[200q20v] Re: Caliper piston extension distances

[Bernie Benz]

Hi Mich,

Very sorry to have misspelled your name.  Engineers are lousy spellers.

> From: "Michael \(Prospeed Motorsports\)" <michael at prospeedmktggrp.com>
> Organization: Prospeed Motorsports
> Reply-To: <michael at prospeedmktggrp.com>
> Date: Thu, 8 Nov 2001 21:21:15 -0600
> To: "'Bernie Benz'" <b.m.benz at prodigy.net>
> Cc: "Audi S Cars" <s-car-list at yahoogroups.com>, "Audi 20 Valve list #1"
> <200q20v at audifans.com>, "Audi Quattro List" <quattro at audifans.com>
> Subject: RE: Caliper piston extension distances
>
> First, I will apologize in advance for the "pissy" attitude I
> demonstrate in the response below.
Expected and accepted.
> I am one of few people in the country
> that specialize in rebuilding and reconditioning Porsche/Brembo
> calipers. I was merely attempting to share the straight facts with the
> group. Once again, I apologize for my response in advance... But I tired
> and its been a long day.
>
> BTW... A big "THANK YOU" to the few guys (you know who you are!) that
> took the time to call today and tell me personally how much they enjoyed
> my knowledge and straight facts when it comes to these brake systems.
>
> Everything I have shared with this list is based up personal hands on
> experience. One of the services I offer is to rebuild Porsche/Brembo
> calipers. I am known in the Porsche market for being the only stocking
> source for individual Brembo parts (seals, bleeder screws, rubber caps,
> etc.) I sell these parts and provide consultation to customers in the
> USA and Europe. And I design Big Brake systems for various Porsche parts
> vendors. Does this mean I know everything... No. But it certainly means
> I spend an inordinate amount of time doing nothing but brake caliper and
> rotor work.
Impressive, but repetitive.
>
> My final comments are interlaced below. I will not address this topic
> any longer except privately.
>
>> -----Original Message-----
>> From: Bernie Benz [mailto:b.m.benz at prodigy.net]
>> Sent: Thursday, November 08, 2001 8:24 PM
>> To: michael at prospeedmktggrp.com
>> Cc: 200q20V mailing list
>> Subject: Re: [200q20v] Caliper piston extension distances
>>
>>
>> Of course, Micheal,
>
> BTW, my name is spelled "M I C H A E L"
>
>> you should practice what you preach, but
>> IMO, your hypothesis is shear speculation on your part, not
>> based on factual failure modes.  Further comment interlaced below.
>>
> I do practice what I preach. I don't do brakes with incorrectly sized
> rotors.
>
>> Snip
>
>> The dust boot cannot disengage from the caliper with the
>> piston installed as the boot lip will not pass through the
>> piston to caliper clearance, which is so designed to retain
>> the boot.  Further, a good boot will not rip before its
>> piston retaining lip will pull from its groove and slide down
>> the piston.  So, no problem with the boot.
>>>
> Wrong Wrong Wrong... I pop these things all the time. If fact this
> morning, when I took the measurements from an 928 S4 caliper, I put an
> air hose to the caliper inlet, applied air pressure and the pistons and
> dust seals popped out. Yep... That's right, the dust seals came off the
> caliper body from hyper extension of the piston.
>
> Try it sometime...  It works!
With air pressure it happens with a quick pop, but the piston first cleared
the bore which in turn released the boot lip from the caliper recess.
>
>> snip <
>
>> What forces do you think exist to retract the piston?  Beyond
>> that of a warped rotor, the only piston return force is that
>> of the pressure deformed elastormeric piston seal returning
>> to its natural cross section.  Good for a few thou only.
>> Further, there are no forces tending to cock the piston
>> within the bore, short of a tapered friction surface on the
>> pad.  And inasmuch as there is no metal to metal contact
>> between piston and caliper bore, even a tapered friction
>> surface will not cause piston binding, in any practical situation.
>
> No again... There is a vacuum that also help to retract the piston...
> Remember this is a closed hydraulic system.
Wrong!  The system pressure on the negative side is limited by the
atmospheric pressure on the resorvoir fluid, the master cylinder seals
purposefully being U seals capable of sealing in one direction only, thus
allowing makeup fluid into the system to compensate for pad wear.

> And for that matter, they don't have to retract far to release the pad
> from the rotor.
Just about as far as the elastormeric piston seal return force is capable of
providing.  Good design.

