Weight is Time

i know that the more weight on a car, the slower it will go (hopefully everyone know that). but, is there like general numbers for how much slower you go with how many pounds you put on? like for every 100 pounds you will go .1 second slower in 1/4 mile, that was just an example of what im looking for. or i guess i could use a formual if there is one. thanx.

i know that the more weight on a car, the slower it will go (hopefully everyone know that). but, is there like general numbers for how much slower you go with how many pounds you put on? like for every 100 pounds you will go .1 second slower in 1/4 mile, that was just an example of what im looking for. or i guess i could use a formual if there is one. thanx.

I dont have the formula for ET estimation in front of me, but I do have one for trap...

trap = 235* cubed root of (hp/weight)

I do remember reading in Hot Rod or something, that you do need to lose at least 100 lbs to make a consistent measurable difference in quarter mile time. There is no realistic linear formula as you suggest as there are other factors at play.
One thing to keep in mind though. A reduction in rotational mass is very important. A one pound reduction in tires and wheel is on average, eqivalent to a loss of 4 pounds of weight elsewhere. Therefore, running the lightest tires, wheels and rotors possible is very effective.

I do remember reading in Hot Rod or something, that you do need to lose at least 100 lbs to make a consistent measurable difference in quarter mile time. There is no realistic linear formula as you suggest as there are other factors at play.
One thing to keep in mind though. A reduction in rotational mass is very important. A one pound reduction in tires and wheel is on average, eqivalent to a loss of 4 pounds of weight elsewhere. Therefore, running the lightest tires, wheels and rotors possible is very effective.

We had a discussion about this topic a few years back. I don't remember exactly how the numbers worked out, but I don't think it came to quite the same figure as the 4:1 rule of thumb. It's important to remember that fast-rotating inertia is more critical than slowly-rotating inertia (by the square of the speed ratio), so reduced crank/rod/flywheel inertia will have a much bigger impact than reduced wheel/tire inertia. Cam assy inertia is less important than crank assy inertia, because of the 2:1 speed reduction, and because there is a lot less of it, but gram-for-gram* it is still pretty important compared to wheel inertia.

(*yeah, I know, kg*m^2 for kg*m^2 would be more accurate)

I figured that the old thread "needed digged up," so I done digged it. It was not as interesting as I was hoping. The conclusion of the blathering was as follows:

So the above formula, combined with the approximation used in the first post, shows us that every pound removed from the [a] tire is worth twice as much acceleration as a pound removed from the body of the vehicle. (times 4 again if you do it to all tires) Mass removed from the wheel does not have quite as much of an effect, but it's still better than a 1:1 relationship.

http://www.automotiveforums.com/vbulletin/showthread.php?t=13944&highlight=the+crazy+hoop

We had a discussion about this topic a few years back. I don't remember exactly how the numbers worked out, but I don't think it came to quite the same figure as the 4:1 rule of thumb. It's important to remember that fast-rotating inertia is more critical than slowly-rotating inertia (by the square of the speed ratio), so reduced crank/rod/flywheel inertia will have a much bigger impact than reduced wheel/tire inertia. Cam assy inertia is less important than crank assy inertia, because of the 2:1 speed reduction, and because there is a lot less of it, but gram-for-gram* it is still pretty important compared to wheel inertia.

(*yeah, I know, kg*m^2 for kg*m^2 would be more accurate)

To get technical, the mass that is further away from the center of rotation is more critical, therefore light weight tires are a more effective change than light weight wheels, pound for pound.
Reducing the weight of the internal engine parts is much more expensive, though. and would change the engine characteristics.
An aluminum driveshaft would help lots and be cheaper, if RWD.

The effectiveness would depend on where the most weight is. My last rod project had 15" steel wheels and heavy all season tires on it. I threw them out and picked up some flea market aluminum rims and put some low profile Pirellis, ( a really thin, light weight tire) The new wheels and tires were 25 lbs lighter EACH. That's 100 lbs overall, of rotational mass. It made a noticable difference.
Also, using a lighter spare tire and car jack in the trunk helped.
I also threw out all the AC stuff, used an aluminum intake manifold and headers which saved another 80 pounds. This car also came from the factory (in the following year) with a much lighter aluminum hood. Its worth 50 lbs saving on its own.
If I wanted to go further, I could use some light weight Recaro seats or something similar and toss out all the tar type sound insulation under the rug.

One thing I have read is taking 6lbs off a car is about equivelant to gaining 1 horse power.

To get technical, the mass that is further away from the center of rotation is more critical, therefore light weight tires are a more effective change than light weight wheels, pound for pound.


Yep, that's why I was talking about "inertia" instead of mass - inertia includes the radius. Also, the flywheel is a big hitter, and I wouldn't count it as an internal engine component...

While gutting your car will gain you a few tenths in the 1/4, ask yourself this. Is it really worth gutting your car for a few tenths in the 1/4? Stick to the rotating mass, and gut your car when the few tenths will actually matter (serious racing).