7 lbs = 1 HP loss?

[HoOn]

I heard and read somewhere that around 7 lbs of weight is around an 1 hp loss?

For example if someone had 20 lb wheels and my wrx had a 27 lb wheel setup I would actually be losing 4 HPs compared to the 20 lb ones?

Did I just confuse you?

Anyone wanna clarify? Thanks!

[WR^2X]

I thought it was 4 pounds of unsprung weight...

[WR^2X]

Here it is:

5 lbs = 1 HP
1 HP = .1 second in the quarter mile

Link

[KoneKiller]

Uuuh.... unsprung weight doesn't affect acceleration any more than sprung weight. Unsprung weight has more to do with suspension performance.

To be accurate, it's rotational mass that has a disproportionate affect on acceleration because not only do you have to MOVE that mass down the track, you have to SPIN it, too.

T = I x alpha

T = torque
I = moment of inertia around the axis
alpha = angular acceleration (which is what we ALL want)

So to accelerate (or stop) faster, you either need more torque or you need less polar moment of inertia.

You get less polar moment of inertia by reducing the amount of mass you spin or by reducing the radius at which the mass spins.

So... lighter and smaller wheels, brake rotors, flywheel, clutch, axles, driveshafts all add up to faster acceleration and braking.

Bottom line is that saving 30 pounds by switching to a fancy battery will gain you much less than shedding 30 pounds off of parts that spin.

[ndw1138]

Originally posted by HoOn
I heard and read somewhere that around 7 lbs of weight is around an 1 hp loss?

For example if someone had 20 lb wheels and my wrx had a 27 lb wheel setup I would actually be losing 4 HPs compared to the 20 lb ones?

Did I just confuse you?

Anyone wanna clarify? Thanks!

KoneKiller laid it out nicely..
But regardless the numbers you are probably looking for is weight on body not wheels is ~7.5 lbs on a 3000 lb vehicle = 1 effective horsepower. This is not true with weight sitting on your wheels (sprung/unsprung). That number is far worse (might be 2.25 lbs per hp, the porsche guys use to have a chart on the rennlist board. Adding weight to wheels is horrible for performance, which is one reason 19" wheels are ridiculously stupid for performance. Lighter brakes, lighter wheels and Im just discovering lighter TIRES..

nathan

[KoneKiller]

Originally posted by ndw1138
KoneKiller laid it out nicely..
But regardless the numbers you are probably looking for is weight on body not wheels is ~7.5 lbs on a 3000 lb vehicle = 1 effective horsepower. This is not true with weight sitting on your wheels (sprung/unsprung). That number is far worse (might be 2.25 lbs per hp, the porsche guys use to have a chart on the rennlist board. Adding weight to wheels is horrible for performance, which is one reason 19" wheels are ridiculously stupid for performance. Lighter brakes, lighter wheels and Im just discovering lighter TIRES..

nathan

Yeah... lighter tires... it's a new frontier for me too....

[WRXMikeH]

Falken Ziex 512...pretty lightweight, check out their site.

[hunts]

Interesting.. now I'm thinking of going to workout and lose some weight myself!

[Zoso]

Doesn't 1lb of unsprung weight = 10lbs of sprung weight or something like that? So if your new wheels were 5lbs heavier each then the old ones that would be the equivallent to having 200 extra pounds in the car.

[nhluhr]

Originally posted by Zoso
Doesn't 1lb of unsprung weight = 10lbs of sprung weight or something like that? So if your new wheels were 5lbs heavier each then the old ones that would be the equivallent to having 200 extra pounds in the car.

sprung vs unsprung has absolutely nothing to do with acceleration.

[Zoso]

Originally posted by nhluhr
sprung vs unsprung has absolutely nothing to do with acceleration.

Could you elaborate on that a little, not sure what you mean?

[nhluhr]

Originally posted by Zoso
Could you elaborate on that a little, not sure what you mean?

this thread is talking about the weight of the wheels/tires/brakerotors affecting the rotational inertia and how that in turn affects acceleration.

The issue of unsprung weight (bottom half of struts, brake calipers, wheels, tires, rotors, hub housing) vs sprung weight (car, top half of strut, passengers, cargo) has nothing to do with this conversation. The unsprung/sprung matter has to do with suspension feel, not acceleration.

[KoneKiller]

Zoso...

