What's So Special about ST Tires?

Aug 23rd 2018

What's So Special about ST Tires?

So you have to tow something behind your vehicle. Maybe you’re moving, maybe you’re taking a boat out for a fishing trip, or maybe it’s a camper or an RV. Either way, if you’re hauling something, you should know something about ST tires. Well, as always, we’re here to help, so let’s talk about what makes an ST tire special.

What is an ST tire?

ST stands for Special Trailer and, as you can probably guess, they’re tires designed specifically for trailers. Unlike regular tires, these are made to be towed. They’re generally constructed from more durable materials and they have a thicker sidewall than tires designed for drive positions. In other words, you wouldn’t want to put one of these on the front of your car. However, they’re great for being pulled along (you just don’t want them leading).

You see, trailer tires are designed to handle a heavier vertical load, which means their sidewalls are a lot thicker than passenger tires. Their overall construction is generally pretty robust. This is great for trailers, because it cuts down on swaying but also makes sure they can withstand more weight coming right down on them. Think about it: if you load a boat on a trailer, what’s going to happen if the sidewall is flexible? They’ll squish way too much!

ST tires are also usually designed with a higher operating pressure, which lets them carry more vertical weight. Their traction is also not designed for a driving position, but it’s just right for keeping your trailer on the road and maneuverable.

There are a few different types of construction for ST tires:

  • RH (Radial): This is a radial tire, meaning it uses ply cords that extend from bead to bead. Unlike bias construction, these plies are parallel to each other and combined with stabilizer belts.
  • LH (Bias): This is a bias tire, meaning it uses ply cords that extend diagonally from bead to bead, forming a criss-cross pattern. They do not have stabilizer belts.
  • All-Steel: These are basically a radial tire with all steel plies throughout the sidewall and body. This makes them especially durable and great for heavy-duty loads.

Bias vs. Radial: How to choose

Bias tire vs Radial tire

Deciding which construction to get isn’t really about which one is the best, it’s about finding out which is best for the job. The fact is both constructions have their uses and benefits. Really, depending on the application, either one can work for you, but you can’t stick one of each on your trailer and hope for the best. So how do you know?

Well to decide it’s best if you know a bit about how the different constructions perform, so let’s take a look:

Bias Tires: The bias construction means that the whole body of the tire is more flexible, instead of just the sidewalls. This results in good traction and stability and tougher, more durable sidewalls. It also means the tread can more easily eject stones while driving, and they’re easier to repair if damaged.

This flexibility can also work against it, though, as bias tires tend to wear out faster. They also have an increased rolling resistance (which cuts down on fuel efficiency) and are more prone to flat spots. They’re also not great for high speeds, as they can lose their stability.

With this in mind, bias ply tires are best if you’re:

  • Going on a short trip at low speeds
  • Travelling over rough terrain
  • Won’t have your loaded trailer sitting for long periods of time

Radial Tires: The radial construction means that the sidewalls are more flexible than the tread area. The biggest advantage of this is a longer tread life, as less flexing means less heat buildup. This also means you can drive faster. They’re more easily controlled and have a lower rolling resistance, making them more fuel efficient, and the stabilized tread area also makes for a smoother ride.

Just as the bias tires, though, these have their disadvantages. For one thing, they’re going to be more expensive. For another, they’re not great at rougher terrain and they’re harder to repair if damaged.

So radial tires are best if you’re:

  • Planning a long trip at higher speeds
  • Driving on highways or paved roads
  • Need better control for turns and city driving

Once you figure out which works best for you, it’s pretty easy to find out how your tires are constructed. Radial tires all have an R in their size, usually right before the rim size (for example 205/75R15). If they don’t have an R, they’re bias tires. It’s also worth noting that radial ST tires are mostly going to be name brand. If they’re budget brand, it’s pretty likely that they’re bias ply.

So why can’t I just use an LT tire?

You’re probably wondering why you need special tires for your trailer when LT tires exist. Well, there are several reasons, the main one being they’re made differently. You see, while LT tires are designed to handle heavier loads, they’re still essentially a passenger tire. They tend to have more focus on traction and comfort, and they have way more flexible sidewalls by comparison to improve both handling and ride quality. However, this means that while they can handle heavier loads, they can’t necessarily handle trailer loads.

Not only will the vertical weight put a strain on LT tires, but they’ll also cause your trailer to sway. Trailers are taller than trucks generally are, which means their center of gravity is higher as well. This is why trailer tires have sidewalls that don’t flex as much. For one thing, they’re not worried about a comfortable ride. For another, if the sidewall flexes over every bump, that’s going to cause the trailer (with its high center of mass) to wave around like one of those tube men you see outside of stores (and that’s generally not something you want in a trailer).If you have an RV, however, LT tires might be a better option, but remember: you’re riding in it. As we talked about above, ST tires are made for towing, not for drive positions, so in a case like that where you need control and traction, LT tires could be an option. However, for a utility trailer or even an RV that’s towed behind your vehicle ST tires are definitely the better choice.

