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Old 12-06-2009, 08:42 PM   #1
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Dexter Self Adusting Brakes (long w/lots of pics and detail)

Hi Fellow Sunliners

This spring I rebuilt my TT axles and I added the new 12 x 2 self adjusting from Dexter. I wanted to wait until I report on them on how good or not they did. I have just over 5,000 miles on them now and I can report, I’m glad I upgraded but I learned some new things about TT brakes I never realized before.

So rather then type so much I’ll do this in pics and less words.

First what do they look like. Well, not a lot different then I have seen before in automotive and they use a similar type of adjusting mechanism as did auto drum brakes. To those who do not know of these this is how they work.

Here is a brake plate before install.


2 installed


How they work. There is a cable attached to the top of one shoe. The cable then goes over a pulley on the other shoe. On the end of the cable is a ratchet adjusting arm. The shoe adjuster has teeth on it that the adjuster rests on.

As the brakes open, the cable is pulled by the pulley attached to the front (primary) shoe. The other end of the cable moves some as well on the secondary shoe. When the cable pulls far enough it pushes the adjuster ratchet 1 click. I have found thru measuring that 1 click is about 0.001 to 0.002” change in brake shoe OD. Or that 1 click will take up about 0.001” to 0.002" wear in brake shoe. It is fine adjustment. The adjustment works in both forward or reverse direction. As long as the shoes open, the system works.

As the shoes wear the magnet arm still pushes them up against the drum as with non self adjusting brakes. When the shoe wear gets to be enough, the cable is pulled enough to complete 1 ratchet click per brake actuation. It will adjust each brake actuation until the shoes expand to the point they took up the wear and the ratcheting action stops until they wear again. This keeps automatically occurring until you run out of brake shoe lining or, the adjuster freezes up from corrosion if not checked on a periodic basis.

Here are some close ups of the parts.


The cable attaching on top


The cable pulley


The ratchet area


They work well. When I installed them I did not adjust them like I normally do, I let them adjust in during the burnish process on the new shoes. By the time I was done doing the stop and go burnish in process on the 1st test run they had adjusted up.

So life was good for about 2,500 miles. Then I had a problem. H’mm. I was on the way to camp and just came into town off the highway. My F350 has the Toqueshift 5 speed and I can tell how it up shifts pulling this camper normally. Well I started out at the traffic light and oh boy it is taking a long time to up shift. It feels Like I’m dragging something. Had about 6 or so stop lights and each time, the same thing. Truck would rev up high before shifting. I pulled over in a parking lot, got out and starting feeling for heat. Sure enough the left side front brake was real hot. I could not touch it, the other 3 where warm but I could touch them. I had a temperature probe in the truck and it was up at 210 F. The others 126F range. I let it cool down and then went on to camp about 3 miles down the road. I nursed it to camp. Once cooled and released they would work but I did not stomp on them hard.

At camp I crawled under and backed off the adjuster 4 clicks. On the way home then everything was normal. That was until about 2 camping trips later it starts acting up again. This time I caught it earlier and at camp only backed it off 2 clicks.

OK something is making this 1 brake be more aggressive in adjusting. Or at least it felt that way. The next week I call Dexter and talked to engineering. I told them what I had and the temps. I asked if that 210 heat was an issue. They said the brakes can take up to 600 and after that the glue starts coming undone. He was not overly concerned with the 210F but agreed that 1 wheel should not be doing what it was. I asked what to look for. They tested this new brake a lot and never had this problem. They had some issue on there older 12 1/4 self adjusters but not these. I asked what about brake drum runout? How much is too much? Dexter has a spec of 0.015" runout. It can work with that. He said that is hard to check on the TT. I said I had 2 old axles I can overcome that part. So that weekend I tore into them. This is what I found.

The front left wheel, the problem one




Well nothing was broke but I had a fair amount of brake dust for only 2,500 miles. The only thing that made any sense was the brake drum had to be running off center. I measured the drum ID when I put theses on. It was true round and not really worn much at all. There may only be 5,000 to 7,000 miles on the TT. So I setup up an indicator with the drum on one of my old axles.


Well, 0.028” runout. Yup, that did not look good. I suspected this but now have some data to help prove it. With the drum running off center that much when the shoe goes into that far off center part of the drum, the self adjusting ratchet can make a click. It takes time but it keeps over adjusting. Then it gets ahead of it self and now there is no very little running clearance. So that brake is now much more aggressive then the other 3. So it trys to stop the entire TT before the others get engaged. Once you get it hot enough it wants to like stick on. This was the drag I was feeling in the F350. It will let go once it releases but the problem is there.

I went down to the local NAPA and asked can they turn this drum. I want it to run true within 0.005" TIR. They said they do them to spec. OK what is the spec and what can they guarantee it will be held within? Well they do not guarantee anything on trailer wheels, automotive they do. It was like $20 to turn it and they would not guarantee anything. Being around machine shop most of my life I can tell for $20 and no guarantee I was asking from something they can’t give. So before they messed it up more, I left. I had problems with them on disk brake rotors on my truck before too. I wanted them to be within 0.0005” and I got 0.009”. Never again.

So I ordered 4 new Dexter drums. They showed up in 2 days. This is where it gets even more interesting. I have new 4 drums, One was 0.003, one 0.006, one 0.012 and one 0.016” TIR. Oh Great. So I said let me pull the rear drum on that same side and measure what it is. By the way, the other 3 brakes where dead on in adjustment. A faint drag just like you set them yourself and all 3 about perfect in adjustment. So the self adjusters where working well other then this one wheel.

Well the left rear old drum ran out 0.015” Oh boy. Well the fact is true that when they say they will run within 0.015 TIR, they do. So I questioned my measuring methods on the new drums and checked them 3 different ways. See here.

