Text 26956, 246 rader
Skriven 2009-01-29 11:13:24 av Michiel van der Vlist (2:280/5555)
     Kommentar till en text av Roy Witt (1:397/22)
Ärende: Trailer brakes
======================
Hello Roy,

On Monday January 26 2009 12:22, you wrote to me:

 MvdV>> Replacing the drum brakes with disc brakes will not solve the
 MvdV>> basic problem which is that when going down hill, the surge
 MvdV>> brake system will always activate the trailer brakes, even if
 MvdV>> one never pushes the brake pedal and only uses then engine to
 MvdV>> control the speed.

 RW> And that "even if" in combination will keep the brakes cooler, longer.

A common misunderstanding, even among caravaneers. Engine braking will help
keeping the brakes of the tow vehicle cool, but it has no effect on the brakes
of the trailer. It does not matter what method is used to control the speed of
the tow vehicle: disc brakes, drum brakes, engin braking or the Flintstone
method; the trailer brakes will activate when going down a steep slope.

 MvdV>> This will cause the trailer brakes to overheat on a prolonged
 MvdV>> steep descent. While disc breaks are less sensitive to
 MvdV>> overheating then disc brakes, disc brakes will overheat as well
 MvdV>> is the descent is long end steep enough.

 RW> But as stated, in combination with low gearing and engine braking,
 RW> disc brakes will stay cooler longer, even with a heavy load in the
 RW> trailer.

No, see above. It seems to me you do not really have much experience with
trailers with surge brakes in combination with prolonged steep descents...

And when I say "prolonged steep descents" I do not mean coming down Blueberry
Hill. I mean coming down from 2000+ metres on a 5 to 10% slope with hairpin
curves every few km of the 20 to 40 km trip.

 MvdV>> Other than that: while disc brakes are superior to drum brakes,
 MvdV>> they have disadvantages too. They are more complex

 RW> I beg to differ. Disc brakes are mechanically as simple as you can
 RW> get.

They are more complex than drum brakes, if only for the hydraulics. The drum
brakes on trailers have no hydraulics, it is purely mechanical.

 MvdV>> and hence more susceptible to maintenance.

 RW> Simple maintenence on a disc brake system is merely replacing the
 RW> pads. To do similar maintenence to a drum brake system would take much
 RW> longer.

Recreational trailers do not make many clicks. The brakes do not wear out. I
have mine for 15 years now and I never had to replace anything. I inspect them
every spring through an inspection hole in the drums.

With hydraulic disc brakes, there would be the rubbers that crack, and oil that
leaks.

It is the same with tyres. It is tear rather than wear that limits their life.
After 6 to 8 years, there is hardly any visible wear. Profile is still
perfectly ok, but little cracks develop on the sides and then it is time to to
replace them.

 MvdV>> the drum brakes on a trailer with surge brakes, are purely
 MvdV>> mechanical,

 RW> If so, they have far less braking power than a hydraulic system.

Another misconception. The drum brakes on my caravan are powerful enough to
completely block the wheels in an emergency stop. That is all the braking power
one needs.

 RW> That was the difference between getting insurance on my 1937 Ford vs
 RW> my 1940 Ford. The former had mechanical brakes, while the latter had
 RW> hydraulic brakes. The latter being considered the safer system by the
 RW> insurance companys.

Apples and oranges.

 MvdV>> no hydraulics, no electrical parts, very little that can go
 MvdV>> wrong.

 RW> In today's world of braking systems, there is very little to go wrong
 RW> or even expect to go wrong with a hydraulic system. Given frequent
 RW> service in any use.

That's one of the problems; frequent service....

 MvdV>> Pus that recreational trailers - we call them caravans here -
 MvdV>> are often only used during the (short) holiday season.The rest
 MvdV>> of the year they are parked in a shed or something like that.
 MvdV>> Disc brakes do not take kindly to long periods of stand still.

 RW> You mean they have a tendency to rust? Rust on a disc is wiped clean
 RW> on the first usage of the brakes.

No, that is not he problem. One problem is "spots" where the brake pads rest on
the disc. Another is stuck cylinders. Had that happen to me once on a car that
was not used in a year.

 RW> Take my 88 Camaro. She sat in the driveway for 18 months before I
 RW> decided to replace the electric fuel pump, which is located in the
 RW> fuel tank. When I drove it from the garage, home, the rust was wiped
 RW> off the disc brakes within the first block. However, while it was at
 RW> the garage, I had to replace both wheel cylinders, have the drums
 RW> turned and new shoes installed before I felt it safe to drive.

There you go. A major operation.

The drum brakes of my caravan never need anything more than a dot of grease on
the linkages at my yearly inspection. I am confident it will be the same when I
inspect it again in March. It has been standing there since September...

 MvdV>> And last but not least there is maximum total train mass: 2430
 MvdV>> kg. In this case it is the sum of maximum trailer mass and
 MvdV>> maximum mass of the car, but for some cars the maximum train
 MvdV>> mass is lower than the sum.

