A hidden brake failure mode on Cobra trailers with surge brakes that's hard to diagnose.

Sorry for the long post. The bottom line: On your Cobra trailer, inspect your Bowden cables when you inspect your brakes. Is the rubber boot intact and in place? Is the cable worn or bent? Is the bell housing damaged? Does the cable retract and seat properly in the "T" fitting when the brakes are released? Read on for more details...

On a recent 2 hr trip over mostly level terrain with my Cobra trailer, I arrived home to discover the left trailer wheel hub smoking hot. The right one was warm, but fine.

After I let things cool down, I pulled the left wheel and drum off to see what the issue was. One of the shoes was worn mostly at the top, and the other had disintegrated with the liner delaminated from the metal shoe. Of course the surge function had stopped working because the left Bowden cable had extended too far due to the shoe disintegration, and the manual brake lever came back too far and did not apply the brakes. Bearings looked fine, but the left drum has some bad scoring.

So I replaced the left drum and the shoes on both the left and right wheels.. I towed the trailer around the hills near my house for a few miles to break in the shoes and make sure things were working properly. Braking felt fine.

Returning home, the left brake was again very hot from just a few miles of driving! The right was fine. When I jacked up the axle, the left wheel was difficult to rotate, even with the brakes fully released.

Under the trailer, I could see that the left Bowden cable was still extended from the "T" bracket they both connect to. I pushed it back into place with my hand, and the wheel rotated freely. Hmmm...

After taking the whole left brake assembly apart again I discovered that there was some friction when I tried to move the cable inside the Bowden sleeve in and out manually. This friction was enough to overcome the spring between the brake shoes which pull the shoes back away from the drum when the brake is released. This meant that the shoes were rubbing on the drum constantly after the first time the brakes were applied, which caused the overheating.

Inspecting the Bowden cable more closely showed that the rubber boot that keeps out road grime on the cable had come loose and had slid away from the cable housing. I also noticed that on the brake end of the cable, the bell housing that mates with the brake assembly had cracked. There was also a very slight bend in the cable near the "T" fitting that was causing some friction, and impossible to see without extending the cable from the sleeve.

I unbent the cable and forced some LPS2 down the cable into the sleeve, and worked it in and out until it ran fairly smoothly. Re-assembling everything showed that the brakes now functioned normally, with the cable retracting after the brakes were released. I also ordered two new Bowden cables from Williams Soaring Center for $38 each.

I suspect that the LPS2 was not a good idea, because given the geometry of the Bowden cable, it could run down the sleeve and into the brake housing and then on to the brake drum. --not good-- I will replace both Bowden cables when they arrive, and post an update if there is interest.

Stay safe out there!

Matt (J3)

Matt - Thanks for the info. Please keep us posted.

Important - Did you happen to take any pictures to share?

John (OHM)

When the new cables come in, I will take some pix of the old ones. What is the best way to post them for the group?

Matt

On Sunday, March 6, 2022 at 9:10:21 PM UTC-8, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
> Matt - Thanks for the info. Please keep us posted.
>
> Important - Did you happen to take any pictures to share?
>
> John (OHM)

On Monday, March 7, 2022 at 12:09:50 PM UTC-6, Matt Herron Jr. wrote:
> When the new cables come in, I will take some pix of the old ones. What is the best way to post them for the group?
>
> Matt
> On Sunday, March 6, 2022 at 9:10:21 PM UTC-8, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
> > Matt - Thanks for the info. Please keep us posted.
> >
> > Important - Did you happen to take any pictures to share?
> >
> > John (OHM)

Matt - Unfortunately, you can't post pictures to USENET group like rec.aviation.soaring.

A couple of ideas are;

- Put them onto one of the photo sharing sites like Flickr, Photobucket, Shutterfly, etc and then share the link.
- Start a new thread on the subject to RAS_Prime (https://groups.google.com/g/rasprime) which is a Google group that you can post images to.

Thanks, John (OHM)

I moved this thread to RAS Prime so I could post some pictures of the damage. Also, I will be posting a way to change the cables on your trailer without the need to remove the wheel hubs and brake shoes. (those clever German Engineers)

Matt

Here is a link to RAS-Prime where I posted instructions on how to replace Bowden cables without removing the wheels: https://groups.google.com/g/rasprime/c/lVbFPvOq_gk

Here is a link to RAS-Prime where I posted pictures of the damaged cables and shoes: https://groups.google.com/g/rasprime/c/ojWDLMLIuRw

There's another brake cable failure mode. The cable can get kinked, either because it catches on an object on the ground or just over time. I could actually see it when I crawled under the trailer, looking for yet another reason the Cobra brakes weren't working properly (I've seen several; see previous posts).

