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Discussion Starter · #1 · (Edited)
I wrote this up as a post on my site because I was working on cutting weight from my rear brake rotor, but I'll just post it here, it's good info that new riders generally don't get exposed to.

There's a lot of importance placed on weight when talking about building a motorcycle for racing. I want to quickly distinguish between the 3 important types of weight, 3 types of mass to be more specific.

There's rider mass, or sprung mass. This is the weight of the rider and the motorcycle and anything else that is suspended by the forks and shock on the motorcycle. The larger the mass, the harder it is for the motor accelerate it forward, and the harder it is for the brakes to stop it. With moving mass also comes momentum, and the more momentum the bike has, the harder it is to change direction, which means less corner speed and more lean angle to make the same turn. If the weight is higher up, it moves the center of gravity up, which also makes the bike require more work to turn (the rider, gas tank, and handle bars are typically the highest points that carry a lot of weight). This weight matters, but it's actually the least important type of weight to focus on when building a race bike. Remove the extra crap from the bike, eat some salads, and keep reading...

The second kind of mass is called unsprung mass. This is the weight of the parts of the motorcycle that are between the suspension and the road. This generally includes the tires, rims, wheel bearings, axles, swing arm, lower half of the forks and shock, brake rotors and calipers, rear sprocket, and part of the weight of the chain. This mass is different because even though it is low to the ground and doesn't greatly effect the center of gravity, this is the mass that the suspension must move up and down quickly to keep the tire in contact with the road. The lower the mass, the faster the suspension can react, and the greater your traction will be. This mass is pretty important.

The third kind of mass is rotating mass. This includes tons of things on a motorcycle, cam shafts, flywheel, gears, sprockets, transmission parts, wheels, tires, brakes, chain, sprockets, stator, etc. When mass rotates, it creates a gyroscopic effect. The larger the mass, the larger the gyroscopic effect. You may have noticed when you drive a motorcycle down the road, it has a strong tendency to continue going straight, even if you take a hand or 2 off the bars. Part of this is because of the rake/trail geometry, but most of it is because of the gyro effect of all the rotating mass. The heavier the particular mass that is rotating, and the diameter of the center of it's mass, the larger the gyro effect. Ever take a hand or 2 off a bicycle handlebar? Much less mass, much less speed, much less gyro effect, it's much easier to make the bicycle turn. If you've ridden a 1000cc bike and a 600cc bike you likely noticed how much easier it is to maneuver the 600cc bike. It feels MUCH lighter. Yet, if you look at the specs, the curb weight of most liter bikes and their 600cc counterparts are generally only marginally different. The reason the 600cc feels so much more nimble is because much of the reduced weight came from rotating parts such as the motor, transmission, brakes, chain, and wheels. This greatly reduces the gyro effect of the 600cc compared to the bigger bike, making it much more nimble, easier to turn, easier to change direction. You can carry much more corner speed with the same lean angle if the gyro effect is lower. Rotating mass is VERY important.

Now, if you look carefully at these three categories, you will notice the most important factor of all...many of the parts on the motorcycle fall into BOTH the unsprung mass category AND the rotating mass category. Bingo!!! These are the most important items that will have the greatest effect on the handling and feel of a motorcycle on the race track. Now, tires seem to make the most sense, as they are heavy and large diameter, but I don't recommend choosing your tires by weight. But wheels, brake rotors, and chain and sprockets as the first things you should consider upgrading to reduce the most important kind of weight on a race bike.

Wheels make a huge difference in the handling of a bike, which is why all professional superbike racers run the lightest weight wheels allowed, they have to in order to stay competitive. However, most production race classes require you to run the stock wheels. The next place to look for weight reduction is the chain and sprockets. The R3 comes stock with a 520 series chain and sprockets. Changing to a 415 series chain and sprocket reduces the life expectancy of the chain a little, but it drops 2.5 lbs from the drive train, unsprung, and rotating mass. This translates to about 1 extra horsepower being transferred to the rear wheel, and better handling and corner speed. Since most racers barely use the rear brake, the rear brake rotor is the next lowest hanging fruit for unsprung, rotating mass. Some race organizations allow you to change the rear rotor, others only allow the stock rotor to be modified.
 

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Ive weighed everything to the gram stripped for the track


Heavy steel sprocket weighs 918 grams with the useless ring thing on it, 778 grams alone.
Plastic Chainguard are 164 grams hanging off the swingarm, reflectors on the forks are87 grams.


And that's just the light stuff, got the rest written down but not typed up yet.


Orig heavy dragging Oring chain robs a horsepower and weighs 1633 grams.


Im all about weight as I hate spending money on motorcycles when efficiency and tuning the balance is more important.


Will do a thread one day when its all finished.
 

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Discussion Starter · #5 ·
Fantastic, I've been weighing most things too, but I also want to put a whole page on the blog dedicated to weights of different parts and mods, do you guys mind if I use some of your info and give you both credit?
 

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Must be a **** of a problem for you guys finding a 0.393700787 inch spanner in your toolbox :)


When I think of 500g grams, Its a big steak, or a tub of butter from the supermarket.
but tell me in pounds and I have to compare a big bag of pot, or enough cocaine for a life sentence.
 

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I get the whole 415 chain retrofit, but why not do a 15/46 combo instead of 18/57 sprocket? Not available?
 

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Good info. Thanks for sharing. I learned something new :)
 

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I get the whole 412 chain retrofit, but why not do a 15/46 combo instead of 18/57 sprocket? Not available?

Because the bigger the front sprocket the less resistance,
This offsets any weight if a non oring is used and alloy rear.
those guys running a 428/420 or even 415 have even more advantage.


I;m stuck on 520 due to race regs (ERT2 MX chain is ideal I this instance)
if this bike can pull a 15T when I get some more mojo out of the top end I may do so, and run 15/43 instead of the 14/41 which my next step.


This is the first bike ive owned that can actually pull its std gearing.
186kph and felt like I needed 7th.
 

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Discussion Starter · #17 ·
I get the whole 415 chain retrofit, but why not do a 15/46 combo instead of 18/57 sprocket? Not available?
I think the reason is actually because the chain links are smaller, so a 15 tooth and 46 tooth sprocket for a 415 chain would be very very small and may have other negative effects, including changing the chain drive angle. The 18/57 keeps both sprockets about the same size as the 15/46 with a 520 chain.
 

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Hi

Very interesting, would love to see a list of the weights in grams ( metric) and non metric.

So what sprocket can you use to reduce weight but not durability and keep gearing more or less the same so no modifications required or disadvantages.

I mean if acceleration is not important but reducing weighr is, but i guess the x2 work together in tandem.
 
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