Physics question: weight loss v. horsepower

ML536 · Jun 13, 2003

Here is something that I was wondering about, the answer to which should be of interest to those of us who are trying to lose weight:

How many pounds taken off of a rider or bike would be the equivalent of a one horspower increase?

Here's what I mean: Obviously, the more the horsepower and the lower the weight, the more quickly one will accelerate. Certainly, that is a generalization, as there are other factors which could come into play.

Nonetheless, it would seem that if the power increases, or if the weight decreases, the bike should be able to accelerate more quickly. (Less weight would also have the benefit of allowing quicker decelleration, too).

With that being said, is the weight and the power related in some way? In other words, is it possible to calculate that a loss of X pounds would have the same effect as the gain of 1 horsepower?

If there were some kind of formula like that, it would sure provide some motivation when you're on a diet. Does anybody know? Thanks.

KiwiBird · Jun 13, 2003

I've heard a generalization of about 5 lbs = 1 hp for bikes.

Given that, I would rather loose 100lbs than gain 20 hp.

CaptainObvious · Jun 13, 2003

Simple math.

Let's assume that you, and all of your gear, weigh 250 pounds and that the wet weight of your bike (full fuel, coolant and oil) is also 250 pounds. That is a total weight of 500 pounds. The other assumption is that you ride a XX250 that makes 40 horsepower at the rear wheel.

500lbs divided by 40hp gives us a power to weight ratio of 12.5:1

Now, lets assume that you found a GREAT modification that will net you TWO horsepower…

500lbs divided by 42hp gives us a power to weight ratio of 11.9:1, almost a 5% increase.

The question is; How much weight would I have to loose to have the same equivalent net effect?

The answer is: Between you and the bike, you have to loose 24 pounds.

476lbs divided by 40hp gives us a power to weight ratio of 11.9:1, the same as 42 HP and a 500-pound total weight.

The difference is that 2hp might cost several hundred dollars while loosing 24 pounds will save you several hundred dollars (in food) and extend your life.

CaptainObvious · Jun 13, 2003

Just thinking...

Two extreme examples. Lets assume you’re pushing maximum density at 300 pounds riding weight and you ride a 125 that weighs 240 pounds that makes 30hp.

540lbs divided by 30hp gives us a power to weight ratio of 18:1

Same 2HP mod…

540lbs divided by 32hp gives us a power to weight ratio of 16.9:1.

To get the same power to weight ratio you have to loose 34 pounds.

The other extreme. A lightweight rider at 160 pounds riding weight riding a modified 250 that weighs 240 pounds that makes 45hp. 400lbs divided by 45hp gives us a power to weight ratio of 8.9:1 Add the 2 HP and now were at 8.5:1. The rider has to loose 18 pounds for the same power/weight ratio. Not an easy thing for a guy who only weighs 160 pounds! In this case I’d have to say spend the money on the 2 HP.

JasonJ · Jun 13, 2003

Not to mention youll probably be in better shape, look better, and be abel to ride longer.
You also get the advantage of increased braking and menuvering performance that no horse power mod will do for you. But wait! theres more! Since your over all vehicle weight is lower, your rolling resistance in mud and sand will be proportionly lower too makeing for a faster ride :) .

I think in Drag racing they say you can shave .25 seconds off your ET for every 100 Lbs you toss.

mx547 · Jun 13, 2003

but what if you're losing traction because of the light weight?

the fat guys in 40+ get all the holeshots. ;)

JasonJ · Jun 13, 2003

"but what if you're losing traction because of the light weight?"

Get a bike with a bigger motor :) that ell weight it down and give it some bite!
Or, It may be cheaper and more fun the drink beer and eat meat loaf and fires to put a traction device under your shirt.

Eric SX Cartman says, "Im not fat, Im just getting the big hole shot!"

sabooo · Jun 13, 2003

But wait! theres more!

You forgot "Chicks dig it."

:yeehaw:

Cheers,
Sabooo

Papakeith · Jun 13, 2003

Lets assume you’re pushing maximum density at 300 pounds riding weight

Bastages!!!
And here I was all excited that I was down to 305! :eek: :confused:

VintageDirt · Jun 13, 2003

Not to worry Keith, big and fast is cooler than just fast. :confused:

5000 HP
1472 c.i.d. Detroit V-16-92
wheelbase 25'
18,000 lbs
264 mph 2002 SCTA World Record Unlimited Diesel Truck

Danman · Jun 13, 2003

I'm a member on another Forum community specific to my bike as well as a number of street bikes. This topic came up and we have started a weight loss group and even named the "mod" Its call the gut-cut mod. Everybody checks in monthly and we are keeping the stats in a spread sheet.

ML536 · Jun 14, 2003

Thanks for your replies.

With the traction issue, isn't it true that a heavy rider will not get more traction than a lighter one? Traction = friction with the dirt. My recollection is that the coefficient of friction is unaffected by mass. Is it possible that the fat guys are also the guys with the best start skills? Or maybe their extra mass lets them get on the gas harder without looping out?

This discussion reminds me of an article in a Hot Rod magazine about 10 years ago where they took a 1970's Cadillac with a big block engine. They measured it stock, then started to cut weight off of it, taking off the fenders, roof, etc. and measuring its 1/4 mile times as the car got lighter. Eventually, it was nothing but a frame, motor, running gear, and seat. Although I don't recall the specifics, the difference in acceleration and 1/4 mile times was amazing.

OnAnySunday · Jun 14, 2003

Originally posted by Kiwi Bird :
"I've heard a generalization of about 5 lbs = 1 hp for bikes.
Given that, I would rather loose 100lbs than gain 20 hp."

This would explain why i can't keep up wit them %@#& XR fitties! :flame:

High Lord Gomer · Jun 14, 2003

So what appendage do I cut off to get that 2 HP?

Signed,
Holeshot Lord Gomer

yzeater · Jun 14, 2003

Originally posted by ML536
With the traction issue, isn't it true that a heavy rider will not get more traction than a lighter one? Traction = friction with the dirt. My recollection is that the coefficient of friction is unaffected by mass. Is it possible that the fat guys are also the guys with the best start skills? Or maybe their extra mass lets them get on the gas harder without looping out?

Well....the total force of friction (the important thing, the coefficient is simply a scalar) is not affected by mass, but it is affected by weight, which is directly proportional to mass in all cases we'd deal with biking. So a heavier rider will get more traction

kbud · Jun 14, 2003

Well, I wouldn't get rid of a leg as prostetics are heavier and how would you shift or brake. The average human head weighs 8 pounds, right? I would go with the head and maybe a couple toes. The middle toes are just filler.

dirt bike dave · Jun 15, 2003

Originally posted by High Lord Gomer
So what appendage do I cut off to get that 2 HP?

Signed,
Holeshot Lord Gomer

 

Gomer,

You could try a breast reduction  ;)   (sorry, couldn't resist).

dirt bike dave · Jun 15, 2003

Originally posted by ML536
This discussion reminds me of an article in a Hot Rod magazine about 10 years ago where they took a 1970's Cadillac with a big block engine. They measured it stock, then started to cut weight off of it, taking off the fenders, roof, etc. and measuring its 1/4 mile times as the car got lighter. Eventually, it was nothing but a frame, motor, running gear, and seat. Although I don't recall the specifics, the difference in acceleration and 1/4 mile times was amazing.

Below is a link to the same concept done to the extreme with a modern car.  Detailed breakdown of the weight savings and dramatic peformance gains.  Pretty funny, too.

http://forums.vwvortex.com/zerothread?id=776885

Click to expand...

Physics question: weight loss v. horsepower

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Physics question: weight loss v. horsepower

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