For most people trailer brakes are just part of the trailer – something we hook up and expect to work. That is great most of the time. However, for those who want to dig a little deeper, especially with tandem axle brakes, there is a whole world of things to consider. In this article we are going to dive into just a few.
While we will touch on brakes in general, this article is more about applications for tandem axle brakes. Specifically, we will look at a few conditions, like surge brakes versus electric trailer brakes, and mixing them. Also, how an axle with brakes can interact with an idler axle (no brakes).
Navigating the world of tandem axle brakes can be somewhat daunting for both novice and experienced trailer owners. Brakes are crucial for safe towing with larger trailers. Yet, with so many options, we need clear navigation through the intricacies of brake types and placement. Because of the scope involved with tandem axle brakes, we won’t address everything, but hopefully, we can demystify a few areas to help minimize risks on the road. In other words, this article is not the complete story.
Let’s start with some definitions.
Surge Brakes / Electric Brakes
For trailers, the term “Surge Brakes” is a class of brakes that use the trailer’s momentum to actuate. Usually they are a hydraulic braking system which engages the brakes when the towing vehicle decelerates. When the tow vehicle begins to slow down, the difference in momentum makes the trailer brakes actuate.
On the other hand, “Electric Brakes” utilize an electric brake controller in the towing vehicle to apply the proper braking force to the trailer’s wheels. When the driver activates the tow vehicle brakes, an electric signal goes to the trailer so the brakes will engage properly, with the right amount of force.
For completeness, we should also mention that an “Idler Axle” is one that does not have brakes.
Function
Surge Brakes operate autonomously. They rely on momentum difference between the trailer and the tow vehicle to engage. As the towing vehicle slows down, the surge brake actuator compresses, sending hydraulic pressure to the trailer brakes and slowing it down with the vehicle.
Electric Brakes are directly controlled by a module installed in the towing vehicle. When the driver applies the vehicle’s brakes, the controller sends an electric signal to the trailer brakes, applying a braking force proportional to the amount of braking applied by the driver. The amount is all based on settings in the controller. Additionally, most systems allow the driver to manually apply the trailer brakes using the electric brake controller.
Application
– Surge Brakes –
Surge Brakes are common on trailers where the tow vehicle is not always the same. Because they do not require special wiring or a brake controller settings, they are well-suited for compatibility across many tow vehicles. Brake operation is set for the trailer, and stays with the trailer, so the brakes will function the same with every vehicle that pulls it.
One issue often reported with surge brakes is a “clunky” feel. This is especially true when the trailer load is different at various times. For instance, if you rent a car hauler from U-Haul, it will probably have surge brakes to adapt easily for any tow vehicle. When the trailer is empty, brakes feel different than when a car is on the trailer. While most systems do a pretty good job of braking based on momentum (more momentum with a heavy load, and less momentum with a light load), there are still “clunky” effects simply because the brakes don’t apply until there is a difference in momentum.
Another issue is backing up. Yes they have control features so brakes disengage for reverse, but that can also conflict, making some weird situations. Pluses and minuses.
Years ago when these systems were not as refined, some local laws were enacted to outlaw the surge brakes. The Surge systems were blamed for accidents. If that is a concern, check your local laws.
One other consideration. When a trailer sways, one good way to counteract it (before it is out of control) is to apply the trailer brakes – without tow vehicle brakes. That is not possible with surge brakes. In fact, applying tow vehicle brakes can actually worsen the problem. Something else to think about.
– Electric Brakes –
Electric Brakes are compatible with most vehicles equipped with a brake controller, and are suitable for heavy loads or situations where frequent braking adjustments are necessary. The settings for braking control can be custom and precise, which makes the ride nice. Electric Brakes are usually the preferrerance when the tow vehicle always pulls the same trailer.
Electric brake control is becoming more and more the default standard. Vehicles come complete from the factory with brake controllers that are reasonably good at adapting. There are still issues, of course, in adjusting the controller so the trailer brakes respond properly. However, when the control settings are correct, the trailer brakes usually work well.
The issue with electric brakes is adaptability to multiple trailers. Every trailer needs slightly different settings to pull flawlessly, so controllers need adjustment for each one. This can become fiddly if you tow multiple trailers with one vehicle. Most people find a happy medium that works OK for the variety of trailers they pull, and though none is perfect, it works acceptably for them.
– Brake Types –
While this does not really fit in this article, I will mention a couple things about the type of brakes. Most trailers still use drum brakes – mostly because they are relatively easy to actuate electrically. Drum brakes also work well with hydraulic actuators.
Disk brakes, on the other hand, require forces in that are not so easy to apply electrically.
