Do new brakes rods really stretch??

BRENT in 10-uh-C

BRENT in 10-uh-C

Well-known member
I read here recently by one poster who stated that the new brake rods have been known to stretch, ...and I started down that road trying to decide whether I want to "buy-into" that mindset/theory.

So let me ask this in this light, ...the new brake rods are .305" in diameter ( 19/64 ) of what I am going to guess is mild steel. I suppose if I wanted to calculate the tensile strength, I could find a formula but I am going to guess-timate that there is a 5,000 yield strength. If someone knows for sure or can calculate this, please do so.

Therefore if that 5k strength on each rod is the case, how is a Model-A brake pedal able to generate that amount of force to effectively stretch a new brake rod? In our discussion around the shop, there are ways that original rods were stretched over time but most of the time we have found the rods were purposely bent to take up slack created by worn internal brake parts. So can anyone tell me of their first-hand experience where their new 50¾ inch brake rod has stretched to 51 or more inches long?

In addition to that, how does one of these brake rods stretch that much yet no one ever complains about the single rod that connects the brake pedal to the cross-shaft. Does it stretch?
 
brent thanks for posting this as i could not comprehend how the rods would stretch. of course i did not do any of the math
 
I'm on the same page. I would be shocked if the rods gained .005" in 5k miles which of course would have no impact. Other brake issues that would falsely leave that impression are more common than not.
 
Maybe as other parts wear over the years there is an illusion that the rods stretch because if anyone adjusts the brakes like I do, there are times when the rods need to be readjusted. I've never concerned myself with stretching rods, just wearing parts.
I'm pretty sure I'm not strong enough to lengthen one all by my lonesome.
But, then, if these things are now being made in china, maybe they're made from lead.
 
Brent,

5/16"...1018 steel yield strength is 40,000 psi,..and the Rockwell is in the
"B" scale...

Bolt torque on a 5/16" "grade 8" is 27 lbs...this part should make you think
about the "grade" and quality of the threads on the rods..

I wonder if Ford used some form of ChromeMoly.....it would be nice to
know whats on the print?....and the forged end?
 
I think it's closer to 50,000 psi, don't know if you missed a zero Brent. Real quick math, 3,500 lbs. force before rod yields. A measly 1/4" bolt will hold 780 lbs. (Before anyone chimes in, I realize there should be a factor of safety and this is for a pure static load!)
 
d.j. moordigian said:
Brent,

5/16"...1018 steel yield strength is 40,000 psi,..and the Rockwell is in the
"B" scale...

Bolt torque on a 5/16" "grade 8" is 27 lbs...this part should make you thinkabout the "grade" and quality of the threads on the rods..

I wonder if Ford used some form of ChromeMoly.....it would be nice to
know whats on the print?....and the forged end?
Click to expand...


So are you suggesting in the cut threads (in lieu of rolled threads) is where they are stretching??
 
From a handbook on my desk-
5/16 -24 thread:
Grade 2- Proof: 3200 lbs Fail: 4300 lbs
Grade 5- Proof: 4900 lbs Fail: 6950 lbs

Even if the rods are fairly low grade, like 1015 steel, ONE should be sufficient to lift the entire car without stretch. Henry's were likely closer to grade 5.

Of course, the driver stompin' on the pedal may have looked like this:
4142s3zCuFL._SL500_AA300_.jpg
 
BRENT in 10-uh-C said:
So are you suggesting in the cut threads (in lieu of rolled threads) is where they are stretching??
Click to expand...

Ya,..I like the rolled threads, with a full grain structure vs. the cut threads!
I just hadn't thought about it much,...I use original rods. Also my car
has the "one piece" non-adjustable rods.

Rod stretch...I just don't know... This could be a good shop project for
some good guy with a bunch of A's in various forms of disrepair, or is
it repair....I guess it depends on which way there going. They could
pick a car with "bent too adjust" rods and a real big fishing scale...


lol
 
Another consideration is what you get is what you think, I have busted, twisted and stretched many of "quality" tools with life time replacement warranty. But the so called stretch maybe from tolerances getting comfy.
 
