My Boat Trailer Is “Jerking”

My boat trailer seems to jerk (clunk around) when I start and stop. What causes that, and how do I fix it?

This is a common complaint, and it usually indicates a problem with your trailer brakes. It is important to investigate and fix this problem.

Most boat trailers are equipped with hydraulic “surge brakes”. When a truck is towing a boat trailer, both vehicles are travelling in equilibrium at the same speed. However, when you apply the brakes in the truck to slow down, the trailer wants to push against the truck causing a “surge”. That’s where the name “surge brakes” comes from.

There is a special hitch on the front of the trailer that converts that “surge or pushing effect” into brake fluid pressure, the same as a brake pedal would do if you were pushing on it. The harder you push the truck brake pedal with your foot, the higher the brake fluid pressure you create in the truck brake system.  The trailer will react the same, because hard braking in the truck creates a big surge on the trailer hitch, which creates high brake fluid pressures on the trailer. They both work together to slow you down, the truck brakes and the trailer brakes. It’s a simple but effective way to slow down the trailer in direct proportion to the urgency of your braking effort in the truck.

The surge brake hitch (called an actuator) on the front of the trailer slides in and out as you travel along. The actuator slides in when you slow or stop, and it pulls out (extends) when you pull away from a stop. This sliding activity is likely causing the “clunk” or “jerking sensation” that you feel as you start and stop. The solution is to minimize how far the hitch has to slide in to activate the brakes on the trailer. The further it needs to slide in, the bigger the “clunk” you will feel when you pull away from a stop. It does not need to slide in a lot to brake the trailer, so your goal is to eliminate excessive movement by finding and fixing the problem that is causing the excessive movement.

A good way to think of this is to visualize a brake pedal that goes WAAAY down to the floor before it does anything.  That is scary if you ever experienced it. This would indicate that drum style brakes need adjusting, or there is air in the brake fluid, very low brake fluid or lastly – severely worn brake shoes or pads. All of these problems can cause the actuator to slide in real far – thus creating a big “CHUNK” when you pull away from a stop.

It is very common for a boat trailer to have little or NO brake fluid in the reservoir. Many coastal boat trailers get dunked in saltwater each time they are used to launch a boat. The brake lines and wheel cylinders simply rust out and all the brake fluid leaks out on the ground.  Please check this before doing anything else. No brake fluid means NO brakes. No brakes is a sure way to feel the clunking/jerking effect while towing. If you are satisfied that the actuator is filled with brake fluid, proceed with the tests and adjustments below to find and fix the clunking.

Get a helper to work with you during this test. Jack up each wheel (one at a time) Use a lever of some type to activate the trailer brakes as shown in this photo and have the assistant rotate the wheel while you activate the brakes.

Trailer hitch

This will prove the surge actuator is working, and that each wheel is braking correctly. Remember to pull the surge actuator out to fully extend it each time you test by sliding it in with your lever.  Watch the distance that the actuator needs to slide in before the brakes work.

We need to point out that brakes with brake drums versus disc brakes have significant differences. Disc brake pads are not retracted by a spring when released. That means that less brake fluid volume needs to be pumped into the brakes to make them work, because they are always close to the brake rotor. Drum brakes on the other hand get fully retracted by heavy return springs every time you release the brakes. As the brake shoes and drums wear, you must move them further and further to make up for the wear, unless you adjust the brakes to make up the difference. This is done by using a brake adjuster tool (sometimes called a spoon) to move the shoes closer to the drum as the shoes wear. If you don’t do this, the actuator must slide in further and further because of wear, and the clunking feeling gets bigger and bigger. Watch any number of “How To Adjust Drum Brakes” internet videos to see how simple it is to adjust drum brakes.

As you adjust (or replace) worn brake shoes, note the reduced travel distance of the surge actuator travel as you adjust the brake shoes closer to the drum. You will see a remarkable reduction or elimination of the jerking or clunking as you start and stop.

Some actuators are equipped with small shock absorbers to reduce the speed of the actuator extending when you pull away from a stop. These shocks (or dampeners) can wear out over time, but our experience is that they get blamed for the jerking when they shouldn’t. Reducing the travel distance to activate the brakes is the most likely (and free) cure of the jerking.

To summarize:

Jerking is due to excessive travel distance of the slide on the actuator.

Excessive Travel distance is caused by low or empty brake fluid reservoir, worn brake shoes and drums, or out of adjustment brake shoes on drum brakes.

Excessive travel distance could be leaking seals in the master cylinder and/or the wheel cylinders. A wet oily stain would be the giveaway that these leaks are present.

We offer another helpful discussion on surge brakes. Read it by clicking this link: http://www.centrevilletrailer.com/how-to/surge-brake-troubleshooting-tips/ 


Based in Centreville, Maryland, Centreville Trailer Parts LLC wants to be a leader for all things needed to keep trailers in good working order. We have parts for all kinds of trailers, such as boat trailers, car trailers, enclosed trailers, horse trailers and large tractor trailers. When a part is needed, we WILL get it for you. Centreville Trailer Parts LLC has local Trailer Parts Stores serving customers in Maryland & Virginia and an Online Trailer Parts store serving customers worldwide.