time to replace brakes?

Chum lee wrote:

JBarca wrote:


A good machine shop can handle 0.005" TIR and less without issue. But, some auto parts places will not guarantee much of anything. I gave up around here trying to get the local auto parts store to turn brake rotors or drums. After getting them back worse than when I gave them to them, I gave up and just bought new brake drums & rotors.



John

In modern times, once brake drums/rotors are out of tolerance, a good machine shop can get them round/flat again, BUT, the problem is that when they remove material, it takes the castings out of tolerance from a thickness variation standpoint. Once the rotors/drums are heated up in use, they usually go out of tolerance again very quickly because the thickness variation causes too much differential heating (expansion/contraction) and the shuddering/pulsing returns. The older "boat anchor" style drums/rotors no longer exist because engineers are trying to save unsprung weight and reduce rotational inertia to the detriment of durability/longevity.

Best bet: once drums/rotors are out of tolerance for any reason, replace them with like and kind. (new) Truck/trailer components are usually more robust, but, . . . . do you feel lucky?

Chum lee

Hi Chum lee,

I fully agree with you. The issue is the trailer industry (the smaller types used in RV's) quality is not like years ago. A brand new brake drum running 0.015" TIR out is, well, poor. The 2 bearing bores can be dead on, just the shoe surface diameter runs out in relation to the bearing bores. The shoe diameter can be dead true also, it is just that the machine was allowed to run off center when turning the shoe diameter. Either the operator just plain set it up wrong or they allow the machine to drift since the spec is, 0.015" TIR, and they let it run.

On a standard manual adjust brake, many folks may never know or realize you have a problem. When you are adjusting the brake, you tweak the adjuster close and listen/feel for the drag while spinning the wheel. Well, when the drag skips, then gets hard to turn, the skip is the drum rotating off center. You are setting the drag to the part of the rotating surface that touches the shoes, whatever portion of less contact in the 360 degrees of the drum. If the shoe surface ran true, you would have even light-drag the full 360 degrees.

Now insert the self-adjusting trailer brake. If the shoe surface runs out much more than 0.015" TIR, and even some may be a little under that, over time when the shoes press into the max out of the round area, the adjuster has enough travel to move a click on the adjuster. The first click is not a problem, that click may only be a 0.0005" movement. But, after enough stopping sequences, the adjuster keeps on adding 0.0005", and soon that turns into a lot of thousands. Now the brake is too tight and there is no way to back it off automatically. You end up with a locked-on mega hot brake.

I have said before, one learns a lot more when things do not work right. If I had not had this happen to me, I never would have realized it. In today's modern machines, holding a brake drum to spin true within 0.005" TIR is a walk in the park. Automotive does it all day long on disk brake rotors that have to run close to dead true. Why does the trailer industry allow such wide tolerances that they ship all the time? It all comes down to $$$.

Sorry for the rant, I come from a machine shop background and it's just poor the stuff we get that could easily be made lots better with just a little more time and pride added.

Thanks for listening...

John