brake conversion-wilwood - the long and the nerdy

Mon May 13 15:25:11 EDT 2002

Comments inserted

>First, as a pad wears, more of the piston
> is exposed to the elements in a non sealed caliper.  This means (SS or not)
> that surface rust forms on the piston,
BB>>If rust forms on a ss piston, it's not stainless.  That's Wilwood's

Not with you bernie.  There are several references wrt ss that don't support
your statement at all.  I haven't seen a piston yet in a caliper that is
"immune" to rust.  That said, below I change rust to "crud" cuz salt has a
way of taking the best of the metals, and turning them to crap accumulators.

> the further the pad wears, the more
> rust area will form.  As this crud gets scraped off,
>>What is scraping it off?  Happens in service or during inept overhaul?

Bernie as a pad wears, you can add shims behind it.  BEFORE a pad wears, you
can add a shim behind it.  I say add enough shims BEFORE a pad wears so that
the pad doesn't touch the rotor in the rest position.  This can be applied to
G60's (that's one THICK pad), big reds, or wilwoods.  You keep speaking of
"service".  "Attention" would be a better word.  You can shim before
servicing any caliper.  WRT shims, we are speaking of optimizing piston
travel (less is better), not rebuilding or "servicing" the caliper, in fact,
not even taking it off the strut.

>>What you absolutely don't want is for the piston seal to scrape it off when
>>the pistons are forced back into the caliper to make room for shims or new

Chickens before eggs Bernie.  Think track event, or think BEFORE the pad has
ever touched a rotor.  Or think before "accumulated" crud.

>wear of the piston bore occurs, more crud = more potential wear =increased
service interval.

BB>>By the piston bore I assume that you are referring to the caliper bore in
>>which the piston seals.  What wear?  In service the piston never touches
>>caliper bore.

I'll chuckle at that statement.  Any piston (especially G60's) can not only
"touch" the cylinder bore, but score it.  Those are the ones I core to Napa
:).  When calipers twist and pistons are hyperextended, all bets are off.  In
service, the only thing attempting to keep a piston from hitting a bore is a
rubber seal.

BB>>Scott, I just don't know where to start on your apparent misconceptions
>>above, but I'll try.

>>First, the pads are rigidly restrained from moving tangentally with the
>>rotor by the carrier, not the caliper.

er, the carrier in a fixed caliper, IS the caliper by definition.  Your
statement only applies to a floating caliper arrangement.

> There is no force resulting from
>braking torque exerted on the caliper!  If braking force is sufficient to
>deflect the carrier and inasmuch as it is mounted to the bearing housing off
>center of the rotor, this deflection would cause an inperceptively slight
>twist at the business end of the carrier which is holding both the pads and
>the caliper.  But the caliper is just going along for the ride with the pads
>and sees no change, twist, relative to the pad or its slides.  There is no
>twist of the caliper relative to the pads or relative to its mounting points
>on the carrier!

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).

>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

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, thermal coefficients of
expansion between piston and caliper as well as seal design and location are
crucial.  If you look at some of the SAE articles published on thermal
coefficients of expansion (try 1999-01-0483 - Numerical Prediction of Brake
Fluid Temperature Rise During Braking and Heat Soaking, or 2000-01-0445 - An
Analytical Method to Predict Thermal Distortion of a Brake Rotor or 942086 -
Evaluation fo the Energy input in Pad and Disc During Brake Application, or
880256 - Approaches to the Thermal Modelling of Disc Brakes), you can see
that piston and bore clearances are affected by temperature greatly.

 >> This latter unusual condition may cause the piston axis to not be
>>congruent with the caliper bore axis, but never enough such that the piston
>>surface contacts the bore.  The piston seal keeps the piston centered in
>>caliper bore, such that there is never contact between the piston and
>>caliper bore and therefore no associated wear.

Opinion Bernie, not supported by fact.  Thermal loads can get high enough
that piston s can touch the cylinder wall, it happens all the time with G60's
at track events.  Look, if you read some of the papers above, you will see
that typical "extreme" heat soaking occurs in under 30 minutes of brake heat
cycling (that's about the length of 1 quattro club run group).  Once that
temperature gets high enough, all bets are off.  What you also find is that
temperatures of aluminum bodied calipers with semi metallic pads generate the
highest "extreme" heat (compared to iron calipers with SM or organic pads OR
aluminum calipers with organic pads.

