REFLECTOR: Matco Brake History and Modifications

[Jim Agnew]

Hi Matco Brake owners,
 
My pictures referenced in the folllowing note have been uploaded to the following site.  The text that follows has not yet been uploaded.  Remember that this is early testing and results are still being evaluated.
 
Jim                                                   Jim_Agnew_2 at Yahoo.Com 
 
http://www.tvbf.org/gallery/?dir=subsystems%2Fbrakes
 
 
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Matco Brake History and Modifications
 
I have never been overly impressed with the performance of the Matco brakes on my Velocity 173 Elite FG and have sought answers and ways to improve the performance.
 
First a little brake education, all brakes have a maximum capacity to absorb and dissipate the energy required to stop. As you approach the maximum capacity the brake components become hotter and hotter and the available PAD-ROTOR stopping friction becomes less and less until it basically becomes almost zero.  The major cause is the melting of the pad material and the resulting liquid acts like a lubricant.  When the brake pads & rotor get hot enough you also get vaporization and you add vapor layer to the liquid so you now have the equivalent of an air bearing.
 
Three things directly impact the amount of energy that is required to stop your aircraft, the amount of energy used prior to braking (i.e., steering prior to an aborted takeoff), gross weight and True Touch-Down Speed.  Brakes that work perfectly well at 70 KTS indicated and maximum gross weight at low density altitudes may be inadequate at high density altitude. 
 
I purchased my kit in 1995 and at that time the Matco brake model WHLB600XT had a three piston caliper with 1.25” bores that were later changed to 1.5”. This change, given the recommended operating brake pressure, increased the caliper pressure from 5,556 lbs. to 6358 lbs a substantial increase. The calipers were attached to the fixed pad with four socket head bolts with tubular spacers.  This allowed the caliper and fixed pad to flex out at the bottom under high brake pressure reducing their efficiency.  Matco replaced the two center spacers with a solid aluminum block that reduced the flexing and improved the braking.  Next they changed to a more aggressive metallic brake pad and this improved the braking.  However, I still felt that the brakes could be improved.  The original Rotors were made of ductile cast iron.  Ductile cast iron is soft compared to steel and contains graphite which acts as a lubricant.  The effect of the graphite on large
 capacity rotors is probably marginal however on rotors operating close to maximum it can decrease stopping power.  If you have older calipers you should check to make sure that all of these modifications have been installed.
 
In the following comments please refer to the pictures in my start of modification process. All of the photos are Hi-Def so you can zoom in closely.  Download the images to your computer and you can zoom them a lot!
 
The old rotor was heavily scored as were the pads after about ten hours with the new metallic brake pads and maybe 60 landing and taxi cycles (see 1-Old Rotor & 2-Wear on Old Rotor).  I decided to consult with my good friend and International racing driver Amos Johnson.  One look at the old rotor and his comment was that the pads were too aggressive for the rotor!  Amos decided to bring two close associates into the discussion, one an expert on brakes and especially pad material and his former machinist that worked for his company building & racing cars for GM, AMC, & Mazda.  Our discussions settled on two approaches, first slotting the rotors to vent the pads and second the possibility of changing to some formulation of Carbon-Metallic brake pads. I decided to go one step at a time to test the improvements. Since the original rotors we in poor condition I decided to order a new set of rotors.
 
The new rotors arrived and to my surprise they were steel (not ductile cast iron) and about ½” larger in diameter (see 3-New Rotor) with the same rim bolt pattern. Note- if you have the old rotors they are the same size as the rims, the new ones stick up about 1/4” above the rim. 
 
I decided to start with slotting the rotors and sent them to Amos’s machinist for slotting. (See photos 4-New Slotted Rotor Inside, 5-Slotting Close-up, and 6-Slotted Rotor Outside) They received three slots, 45 degrees to the pad path and 120 degrees apart and slightly wider than the actual pad path to minimize material removal.  The opposite side slots are offset 60 degrees from the other side.  The slots are 1/8” wide and 0.025” deep and their wiping action allows the liquid and gas to vent from the area between the pad and rotor thus increasing friction and assisting cooling.  The slots also act as rotor wear indicator and when they are gone the rotors are at minimum specs.
 
Preliminary results with a new set of your pads (after brake-in) show a very even pad wear path, smoother steering (differential braking), and during high speed (70 kt) braking (two stops back to back) with little fade I still had adequate braking for steering and taxi.  Using a laser optical IR pyrometer showed reasonable pad & disk temperatures. 
 
I feel at this point that the slotting is lowering the pad & rotor surface temperatures by allowing the liquid & gasses from the melting pads to escape not to mention the lubrication effect of these products.  I will put more time into high speed testing and landings before I go to Carbon-Metallic pads if necessary as well as watching for excessive scouring. 
 
Since Carbon-Metallic pads need to heat up for maximum efficiency this is one consideration and some all out racing are very hard to modulate the recommendation at this time should I continue with C-M pads is to use “street stock” racing pads.
 
I will post additional information.  
 
Jim 
 
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 *** Note ***  One new high speed taxi test at 70 kt with additional  300 pound load showed no appreciable difference in braking.  Note, braking efficiency may be improving due to additional pad seating.  There is some difference in the R-L pad path that shows a minor difference in seating.   
 
 
 
 
 
 
 
 

 
James F. Agnew
Jim_Agnew_2 at Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying 
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