Question on adding lightness.

In the grand scheme of things, which is better? Removing weight from the car as much as possible or getting as close to a 50/50 weight balance as possible?

I realize that doing both is probably the best answer but on my car (2nd gen f-body) there's a lot more that can be removed from the front and some that can be moved to the back.

I can probably take about 100 lbs out of the front of the car but lightening the back as well would probably only get me 20-50 lbs at most and wouldn't help to even out the balance much at all.

Thanks.

Shawn

Is the car biased to the front already? Seems most stuff usually is.

Joey

Lighter > balanced.

I lean more towards balance. Here's why: all else being equal, a car with a good PMOI (Polar Moment Of Inertia) will handle better than another car with a poor (or at least worse) PMOI. So the more stuff you can keep inside the wheelbase, the better. Honda called it 'mass centralization' with motocrossers back in the mid '90's and yes it did have a measureable effect. Their bikes were no lighter than the competition's but were noted for having a better 'feel' and this led to rider confidence which led to ??? Then profit. Look at it another way: say you have a ~1500 pound car but 300 pounds of it sticks out past the front axle (that's real world, an early VW Rabbit) or you have a similar weight car with ~ 200 pounds sticking out past the front axle, which one is gonna handle better?

Yes, the car is front biased, they're about 60/40 stock.

It can be improved by moving the battery to the trunk, above the axle and removing front inner sheetemetal, bumper supports etc.

If I lighten the rear as well, it will make the car lighter but maintain the front bias.

Shawn

The mid-engine Mini I built was just amazing for fast lateral transisions.

My favorite analogy is the shopping car with a case of beer in it. The overall weight of the cart is constant, but move the beer from the front of the cart to over the rear wheels and it makes an amazing improvement for changing directions quickly. I'm sorry, what was the question?

Question was, essentially, whether it was more important to have a light car or a better front/rear weight bias.

It sounds like I should be lightening the car and moving as much unremoveable wieght as close to the center of the car as possible.

Is that about right?

Trans_Maro wrote: Question was, essentially, whether it was more important to have a light car or a better front/rear weight bias. It sounds like I should be lightening the car and moving as much unremoveable wieght as close to the center of the car as possible. Is that about right?

No. You move it as far back as you can until you hit 50/50, then you start moving it to the middle.

While you have your forward bias at the moment, the best way to counteract that is to move as much as far back as you can. At least that's what my thought process on physics is telling me. I could be wrong.

kb58 wrote: The mid-engine Mini I built was just amazing for fast lateral transisions. My favorite analogy is the shopping car with a case of beer in it. The overall weight of the cart is constant, but move the beer from the front of the cart to over the rear wheels and it makes an amazing improvement for changing directions quickly. I'm sorry, what was the question?

I get it now! I need to put a case of beer in my trunk!

Everything has to work together, but being light helps in acceleration, braking, and handling. Weight distribution depends a bit on how the car is handling. Taking weight out of the middle is not as effective as removing weight on the outside of either axle, which is why most of us move the battery from the front and put it right behind the driver's seat (if practical). Car weighs the same, but we took weight off the nose and put it towards the center.

Porsche bragged about having lumps of weight on each axle in the 928. The effect on handling was compared to holding a dumbbell and rotating it back and forth compared to rotating a heavy object with the weight centralized. The dumbbell has a more distinct and controllable rotation compared to the central weight object which just spins freely. Who knows if it is just marketing hype, but it sort of makes sense.

two comments:

1. lighter > balanced. 911s are fast on race tracks, and their static weights are about 40% front / 60% rear. CRXs are fast on race tracks, and their static weights are about 60% front / 40% rear. if your cross-weights are good, you're good.

2. no way is a stock f-body of any generation 60/40. at worst i'm betting 55/45.

Go for lighter first. Straight line acceleration is determined by power to weight ratio until high speed bring aero drag into the equation. Think of it this way, a 5000 lb. car with a 500 hp. motor accelerates as fast as a 2500 lb. car with a 250 hp. motor or a 1000 lb. car with a 100 hp. motor, until you get to the first corner.Then the lightest of the cars will be able to drive much further into the corner (brake later), maintain a higher cornering speed, and start the next straight already going faster. Ultimately winning every time. Balance and low polar moment of inertia are both important, but low weight trumps them all. I'm sure your impressed with your F body an I'll assume a 350. Put that same 350 in a Caprice It's less impressive. A 4 door Suburban even less so. Put it in a Monza or an early Chevy II and you've got a wild ride. Put it in a Vega or (mid) Corvair and you'll fill your pants!

Angrycorvair:

I've seen stripped, race f-bodies listed at 55/45 but couldn't find numbers for stock vehicles. I assumed closer to 60/40 just to be on the safe side.

Time to check corner weights.

Smitty:

Power to weight ratio, thanks, I'm familiar with the concept. I owned a Starlet for quite a while. Just started autocrossing in American iron last year. I don't run a smallblock though.

It's a 1981 Turbo T/A with a worked over 301T. Makes plenty of power for this chassis. Power vs weight isn't my problem, power vs traction is.

