From: Bjorn Brandstedt (super_deuce@hotmail.com)
Date: Wed Aug 03 2005 - 03:58:50 PDT
MV,
It's not an unlikely situation at all. When you are operating your vehicle
on a side slope and the drums lose their efficiency for some reason (water
perhaps), the pinion brake will easily make the wheel(s) on the "uphill
side" spin.
As far as the deuce's hand brake being a pinion brake, that's correct, but
you have separate control over it. It doesn't operate via the master
cylinder. The same goes for engine braking, which is driveline braking, but
you have separate control over it. Automatic transmissions and ABS makes
things more complicated and yes, locking diffs would help a lot off-road,
may be dangerous on the road, don't know, no experience here.
Driving slowly and off-road probably wouldn't create the need for extra
braking power anyway. It's the emergency situations and long downhill runs
that needs extra braking power.
Going downhill can almost always be accomplished without brakes with the
right speed/gear combination so, we are down to emergency situations,
transmission pops out of gear while driving downhill or the need for a
sudden stop. Those would be (are) my primary concerns.
The drum brakes SHOULD be able to handle the sudden stops (clean, dry
brakes) perhaps with the aid of the emergency brake. It's that long downhill
run with the transmission out of gear that will need the extra brakes for
heat dissipation. Here, the pinion mounted disks will help. Keep in mind
that the disks would have to be rather large to equal the heat dissipating
ability of the drums on the deuce otherwise you will have fading here as
well.
How large I don't know, but it should be calculated by the designer and made
known to the user.
There are aslo electric driveline brakes (Telma), or "retarders" as they are
called, but they are operated separately from the service brakes.
The drums can be made more efficient at dissipating heat by drilling holes
along the shoe/drum contact area. This will dissipate the gases formed when
the pads get hot and also make them essentially self cleaning.
To be safe if there is a mechanical failure, install a split braking system.
Bjorn
MVPA19212
The Blue Ridge Mountains of Virginia
>From: MV <MV@dc9.tzo.com>
>Reply-To: MV@dc9.tzo.com
>To: Bjorn Brandstedt <super_deuce@hotmail.com>
>CC: Military Vehicles Mailing List <mil-veh@mil-veh.org>
>Subject: Re: [MV] disc brake pressure, et al
>Date: Wed, 03 Aug 2005 01:55:42 -0500
>
>Logically what you say makes sense.
>
>But, think about this scenario:
>
>An M35 with moving forward.
>All 9x20's spinning in the same direction.
>The brakes are applied fully and for some reason the wheel brakes don't
>work- yet the pinion brake works and locks the drive shaft (better have the
>clutch in fully?).
>One wheel in the dual tandem set has tremendous traction, the other is on
>ice.
>The wheel on ice decellerates, stops, and reverses direction?
>
>I think this scenario is very unlikely realistically. There may be a
>tendency for one wheel to slow and begin to stop since it has little
>traction but I seriously doubt it would get anywhere close to reversing
>direction. Besides if the road conditions are that extreme perhaps the M35
>should not be on the road anyway, or at least it should be moving very
>slowly??? Even if it did reverse direction, the worse thing that would
>happen is that the effect of the pinion brake would be negated or
>non-effective in that situation. I can see why pinion brakes are not used
>as main vehicle brakes, however I think it is a litle ironic that the
>safety brake or parking brake is effectively a pinion brake.
>
>The same type of criticism has been made against vehicles with
>positraction. If you get in a condition where one wheel is on ice and the
>other is on pavement and you hit the gas, the vehicle will tend to turn
>itself due to the uneven traction. The argument against positraction is
>that it would be better to simply have the one wheel slip and the vehicle
>lose traction and rather than have the vehicle swerve under acceleration.
>But from a practical standpoint Positraction can keep you from getting
>stuck in marginal conditions.
>
>Dave
>
>Bjorn Brandstedt wrote:
>>"And as far as one wheel spinning backwards, what is there in the stock
>>system to prevent it? Yet the issue is never raised unless pinion
>>braking
>>is mentioned."
>>
>>
>>This is how I see it:
>>If the driveshaft is locked then the "sum" of the two wheels spinning must
>>equal zero. That's how a differential works.
>>So, the two wheels can drag on the pavement, which would produce a zero
>>"sum" or they can rotate in opposite directions and produce a "sum" equal
>>to zero. The wheel with most traction will force the other side to turn in
>>reverse to produce a "sum" equal to zero.
>>
>>When the wheels are braking at the wheels (service brakes) then the drive
>>shaft is still spinning from the engine/transmission. If one wheel locks
>>up, there is nothing making the other wheel reverse direction unless the
>>transmission is placed in reverse. The drive shaft will change speed
>>that's all.
>>
>>Does that make sense?
>>
>>Bjorn
>>
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