I was in a hurry so I removed the cables, dripped/injected lubricant from the front end (where the rubber boot is and where water is most likely to enter), then chucked one end of the cable in an electric drill and spun it briefly while I worked the inner cable in and out. I could really feel the difference in force required to actuate the cable before and after. I could also feel the difference when I flexed the cable housing where it was kinked.. When I reinstalled everything, I did so in a way that the cables bent the opposite direction they had for almost 30 years. I think that means I swapped left and right but I honestly can't remember; I might just have rotated the cables. Everything has been fine since then, except for the ongoing problem of the surge brake cylinder binding in the outer tube, cause by the large plastic/fiber bushings swelling over time from moisture (see numerous other posts).

This is slightly off topic (forgive me RAS; I know that's a grievous sin), but I've lost count of the number of times I've had problems with the surge brake system installed on my Cobra trailer. It works well when everything is adjusted properly and maintained and when the trailer is kept indoors and never used on wet roads and driven less than 6,000 km per year. That's a sarcastic comment, of course, but my experience is that the AL-KO brake system is an elegant solution to a European problem and yet another example of how the American market is very different in some ways to the Western European one. Yes, I know plenty of pilots who've had no problems with their trailers. Maybe they park them in a hangar so they stay dry. Maybe they drive less than I do. Maybe their trailers are younger (mine was new in 1992, although my problems started a few years into it). Maybe the newer ones are of a better design (I know there are differences in the surge mechanism inside the tongue in newer trailers).

This is making me tired already and the season hasn't even started yet. :) Happy trailering. FWIW, I check the hub temps every time I stop, which is every few hours now. If I have to make a hard stop, I pull over immediately and check to make sure the brakes have released (they haven't on two occasions, once when the glider was almost new and more recently). I've even disconnected the brakes several times to get to or from a contest and then fixed the problem later, although I don't like to do that even pulling with a full-size Chevy van.

Chip Bearden
JB

On Friday, March 11, 2022 at 1:25:02 PM UTC-7, chip.b...@gmail.com wrote:
> There's another brake cable failure mode. The cable can get kinked, either because it catches on an object on the ground or just over time. I could actually see it when I crawled under the trailer, looking for yet another reason the Cobra brakes weren't working properly (I've seen several; see previous posts).
>
> I was in a hurry so I removed the cables, dripped/injected lubricant from the front end (where the rubber boot is and where water is most likely to enter), then chucked one end of the cable in an electric drill and spun it briefly while I worked the inner cable in and out. I could really feel the difference in force required to actuate the cable before and after. I could also feel the difference when I flexed the cable housing where it was kinked. When I reinstalled everything, I did so in a way that the cables bent the opposite direction they had for almost 30 years. I think that means I swapped left and right but I honestly can't remember; I might just have rotated the cables. Everything has been fine since then, except for the ongoing problem of the surge brake cylinder binding in the outer tube, cause by the large plastic/fiber bushings swelling over time from moisture (see numerous other posts).
>
> This is slightly off topic (forgive me RAS; I know that's a grievous sin), but I've lost count of the number of times I've had problems with the surge brake system installed on my Cobra trailer. It works well when everything is adjusted properly and maintained and when the trailer is kept indoors and never used on wet roads and driven less than 6,000 km per year. That's a sarcastic comment, of course, but my experience is that the AL-KO brake system is an elegant solution to a European problem and yet another example of how the American market is very different in some ways to the Western European one. Yes, I know plenty of pilots who've had no problems with their trailers. Maybe they park them in a hangar so they stay dry. Maybe they drive less than I do. Maybe their trailers are younger (mine was new in 1992, although my problems started a few years into it). Maybe the newer ones are of a better design (I know there are differences in the surge mechanism inside the tongue in newer trailers).
>
> This is making me tired already and the season hasn't even started yet. :) Happy trailering. FWIW, I check the hub temps every time I stop, which is every few hours now. If I have to make a hard stop, I pull over immediately and check to make sure the brakes have released (they haven't on two occasions, once when the glider was almost new and more recently). I've even disconnected the brakes several times to get to or from a contest and then fixed the problem later, although I don't like to do that even pulling with a full-size Chevy van.
>
> Chip Bearden
> JB

A few years ago, I had a similar experience when a kink in the cable from maybe catching on brush or something that caused a dragging brake. Fortunately, I had stopped and checked the hubs after a few miles and one was very hot to the touch, but didn't cause damage. Like Chip did, the trip was made by disconnecting the cable from the brake on that side. I highly recommend checking the brake hubs at regular intervals to prevent disaster.

