Older Cobra trailer brake oscillation

[Brian Bange[_2_]]

I just towed my new (to me) Cobra for the first time and when I
brake even moderately, the tires lock up on the trailer, then let
go, then lock up in a repetitive cycle of about 3Hz. What do I
do?

Brian

[Darryl Ramm]

On Mar 7, 7:30*pm, Brian Bange > wrote:
> I just towed my new (to me) Cobra for the first time and when I
> brake even moderately, the tires lock up on the trailer, then let
> go, then lock up in a repetitive cycle of about 3Hz. What do I
> do?
>
> Brian

Start by adjusting the brakes properly. Follow these well written
directions from Spindelberger...

http://www.cobratrailer.com/Bremseinstellung_mit_Bildern_englisch_1.pdf

You don't say how old the trailer is and how many miles it has
traveled. It could be it needs more work in addition to this, but this
is the place to start.

When you break something Williams Soaring has lots of Cobra parts in
stock (there is a free plug in return for an excellent recent trailer
maintenance seminar).

Darryl

[JS]

If the trailer is more than 8 or 10 years old, there's a good
possibility that the shock absorber in the tongue of the trailer is
shot.
Tim Mara has posted some great info on his website on how to remove
and replace this part.
http://www.wingsandwheels.com/Tongue%20detail.htm
I believe that Williams should be getting some of those parts in
soon. But if you have not already removed and examined the shock you
must have all the info for your trailer in detail to figure out which
shock you need. Send all the numbers from the left side of the trailer
tongue to Agnes at Cobra. She'll set you straight.
The link Darryl posted is excellent instruction for adjusting the
brakes. Many people adjust the wrong place in the system. You should
never adjust the threaded rod from the tongue to the cable, unless
you're undoing a previous maladjustment.
Jim

[Eric Greenwell]

Darryl Ramm wrote:
> On Mar 7, 7:30 pm, Brian Bange > wrote:
>> I just towed my new (to me) Cobra for the first time and when I
>> brake even moderately, the tires lock up on the trailer, then let
>> go, then lock up in a repetitive cycle of about 3Hz. What do I
>> do?
>>
>> Brian
>
> Start by adjusting the brakes properly. Follow these well written
> directions from Spindelberger...
>
> http://www.cobratrailer.com/Bremseinstellung_mit_Bildern_englisch_1.pdf
>
> You don't say how old the trailer is and how many miles it has
> traveled. It could be it needs more work in addition to this, but this
> is the place to start.
>
> When you break something Williams Soaring has lots of Cobra parts in
> stock (there is a free plug in return for an excellent recent trailer
> maintenance seminar).

And possibly, the gas spring in the tongue is shot. It's easy to check.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[Brian Bange[_2_]]

Thanks fella's, this information gives me a pretty good start.

At 06:47 08 March 2009, Eric Greenwell wrote:
>Darryl Ramm wrote:
>> On Mar 7, 7:30 pm, Brian Bange wrote:
>>> I just towed my new (to me) Cobra for the first time and
when I
>>> brake even moderately, the tires lock up on the trailer,
then let
>>> go, then lock up in a repetitive cycle of about 3Hz. What do
I
>>> do?
>>>
>>> Brian
>>
>> Start by adjusting the brakes properly. Follow these well
written
>> directions from Spindelberger...
>>
>>
http://www.cobratrailer.com/Bremseinstellung_mit_Bildern_engl
isch_1.pdf
>>
>> You don't say how old the trailer is and how many miles it
has
>> traveled. It could be it needs more work in addition to this,
but this
>> is the place to start.
>>
>> When you break something Williams Soaring has lots of
Cobra parts in
>> stock (there is a free plug in return for an excellent recent
trailer
>> maintenance seminar).
>
>And possibly, the gas spring in the tongue is shot. It's easy to
check.
>
>--
>Eric Greenwell - Washington State, USA
>* Change "netto" to "net" to email me directly
>
>* "Transponders in Sailplanes" http://tinyurl.com/y739x4
> * Sections on Mode S, TPAS, ADS-B, Flarm, more
>
>* "A Guide to Self-launching Sailplane Operation" at
www.motorglider.org
>

