TRS (Transverse Rupture Strength) measures tensile strength of the outside surface.  Regarded as an indication of rivet holding capacity.  Average readings taken on any one part should be above 5,000 pounds per square inch.  The stronger the better.

The Chase machine is well established as a means of determining friction levels.  AAMVA certification and edge codes are based on the results of this test.  It does not, however, accurately predict how a friction material will perform in a brake.  A sample for Chase testing is only 1"x1"x1"!  A sample this size may not be representative of a full set of blocks.

Look at the three typical Chase test results: A, B & C.  Let us analyse them.

The test procedure is according to SAE standard J661a.  It begins with a bedding in of 20 applications, 10 seconds on, 20 seconds off, with friction readings at every fifth application.  This is followed by a drag test where the test drum temperature is allowed to rise to 550°F.  Friction reading are taken at 50°F intervals.  During the recovery part of the test the drum is allowed to cool, and the brake is applied, and friction readings taken, at 100°F intervals.  The wear portion of the test consists of 100 applications at 400°F, 20 seconds on, 10 seconds off.

This is followed by a second fade and recovery test.  Similar to the first, but with temperatures going up to 650°F.  Finally a baseline like the one at the beginning.

Test samples are weighed and thickness is measured before and after testing to get some idea of wear.

AAMVA edge codes indicate the normal and hot friction values based on this test - see "What is an Edge Code?" (SAE J866a)

Friction Characteristics

Materials A, B & C are all rated as FF.  This means the average friction up to 400°F (normal) and generally over 400°F (hot) are both within the range .351 to .450.  All three materials exhibit basically the same friction level during the 100 wear applications.  Material A tended to lose friction level drastically above 500°F in the second fade portion - above 600°F the friction was so low that no work was being done - not enough to lift the drum temperature to 650°F for a final reading!  And it took quite a time for the original friction level to be regained upon cooling.  This low friction at high temperatures results in a FE rating for this sample.

Sample B exhibits higher friction during the fade portion around 400°F, but does not fade below its initial value.  A material that does not fade when temperatures get high can be dangerous.  A driver is not aware the brakes are hotter and does not adjust his driving to compensate.  Brakes can get hotter and hotter until the tires burst, or the brake lining catches fire.

Sample C exhibits a steady friction level throughout the test with just about the right amount of fade above 550°F.

Durability

Examine the wear data.  With a stable material the thickness loss and the weight loss should be relative to one another.  If thickness loss is proportionately less than the weight loss then the material is swelling.  Note samples A and C both have some degree of swell.  Sample B looks as though it could be shrinking!

Comparison of SAE J-661a Tests on Samples A, B & C

Effectiveness test measuring the efficiency of the brake at different line pressures. The minimum effectiveness permissible is indicated by the dotted line.

Fade test where ten stops are made at a high deceleration rate.  The line pressures needed to meet the required deceleration rate are displayed on the graph.  A high line pressure is indicative of a less effective brake; the maximum pressure should not exceed 100 psi.

Recovery test where 20 stops are made at lower deceleration rates.  The maximum pressure allowed is 85 psi.

Factors affecting the performance of a brake are the gross axle weight, the tire rolling radius, the AL factor (air chamber size x slack adjuster length) and the drum weight. (A light weight drum gets hotter than a heavy one during braking).  So it is important when comparing test results all these factors are checked - that we are comparing "apples with apples".

These three tests were carried out on the same dynamometer under the same testing conditions.  All tests were done within a few days of one another.  They should be directly comparable.

The three test results are basically very similar.  Variations which might have been predicted from the Chase tests on blocks taken from the same box did not materialise.

Effectiveness

Sample C is slightly more effective than the other two; comfortably above the minimum requirements.  Expect this material to perform well at a higher axle rating.

Fade (Power Stops)

Sample B fades during the fade (power stops).  (It requires more line pressure to maintain the same level of deceleration).  This is contradictory to what would have been forecasted from the Chase results.  Although less effective that the other two, pedal pressure does not exceed the 100 psi maximum.

Recovery

Samples A and C behave quite similarly during the recovery portion.  Sample B exceeds the permissible maximum on the first stop, but regains its friction level by the end of the test.

Durability (Drum and Lining Wear)

Wear results on a short test like this do not mean a great deal.  Samples A and C have similar lining and drum wear.  Yet the difference between drum wear with samples B and C is only .002" on diameter - .001" in drum wall thickness.  Anyone who has measured a drum will appreciate the difficultly in measuring to such accuracy with normal drum micrometers.

The leading shoe data on sample A and C suggests some swell and growth, but this is contradicted with the data on the trailing shoes.  The similar weight loss of sample C, in spite of its higher specific gravity, would suggest it may last longer.  In practice do not expect to see much difference in overall performance between all three.

Inertia Dynamometer Test Conditions

Comparison of FMVSS-121 Test on Samples A, B & C