pounds and horsepower

[TommyG]

 So I should have started my diet last year?  ;D

No Garth. Just ride the 1000!

[Jeff]

Dave, you took the words right out of my mouth...

I was originally going to reply saying "the weight loss won't make any of you suck any less than you already do", but I felt it would have been interpreted as a negative post.

[Team-G]

"the weight loss won't make any of you suck any less than you already do",

So much for my motivation, back to cheese burgers and Tommy's Tripple D's  :o

[tzracer]

Let's say that my bike weighs 380 pounds, I weigh 170 in gear..So you add the two together and get 550 pounds. Now divide the 550 by 119hp and you get 4.621848739. I am assuming that is how many pounds I would have to lose to gain 1hp...If that is the case and I am 10 pounds lighter then last year(saying that the bikes weigh the same then I would have a gain of 2.16 hp over my last years weight. Did I do something wring here Brian? Please correct me if I did...

Everything looks OK.

[tzracer]

But what if you were to take, say, 12lbs off the wheels?

You get more benefit from reducing rotating mass. There is no general relationship between losing rotating weight and gain in power because it is a function of the weight reduction and where the weight is lost. To calculate for a wheel you would need to know the moment of inertia of the wheel before and after the weight loss.

Since I don't know where on the wheel you are losing the weight, I cannot tell you how much of a difference it would make.

[tzracer]

I have to disagree a bit here... let's see if I have what you're saying right.

For example... I've been a lazy fat a$$ during the off season and I weigh 180 pounds.  Let's say that in addition to my R1 superbike I also have a RS125.  You're saying that I will proportionally go faster on the R1 if I lost, say, 15 pounds?  I know there are a lot of little people that race 125s that when they move up to a bigger bike don't do as well because they lose their advantage.  It seems like you have more to gain by losing weight the smaller the bike you ride is.  I mean, that's half the reason I bought a rocketship... so I wouldn't have to starve myself... are you telling me that I may have to start eating Broccoli again?  ;D

I never said anything about going faster. Going faster is up to the rider.

Think of it this way. By losing weight you are trying to 'free up' horsepower.
If your bike has a ratio of 8lb/hp, you would have to lose 8 lbs to free up 1 hp.

Yes it seems counterintuitive. I often tell my students that physics common sense and common common sense are often not the same.

[tzracer]

of course it a sliding scale and you would need to re-equate the new mass plus HP to get a more accurate estimation... right??  

Yes, as the weight to power ratio changes, the amount of weight needed to gain 1 hp changes.

I flunked college algebra 3 times I am in finance....  :-/

thank the lord for calculators  ;D

Calculators don't do any work for you. They are only as good as the operator. GIGO  :)

[tzracer]

All makes sense....but,  isn't torque more important than hp?  Not saying you don't need hp, but torque is the ability to change velocity (accelerate) and hp maintains velocity (constant rear wheel speed) under dynamic load conditions.

Hmmm, how do I say this nicely......NO.
Power the rate at which work is done (P=W/t). Your change in kinetic energy (due to acceleration) is the work done, the t is time. Power is essentially how quickly you can gain kinetic energy and hence acceleration.

Torque and power are directly related, so your above statement really doesn't make any sense.

And, if you do the flywheel calc's, its better to have more rotational mass in the rims (fly wheel effect) for torque and loose the weight in my a$$ (non-rotational mass; unless of course, I crash).  So save the money on the light wheels and quit eating pasta...right??

Actually losing rotating weight gives the most bang for the buck, but losing too much can make it difficult to harness the power (wheel spin, revs will rise more quickly).

Maybe I need to teach a motorcycle physics course in the evenings at the track.

[tzracer]

Ok, so you're a very good rider.  And you race a two bikes. One bike makes about 40% more power, which is the difference between a 600 and a 1000.  But it only nets you .75 of a second, which is the difference between a 1000cc four vs a 600cc four at some smaller tracks.

You'll have to loose 175 pounds on the 600 package to make up the power to weight ratio...for less than a second.

Hmmmmmm

Never said there was any direct correleation to lap times did I?

Like I have said before, comparing different classes of bikes is apples to oranges. It will give no indication of the lap times of a 600 with the same weight to power ratio as a 1000. They are different bikes with different dynamics. To try to get a correleation you would need to compare to bikes with identical engines, one heavier, one lighter. That is how science is done.

Less weight has other advantages, less tire wear, less crash damage, less brake wear, suspension action can improve (unsprung weight), etc.

I was just correcting a common misconception. One down many more to go (damping not dampening). A physicists work is never done.

[tzracer]

I've also heard this theory stated as:

"For every 1 lbs of weight you lose you gain X HP on ACCELERATION"

Meaning a 340lb 75hp Hawk is going to accelerate up to it's top speed faster than a 380lb 75hp one. Not a 340/75 Hawk going to have a faster top speed than a 380/75 one.

Sort of makes sense to me.. again just another way I've heard it explained.

Personally I'd rather gain that Mythological HP on acceleration over top speed anyday.

Weight has nothing to do with top speed. Top speed is determined by drag vs. power. Assuming that the additional 40 lbs in your example do not affect the aerodynamics of the bike, the 2 Hawks would have the same top speed, the lighter one would reach top speed sooner.

I was calculating based upon acceleration. If you only lose weight and do not increase the power of your bike and do not change the drag, your bike will have the same top speed. If you want to increase top speed you need to increase power or reduce drag.

[tzracer]

Dave, you took the words right out of my mouth...

I was originally going to reply saying "the weight loss won't make any of you suck any less than you already do", but I felt it would have been interpreted as a negative post.

True, but the point of my post was not to help lap times but to correct a common misconception. Should have done it sooner, the calculation is done with high school physics and algebra.

I enjoy the technical side of motorcycling as much as I do riding. I enjoy building a bike almost as much as I do riding it. Part of building is reducing unwanted/needed weight and also relocating it (both to get a better front to back weight distribution and the center of mass at a proper height - there is a reason that Yamaha raised the CM of their bike after Rossi joined the team). Much of it is not horribly expensive, it does take time and the proper machines (which I have access to and know how to use - which is part of the fun - making stuff no one else has).

[tstruyk]

Yes, as the weight to power ratio changes, the amount of weight needed to gain 1 hp changes.


Calculators don't do any work for you. They are only as good as the operator. GIGO  :)

who you calling a gigo?    ;D