Weight distribution

[Hancha Group]

Originally Posted by 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.

I'm going to cop out and give you the blanket response, "it depends." It depends on the cornering stiffness, which depends on your tires and suspension. Height depends on suspension geometry adjustability and aero setup. It is really a case-by-case basis.

What is your current setup?

[Huehuecoyotl]

Thanks for your reply and your ear

Adding power has complicated my assessment of everything, and I tend to drive at 9/10s alot, am intermediate at best, and trying to learn more techniques. I'm good at working on my car, but openly admit I am clueless about how to dial this in for the track any further with any level of confidence past as far as I have gotten on my own-which is here. I want solid direction on what i need to spend and what my targets should be from where it sits. yes yes, i am always in the ongoing process of upgrade the driver too.

I'm on either R888(34f/36r) in OE size or direzza2(33 all around) in 235/40 17 sprung with 430F/350R on Koni race dampers front and rear, dual adjusts up front, rebound full fast in the rear and just a hair slower up front. Front compression is soft to medium. While I have tracked the car, i have NOT since adding the new coilovers. I have torn up some canyons, but thats all so far since I have had my car back for a few weeks now.

Factory LCAs f&R, eccentric bolts in rear LCA. Front LCA have the perrin PSRS that added +1.2degrees of caster, it sits at 7.2 both sides now, about a half more than I 'like' but it will prolly feel just right out on real tarmac. Ride height is about 27mm lower than OE up front and maybe 37mm in the rear. 22mm/16m sways at full soft. I played witha firmer front setting but it gets twitchy/too sensitive, I prefer that decoupled feel i think.

I tried both a track-ish alignment with toe out 1/16 and camber neg2.5 up front, rear was 1/32 in and neg1.9 both sides(more than I want)but havent had chance to get to a real track

I think dialing back the front camber and setting both toe to zero felt great for street, and made me 2nd guess the toe settiings up front, I may go back to zero toe for track?
I need to figure how to go from street to track or just leave it set somewhere, but i'm thinking that wont work

help welcome. I couldnt find but 1 shop in PHX who has corner scales, and I need help dialing in alignment, recommending what needs to get swapped out from stock to get me the setup I want-drivable to track, for now. most tracks a 100 to 300(+) mile trips, i am fine rolling on the dunlops and picking up a set of hoosier too, if that matters. The r888 is a great compromise, but not the best of either world.

[Dimman]

Originally Posted by 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.

If we start simpler, for example with simple weight transfer based on no suspension, static weight distribution and tire friction curve/efficiency (just vertical vs lateral load) in a steady 1g curve and then gradually add detail, it could help ease people in to the more complex stuff.

[SubieNate]

@Hancha Group-now that's what I'm talking about!

I wanted so badly to take ground vehicle dynamics as an elective my senior year but I had garbage priority and it's a ridiculously popular class.

[Hancha Group]

Originally Posted by Huehuecoyotl

Thanks for your reply and your ear

Adding power has complicated my assessment of everything, and I tend to drive at 9/10s alot, am intermediate at best, and trying to learn more techniques. I'm good at working on my car, but openly admit I am clueless about how to dial this in for the track any further with any level of confidence past as far as I have gotten on my own-which is here. I want solid direction on what i need to spend and what my targets should be from where it sits. yes yes, i am always in the ongoing process of upgrade the driver too.

I'm on either R888(34f/36r) in OE size or direzza2(33 all around) in 235/40 17 sprung with 430F/350R on Koni race dampers front and rear, dual adjusts up front, rebound full fast in the rear and just a hair slower up front. Front compression is soft to medium. While I have tracked the car, i have NOT since adding the new coilovers. I have torn up some canyons, but thats all so far since I have had my car back for a few weeks now.

Factory LCAs f&R, eccentric bolts in rear LCA. Front LCA have the perrin PSRS that added +1.2degrees of caster, it sits at 7.2 both sides now, about a half more than I 'like' but it will prolly feel just right out on real tarmac. Ride height is about 27mm lower than OE up front and maybe 37mm in the rear. 22mm/16m sways at full soft. I played witha firmer front setting but it gets twitchy/too sensitive, I prefer that decoupled feel i think.