I too doubt that straight rods stretch. I suspect the statement "stretched rods" might come from the later Ford mechanical brakes that used cables rather than rods. It is more likely that the cables would stretch and the term "stretch" became attached to mechanical brakes in general.

Just my thoughts
 
Rolling the threads pre-disposes the grain structure towards more easily stretching than having cut threads which is more likely to produce stress 'fractures', but I believe the reason for going to rolled threads had more to do with not having to pay the additional labor to clean up the thread shavings and to haul them off than for any engineering concern. Also saving in re-cycling costs.
I am estimating all the numbers here, but at the 50,000 psi tensile strength number, and Area = (pi x r squared) the bottom of a 5/16" thread is closer to 1/4" in diameter and has about .08 square inches in cross section. .08 time 50,000 psi equals about 4,000 pounds. If the arm below the pedal shaft is about 2" and the pedal swing arm above the pivot shaft is about 12" long, than the force 'multiplier' at the brake pedal is 6. 4,000 pounds divided by the pedal arm 'multiplier' brings the load required at the top of the pedal to 'stretch' the metal to about 660 pounds. Not likely to stretch brake rods as an ever day occurrence, but a 'big boy' having a 'bad day' on a 'bumpy road' might be in a position to coerce a little elongation in the rolled threaded area.
Just a thought!
 
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As steel gets longer, even a minuscule amount per inch will eventually add up to something in 50"
For example railroad track elongates in the heat and if rail was not fastened to every tie block, the track would never be straight.
Or take cable on a crane... It stretches.
The eyes have no space to expand, but 50" does.
I will pull the books out tomorrow, but we need to know the moment force the foot puts on the pedal to the rod.
I for one believe it stretches
Tom
 
Mr. Maxwell, just so we are clear, I am speaking of a 'stretch' that does NOT return, ...not like a rubber band that stretches but returns. Please let us know what your findings are. Tomorrow morning when I get to the shop, I will take a picture of a brake pedal listing the measurements.
 
With brakes as one of my forte's I still never quit learning.... You guys amaze me... I am looking forward to findings as metallurgy in that sense is not my forte... I have done many a Model A brake job/restoration, and honestly the wear factor on the linings/drums and other parts I would think would be much more significant, and account for the majority in what is necessary to keep the brakes dialed in adjustment. Under normal circumstances following proper initial adjustment, wedge adjustment is the primary periodic adjustment, although if your brakes are not activating properly rod adjustment should/coulc be in order. I do concur that all rods, almost especially the service to pedal rod has the greatest amount of force applied to it. Thinking about the force and pivot nature of the cross shaft/bushings, etc, common sense would lend itself to believe that a significant amount of wear could occur there first with other soft bushings/shafts. That accounts for the wear at the rear service brake shafts.

We have seen some pretty horrible reproduction brake parts in the past, however, I will say that in my experience that a good amount of the current reproduced parts seem to be of very good quality these days. I compliment the suppliers like Brattons, Snyders, for improving the parts for us today from the junk of years past. A little adjustment is in order anyway. Unless a brake rod breaks, threads pull ( I've never seen that), then I'm not too worried about it. I am much more concerned about the single master cylinder on my '40 or like I've experienced, a stop light switch blowing out the back to lose all fluid ( fortunately that happened in my driveway).
Think about it.... the simplicity and knowing there is a solid pedal to drum contact on "A" is really a great feeling. I never worry about having good brakes!

Larry Shepard
 
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the rod attaching to the pedal would be under MUCH more pressure.

I have never heard of any rods stretching. Ain't gonna happen.

Own mayn rods that have been shortened , due to worn out brakes, but I own porbably 1000 original brake rods, and I doubt ANY of them have been stretched, and I see no reason why the new ones would either.

Where did this idea originally come from?
 
Interesting how these urban myths start. The mechanic who does most of the local veteran and veteran motor rebuild work and who I respect greatly stores all his cranks vertical as he claims they will deform if stored horizontal. It just can't be true but I can't convince him otherwise . I asked him if they would warp under their own weight why doesn't a camshaft bend form the force the open valve springs exert on it when the car is stopped - No answer yet on that one -Karl
 
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