[enkeivette]

Originally Posted by BADDASSC6

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.

Ok, you have a C6 and you race it. I won't argue, if you say they understeer, they understeer.

So do you think it would be better to have more of a 47/53 46/54 weight balance?

[Vettezuki]

Originally Posted by BADDASSC6

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.

What's the primary design reason for the purpose built race cars (formula whatever) and high end exotics for having rather strident rearward weight biases? Or is that just a function of some other design requirement? (I understand the goal of mass centralization around the COG to optimize PMI.) Obviously for one thing they're using aero to create down force for grip which changes what they can do in the first place.

Obviously the ZR1 and ACR do pretty well stock to stock against the exotics at the Ring, so they certainly manage Though, while I have no experience, the thing that seems to be repeated as an observation comparing say a 430 to a ZO6 is that it is much harder to drive the ZO6 at its limits. The Ferrari requires less controlling ability. This is comparing relatively similar levels of non-driver assistance. (i.e., it's not a GT-R that drives itself.)

[BRUTAL64]

Originally Posted by BADDASSC6

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.

Good stuff!

[Ultraperio]

Originally Posted by enkeivette

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.

IRAC-RAFADC!

A cars cornering limits (what a lateral G on a skidpad tests) are dictated by far more than the cars CoG and is a separate discussion to the current one. We're discussing a cars tendencies approaching and past its limits, due to the location of its CoG. An under-steering or an over-steering car can post very good lateral G numbers but it 's lateral G numbers aren't necessarily a function of its under/over-steering tendencies or its CoG location. And its lateral G numbers aren't necessarily a good indication of how well handling a car it is (tho lateral road holding is definitely a contributing factor to a well handling car).

I think calling 50/50 the "perfect" distribution is a misnomer or possibly a bit vague as to what it is "perfect" at. I will not argue that several of the best handling road cars were designed with 50/50 weight distribution as a goal. However you only hear this as a goal for front engine, rear drive cars. I've never heard of Ferrari putting mass forward to try to 'correct' its rearward biased cars. My personal favorite, the 3rd gen RX-7 (one of the best street/track cars of the 90's), was even marketed with its 50/50 weight distribution (also another under-steering car) as one of its main selling points. 50/50 weight distribution may be "perfect" in that it is balanced front to rear and it may be "perfect" in that it is the best compromise between stability/safety and and good responsive handling but It is not "perfect" when building a car for maximizing its handling potential.

Another interesting thing to consider is most/all 'exotic' mid/rear engined sports cars either have a base AWD option (Gallardo, Mercialago, 911 turbo), have a plethora of electronic nannies (every Ferrari out there) to keep the car stable, or try to kill you at every less than perfect attention opportunity (aforementioned Mosler). Possibly because the additional responsiveness comes at the price of less predictable, more dangerous handling characteristics at the limit (i.e. snap over-steer, something that has plagued Porsche's for years) which would be acceptable on a race track with a savvy driver but unacceptable on public roads with your typical driver. Thus requiring either a more deft driver, some under-steer inducing AWD, or little black box to keep your foot out of it and the nose pointed in the direction you're traveling.

Remember the trouble they had with keeping MR2's on the road?

Another thing that I would contest is the idea of "add weight receive grip" in regards to corner weighting. I think its a little more complicated than that but that might be beyond the scope of this thread.

[BRUTAL64]

"snap over-steer"--the first time I heard that was in the 60s. It was in reference to a Porsche.

C2 and C3 Vettes can get the same problem because of great camber and toe out changes at full travel. That why I limited my rear travel with a sway bar.

[Vettezuki]

Originally Posted by BRUTAL64

"snap over-steer"--the first time I heard that was in the 60s. It was in reference to a Porsche.

C2 and C3 Vettes can get the same problem because of great camber and toe out changes at full travel. That why I limited my rear travel with a sway bar.

How bout stopping in a straight line? C3s in stock form can do some mighty exciting things if you hit the brakes at high speed.

[BRUTAL64]

Originally Posted by Vettezuki

How bout stopping in a straight line? C3s in stock form can do some mighty exciting things if you hit the brakes at high speed.

Yep, the same changes in toe and camber can happen with the rear suspension under braking load. BUT, the rear suspension does not move as much as it does on hard cornering.

The front suspension has it bump steer quirks to. But not quite as bad as the rear. Limiting travel helps there also.

[Ultraperio]

Originally Posted by BRUTAL64

"snap over-steer"--the first time I heard that was in the 60s. It was in reference to a Porsche.

C2 and C3 Vettes can get the same problem because of great camber and toe out changes at full travel. That why I limited my rear travel with a sway bar.

I'm not familiar with C3 suspension. Is the toe change due to changes in geometry through its travel or load induced bushing deflection?

[BRUTAL64]

Originally Posted by Ultraperio

I'm not familiar with C3 suspension. Is the toe change due to changes in geometry through its travel or load induced bushing deflection?

C2 an C3 have basically the same chassis.

It actually both. But. most comes from the toe change as the rear moves in an arc from lowest to highest. The camber change can be as much as 5 degrees also.



Over that last 35 years I learned to use the snap over steer to my advantage.

At any time over 120 mph (road pitch change) you can end up 180 degrees different than you wanted-- yep, did that on Mar Vista in Whittier / 1976.

[Ultraperio]

RX-7's and possibly 300zx's if i remember correctly had 'passive rear steer' where strategically placed bushings allowed passive toe out on the outside rear under load when in a corner giving the car over-steer like behavior without loss of traction. Problem is, when the bushings inevitably wear out you have some wacky toe situations which unsettle the car.

Solution is delrin or urethane bushings to stiffen up play or aftermarket solid toe links to eliminate the setup entirely.