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Is your bike ready for the FAT Farm!

By Bret Tkacs

Ok, lets get straight to the point - does your bike really need to shave off some pounds after the holidays or just you? Does it matter where the weight is or is the quest to shave the pounds just a way to make your fat wallet skinny (pun intended). There must be some truth to the benefits of shaving weight or the bike manufacturers wouldn't be bragging about how they shaved four ounces off the crank and made the bike 5mm narrower... right? There can be real gain by weight savings but all weight is not created equal.

The Ducati above went on a diet. See if you can count all the ways it shed weight?

Let's take a look at your bike and see where that weight really is. You may have heard of sprung vs. un-sprung weight as well as the term reciprocating mass? To understand weight saving we must first understand a little more about the weight itself. Sprung weight vs. un-sprung weight simply refers to where the weight is located on the bike. Sprung weight is all of the weight that rides on top of the springs (suspension) whereas un-sprung weight rides below the spring. If you look at your front suspension, everything above the fender is sprung weight; the fender, wheel, brakes, etc are un-sprung. On the back of the bike the dividing point is about mid shock, so your swing arm, sprocket, and so on is included as un-sprung weight.

If you have priced "light weight" aftermarket components, you already know how expensive it is to purchase items to reduce your bike's un-sprung weight. A set of carbon fiber wheels will cost you around $4,800 whereas a more economical set of magnesium or forged aluminum may "only" cost $2,800 to $3,500 a set, about the same cost as an inexpensive track bike. The reason that shaving un-sprung weight is so desirable is that it affects two critical aspects of how your bike performs. One is that you have less reciprocating mass (parts the spin) which means that your engine has an easier time accelerating and the bike can turn into corners more quickly. The second and even more desirable effect is that by lightening the parts of the motorcycle that need to stay in contact with the road, your suspension can respond to changes in the pavement quicker. This results in better traction as your suspension can work better (more traction is always good). Less weight means that your springs can "push" the tire down to stay in contact with the road and when it has to respond by compressing (hitting a bump) it can do so in less time. Of course we need to have our suspension properly set up with proper spring sag, spring rates, etc� which I'll cover in a future article.

The KLX shown here has just 92 miles on it. The motor is out losing weight and the swing arm will be next to go.

The effect of reducing sprung weight allows us to use lighter springs in our suspension which is a good thing since this also reduces the overall weight of the bike as well as allowing our suspension to be more compliant. Sprung weight also reduces the amount of static weight we have to move to turn the bike into a corner and how much mass our motor has to move to accelerate and how hard our brakes have to work to slow everything down.

Reciprocating mass is any thing that rotates; tires, pistons, clutch baskets, and flywheels are all examples of reciprocation mass. For example, by making parts and pieces that move inside the motor (flywheel, piston rods, etc.) lighter, it allows the motor to rev more freely. In your quest to shave off the weight, don't forget that just like New York fashion models, there is such thing as too skinny. Your motor needs to have some weight for its internal reciprocation mass to help keep things running smoothly in traffic and at idle. If you go to the race track, you may notice that many of the race-built motors idle very high. One reason for this is they do not have much reciprocation mass to store energy to keep things running smoothly at lower RPMs. A light motor that runs well at 15,000 rpms may be all but un-ridable on public roads. Those $4,800 wheels may be the greatest thing on the track and make the bike feel like magic but how will they hold up the first time you hit a pothole or that rock in the road? Everything comes with a price; remember to consider what it is you really want to achieve and keep your goal in sight.

Old vs. new. The swing arm in the foreground is made of much light materials and is drilled out at the lower stress point on the cross bar.

Will three pounds really make a difference? If it is saved by tossing those oversized turn signals, probably not. But if it is from the purchase of a new set of rims then you will most likely really notice the difference� but at what cost? As Kevin Cameron so clearly stated in his book SportBike Performance Handbook "� it's possible to go stark raving catalog-mad unless you impose some discipline on yourself. What do you need from your bike? The more you modify your bike, the harder it is to ride, and the harder it is to get service�" The point is� if you want a faster and better handling bike, many times the cheapest and most reliable way to do this is to buy a production bike that is fast and better handling.

The easiest, cheapest and healthiest way to shave weight is to eat one less cheeseburger and lay off sugary soda! Not only is it the cheapest way to shave sprung weight but shaving a few of those pounds from the pilot will make for a more comfortable ride, and let you ride longer. Try pricing out how much it would cost to shave a mere 10 pounds off your bike compared to just trimming back at meal time. In the end, the best investment if you want your bike to perform better is to invest in yourself. Take the money you save and attend a riding school and learn how to use the performance that your bike already has.

Most riders ride well below the capabilities of their machines. Ask yourself if Nicky Haden or Valentino Rossi could ride faster on your bike or would that new carbon fiber fender really make the difference?


Bret Tkacs is the president of Puget Sound Safety , a Northwest company that provides riding skills classes as well as do it yourself maintenance programs.


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