THE 20-INCH TIRE REVOLUTION

February 9, 2000
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Why are so many of the factory riders testing 20-inch front tires????.
Are they guinea pigs? What are the advantages and disadvantages of the new tire size? Is the 20-inch actually smaller than a 21-inch tire? Are consumers going to have to dig into their pockets to pony up for a 20-incher next year? What’s all the hype about?

WHERE DID THE IDEA COME FROM?
The truth is, Honda asked Dunlop and Bridgestone to build 20-inch front tires. Neither tire giant was pushing the idea-Honda wanted it and both Bridgestone and Dunlop responded. Why did Honda ask both companies to work on the project? Business! In Japan, Bridgestones come stock on CR’s, while in the U.S. Dunlop is standard equipment (on both the production bikes and the race team bikes).
Honda doesn’t want the 20-inch front tire to be a one-off works part available only to Tortelli, Lusk and Windham. They want all aluminum-framed CR’s to have 20-inch front tires as standard equipment, and to do this they need both Bridgestone and Dunlop to manufacturer them.

WHY IS HONDA SO HOT ON THE 20-INCH FRONT TIRE?
In the simplest terms, they believe it will take the harshness out of their aluminum frame. Honda asked its engineers what they could do to make the CR frames more absorbent, and the answer had nothing to do with the frame. The engineers said that a larger front tire might take the burden off the frame. Hence, we have the 20-inch solution.

HOW CAN IT BE LARGER IF IT’S ONE INCH SMALLER?
A standard-issue front wheel has a 21-inch rim and stands approximately 27.5-inches (from the ground to the top of the tire). Honda’s idea was to produce a 20-inch rim, but make the sidewalls of the tire taller so that it would still stand 27.5-inches tall. There is no significant outside diameter difference between a 20-inch and 21-inch front wheel. The main difference is in air volume. A fatter front tire will absorb the bumps that Honda’s front suspension can’t handle.

WHY COULDN’T THEY JUST MAKE A LARGER 21-INCH TIRE?
Honda was caught between a rock and hard place. They wanted to fix their frames, but they didn’t want to change the frame’s geometry. A larger 21-inch front tire would make the head angle slacker. Conversely, a 20-inch wheel with a normal-sized front tire would steepen the head angle significantly. In order to keep the frame geometry as it is, Honda couldn’t go to a larger 21-inch tire (unless they wanted a chopper).

DIDN’T HONDA HAVE A 23-INCH FRONT WHEEL A FEW YEARS AGO?
Yes, 20 years ago. The 1979 Honda CR125 came stock with a 23-inch front wheel. The larger-diameter wheel was supposed to roll through bumps better, offer a larger contact patch and improve steering. Unfortunately, it was a failure of immense proportions (many ’79 CR125 owners cut their frames in half and changed a section of backbone so that they could run a 21-inch front wheel).
The premise of the 23-inch front wheel is still valid, but it would not address the frame stiffness problems that Honda is worried about.

HOW DIFFERENT IS THE 20-INCH FRONT TIRE?
Very different. (1) It has a smaller inside diameter (20 inches instead of 21). (2) The sidewall is about 20mm taller than a standard 21-inch front tire. (3) The 20-inch rim is 1.85-inches wide instead of 1.60-inches. (4) The footprint of the tire is 15mm wider than the standard 21-inch tire. (5) Although Honda wanted the outside diameter to be as close as possible to that of a 21-inch tire, Dunlop’s 20-incher is 14mm smaller (which means the front axle is 7mm closer to the ground). To compensate for this, the forks have to be pushed down in the clamps about 5mm. For guys who like to run their forks flush with the triple clamps (Larry Ward) the 20-inch front tire is out of the question. On the other hand, Bridgestone’s 20-incher is within a few millimeters of the height of a 21-inch tire (no compensation with the triple clamps is required).

ARE THE OTHER MANUFACTURERS JUMPING ON THE BANDWAGON?
Yes, but not because they feel that their frames are flawed. The 20-inch front tire is a new idea and they want to be sure that it doesn’t work better than what they already have. Every company is testing 20-inchers, but only Honda and Kawasaki are testing them seriously for 2001.

