Figured I'd post this here in the brakes, tires, and suspension section because most of the guys in here are into spirited driving, roadracing, autocross, etc. Here it goes:

Everybody knows that on a traditional front engine, rear wheel drive car, a LOT of weight can be removed from the rear of the car in the form of spare tire, jack, extra underbody metals, lightweight exhaust system, rear seat delete, gutted trunk, etc. and not as much from the front.

My hypothesis is as follows (and it's pretty general/simple) just want some other ideas from experienced roadracers or technical people:

Remove everything you don't need from the car (or whatever you're willing to part with) be it front, rear, whatever (I'm neglecting side to side for simplicity). Then calculate your weight distribution, and add weight accordingly. Here's the catch: you just took out a LOT of weight from the rear, and very little from the front, so your weight distribution is worse on a F/R car.

The idea: add weight to the extreme rear of the car to multiply weight realized at the rear wheels, and reduce weight from the front wheels.

Assuming a car has a wheelbase of 9' and an overall length of 15' (with a 2' long front) and a 4' long rear (G35 specifications, approximate), statics says that if you add weight to the cantilevered extreme back section of the rear end, weight will be multiplied at the rear wheels, and removed from the front wheels - altering your weight distribution to more favorable specifications.

Example: A G35 Coupe weighs approximately 3,500#. Through extensive weight reduction, you have been able to remove a total of 200# out of the car, 25# from the front, and 175# from the rear of the car, effectively. Before modification, the weight distribution was 54/46, so the front weight distribution was 1890#/1610#.

Now, with reduced weight: 1865#/1435# and a balance (worse) of 57/43.

The idea is to add back 50# to save a total of 150# (a whole person dropped out of the car) to regain a better distribution.

Addition of 50# to the extreme rear of this car will result (statics) in a lift of 22# on the front wheels, and an extra weight of 72# over the rear wheels. Effectively, a 94# weight distribution advantage. Weights change to 1843#/1507# and balance changes to 55/45 - much closer to stock balance, and still 150# lighter.

Removing weight from the extreme front end of the car will similarly increase weight to the rear by statics. Changes in the center of the car are less noticeable.

So, what do you guys think?
Can you add to the above and help me further balance out my car?
Ideas on lightening the front end?
What is the ideal weight distribution for a F/R car? 50/50, more like 55/45, or 45/55? (In regard to handling, grip, and balance.)
Is the added weight to the extreme rear going to make the rear want to step out more or less due to added rotational inertia?
Other ideas to boost efficiency - faster without necessarily increasing engine output?

Let's discuss

 

From a front/rear weight distribution standpoint, that does make sense, but one major part that you are forgetting is how putting weight at the extremes of the vertical axis of rotation (can't remember if it's the x, y, or z axis), you will be increasing the moment of inertia of the car through the vertical axis. This means that the car will have a greater resistance to rotate (ie. wants to keep traveling straight more than before).

On one hand you are evening out the weight distribution, and on the other you are hurting the moment of inertia. Which one is more important/affected greater, I am not sure.

 

Watch out for polar moments of interia changes, keep all additions inside the springs. all reductions outside the springs.............never add weight to bumpers.

Never reduce the weight of brake rotors without understanding how a 10% reduction increases their temperature by >10% under the same braking conditions.

Static weight balance is far from that of accelerating or braking in a curve or straight. With a 0.5G acceleration what is the balance; with a 0.95 G deceleration what is the balance?

 

Didn't forget it - just don't know enough about it to quantify benefits/losses:

"Is the added weight to the extreme rear going to make the rear want to step out more or less due to added rotational inertia?"

I'm trying to find that balance you're referring to as far as where to add the weight back.

 

That's the info I'm looking for (and more of it is welcome). I'm sure there is a benefit to not adding weight so far back so it doesn't affect the car's "rotating" characteristics. From what I'm gathering from above, the extra say, 22# lift in the front and extra 22# weight in the rear isn't worth the added rotational inertia of hanging the weight 4' behind the back tires.

Can someone quanitfy the effects of the increase in static friction on the back tires vs. the tendency for the rear to resist motion due to the added inertia? If it could be quantified (of course in a simple model) that would be really helpful.

 

The problem is the static friction is not what you want rolling, as static is just the instant of launch under a few mph. Once the tire deforms rolling another friction comes into play.

Study Pacejka’s magic tire formula
http://www.racer.nl/reference/pacejka.htm

You might find GAME software useful: Read all posts it gets good and technical after the middle
http://www.gamedev.net/community/for...opic_id=462784

 

If you increase the polar moment of interia, you car will resist rotation. The axis of rotation is a vertical line through the center of the front axle. If you increase the polar moment of inertia, this means that the car will not want to as easily rotate around this axis. This has has less to do with the "tail stepping in/out" than it does with the car not wanting to turn in the direction that you want it to go.

Again, all my knowledge in this is what I learned in college (BSME), but there are many more people on here tha have a deeper, more practical understanding and first-hand experience with all this.

 

Reducing weight at the front is definitely possible. Things like a lightweight battery, lighter headers, removing airbags, lighter seats, emptying washer fluid, a lighter hood.

Personally for most racing applications, I would probably want to reduce the weight as much as possible and tune the car (shocks, sway bars, tires, etc) from there.