Move battery from front to smuggler's box, worth it?

Colby · 04-22-2002, 06:36 PM

Ok, '84 Carrera. Setting up for spirited weekend racing. Have already done substantial lightening. I have the stock size (huge) battery up front. I have read about moving a smaller one to the smuggler's box. It seems like a good idea from a polar moment of interia perspective, but bad from a weight distribution perspective (front to rear balance). What are ppl's opinion on this? Worth it? Any significant difference? Don't do it? Why?

Colby

pwd72s · 04-22-2002, 06:56 PM

Quote:

Originally posted by Colby
Ok, '84 Carrera. Setting up for spirited weekend racing. Have already done substantial lightening. I have the stock size (huge) battery up front. I have read about moving a smaller one to the smuggler's box. It seems like a good idea from a polar moment of interia perspective, but bad from a weight distribution perspective (front to rear balance). What are ppl's opinion on this? Worth it? Any significant difference? Don't do it? Why?

Colby

Polar moment of inertia perspective mean keeping the weight balance of the car as close to 50/50 as you can, plus having the balance point mass in the center as much as possible, right? So, in a rear engined car, it makes sense to move the weight of the battery more aft??? Less battery weight sounds great. Less weight is usually great. But where is the advantage of moving it aft? Just food for thought....

Colby · 04-22-2002, 07:01 PM

Yes polar moment of inertia is like spinning a top. You can have a 50/50 weight distribution, but where is all the weight? On the ends, or close to the center? If it's on the ends, you'll spin like a top. So from a polar moment or inertia perspective, moving the weight of the battery inward is good, even if it makes the rear more heavier than the front.

Colby

pwd72s · 04-22-2002, 07:13 PM

Oh, I see...that's so one end of the (car) "top" tends to spin faster than the other? The more weight in the back, the more likely that is to happen, I'd think. In other words, the more weight in the back, the easier to enter "terminal oversteer". Bottom line, way I see it, is if lowest polar moment of inertia is your goal, you need a mid engined car. Otherwise, go for the best weight distribution you can get? Again, just food for thought...

Colby · 04-22-2002, 07:19 PM

Weight on EITHER end (front or back) promotes spin. Can't move the weight of the engine forward, but you can move weight from the front more inward, such as the battery. However, you can lighten the rear in other ways (remove A/C, rear bumper, etc) which also in effect shifts the weight balance more inward.

Colby

pwd72s · 04-22-2002, 07:31 PM

Colby...so the more in the better?

Colby · 04-22-2002, 08:25 PM

Hehehehe, you got it!

Colby

911nut · 911nut's Garage

With a 911 we all agree that much can't be done with reducing the polar moment of inertia because of the layout. The next goal should be to make the handling and balance of the car more predictable. Early, short wheelbase 911's had ballast weights added to the front bumper to improve balance. Early long wheelbase 911's had two small batteries to accmplish the same goal. Other front engine cars such as the Ferrari Daytona, 928 and the current C5 Corvette employ transaxles to do pretty much the same thing.

Colby, I would recomment that you leave the battery where it is and spend the time/money on further weight reduction.

Colby · 04-23-2002, 04:57 AM

Now, correct me if I'm wrong, but weren't the weights on the front of the early cars to try and keep the front end's on the ground? They hadn't yet developed a good valance/tail combo and the early cars were known to get light at high speeds. The weights were there to counter this. And even then they realized that it was bad to have weight on the ends, but spinning is less scary than leaving the ground.

Colby

911nut · 911nut's Garage

According to the book "Porsche 911 Story" the weights were added to help remedy the severe trailing throttle oversteer that the short wheelbase early cars had that suprised the average driver. Road car aerodynamics related to downforce weren't really considered back then.

widebody911 · widebody911's Garage

Actually, I've heard of a trick where they modify the mounts and move the whole engine and trans an inch or so forward.

Quote:

Originally posted by Colby
Can't move the weight of the engine forwardColby

304065 · 04-23-2002, 10:10 AM

Check the archives here and on Rennlist, there have been a few active discussion. I too was considering moving the battery aft to the Smuggler's Box, but ended up putting my fuel pump in there. I went instead with a "Stinger" battery which is very light and compact and that satisfied me.

I agree with your thoughts re: polar moment of inertia. The more you can do to move weight from the ends into the middle, the more rapidly the car will rotate. e.g. 914/Boxster. The great thing about a 911 is, with the nose weighing very little, when the tail does begin to step out, you can very easily open up the steering wheel and increase the radius that the front wheels are describing, allowing you to PREVENT a spin, while at the limit. This is one of the things that makes a 911 FAST.

Including the cast iron weights, then the dual batteries (some say due to the fact that you couldn't buy a single battery with enough juice to crank over a high compression 6-cylinder in the late sixties) or the post 74 bumpers, was a way for the factory to REDUCE the speed of rotation of the car, to make it easier for drivers on the street to prevent oversteer. The theory being, the more weight up front, the harder it is to get it to move, and once it moves, it will move slowly and be difficult to stop. Kind of like the 944/928- lots of polar moment and lots of resistance to a spin in the first place.

With a ducktail, carrera tail or IROC tail or better, with reduced rear lift the tendency to oversteer should be reduced-- some say the whole oversteer thing was ONLY a function of aerodynamics- so I've gone ahead and replaced the factory dual batteries with a stinger and the factory fuel tank with a fuel cell, removing considerable mass from the front while remaining within the letter and spirit of the PCA club racing rules. And I would say the that the net effect on handling is. . .

. . . negligible. But it was fun working on the car.

Bill Verburg · Bill Verburg's Garage

The most obvious example of polar moment in action that most people have seen is a figure skater controlling her rate of spin by moving her arms inward or outward. There is obviosly no change in weight or weight distribution. Moving the arms outward moves mass away from the axis of rotation, increasing the polar moment, since no energy is added to the spinning system, the rate of rotation will decrease, moving the arms inward has the opposite effect.

Saving weight at the ends of a car will lower both the total mass, and polar moment of the car(both are very good objectives), it may or may not affect weight distribution. When I swapped out the stock Carrera bumpers for f/g I saved ~20# f and ~15# r. the weight distribution was adversely affected, but not noticeably so. The polar moment was greatly reduced because of the distance from the cg of the car, the affect on turn in was dramatic, as was the improved response to vertical changes in terrain(bumps and undulations in the pavement).

Moving a smaller battery further to owrds the cg will have a similar effect. It will however adversly affect the useability of the trunk for storage on a street car.

The purpose of the weights in the ends of the front bumpers was as mentioned, an effort to increase the polar moment. Any increase in the polar moment will slow down the rotation of the chassies about any axis through the cg. The intent was that by slowing chassis rotational tendancies the cars would be less twitchy and more manageable by normal drivers.

Later efforts along those lines were the increased wheel base of the post 69 911s anddifferentialy sized wheels and tires, along with years of suspension tweaks.