brake conversion-wilwood - the long and the nerdy

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Bernie, edited and comments inserted...

In a message dated 5/14/02 11:14:52 AM Central Daylight Time,
b.m.benz at prodigy.net writes:


>Note to the List:  Scott and I have been known to get into a little techno
>banter, sparring, now and again. (?)  If this stuff is not of general list
>interest, speak up please.  Then we could all go back to "Was that you I saw
>on Route 69 yesterday?"

Nice load of the gun Bernie  :)

>>Stainless is is only stainless relative to the application.  That
apparently
>>is one of Wilwood's problems.  But, I like your "crud" description.

SS has many iterations, few of which are immune to surface rust.  Use crud if
you like.  All that said, I'd take ANY stainless over the mild steel/chrome
piston arrangement audi uses.  Dust booted (as apposed to the Denver type) or
not.


>>sufficient to keep the piston centered in the caliper
>>bore if there are no latteral forces on it.  And there are absolutely none
>>on an opposed piston caliper and should be none on a sliding caliper if the
>>guide pins are straight, perpendicular to the plane of the rotor.

Lots of "if's" we should be looking at the documentation for.  The guide pins
AREN'T straight, that's one of the reasons floating caliper arrangements are
rarely found in racing.  That's LOT of stress on a guide pin, under a LOT of
thermal stress as well.  Drag torque is a HUGE problem in heat cycling of
brakes bernie.  Add to that a deforming rubber piston, differing metal
expansion rates pison/caliper/carrier/carrierpin/pad, you start speaking
*ONLY* in theory.  Right now, the current brake theory modelling (as late as
SAE 2002) gives piston and carrier/caliper distortion as "fixed".  BUT, that
is only for the modelling exercises, we KNOW (and the paper authors
acknowlege) this just isn't so.

> er, the carrier in a fixed caliper, IS the caliper by definition.  Your
> statement only applies to a floating caliper arrangement.
BB>>You are correct.  Most on this list are more familar with the floating
>>calilper, hence my emephsis.

Hmm, where I originally was asking the question.  You are speaking of
floating (when you responded to Keith, you were speaking of fixed), Keith is
speaking of fixed, and MGW is speaking of fixed.  I'm happy to discuss
floating, as all stock audis use this arrangement.

> Again, you are thinking in terms of "floating" but even there the thinking
is
> not quite correct.  The forces on a pad are NOT even, and the stresses on
> calipers includes designing anti-clamshell tendencies of the caliper
bridge.
> One area where Posrche found Brembos stock application to be lacking (used
> larger and better grade bolts).
>>You are talking caliper "clam shell" deflections to the extent that they
may
>>alter the caliper bore axis from being perpendicular to the rotor and the
>>pad backing plate.  Firstly, this type deflection should be negligable in
>>any caliper worth its salt. But maybe not Wilwoods.

Explain that to Porsche, or better yet, explain that to Brembo.  Porsche
didn't like what they saw, and spec'd a fix.  That said, I'll assign a
chuckle to "any caliper worth it's salt", since we are now speaking of
floating G60's.  Assigning an F1 key to G60's = No worth it's salt.
>
>> Therefore, the piston instability problem to which you allude does not
>> exist.  The piston axis is always perpindicular to its mating backing
plate,
>> therefore is always on axis with the caliper bore (unless the pad has worn
>> tapered).
BB>>>To your above point I will add to my statement: (and unless appreciable >
>clam shell deflection exists)

I'd need a quantification on "appreciable" Bernie.  ANY is critical,
especially in the context of extreme heat cycles.


>
> Bernie  There IS drag of the pad against the disk which tends to "cock" the
> piston in its bore so the piston to bore clearance,

BB>>This effect that you claim is not true, Scott!  The reaction force
>>restraining the pad against braking torque is supplied by the caliper or
>>carrier in which the pad is restrainded.  The caliper bore is an intergal
>>part of, or closely coupled to this restrainment.  Thus, braking torque
>>causes no significant motion between pad backing plate and the caliper bore
>>axis and, inasmuch as the piston is pressure coupled to the backing plate,
>>it is not subject to either a cocking force nor latteral motion within the
>>bore.

Hmm.  Bernie, what happens when you warp a rotor?  What happens to that
warpage as it crosses over a large lateral pad?  If this was caused by
extreme heat,  then we also might consider that the seal is also compromised
= deformed (it is by definition).  Your theory is not supported by the
documentation.  OR, someone needs to explain to me why I have nicks in G60
bores.  "It's not possible" seems a bit weak for me to argue.