 RW> That is how it is here too, but they also regulate how long the train
 RW> may be.

18 metres here. But that is too big for comfort anyway for a recreational
trailer. You want to get at places, not merely drive it on the freeway.

 MvdV>> http://www.youtube.com/watch?v=s4MQEYe4_0Y

 MvdV>> In most European countries there is a maximum speed for
 MvdV>> trailers. Normally between 80 end 100 km/h.

 RW> That's how it's regulated here too. 55mph. However, you will see a lot
 RW> of those vehicles going faster than that.

Here the automatic speed traps detect vehicle length. If it is over nine
metres, it assumes it is a truck or a combination and it triggers at the lower
speed allowed for trucks and trailers.

 MvdV>> Circumstances that do not really make it worth while to install
 MvdV>> disc brakes on trailers.

 RW> You can see some very large two vehicles towing some just as large
 RW> trailers here. Imagine a vehicle the size of a bus towing an equally
 RW> sized trailer.

Different league. We were discussing trailers behind passenger cars.

 RW> US DOT (department of transportation) regulations specify that
 RW> trailers with brakes must be fitted with an actuator that allows the
 RW> tow vehicle driver to operate the trailer brakes independent of the
 RW> tow vehicle brakes. That is usually ignored by those using surge
 RW> brakes in the US, but that could effect insurance premiums if you had
 RW> an accident while using just surge brakes.

Different country, different rules. Here it is surge brakes only for trailers
behind passenger cars.

 RW>>>  since they're programmed for trailer weight v tow vehicle
 RW>>> weight. Electric brakes must have a controller in the tow
 RW>>> vehicle, as there is a manual override switch in case of
 RW>>> electric failure.

 MvdV>> And that is the problem. They must be programmed for the
 MvdV>> specific combination.

 RW> That's not hard to do. Enter the two vehicle weight and the towed
 RW> vehicle weight and you're home safe. Even if you miss it by 500 lbs,
 RW> the braking ratio is set close enough.

What is not there can not break. What can not be programmed can not be
programmed wrong. With surge brakes, there is no controller and so there is
nothing to programme. Nothing to break, nothing one can do wrong.

 MvdV>> Do it wrong and the results may be worse than without. Also it
 MvdV>> requires cabling which is susceptible to wear and tear.

 RW> You have cables no matter which braking system you have. You do have
 RW> lights on your trailer, right?

Of course. But a failing light is seldom directly fatal. A failing brake
OTOH...

 MvdV>> The philosophy here is that  any trailer can be safely hooked
 MvdV>> up behind any car and that only only has to observe the mass
 MvdV>> limitations, nothing else.

 RW> Tail and brakes lights do count?

Plus turn indicators and rear fog light.

Connections are standardized (ISO 1724) so there is little that can go wrong.
It is recommended - but not required - to check the lights every time the cable
has been unhooked. I do.

 RW>>> Using engine braking and lower gears should allow the tow
 RW>>> vehicle to regulate the trailer braking. Using the trailer brake
 RW>>> to slow you down and the tow vehicle brakes only when really
 RW>>> needed will help to keep both sets of brakes cool.

 MvdV>> Nope. Not with surge brake system.

 RW> Yes, with surge braking systems too.

I really start to wonder if we are talking about the same thing. Are you sure
that what you call "surge brake" is the same as what I know as "oplooprem"?

 MvdV>> There have been experiments with systems that deactivate the
 MvdV>> surge brake system during descent:

 MvdV>> http://www.caravantrekker.nl/algemeen/adrem.php

 RW> I didn't imply deactivating the surge system. By using the engine and
 RW> gearing as a braking system, the trailer would still be braked by the
 RW> surge brake, as the trailer weight is still there pushing on the
 RW> activation system. However, it would give the towing vehicle's braking
 RW> system a chance to cool off.

The problem when driving in the Alps however is the *trailer brakes*
overheating....

 MvdV>> They seem to be effective, but for the moment their legality is
 MvdV>> questionable.

 RW> It's in Dutch, so I can only look at the pictures and surmise what
 RW> they're doing.

What it boils down to is an electromagnet that blocks the surge brake system.
When the brake lights come on, the current to the coil is interrupted and the
surge brake system becomes operational again. Also the electromagnet is not
strong enough to maintain the block if the force is in excess of 300 kg. So the
surge brake wil activate in an emergence stop even with the coil activated.

The system has been tested by several reputable magazines and it does indeed
seem to solve the problem of overheated trailer brakes during prolonged steep
descents, but it has not been certified for use on public roads. (yet) The
manufacturer claims certification is not required as it leaves the surge brake
system fully intact. (What else can they say?). But lawyers versed in the
matter say otherwise. Transport authorities remain silent for the moment. So it
is a grey area. Which makes it a near certainty that insurance companies will
refuse to pay when something happens.

Certification is a long and slow process. Much red tape to be cut. Let us hope
the manufactures of the "caravan Descent Control", as it is called, will not go
broke before it gets officially certified for use on public roads.


Cheers, Michiel

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