IS THE 20-INCHER BETTER THAN A 21?
So far, there have been no significant improvements in lap times. However, the 20-inch front tire does improve front wheel damping, braking and traction because of its larger footprint. On the downside, test riders report that it wallows more, and with increased front wheel traction, the rear wheel tends to drift more. Kevin Windham reportedly doesn’t like the fact that the 20-inch front tire limits his ability to get the bike into a two-wheel drift exiting corners.
As expected, the testers have fallen into different camps. Ryan Hughes and Sebastien Tortelli love the 20-inch front tire and want to run it. Robbie Reynard doesn’t want to see a 20-inch front ever again. Windham, Albee, Dubach and the rest of the riders continue to run 21-inch front tires because they’d rather run what they’re comfortable with until the 20-incher proves itself.

WHAT ABOUT THE 20-INCH REAR TIRE?
What! You haven’t heard about the 20-inch rear? We’re not joking. It exists. Not only could we be using 20-inch front tires in the future, but 20-inch rears as well.
Honda is not to blame for the 20-inch rear tire movement. That credit goes to Suzuki. Four years ago, Suzuki requested a radial rear tire. The goal was not a 20-inch rim size, but a radial (as opposed to bias ply) lay-up. When Bridgestone punched the footprint, radial belting and physical requirements of a rear motocross tire into their computer, it spit out a radial tire that required a 20-inch rear rim.
Again, a 20-inch rear tire’s overall diameter has to be the same as the 19-inch rear tire it is replacing. Unlike the front tire, that means shrinking the sidewall to keep the frame geometry the same.
It should be noted that different size rear tires have been tried before. In recent history, there have been competitive motocross bikes built with 17-, 18-, and 19-inch rear tires.

WHY DID SUZUKI WANT A RADIAL REAR TIRE?
Weight savings. Radial tires are much lighter than bias ply tires. The weight savings comes mainly from the construction of the tire. Instead of using multiple plies of chords at 45-degree angles, a radial tire uses two plies. The first ply runs at a 90-degree angle (from one side of the tire to the other), while the second ply runs lengthwise down the middle of the tire. Combine this with a smaller sidewall and you get a 4- to 6-ounce weight savings.
Another appealing aspect of radial tires is their damping. Radial tires have more give than standard, bias ply tires. In order to reduce the amount of flex, both Dunlop and Bridgestone have been testing with wide rim widths. They’ve gone as wide as 3 inches (a standard 250 rear rim is 2.15-inches wide).

WHAT’S TAKEN SO LONG?
So why hasn’t Suzuki raced on a 20-inch radial in the last four years? Because they gave up on them. Since Suzuki had proprietary rights to the 20-inch rear tire idea, Bridgestone and Dunlop didn’t envision a large enough market for an expensive development program that would only fit on one brand of bike-so they simply waited until Suzuki’s rights ran out before resuming their development work.

WILL ANYONE RUN THE 20-INCH REAR?
You won’t see 20-inch radials on any stock bikes in 2001. Why not? The wider the rim, the shorter its lifespan. At the moment, the 2.50- and 3-inch wide rear rims only last one moto before they look like they were worked over with a hammer. Until rim manufacturers can make a rim that lasts, the 20-inch rear tire will remain a test bed product (although Bridgestone would like to get a race team to race the tires-they have been testing with Team SplitFire).

WHAT’S THE FUTURE OF THE 20-INCH MOVEMENT?
If you are a betting man, bet the farm that next year’s Hondas will come stock with 20-inch front tires (probably on the CR250 first). Whether the rest of the manufacturers will spec 20-inch front tires is up in the air. If Dunlop and Bridgestone can’t get Kawasaki, Yamaha or Suzuki to join Honda in the OEM tire business, their bottom line will be very tight. Just like the 20-inch rear tire, the 20-inch front will be doomed if it can’t be built in quantities large enough to bring the economies of scale into effect.
As for the 20-inch radial rear tire, we won’t see that on a production bike for years. The factories may start testing them with the race teams in 2000, but more than likely it will be 2001.

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