[[email protected]]

On 8 Mrz., 04:30, Brian Bange > wrote:
> I just towed my new (to me) Cobra for the first time and when I
> brake even moderately, the tires lock up on the trailer, then let
> go, then lock up in a repetitive cycle of about 3Hz. What do I
> do?
>
> Brian

HGi Brian
In your case, most likely the wheels are too big in diameter.
Overrunning brakes are a self regulating system, and they are servo-
sustained.
Self regulating means, that a trailer with a certain weight needs a
certain diameter of the brake drum and a certain diameter of the
wheels.
The dynamic wheel diameter for a trailer with wheels with 4 bolts can
be between 572 and 602 mm, so the difference between minimum and
maximum diameter is just slightly more than one inch.
The dynamic wheel diameter is the diameter with 100 km/h ( about 60
miles an hour.) Here the wheel is about 12 mm( 1/2 inch) more in
diameter as the trailer resting.
As mentioned before, the brake is servo sustained. This is done with a
certain position of the brake shoes inside the brake drum. So, a
certain amount of the
force of the turning wheel is used to sustain the brake force.
If your wheels are too big in diameter, this servo system just
receives too much force from the wheel and overreacts, blocking the
wheel. This means no force anymore to the servo brake, = brake opens.
Wheel turns again = too much force to the servo brake = blocking
again.........
If you tell me the wheel size you currently use and if you have a
brake drum with 4 or 5 lugs, I can give you some advise
The whole system is designed to work perfect at 90 % of the maximum
weight of the trailer. It will run up too light and brake too weak at
50 % and it will run up hard ( and brake weak again ) with 100% of the
trailers maximum weight.
Alfred

[Brian Bange[_2_]]

Alfred,

I towed the trailer home with just the wings in it. I left the
fuselage in the hangar. That would account for about a 300
pound lighter load. Would this cause my problem?

The trailer was probably built in 1983 and has housed an
ASW20B since it was new, as nearly as I can tell. It has
P165/80R13 tires on 4 lug wheels.

Thank you for your very detailed post.

Brian Bange
At 17:11 09 March 2009, wrote:
>On 8 Mrz., 04:30, Brian Bange wrote:
>> I just towed my new (to me) Cobra for the first time and
when I
>> brake even moderately, the tires lock up on the trailer, then
let
>> go, then lock up in a repetitive cycle of about 3Hz. What do I
>> do?
>>
>> Brian
>
>HGi Brian
>In your case, most likely the wheels are too big in diameter.
>Overrunning brakes are a self regulating system, and they are
servo-
>sustained.
>Self regulating means, that a trailer with a certain weight
needs a
>certain diameter of the brake drum and a certain diameter of
the
>wheels.
>The dynamic wheel diameter for a trailer with wheels with 4
bolts can
>be between 572 and 602 mm, so the difference between
minimum and
>maximum diameter is just slightly more than one inch.
>The dynamic wheel diameter is the diameter with 100 km/h (
about 60
>miles an hour.) Here the wheel is about 12 mm( 1/2 inch)
more in
>diameter as the trailer resting.
>As mentioned before, the brake is servo sustained. This is
done with a
>certain position of the brake shoes inside the brake drum. So,
a
>certain amount of the
>force of the turning wheel is used to sustain the brake force.
>If your wheels are too big in diameter, this servo system just
>receives too much force from the wheel and overreacts,
blocking the
>wheel. This means no force anymore to the servo brake, =
brake opens.
>Wheel turns again = too much force to the servo brake =
blocking
>again.........
>If you tell me the wheel size you currently use and if you have
a
>brake drum with 4 or 5 lugs, I can give you some advise
>The whole system is designed to work perfect at 90 % of the
maximum
>weight of the trailer. It will run up too light and brake too weak
at
>50 % and it will run up hard ( and brake weak again ) with
100% of the
>trailers maximum weight.
> Alfred
>

[Bob Kuykendall]

On Mar 9, 10:45*am, Brian Bange > wrote:

>...It has P165/80R13 tires on 4 lug wheels...

