Brake Drum TIR
 

I'm working to correct some issues with the front brakes in my Fordor. For background, I have installed cast iron drums and molded brake shoes. The drums were turned to be concentric with the hubs, and the shoes shaped to the drums. I've had some pulsing in the brakes recently that I would like to remove.

In pursuing this goal, I planning to check the concentricity of the drum to the center line of the hub. I'm using a Test Indicator that measures to 0.001".

This has led to a question: what is an acceptable amount of Total Indicated Runout? I have looked in several reference books but haven't found any information; it'd be great to locate some Ford specs....or some informed opinions!

Thanks very much for your help with this.

David

Re: Brake Drum TIR
 

I don't know what the specification is for TIR on the drums but any measurable amount with your Test Indicator would be too much, in my opinion.

How are the bearings? Is the pulsing coming from the front or rear? Are the journals on the rear end where the bearings ride worn?

Measure the runout of the drums but also look for other possible causes.

Re: Brake Drum TIR
 

David, where is that runout number projected to be coming from?


I personally true each hub in a lathe using new bearing races as my datum. Does this operation need to be done? Probably not, but it generally ensures the new drum has been installed very close to perpendicular with the centerline of the Spindle bearings.

In your case, if the replaced Drum was trued on a lathe yet it is out-of-round causing an intermittent contact, then likely a poor job of machining (-or setup) was done. Hypothetically, even if the drum were installed onto the hub at a slight angle in both the X and Y directions, machining the Drum & Hub as a unit should have straightened this to the point where the shoe surface was coaxial.

As far as what I would want, ...somewhere around 0.0005", -or about ½ of what your indicator would read. Could it function with a greater reading than that? Yes, but you asked what I would want to see.

Re: Brake Drum TIR
 

I agree with Brent re incorrect machining setup. The concentricity is all relative to the ID of drum where it mounts on the axle. I would have the drum remachined. Before it is resurfaced I would ask the machine operator to indicate the ID of the drum once it has been setup. This should show how much runout is present and also the amount of material that is needed to be removed.

Re: Brake Drum TIR
 

One way to test the TIR of the drum, but this is a lot of work, would be to take the backing plates off and mount the hubs/drums and then use a magnetic base for the indicator. You would only need to take the backing plates off on one front axle and one rear axle.

Re: Brake Drum TIR
 

Thanks for all that! Much to think about there.

We're talking front brakes; bearings and races are not new, so it may be meaningless to test with them!

I'm a little unclear about Brent's question about the source of the non-zero TIR IF it's a meaningful number....I'm thinking that it could be bearings/races, or it could be that the drum wasn't machined to be concentric with the spindle center line. More investigation is needed....

I will think about your advice and discuss it with a couple of local Model A folks to figure out how to proceed.

Thanks again.
David

Re: Brake Drum TIR
 

Again, this could be the results of several different issues. A great example is one of my arbors got bent once likely from an employee dropping it, -or something on it. Because the shaft is only ¾" in diameter on the end, it does not take too much to bend it. If it had 0.002" runout on the shaft, that could cause 0.004" of run-out. Also, if the race was not installed correctly, -or if there was some dirt/grit between the mandrel and the bearing race, this could skew the drum enough to cause a pedal pulsation. Dave, do you even know which side is causing the pulsation??

Re: Brake Drum TIR
 

To find out which front brake is causing the problem you could back off the adjuster on one side at a time and tighten up the other one until it dragged. Then spin the wheel while it is up in the air. Count and keep track of the number of clicks you move the adjusters so that you can put the brakes back to where they were.

Randy Gross is the goto person for brakes. If you send your drums mounted on the hubs to him they will come back true. Send the shoes too so that he can arc them to the drums.

Re: Brake Drum TIR
 

Teachable moment here:

Lots of folks save money by NOT replacing bearings and races when they install new drums. Their basis for this seems to be that neither the bearings or races are pitted and the bearings pass the "old mechanic" test of taking the bearing in one hand and spinning the bearing cage against the inner race with the other hand while exerting inward pressure. If no resistance is felt anywhere in the travel of the bearing cage, the bearings are deemed OK. Now, down the line after replacement of drums, a scenario as outlined here develops. Now we think .................... Hmmmmm, maybe it's the bearings????????

If you know the bearings and races were replaced 100 miles ago, you're probably OK re-using them. Most of us can only guess as to whether the bearings and races were EVER replaced. In that case, the "old mechanic" test may not be conclusive. My rule of thumb ..................... if you replace the drums, set new studs in place, go out of your way to have the new drums trued ..................... spring for the new bearings and races. Then, you have a clear frame of reference when you have to find a solution in reverse.

Re: Brake Drum TIR
 

Update....TIR left 0.006"; TIR right 0.007". Brent - based on behavior/feel while driving, I believe it's the right front.

The original style adjustment wedges had been replaced by floating type; I am converting back to original.

Continuing to investigate.... I appreciate your suggestions.

Re: Brake Drum TIR
 

In my opinion, and I think Brent would agree, 0.006 and 0.007 are both too much. My suggestion is to send them to Randy Gross along with the shoes.

Re: Brake Drum TIR
 

I don't think that is going to solve anything. Just make sure the pin inside of the stud is free to allow the wedge to float.


So that we are clear here, is the measurement taken on the friction surface where the shoe lining is applied, ...or are you measuring the OD of the Drum? If you are measuring the outside, that is a realistic reading. To measure these on the vehicle, you will need to remove the Brake Housing (Backing) Plate so you can access the inside of the drum with the indicator.

Re: Brake Drum TIR
 

Both of those drums need to be turned on a drum lathe. .005 and .006 are way too much runout. Something I ran into recently was whoever installed the new drums did not have a big enough press as the swedging was not done fully. The drums were not tight on the hub. After swedging properly one drum had some runout so about .005 cut cleaned it up.

Re: Brake Drum TIR
 

Another update: with original wedge shafts installed and adjusted, pulsing (and occasional brake locking on at a stop) and both gone - even with the TIR's noted above. Reconciling this with the opinions above seems a challenge, at least in my mind....unless this outcome suggests that the TIR measurements are incorrect. That could be possible given my inexperience with making these measurements.

Re: Brake Drum TIR
 

Glad you got it fixed. It would seem that the TIR was either not measured correctly or that there is some other factor at play. My car has the Flat Head Ted floaters and does not pulse or have problems with the brakes locking up.

Re: Brake Drum TIR
 

nkaminar, I agree with your comment about my TIR measurements! I guess that I need to work on my capabilities.......but in the meantime it's been an interesting discussion.

Thanks to all who commented.
David