Wide Tires on Narrow Rims — again
** I have read that the AMA XR1200 racers are forced to use one race tire on the rear, a 190/55 on a 5.5-inch rim. I have read your article on the cons of using a 190/50 tire on a 5.5-inch rim due to profile issues with the smaller rim, and seen your graphs that illustrate this issue. However, if this is the case on the lower profile tire how does this change on the 190/55 tire? Moreover, if it is still unstable why does the AMA require the XR1200’s to use this tire on the racetrack? Of course, now I will ask the “other” question: how will this tire work on my ‘07 Suzuki GSF1250, which also sports a 5.5-inch rim.
Via SR mail
The AMA’s spec tire for the XR1200, Supersport and Daytona SportBike classes is the Dunlop Sportmax D211 GP-A, in a 190/55 size for the rear. The XR1200, along with most bikes eligible for the Supersport and Daytona SportBike classes, use a 5.5-inch rear rim, and the tire size is seemingly at odds with what manufacturers recommend. However, most tire companies are using the larger tire for racing applications for the larger contact patch and better grip. The downside is higher steering effort, slightly slower steering and reduced stability due to the larger, heavier tire — still a worthwhile sacrifice in racing. Note that the spec tire is a 55 series; a 190/50 tire, with its lower profile, significantly exaggerates the disadvantages that arise from using the wider tire on a 5.5-inch rim, and we have often stated previously that this is a bad combination. All that said, unless you are racing your GSF1250 (and I hope you are not…), stick with what Suzuki and the tire manufacturers recommend: a 180/55 rear tire. In general, the marginal extra grip is not worth the tradeoff in steering and stability for a streetbike, and — specific to your GSF — you may run into clearance problems with the bigger tire.
** Are the horsepower and torque numbers you publish for the bikes you test on your SuperFlow dyno derived from inertia sweeps or are they derived from loaded runs reading torque directly off your brake? Also, what correction factor do these numbers correspond to? I ask because I’ve been referencing the graphs (a valuable resource) to get a ballpark figure on what a bike should be making relative to my L&S; dyno.
_**Our SuperFlow CycleDyn dyno does have an eddy current module, which allows us to use a number of loaded tests in addition to inertia runs using the dyno’s drum only. For all the charts shown in the magazine and online, we use a timed run that does use some eddy current load — the dyno’s software adds load as necessary to force the bike to take a given amount of time to go from a set rpm to redline. Typically, we program the run to last a couple of seconds longer than an inertia run would take, as this adds some load without unduly taxing the motor. The results are rarely different from an inertia run (which we also perform for each bike), but they are more consistent and at times show more detail at lower rpm than you would see from an inertia run. We use the SAE correction factor, which modifies the results to represent conditions of 77 degrees Fahrenheit, no humidity and approximately one standard atmosphere of air pressure. Conveniently, our dyno is situated in an area of Los Angeles that sees almost ideal conditions year round, giving a minimal correction factor. Additionally, we simply don’t test when conditions are at an extreme, such as excess heat, humidity or rain. Unfortunately, without taking some bikes directly from our dyno to yours and making some test runs, it’s almost impossible to know exactly how the two dynos compare; even two supposedly identical dynos from one manufacturer can show significant variances, depending on location, setup, condition and even the operator. **SR**** _