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Re: [Fot] Alighnment killed my TR4 rotors?

To: toodamnfunky@comcast.net
Subject: Re: [Fot] Alighnment killed my TR4 rotors?
From: Bill Babcock <billb@bnj.com>
Date: Tue, 19 May 2009 09:21:33 -0700
It's always possible to get a bubble somewhere that just won't let go.  
Motorcycle brakes are infamous for this because of the small system  
volume, and we had very complex methods for bleeding. One of the  
reasons I like speed bleeders so much. You can pump fluid through so  
fast that there's a much lower likelihood of a trapped bubble. But if  
you want to do it right, bleed from the top down, bleeding at every  
connection under strong pedal pressure. It's messy, but it works. You  
almost always gets some farting from the bubbles of the upper  
connections.

I wouldn't discount the bedding issue either. On higher performance  
race cars it becomes really obvious when brake pads haven't been  
properly bedded--they go away very quickly and/or develop hot spots  
that make the material fail, usually crazing and hardening  the  
surface but in extreme cases making the pad crumble. Follow the  
manufacturers recommended process exactly, and afterwards pull your  
pads and look for high spots. You need to transfer material from the  
pad to the rotor and you need the pad to be flat on the rotor face.  
High spots are hot spots. I see people all the time with pads that are  
obviously not working. But I rarely say anything because no one wants  
to hear that they need to toss them and do the bedding right.

You CAN sometimes reuse a pad that''s been tapered or has hot spots. I  
use a method I copped from Kas--a cupped grinding wheel in a drill  
press with a dead flat surface (an old piece of inconel plate from a  
nuke plant that I've hauled around for thirty years)  to slide the pad  
on. Grind till it's flat. Wear a respirator. Then rebedd it.

If your manufacturer doesn't give a recco, here's the standard approach:

Get on the track, accelerate to a good speed, press the brake gently  
and slow to nearly a stop. drive a little more without using the  
brakes and let things cool a bit, get going fast and then brake HARD  
to nearly a stop. Cool again, brake hard to nearly a stop. Repeat  
about five times, cooling in between. Return to the pits, pull the  
pads and look at them. If you have high spots (isolated, shiny spots  
with no wear around them) your pads aren't flat or your calipers  
aren't pressing evenly. If they got really hot then your seals aren't  
drawing the pads away from the rotor or you have too much residual  
pressure.

Don't assume you don't have residual pressure. The weird little  
Triumph brake line junction is a residual pressure device, and many  
master cylinders come with a residual pressure device installed.

The calipers are supposed to withdraw the pads from the rotors with  
the elastic stickiness of the seal lips. Worn or hard seals, corroded  
pistons, or bowed calipers compromise this.

Girling calipers suck, by the way. For as heavy as they are, they  
don't have a lot of rigidity. A Wilwood ultralight is ten times  
better, give you brakes that can stop on a dime and give you nine  
cents change, but we can't use them. Sure are nice though.


On May 19, 2009, at 8:51 AM, toodamnfunky@comcast.net wrote:

> Michael,
>
>
>
> The masters and calipers are rebuilt with only two races on them  
> prior to this
> past weekend.
>
> jg
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