What's 100lbs worth?

[erlrich]

Just wanted to get some opinions; with all this talk about tweeners and dual classification and the formula and everything else, the comment has been made more than once that 80lbs, or 100 lbs, or whatever lbs is within the acceptable "noise" range for classifying cars. As someone who is gravitationally challenged (both car and driver) I got to wondering what 100lbs is worth, in terms of lap times. For sake of comparison, let's use as an example a 2-mile "horsepower" track (e.g. Summit Point), and an ITA car (purely coincidental choices I assure you ). Moreover, as I assume the weight would affect different types of cars differently, why don't we consider a couple of cars; a handling car like the Miata, and a horsepower car like the NX2000 or Integra.

So how about it - what's 100lbs worth?

[Knestis]

In terms of Newtonian physics, that mass needs to be accelerated in longitudinally (engine power, positive and braking, negative), and both left and right around corners, so it impacts performance everywhere the car is on the track - except where the driver's being a wuss and coasting.

The math to determine the influence on lap time isn't theoretically complex - it just requires a gazillion calculations. The best way to do it might be to boot up the race car dynamics simulation software and change just that variable. I don't have that software but I'll be that someone around here does.

In reality, it's got to be considered as a percentage change in all respects, depending on what the total mass of the car is. It might be a meaningful difference if 100# is a 5% change (2000# car) rather than a 4% change (2500# car). Your designations of "handling" vs. "horsepower" are proxies for this, I think...

The REAL question however - and the assumption on which the 100# noise tolerance is predicated - is whether or not the variance in lap times resulting from that weight is smaller than the difference between what a car is actually capable of, and what the typical SCCA club racing driver can get out of the hardware. Unless that influence is the determining factor in whether one make/model car beats another make/model car - with ALL of the other variables being held constant - it's just too small to try to attend to.

Similarly, if that weight contributes less to a lap time than the real lap-to-lap variance of the driver, it's meaningless. This is about repeatability.

Either (or even both) can be issues in any given case. A driver can be inconsistently fast or consistently slow (relative to what the car is capable of) and in either situation, it's not weight that's the issue.

K

[spnkzss]

I know this is apples to planets, but I found it interesting that on a 3 mile course for a F1 car they burn about 5 lbs of fuel and that 5 lbs was worth a tenth. Totally extreme, but interesting enough.

[Conover]

$203.08

:P

[JohnRW]

$203.08
[/b]

Yeah, but the bank is going take a cut of that as an 'exchange fee'.

[gsbaker]

^ Damn, beat me to it--although I have $203.11

[Greg Amy]

"It's worth what someone is willing to pay you for it."

- Long-standing Improved Touring Forum "wisdom"

[erlrich]

Kirk, I would have been terribly disappointed with anything less from you; thank you. And while I can't argue with any of your comments, I did specifically place the word "opinions" in bold/underline in the original question for the very reasons you point out; that there are too many variables in play to make an accurate calculation likely.

And I also agree that the average club racer contributes more to the variance in lap times than the weight difference ever would, but I also believe if you were to use best times as a comparison you would see much less variance. By way of example, for the Labor Day races last week we ran two qualifying sessions Saturday, with races Sunday and Monday. Looking at my lap charts, and picking out the laps where I was unobstructed by traffic, my lap times varied by well over a second. But, my fast lap for each session was within .3 seconds. This is what I was asking - how much of a difference would you expect to see on your best possible time for any given driver/car?

$203.08

:P [/b]

10,000 comedians out of work...

Damn...last time I checked it was in the $1.90s

Guess I won't be going to London any time soon

[gsbaker]

Yeah, but the bank is going take a cut of that as an 'exchange fee'.
[/b]

Tell me about it. I just got off the boat and took a 10-15% haircut on the buy/sell spread.

[JamesB]

earl,

Its worth the effort to get into shape (helps you during the race and you might drop a few # in the process.) and while your at it get under that car and start scraping away that undercoating. the combination of weight reduction should help you out in a multitude of ways.

[erlrich]

"It's worth what someone is willing to pay you for it."

