Adjusting Brakes...Why Equal Temperature ?

marc hildebrant

marc hildebrant

Well-known member
Group,

I just finnished using the adjusting board from Les Andrews book to set my brakes. Although the terms are somewhat vague ( just holding...not locked), I think I have it set. I did modify the method and just jack up the rear, adjust rear, drop, jack up front, adjust front, and then drop.

In the past, I tried to have the wheels all at the same temperature (using my calibrated finger) after a stop,but,wouldn't the temperature's of the brakes be different between the front and rear ?

After all, the rears are set to come on first.

Using the adjusting board was a pain...there has to be a better way.

Marc
 
Marc:
Thanks for the post on my question last week. I decided that it was not possible to use the primary/secondary shoe arrangement when the actuation was from the bottom of the front wheel.

To get to your question - I wrote an article for our club newsletter that outlined how to adjust the brakes side to side using an infrared pyrometer similar to what Harbor Freight sells. I have not gotten around to researching and writing an article about the rear brakes. There is a lot more mass to the rear drums compared to the fronts and how to allocate the energy put to the brakes drums causing the temperature rise is a question in my mind. The $30 tool is a lot better than a calibrated finger.

Vic
 
Marc:
I think that I did not answer your original question. The answer is that the function of the brakes is to take the kinetic energy of the car and convert it to heat energy. If the side to side temperature is equal then the brakes are doing an equal amount of converting that energy to heat. To me that is a better method of adjusting than seeing how much torque it takes to turn the wheel with the brake pedal partially applied. I believe that the temperature method is better than any other method of side to side adjustment.

I am not sure that I completely agree with the written word that the rears need to come on first. I have been playing with the idea of having both front and rear come on at about the same time. Maybe this is the subject for further input from the experts.

Vic
 
Vic,

Is there a way to see your article ?

So, what you do is make the two front brake drums the same and the two rear drums the same BUT the front and rear are not equal ?

Marc
 
You definately DO NOT want the fronts to come on before the rears. The rear brakes help keep the car going straight during braking. It is also best to have the fronts lock up before the rears but the way the Model A Brakes are designed, this is difficult to do. If the rears lock up first, it is easier to turn the car around during hard braking.

With Hydraulics, the fronts and rears come on at the same time but with the bias built into the system is such that the fronts will have more braking effort then the rears for a given pedal force. Under hard brake application, the fronts almost always will lock up first.

It is VERY important to have equal braking side to side, especially on the front. If not, the car will want to pull to one side or the other during braking.

Chris

Chris
 
Chris,

Indeed, I'm using the method in Les Andrews book with the adjusting board so that the fronts come on after the rear.

It's still a tough way to set them up, as you have "just holding"....whatever that means !

Marc
 
I've been using the infared pyrometer method after the mechanical set up with the new brakes. It will take a few test runs to balance things out and I test the temps. within minutes of using the car because of the heat dissapation. I just record on a piece of paper each wheel and use it as reference for the next test ride. Should work but we'll see.
 
Marc:
It is very easy to do the side to side adjustment on both the front and back using the adjusters and the rod lengths a little because I have found that the adjusters are a little coarser than the rod lengths. I never take more than a half a turn on the clevises.

Vic
 
Vic,

Hummmmmm....... I did not think that the rod length should be changed from the "initial" setting to make the lengths the same/front at 15 degree's.

I do find that the brake adjustment is coarse and it would be nice to have a "mid" position between settings.

Marc
 
I understand that the goal is to balance the drum temperature on the driver side and passenger side. What would you expect to be an acceptable temperature that would give you good stopping results?
 
Maybe it is just me! who cares what the temperature is, what counts is the ability to stop and stop evenly [no pull]. When set up by the book ie: 15 degree angle slight drag on the drum it is then the road test [real world] that counts..I adjust the fronts so that the clevis pin just drops over the brake actuator at the 15degree angle thus the "delay" is set rear to front and take it out on a dirt road for preliminary test, once rears lock and we stop stright we confirm on paved roads. Once satasified done deal! If the temps are different but it stops fine what are you going to change and why ????????
 
