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MXA PRODUCT TEST: MAXXIS SI RADIAL TIRES: Maxxis Tried To Achieve The Impossible; It’s No Surprise That It Needs More R&D

December 1, 2009
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WHAT IS IT? –The Maxxis Radial SI is the first publicly available motocross tire to utilize radial technology. Normal motocross tires have layered plies of chord in the rubber that are laid diagonally. These are called bias ply tires. Radials have parallel ply chords that extend from bead to bead at roughly 90 degrees to the direction of travel. Theoretically, radial construction provides a larger contact patch and a more absorbent feel. On the other hand, bias ply tires provide the firmness needed to take the abuse of offroad conditions. In addition to the two nylon belts in the original SI, Maxxis added two Kevlar belts to the radial to help recover strength with minimal weight.

Although radial tires have dominated the automotive industry, motocross tire manufacturers have struggled to harness the technology for offroad bikes. All the major companies have tried and failed. Maxxis spent five years developing their radial to take advantage of a larger contact patch that better conforms to the ground.

WHAT’S IT COST? $122.09 (rear), $104.65 (front).

CONTACT? www.maxxis.com or your local dealer.

WHAT STANDS OUT? Here’s a list of things that stand out with Maxxis Radial SI tires.

(1) Installation. The Maxxis Radial’s flexible sidewalls made these tires the easiest we’ve ever installed.

(2) Rear performance. The rear offered good traction in acceleration and under braking in light chop, but the rougher the terrain, the more the Maxxis Radial SI’s bounced. Every MXA test rider chased tire pressures to maintain the absorbent feel, but lessen the tire’s tendency to rebound off square-edged bumps.

(3) Front performance. Our first impression was that the Maxxis SI Radial front tire was flat. It squirmed around a lot when inflated to normal tire pressures. We kept increasing the pressure to get a secure feel in the corners, but stopped at 30 psi because the ride got very harsh.

(4) Overall performance. In all honesty, we aren’t surprised by the lack of sidewall support on the radial tires. Whereas bias ply tires get extra sidewall support from their overlapping layer, the radials don’t. Every motocross tire manufacturer has tried to build a radial motocross tire and none have come to market. They have all faced the some problems. Maxxis was at least brave enough to try. On very soft soil and in sand, the Maxxis Radial SIs might be in their element, but they don’t like flat corners, square-edged bumps, big jumps or excessive lean angles. There is not enough sidewall support for cornering (when running air pressures that allow the tire to absorb bumps).

(5) Durability. As a rule, Maxxis tires offer great durability, but with the Radial SI’s we were more concerned with the durability of our rims. Our front wheel took an especially bad beating.

(6) Weight. Radial tires should be lighter than bias ply tires because of the creative plies, but the effort to regain sidewall strength must have added some weight. The original SI front weighs 7.75 pounds and the rear weighs 13.15 pounds. The Radial SI front weighs 9.95 pounds and the rear weighs 13.6 pounds. The Radial SI front weighs over two pounds more than the standard SI.

WHAT’S THE SQUAWK? We have three quibbles. (1) Sidewall stiffness is less than what we are comfortable with on intermediate terrain.(2) The price is rather steep.(3) The rear tire is a little porky, and the front is downright obese.

We applaud Maxxis’ effort to embrace new technology for motocross, but their Radial SI tires need more development.


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