AT, AP, and MT Tire Tech
Rubber Side Down – AT, AP, and MT Tires
By Justin Banner
The first tire you probably think of when it comes to your Jeep is probably the Mud Terrain (MT) tire. What makes a MT such a specific tire? Just was is an All-Terrain (AT) Tire? Find out in this tire tech article.
A Mud Terrain tire is designed for extreme off-road terrain. Despite its name, the MT is used in more than just situations than muddy conditions. The focus of this tire type is debris ejection, be it mud or stones. Clearing out the grooves naturally with tire rotation allows the tread block leading edge to grab the next portion of the surface and “claw” through it. So, the grooves act sort of like a scoop. Otherwise, the grooves fill up and the tire loses traction.
The Mickey Thompson Innovation
However, an innovation made by Mickey Thompson in the 1960s and featured on nearly every MT tire since is the sidewall tread. Allowing the tred to continue down the sidewall of the tire gives the MT another area for traction in rocky and silty sand conditions. When aired down, the MT’s sidewall tread also helps to increase the tread width as the tires flatten out under vehicle weight.
Because it’s made to be aired down, the MT tires carcass is also designed much differently than a regular street tire. It typically features more belts to deal with the additional stress airing down creates on the sidewalls. Those belts are also designed to flex despite adding more of them.
The beads are designed to hold on to the rim at lower pressures, usually down to about 20-PSI before needing beadlocks. However, that’s not true for all MT tires. Some can go lower, some can’t go that low without a beadlock. Again, it’s up to the design so always follow the recommendations and warnings from your tire.
Unlike the MT tire, the All-Terrain is a compromise between on-road comfort and off-road capability. AT tires don’t generally excel in either area but work at their best in either. They feature smaller tread blocks for the best on-road noise and wet surface grip, but the blocks are still large and aggressive enough to be used in dirt and light rock off-road conditions. However, there has been a change in how an AT tire is designed and now there are two types of tire designs within the AT tire class. Each type of AT gets closer to the MT design.
AT vs All Purpose (AP)
The classic AT is designed with more on-road performance than off. Some will call this a Trail-type or even an All Season but there is a specific All-Purpose (AP) tire category. So, calling an AT and AP tire is technically wrong.
This is mostly because AP tires are capable of going off-road, but its smaller tread blocks and grooves allow mud to “stick” to the tire more. This reduces traction in that condition, but the smaller tread and harder tread compound mean that it won’t do well in rock crawling conditions.
The tread also doesn’t travel down into the sidewall and it features less belts than the MT tire. The sidewall ply and bead design will also only allow for normal tire pressures of 30-PSI and above. When people think of “truck tires,” like what you’d see on stock mid- and full-size pickup trucks and SUVs, this tire design is what they will picture. It’s perfect for trucks and SUVs that don’t see much off-road action, but if it does, it’s only going to be down a dirt road.
One of the biggest down falls of the MT tire is noise. Large tread blocks compress air into the ground, putting it under extreme pressure at the microscopic level. When the tread rotates, that highly compressed air shoots out at Mach speeds and creates the howling noise that’s typical of a very aggressive and blocky tread pattern. The other disadvantage to those large tread blocks is squirm, traction in wet road conditions, and rubber compound life.
Squirm is the movement of the tread on the road surface as the tire drives down the road. Because of its large size, the large lugs will squirm more and create heat. That heat travels through the lugs to spots where it can’t cool off and creates hot spots.
The combination of squirm and hot spots creates weaknesses in the lug and can cause chunking. Squirm is typically worse on the steering axel than the drive axle, but the drive axle can still see some squirm as you accelerate on changing road conditions.
Wet Traction and Rubber Compound
Despite its great off-road traction, wet asphalt or concrete surfaces will be its weakest points. Those large tread blocks with no grooves have a reduced amount of water removal. While the water can travel around the blocks, the blocks contacting the surface is trying to squish down water that’s between it and the road surface.
Water is a nearly incompressible fluid, so that treads ride above the surface. This causes hydroplaning and reduces traction to zero because the rubber can no longer form with the road, which is what creates grip.
A tire’s rubber compound, which arbitrarily describes the softness or hardness of rubber in tires, can also increase grip if its softer. Many MT tires are softer than their road cousins due to the requirement of traction in sand and rocks. That also means that a MT tire won’t always last as long as regular road tires. Not always, but a majority will not.
However, modern MT tires like the Nexen Roadian MT-X and Yokohama Geolandar X-MT are designed with mixed surfaces in mind. So, while the tread blocks are still larger than a standard road tire, they feature additional grooves and purpose made sipes. The sipes allow the tread to move in smaller sizes, reducing the squirm when compared to a fully solid tread lug.
The combination of grooves and sipes also helps in removing water so the tread can grip on wet asphalt and concrete roads. They also help reduce road noise by giving air an escape route before being compressed into the road.
For a many people, the MT design is too aggressive, but an AT isn’t aggressive enough. So, the next step in the AT ladder is the AT-X (or even referred to as an AT-R or “hybrid”) tire like the Mickey Thompson Baja ATZ P3. This type features larger blocks than the standard AT but still features sipes and grooves like a standard AT tire. The tread blocks are much more aggressive as well as larger and the sidewall of an AT-X has some tread, but not to the extent of a full MT tire. It also features more siping than a MT, but not as much as the AT.
Again, the siping is there to reduce squirm and improve wet road surface traction by giving water an evacuation path. The AT-X type All-Terrain tire is perfect for vehicles that see more off-road surfaces but still travel mainly on surface streets. It’s probably not going to work well as a rock crawler or dune tire, but you’ll be able to get to your favorite off-road and camping spots with no issues.
What both types of AT tires feature is reduced road noise. This is an integral feature of any tire that has smaller tread blocks and more grooves and sipes. When the tread rotates onto the road surface, it compresses the air.
That loads up air like a spring and when it escapes, it does so at Mach speeds from the energy it gains from being compressed. If the air has a path or pocket to escape to, it reduces that compression and potential energy. That slows down the air’s speed and you no longer hear the howl like you do of a MT tire.
Which One Do I Need?
Do you need a MT tire? Maybe. Maybe not. The only way to answer that is to ask yourself this question. “Where am I using my vehicle the most?” If you’re mostly running on surface streets with little to no off-road use, then you don’t need a MT tire.
If your response is the opposite, how often are you on those off-road conditions and can you deal with more road noise produced by those tires? If you just want a tire that looks cool and don’t care about noise and wet surface traction, you can’t beat the aggressive looks of the MT but don’t overlook those Hybrid ATs.
The AT tire is best compromise of on-road manners and off-road capability. You’re not going to be crawling up Backdoor with either AT tire, but you’ll produce less noise than the MT tire. You’ll be able to get to a spot where you can watch your favorite off-road racer and drive home with more wet surface traction than the MT.
If you want more off-road traction, then the AT-X will be a better choice. You’ll get a more aggressive look than the AT and better performance off the asphalt, too. However, if you need absolute off-road traction, then you’ll have to consider something more aggressive like the MT.
In short, the tire you want to run should match what you’re looking to achieve. Good thing we here at Vicious Off-Road have the tires you need from brands like Mickey Thompson and many others in stock in our physical store. Soon, those brands will also appear online, so be sure to keep your browser pointed here to see when the latest tires and other products come to our online store, too.