Weight distribution

[SubieNate]

Was talking about this with David from CSG yesterday and thought I'd get you guys' thoughts as well.

Have any of you played around with the weight distribution by adding/removing ballast or relocating the battery or adding/removing the spare tire etc. and what effects have you noticed, if any? Is there any significant performance gain to be had by moving things closer to 50:50 or even a rearward bias?

For the "fun" factor as designed, it makes sense for the car to have the weight bias it has. By biasing things just slightly to the front, you lock down the feel/bite of the front tires, improve their grip (to a point), make the rears easier to break loose with the low HP the car has, and make things easier to handle once sliding (Think this car vs vintage 911 oversteer). But, is it optimal from a tracking standpoint? Would there be anything to gain by making the car more neutral with weight instead of simply tweaking the suspension to be so? I know at high speeds a light rear end can be a major issue both entering and exiting a corner, especially for a RWD car that wants to put it's power down through those tires.

At what point does biasing the weight forward stop improving the front tires bite and start causing understeer? When does it start to affect the ability of the rear tires to hookup under power/cause excessive looseness under trail braking?

Just thinking about things.

Cheers
Nathan

[CSG Mike]

At what level of aero?

Some more food for thought: What exotics and racecars have a 50/50 distribution? What about rotational (yaw) moment of inertia?

[SubieNate]

Good point. I wouldn't even be opposed to experimenting with rearward biased setups, which most super/hypercars tend to be. The problem is that any kind of rearward ballast increases the moment of inertia, unlike a MR car that can naturally have a rearward bias without hanging things off the very back.

Food for thought:

Pagani Huayra: 44F:56R
Koenigsegg Agera R: 44F:56R
Ferrari 458 Italia: 42F:58R
Ferrari F12: 46F:54R
Lamborghini Avendator: 43F:57R
Porsche Carrera GT: 41F:59R
Porsche Cayman R: 45F:55R

All but one (F12) are mid engine. Is there a different ideal weight distribution depending on layout, or is it a simple matter of matching power level and grip needs front:rear?

Cheers
Nathan

[CSG Mike]

S2000CR is 49/51 or 48/52 depending on the day... :p

[Huehuecoyotl]

Funny this thread popped up, just last night I was reading a bit on suspension tuning and well, rear grip is a challenge, so I raised the front end up. Hooks up better but steers a bit funny. I'm aiming to get corner weighted soon.

[SubieNate]

Originally Posted by CSG Mike

S2000CR is 49/51 or 48/52 depending on the day... :p

That's 49F:51R or the other way around?

[CSG Mike]

Yes, rear heavy :p

[SubieNate]

So all of these cars have their bias at least slightly in the opposite direction as our car. What does that tell us?

[Rayme]

Originally Posted by SubieNate

So all of these cars have their bias at least slightly in the opposite direction as our car. What does that tell us?

Under hard braking / cornering the weight is more evenly distributed than standing still?

[SubieNate]

And is there any reason to have it the other way around for track driving as opposed to "fun" and drift type driving? (AKA, what our cars have)

[Hancha Group]

It's just due to layout and where the engine is placed. If any of you have done FSAE and ever spoken to Steve Fox, you might remember his 911 example The 911 works so beautifully, even with all the weight over the back, because only 2 wheels do the accelerating, but 4 wheels do the braking. Dynamically, you have a much better weight balance under braking, and loads of traction under acceleration. Statically, this seems ridiculous, and the moment of inertia makes the car like a pendulum. Throw in some wider front tires, and you regain some front traction under cornering. The 911 is an extreme example and due to the engine layout; you're not ever going to get that kind of a moment of inertia. However, the same concept applies if you continually moving the weight balance rearwards.

For cornering, at the simplest level you are trying to tune the yaw characteristics of the car.

[SubieNate]

Thanks for the post Hancha. I guess my main question is, is weight distribution something worth looking into tuning for this car or are we better off concentrating on aero and suspension only?

Obviously the similarities between an 1000 hp hypercar and our 200hp sportscar are limited. The need for extra weight over the rear wheels is greatly reduced (By 5x ) but it still seems interesting to me that almost all high performance RWD cars tend to put as much or more weight on the rear tires and we have the opposite.

Nathan

[Hancha Group]

Absolutely! It's just another tuning tool to add to your repertoire. Don't break your back trying to get an "ideal" weight distribution because you can compensate by changing cornering stiffness.

*WARNING: MATH AHEAD*

You can approximate the steering angle as:

delta = 57.3*L/R + (Wf/Caf - Wr/Car)*V^2/(g*R)

this means changing the weight distribution will affect the amount of steer required. Using the stability factor (understeer gradient), you can determine the vehicle's characteristic or critical speed, allowing you to tune its yaw stability as well.

k = - (C/W)/((Cf/W)*(Cr/W))*SM*Ay
k > 0, vehicle is said to have US
k = 0, vehicle is said to have NS
k < 0, vehicle is said to have OS

Vchar = SQRT(1/k)
Vcrit = SQRT(-1/k)

delta = steering angle
L = wheelbase
R = radius of corner
W = vehicle weight
Wf = weight front
Wr = weight rear
C = vehicle cornering stiffness
Caf = cornering stiffness front
Car = cornering stiffness rear
V = velocity
g = gravity
k = US gradient
SM = static margin
Ay = lateral acceleration

It's been a while since my vehicle dynamics class, but that's what I remember and found sifting through RCVD and old lecture notes. I have a crude Excel calculator for basic vehicle dynamics parameters I could send. If you're interested just PM me.

[Huehuecoyotl]

so with all the fancy tools, what is the optimal corner balance and height setup, software wise for the 86 ?

I've moved my weight back by raising the front, the tricky part wasnt determining rear height, I actually think I got that, but I am having to keep setting the front perches one turn lower at at time and re-test drive to see where it feels 'even' then dial it back a bit-just how much is that 'bit' is tricky to determine for me.

I have a bunch of time in a c2 and a 911c4, very different car to try and dial in, again that was something I turned over to a pro shop to setup,I can only get so far on my own here.