Joe Weinstein

Hi all. I would love to know roughly how much one should expect
to gain from lightening the car. I do know that it depends
on where the weight is... Removing weight high up on the car,
or off the unsprung or rotating mass (wheels are both, so
are especially important) are best, and if a car is
significantly unbalanced, lightening the heavy end is better
than the other end...
Is there a rough road-race/autocross formula for what removing
a generic 1% off the total weight would show on the clock? Or how
much I'd see removing 7 pounds off of each wheel?
I am sure there are folks with detailed notes and a long history
of devoted auto-lightening... Drag racers have a simpler formula
and do have an approximate formula.
Oh, and in the 7-lbs per wheel case, the wheels would
go from 26lbs and 30lbs to 18lbs and 22lbs, so it's about 35%
less rotating and unsprung weight, and the wheels are 18" so
the weight's polar momentum is high.

thanks in advance,
Joe Weinstein

hayaku

not the right way to do it, but you can get some guestimates from some basic formulas...

each unsprung 10 lbs of weight loss = 1 hp regained at the wheels

1/4 mile acceleration formula:
ET = (weight / rwhp)^1/3 * 5.825
Trap Speed = (rwhp / weight)^1/3 * 234

mitch236

The mathmatical variables are so expansive that a simple formula is impossible.

mein51

Correctamundo. Removing weight changes everything (CG, brakes, power, effeciency, etc...).
I've heard a drag racer rule of thumb is that for each 100lbs you lose, you can gain a second in the 1/4. Not that its helpful in a road race scenario, but it can give you an idea.

TheOtherEric

On a 90 second track, I lose roughly 1.5-1.8 secs with a 170 lb passenger. YMMV.

J Silverman

Its actually 100 lbs for a tenth of a second in the 1320.

Larry Herman

Really, I have actually picked up time. I tend to drive faster when they're screaming.


Seriously, it will be interesting to see, because with the new PCA rules I can take about 100 lbs out of my car for this season.

Joe Weinstein

and what is the running weight with and without the passenger?
100 lbs off is a lot different in a 2000lb car than a 3500lb car.
thanks,
Joe

Larry Herman

I'm smack dab in the middle at 2750.

TheOtherEric

My car weighed in at 3020 lbs.

Geoffrey

I have a very lightweight 964RSR and the difference between it and a standard factory RSR is substantial in the braking, turn in and corner exit.

Jim Child

I don't think its possible to answer your original question due to too many variables at work. However you can do some interesting lbs/hp calculations.

Assume you can take 100 lbs out of a 3,200 car with 215 whp.

Original lbs/hp = 3,200/215 = 14.88
New lbs/hp = 3,100/215 = 14.42

You can now say that all else being equal, removing 100 lbs. was the same as gaining 6.9 whp. You calculate this by dividing the original weight by the new lbs/hp:

3,200/14.42 = 221.9 whp ==> an increase of 6.9 whp

This answer applies only to this car. Answers will vary depending on the weight and hp assumptions used.

Of course in real life all else is not equal. A lighter car will also corner and brake better. Translating all of that into an impact on lap times around a road course is where it gets impossible. Consider the fact that no two road courses are the same and it gets even more complicated.

Your best bet to answer your orignial question is to find a known car that is comparable to where you want your car to be and find out what lap times it is turning on the tracks you plan to run.

Joe Weinstein

Yep. I'm asking in autocross groups too, because that application would maximize
the difference. It's all acceleration at low speeds where weight is the issue (no
aero component), maximum cornering, and braking.
Joe

mein51

Sounds more realistic - I'm no drag racer

TD in DC

Removing weight is dangerous because it can take away a primary potentially valid excuse for why you are slower than others. I removed 200 lbs from my car this winter . . .