First I put the indicator down on the bearing surface. It’s dead on like a bearing should be. Less then 0.001” TIR. OK the method is sound




Then I went to the seal surface. OK it’s now making sense




And then to the brake drum


So I changed to the worse new drum and used a different indicator setup




Me looking at this, it is a machine setup problem when they turned the drum. The seal area and the brake shoe area are running dead on the same center, just shifted from the bearing bore. Those 3 diameters had to be bored in 2 setups. The drum was dead on true round, just machined exactly off center. What ever robot or person was not holding the part correctly in relation to the bearing bore.

So I called Dexter back. He agreed the 0.028" TIR was out of spec and might cause what I was seeing. I told him about the 4 new drums. He did not know what to say. He said they are in spec expect the one that was 0.016” but had no idea on how the 4 of them where that much different. He gave me the name of the factory QA person where they make them to call and ask about how they make them. He was very helpful just did not have an answer to the difference in the 4 drums and if I was not happy with them to return them. There would be no problem returning.

I did ask, why are these not machined to at least within 0.005”? It is not that hard to get. Automotive does it all the time. He stated no one rides in a trailer. The runout does not have to be that close. No one feels the vibration. I thanked him and we ended the call. Basically like everything else in the RV industry, precision costs more and the industry is so competitive they only do what they have to just work.

So I went ahead and installed the 2 better new drums on the one side since I had it all apart.

Here is the left rear that was working well. Very little brake dust.





Here is the left front, not working well. A lot of brake dust.


And the 2 new drums on.


I am now 2,500 miles more since that drum fix. No problems and the brakes work well.

So is this worth the up-grade? In my opinion, yes. However before you do do this upgrade you need to know the runout of your existing brake drums. 0.015” TIR and less works. Something above that might, 0.028" does not. You can check your own by pulling the brake plate and put the drum back on with no brake plate. Then just indicate in place.

On the non adjuster older ones, runout is not even found. You adjust them until it drags and stop. Since it will never adjust itself, the thing just works running out of round. Not as effective once it starts wearing, but it works and unless you are really on top of the adjustment every 3,000 miles, most will never know there drums can be out of tolerance. I never did.

Hope this helps someone

John
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Old 12-07-2009, 12:45 PM   #2
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thanks John very good info

Dan
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Old 12-07-2009, 01:55 PM   #3
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Brake drums are surely not an ideal form of braking for many reasons. There always seems to be more runout than there should be with them, even when new. I wonder if they machine them so fast that they tend to warp, some more than others?

I bought new Raybestos cast iron drums for my 1966 Dodge Charger and the 2 for the rear were so warped that they were unturnable! That was an expensive lesson that taught me that new parts are exactly that: New, and that alone!
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Old 12-07-2009, 09:12 PM   #4
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Rick

Pure skill is getting lost now a days. I issue drawings out every day for parts that have tolerances 15 times tighter then these things and the machine shops hold them no problem.

However it comes down to cost. A machine shop at $65 a hour works on skill and in tolerance as that is how they live. Auto parts stores live on parts sold as cheap prices and the quality goes down hill with it. They are just as happy selling you low end quality as they are high end quality.

Since NAPA only wanted $20 a drum to turn it and would not guarantee the outcome that tells me they do not spend the time to indicate the drum to run true before they cut it. It’s not that they are heat warped, cast iron can take tons of heat, it is that they are not machined right from the get go.

It’s like brake rotors. I got burned once on run of the mill new rotors. They have to run dead right or they burn up the pads in a week. Yet still you can buy brand news rotors that are 0.005” out. So from now on, I spend the money and buy the higher end name brand and for sure I indicate the new rotor before the truck ever leaves the shop.

John
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Prior Sunlines: 2004 T2499 - Fern Blue
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Old 12-08-2009, 10:14 AM   #5
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JohnB,

You are certainly correct. The capability to do quality work is there. Unfortunately, in many cases the one major thing that stands in the way of accomplishing that is corporate greed. Greed for profits, among other things, means that unqualified workers are hired and not trained properly. The consumer gets the end result: out of spec new parts which may be worse than the parts they replace. The rear drums I mentioned were $100 a piece so I think that they could have done better. Even the new front drums had to be cut (they were salvageable) to brake smoothly.

My equipment trailer came in this last Summer and did not track properly behind the truck. Thanks to your numerous posts about axle alignment, I was able to ascertain (not with your precision, I might add) that the axles were indeed bent. Whether it was through shipping or original manufacture, the end result was still the same. I brought the trailer back and even though I showed them why I thought it was the axles (2 of the wheels appeared clearly out of line with the frame of the trailer) they ended up making the manufacturer build a new trailer to replace the 1st.

I wondered why they did this instead of replacing the axles, which were the problem, and later realized that their service department was somewhat of a joke and could never have handled the job. Another case of hiring cheap poorly trained labor. The replacement trailer was much better although, after measuring everything, the spring mounts were not welded in exactly the same position side to side. It is close enough to be acceptable since the trailer has been working fine and tracks true with the tires wearing even. To be fair, my measuring methods could also be off by the tiny amount I'm referring to.

It's interesting that the trailer store said that my trailer was the first defective one they ever got from the manufacturer. I wonder if that is true or if I'm the only one that actually looked the trailer over carefully enough to see the problems. Certainly the trailer store didn't or THEY would have caught the problem.

Anyway, not meaning to highjack your post and start a philosophical discussion, I'm glad that you have the interest and capability to examine these problems and determine the real root cause. Apparently, the manufacturers don't seem to be willing or able to do it. Thanks again for your great diagnostic skills!
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