optimal weight distribution

Shuie · 11-12-2003, 05:54 AM

The car is a stripped 73 with a mag 2.4 and a mag 915. It’s in a tub state right now, but it’s being rebuilt as a less than practical street car with some track time in mind. I’m trying to either trim or re-distribute as much weight as possible.

I’m keeping the mag engine and tranny and am thinking of doing alloy trailing arms, glass rear bumper, glass doors, glass decklid, dry cell battery in the smugglers box, metal front bumper, and stock front fenders.

First off, is it worth it to even worry about the weight distribution considering the cars intended purpose?

If it’s worth it, how close to 50/50 weight distribution can I get with my 73?

Any other suggestions for better weight distribution?

Would using a lot of glass on the front half of the car disrupt things significantly, or would you have to really be able to drive the wheels off one of these cars to notice it?

Any help is appreciated

Dantilla · 11-12-2003, 07:24 AM

The solution for my hooligan '73 was to add rear flares so I can run wide rear tires to compensate for the rear weight bias.
I have a 3.2, mag 915, aluminum banana arms, and the car weighs about 2200 lbs.

Chuck Moreland · 11-12-2003, 08:45 AM

Continuing Dantilla's theme, there is nothing wrong with a rear weight bias provided the suspension is set up properly. 911s run about 60/40.

You really can't get to 50/50 while still building a lightweight. Most of the weight reduction potential is on the front end.

I'd focus on removing weight from the extremities; front bumper, rear bumper, canopy. Removing weight in these locations does the most to reduce polar moment and roll, resulting in a car that responds quickly.

island911 · 11-12-2003, 10:28 AM

Porsche and the Wright Brothers both understood that designing in too much stability will sacrifce manuverability.

Bill Verburg · Bill Verburg's Garage

You can have perfect 50/50 weight distribution in numerous ways. a 928 does it w/ masses spread out towrd the ends, a 914 does it by concentrating the mass in the center. Both will have close to perfect weight distribution but the higher polar moment of the 928 will make it more stable and harder to rotate throufgh a turn. The 914 will be the opposite i.e.lower polar moment and thus far easier to rotate.

As Chuck and Island have stated know your design goal(and rational), eliminate the weight at the ends and up high, tune for whats left.

Shuie · 11-12-2003, 03:23 PM

Thanks guys! Sounds like I might be buying some more fiberglass than I had originally thought

Randy Webb · 11-12-2003, 05:33 PM

you can concentrate your wt. reduction on the rear and get a bit closer

or... you can put ina 912 engine....

petevb · 11-12-2003, 05:53 PM

A rear wt bias is actually a plus for a rear wheel drive race car- to a point. If you look at pure race cars with somewhat free rules you'll see f/r wt bias around 55% rear wt. The more power the more weight you want over the rear (for traction). Clearly there is a point where it becomes a problem, though, and a highly stripped 911 can be an example of this. Concentrating on removing weight from the rear (particularly behind the rear wheels) first, as you suggested, will help you balance the car. Look to the exhaust in addition to the things you mention, and in a race car you can move the oil tank to the front of the car. My gut feel (and I'd like to get other's reactions to this) is that you should start to worry when you get much beyond 60:40. I can tell you that my car was almost 64:36, and it was a beast.

Randy Webb · 11-12-2003, 09:08 PM

A magazine did a test on exotics with different engine placements and the rear engined car did very well -- unfortunately I can't rember what magazine or exactly when, but it was within the last few years and might have been R&T or Automobile.

I'll post the gist if I run across it.

RallyJon · 11-13-2003, 06:07 AM

You should be near 40/60 already with that combo. I have steel fenders all around, both glass bumpers, glass decklid, steel trunklid, one light battery, no spare, magnesium drivetrain, and I'm right around 40/60.

Fiberglass rear quarters and some lexan in back would do the trick.

jluetjen · jluetjen's Garage

I'd argue that the "ideal" weight distribution will depend on the tire and wheel sizes which are allowed by the rules. If the rules require that the front and wheel tires and wheels are the same size, then 50/50 or a slight rear weight bias is most likely best. If the rules allow larger rear wheels and tires, then a larger rear weight bias is most likely not a big deal.

Conceptually in a car where the front wheels steer and the rear wheels drive, you'll want your weight distribution roughly similar to your front/rear tire patch area proportions.

Then you need to consider your polar moment -- ie how far from the CG you place your masses.

Low polar moment = quick responses like 914's. It also makes it hard to catch the car when it spins.

high polar moment = a stable car like the 944 and 928. They can be somewhat sluggish to change directions, but are forgiving.

The art is how you put the different aspects together.

911pcars · 11-13-2003, 11:13 AM

Obviously, there are a multitude of variables at work in keeping a chassis stable during cornering; but I think a vehicle will ultimately understeer or oversteer based on the front/rear weight distribution.

Sherwood
http://members.rennlist.org/911pcars

petevb · 11-13-2003, 12:00 PM

As you say, Sherwood, there is a lot too it...

Generally I agree with John- it's more about how much grip you have for the weight of that end of the car, rather than the relative amount of weight. You can think about it this way- if you put bicycle tires on the front of your car and fat tires on the back (as they basically do with some dragsters) the car is always going to under-steer despite the fact that the car might be very rear heavy. So what the car will ultimately do (understeer, oversteer, neutral, etc) is not determined so much by the weight distribution, but rather by the setup.
Key factors to this would be tire grip and relative roll stiffness front to rear. If you’ve got a few hours to kill this Excel page/ link, from Dr. Steven Timmons of instant-g, is a great way to play with the variables (warning- you need to be pretty hard core to enjoy this):

http://www.me.udel.edu/meeg467/Vehicles/TireData2002Values.xls

You can see how you can make even a rear heavy 911 with the same tires all the way around balanced in a corner (up to a point, anyway) by stiffening the front swaybar significantly. The transitions (not max cornering, but combined cornering and braking, for instance) are where things get tricky doing this, however, and where you are usually better off going with staggered tire widths in addition.

I’d love to hear people’s personal experiences with regards to how much of a rear weight bias race 911 owners have while still being able to set the handling up. I imagine those ultra-light cars with 3.6 motors in the back would have almost 2/3 of the car’s weight on the rear. How'd that go?