QUOTE(ConeDodger @ Aug 25 2016, 04:20 PM)
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...
QUOTE(NeunEinVier @ Aug 25 2016, 05:35 PM)
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.
QUOTE(ConeDodger @ Aug 25 2016, 04:20 PM)
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.
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.
QUOTE(mepstein @ Aug 25 2016, 04:38 PM)
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!