Question for Mr. Dorn or any race tuner on the old wives tale

[dinolee]

So the old wives tale that everyone knows is that a rider losing 7 lbs of weight ( or just losing 7 lbs of sprung weight in general) is equal to freeing up 1 rear wheel horsepower. Just how true is this, if at all?

And, say if a major weight savings is made in unsprung weight (like gazillion dollar carbon fiber wheels ), does that unsprung weight savings equate to a higher top speed, or just faster acceleration to the same top speed?

[Mr Sunshine]

The 7lb tale is based on power to weight ratios.

For example lets take my SV. 400lbs (or so) + 200lb rider = 600lbs. Then I have 78hp in that bike so 1hp is pushing 7.69lbs. If I took a GSX-R1000 which will weight about the same or less than the SV but puts out 175hp...1 of its hp is only pushing 3.42lbs. So on the GSX-R you have to loose less weight to get a HP. But it isn't 3.42lbs you have to loose. I just don't feel like doing the math.




And unsprung weight is all about changing velocity and not terminal velocity. Friction affects top speed....less friction...higher top speed (everything else being equal). Friction exists in many places including wind friction and tire to pavement friction.


Party on! :beerchug:

[Luke]

Thank God I only weigh 160, I like to wave at all the other 600 riders as I passed them down the straight with my 2007 cbr600!:firedevil:
I wish I could stay 19 forever.......

Edit: I know this is OT but anyone got a job for a kid that would spend all the money for track days? I need money for racing! I got no bills to pay right now so its the best time for a little extra cash. Looking for something where I could do track days weekly! I know I might be asking for much.

[Rubber Ducky]

Thank God I only weigh 160, I like to wave at all the other 600 riders as I passed them down the straight with my 2007 cbr600!:firedevil:
I wish I could stay 19 forever.......


Hey Luke you don't HAVE to get fat as you age. I have been 155 lbs since when I was 20 . . . I am 42 :banana:

[geddyt]

Sunshine's right on this one. Weight becomes less and less of a factor the faster you go. Wind friction increases exponentially in proportion to speed. Trying to hit a higher top speed is largely an issue of horsepower and coefficient of air friction.

From Wikipedia:
Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

I was reading a Road and Track or something like that recently at work and they held a drag race to 200mph between five cars advertised to be able to do so. Of course they had the Bugatti Veyron and a couple of other cars and they were all trying to break the 200mph mark. Just for fun, they threw in the new totally blown Hennessey Venom, putting 1100hp to the ground. Well, shit, why don't I just dig up the article...
Here it is (http://www.roadandtrack.com/article.asp?section_id=31&article_id=5680):
Also, bear in mind that it takes gobs of power to reach 200 mph. The air resistance grows with the square of speed. Stick your hand out the side window of your car at 60 mph and feel the resistance. At 180, that force will be nine times stronger, should you be foolish enough to give it a try. Don't. And at 200, it will have more than 11 times the force! What's more, because the power requirement of a car grows with the cube of speed, this means it will need 37 times more power to go 200 mph than to maintain 60. It boggles the mind.


Those lightweight wheels will spin up more quickly, though, for acceleration, and turn more quickly.

[Luke]

Speaking of air resistance, at a track day a couple of months ago my visor flipped open between 1a and 1b, tried to to close going about 160 and my hand just flew back and thought I was gonna be pulled off the bike! I will never do that again......

[Hypnotiq]

You must be talking about Portland because Pacific doesn't have a 1a and 1b. ;)

[jkaiser]

Those lightweight wheels will spin up more quickly, though, for acceleration, and turn more quickly.

Light weight wheels are a bonus in other ways as well. One of the biggest being that they let the suspension respond faster to input from the road, resulting in better handling. Brakes are often upgraded for the same reason, they provide better braking but they are also much lighter as well (i.e. carbon fiber rotors)

On the subject of weight I wonder if there isn't more to the story of rider + bike. For example, Jason Pridemore is 6'0" and 180lbs and he's definitely no slouch. We know the obvious disadvantages to being heavier, but there must be some advantages as well? I know for example that heavier riders don't get high sided quite so easily allowing them to push the exits hard. Although it's obvious that a 120lbs rider has an advantage over a 200lbs rider in terms of mass, I wonder if the 120lbs rider also suffers in terms of control. If the 120lbs rider is on a 350lbs bike for a total bike + rider weight of 470lbs, the rider in this case only makes up only 25% of the equation. In the case of the 200lbs rider on the same bike, he makes up 36% of the equation and I wonder if this allows him greater control over the bike's mass. I'm also thinking that the riders mass is not static like the mass of the bike. He can move his mass around to affect the performance of the bike. I'm just thinking out loud here, but I suspect the mass issue is a bit more complex than the lightest rider wins. I think this may be another reason why lightening the bike is a big advantage. I suspect the higher percentage of weight the rider makes up in the bike + rider equation the better. This can be accomplished by adding weight to the rider or reducing the weight of the bike. Reducing the weight of the bike also provides a mass advantage and so this is where we focus. But again I think there is more to the story.

[Mr Sunshine]

JKaiser thanks for giving me an excuse to not go on a diet. :)

[jkaiser]

JKaiser thanks for giving me an excuse to not go on a diet. :)

My winter sport is speed skiing, it's the only speed related sport I know of where more weight will actually make you go faster.

[flyinglow]

jkaiser-
you have a point on the bike vs. rider weight, but you are also right that it is not that simple. the more the total weight, the harder your tires are working in the corners. more weight also means slower acceleration (or more power required for the same acceleration).
overall, a lighter combination of bike and rider will result in better performance through higher potential corner speed and quicker acceleration; hard to beat that combination.

for weight vs. power... unsprung mass is 4 times more critical than spring mass. so loosing 1 lb in those lightweight wheels is "equal" to 4 lb of your weight (or sprug weight of the bike).