mhax

I read some thread about weight reduction and i wanted to see what would be the gain from a 80 kg reduction on the 1/4 time. I used some physics formula (in SI unit), i think i did it right, but if i'm wrong, let me know.

-2001 integra gsr
-Weight of the car stock = 1150 kg
-1/4 time = 15.0 seconds
-1/4 miles distance in meters = 402.336 meters

distance = initial distance + initial velocity * time + 0.5 * acceleration * time ^2

initial distance = 0
initial velocity = 0

so

distance = 0.5 * acceleration * time ^2

considering the time = 15.0 seconds and the distance = 402.336 meters, the average acceleration = 3.54632 m/s^2

Using the well known F=m*a formula, F = 4112 N
If we remove 80 kg, the new mass = 1150 - 80 = 1070 kg
4112 = 1070 * a, so the new acceleration = 3.843708
We put this in the distance formula to find the new time value :
402.336 = 0.5 * 3.843708 * t ^ 2
t = SQRT(402.336*2/3.843708) = 14.46886

Conclusion, a weight reduction of 80 kg will make you gain 0.5 second on your 1/4 time.

Limitation of this calculation : the acceleration is considered as constant
But it still gives a good approximation.

wolf95

I just took my physics final today on all that stuff. it looks right to me.

94VTEC2TONE

wow talk about some **** right there, and thats alot of weight to drop though

sebTeggy

Using a simplified version of that formula, yeah you are probably right.

For it to be accurate, id say you have to use integration since acceleration (as you said) varies.

Although, it is approximate (for very very small masses).

Pretty cool tho :D

SleeperGSR

Curious.....what is your F?

Something doesn't seem right in this. I think there are alot of things not being taken into accout here. Drag increases exponetially vs speed is one for example.

OldSchool Honda

I think the rule goes something like: For every 100 lbs dropped, you lose 0.1 seconds on the 1/4 mile.

This is mostly in part that yoru are not aerodynamically doing anything different, and at higher speeds, aerodynamics kicks weight's but.

Weight loss makes a HUGE difference in handling. I just took out 145 lbs out of my car, and it grabs corners way more.

94VTEC2TONE

I talked to a tuner and he dropped 85lbs in his hatch and gained 3 tenths so its not always true

youmadbro

In a simplified "world" you would be totally spot on. But as mentioned the reason the answer doesn't seem right is because there are a lot of forces at play here. Friction and air flow, as well as power lost due to mechanical parts are all stuff that work against your car, and ultimately your time. Hopefully there's a engineer that actually works for Honda in here. lol

mhax

That's right, i should tweak it to use drag effects. I could remove the force caused by the drag as a function of speed.

mhax

I thought it would be more than 0.1 for 100 lbs...

mhax

In my example it is 80 kg not lbs.

mhax

Since i'm using a specific 1/4 time for the acura (15.0 seconds), the time takes care of the drag and road friction. If the friction/drag was lower, the 1/4 would be faster and the opposite is true as well.

The force, acceleration and drag are all average values on the period of time.

Even with considering the drag affects the default 1/4 time, i'll try another way to calculate it.

mhax

Lol forget everything i said. The drag force is function of front surface area, the drag coefficient of the integra and the speed.
F_drag = -1/2 * air_density * velocity * front_car_area * drag_coefficient

One could consider a difference of 5 mph in the final speed for a difference of 0.5 second on the 1/4 time.

Air density = 1.3
Velocity = 95 mph for the 14.5 sec 1/4 time and 90 mph for the 15.0 sec 1/4 time
front_car_area = ...2.0 m²?
drag_coefficient = 0.31 ?

We find a difference of 240 Newtons between the two final speed. I'm not sure if we could consider this negligeable. I'll have to think about it hehe!

mhax

Haha that's funny :
The first F i calculated was equal to 4112 N.
Using this formula Power = Force * distance / time
i get a force = 3918 N with power = 143 whp.
I used the metric conversion of 1 HP = 735 Nm/s.

youmadbro

You could always try energy as well, granted all you would need is the frictional force on the floor which a quick search online would take care of that. You would have to assume you don't slide at any given point to simplified the situation.

94VTEC2TONE

yes i can read your posts. I just stated the guy i talked to cut 85lbs and 3 tenths, so the 100lb per tenth isnt always true. just stating a point

OldSchool Honda

Ya, it depends on the car too I guess. Cutting 100 lb from a mustang vs cutting 100lb from an integra are two totally different stories.

b18c5ness

The reason for why the equation is off is other missing variable such as "traction". If traction, aerodynamic and shifting is constant then all is true.

dc4ok

So much mathematics. I grew up learning street, I didnt pick up a book to learn. Really is all
obvious. Too heavy to much drag. Too light too much slip. I'd just tweak with it and leave
the math to the japs that build race cars for a living lol.

mhax

Drag is not function of mass. Friction on the road is function of mass. Drag is only affected by the front area of your vehicule and the speed you are going. Drag doesn't know about weight.

OldSchool Honda

Not true.
Depends indirectly on your weight.
A heavier object will cut through air straighter, making a more uniform airstream.
A lighter object will tend to "wobble" or "bend", making a less uniform airstream.
Example: a ping pong ball and a tennis ball flying through the air.

Also, F=ma counteracts Ffriction of air. Ex. Why a sheet metal kite does not fly where as a cloth one does.

I think this can be ignored when your measuring acceleration for cars 300lbs different in weight.
Thats why we always ignored it in physics: only made a 0.01s difference.

mhax

You are talking about surface drag which is caused by friction of air with the material of the vehicule. If removing weight inside the vehicule, you won't see any effect on the drag, except that if you remove weight, you will go faster, and if you go faster, you will have more drag.

OH_1fstgsr

iTrader: (2)

So why do I want to make a Rear Diffuser to but on my GSR? Because that rear bumper acting like a parachute is creating air turbulence and drag.

SJC

has no one said anything about gears yet. acceleration will never be a constant, and there is a MUCH easier way to figure this out... go to a track, run your car, then take off whatever it is your taking off to lose weight, then run it again. no calculator needed. and it's highly unlikely your going to drop almost 6 tenths of a second by taking off 80kg. just saying