Disk brakes are pretty universal in cars and trucks now, but slow for adoption in trailers, I believe because they add cost and complexity. They do appear, like in this image of tandem axle brakes from PJ Trailers. Most, however, are either electrically actuated hydraulic brake systems (electric over hydraulic), or surge actuated for trailers.
Neither of these systems, hydraulic or electric are better or worse suited for tandem axle brakes, or single axle brakes. Just make sure the system fits the axles and load capacity you need.
We can write more on brakes in a later post if that is what you want to see. Just tell us in a comment.
Maintenance
Surge Brakes typically require less maintenance compared to electric brakes since they have fewer components and do not rely on electrical connections. However, periodic inspection of hydraulic lines, brake fluid levels, and the surge actuator is essential to ensure proper function.
Electric Brakes may require more maintenance due to their reliance on electrical connections and components. Regular inspection of wiring, brake magnets, and the controller is necessary to detect and avoid problems. Additionally, we often need adjustments to brake sensitivity and braking force to optimize performance based on trailer load and road conditions.
If we are comparing, I would call maintenance a toss-up. The truth is, every brake system needs attention, and, it pays big dividends to make sure your brakes are working. That is true of both surge brakes and electric brakes.
Safety / Break-Away
In most jurisdictions a breakaway system is required for trailers that have brakes. Since we have already discussed Breakaway systems, I will just point out that electric breakaway systems are easier to implement simply because the brakes are actuated electrically. Hydraulic brake systems achieve the breakaway function in other ways. Some people really like the manual interface, while others prefer an electrical interface.
Personally, from a breakaway standpoint, I’m not sure one is better than the other. They are just different.
Brake Axles / Idler Axles
The difference between Brake Axles / Idler Axles is pretty obvious, but you might not be so familiar with the terms. A Brake Axle has brakes at each wheel hub. An Idler Axle does not have brakes.
In single axle applications, brakes are required typically if the trailer is 3,000 lb GTWR or more. That makes it simple. So, we often find an idler axle on smaller, light trailers. For a heavier trailer, a brake axle is required. (Read more about this in the article “Does My Trailer Need Brakes?“)
On the other hand things can get a little weird when we get to tandem axle brakes. In most cases both axles will have brakes. However, it is not uncommon to find tandem axle brakes where only one axle will have brakes, and the other will be an idler.
We will focus the remainder of the article on this area of Tandem Axle brakes.
Tandem Axle Brakes
When it comes to determining which axle should have brakes in a tandem axle trailer, there are indeed several factors to consider.
The safest, and most common is to have equal brakes on both axles. This is especially true if the trailer is particularly heavy. (Yes, I know, the word heavy is relative. In this context we will describe it as approaching the weight of, or exceeding the weight of the tow vehicle.)
Both axles with brakes. That is our recommendation, with very few exceptions.
While that is a practical recommendation, we are also willing to explore other possibilities from a theoretical standpoint.
Tandem With One Idler
While it is not so common, we do see trailers where one of the tandem axles is an idler. The most obvious negative to using just one brake axle: You have half the potential braking capability.
Brakes work in part based on how much force the wheel is carrying. More load on the tire means more friction capability between the tire and the road. With a tandem axle the weight on the braking wheel does not double, so the braking force available is half that of having both brake axles.
What benefit is there to having one axle as an idler? Cost is the main reason for this choice from what I understand. To me, cost savings by sacrificing safety is questionable. Safety? Well, the real sacrifice is stopping distance, which relates to safety.
So, even though we recommend against it, we will follow this idler question a little deeper. The question often arises which axle should be the brake axle, and which should be the idler? Front or back?
Rear Tandem Axle Brakes
Let’s look at the situation where the rear most tandem axle has the brakes.
Condition 1
One argument says that having brakes on the rear axle could potentially cause the equalizer bar to unload the front axle when braking. This makes some theoretical sense. However, it depends strongly on the suspension setup.
In a traditional tandem axle setup, practical considerations such as the compliance of the tow vehicle’s suspension, the trailer suspension, and the weight shift during braking will more than mitigate this effect. Look at the mechanism around the equalizer. If the rear axle could potentially cause the equalizer bar to unload the front axle, it would have to apply a force on the equalizer sufficient to push it forward. With a traditional linkage, considering the pivots at each shackle, that really can’t happen.
On the other hand, sometimes the rear springs attach directly to the equalizer – like a typical slipper spring setup. In these cases the argument holds more water. It would have a little bias toward lifting the front axle, but honestly, very little. And, the benefit is putting more weight on the braking tire which would give slightly more braking power. This is a small effect.
