Do new brakes rods really stretch??

[Randy in ca]

I'm confused about this subject. is the conclusion that repro parts will break but not stretch?

Bent or broken yes, but stretched ? If so I am sure it would take a micrometer to measure the result !
Wayne

I have trouble believing brake rods stretch.......

An object will always distort (ie compress or stretch) before it breaks. Even the most minute force will cause some distortion - it may be so minute as to be virtually undetectable, but it still happens. The elastic properties of the object determine how much force can be applied before it doesn't return to its original shape when the force is removed. This property varies greatly in different types of steel - not quite like comparing bubble gum to writing chalk, but still can be significant.

When the force applied to an object becomes high enough, such that it no longer returns to it's original shape when the force is removed, it is said to have attained it's Yield Strength, after which permanent deformation (stretching) starts. If further force is applied, the object will continue to distort until it eventually breaks. This force is referred to as the Ultimate Tensile Strength.

The Yield Strength of steel is always lower than the Ultimate Strength, but the difference in the two can vary greatly from one type/grade to another. Some will break soon after they start to distort (like chalk) while others will stretch a lot before they eventually break (like bubblegum).

For a brake rod to stretch and not return to it's original shape, it must be subjected to a force greater than it's Yield Strength and additional force will always be required until the Ultimate Tensile Strength is applied and the rod breaks.

In short, the brake rods are constantly being stretched with every brake application. The question here is if the force on them ever becomes great enough that the don't return to their original shape and if enough additional force can be applied to result in breakage.

I hope this makes sense and I haven't miss worded or screwed something up too badly!!

 

[BRENT in 10-uh-C]

Randy, I am not going to copy what you wrote due to the lengthiness but the original reason why I posted this was because someone claimed that repro brake rods were stretching thus requiring additional adjusting. I do agree there is some in detectable stretching/compression from components throughout the system however I'm not sure any of those would require readjusting the brakes.

FWIW, sometimes it is fun to watch how a topic will receive an answer or two and then find itself making a 90 degree turn to chase a different --but quasi-related subject, ...such as the crankshaft deflecting while laying on the table. So for those of us who played with the Slinky when we were kids, shouldn't it be more disconcerting for a crankshaft left hanging on a rack to stretch longer? Remember what happens when a Slinky was picked by the top few coils and pulled upward. Surely it is plausible that a crankshaft hung by the flange would stretch over the length of the shaft just due to its' own weight! Why aren't we addressing that??

 

[RockHillWill]

I think that the issue with a crank 'distorting' radially from laying horizontally is of much more concern due to the fact that the highest loads are applied radially when in operation, whereas the fact that horizontal 'tolerances' (endplay) are actually part of the built in function of the crank. If the crank is balanced, the offset loads due to the rod journals are equally spaced, minimizing any radial deflection due to those offsets.

 

[BILL WILLIAMSON]

D.O.T. doesn't test EVERYTHING!! They just pretend they DO! It's called, JOB SECURITY & GREAT RETIREMENT BENEFITS!!
Bill W.

 

[briphaeton]

I agree with Bill. My machine shop teacher said the whole world is rubber, everything bends. While he was teaching at Lawrence Lab in Livermore, they measured someone leaning on the headstock of a lathe. Only the lab would have the ability to measure something so minute. My neighbor bracket raced a early camaro and painted a line on his axles so he could see how much the axles would twist. Those old mechanics and machinist really have a understanding how things work and we are losing so much knowledge as each one passes. Old Henry had many, many engineers working to give us this great car to enjoy today.
Brian

 

[Smog Tech]

When I was much younger we would paint the line on axles used in drag racing cars. Quarter turn, replace the axle.

 

[BILL WILLIAMSON]

When I was much younger we would paint the line on axles used in drag racing cars. Quarter turn, replace the axle.

Trick idea! My drag racin' cars never developed enough power to wurry about axle twist. Unlike them ooooold stock Chevys, that would "POP" an axle when you backed up. A friends '29 Chev popped one on a tour, a welder/machinist slipped a water pipe coupler over it & welded it, then straightened it in a lathe. Drove it that way for a looong time.
Another time, he popped a sector shaft while making a "U" turn which resulted in a BROWN spot in his seat
Bill W.

 

[Patrick L.]

When I was much younger we would paint the line on axles used in drag racing cars. Quarter turn, replace the axle.

Yup ! It sure beat buying those super duper expensive Summers Bros units which would just shear at the splines.

 

[Patrick L.]

Randy, I am not going to copy what you wrote due to the lengthiness but the original reason why I posted this was because someone claimed that repro brake rods were stretching thus requiring additional adjusting. I do agree there is some in detectable stretching/compression from components throughout the system however I'm not sure any of those would require readjusting the brakes.

FWIW, sometimes it is fun to watch how a topic will receive an answer or two and then find itself making a 90 degree turn to chase a different --but quasi-related subject, ...such as the crankshaft deflecting while laying on the table. So for those of us who played with the Slinky when we were kids, shouldn't it be more disconcerting for a crankshaft left hanging on a rack to stretch longer? Remember what happens when a Slinky was picked by the top few coils and pulled upward. Surely it is plausible that a crankshaft hung by the flange would stretch over the length of the shaft just due to its' own weight! Why aren't we addressing that??

Now I know why all my cranks shrunk/shrink ! I'm one that sets them on the flange, so, thats why they get shorter !

 

[KR500]

Brent, If they aren't scrutinizing the drums, shame on us for having just another government subsidized useless department with no concern for our safety. Grade 5 bolts which aren't and the list goes on. I must agree the government doesn't give a hoot about us. I guess the manufacture of tires and tubes should be included.After hearing of General Motors faulty ignition switches any junk somebody turns out gets dumped on us poor fools.