There are a couple things worth noting. First, the front levers are NOT set at 15° by the length of the brake rods. If internal parts are not worn they will be at 15° with all free play removed i.e., the lever pulled back to the point that any further rearward motion will begin to push the operating wedge off it's seat. If you have internal wear then you can use "brake pills" to effectively lengthen the push rod until the lever is near it's proper 15° position as described above. The real purpose for that position is to maximize the leverage which means the lever is close to, but never past vertical with the brakes fully engaged.

Second, you MUST adjust the brakes so the rears BEGIN to engage first. The rear brakes engage at a slower rate than the fronts. Maybe it's more clear to say the fronts engage at a FASTER rate than the rears. That means the fronts actually "catch up" to that rear brakes or very close to it. If you were to adjust the fronts to begin engagement at the same time as the rears, the fronts will be fully engaged before the rear brakes are and your brake pedal will travel no further.

Drum temperature will likely be a nearly useless test as far as adjustment goes but will tell you if you have a serious problem. I'd be surprised it 1 in 500 people actually get there brake shoes centered with 100% shoe contact until many miles of use and several adjustments. The amount of shoe contact will typically vary from wheel to wheel and thus the temperature and thermal expansion will vary but not necessarily commensurate with the stopping effectiveness.


 
Marco:
Lets look at the 15 degrees. I have not yet calculated all the dimensions and angles in the front brake system. The force acting on the shoes will change slightly as the angle is reduced from 15 to 0 and then the other way. Is this more or less significant than the actual pressure on the shoe? The change in the lever arm length is the important factor. As you push harder on the pedal past 0 degrees, the force actually goes down. That is why the shoes need to be arced and centered.

I need to look into your statement that if the fronts are engaged before the rears, the fronts will be fully engaged and the brake pedal will travel no further. I think that the travel can be calculated but practical experience suggests that it may not be significant.

I stand by my statement that the purpose of all brakes is the conversion of the kinetic energy of the car when moving to thermal energy in the brake drum when stopping. If the temperatures are even then the brakes are balanced. To take this a step further assume that you have only one shoe in contact with the drum on one side and both on the other side. Is it realistic to assume that the car will stop without swerving and the drum temperatures will be even? One side will be doing more of the work. This situation can be brought closer to even by using a pyrometer on the drums. The best method is to arc and center the shoes, replace all of the loose clevises, etc. However much we intend to do this, we do not always complete all of our resolutions.

I use molded and bonded linings with cast drums on all my cars. I have had difficulty getting someone to arc the linings to the drums. A friend recently purchased an old arcing machine. I recall that there was an article maybe 15 years ago in Skinned Knuckles reviewing the friction characteristics available for linings. Every shop that I speak with does not know what I am asking for and say that they use what they have. I have been looking for lining material that has the highest friction coefficient so that I can stop with the lowest pedal pressure.

Your comments are welcome.

Vic
 
Marco Tahtaras said:
There are a couple things worth noting. First, the front levers are NOT set at 15° by the length of the brake rods. If internal parts are not worn they will be at 15° with all free play removed i.e., the lever pulled back to the point that any further rearward motion will begin to push the operating wedge off it's seat. If you have internal wear then you can use "brake pills" to effectively lengthen the push rod until the lever is near it's proper 15° position as described above. The real purpose for that position is to maximize the leverage which means the lever is close to, but never past vertical with the brakes fully engaged.

Second, you MUST adjust the brakes so the rears BEGIN to engage first. The rear brakes engage at a slower rate than the fronts. Maybe it's more clear to say the fronts engage at a FASTER rate than the rears. That means the fronts actually "catch up" to that rear brakes or very close to it. If you were to adjust the fronts to begin engagement at the same time as the rears, the fronts will be fully engaged before the rear brakes are and your brake pedal will travel no further.

