Difference in Surge brake actuators

[Timm R Oday25]

The surge brake actuator on our trailer is getting long in the tooth .
While at rather large boat /rv warehouse ,I found a replacement acuator from Tie Down engineering .
What I don't understand is the difference between a surge brake actuator for drum brakes vs one for a disc setup .
Is it volume ,pressure ? The various websites don't explain the difference .
Thanks

[Charles Brennan]

Timm, Your question:
"What I don't understand is the difference between a surge brake actuator for drum brakes vs one for a disc setup. Is it volume, pressure?"   

The answer is: Yes.
Pressure first: Drum actuators range between 400 - 800 PSI, depending on braking load capacity (i.e. A Big-@$$ trailer vs a smaller one), whereas disc brake actuators range from 800 - 1200 PSI and as much as 1500 PSI.
Volume next:
The volume for drum actuators is marginally greater than disc actuators and also varies by load capacity (actuator size). 
Finally, (although you didn't ask the question and should have!)  Drum actuators have more piston excursion (throw) than disc actuators.
An excellent reason for making sure you purchase the correct hydraulic actuator for your application.
Seems to me, there is ample opportunity to mess up! 

"The various websites don't explain the difference."
You need to visit better web sites. 
The trailer web sites are of necessity, pretty vague in our Litigious Society, thanks to all the lawyers.  But the actual actuator manufacturers publish specs on their web sites, if you can manage to dig through enough layers to reach them and understand what they are saying.

Hope this helps,
Charles Brennan

[Krusen]

First off, I do not know.

I will give you what I do know in general about the two systems on cars, plus my surge drum brakes.

On traditional drum brakes, there is a lead and a lag shoe.  Going backwards, the action reverses, with the lead becoming the lag shoe.

The front piston of the cylinder pushes the end of the lead shoe against the drum, and the turning friction causes the "free end" to move against the "free end" of the lag shoe, causing it to press much harder on the drum.  The opposite end of the lag shoe is against a fixed anchor.

Because the lag shoe has very much more pressure on it, the lining is much thicker, so it wears out at the same time as the lead shoe.

There are springs to retract the shoes when no pressure is present. The link between the free ends is adjustable in length to compensate for wear of the lining.

Backing up, the opposite end of the cylinder has an identical piston, and all works in opposite manner, with the lead shoe end against the fixed anchor, and the piston on that end of the cylinder is full pressed in by the multiplied force from the friction.

This force multiplying is unique to the drum brakes.


Disc brakes have much larger cylinder areas, and I believe, higher hydraulic pressure.


Surge master cylinders have a spring resisting the force activating motion of the piston, and that spring establishes the pressure the master piston produces for a given braking action of the tow vehicle, while the area of the piston controls the volume.

From my ASSUMPTION that the pressure and volume are different, the area of the cylinder and spring strength would not be compatible.

I hope this helps a little.

Krusen, whose trailer had drum brakes, but all late model cars had disc, starting in 1970

[Timm R Oday25]

Thank you both