914 6 calipers rear limited reproduction run GB?, Built by PMB $799 A set Options

[Mikey914]

We decided to make some steel calipers for Eric, as we are already dong the aluminum ones, and it turned out to be a fiasco!

My foundry guy decided to to use steel instead of a grade of iron that was hard-enable.

Didn't find out unit I got the bill from my machinist as about 3x what we thought it should be. Long story short, we have about 4 sets of calipers that are very nice. The only difference you will notice over factory is the hardware is 8mm not 7mm.

Looking to get $925 a set. Unfortunately, this still is selling at a loss.

Eric has built up one set, and has the spacers for the vented ones, so just looking to cut some losses as they do me no good setting on a shelf. I will post in classifieds, but wanted to put this up here to give a little back story. I have one built set now and can have Eric build vented / non vented sets to order.

These include the upgraded duro-anno pistons also!

[NeunEinVier]

The cast alloy calipers appear to be made from patterns matching the original calipers.

Shouldn't the patterns have been substantially enlarged (increased wall and rib thicknesses, etc) to account for use of the weaker alloy aluminum material?

[mepstein]

The cast alloy calipers appear to be made from patterns matching the original calipers.

Shouldn't the patterns have been substantially enlarged (increased wall and rib thicknesses, etc) to account for use of the weaker alloy aluminum material?

Early Aluminum S calipers weren't really any larger than steel M calipers. The 914-6 GT seemed to do just fine. The rear aluminum calipers seem to be plenty beefy for the job. Probably overkill. Don't knock it till you try it.

[jd74914]

The possibility of vibration or flex has been handled with the larger hardware.

I read the fastener size increase on PMB's website, but I don't quite know what is has to do with anything in terms of caliper stiffness. I get increasing fastener size as a way to reduce bearing stress on the fastener surface since larger diameter fasteners also have larger surface area bolt/nut heads. The would help if there was concern of boss cracking after some number of cycles.

It seems easier to get 8mm fasteners in all strengths which would be a primary driver for me to change fastener size...

The engineers of the OEM calipers must have designed the iron caliper to be many times stronger than necessary, so it's still adequate when cast in aluminum.

You'd think the engineers would have designed the iron calipers to be as light as possible (for performance and less manufacturing material), but apparently they wanted a larger safety margin and went for overkill.

Yeah, I would have thought they would have done some more optimization too. On the other hand, metal was cheaper in the '70s and 3D FEA wasn't really a thing so maybe the cost of optimization through prototyping was too high?

Are they iron? Or steel? I feel like they are probably steel since it has so much better resistance to crack propagation from casting porosity, etc.

What is it you think might happen?

My concern is always fatigue life with aluminum parts. The problem is that you don't know there is a problem until maybe years later if you are in the high cycle fatigue region. Aluminum has a significantly smaller stress amplitude limit (about 3x less) than steel for the no endurance limit case (ie: the point they will never ever break).

PMB says the calipers flex 0.002" (not really meaningful since it doesn't equate that to any clamping force, etc. but) which is an order of magnitude less than some racecar calipers I've used, but we replaced those pretty frequently too. (IMG:style_emoticons/default/rolleyes.gif)

From a sudden failure perspective, IIRC aluminum's fracture toughness/yield strength ratio is a bit better than cast iron so you're more likely to deform the things than crack them from a casting imperfection which is probably good. Of course, it the OEM calipers are steel the converse is true.

The rear aluminum calipers seem to be plenty beefy for the job. Probably overkill. Don't knock it till you try it.

Just so everyone knows, my comments here aren't to knock them. They just worry me a bit long term since the amount of analysis into them isn't publicized. I'd love to see it though! (IMG:style_emoticons/default/biggrin.gif)