Downforce -vs- lightening

The thread showing the E46 with a CF trunk and a wing got me thinking (someone else posted part of this on that thread too)

Why lighten the trunk then put a wing on it for downforce? Isn't that counter productive?
Also I see many people pull the spare tire out of their car on track days. You have now lightened the rear and not only messed up the weight distribution (moving the CG forward) but now have less weight over the rear wheels. And how much faster are you going to be with a 20+ lb weight loss?

While the general thought is right (lightening the car) I think the execution is wrong. Kinda like sunday bike riders buying a racing bike "to be faster" or beginner/intermediate skiiers buying racing skiis...if you are not performing at that level you are not going to see any benefit and in the case of Skiis actually see determent.

IMHO,
Haus

Originally posted by in_d_haus
The thread showing the E46 with a CF trunk and a wing got me thinking (someone else posted part of this on that thread too)

Why lighten the trunk then put a wing on it for downforce? Isn't that counter productive?
Also I see many people pull the spare tire out of their car on track days. You have now lightened the rear and not only messed up the weight distribution (moving the CG forward) but now have less weight over the rear wheels. And how much faster are you going to be with a 20+ lb weight loss?

While the general thought is right (lightening the car) I think the execution is wrong. Kinda like sunday bike riders buying a racing bike "to be faster" or beginner/intermediate skiiers buying racing skiis...if you are not performing at that level you are not going to see any benefit and in the case of Skiis actually see determent.

IMHO,
Haus

My take is lightening increases acceleration, while the aerodynamic/downforce creaters work at high speeds to increase stability. That's the way it's supposed to work. Make the car as light as possible standing still, and as heavy as possible (at the tire contact patch) when going fast around a corner. The weight has a minimal effect on top speed (although the additional drag sure does)

That's true Josh but how much difference is a few pounds to a weekend warrior and what is the disadvantage of messing up the 50/50 weight distribution of the car?

Originally posted by in_d_haus
That's true Josh but how much difference is a few pounds to a weekend warrior and what is the disadvantage of messing up the 50/50 weight distribution of the car?

Don't get me wrong Indy, I'm not saying the guy has a clue what he was doing to his M3 (I guarantee the weight of the wing standing still is more than the savings of the CF trunk), I'm just saying from a greatly overly simplified optimum engineering standpoint that's how you want your car to be (light at low speeds, heavy at high ones).

Originally posted by in_d_haus
That's true Josh but how much difference is a few pounds to a weekend warrior and what is the disadvantage of messing up the 50/50 weight distribution of the car?

He will notice the weight savings about as much as he will notice the weight distribution change.

I'm not meaning to make a statement on anyone in particular. I'm just wondering if taking out a spare tire/ removing weight is worth it on a daily driver.

Exporatory reasoning...

It depends on the application. Having less mass to move under acceleration in all six axis is always a good thing. By removing it from the rear, it allows more weight to transfer from the front to rear before the suspension components are overloaded when on the gas. The question is whether the benfeits outweigh any negative impact shifting the static weight distribution has on braking or other handling characteristics. The truth is in the lap times.

Going to a CF trunklid plus wing will increase downforce at speed with a smaller static weight penalty at low speed than adding a wing to a steel trunklid. Of course, adding a wing also raises the CG which comes with its own negatives. Again, the truth would be in the lap times.

Originally posted by in_d_haus
I'm not meaning to make a statement on anyone in particular. I'm just wondering if taking out a spare tire/ removing weight is worth it on a daily driver.

Probably not on a car as heavy as an e46. What's the spare weigh?? 45 lbs? Taking it out lowers the weight of the car by less than 1.5% Do you think you'd notice that difference in power to weight ratio? Now if it was part of a large scale weight saving project on something light (like an e30), then it would be worthwhile.

Have you ever noticed a performance difference between a full tank of gas and a half full one?

Originally posted by Josh (PA)


Have you ever noticed a performance difference between a full tank of gas and a half full one?

Actually yes I have...well maybe not half full but 1/3

Originally posted by in_d_haus


Actually yes I have...well maybe not half full but 1/3

Maybe it's time for you to take your spare tire out then

Originally posted by in_d_haus
That's true Josh but how much difference is a few pounds to a weekend warrior and what is the disadvantage of messing up the 50/50 weight distribution of the car?

20lbs out of 3,000 is like what, .6%? So now instead of your 51.8/48.2 distribution you'd have 52.4/47.6?

It doesn't make that much of a difference in the distribution of weight. However, I read somewhere (might have been from Rob Levinson) that for every 10 lbs sprung weight extracted is like 1 additional HP. On a small displacement car, 2 hp is a 1% gain in power.

Sometimes every bit of advantage helps.

I've seen a few wind tunnel tests of those little spoiler wings that a lot of tuners put on road cars and they generally don't do a heck of a lot. For a front engine RWD car I'd lighten that trunk up to move the CG forward and skip the wing altogether. Unless I get some high-speed oversteer and then I'd put a WING on it and not one of those little ricey spoilers.

Think about it this way for lightening. How many pounds per HP in your car? Then thnik of the wieght loss as being the same thing as a HP gain.

An E46 M3 is about 10 pounds per HP. So removing 45 pounds would be like gaining 4.5 HP. And how many people would jump through many hoops and spend lots of money for a 4 HP gain?

50/50 weight is not a holy grail. Look at F1 cars, nowhere near 50/50. It is more of a marketing ploy than a reality.

Another point, polar moment of inertia. By removing mass from teh ends of the car, you make it easier to rotate the car. The classic exampel is a bar bell versus a bowling ball. At the same weights, which is easier to rotate and stop rotating? This is wy mid engine cars work so well, move the weight towards the center of the car.

As to downforce versus weight, remember, that the ultimate cornering force is based on teh tire delivering so many POUNDS Of cornering force, that based on teh wiehgt translates to a G force. So by making the car lighter you get higher G for the same tire cornering load. Yes, by going lighter, you loss some cornering force (pounds), but not as much as you gain by going lighter. But for "something for nothing" look at downforce. This increases the tires cornering ability by increasing the load on the tire, but doesn't increase the amount of weight that has be accelerated (remember corning is accelerating the car sideways, Nwton's laws, etc).

So bottom line, remove weight make the car faster, both in accleration and cornering. Add downforce and not hurt acceleration and increase cornering ability. Only downside is top speed loss due to drag. And if you can tune the downforce, you change for different tracks.

And finally, if it didn't work, you think that all the racing teams in ALL classes wouldn't be spending lots of time and money trying to make their cars as light as possible with the most downforce? Everything from A Mod autocrosses, through NASCAR through F1.

Final thing, if you have to have a given amount of wieght (class rules) the best thing is to make the car as light was possible, then ad the weight back as ballast. Then mount it low and in the center of the car. If the driver is offset use it to tuen the left right weight. Oh, and talk ti any serious race team, they don't worry about front/rear eight distribution, they worry about corner weighting