- Long-standing Improved Touring Forum "wisdom" [/b]

10,000 philosophers out of work....

earl,

Its worth the effort to get into shape (helps you during the race and you might drop a few # in the process.) and while your at it get under that car and start scraping away that undercoating. the combination of weight reduction should help you out in a multitude of ways. [/b]

Hey now James... I didn't say how much of that was driver, and how much was car .

You're absolutely right of course.

[tnord]

my take;

if your brakes don't fade, 100lbs won't hurt you under braking.
if you're in a car with low torque, and the longest straight has a slow 2nd gear corner right before it, it'll hurt.
if you can add the 100lbs on the passenger floor, and still make cross weights, it won't hurt you much at all in cornering. so long as you don't use up your tires due to the added weight.
if you're at a high speed, flat, momentum track (Nelson Ledges perhaps?), the weight won't hurt much overall

the weight becomes a bigger problem for endurance racing obviously.

i would guess that 100lbs would be worth, on average across all tracks and all ITA cars, .1-.5s/lap. you might want to wipe the poo from that before you use it, as it came directly from my butt.

[Jake]

For most ITA cars, it's about the same as 5hp.

Has anyone seen how much their times went down from a 5hp power add?

[Ron]

Our first ITB mustang went on a diet between two races at Roebling Road. We took out 110 pounds of weight and the lap times went down by almost a second. In the April race David Leria was there and he was a second faster. two months later he was there again and we were equal. So in this poorly written statement on a 2 mile fairly fast and flowing track we were 1 second faster.

[RSTPerformance]

I have never felt that weight had much of an effect on a car like mine, but 100lbs on a heavy car with 75% of the weight over the front wheels does not have the same effect as a well balanced light car... just my experience.

With that said I must admit that even with the thought that it has no effect I still weigh the car between sessions and calculate lbs per lap used at X track so that when the next session is over I am within 10lbs of my legal weight.

Raymond

[Knestis]

... you might want to wipe the poo from that before you use it, as it came directly from my butt.
[/b]

Oh, thank YOU for that mental picture.

Seriously - I think someone could run the sim for you can find the software. You might post over at http://roadrace-autox.com and ask who has it installed. SOMEONE does, I just don't remember who.

K

[Andy Bettencourt]

Golly, I just think that unless you test the same day, the results are moot. I was fortunate enough to grab the track record at LRP last fall and came across the scales 40lbs over min (was a long race and didn't want to get bounced so I made dang sure we would be over). I was also 7hp down from what I am this year with the new ECU. This year I have not gotten real close to that time - even 20lbs lighter. Same amount of heat cycles in tires, etc.

There are SO many variables (track condidtion, track temp, air conditions, etc) that unless we are bolting in and out between sessions, I just can't see how we can get something real.

It's definately worth SOMETHING - would love to see a sim on it.

[JeffYoung]

With all due respect to Earl, not sure this is a helpful thread. 100 lbs means so many different things to lap times depending on car, track, driver, hp, tq., temp, tires, etc.

That said, I run 50 lbs over to be safe and have not seen any significant difference in lap times vis a vis when I run without the ballast. I think however that 50 lbs probably means a lot less on my car given the torque. On a low torque car it may be more significant.

But the bottom line for me is a lot of lap records are being set by cars that are not even close to min. weight. I think that speaks for itself. Quit worrying bout the 50 to 100lbs and start worrying about the driving.

[Z3_GoCar]

With all due respect to Earl, not sure this is a helpful thread. 100 lbs means so many different things to lap times depending on car, track, driver, hp, tq., temp, tires, etc.

....
[/b]

That's too true Jeff. I bolted on 60lbs and got faster The hard top added 60 lbs to the rear and high up but greatly reduced drag that my top end was just so much better that it more than made up for the weight to accelerate, and with larger sway bars and urethane bushings in the sub-frame and trailing arms and proper toe in on the rear and it was much better, untill I swapped ends and went round. All because I couldn't keep up with a 400+ hp Firebird

[dickita15]

The only real world hearsay I know about on this is from World Challenge with regard to rewards weight. It is often said that the extra weight has little effect of a single qualifying lap but greatly contributes to the tires going off over the course of a race. My limited time with my new lighter car seems to back this up. The heavy car went off a lot more near the end of a race.