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 > Dexter Self Adjusting Brakes (long W/pics and details)

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JBarca

Radnor, Ohio, USA

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Posted: 12/06/09 08:13pm Link  |  Print  |  Notify Moderator

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.
[image]

2 installed
[image]

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.
[image]

The cable attaching on top
[image]

The cable pulley
[image]

The ratchet area
[image]

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
[image]

[image]

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.
[image]

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
[image]

[image]

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

[image]

And then to the brake drum
[image]

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

[image]

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.
[image]

[image]


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

And the 2 new drums on.
[image]

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

* This post was edited 12/06/09 08:27pm by JBarca *


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downtheroad

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Posted: 12/06/09 08:24pm Link  |  Print  |  Notify Moderator

John,
It amazes me that you take on projects like this and have the talent to do so..I'm always willing to look and learn..Great write up and very interesting post.

You ended the post with "Hope this helps someone."
It helped me to know that I probably won't be taking on a job like this...when it's time - it's off to the dealer.


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Posted: 12/06/09 08:53pm Link  |  Print  |  Notify Moderator

That's one of the most really interesting posts that I've seen!! You have explained it very well, and the pictures are an excellent addition. I've thought about doing something similar, but you've convinced me that adjusting them myself when I do the bearings (every year or so) is good enough for me. I don't put on enough miles to make this a priority job when a manual adjustment isn't necessary very often. Thanks for the education!!


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BenK

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Posted: 12/06/09 10:05pm Link  |  Print  |  Notify Moderator

Great detective work and add that there are other variables to take
into account.

The whole system depends on each component to be within spec as you
have found out.

The other components in that food chain are the cable and it's pivot
and sliding points. They have to within spec, other wise they will NOT
transmit enough or too much force on that pawl that moves against the
star wheel of the adjuster.

That pawl also has a spring (on cars and trucks, and don't know if
trailer brakes has similar, but assume they do). That spring is another
preset so that it doesn't over tighten. If no spring, then non issue
on that component. If there is a spring, then it must be the same for
all or the weaker/stronger one will adjust at a different rate.

I've also seen that spring hooked onto the wrong tab or hole, so it
has either too much or not enough preset tension.

Then the adjuster and how freely or how much it takes to move the
star wheel. Yours is new, so not an issue, but I've worked on some
that were so rusted, couldn't be turned in a vice. Not expensive
and easy to replace, which I do on most all or rebuild them with
anti-seize after cleaning off the brake dust and rust.

If it moves with less force than spec, then it will over tighten.
Just like too much friction and it won't adjust enough.

Be sure to have the leading and trailing shoes in the right place.
One has short arc of friction material and the other has thicker friction
material and also longer arc of material. Normal for dual action shoe
design and folks mix them up all the time. As the metal shoe is
interchangeable between the leading and trailing or primary and secondary shoe.

Finally, that adjuster pawl can wear and/or comes from the factory
with too much of a sharp edge. Ditto the adjuster star wheel. That
means the pawl will have more or less bite to adjust the star wheel.
I've seen some pawls worn to point it won't adjust the star wheel no
matter how much that cable pulls (some has a large gauge wire or
a stamped sheetmetal in place of that cable).


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BenK

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Posted: 12/06/09 10:23pm Link  |  Print  |  Notify Moderator

Forgot to add that the radius between the shoes and drum important.

Why proper bedding in more than the transfer of friction material onto the cast
iron, but to wear down and match the shoe radius to the drum radius.

Point contact is what happens till the shoe material beds into the drum.

Few shops if any, will grind and match the friction material to the
drums anymore.

coolbreeze01

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Posted: 12/07/09 07:32am Link  |  Print  |  Notify Moderator

As I shop new trailers, I'm seeing some with the self adjusting Dexter's. After reading your experience, now I'm wondering if the self adjusters are a plus or minus.
Good report.


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BenK

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Posted: 12/07/09 10:37am Link  |  Print  |  Notify Moderator

[image]

Notice the oil/grease spot on the friction material. Ruined it and needs
replacement.

If oil/grease, slippery and lessens friction. If brake fluid, will cause
the shoe to be 'grabby'

It appears to be the secondary shoe which has this wet spot. Hope it
is something non-dino oil based so not an issue.

To the PM asking why one shoe has more friction material than the other.

The 'primary' shoe has less material than the 'secondary' shoe because
the primary shoe adds more pressure to the secondary shoe.

