jim lockwood

I'm in the hunt for a single UNUSABLE front RPO 684 drum. Yes, unusable, as in worn out, broken, missing some fins, whatever..... just as long as it used to be a Real Deal RPO 684 drum once upon a time. It can be a '57 drum with the wrap around fins or the later style. I don't care.

Have you got one or do you know someone who does? Please let me know.

Thanks in anticipation....

Jim

john neas

Jim
Are you needing it long term or short term?
Regards

jim lockwood

Hi John,

Probably should be considered long term.... well, permanent actually. In all probability it would end up being cut and sectioned. I'd hate to do that to a good, usable drum; in fact I wouldn't do it. My conscience wouldn't give me as bad a time about mutilating an unusable drum.

Do you have a drum that's bad enough to destroy or do you know someone who does?

Regards,

Jim

john neas

Jim
I dont have one that falls in the category that you describe. Even the junk drums are suitable for cars in museums or flight judging. You had asked for a front drum. By memory the front and rear drum are the same except for the plate offset. If I run across or remember a candidate I will get with you. Good luck.
Regards

cameo

Maybe we should have these recast? Why do you want to section one. curious

jim lockwood

I've been talking to a fellow who has foundry connections about (wait for it.... wait for it......) having some drums made. Although I have a good, usable '57 drum from which a pattern can be developed, we may also need a drum to section to generate more accurate dimensions. Clearly then, the sacrificial drum needs to be one that you wouldn't even want to use as a door stop.

Something may come of this or nothing may come of this. I'm no stranger to the process of designing parts and having them fabricated, but having a part cast is a new one for me. I'm learning as I go.

So, back to my original plea..... Anyone got a near-trash-grade RPO 684 drum? Or even a significant piece of one? As John pointed out, front and rear are the same in the cast portion, so don't be shy about offering up a rear (or piece thereof).

Jim

cameo

a lazer scan of the part does not destroy the part.

john neas

Jim
Ken kayser has a blueprint for the first design RPO 684 drums on page 537 book " The history of GM's Ramjet Fuel Injection on the Chevrolet V-8 and its Corvette Racing Pedigree". The drums were a slight change from the 56 finned SR drums. I believe you would get the dimensions that the original foundry(s) had. Will your drums be suitable for racing use? Best of luck.
Regards

jim lockwood

Absolutely. That's the entire focus of the project, if anything comes of it.

For 20+ years, I've been involved with friends who vintage race C1s and the chronic problem they all face is what to do about brakes. I've seen some really off-the-wall "solutions" that are so scary that I wouldn't drive the cars on the street, to say nothing about racing them.

Now this is a dumb problem to have because there is nothing magic about the RPO brake drums. They are just brake drums. Old technology. Well understood technology.

Of course, as I get smart about what's involved in having parts cast, I may change my tune. We'll see.

Jim

john neas

Jim
Please keep me posted. Have you found a Detroit or other greyhair that has extensive experience with drums?
Regards

cameo

I have a couple grey hairs, but I have worked in tool and die for 15 years and now the mold industry for 3. I have made patterns for cast iron die shoes out of foam. Cast iron pattern are made with 1/8" per foot manufactured in. So the patterns have to be 1/8" per foot larger than the finished part. Things shrink when they cool. So to make a casting from a casting would not really work, because the finished part would be slightly smaller. So you need to account for expansion (or constriction depending how you look at it) in your patterns.

Do you know if the 684 backplates are the same as stock with cooling holes cut

or are they completely different?

john neas

The 56 SR and the 684 front plates are made from Olds backing plates. There may be more modifications than just the cooling holes. Ken has prints in his book. The rear may be the same except there is an additional spacer under the unique eccentric anchor to space the 2" shoe out. The SR back stayed at 13/4" shoe so it didnt need the spacer.
Regards

cameo

thanks, John. I just ordered Kens book.

cameo

anyone have any pics of the finned drums? Do you know how many fins they have? How much do they weigh?

gord holden

send me your e maill address and i will send you pictures.

john neas

Jim
Have you considered making the drums thinner and lighter? Some of the fast C1s are using non finned drums. Maybe thinner drums (if safe) would run cooler, have less unsprung weight and have less enertia to overcome. I believe that part of the original design, thickness was to compensate for the aggressive metalic brake lining that chewed the drums up. My 56 Sebring drums after 4 races in 56 and about 14000 street miles are .500" oversize.
Regards

jim lockwood

John,

I've seen one '59 here on the left coast that allegedly races with inexpensive, narrowed Sting Ray drums. The car goes and stops with the very best of them. That is undoubtedly due in large part to the driver's skill, regardless of what the brakes contribute.

By and large, the other C1s I know of use some kind of finned drum. Paul R., for example, admits to using some left over Z06 drums. A friend with a '58 uses the station wagon drums similar to those that Tom Parsons has on his '56. Russ's '57 has a set of the later, factory finned drums and Velvetouch shoes. Everyone, it seems, has a different solution to a tough problem.

As far as thinner drums being cooler running.... my intuition is that they would run hotter because there is less thermal mass within which to dissipate the built up heat.

The only confirming analogy I have to back up my thinking is the disc brake setup I have on my repop Grand Sport vis a vis the now-former brake setup on my wife's repop Grand Sport. The rotors on my car are a good 7 lbs heavier than the rotors that my wife's car used to run. We once ran her car completely out of brakes at Laguna; mine, on the other hand has never, ever had any brake issues despite being driven much harder. I attribute the difference to the mass of the rotors more than anything else.

This said, I agree completely that lighter brakes would be all around goodness for handling and reduced rotational inertia. I believe that was Zora's lament decades ago, wasn't it?

The thought of drums wearing half an inch over size just makes my jaw drop. I'm (almost) speechless!

As an interesting comparison, the Bondurant '57, which you know well, has the same drums and shoes now that were on it when Mickey bought it in '62. Those brake parts survived 4 years of competitive SCCA racing back in the day and 20 years of vintage racing.

Anyway, bottom line is that overall, my bias is towards a source of shoes that replicate the look and performance of the originals as closely as possible.

Regards,

Jim

john neas

Jim
My jaw droped also. It probably wasnt wear only. Possibly some resurfacing. Frank Burrell was the mechanic during and after Sebring. The car raced with cerametalic shoes which were more aggressive than the metalic shoes.
To make shoes we put on thick linings and had them arched(sp).

The 59 uses driving skill and good use of components to be successful.


On the heat transfer issue. I will attempt to run this past some tenants who work with heat transfer in refineries for their take. I was thinking about the heat sink issue.
Regards

DZAUTO

I don't "race" the 56, but sometimes I do "drive" it HARD. Many times I've really clamped down on the brake pedal and punished the brakes. So far, so good. They're still just fine. I haven't even replaced the big Olds brake shoes that I put on several years ago (although, I have a set under the workbench ready to go if needed).
Also, when needed, I do some serious down shifting to augment the brakes. Since I do all my own mechanical wok, if I blow up a Muncie, I'll just pull out one of the spares and stick it in.

cameo

Anyone know how many fins they originally had? Were they bigger than 11" ?