The replacement Bowden cable for Alko trailers has changed in design from the original that was on my 2000 Cobra trailer. The new style uses a nylon sheath over the cable that is advertised to last much longer, but the cable friction is also reduced dramatically. I wouldn't recommend using lubrication on this style of cable as it would probably only act as a dirt magnet. After three years the cable seems to still slide as new, even though I leave the trailer outside year around.
Bob Faris
CX

Gentlemen,

Spindelberger has extensive maintenance instructions for the wheel brakes on their website: https://www.cobratrailer.com/?page_id=1257&lang=en. The document about adjusting the wheel brakes includes a reference on how to replace the bowden cables. In our club, we are servicing the trailers accordingly every 2nd year. I don't know how vehicles are checked over in the US.. In Germany, trailers need to have an official checkover every 2 years, including a brake test. Stuck bowden cables or brake shoe failures are normally found in these tests when the brakes do not release properly. It seems they need replacement after around 10 years if a trailer is rarely used.

Best regards
Christoph

Thanks Christoph,

I read through all of those manuals I thought were relevant. I didn’t find anything on changing the Bowden cables. Could you point that one out to me?

Thanks,

Matt

On Saturday, March 12, 2022 at 12:41:20 AM UTC-8, Christoph Barniske wrote:
> Gentlemen,
>
> Spindelberger has extensive maintenance instructions for the wheel brakes on their website: https://www.cobratrailer.com/?page_id=1257&lang=en. The document about adjusting the wheel brakes includes a reference on how to replace the bowden cables. In our club, we are servicing the trailers accordingly every 2nd year. I don't know how vehicles are checked over in the US. In Germany, trailers need to have an official checkover every 2 years, including a brake test. Stuck bowden cables or brake shoe failures are normally found in these tests when the brakes do not release properly. It seems they need replacement after around 10 years if a trailer is rarely used.
>
> Best regards
> Christoph

A Dexter axel with electric brakes. Never burn your up brakes again.

On Saturday, March 12, 2022 at 11:11:44 AM UTC-6, Matt Herron Jr. wrote:
> Thanks Christoph,
>
> I read through all of those manuals I thought were relevant. I didn’t find anything on changing the Bowden cables. Could you point that one out to me?
>
> Thanks,
>
> Matt

Matt,
You may have been reading the general Cobra Manual. Like Christoph said, it's in the Brake Wheel Adjustment manual linked below. Also; from the manual:
https://www.cobratrailer.com/wp-content/uploads/Maintenance_Manuals/Bremsenwartung_ALKO_Bremsanlage_E_v01.pdf
Phyllis

"Too much friction at the bowden cables? The bowden cables can get removed without opening the brakes. Screw off the connection at the
equalizing lever, at newer trailers just unhinge. Screw then off the other end at the axle: Loose the
triangle socket by removing ring of the wrap. Unhinge now inside.
Check for bends, damage and easygoing. Moving must be possible easily with 2 fingers. Replace if
necessary."

Thanks.

I read that manual, but missed the paragraph. I guess I was looking for picture instructions :-)

Matt

On Saturday, March 12, 2022 at 10:10:40 AM UTC-8, phlyi...@gmail.com wrote:
> On Saturday, March 12, 2022 at 11:11:44 AM UTC-6, Matt Herron Jr. wrote:
> > Thanks Christoph,
> >
> > I read through all of those manuals I thought were relevant. I didn’t find anything on changing the Bowden cables. Could you point that one out to me?
> >
> > Thanks,
> >
> > Matt
> Matt,
> You may have been reading the general Cobra Manual. Like Christoph said, it's in the Brake Wheel Adjustment manual linked below. Also; from the manual:
> https://www.cobratrailer.com/wp-content/uploads/Maintenance_Manuals/Bremsenwartung_ALKO_Bremsanlage_E_v01.pdf
> Phyllis
>
> "Too much friction at the bowden cables? The bowden cables can get removed without opening the brakes. Screw off the connection at the
> equalizing lever, at newer trailers just unhinge. Screw then off the other end at the axle: Loose the
> triangle socket by removing ring of the wrap. Unhinge now inside.
> Check for bends, damage and easygoing. Moving must be possible easily with 2 fingers. Replace if
> necessary."