Weight Bias and Handling
 

Branched off the Cool C3 thread.

That's actually closer to a race car ideal. It would help a lot with planting power as the weight is right over the drive axle.

More weight in the back can actually make the car more tail happy, overloading the rear tires in a turn.
The main reasons you want a more rearward center of gravity is you are giving the front wheels more leverage authority over the cars moment of inertia, i.e. the car wants to change direction easier.

And having a rearward center of gravity allows more equal tire loading under braking allowing more total braking force to be applied.

My C2 is 43/57 from the factory.:drink:

Your front tires have enough to do with sharing friction with both turning and braking.:boggled:

Giving the front tires more leverage authority allows a greater change in direction with less tire loading.

Shifting your center of gravity rearward allows the rear tires to account for a greater percentage of total braking force. Offloading the front tires.

Thank you for the additional wholesome goodness of rearward COG bias. I was going for the simple dumb answer of launch traction. :mullet:

2010 Viper ACR: 49.5/50.5.
2005 Corvette: 51/49

:leaving:

Sorry guys, I believe the makers of these front engine rear drive supercars over yall when it comes down to what's best for a front engine rear drive car.

Seems simple enough to me, more weight in the front, more load on the tires, more understeer. More weight on the back, more load on the rear tires, more of a tendency to oversteer.

Equal weight distribution, neutral steer. Front engine rear drive cars always have slight oversteer. Putting more weight in the back would... make it worse. :nuts:

Yes, Ferraris and Porsches have a rear weight bias, but they also have the engine and your fat ass back there. I'm surprised they don't have more of a rear bias.

These are street cars home slice. I was talking about idealized race cars. Ultraperio described it in more detail. And here's still some more.

In short, if you need/want a car that will easily change direction, you want a rearward weight bias. You'll also tend to get oversteer in steady state cornering, but that's not the reason for the rearward bias.

Your bold text says race cars and sports cars. Further, I don't see the difference. Still siding with Chrysler and GM on this one, sorry. :judge:

Thus allowing the front tire to use most of it friction capabilities for stopping and turning the vehicle.:)

Of course there cars work just fine. But if you give a pencil and blank piece of paper to a race car (hyper sports car) designer and say, make this handle as well you can on a course without restrictions, you're going to get a rearward bias NOT a 50/50 bias. Fin.

Front engine Ferraris also have the engine behind the front axle. This not only shifts weight to the rear but reduces the polar moment of inertia allowing the car to respond to steering inputs better. If you do any driving in a Porsche or a Corvair, it can be difficult to get the car to turn especially when you are accelerating and if the rear does come around too much, oversteer, it is difficult to get the rear end from swapping very quickly. Mid engine cars have both low polar moment and some rear bias to help with accelerating and stopping.

Bob

Fin? :sm_laughing: Going to try that in front of a judge someday. IMHO, if GM can put the motor up under the dash in a van, they can move the motor as far back as they want in the Vette. And they do have a blank piece of paper, they're GM.

I'll plead the 5th to a judge. :smack:

BTW, you may recall that ZAD, peace be eternally upon him, argued almost violently for a rear engined Vette back in the 60s. Just say'n. The marketing boys rightly or wrongly, insisted on sticking to the formula. Fiberglass. Front Engine. RWD. Two Seats.

That Vette looked stupid. They tried to keep the hood longish and it just looked retarded. They also tried to put a rotary in it, doesn't mean it was a better engine than the piston engine.

You're thinking of the Aerovette. It was much earlier.

Can't we all just get along?

Nnnnnnnnno!

Oh good times!! :laugh:

"Sorry guys, I believe the makers of these front engine rear drive supercars over yall when it comes down to what's best for a front engine rear drive car."

Street cars, even spots cars, are full of compromises and may not be setup for absolute cornering performance in the name of stability and safety for their generally less than capable occupants. Compromise wise, front/rear weight balance is a trade off between passenger compartment space, car size, and available/designated engines. Safety wise, all street cars are built with a focus on under-steer because an under-steering car is safer and more controllable/recoverable than an over-steering car.