This is why shoe/drums of old were known to lock up and never release
till you stopped. The springs are engineered to pull the shoe off the
drum once the pedal pressure is removed. One reason why disc won as
they can be modulated much better than shoe/drum. Modulated meaning
pressure and release during a panic stop. With shoe/drum, used to be
along for the ride skidding till it stopped or hit something. Also
why ABS doesn't work well with shoe/drum...that modulation thing...

Since the primary shoe adds pressure to the secondary shoe, if larger
it will place too much pressure on the secondary shoe. It is designed
to provide a proportional force onto the secondary shoe to the pedal
pressure.

The secondary shoe does the majority of braking, but the primary also
provides some braking. Again a ratio dialed in by the OEM. Also why
the secondary shoe's friction material is usually thicker. On a trailer
electric brake, the lever arm ratio of the magnet arm pivot is that

On a TV/car, the brake cylinder bore vs MC bore ratio also dictates
that pressure.

Folks who change to higher performance friction material alter this
pre-engineered ratio/performance. So they must be sure that the supplier
has done their re-engineering correctly.

JBarca

Radnor, Ohio, USA

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Posted: 12/07/09 07:13pm Link  |  Print  |  Notify Moderator

Hi Ben

Ah I see you remember the world of automotive from years past. I’m not disagreeing with you, just bringing you up to date on what the RV trailer system has and is.

Remember up until this new Dexter “upgrade” (I guess I have to call it that…) the only thing normally available on medium to smaller TT’s is plan old non adjusting brakes. Higher end 5th wheels can have more advanced systems, but unless one upgrades themselves, the RV industry is just barely catching up.

What I have is an upgrade to the normal RV industry manual adjust system and that is about it. Keep in mind the entire brake plate, magnet arm, coil, shoes, adjuster etc costs $39.95 a wheel for this upgrade….. That’s it. This is not a rocket science upgrade or a leap in braking technology, it mealy makes the ele brake setup as good as it can be. The brake shoes are still just as thin as the older standard ele brakes ones on TT. The power of the magnet arm is still the same force applying the braking. The benefit is, the system is adjusted to be at it’s best operating condition all the time. Keep best, in perspective. I can tell by your comments you are giving these things more credit then they have.

On the manual system it is best the day you adjust them. Then as the months go by they get weaker and weaker until you either readjust them or they stop working as they are so far out of adjustment there is no more lever arm stroke to push the shoes apart….

See some comments here.

BenK wrote:



That pawl also has a spring (on cars and trucks, and don't know if
trailer brakes has similar, but assume they do). That spring is another preset so that it doesn't over tighten. If no spring, then non issue on that component. If there is a spring, then it must be the same for all or the weaker/stronger one will adjust at a different rate.

I've also seen that spring hooked onto the wrong tab or hole, so it
has either too much or not enough preset tension.

Then the adjuster and how freely or how much it takes to move the
star wheel. Yours is new, so not an issue, but I've worked on some
that were so rusted, couldn't be turned in a vice. Not expensive
and easy to replace, which I do on most all or rebuild them with
anti-seize after cleaning off the brake dust and rust.

If it moves with less force than spec, then it will over tighten.
Just like too much friction and it won't adjust enough.



The is no spring on the cable like the auto drum brakes. It’s not that sophisticated. The cable is rigidly connected to the adjuster pawl. If the cable stretches, then yes it will take more shoe movement to operate the pawl. They do have a spring, it’s in blue in the pic, that pushes the pawl against the ratchet teeth of the adjuster.

See here
[image]

[image]

A frozen adjuster, yup dead on. Again keep it in perspective. The manual adjusting brakes can have the adjuster frozen just as good as this self adjuster. These things need to be checked routinely regardless if self adjusting or manual. That problem has not changed.

Putting the adjust cable in the wrong hole. Well yes on auto shoes you have choices. Here they only give you 1 hole… Well technically I guess you can hook the cable on the wrong shoe but then the cable is so sloppy I would hope the rebuilder would see that and say hey, that’s not right…

BenK wrote:



Be sure to have the leading and trailing shoes in the right place.
One has short arc of friction material and the other has thicker friction
material and also longer arc of material. Normal for dual action shoe
design and folks mix them up all the time. As the metal shoe is
interchangeable between the leading and trailing or primary and secondary shoe.

