Hybrid Cam Drives and Bald Tires | Ask the Geek

Hybrid cam drives
I have noted in many pictures of the new hybrid gear/chain valvetrain system of the 2012 Ducati Panigale that the ratio between the two sprockets connected by the chain doesn't look like a 2:1 ratio. Do they compensate the difference with the gears' ratio to spread the big pulses of the power stroke of the V-twin throughout the full length of the chain, to prolong its life, considering the different nature of the V-twin to an inline four?
Sal Costabile
Ottawa, Ontario

The hybrid layout, and the sizes of the gears and sprockets used, is all about packaging the drive system into as little space as possible. In a traditional all-chain setup, the sprockets on each camshaft must be twice the size of the sprocket on the crankshaft, so that the cams spin at half the speed of the engine. Even using the smallest possible sprocket on the crankshaft, the camshaft sprockets can end up large enough that they force the camshafts to be placed farther apart than desired, affecting overall size of the cylinder head. With a system using all gears or a combination of gears and chain, the gears can be used to obtain the 2:1 ratio, and smaller sprockets or gears on the camshafts can be used. This, in turn, allows the cams to be closer together and the cylinder head to be smaller. In the case of the Panigale’s hybrid drive, the chain/sprocket part of the drive accounts for some of the ratio, while the gear part accounts for the remainder. Note in the image that even in the Panigale’s case, the crankshaft sprocket is as small as possible given the size of the crankshaft at that point; based on that, and all-chain drive would definitely push the camshafts much farther apart.

There is stress in the chain from power pulses in the engine, and I’m sure they would play a part in a V-twin engine more so than in an inline four; however, the Panigale is a special case because it uses Ducati’s signature Desmodromic valve actuation. In an engine with standard valve springs, it takes energy to compress each spring as the engine rotates. Most of that energy is given back when the valve closes and the spring extends back to its original length. If you’ve ever turned an engine over by hand or with a wrench, you’ll have experienced how extreme these forces can be by how the engine can turn itself each time a valve closes. This constant reversal of force in the cam chain can also cause a lot of wear and tear. But in the Desmo system, there are no valve springs, and these forces are largely absent; the cam chain will have a much easier life.

Bald tires
** I have a question about using the wear indicators on street tires as a measure of tire life for strictly dry-weather use. From my understanding, the grooves in street tires are for displacing moisture during wet weather riding, which is why they are absent from racing slicks that will be used on a dry closed circuit. Now, if you are planning on riding in strictly dry conditions, do you really need to be concerned if your street tire tread is worn down to the wear indicators? Because I have always been taught that it’s not worth trying to squeeze every bit of mileage out of a tire; I have always bought new tires immediately when the tread is at the wear indicators, sometimes even before. But I never ride in the rain. And, here in Southern California, the dry season is long so there are months where there isn’t even the possibility of being caught in an unexpected downpour. I know that street tires are made to withstand many heat cycles. I also know that when manufacturers of motorcycles and motorcycle accessories issue things like maintenance intervals, stock settings, and other standards, such as suggested tire pressures, they typically try to establish figures that take into account applications across a wide range of riding conditions and environments. Is it possible to get more mileage out of a typical supersport tire than I currently do by using the wear indicators as a measure of the tire’s life when using them in strictly dry conditions? If so, how many more miles?
**Eric Swidler
Simi Valley CA

You're doing an awfully good job of convincing yourself that it's OK to use your tires past their wear indicators and until there is no tread left, but you are missing a few key points. Using tires that are close to worn out is risky, even if you ride only in dry conditions. By law you must have a certain tread depth on your tires, so if you get pulled over or have an accident you could end up getting some grief over your worn-out tires. Even if you don't plan on riding in the rain, you can't guarantee that you won't end up, at some point, on a wet road. In California in the summer, we have been surprised many times by a freak rainstorm or water running across the road. The last thing you want to be thinking about if that happens is the condition of your tires. The handling of your bike will degrade significantly in those last few millimeters of tread depth, especially if the wear is uneven between the front and back tires. And finally, worn tires are more susceptible to damage or puncture than newer tires. To me, it just doesn't seem worth it to risk that much for the small savings you'd get by stretching the mileage on your tires. Stick to your policy of changing your tires as soon as possible once they reach their wear limits. SR

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