>>Your references point to the fact, and I agree that there are major thermal
>>problems in brake design but IMO, the least of which is differential
thermal
>>expansion between piston and caliper bore.  Current seal designs allow a
>>relatively large clearance between the two, and they enjoy close thermal
>>coupling because of the surrounding fluid.  Wilwood may have greater
>>problems with differential thermal expansion because of their use of
>>dissimilar materials.

Again, your "opinion" Bernie is just that.  Current seal design doesn't
support you.  The seal groove is tapered, and the seal isn't made of any
fantastic material, which means it's sensitive to heat.  As a pad wears, you
get a lot more of it transmitted INTO the caliper, THRU the piston.  The heat
of that piston affects the retraction properties of a deformed piston seal
trying to retract an extended piston.  Result is more drag torque = more heat
= failure....

On extension a piston seal is deformed, that's it's design.  A whole bunch of
factors (age, temp, clamp vs sliding force) affect the character of the
deformity.  Right now, the current research is focusing on keeping age and
temp constant and focusing on clamp vs sliding force.  Once that's done,
enter the heat cycle and the cray supercomputer.  I *like* your theory, wish
it could be supported.

>The piston to bore clearance will tend to increase with temperature rise in
>aluminum calipers.

Ok cuz aluminum expands faster than steel...  Doesn't that affect the seal
and retraction properties?  If that clearance gets enough, and the resulting
drag torque generates even more heat, a cocked piston could become a higher
risk problem....

>>Here you admit that it is the piston seal that is responsible for aligning
>>the piston in the bore.  I would add "exclusively".  "heat accumulation"
can
>>not cause higher heat.

True, but convection of more heat sensitive parts becomes a problem.
Quickly.  The piston seal is heat sensitive.  So is brake fluid.

>> Increased heat input will cause increased
>>temperature rise.  Piston retraction is caused exclusively by the memory
>>return characteristics  of the distorted elastermeric seal, the recovery of
>>which is degraded by temmperature.

...  And age, and clamping force vs sliding force, and piston extension to
retraction changing the strain energy.  A huge matrix here.

BB>>  Do you use Viton seals for your racing
>>applications?

I don't think current technology is there yet Bernie.  Designing braking
systems will be tough to accomodate viton IMO.  The problem is that the
matrix of heat includes the heat absorbed by a specific rotor on a specific
hub with a specific wheel.  Right now the current thinking is to leave the
"seal" in the "rubber" category, and keep the heat FROM getting to it.  Someti
mes that's possible.  Many times not.

> adding shims to 2/3 worn pad (especially heat reflecting or
> grate type) will reduce the temps at the piston and caliper compared to a
pad
> with full service on it (a conclusion in the paper)
>>This response is common sense Scott, if one is not nit picking terms.  But
>>it does not address the point of my statement.  If cost were no object we
>>would all be using low conductivity, composite backing plate pads (without
>>shims).

So, IN THE MEANTIME, shims sure appear to be a decent answer.  Or just keep
putting new pads on during "extreme" usage.  There are ceramic coated backing
plates available, even for the G60.  IME, that didn't solve the problem in
the G60, but that's common sense too, I suppose.

>>Scott, you've switched the point of discussion away from my objection to
>>your contention of "kinked and cocked" pistons being caused by braking
>>torque as a reason for employing shims, to one of thermal considerations in
>>brake design.  Different subject.

Hardly.  Try a correlation of 1.  Kinked or cocked pistons will increase the
drag torque, due to the increase in time for the piston to return to it's
original (or close to) rest state.  Drag torque creates a heat problem.
Thermal consideration in brake designs are currently focused on drag torque
(minimal) and piston extension (minimum).

BB>>We started this thread by pushing "cruded" Wilwood pistons into their
seal
>>interface to make room for shims!

You claim the wilwood piston is crudded, not me.  I say use the shims until
you get drag torque.  At a track event this can be a great way to keep the
caliper from failure *before* crud accumlates.  You are focusing on sending a
crudded piston into a seal.  In a G60 with chrome plating and heat cycles,
without piston replacement you are doing that anyway.  I say keep the piston
travel to a minimum.  Keith's point as well.  Do that, only good things can
result.  Once you have crudded a piston, the game is over anyway.

BB>>You must then have a list of Porsche and Audi factory approved PNs for
these
>>"service shims"?

The textars are easy, those shims are available separately, or you can
accumulate a pile of them.  The G60's are too.  Nice that antisqueal plates
can serve a dual function.

SJ