Weight... and the lack of it

Weight is often the forgotten aspect of performance. Reducing the weight of the motorcycle can reap performance rewards in improved acceleration, braking, handling, and cornering.

Weight can be seen as static – the weight of the bike and rider; rotational – the mass of rotating parts; unsprung – the weight of the suspension parts that are not suspended by the springs and shocks; and gyroscopic – the effect of rotating weight. Also the weight balance of the bike can make two bikes of the same weight feel and respond differently.

Inertia:
The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force.

This means that a weight will resist acceleration, changes in speed, and changes in direction once moving. Reducing the weight makes it easier to accelerate, slow down, and change direction – all things we want our bike to do well.

Static weight:
The overall weight of the bike and rider effects performance more than you may think.

There is an old rule of thumb in drag racing that a 2800 pound vehicle requires 10 hp more, or 100 pounds less, to be 0.1 seconds quicker in the ¼ mile. It is not perfect, but it comes close. This can be roughly estimated to relate to lighter vehicles:

For a 450 pound bike and a 165 pound rider, it is 2.20 hp or 22.0 pounds.
For a 450 pound bike and a 185 pound rider, it is 2.27 hp or 22.7 pounds.
For a 450 pound bike and a 200 pound rider, it is 2.32 hp or 23.2 pounds.
For a 450 pound bike and a 230 pound rider, it is 2.43 hp or 24.3 pounds.

This doesn't sound like much of a difference does it? Well, keep reading.


Power to weight:
The power to weight ratio (P/W) shows how many pounds each horsepower has to move. This allows us to compare vehicles of different weights and power. For a 165 pound rider on a 450 pound bike that makes 112 hp, the P/W is 5.491 pounds for each hp. A 230 pound rider on the same bike would need around 123.84 horsepower to have roughly the same acceleration.

For the same 165 pound rider and bike making 117.5 hp at the rear wheel (silencers added maybe?), a 230 pound rider would need around 130 hp at the rear wheel to accelerate the same. This is a big difference, and does not even look at the handling differences. “Light makes might” really pays off for the runt riders. This may be why the 'corn fed', or simply larger, riders may want more power. ;D

This is also why many forms of racing have classes that mandate a specific MINIMUM weight, in order to more evenly match the light and heavy riders/drivers.



Balance:
Let’s say you remove the dual high mount silencers on an S3, and replace them with a single low mount silencer. You have done two things that are both good. First you have reduced static weight, and second you have moved the remaining weight lower. This relocation of weight makes the bike easier to lean and throw around than weight that is located higher on the bike. A pound removed up on the high mounted silencers is more effective than a pound lost in a lighter header that is located low on the bike.

From all of the above, you can really see how the rider’s weight (high on the bike) is important in many different ways, and how shifting weight around on the bike can pay off in balance and feel.



Rotational weight:
Weight that spins has a dual effect. It is static weight, and its mass also effects how easy the bike is to accelerate faster and slower (braking). The faster a part spins, the harder it is to change speed and direction, and the more effective weight reduction becomes - especially in the lower gears where torque multiplication is higher.

Internal engine parts spin the fastest –the flywheel, clutch, pistons, rods, crankshaft, gears, etc. can all be lightened. However these parts are expensive to lighten, and even then, only a few pounds can be removed in total before life and drivability suffer. For many street riders, this is not very realistic.

The chain and sprockets can be replaced with lighter 520 parts and the sprockets can be skeletonized to reduce the weight even further. I have seen improvements on the dyno of 1.5-2% from these changes alone.

The brake rotors can be cut in non-essential areas (skeletonized) to reduce their weight, but if the material is removed in the wrong areas, or too much area is removed, the pad contact areas can be reduced to the point that braking performance suffers. Rear rotors are usually a prime candidate for skeletonizing. Lighter rotors also reduce the unsprung weight, and allow the springs and shocks to do less work to keep the tire in contact with the road, and can allow lighter springs and shock valving to be used effectively.

Lighter wheels can work in four ways. They reduce static and unsprung weight. They are large diameter, so they generally have a lot of rotating mass that is reduced, and they also reduce the gyroscopic effect that prevents the bike from leaning into and out of a turn. But this safe thing also reduces their stability at high speed. When looking at lighter wheels, the hub weight is not nearly as important as lighter rim weight.

The tires are on the very outside of the wheels. A little weight saved here, can be very effective.


Putting it all together:
How you ride can alter what weight is important to you. If you like occasional high speed runs, you may want to stay away from light wheels and internal components. But if you like to wheelie often, lighter front end components may be just what you want. If carving corners is your thing, many aspects of weight reduction may be what you are looking for. If acceleration lights your hair on fire, all of the above may help. Look at what you want/need, and what you can afford, and make a weight loss plan that best matches your riding style and budget.

And remember that you can go too far, and make the bike very hard to ride – especially on the street. [Ever seen the tank-slappers and front end wobble issues on race bikes - this is the problem with going too light, on too many parts. It's fast, but tougher to ride. Find a reasonable middle ground that works for you.]

I am working on a weight loss plan for my S3 and self that includes getting in better riding shape, lighter wheels, lighter 520 chain and sprockets, lighter rotors, lighter wheels, lighter bike parts, and an improved balance to my bike. I hope to give accurate accelerometer data, dyno graphs, and riding impressions along the way as the riding season progresses.

This is yet another ‘book’ on tech related BS that is just a basic overview of weight loss and why its location is so important. Like always, I hope it helps, and that others will add input and corrections as needed.




:wrench: For the metric world, 1 pound = .455 kg, and 1hp (SAE) = 1.014 hp (PS) = .7457 kw.......you do the math. My brain hurts.