I'm not sure what 'ratio' would be accurate, but the phrase should look more like:

X number of pounds in rotation = Y pounds of total car weight in its effect on acceleration/deceleration.

And we are not just talking wheels/rotors, but EVERYTHING your motor has to spin to make you go.

This is part of the issue of why a Porsche is so much quicker than cars that have similar or higher horsepower numbers -- Porsche does a great job of reducing polar moment of inertia in every little bit that spins.

[WR^2X]

The problem here is that most people (myself included), mean "parts that rotate" when they say "unsprung weight", because the majority of your unsprung weight comes from parts that rotate... So when used in that sense, "unsprung weight" has a lot to do with acceleration. Before this thread I had never heard or seen anyone use more than just "sprung" and "unsprung" weight, there was never the third factor, "polar moment of inertia". I can see where my "unsprung weight" generalization is wrong, but I will continue to just say "sprung" and "unsprung" rather than explain physics to people.

[turbine]

1 Hp does not equal 0.1 seconds in the quarter. More like 0.01 seconds.

[WR^2X]

Originally posted by turbine
1 Hp does not equal 0.1 seconds in the quarter. More like 0.01 seconds.

I'm just going by what Luke said in his post.

[nhluhr]

Originally posted by WR^2X
but I will continue to just say "sprung" and "unsprung" rather than explain physics to people.

So you are actually choosing to continue propagating bad information? great job.

There is a VAST difference between unsprung weight and rotaing weight.

Lots of rotating weight is NOT unsprung and lots of unsprung weight is NOT rotating.

For instance:
flywheel, clutch, driveshaft - all rotating but NOT unsprung weight.

control arms, hub housing, brake caliper - all unsprung but NOT rotating weight.

To say that reducing "unsprung weight" helps acceleration would be true, but only because you are reducing total weight. In that sense, you are wasting breath when you say "unsprung". Because, in reality, reducing "sprung weight" ALSO helps accleration to the same degree.

The correct notion is that reducing rotating weight has a profound effect on acceleration, which many people approxmate as - for every 1lb of rotating weight you lose, the increase in acceleration is as if you are giving up 4lbs of total weight.

Really, if you had never heard the term "polar moment of inertia" before and you don't have a good understanding of how it's determined, then you have no place explaining physics to anybody

[nhluhr]

Originally posted by WR^2X
I'm just going by what Luke said in his post.

5 lbs = 1 HP
1 HP = .1 second in the quarter mile

if luke said that, then I will flat out announce that "Luke is completely wrong"

[WR^2X]

Originally posted by nhluhr
So you are actually choosing to continue propagating bad information? great job.

Since this is a tire and wheel forum, both of which are unsprung weight, and the original question was no doubt asked regarding unsprung weight, then yes, I will continue to say unsprung weight. He wasn't talking about anything else.

Nobody has ever corrected me when I said "unsprung weight", and in all of the posts I have read, nobody has ever corrected anyone else when they said "unsprung weight"... When you're posting to this forum, it's understood that you're referring to weight that truely is unsprung - tires and wheels. Nobody posts talking about unsprung weight in this forum and wants opinions on getting a lightweight driveshaft.

[WRXMikeH]

Originally posted by nhluhr
So you are actually choosing to continue propagating bad information? great job.

There is a VAST difference between unsprung weight and rotaing weight.

Lots of rotating weight is NOT unsprung and lots of unsprung weight is NOT rotating.

For instance:
flywheel, clutch, driveshaft - all rotating but NOT unsprung weight.

control arms, hub housing, brake caliper - all unsprung but NOT rotating weight.

To say that reducing "unsprung weight" helps acceleration would be true, but only because you are reducing total weight. In that sense, you are wasting breath when you say "unsprung". Because, in reality, reducing "sprung weight" ALSO helps accleration to the same degree.

The correct notion is that reducing rotating weight has a profound effect on acceleration, which many people approxmate as - for every 1lb of rotating weight you lose, the increase in acceleration is as if you are giving up 4lbs of total weight.

Really, if you had never heard the term "polar moment of inertia" before and you don't have a good understanding of how it's determined, then you have no place explaining physics to anybody

I think you're reading into it too far here... When people refer to unsprung weight, they are referring strictly to rotational weight. Sprung weight is another way of describing the gravitational weight (m*g). To say that something is unsprung because it is physically below the spring on the car is just silly.

