Text 26971, 369 rader
Skriven 2009-01-29 10:20:11 av Roy Witt (1:397/22)
  Kommentar till text 26956 av Michiel van der Vlist (2:280/5555)
Ärende: Trailer brakes
======================
29 Jan 09 11:13, Michiel van der Vlist wrote to Roy Witt:

 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,
 RW>> longer.

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

I think you misunderstand the concept. By alternating between systems, the
brakes of both vehicles will be kept at a minimum. Engine braking will
keep both brake systems from engaging until they're needed, the trailer
braking system being nuetralized by the speed. By shifting up a gear, the
trailer brakes are then engaged, as the tow vehicle picks up speed.

 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.

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

I have enough experience to know about it.

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

You mean like the Rocky Mtns? Hiway 95 from Yuma to Flagstaff, AZ? Yes,
those are treachurous mountain roads, many of them overlooking deep
gorges, far below.

 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.

 MvdV> They are more complex than drum brakes, if only for the hydraulics.

Drum brakes also have hydraulics, not to mention a much more complicated
application mechinism. We don't have mechanical trailer brakes here.

Anybody who does is endangering their lives and those of others on the
road.

 MvdV> The drum brakes on trailers have no hydraulics, it is purely
 MvdV> mechanical.

Bad news...

 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
 RW>> much longer.

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

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

Maintenence of hydraulic brakes on you trailer would be no different than
the hydraulic brakes on your tow vehicle. i.e. those 'neoprene' seals
don't crack over a season. It takes many years before they deteriorate to
such a state. If you see oil, then you haven't done any maintenence on
them.

 MvdV> It is the same with tyres. It is tear rather than wear that limits
 MvdV> their life. After 6 to 8 years, there is hardly any visible wear.

On a trailer with very little weight to tire ratio.

 MvdV> Profile is still perfectly ok, but little cracks develop on the
 MvdV> sides and then it is time to to replace them.

Those cracks aren't from use, they're from environmental conditions. Tires
age exponentially in the sun vs your garage.

 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.

 MvdV> Another misconception.

Wrong again. It is a proven fact that mechanical brakes will not have the
stopping power of a hydraulic system. That's why mechanical brakes were
outlawed here in 1939...

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

That's the problem. Locking up the wheels isn't proof of superior stopping
power. If you think so, that's foolish thinking.

 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
 RW>> the insurance companys.

 MvdV> Apples and oranges.

Mechanical vs hydraulic brakes. Hydraulic known to be superior.

 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
 RW>> wrong or even expect to go wrong with a hydraulic system. Given
 RW>> frequent service in any use.

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

A once a year inspection isn't too much to ask.  Consider it the same as
the pre-flight inspection you have to do to fly an airplane.

 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.

 MvdV> No, that is not he problem. One problem is "spots" where the brake
 MvdV> pads rest on the disc.

Pads don't rest on the disc. They're always a few mm away from them. If
you find that they're touching the disc, you haven't done your
maintenence.

 MvdV> Another is stuck cylinders. Had that happen to me once on a car
 MvdV> that was not used in a year.

Another ill maintained brake system. The disc brakes on my 1988 Camaro
worked out of the box after sitting for nearly two years. Meanwhile, the
rear drum brakes needed a complete rebuild.

 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.

 MvdV> There you go. A major operation.

Remember, it sat for nearly two years and had no maintenence for at least
3 years.

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

Which gives you a false feeling of safety.

 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
 RW>> train may be.

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

60 feet! That's longer than a semi-trailer's trailer.

 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
 RW>> lot of those vehicles going faster than that.

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

Meanwhile law enforcement is having coffee and donuts at the local donut
shop.

 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.

 MvdV> Different league. We were discussing trailers behind passenger
 MvdV> cars.

Towing trailers isn't exclusive to passenger cars. There are 1 ton
'dually' pickups towing 40 foot trailers on the road on a daily basis.

Dually > four wheels on one axle.  ||___O___||
                                   ||       ||

 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
 RW>> had an accident while using just surge brakes.

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

Agreed.

 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.

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

Don't bet on it.

 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?

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

On a trailer is also seldom fatal. There are the 'hydraulic' brakes of the
tow vehicle that will do the emergency stopping regardless of what happens
to the trailer brakes.

 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?

 MvdV> Plus turn indicators and rear fog light.

Is your fog light white? White lights to the rear are illegal here.

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

Standardized connections are used here as well. If your tow vehicle didn't
come with a factory installed towing system, it can be purchased and
installed by the dealer, yourself or by the trailer sales people. My
pickup didn't have the towing harness installed when I bought it, so I
installed it myself when I installed the tow hitch.

 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.

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

A surge brake here has a hydraulic master cylinder mounted in the front of
the trailer's tow bar and is attached to the coupler. The master cylinder
has a floating connection with the rest of the trailer's frame. When the
tow vehicle slows down, the trailer brakes engage due to the momentum of
the trailer, as it tries to continue at the tow vehicles speed. By the
same token, the surge brake will engage when the vehicles are going down a
steep grade, whether the tow vehicle applies its brakes or not.

This system is inferior to the electronic controlled system in that it is
engaged without the control of the driver. Used mainly on light trailers
that won't see any heavy loads. i.e. you won't see this system on a large
camping trailer or long boat trailer.

 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
 RW>> and gearing as a braking system, the trailer would still be braked
 RW>> by the surge brake, as the trailer weight is still there pushing on
 RW>> the activation system. However, it would give the towing vehicle's
 RW>> braking system a chance to cool off.

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

I think it's a matter of perspective: one driver's habits vs anothers.

 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.

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

Ahhh, an electronic surge system. I've heard of those, but never
encountered one.

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

Something to look into.

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

If it works and is safe, then it should be certified.

                R\%/itt



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