Text 26872, 228 rader
Skriven 2009-01-26 12:22:09 av Roy Witt (1:397/22)
Kommentar till text 26826 av Michiel van der Vlist (2:280/5555)
Ärende: Trailer brakes
======================
25 Jan 09 00:23, Michiel van der Vlist wrote to Roy Witt:
MvdV> Hello Roy,
MvdV> On Tuesday January 20 2009 13:23, you wrote to me:
RW>> The difference in weight would be the reason to use disc brakes in
RW>> lieu of drum brakes. The more weight you tow, the more down-hill
RW>> force you will have. Thus, you'd do well to have disc brakes on a
RW>> larger trailer.
MvdV> Replacing the drum brakes with disc brakes will not solve the basic
MvdV> problem which is that when going down hill, the surge brake system
MvdV> will always activate the trailer brakes, even if one never pushes
MvdV> the brake pedal and only uses then engine to control the speed.
And that "even if" in combination will keep the brakes cooler, longer.
MvdV> This will cause the trailer brakes to overheat on a prolonged steep
MvdV> descent. While disc breaks are less sensitive to overheating then
MvdV> disc brakes, disc brakes will overheat as well is the descent is
MvdV> long end steep enough.
But as stated, in combination with low gearing and engine braking, disc
brakes will stay cooler longer, even with a heavy load in the trailer.
MvdV> Other than that: while disc brakes are superior to drum brakes,
MvdV> they have disadvantages too. They are more complex
I beg to differ. Disc brakes are mechanically as simple as you can get.
Not to mention that a disc will always be there hot or cold, while a drum
will have a tendency to expand when heated. The very nature of drum brakes
makes them inferior to the disc brake system, 2000 fold.
MvdV> and hence more susceptible to maintenance.
Simple maintenence on a disc brake system is merely replacing the pads. To
do similar maintenence to a drum brake system would take much longer.
MvdV> the drum brakes on a trailer with surge brakes, are purely
MvdV> mechanical,
If so, they have far less braking power than a hydraulic system. That was
the difference between getting insurance on my 1937 Ford vs my 1940 Ford.
The former had mechanical brakes, while the latter had hydraulic brakes.
The latter being considered the safer system by the insurance companys.
MvdV> no hydraulics, no electrical parts, very little that can go wrong.
In today's world of braking systems, there is very little to go wrong or
even expect to go wrong with a hydraulic system. Given frequent service
in any use.
MvdV> Pus that recreational trailers - we call them caravans here - are
MvdV> often only used during the (short) holiday season.The rest of the
MvdV> year they are parked in a shed or something like that. Disc brakes
MvdV> do not take kindly to long periods of stand still.
You mean they have a tendency to rust? Rust on a disc is wiped clean on
the first usage of the brakes. Take my 88 Camaro. She sat in the driveway
for 18 months before I decided to replace the electric fuel pump, which is
located in the fuel tank. When I drove it from the garage, home, the rust
was wiped off the disc brakes within the first block. However, while it
was at the garage, I had to replace both wheel cylinders, have the drums
turned and new shoes installed before I felt it safe to drive.
MvdV> Regarding the ratio of the mass of the trailer vs the mass of the
MvdV> tractor, it is limited by regulations. Note: whatever I mention
MvdV> next regarding rules and regulations applies to trailers drawn by
MvdV> passenger cars end/or light deliver vans. For trucks ( max mass of
MvdV> tow vehicle greater than 3500 kg) the rules are different.
As they are here. i.e. my 1/2 ton pickup truck isn't allowed by the Uhaul
company to tow any of their car trailers, since they feel it is unsafe to
do so with a 'short' wheelbase pickup. If it had an 8 foot bed rather than
a 6 1/2 foot bed, they'd allow it. Wheelbase on mine is 116 inches, while
on the longer bed pickups, it's 125 or 137 inches. Although it is legal to
tow a car trailer with my pickup, this is a Uhaul only thing.
MvdV> For every car, there is a maximum mass of the trailer. It is
MvdV> printed on the license. For example my Volvo 345 has an empty mass
MvdV> of 950 kg. The maximum allowed mass (loaded) for the trailer is
MvdV> 1000 kg. There is also a maximum for the total mass of the car:
MvdV> 1430 kg. It is even split out for the front and rear axle. 665 kg
MvdV> for the front axle and 785 for the rear axle. Note that the sum of
MvdV> those is more than the maximum total of 1439 for the whole car.
That is printed on the VIN plate on every US manufactured vehicle since
1981.
MvdV> And last but not least there is maximum total train mass: 2430 kg.
MvdV> In this case it is the sum of maximum trailer mass and maximum mass
MvdV> of the car, but for some cars the maximum train mass is lower than
MvdV> the sum.
That is how it is here too, but they also regulate how long the train may
be.