I'm trying to get a more balanced, more predictable car. SFC's and G-braces are in the very near future. Since I'm going to be lightening the car as well, I wanted to find the best approach.

Thanks guys.

Shawn

MrJoshua wrote: Porsche bragged about having lumps of weight on each axle in the 928. The effect on handling was compared to holding a dumbbell and rotating it back and forth compared to rotating a heavy object with the weight centralized. The dumbbell has a more distinct and controllable rotation compared to the central weight object which just spins freely. Who knows if it is just marketing hype, but it sort of makes sense.

Since the 928 was a GT, that makes sense. You want that stability. The flip side is that when the car gets out of shape, it takes a lot longer to bring it back. The dumbbell analogy is good for illustrating the polar moment of inertia. Mazda went to great efforts on the new MX-5 to stuff the engine as far back as possible to minimize polar moment. Nissan brags about their "front mid-engined" design on the 350Z, IIRC, and you can ask Audi what the biggest problem with the original Quattro as a rally car was - the engine stuck out in front of the drive wheels.

I'd go for light weight over balance, as long as things don't get too far out of control. I have to admit I once installed a spare tire in a race car to get some weight on the very light right rear though :)

Trans_Maro wrote: It's a 1981 Turbo T/A with a worked over 301T. Makes plenty of power for this chassis. Power vs weight isn't my problem, power vs traction is.

sounds like it's time to add some traction, then. what tires are you running?

Do you have aluminum heads on it? Good way to save some pounds.

Lightness..

If you took a front wheel drive car and kept adding weight to the rear to bring the balance closer to 50/50, you'll go slower.

Running 255/60 15 tires at the moment, as soon as the tire budget allows, I'll be adding wider rubber.

255 is about as wide as I can go on the front but I can squeeze 275's in on the back with the use of a random impact generator.

No aluminum heads available for this engine.

Shawn

LocostSmitty wrote: Go for lighter first. Straight line acceleration is determined by power to weight ratio until high speed bring aero drag into the equation. Think of it this way, a 5000 lb. car with a 500 hp. motor accelerates as fast as a 2500 lb. car with a 250 hp. motor or a 1000 lb. car with a 100 hp. motor, until you get to the first corner.Then the lightest of the cars will be able to drive much further into the corner (brake later), maintain a higher cornering speed, and start the next straight already going faster. Ultimately winning every time. Balance and low polar moment of inertia are both important, but low weight trumps them all.

Unless you are comparing 0-60 times aero has more of an effect.

Ex. A 2500lb RX-7 with 200hp vs a 3300lb RX-7 with 265hp. (same power: weight ratio)

They have the same coefficient of drag and frontal area, so the 3300lb RX-7 will pull away on the straights. The lighter vehicle will be quicker through the corners. On a given track either could be the faster vehicle.

Polar moment on inertia is important. It is a reason why the 240SX is the most popular drift car. The big lumps of mass are situated near the ends of the vehicle. This makes drifts easier to control. The RX-7 with its front mid engine layout has the mass situated closer to the center of the vehicle. The rear steps out much quicker. Even though it has a 50:50 weight distribution it is more difficult to drift than the 240.

With the same power to weight (not just peak but under the curve, assuming same traction, etc...) they will accelerate the same.

I haven't seen anyone mention sprung vs. unsprung weight here yet. I had this same debate over installing aluminum rear control arms in my T-Bird, a car with a ~57/43 weight bias. I wondered whether pulling 30 pounds off the already lighter rear was worth it until I realized that since the springs don't feel the weight of the lower control arms it's somewhat irrelevant. Yes, the swap made the weight balance "worse" on a set of chassis scales but I think in this case the balance numbers are misleading.

Will wrote: I haven't seen anyone mention sprung vs. unsprung weight here yet. I had this same debate over installing aluminum rear control arms in my T-Bird, a car with a ~57/43 weight bias. I wondered whether pulling 30 pounds off the already lighter rear was worth it until I realized that since the springs don't feel the weight of the lower control arms it's somewhat irrelevant. Yes, the swap made the weight balance "worse" on a set of chassis scales but I think in this case the balance numbers are misleading.

I am going to agree. You should focus on unsprung mass as much as possible before anything else.

YaNi wrote: Unless you are comparing 0-60 times aero has more of an effect. Ex. A 2500lb RX-7 with 200hp vs a 3300lb RX-7 with 265hp. (same power: weight ratio) They have the same coefficient of drag and frontal area, so the 3300lb RX-7 will pull away on the straights. The lighter vehicle will be quicker through the corners. On a given track either could be the faster vehicle.

Based on my experience with Locosts, the light car is going to have a good advantage all around. Braking and cornering will make up for a lot of top-end acceleration. Remember that a faster cornering speed means a faster entry speed on to the straights, so the heavy car is already at a disadvantage before the higher horsepower comes into play. Also, the lighter car gets to stay on the gas longer before braking.

The inner front bumper on my 78 Camaro I took out-it weighed 70 lbs. This helped traction greatly. Also, the front bucket factory seats are very heavy also.

The 74-81 Camaro/Firebirds were very heavy cars - especially the 74-77 metal bumper cars. The all need to be put on a disco diet.