> And there
> are lateral forces acting on the piston... Remember, the wheel/rotor is
> rotating and there is friction between the pad and the rotor, so if the
> pad wasn't held in place the friction would move it in a lateral motion.
That statement IMO, is naive!   All who have ever replaced a pad realize
that the pad is restrained laterally only by the carrier and in no way by
the unattached piston.

> So, even a pad contained in a caliper has a slight bit of lateral
> movement.
A pad is not contained by a caliper, except from falling out the back side.

> This force can cock the piston. Granted, not by much, but the
> clearance are so tight it doesn't take much to cock the piston in such a
> manner that it will not move in the bore at all.
BS!  The clearances are not tight relatively speaking, and even with the
piston fully within the bore there is ample angular clearance for the
piston's latteral movement without metal to metal contact, piston to bore.
Upon pressure release the elastormeric piston seal will again center the
piston axis in the bore.
>
> And, in the earliest S4 caliper designs, there was NOT a dust seal as we
> now know it. There was a second pressure seal located at the top of the
> piston bore. Porsche called it a "scrapper ring"
Porshe used the correct technical terminology, and it is not a second
pressure seal.  All hydraulic cylinders including automotive shock
absorbers, not using a dust boot, use a rod scraper to exclude dirt and
protect the pressure seal.
> So in this case there
> were not be the deformed elastormeric piston seal you previously
> referred to.
By design, all elastormeric seals must deform to seal.

> And the Brembo calipers used by most, if not all of the
> NASCAR teams do not have dust seals or scrapper rings.
Not necessary for sealing purposes.
> They have an
> inner pressure seal and that's all. This would also blow your deformed
> elastormeric piston seal theory.
Not in the least.

> Granted they rebuild the calipers
> following every set of laps (i.e. qualifying laps, practice laps, race
> laps, etc.)
Probably necessary without dirt exclusion devices.

> I probably talk to the world's largest Brembo race
> distributor (their self description) at least once every couple of
> weeks. I have even recently spoken with the guy in charge of Brembo's
> racing program as it relates to NASCAR. He's a very nice guy...
Obviously, talk does not an expert make.
>
>>>
>> IMO, electrical pad wear sensors are totally redundant
>> (unless you like to see red lights) excepting for the
>> unlikely special case of worn out pads being used on new or
>> remachined rotors.  The rotor/pad configuration is its own
>> best warning system.  Any rotor wear will leave raised ridges
>> either side of the wear surface, which will be the first
>> surfaces to contact the backing plate with a loud warning
>> that pads are ready to replace, long before the wear surface
>> is damaged.  This system works, I use it.
>
> I have no idea where this came from.
Just threw it in to see if you'd bite.

>.. And frankly don't tell me. I
> don't care. I did not ever mention electrical pad sensors and in the
> scenarios I described there was still enough pad material remaining that
> there would not be any metal to metal (rotor edge to backing plate)
> contact. For that matter, in the 996tt with 30 mm rotor scenario, if the
> rotor was worn so much as to produce a ridge of several mm and the pad
> was also wore down to the point the few mm of rotor ridge would contact
> the backing plate, you would be losing the piston from its bore.
A bad design!
>
> BTW, that is an extremely unsafe system you admit to using. Hard to
> believe someone that thinks they know so much about the flaws in my
> rationale would tell the group they basically don't inspect their
> brakes.
Never said that.

>.. They wait until the pads/rotors are making noise from
> metal-to-metal contact.
>
>>
>> I have seen Ford vented rotors worn to the point of the internal
>> vents were showing through the wear surface (slotted rotors
>> are good, yes!) without the hydraulic system being compromised.
>
> So what??? We're not talking about the Ford caliper/rotor system. I have
> no idea what the tolerances for such as system are and don't care. I
> don't do Ford brakes.
>>
>> Don't speculate on nonexistant problems,
>
> I'm not speculating. I am 100% positive that with the correct variables,
> brake system failure will occur.
Sounds like some kind of a double negative speculation to me.

Are we having fun?

Bernie
>
>> and "Don't fix it if it ain't broke"
>>
>> Bernie
>>
>>
>>> Michael G. Wachholz
>>> Prospeed Motorsports
>>>
>>> 952.249.1169 office
>>> 952.475.3267 fax
>>>
>>> 400 Pond Ridge Circle
>>> Wayzata, MN 55391
>>>
>>> mailto:michael at prospeedmktggrp.com
>>> www.prospeedmktggrp.com
>
>