ST Speed Ratings

Now let’s talk about speeds. Just like regular tires, trailer tires have a speed rating. Some, like bias ply tires, may be rated to lower speeds than others, but generally the maximum speed is 65 mph. Going over this speed, especially for long periods of time, increases the stress and heat buildup on the tire, both of which will just increase the tire’s risk of failure. These tires were designed for towing, after all, not racing.

There are a few models that, under certain circumstances, can be used up to 75 mph. This usually involves inflating the tire to 10psi over its recommended cold inflation pressure. This, however, can only be done if the wheel allows for a pressure increase. If not, you’re stuck at 65 mph. It should also be noted that the increased pressure doesn’t increase the load carrying capacity of the tires, only their speed. You also can’t go up any higher than 10psi, so don’t think you can just increase it more if you want to go faster. Like I said, these aren’t racing tires.

ST Life Expectancy (and tips for improving it)

A red horse-carrying trailer

So how long to trailer tires last? Well just like regular tires, it depends. The industry recommendation is that you change your trailer tires every 3 to 5 years, but that’s assuming they’re in ideal conditions. There are a lot of factors that can cause them to fail or shorten their life, and to be honest a lot of these are user error. So let’s take a look at some of the main ones:

  • Overloading: While ST tires have some of the highest load ratings, that doesn’t mean they can’t be overloaded. The load rating is the maximum weight the tire can carry at the inflation pressure, but this isn’t cumulative. For example if your tires can support 1000 lbs, this doesn’t mean you can load up 4000 lbs because you have 4 tires. If the weight isn’t distributed evenly, the individual tires can end up carrying more than their share.
    • How to avoid it: Make sure you know the weight of your trailer and the maximum weight each tire can handle. It’s also best to make sure you get the proper tires. If you need a heavy-duty tire, get it! Also, if one tire blows out, whether it’s on a single or dual axle, it’s always best to replace the other tire (either the one next to it for a dual axle or across from it on a single). Even if it doesn’t look damaged, it could have internal structural damage from taking that excess weight.
  • Over/Under Inflation: Having the wrong air pressure for the load can also lead to failure. It can make the tires wear faster or unevenly, and the handling can take a hit as well. Even if the tire looks fine, the internal structure can be under undue stress or even damaged. Another reason it’s important to know the weight of your load!
    • How to avoid it: Know how much to inflate your tires for the load you’re carrying. Luckily, manufacturers usually print load and inflation tables you can consult for the correct pressure. On top of that, it’s important to check your psi regularly, especially if the temperature changes.
  • Improper Weight Distribution: If the manufacturer did their job, the trailer itself should have an even weight distribution. It’s when you load it that you can hit a problem. This connects back to overloading, in that if the weight is distributed wrong, you could put pressure on one tire over the others. So you don’t want to put a couch on one side and a feather pillow on the other!
    • How to avoid it: Like I just said, don’t put all the heavy things on one side of the trailer. Try to balance it out as much as possible, so one side or tire’s not holding more than the others.
  • Excess Heat Buildup: This can be caused by over/under inflation, but it can also be the result of driving at speeds higher than recommended. Excess heat is an early death sentence for a tire. It can break down the whole construction and cause it to wear faster.
    • How to avoid it: Make sure your tires aren’t overloaded, over/under inflated, and are the right ones for the application. Also, don’t exceed the recommended speeds.

As you can see, a lot of these can be avoided with some common sense: mainly don’t overload your tires and pay attention to the manufacturer recommendations. But what about when your tires aren’t in use? Sitting for a long time can sometimes be just as bad. So, if your trailer’s going to be sitting for a while, here are some tips to keep your tires in shape:

  • Keep them covered. UV rays, weather, and heat can all lead to a tire aging and degrading faster than it should, especially when it’s sitting still. ST tire compounds are especially susceptible to UV rays, so if it’s going to be parked for a while it’s best to keep it in the shade. You can also get tire covers to protect them from the elements.
  • Lower the air pressure. This lets the tires flex a bit more and evenly distribute the weight across the footprint. If the air pressure is kept at the maximum, it can increase the occurrence of flat spots and tire deformation.
  • Park on a hard surface. This also ensures the integrity of the tires. Parking on a soft surface can cause the tire to distribute its weight unevenly over time. A hard surface gives it a strong base to make sure it can evenly distribute the weight on top of it.
  • Put the trailer on blocks or stands. If it’s going to be sitting for a really long time, or if you have to keep it fully loaded, it might be best to put the trailer on blocks. This takes the weight off of the tires, so they won’t deform.
  • Unload the trailer. This might not always be possible, depending on the circumstances, but when it is it’s certainly better to just have the weight of the trailer on the tires while sitting. The less weight pushing down on them, the better.

Just like regular tires, ST tires will last longer with proper care and maintenance. Hopefully the information we’ve provided here will not only help you find the right ST tires for your trailer, but also get the most out of them. Because, while they’re durable, ST tires aren’t invincible, and understanding a little about how their made and how they work can save you some hassle. As the saying goes: a little knowledge goes a long way!

So why not use your newfound knowledge and shop our trailer tires?

Drive safe out there!

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