MvdV> So you won't find combinations with a very heavy trailer and a
MvdV> light car.
Not necessarily the case here. i.e. you will find Toyota mini-pickups
equipped with a camper that would be a large load for a normal sized, or
standard size, pickup.
MvdV> For every combination, there is a critical speed, above which it
MvdV> becomes unstable. The critical speed decreases if the ratio of
MvdV> trailer mass/tractor mass goes up. It also decreases when going
MvdV> downhill. hers is what happens if you exceed the critical speed:
MvdV> http://www.youtube.com/watch?v=s4MQEYe4_0Y
MvdV> In most European countries there is a maximum speed for trailers.
MvdV> Normally between 80 end 100 km/h.
That's how it's regulated here too. 55mph. However, you will see a lot of
those vehicles going faster than that.
MvdV> Circumstances that do not really make it worth while to install
MvdV> disc brakes on trailers.
You can see some very large two vehicles towing some just as large
trailers here. Imagine a vehicle the size of a bus towing an equally sized
trailer. They have disc brakes on every axle. I see these at the local
drag racing facility and they take up a lot of space.
MvdV>>> The only officially allowed system for trailers behind
MvdV>>> passenger cars is the push brake system.
RW>> That's odd. There are elecrically operated brakes that do a better
RW>> job,
MvdV> Electric braking sytems are available as an option, but the
MvdV> regulations only allow them as an *addition*, not as a replacement
MvdV> of the surge brake system. The surge brake system *must* remain
MvdV> operational at all times, so electric brake do not solve the
MvdV> problem of overheating during during long steep descents.
US DOT (department of transportation) regulations specify that trailers
with brakes must be fitted with an actuator that allows the tow vehicle
driver to operate the trailer brakes independent of the tow vehicle
brakes. That is usually ignored by those using surge brakes in the US, but
that could effect insurance premiums if you had an accident while using
just surge brakes.
RW>> since they're programmed for trailer weight v tow vehicle weight.
RW>> Electric brakes must have a controller in the tow vehicle, as there
RW>> is a manual override switch in case of electric failure.
MvdV> And that is the problem. They must be programmed for the specific
MvdV> combination.
That's not hard to do. Enter the two vehicle weight and the towed vehicle
weight and you're home safe. Even if you miss it by 500 lbs, the braking
ratio is set close enough.
MvdV> Do it wrong and the results may be worse than without. Also it
MvdV> requires cabling which is susceptible to wear and tear.
You have cables no matter which braking system you have. You do have
lights on your trailer, right?
MvdV> The philosophy here is that any trailer can be safely hooked up
MvdV> behind any car and that only only has to observe the mass
MvdV> limitations, nothing else.
Tail and brakes lights do count?
MvdV> The electric brakes offered here have the controler in the trailer
MvdV> and only in the trailer. They are intelligent and have "learning
MvdV> capabilities". They offer advantages, but as noted before, they do
MvdV> not solve the problem of overheating during long steep descents.
Too bad they didn't have the foresite to put the trailer brake controller
in the cab of the tow vehicle. One can use either or both braking systems
to keep the brakes cooler on both vehicles.
MvdV>>> When the car decelerates, the inertia of the trailer makes it
MvdV>>> push against the car and that activates the brake. It is a
MvdV>>> pureley mechanical system. Very reliable and it works with
MvdV>>> every trailer and every car combination.
RW>> Unless the tow vehicle is a light weight vehicle and the trailer
RW>> isn't.
MvdV> See above: regulations limit the mass ratio.
True enough. But, having independant control over both braking systems is
a much safer system.
MvdV>>> The only problem is that it automativally activates when going
MvdV>>> downhill on contstant speed. Even if you never touch the brakes
MvdV>>> and do it all with the engine, the trailer brakes will still
MvdV>>> activate when going downhill. In the high mountains, this is a
MvdV>>> sure recipe for overheating the brakes.
RW>> Using engine braking and lower gears should allow the tow vehicle to
RW>> regulate the trailer braking. Using the trailer brake to slow you
RW>> down and the tow vehicle brakes only when really needed will help to
RW>> keep both sets of brakes cool.
MvdV> Nope. Not with surge brake system.
Yes, with surge braking systems too.
MvdV> There have been experiments with systems that deactivate the surge
MvdV> brake system during descent:
MvdV> http://www.caravantrekker.nl/algemeen/adrem.php
I didn't imply deactivating the surge system. By using the engine and
gearing as a braking system, the trailer would still be braked by the
surge brake, as the trailer weight is still there pushing on the
activation system. However, it would give the towing vehicle's braking
system a chance to cool off.
MvdV> They seem to be effective, but for the moment their legality is
MvdV> questionable.
It's in Dutch, so I can only look at the pictures and surmise what they're
doing.
R\%/itt
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