I tried both a track-ish alignment with toe out 1/16 and camber neg2.5 up front, rear was 1/32 in and neg1.9 both sides(more than I want)but havent had chance to get to a real track

I think dialing back the front camber and setting both toe to zero felt great for street, and made me 2nd guess the toe settiings up front, I may go back to zero toe for track?
I need to figure how to go from street to track or just leave it set somewhere, but i'm thinking that wont work

help welcome. I couldnt find but 1 shop in PHX who has corner scales, and I need help dialing in alignment, recommending what needs to get swapped out from stock to get me the setup I want-drivable to track, for now. most tracks a 100 to 300(+) mile trips, i am fine rolling on the dunlops and picking up a set of hoosier too, if that matters. The r888 is a great compromise, but not the best of either world.

Unfortunately, I don't have enough (tire) data to pump those numbers in to give you a magic response. What do you feel is lacking with your current setup? Where is the problem and what phase of the corner?

What shop in PHX? I have a couple acquaintances that are track junkies out there that might know a place for help with setup, etc.

Originally Posted by Dimman

If we start simpler, for example with simple weight transfer based on no suspension, static weight distribution and tire friction curve/efficiency (just vertical vs lateral load) in a steady 1g curve and then gradually add detail, it could help ease people in to the more complex stuff.

That was the simplified, bicycle model I used. No offense to you, but if you want something simpler so you can ease into it, shoot me a PM or send me an email with any questions you might have and I'll try my best to help you out. As for starting simple, the math's fairly complex any way you cut it. Load transfer doesn't tell you much by itself because it's only one component to the US gradient.

K = Ktires (tire cornering stiffness) + Kcamber (camber thrust) + Krollsteer (roll steer) + Klfcs (lateral force roll compliance steer) + Kat (aligning torque) + Kllt (lateral load transfer) + Kstrg (steering system)

Even the lateral load transfer has a sprung mass, unsprung mass, and geometric component to it. That is why it is different for every car.

What you can take away from this is is that the tire is by far the most important factor. What a good suspension setup will do is allow you to extract the maximum out of the tire. Unfortunately, most chase a certain feeling from the suspension. Not entirely wrong though, since the driver is a part of the feedback loop. This system is something I hope to one day quantify in grad school.

- Earl

[CSG Mike]

Originally Posted by 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.

I'm gonna mirror the "that depends".

Do you have datalogs and video that show your driving style, and feedback on how you'd like to change the car? What do you like that it currently does, and what do you dislike that it currently does? A lot of places like to tune based on theoreticals and what they believe to be best. I prefer to dial in a car to the driver.

Move to Cali where you have access to us, Robispec, and so many good shops that I can't name them all off of the top of my head

[Hancha Group]

Originally Posted by SubieNate

@Hancha Group-now that's what I'm talking about!

I wanted so badly to take ground vehicle dynamics as an elective my senior year but I had garbage priority and it's a ridiculously popular class.

Don't worry, aerospace > motorsports engineering. If I didn't **** around in high school, I would've tried to go to MIT or UIUC and would've been happy to fetch coffee for the guys at NASA.

- Earl

[Dimman]

Originally Posted by Hancha Group

Unfortunately, I don't have enough (tire) data to pump those numbers in to give you a magic response. What do you feel is lacking with your current setup? Where is the problem and what phase of the corner?

What shop in PHX? I have a couple acquaintances that are track junkies out there that might know a place for help with setup, etc.



That was the simplified, bicycle model I used. No offense to you, but if you want something simpler so you can ease into it, shoot me a PM or send me an email with any questions you might have and I'll try my best to help you out. As for starting simple, the math's fairly complex any way you cut it. Load transfer doesn't tell you much by itself because it's only one component to the US gradient.

K = Ktires (tire cornering stiffness) + Kcamber (camber thrust) + Krollsteer (roll steer) + Klfcs (lateral force roll compliance steer) + Kat (aligning torque) + Kllt (lateral load transfer) + Kstrg (steering system)

Even the lateral load transfer has a sprung mass, unsprung mass, and geometric component to it. That is why it is different for every car.

