cars better be put together RIGHT! At our event last Thursday, a Miller Motorsports instructor posted lap times 30 seconds faster in one of our cars than a pretty experienced (although ametuer driver) in a HIGHLY modified Ford GT. When you ride with one of those guys (or girls) you simply can't believe what the car can do. Like I said, I only see as far as I do because I stand on the shoulders of giants.

Lowell,

I am happy to share what we have learned over the years. Hopefully it will just make the entire industry better. Over the years many, many people have shared freely what they do with us. That is where I got all the information. Like my mother always said, "There'll be more for all of us if you share."

David

[cobred]

I enjoyed this thread very much, thanks for taking the time to post it. Any videos of the instructor doing some hot laps in a Kirkham?

[Lowell W]

David,

THANK YOU SO MUCH for taking the time and effort to post and explain all this information that you had to learn the hard way!

Lowell

[brobehr]

this is like a history/metallurgy/mechanical engineering class all in one..

awesome!

[sshurts]

At our last event last Thursday, a Miller Motorsports instructor posted lap times 30 seconds faster in one of our cars than a pretty experienced (although ametuer driver) in a HIGHLY modified Ford GT. When you ride with one of those guys (or girls) you simply can't believe what the car can do.

As someone who personally experienced one of these rides, I will say that these cars perform absolutely remarkably in the hands professionals.

David, please thank Cindy (again) for the absolutely awesome ride in the new track car. The track car has a second rollbar with a six point harness and kirkey racing seat on the passenger side. It held me firmly in place to savor every bit of g-force that the car delivered with Cindy's precise control. This ride was on the more technical east track.

I'm hopeful that the early discussions that took place between the head instructor and Thomas will lead to overlapping of the track day and training from these instructors. And of course, more rides with the instructors too.

Thanks for the track day
Scott

[big-boss]

David nice thread, very nice. I have already sent a couple of my engineering friends to the thread- they were very impressed. The rocket sicentist for Northroup- emailed and said
"I didn't think those models you put together had that much tech".
He said you are right on about the corrosion-Jim said they not only use heat treating when they can-but this is where the bad boy coatings come into play their big part.
David thanks again- This is why you are selling cars like hot cakes!

[jperickson2001]

I am humbled by your high tech. Very fun and informative to read. -John

[CobraDan]

David I have really enjoyed ready this thread but then the reference to Florida and rust and now talking about Jamo’s ugly butt, Well there I have to agree.