PRODUCT EVALUATION STI BLACK DIAMOND XTR TIRES Aggressive rubber from a new source

q From what we’ve seen, the sport of ATV riding is as strong as ever. Even if quad sales are down, people are still riding every weekend; it’s just that they’re doing it on older machinery. And as long as people are still riding, tires are still wearing out. Despite that, we haven’t seen much happening in the world of rubber; very little advancement has taken place recently. The one exception, however, is STI.

STI is the in-house brand from Motorcycle Tires & Accessories, a large wholesale distributor. This year, the company announced an entirely new line of tires for ATVs and UTVs. The Black Diamond utility tires look great, and we were anxious to give them a try.


A few key people from ITP recently jumped ship and designed the new Black Diamond tires for STI. The first priority in the new tire was to have an aggressive tread with lots of rubber. The tires that come on 4WD quads are conservative for a reason: the manufacturer doesn’t know where they’re going to be used. It could be mud, hardpacked clay or even a paved equipment yard. The STI Black Diamonds are designed for use in dirt and mud, not on concrete or pavement, so the designers made them very aggressive with tall, deep lugs. The tread is 1 1/8-inch-deep on the XTR models and 3/4-inch deep on the ATR models. Both tires have a wide “V” pattern with full-depth lugs alternating with more reinforced clusters of rubber. The full-depth sections allow for more bite in the mud, while the reinforced sections have less flex and are less likely to roll on hardpacked dirt. The “V” pattern is designed to clean itself in use so that mud buildup doesn’t turn the tire into a smooth, round doughnut.

The same tires are designed for use on ATVs and side-by-sides, so a radial case is used with a six-ply. It’s a tough tire; this is not a tentative launch with only a few applications. There are already 30 different sizes available.


All the theory in the world is fine, but it all comes down to actual use. We mounted a set of Black Diamond XTRs on a Polaris RZR 800 and a set of ATRs on a Suzuki KingQuad 500. First off, we like the way they look on the two machines. This isn’t to be taken lightly. Owners spend hundreds and thousands of dollars on cosmetic parts that have much less impact. ATVs have huge tires that dominate their overall appearance.

Our test loop had a rocky, wet sand wash, hard-packed fire roads, and a bottomless mud bog. We even turned a lap or two on a Lucas short course for trucks. Despite having the more aggressive tread depth on the RZR, cornering was excellent. There was no real feeling of tire roll even on the hardpacked fire road. In general, a stock RZR doesn’t really push its tires very hard; it takes a lot of horsepower and hard driving to exceed the tire’s limitations, and we never felt like we got close.

Feedback from the KingQuad was much easier to interpret. The most obvious result was that we almost couldn’t get the quad stuck in a very familiar mud bog that gave us fits just a few days earlier. With the stock tires, the KingQuad was only good for two or three passes on the same line. The ATRs went through the same spot time after time without sinking.

To be fair, we did mount very wide front tires on the Suzuki. We put 25x10R12 tires on both ends, whereas the stock fronts were 25x8R12s—this made a huge difference in the mud. The drawback was that the wider tires could throw mud on the rider outside of the fenders, and they also gave the rider more feedback through the bars in rocks. For kicks, we then mounted ATRs in the stock size up front and went back to the same mud hole. The Suzuki still refused to sink, and all the drawbacks disappeared.


We have about 12 hours on the ATRs now, and they still aren’t showing much wear. We don’t expect them to yet, especially since it’s been muddy on most of our rides. We’re keeping track of the use that we logged on the tires and plan on doing a long-term report, but so far, so good. The new Black Diamonds are performing like premium-quality tires for us, which is pleasing because they aren’t priced that way. The prices vary by size, but for reference, the ATR rears used on the Suzuki were priced at $127.79 apiece, and the 26x11R14s on the rear of the RZR were $158.19. Prices are typically lower on the street, but the STIs seem to be about 10 percent less expensive than comparable products. That makes them especially good for us. For more information, ask any retailer that carries MTA products, or go to q
We found the Black Diamonds to be a huge improvement over the stock rubber on the Suzuki, especially in deep mud.
This was our first chance to check out STI’s new HD2 alloy wheel. It’s an attractive eight-spoker with a lifetime warranty. The MSRP is less than $100 each, which is way under most premium wheels on the market.
We tested STI’s new Black Diamond tires on both a RZR and Suzuki KingQuad. The RZR was fitted with the deep-treaded XTR versions and HD2 cast-alloy wheels, which made a good-looking addition to the machine.

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