Finally, that adjuster pawl can wear and/or comes from the factory
with too much of a sharp edge. Ditto the adjuster star wheel. That
means the pawl will have more or less bite to adjust the star wheel.
I've seen some pawls worn to point it won't adjust the star wheel no
matter how much that cable pulls (some has a large gauge wire or
a stamped sheetmetal in place of that cable).


Yes, you can screw up which shoe goes where. The primary shoe is a shorter pad length. It helps not lock the lining in the drum up top. The secondary shoe has a longer lining.

Now the pad thickness, again these things are dirt simple. Both linings are the same thickness. And they are really thin to start with. I was shocked the 1st time I saw TT ele brakes and they where new shoes. They are not like automotive that are thicker and more sophisticated. Again keep it in perspective, on a auto you have hydraulic wheel cylinders that generate high force as opposed to this magent setup. As I recall, 40K miles on a car on drum brakes and you could be in for new shoes on the standard OEM linings. Well that is 40K miles with high stopping force. Here 40K miles is a lot on a TT and then the weaker engagement strength and the pads do not wear as much. So they do not put very thick linings on. And they do not stop as good either… so the wear is less.

The adjuster or the pawl rounding itself, yes in time this will happen. But your perspective is automotive. The auto's wear more as they stop more so they adjust more. On TT’s they do not stop as good, do not wear as much and thus take longer to wear the sharp edges down. It’s not that they won’t or don't wear, it’s it will take longer to get there. For many RV’ers they may sell the TT by then….

BenK wrote:

Forgot to add that the radius between the shoes and drum important.

Why proper bedding in more than the transfer of friction material onto the cast
iron, but to wear down and match the shoe radius to the drum radius.

Point contact is what happens till the shoe material beds into the drum.

Few shops if any, will grind and match the friction material to the
drums anymore.


Yes you are right. These will fit better as they wear in and more contact comes that creates less spot loading heat. Boy I’m not old enough to ever remember shops grinding linings to match the drum. Again on automotive. What came out of the Raybetos box went on the car…

BenK wrote:

[image]

Notice the oil/grease spot on the friction material. Ruined it and needs replacement.

If oil/grease, slippery and lessens friction. If brake fluid, will cause the shoe to be 'grabby'

It appears to be the secondary shoe which has this wet spot. Hope it
is something non-dino oil based so not an issue.



Ben, the photo tricked you… sorry. It is for sure not brake fluid or oil…. And it is not grease either. That pic is from the actual good wheel. Good eye, but the blackend spot is water. In this saga of drum runout I had a tarp over the axles and brake ass’y for about 2 days. And it rained. I took the tarp off to take the pic's before I cleaned up the dust, resanded the shoes and cleaned the brake plates back up. The tarp splashed and I rubbed it getting it off. It is a mark on the brake plate too.

On precision made parts or even local auto shops re-maching the drums, this one urked me. The shop that wanted to charge $20 and no guarantee for run out on a trailer drum tells me all they can do and make money is slap the drum on a resurfacer, tighten the hub and let it cut. They do not indicate the drum in before they cut and they do not adjust if it is not spinning on the bearing cones right. Thus you get a drum, resurfaced and runout that is hit and miss. And what is worse is a brand new drum that runs out 0.016”?? The seal and the lining surface are dead true to each other, just off center with the bearing??? That lathe can cut perfect just as easy as it can cut off center. The difference is the time/skill to indicate the part in before you cut.

You have a good write up,. Just these things are not that sophisticated.

Thanks, good info.

John

* This post was edited 12/07/09 08:50pm by JBarca *

JBarca

Radnor, Ohio, USA

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Posted: 12/07/09 08:55pm Link  |  Print  |  Notify Moderator

coolbreeze01 wrote:

As I shop new trailers, I'm seeing some with the self adjusting Dexter's. After reading your experience, now I'm wondering if the self adjusters are a plus or minus.
Good report.


Hi Coolbreeze

While yes I had an issue with an out of spec brake drum, I for sure would buy these again and will if I ever wear them out.

The brakes work all the time just like they do right after you use to manually adjust them. If your TT does not stop well for you right after you adjusted the manual brakes, this upgrade will not do you much good. It may be a wiring issue.

Out of curiosity, what brand now offer these are new? I’m hoping they have at least started making it into the industry.

Thanks

John

time2roll

Southern California

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Posted: 12/07/09 09:13pm Link  |  Print  |  Notify Moderator

For all that hassle it makes disc conversion look easier. I was kinda thinking of the 10" self adjusting system but now I hesitate.


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