>>The only heat generated in the braking system is generated by friction
>>between the pads and the rotor.  There is no other source of heat!  Twists
>>and kinks, real or unreal, do not generate heat.  You have kinky pistons?
>>Kinky pistons must be bad!

A true statement, there is no other *source* for heat.  But the resultant
heat acuumlation can cause higher heat, and can be measured in the
caliper/rotor/fluid/piston.  What you find with "cocked" pistons (hey, that's
the term SAE uses)  or "overextended pistons", is that the ability of the
piston to retract becomes very high.  That causes more heat to build up,
which leads to premature failure of dust seals, as well as the ability of the
piston seal to properly align the piston in the bore.

>>My comments are general, applicable to any braking system excepting where
>>stated as Wilwood related.  Piston travel relative to its seal interface is
>>related only to pad wear, therefore with pad wear the seal is moving onto
>>the clean internal piston surface, never the other way, excepting when one
>>forces the piston back into the caliper bore (without overhaul) for shim
>>addition or pad replacement, thus forcing the dirty external surface
>>the seal interface.

Hmm, I don't read a lot of application, only theory here Bernie.  The optimal
pad in terms of heat, is a new one.  As you move a pad closer to the rotor
(wear), the heat *thru* the backing plate gets higher (see -0483 above).
Without question, 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)

> When you speak of ANY aftermarket brake combination, shims are a key factor
> in them.  For more on this ck the archives, I went a round with MGW on this
> exact point.  Bottom line:  The less piston travel (*however* you achieve
> shims are the easiest way) over the life of a given pad, the less service
> caliper will require.
>>As I demonstrated above, there is absolutelly nothing negative about
>>piston movement compensating for pad wear, nor does it result in any
>>required caliper service.

No Bernie
-0483 "Case 3:  Brake system with aluminum caliper and two thirds worn NAO
(organic) friction material"

"As friction materials wear out, more frictional heat transfers to the
caliper resulting in high brake fluid temperature and hence the brake system
with worn friction pad should be considered"

Table 2 shows the operating fluid temps increased by 50C at 30minutes, and
Max fluid temp during heat soak increased by 53C to 147C (150c is the wet
boiling point).  If you look at SM pads, a new pad is already over the 150C

>>Smart caliper service is performed in preperation
>>for new pads,   Not so smart caliper service must be performed as a result
>>of caliper seal failure caused because of dirty pistons being forced into
>>the caliper bore (obviously such seal failure is much less frequent with
>>booted pistons than open).

Me, I'd be pulling the boots off and replacing them before and after track
events.  It's not "dirty" pistons we are speaking of.  It's worn pads and
heat Bernie.

>>Scott, I have never questioned, and have high respect for your service
>>experience.  But IMO, the use of shims is an old wive's tale (tail),
>>excepting possibly in the use of unbooted pistons on dirt track, aka

I routinely shim booted piston calipers Bernie, even porsche does with the
Big Reds (stock application).  It's NOT a wives tale, it's sound and good
practice over the "service life" of any caliper pad.

>>How does one run "out of master cylinder"?   Are you talking about running
>>out of pedal stroke?  No, the pad to rotor clearance established by the
>>flexing of the piston seal remains constant through out the full thickness
>>life of the pad.  No, not the brake fluid resorvoir volume, as it is
>>sufficient to allow backing plate to rotor contact on extreemly undersized
>>rotors without sucking air.

Bernie, you can easily run out of brake master volume.  Your above only holds
true of equal total piston area.  As it the case with most 4 pot brakes,
accomodating the front conversions is *usually* adequate, accomodating 4 x 4
pot usually isn't.  It's strickly a surface area/volume of fluid equation.

>>I hope that we are still having fun.

Bernie, I enjoy reading your opinions, but here, the documentation supports
little of your opinned theory.  Even after the multiple reads, Keith Tackett
appears to be closer to practice *and* theory.

Shim away, I say.

Scott Justusson
t44tqw big reds with shims

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