Sure you could jam the motor under a sports cars dash van style, but where are your legs going to go? how difficult is it going to be to service that motor? How are you going to deal with the added height needed for the clearance?

"Seems simple enough to me, more weight in the front, more load on the tires, more under-steer. More weight on the back, more load on the rear tires, more of a tendency to over-steer."

Obviously a cars over/under-steering tendencies are based on more than its CoG location. What the CoG's location dictates, and is harder to tune in with tires and suspension tuning, is its turn in behavior and how quickly it will rotate about tis center of gravity. This is mainly dictated by the cars weight and where that weight is carried relative to the the force used to rotate it (the deflection of the front tires)

"Yes, Ferraris and Porsches have a rear weight bias, but they also have the engine and your fat ass back there. I'm surprised they don't have more of a rear bias."

You have to consider the reason these 'money no object' sports cars are designed with the motor in the mid/rear. Its because of the ideal weight distribution for high performance handling and the easiest way (car design wise) to attain that distribution.

Porsche is kind of the oddity, they rear mount their motors just as much (possibly more) for nostalgic reasons as for performance.

You already have to drop the motor out of the bottom of the car and dismount it to install long tubes, so how much more difficult can it get?

And no, all street cars are not built with understeer. Most are, yes. But the two examples that I've been leading with both have an oversteer bias from the factory.

Your philosophy sounds intelligent, but any way you slice it, with a front engine rear drive car that already has a tendency to oversteer, putting more weight in the back is just going to make that worse. Bet on it.



I'm not making any claims about rear engine cars.

You have to separate over-steer due to chassis setup and oversteer due to which wheels are driven.

When I talk about over-steer and under-steer I'm talking about steady state cornering tendencies with no throttle influence. Sure you can get any reasonably powered RWD car to over-steer very easily, but that is an entirely separate issue from the chassis tendencies. I haven't driven a viper so i cant speak from experience, but i do know at the limit(however high) late model corvettes tend to under-steer. You're not going to get a corvette to over-steer unless you give it some power, turn in under hard braking, or give it a scandinavian flick.

You are making blanket statements about over-steer. A rear biased car will not always over-steer more than a front biased car. For instance again under hard braking in a turn a front biased car can unload the rear wheels to the point of loss of traction. In a rear biased car the additional weight over the rear wheels might keep them loaded up and prevent over-steer.

Another example on the other hand is what jedhead alluded to. A rear heavy car will tend to under-steer on corner exit under power. This is because of the weight transfer unloading the front wheels. A front heavy car will plant more effectively on corner exit under power. This is actually the current GT2's achilles heel as it has too much power and too little weight in the front causing near loss of control(due to under-steer) under hard acceleration in a turn.

No blanket statements. Statements from reading motor trend magazine for years. They tested all the hot cars a few years ago and published a report. Vipers and Vettes had slight oversteer. And I'm pretty sure they know the difference between breaking the tires loose with the motor and breaking the tires loose with lateral g force.

So, yes, a Corvette will oversteer in neutral. Wanna go for a ride? I'll show ya. And as physics would have it, more weight in the rear would make that bias worse.


The Corvette frame has been completely transformed 3 times since my frame was conceived in 1963. The front end is about 2 feet shorter now and the transmission is in a different spot. I don't believe that maintaining a 50/50 weight distribution again and again was an accident. Too much has changed and the weight distribution is no accident. The fact that the other American front engine rear drive supercar has almost the exact same weight distribution with a different frame, engine, and transmission location is no coincidence either.

I'm not saying they got it exactly where they wanted it, I agree there is always a need to compromise. But I'll bet 50/50 was and always has been their goal.

:popcorn:

Well, I'd like to agree-- but my C2 may have it own ideas. It goes into the corner with a little under steer and goes to OVER STEER in the middle and neutral out.