What you can take away from this is is that the tire is by far the most important factor. What a good suspension setup will do is allow you to extract the maximum out of the tire. Unfortunately, most chase a certain feeling from the suspension. Not entirely wrong though, since the driver is a part of the feedback loop. This system is something I hope to one day quantify in grad school.

- Earl

We're starting with basic F:R split in this thread, right? And 4 identical tires. With just that and 1g for weight transfer, plus a hypothetical vertical to lateral tire load curve is an excellent and fantastically simple starting point for how weight distribution can affect cornering grip.

Plus the math isn't too hard.

I'll put something up for comment when I get home.

[Hancha Group]

Originally Posted by Dimman

We're starting with basic F:R split in this thread, right? And 4 identical tires. With just that and 1g for weight transfer, plus a hypothetical vertical to lateral tire load curve is an excellent and fantastically simple starting point for how weight distribution can affect cornering grip.

Plus the math isn't too hard.

I'll put something up for comment when I get home.

I think I understand what you mean. In regards to weight balance, the first equation is about as simple as it gets while remaining practical.

A simple breakdown is, that as you increase lateral acceleration, you must increase the yaw velocity in order for the rear tires to generate the necessary slip angle. Think about if the front were to turn-in sharply, but the rear could not keep up.

So, as a generality, the greater the rear weight balance gets, the smaller the steering angle required becomes, increasing lateral acceleration gain. You also decrease the static margin by moving the CG back, making the car more neutral.

[Scooby South]

Fighting weight -another 37lbs..*Gas, From this

[SubieNate]

Is that with or without a driver? It's puzzling to me that almost every corner balance sheet or readout I've seen has the car at around 42R:58F. Seems pretty far off from the advertised balance...

[Racecomp Engineering]

Originally Posted by Huehuecoyotl

I've moved my weight back by raising the front,

Full stop. Raising the front does essentially nothing for your static weight distribution. You have altered your roll-axis (depending on whose book your read) and altering dynamic load transfer, but you are not moving weight around.

- Andrew

[Huehuecoyotl]

Originally Posted by Racecomp Engineering

Full stop. Raising the front does essentially nothing for your static weight distribution. You have altered your roll-axis (depending on whose book your read) and altering dynamic load transfer, but you are not moving weight around.

- Andrew

Thanks for correcting me- youre right< what I have changed is how I read the diagonal weight transfer of the car, during corning.> Where I have played with it and set it isnt bad, but its not like I know what i am doing. I know having the front 'too high' makes the weight feel like it wants to stay back there and the car doesent seem to turn as willingly to smaller steering inputs. Youd know why etc.Too low up front and it out tracks the rear, which comes loose sooner than I think I like.

I dont know the right technical terms, at all, but I'm due for help past where I'm at setting it up by 'driving braille'. Turn the coil seats, drive it, nah that was too tall, that was too short, etc. I seem to like faster rebound at the rear axle, and almost as fast but a hair slower up front. I dont have heave and pitch all symmetrical, and I'd like to get it to feel more symmetrical at each wheel, but so far this is working better, I think.

No one in PHX knows the car, so it seems like I'll be driving over to Robispec for some help with corner weights, alignment, ride heights, etc.

[Racecomp Engineering]

Originally Posted by Huehuecoyotl

Thanks for correcting me- youre right< what I have changed is how I read the diagonal weight transfer of the car, during corning.> Where I have played with it and set it isnt bad, but its not like I know what i am doing. I know having the front 'too high' makes the weight feel like it wants to stay back there and the car doesent seem to turn as willingly to smaller steering inputs. Youd know why etc.Too low up front and it out tracks the rear, which comes loose sooner than I think I like.

I dont know the right technical terms, at all, but I'm due for help past where I'm at setting it up by 'driving braille'. Turn the coil seats, drive it, nah that was too tall, that was too short, etc. I seem to like faster rebound at the rear axle, and almost as fast but a hair slower up front. I dont have heave and pitch all symmetrical, and I'd like to get it to feel more symmetrical at each wheel, but so far this is working better, I think.

No one in PHX knows the car, so it seems like I'll be driving over to Robispec for some help with corner weights, alignment, ride heights, etc.

Gotcha. I just see that a lot and it bugs me.

- Andy