Ask the Geek: Tire Pressures and Aftermarket Shock Links

Your abnormal guide to things abnormal

Mythbusters
My buddy and I were discussing tire pressures when he hit me with this information gleaned from the web of infinite truth and limited facts: Japanese sportbikes seem to have an extra 4-6 psi specified for their tires, compared to the equivalent Ducati. Why? To answer the question about higher recommended tire pressures for Japanese inline fours versus Ducati twins - inline fours heat up their tires more than a twin so a higher starting pressure is needed to prevent overheating the tires, particularly the rear tire. Years ago, superbike racers discovered that it was easier to modulate the power to prevent wheelspin on the Ducati V-twins than it was to do the same on the Japanese inline-fours. This is because there is a longer interval (in terms of both time and crankshaft rotation) between cylinders firing, which gives the rear tire a "break" - time to recover traction and match its speed to that of the motorcycle. More recently, more sophisticated traction control systems have been tried to reduce tire temperatures, improve tire life and lap times. The InterWeb is full of junk (with exception of your fine web publication), so I thought I would ask you guys to check the veracity of this information.
Darron Fuller

Your question includes a nice summary of some of the differences between a V-twin and an inline four-cylinder, but tire pressure is not one of them. A sampling of recommended tire pressures in the owner's manual shows that Ducati does list lower numbers for the 1198 compared with other manufacturers: 30-33 psi front and 32-35 psi rear for the 1198 vs. 36/42 psi for, as an example, the Kawasaki ZX-10R. But other twins have equally high pressures: 36/42 psi for the Honda RC51, and 36/36 psi for the KTM RC8 (36/42 psi with a passenger). Here at the magazine we've long used a single setting with variations based on machine weight, load and conditions, but never engine configuration. And for track use we base pressure on the tire manufacturer's recommendations, which is almost always independent of what bike the tires are mounted on. I also called Corey Neuer at CT Racing (www.coreytaylorracing.com), Pirelli's West coast roadrace distributor, who says "We adjust pressure for conditions and compound, but never for something like a Ducati vs. a GSX-R1000."

Most manufacturers list pressures based on maximum load of rider, passenger and luggage, giving numbers on the high side compared to what you'd use for best performance on the track with just the rider onboard. Ducati has listed tire pressures based more on the performance side for the 1198, and you'd definitely want to use slightly higher pressure for street riding with a passenger or luggage. Note that the Streetfighter's recommended pressures are higher, 36/36 psi, and that Ducati owner's manuals include the caveat "To avoid front wheel rim distortion, when riding on bumpy roads, increase tire pressure by 0.2 - 0.3 bar (3-4.5 psi)." Take that information into account with the 1198's numbers, and you'd be at almost exactly the typical 36/42 psi specified for many other bikes.

Aftermarket Links
If I go to an aftermarket rising rate shock link on my track bike (an '08 GSX-R1000), from the stock street link, should I expect to have to make significant adjustments to the shock settings? Any changes at all?
Al Martz
Las Vegas, NV

Most aftermarket shock linkages are designed such that you won't have to change the spring rate or ride height when you install the new link, but count on checking your sag and changing preload, as well as making some adjustments to both compression and rebound damping. Most stock links are made to work over a wide range of rider weight and conditions, with a fairly soft initial rate ramping up to a stiffer rate further into the suspension's travel. A link intended for track use will be more linear, with a stiffer initial rate and a softer rate near the bottom of the travel. This will make the suspension work differently in those ranges, requiring some changes to match.

While the stock shock will work with a non-stock link, you should already have an aftermarket shock if you're considering the link, as well as being familiar with its adjustments and how they affect your bike's handling.

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