Condition 2
The next argument considers forces from the brakes to the trailer. In a traditional tandem setup, if the rear axle has brakes, then forces to the trailer must go through the back portion of the leaf springs. (Shackles at the front of the rear leaf spring can’t really transmit forces in the fore and aft direction.) That puts the rear portion of the rear leaf springs in compression. Looking at the Arc of the spring, that means those forces will try to roll up or curl the rear leaf spring.
For cases where the rear springs attach directly to the equalizer, (like the slipper springs above), this becomes a non-issue.
Front Tandem Axle Brakes
What happens with weight shift when braking for a tandem axle trailer? Certainly there can be more load at the tongue, which shifts more weight to the tow vehicle. Yet, if the tandem axles are properly equalized, they will both carry roughly the same amount of weight.
Is there an argument for having the front tandem axle with brakes?
Perhaps the best argument is the opposite of what we said about the rear tandem axle brakes. When braking forces apply, the front axle must act on the front portion of the leaf spring, meaning that portion of the spring is in tension. Since the leaf spring is made for straightening loads anyway, it will handle tension forces arguably better than the compressive forces discussed above for the rear axle.
It effectively makes the front axle and leaf springs act like a trailing arm suspension which is a proven, stable configuration. This is true with all the leaf spring configurations: eye-eye, slippers, etc.
Please note, when looking at the front axle, it is similar to that of a single axle leaf spring suspension.
So, if you must use an idler axle in a tandem axle brake situation, I would make the back axle the idler, and the front axle with brakes. That is true of eye-eye and slipper springs.
Mix And Match – Surge & Electric Trailer Brakes
A question from a reader: Can you mix Surge and Electric for tandem axle brakes?
“If one axle has electric brakes, and the other surge brakes, it could largely mitigate the clunks of the surge brakes and gain the ability to manage trailer squirreliness using the perks of the electric brake controller. Would it work?”
The consideration is understandable. Electric brakes can provide more precise control, particularly in emergency situations. Surge brakes allow at least one axle with brakes even if the tow vehicle does not have an electric brake controller.
Such a mixed axle scenario will make one axle an idler in various situations, while the other provides braking. If the electric is not connected, the surge brakes will be the brake axle, and the electric axle will be the idler. Pretty simple.
In conditions like backing the trailer, the electric brakes will work all the time, even when surge brakes become a little confused. That effectively makes the surge axle an idler, and the electric one the brake axle. I would also guess that application of the surge brakes will delay in normal driving because the electric brakes will minimize the momentum differences thus delaying or reducing surge brakes applying. That might feel good in light braking situations, and it might limit the surge brakes full application in hard stops. Hard to say where the crossover will be and how that might feel.
It could be a fun, though expensive experiment.
What Are The Issues?
- Most prominent, You will have only half the braking capability when not hooked to an electric brakes controller. That means longer braking distances when using surge only.
- You must maintain two different brake systems. They have to be independent, so you must have both full systems.
- Depending on the Surge system installed, it may require two Breakaway systems. Maybe you can get away with just the one for the Surge brakes? Something to consider.
- While I can’t say exactly, I believe there will be some undesirable interactions when both systems are in play. (See the section above.)
- More tire wear on the primary brake axle, and especially on the tow vehicle.
- Particularly if the trailer is heavier than the tow vehicle, an idler axle is a bad idea. Even if it is only some of the time. IMHO.
As A Positive
- It is very unlikely that both systems will fail at the same time.
Summary For Mixed Tandem Axle Brakes
Ultimately, if you are willing to manage multiple braking systems on the trailer, theory says it could be beneficial. But you must accept significantly reduced braking in non-electric situations.
While the concept of mixing surge brakes with electric brakes for tandem axle brakes is intriguing, I think there would be a lot of experimentation to make it work well. Of course I haven’t tried it, but that is my gut feel.
Oh, and put the Surge axle in front since that one will always be in action. Good luck.
Moving Slightly Off-Topic
Side Note: Just for grins I gave some of these questions to ChatGPT. The answers I got sound so good, but some of the information was horribly misleading. I love the concept of AI giving immediate access to information, yet it makes a lot of mistakes, or gives incomplete answers. Do be careful.
2nd Side Note: In doing some other research, I have also found there are many websites full of the same kind of BS I got from ChatGPT. Maybe they use AI to write articles? What a terrible situation we are in. The internet is so full of information, but we don’t know what is good and what is not. This may be most true with technical-ish topics.
With the above said, if you find things with our articles that are not correct, please let us know. Our intent is to provide accurate and complete information in our articles. Please, if you see anything wrong with our information, bring it to our attention. Thank you.
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