Text 26826, 124 rader
Skriven 2009-01-25 00:23:00 av Michiel van der Vlist (2:280/5555)
  Kommentar till text 26568 av Roy Witt (1:397/22)
Ärende: Trailer brakes
======================
Hello Roy,

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 force
 RW> you will have. Thus, you'd do well to have disc brakes on a larger
 RW> trailer.

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

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

Other than that: while disc brakes are superior to drum brakes, they have
disadvantages too. They are more complex and hence more susceptible to
maintenance. the drum brakes on a trailer with surge brakes, are purely
mechanical, no hydraulics, no electrical parts, very little that can go wrong.

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

Regarding the ratio of the mass of the trailer vs the mass of the tractor, it
is limited by regulations. Note: whatever I mention next regarding rules and
regulations applies to trailers drawn by passenger cars end/or light deliver
vans. For trucks ( max mass of tow vehicle greater than 3500 kg) the rules are
different.

For every car, there is a maximum mass of the trailer. It is printed on the
license. For example my Volvo 345 has an empty mass of 950 kg. The maximum
allowed mass (loaded) for the trailer is 1000 kg. There is also a maximum for
the total mass of the car: 1430 kg. It is even split out for the front and rear
axle. 665 kg for the front axle and 785 for the rear axle. Note that the sum of
those is more than the maximum total of 1439 for the whole car.

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

So you won't find combinations with a very heavy trailer and a light car.

For every combination, there is a critical speed, above which it becomes
unstable. The critical speed decreases if the ratio of trailer mass/tractor
mass goes up. It also decreases when going downhill. hers is what happens if
you exceed the critical speed:

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

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

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

 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,

Electric braking sytems are available as an option, but the regulations only
allow them as an *addition*, not as a replacement of the surge brake system.
The surge brake system *must* remain operational at all times, so electric
brake do not solve the problem of overheating during during long steep
descents.

 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 is
 RW> a manual override switch in case of electric failure.

And that is the problem. They must be programmed for the specific combination.
Do it wrong and the results may be worse than without. Also it requires cabling
which is susceptible to wear and tear.

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

The electric brakes offered here have the controler in the trailer and only in
the trailer. They are intelligent and have "learning capabilities". They offer
advantages, but as noted before, they do not solve the problem of overheating
during long steep descents.

 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.

See above: regulations limit the mass ratio.

 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 down
 RW> and the tow vehicle brakes only when really needed will help to keep
 RW> both sets of brakes cool.

Nope. Not with surge brake system.

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

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

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



Cheers, Michiel

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