That's with a 47/53 weight. Over the years it has always been this way.:boggled:

50/50 may have been the goal. 50/50 weight distribution may indeed be the best compromise for a street car but this does not make it ideal. What i am saying is in terms of outright performance a rearward weight bias allows for quicker changes in direction and better braking performance which is proven time and again by purpose built race cars and very high performance street cars.

Again, over/under steering tendencies are not just effected by weight bias. Suspension setup/alignment, damper and spring rates, and relative tire sizes pressures and compounds all greatly effect a cars tendencies around a corner.

Besides, 47/53 isn't exactly what I'd call 'tail heavy' ;)

The Viper broke the lateral G record in 97 with its 50/50 weight distribution, not sure who has it now. Probably the Mosler, but that's not really a fair take since the Mosler comes factory with slicks.

I won't argue your point about race cars, because I really don't know. And maybe a rear weight bias works better with Porsches and Ferraris, I really don't know. But it doesn't with Vettes and Vipers, so I really can't agree that your theory has been proven time and time again with very high performance street cars. Unless you can provide examples like I've been doing. :p

IRAC-RAFADC! :suicide:

I am with Brutal on this one!

:popcorn:

Actually, it's called physics. Different discipline.

Out of curiosity, what mystical life force differentiates a FRWD with say 45/55 and RRWD with 45/55 all other things being equal? We're controlling for weight bias (and punting on mass centralization a bit).

I don't think this thread has enough hostility yet. I'm inviting Carlos. . . stand by. (I may regret this. It's sort of like releasing the Kraken.)

Ok, I was wondering when we would get to this. First with all the aluminum I have up front --heads, intake (plus stuff removed) I would have even less on my front tires.(all the stuff I removed lifted my spring height 1 inch)

Ok, slightly positive camber in front with as much caster I could get into it.

Negative camber in back, plus over size tires and a sway bar.

Yep, this all makes a difference in an OLD C2 that gives the newer cars a real bad time on the twists!

Now, if I only had some brakes.:boggled::inout:

I like Carlos. I welcome his input.:thumbs_up:

I totally agree, although his philosophy sounds great, physics is what really matters. :p

I feel like I'm in a trench and you just announced that you're throwing a grenade in here. We may learn a valuable lesson, but at what cost? At what cost Ben?

Adam posting on Motorgen at 4 am, it could only mean one thing... paper due tomorrow!

Glenn posting at 3:48 am, it could only mean one thing... X Box!:boggled:

I went to bed at 7:03, then still had to wake up for class. Beat that!

Better you than me.:lmfao:

I went to school at one time. I know how tough it can be.:hail:

Unlike me, I know it will do YOU some good.:bigthumbsup:

I know you will make the best of it.:drink:

There is a lot of miss information here. I'm on my blackberry so bear with me. There are obviously a lot of factors, but let's focus on the ones that are most pertinent. Let's talk about center of gravity, polar momentum, and corner weights. The center on gravity of a car is actually three dimensional. It is a point in the X Y Z axis of the car. For this discussion I will be referring to X or longitudinal axis. The moving the center of axis forward or aft will change the ratio of effort that the front and back wheels must do to rotate the car around it's COG. Polar momentum is a function of how hard to rotate the car the closer the mass of the car is to the COG the easier it is to rotate. Corner weights are affected by the COG and the overall car weight. Corner weight are the amount of force applied to the tires. As more weight is applied the greater they grip. let's apply this to a corvette. It has a slightly forward COG with relatively high polar momentum and has perfect weight distribution. Read a magazine and you would believe that it oversteers, that's flat out wrong. C6 understeer! They have tons of torque so if you rape the pedal it will step out, but driven correctly you can go flat out very early. Most vettes races run almost square tires. I run 275 fronts and 295 rears. The car is corner balanced 50/50, but before my new rims (245 fronts old) I Rand slight front heavy for more front end grip. Remember that changing corner weights changes the force applied downward to the tire, but does not change the polar momentum or the amount of grip required to move the front or rear. if you can drive then you are consiously managing the shifting weight of the car to make it turn in track out slide whatever.