The nominal OD of that tire size (independent of "dynamic" effects)
is:

165*0.8*2+13*25.4 = 594mm.

Brian, you probably already know this, but for everyone else the parts
of a tire size like "165/80R13" are:

165 - Width in millimeters
80 - Aspect ratio; ratio of height to width
R - Type and/or speed rating. R=radial
13 - Wheel size, same as the tire ID in inches (yes, inches)

Nominal OD = (width*aspect)+(wheel size*25.4)

Thanks, Bob K.

[Bob Kuykendall]

On Mar 9, 1:46*pm, Bob Kuykendall > wrote:

> Nominal OD = (width*aspect)+(wheel size*25.4)

Oops, I got that wrong. The correct expression is:

Nominal OD = (width*aspect*2)+(wheel size*25.4)
^^
Sorry for the confusion.

Thanks, Bob K.

[[email protected]]

On 9 Mrz., 18:45, Brian Bange > wrote:
> Alfred,
>
> I towed the trailer home with just the wings in it. I left the
> fuselage in the hangar. That would account for about a 300
> pound lighter load. Would this cause my problem?
>
> The trailer was probably built in 1983 and has housed an
> ASW20B since it was new, as nearly as I can tell. It has
> P165/80R13 tires on 4 lug wheels.
>
> Thank you for your very detailed post.
>
> Brian Bange
> At 17:11 09 March 2009, wrote:
>
>
>
> >On 8 Mrz., 04:30, Brian Bange *wrote:
> >> I just towed my new (to me) Cobra for the first time and
> when I
> >> brake even moderately, the tires lock up on the trailer, then
> let
> >> go, then lock up in a repetitive cycle of about 3Hz. What do I
> >> do?
>
> >> Brian
>
> >HGi Brian
> >In your case, most likely the wheels are too big in diameter.
> >Overrunning brakes are a self regulating system, and they are
> servo-
> >sustained.
> >Self regulating means, that a trailer with a certain weight
> needs a
> >certain diameter of the brake drum and a certain diameter of
> the
> >wheels.
> >The dynamic wheel diameter for a trailer with wheels with 4
> bolts can
> >be between 572 and 602 mm, so the difference between
> minimum and
> >maximum diameter is just slightly more than one inch.
> >The dynamic wheel diameter is the diameter *with 100 km/h (
> about 60
> >miles an hour.) Here the wheel is about 12 mm( 1/2 inch)
> more in
> >diameter as the trailer resting.
> >As mentioned before, the brake is servo sustained. This is
> done with a
> >certain position of the brake shoes *inside the brake drum. So,
> a
> >certain amount of the
> >force of the turning wheel is used to sustain the brake force.
> >If your wheels are too big in diameter, this servo system just
> >receives too much force from the wheel and overreacts,
> blocking the
> >wheel. This means no force anymore to the servo brake, =
> brake opens.
> >Wheel turns again = too much force to the servo brake =
> blocking
> >again.........
> >If you tell me the wheel size you currently use and if you have
> a
> >brake drum with 4 or 5 lugs, I can give you some advise
> >The whole system is designed to work perfect at 90 % of the
> maximum
> >weight of the trailer. It will run up too light and brake too weak
> at
> >50 % and it will run up hard ( and brake weak again ) with
> 100% of the
> >trailers maximum weight.
> > * * * * Alfred- Zitierten Text ausblenden -
>
> - Zitierten Text anzeigen -

Hi Brian!
It seems,that I was wrong with my assumption - your wheels are the
right size.
So, it may be an extremly worn brake lining, may be together with a
wrong adjustment of the brake rod.
Please check first the lenght of the brake rod and look at the
linings. Use a light to see if the lower edge of the chamfer of the
lining is still visible.If not, new linigs are needed.
Please do not hesitate to contact me if you have no efforts.

Best regards, Alfred