EVERYTHING on the car is "sprung" because gravity affects everything...the term unsprung is really a misnomer. Both types of weight affect acceleration, and lightweight rotating parts remove both types of weight, hence why they help more than a lightweight battery for example. Anywho, not trying to criticize you here, just wanted to point out your misinformaion like you pointed out to WR^2X.


For the sake of all, people should just say gravitational and rotational.

[Rich10]

Originally posted by turbine
1 Hp does not equal 0.1 seconds in the quarter.

If 1hp equalled 0.1 seconds in the quarter mile, you could add 30hp and run 11's.

[WRXMikeH]

That's not what the quote is saying..

For every removal of 5 lbs of ROTATIONAL weight, you will pick up .1 sec in the 1/4 mile. Who knows if this is true, but it sounds about right to me. Obviously, this is a law of diminishing returns...as horsepower required to drop 1/4 mile times increases exponentially as you get faster. We would have to know what 1/4 mile time range Luke was talking about in his approximation.

[Mister Toro]

nhluhr,

Glad to see you're a voice of reason here. When the facts are simple and clear, why do so many take the attitude that "I'm comfortable being wrong because that's the way I've always done it."??? You are right on with your last post.

Originally posted by WR^2X
Nobody posts talking about unsprung weight in this forum and wants opinions on getting a lightweight driveshaft.

That is because there is a difference between 'unsprung mass' and 'rotational mass'. Our cars are made of unsprung mass, rotational mass, unsprung-rotational mass, and lots of mass that is neither unsprung nor rotational. There is a reason to be precise in just which you are talking about. While we can make assumptions about what the original poster intended to ask, he didn't mention any of these terms and so was not wrong about them. Others however have used them incorrectly in the rest of this thread.

Originally posted by WR^2X
I will continue to just say "sprung" and "unsprung" rather than explain physics to people.

Hey, that is ok, but when you use it incorrectly like you have here, please be prepared to be hassled becuase you are wrong and misleading.

Originally posted by WRXMikeH
To say that something is unsprung because it is physically below the spring on the car is just silly. EVERYTHING on the car is "sprung" because gravity affects everything...

[ralphwiggum]My brain hurts.[/ralphwiggum]
I guess there is a reason why physicists are such a lonely and longsuffering bunch.

Originally posted by Rich10
If 1hp equalled 0.1 seconds in the quarter mile, you could add 30hp and run 11's.

Good point Rich, but why stop there? Just add 150hp and finish before you start!

[Farfrumwork]

I believe there was a Grassroots Motorsports article last year sometime on Rotational and vehicle weight, and the effect they had on acceleration....

IIRC they added 8lbs disks to each wheel (solid disk, not concentrating weight at the outside or inside) and came up with a .15sec (or so) increase to the 1/4mi time. so ~32lbs = .15sec.

I can't remember what they gained in traps - but they did also affect mph

Then they added weight to the car itself, my memory is fuzzy here, someting like 100 or 150 lbs and came up with the same effect as the 8lbs wheel addition... ~.15sec (again from partial memory).

I rememver thinking... hey, if I get those Rota's I want, paired with my Khumo MX's (suposedly lighter), I could knock around .1sec off my 1320 time -being conservative. My Leg GT wheels weight 19.5lbs


anyhow... even 5lbs=.1sec is VERY hopefull. I think in reality (making many assumptions for any of these hypothesis; location of weight, "I" being very complicated for a most wheels, etc..) I think the best you could do would be >

for every 25lbs of Rotational weight you can shave 0.1sec off your 1/4 mile time


-chad

[filbert]

Originally posted by WRXMikeH
For the sake of all, people should just say gravitational and rotational.

Well... since we're getting all geeky... it wouldn't technically be gravitational because your weight changes based on gravity, but your mass does not. If you were in space and had zero gravity and zero weight... your mass would still be the same and would require the same amount of force to accelerate the mass of your car... so really, it should be rotational mass and non-rotational mass.

That's one of the few odd things about the metric system. Everyone talks about your weight in kilograms, which is incorrect. KG is a measure of mass, but weight is a force which changes with gravity. So technically, we should weigh people in Newtons in the metric system. In the US, we use pounds for weight, which is correct since it is a unit of force. The US standard unit of mass is called the slug, but no one seems to use that.