Depending on the condition of your license, it might be good or bad that shorter final-drive gearing will make wheelies easier.
I have an Aprilia Tuono V4R, and just like you guys have mentioned in your tests of the bike, it’s a struggle to get going from a stoplight with the tall first gear. I know I can change overall gearing to help this, but what sprockets should I use? And are there any downsides to using shorter gearing?
Changing to shorter gearing is a common modification for street or track riding, and this is not limited to just your Tuono; many sportbikes can benefit from this modification. Over the past couple of decades, sportbikes have been equipped with gearboxes having closer and closer ratios. This is good for track riding and racing because the closer ratios mean engine rpm doesn’t drop much every time you shift, keeping the engine spinning in a more optimum rev range. What this means, however, is that first gear ends up being a very high ratio; on many literbikes, it’s good for more than 100 mph at redline. This leaves the engine lugging at very low revs even in first gear at around-town speeds or in slow corners on the racetrack, and a lot of clutch slip is required to leave a stop. The situation is compounded by bikes becoming more powerful with higher top speeds over time. Sixth gear must be accordingly taller to avoid over-revving the engine at maximum speed, and in turn this makes first gear even taller. Adding to all this, today’s better top-end performance has come at the expense of midrange and bottom-end power, and many sportbikes are quite lethargic at low revs compared to their older-generation equivalents. All this combined can make riding around town or at a short, tight track on a current sportbike a real chore.
Even though your Tuono has a different gearbox from its sporting cousin the RSV4, with shorter first, second, and third gears, it could still benefit from some shorter overall gearing. Shorter gearing is attained by using a smaller front or larger rear sprocket—both mean the engine must turn more revolutions for a given distance traveled and hence more rpm for a given road speed. A typical modification is a one-tooth-smaller front sprocket (from 16 to 15 teeth on the Tuono), which is the cheapest and easiest way to change the overall ratio. The smaller sprocket will require a readjustment of the chain tension, but unless your chain is very used there should be plenty of room available to make up the difference. Alternatively, you can fit at the same time a one-tooth-bigger rear sprocket, which will negate some of that.
The smaller front sprocket will make a significant change, and your engine will rev about 6 percent higher than before at any given road speed. Because the rear sprocket is much larger to begin with (42 teeth on the Tuono V4R), a one-tooth change has less effect, just 2 percent. Even just the front sprocket swap will make leaving a stop easier, with less clutch slip required, and your bike will definitely be peppier around town. Note that if your chain is at all worn, it’s generally a good idea to replace it and both sprockets as a set so the used chain doesn’t prematurely wear the new sprocket or sprockets.
There are some downsides to keep in mind with the change to shorter gearing. Your bike will be revving higher at freeway speeds than previously and might vibrate more; fuel economy might also suffer. If you ride on a track with a long straight, you could end up hitting the rev limiter in sixth gear before the end of the straight. The Tuono’s speedometer works from a sensor on the rear wheel, so your speedometer will still read correctly; on bikes that have the sensor on the countershaft, the speedometer will have to be recalibrated to match new gearing using a device such as the SpeedoHealer (healtech-electronics.com). On Aprilia models with APRC, changing gearing (or to a different type of tire, for that matter) requires that the APRC be calibrated to optimize its functionality. Refer to your owner’s manual for the complete procedure, which requires you to ride at 25 mph in second gear for 10 seconds to complete the calibration.
Got a question for the Geek? Email him: firstname.lastname@example.org