Drum temperature will likely be a nearly useless test as far as adjustment goes but will tell you if you have a serious problem. I'd be surprised it 1 in 500 people actually get there brake shoes centered with 100% shoe contact until many miles of use and several adjustments. The amount of shoe contact will typically vary from wheel to wheel and thus the temperature and thermal expansion will vary but not necessarily commensurate with the stopping effectiveness.
Click to expand...
Marco; I have never heard that the front and rears engage at different rates before! can you elaborate further? Chuck
 
Marco, Chuck and anyone else following this thread:

I measured the lengths of the arms on my cross shaft and front and rear actuators.

Cross Shaft - both front and rear arms are 2.5 inches from the CL clevis to the CL of the arm.
Front actuator arm - 2.5 inches
Rear actuator arm - 4 inches

I am beginning to agree with Marco that the rears need to come on a little before the fronts. The ratio of force will be 4/2.5 = 1.6 favoring the rear. I do not know what the designers were thinking. Comments are welcome.

As far as the 15 degrees is concerned. The front lever arm length increases from 2.4148 to 2.5 inches. 2.5 times cos 15 degrees. This is 3 percent in the right direction but it is balanced by a negative 3 percent in the cross shaft.

I am looking for the quickest and dirtyest method of making some braking improvement in the 499 out of 500 of us that do not have our brakes set up perfectly.

Vic
 
Vic in E-TN said:
The ratio of force will be 4/2.5 = 1.6 favoring the rear. I do not know what the designers were thinking. Comments are welcome.

Vic
Click to expand...

I think more braking effort being designed into the Model A rear brakes is a hold over from the Model T days when Ford only had rear wheel brakes.

When the Model A and it's four wheel brakes were designed, I do not think there was a good understanding of "weight Transfer" during braking and the need to have more brake effort in the front for an overall balanced system. Most modern (Front Engine) cars will have 60-70% of the total braking effort in the front.

Ideally, for the best control of the vehicle during maximum braking in an emergency situation, you want to be able to apply the brakes just short of lock up. If the front brakes lock up first, you loose the ability to steer the car. If the rears lock up before the fronts, the vehicle wants to swap ends. If you lock up all four wheels, you loose all directional control of the vehicle and are just along for the ride. If you do get brake lock up during maximum braking, it is best to have the fronts lock up first. This way, you may loose steering but if the rears do not lock they will help keep the vehicle going straight ahead.

Drivers of Road Race Cars learn to apply the brakes just short of lock up. This results in maximum braking while keeping directional control "Steering" of the vehicle. However, this is a skill that takes time to learn. For most "Civilian" drivers, the only time they apply maximum braking is in an emergency situation and most often, they will just jump on the brakes and lock up all four wheels. In the process, they loose directional control (steering) of the vehicle. That is why anti lock brakes were developed.

I hope I did not get too "Long Winded" in my reply. Brake design, while it seems to be a very simple issue is quite complex.

Thanks for your patience,

Chris
 
Marco and Others,

Good comments. I agree that the linings on the brakes will not be centered until they are worn or arched correctly. However, if heat is equal to the amount of energy removed by the brakes, then wouldn't the proper method be :

1. Set up the brakes correcly with new linings/arch the linings to the drum.

2. Adjust per the Ford Service bulletins/Les Andrews so that the rears come on first.

3. Road test and adjust the brakes so that the fronts have equal temp and the rears have equal temp.

4. Continue to monitor/check temps as the brakes wear-in and make small adjustments.

5. Reset to step 2 after , say 200 miles.

6. Minor adjust again for equal temp. to the fronts and back.


What I'm getting at is that brake temp always indicates the amount of stopping force or car energy converted to heat by that brake, so as the brakes get properly set, you would have to make minor adjustments so that the fronts are stopping the same and the rears are stopping the same.

Also, the "rear first" is important since the braking system was designed that way...unlike today's cars.

Marc
 
Some of you guys have way too much time on your hands. You've taken adjusting model A ford brakes and tried to make rocket science out of it. It's levers and links pushing/pulling a friction type material against a steel surface until that surface stops. JMO
Paul in CT
 
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