JDavenport

iTrader: (33)

After my weekend at cmp, your comment on rear camber rings a bell. 2.1 rear does not appear to be even close to enough on that course to give any kind of happy tire.

killerpenguin21

iTrader: (20)

the more we talk about this, the more my slight understanding is going out the window. i think part of the issue is at some point the basis of my understanding of vehicle dynamics was that raising fron spring rate increases understeer and locking tires. is the issue here that there is a crossover where too soft overloads the tire, and too hard will go to locking up? obviously im concerned about how drastic a change jumping 8k to 11k may provide...

using the math it looks like on a stock weight evo 8k/10k gives approx. 1.83hz/2.03hz, 11k/10k gives 2.07hz/2.03hz and 11k/11k gives 2.07hz/2.11hz.

-if i arbitrarily take weight off the rear the frequency goes up, which is good for me to understand.
- just from that it seems swapping my front springs to 11k would be a positive change


-how does one calculate how much roll stiffness is provided by the sways?
-does anyone have this data for whiteline bars?
-does anyone have this data for STOCK bars, since whiteline lists percentages stiffer than stock?

i feel as tho i should not touch alignment right now, and get the dynamics correct and then use alignment to dial in the tires and fine tune characteristics...is that heading down the correct path?



i may have a breakthrough in understanding this any second...

Dallas J

iTrader: (1)

Make sure you're subtracting unsprung weight from your calculations, it's not insignificant

03whitegsr

iTrader: (8)

I'm just about 100% certain that 0.92 figure for front motion ratio is completely wrong at low ride heights. The evo LCA is damn near flat with the ground at stock height. As you move it further into the compression curve it tilts the tire towards positive camber and it does so in a manner that makes tire centerline move less than the spring...

It's probably pretty wrong for roughly 1" drops too. I'd also question the figure for the rear motion ratio, but I don't have the data to prove it so will just assume it is correct.

At 10k f/r with 3400 pounds and 62% front bias, I get 2.2/2.5Hz. If you go by the 0.92 then it drops to 2.0/2.5 Hz. This is using 3500 lb/in for tire stiffness...which I have no clue if it is actually a good estimate for modern day radial tires.

Just for reference, here is what the front looks like when the tire is just touching the unibody seam above the tire in the wheel well. LOTS of LCA angle. The Car really isn't as low as you might think at this point either. I think it is tucking the 25.6" tire like 1.25" or so.

Dallas J

iTrader: (1)

I measured 0.96 front and 0.73 rear around my ride height FWIW. Also, maybe its cause I have an SE but with pulling the spare, small battery, and normal bolt-ons my car was 3100 lbs with roughly 990lbs per front wheel and 560 per rear as it was measured at Tour (no guarantee's on the pads being level, so no way to really see corner balance other than front/rear distribution).

killerpenguin21

iTrader: (20)

i didnt since i was just playing with numbers. unsprung would be coilover/control arm/rotor/caliper/wheel/tire correct? may actually be attainable.

really 3100 with just that? because thats where i am mods wise.

i also was unsure of tire spring rate, the calculator autopopulated with 1800 so i just used that, i fear this will be a somewhat inexact science without A LOT of foot work. not even sure where to begin finding tire spring rate.

Dallas J

iTrader: (1)

Heh, Good luck with tire spring rates. There are just too many variable for any manufacturer to measure each combo. It would be nice to have a few datapoints, but I havent been able to find anything more than a good street tire can be around 1000lb/in and race tire 1500-2000 lb/in.

I would be sweet if there was someone who had a machine to test tire spring rate and could give a grip/scrub angle plot. I know I've seen videos in the past of tires being tested, but not sure where.

killerpenguin21

iTrader: (20)

any thoughts on a number to use for unsprung weight just to mess with the calculator?

griceiv

iTrader: (2)

100 lbs is close enough for unsprung weight.

03whitegsr

iTrader: (8)

Yeah, it's pretty close to 100 pounds a corner on unsprung weight.

Unsprung weight accuracy has a bigger effect on calculating rebound damping then anything else.

killerpenguin21

iTrader: (20)

so i played with some more numbers.
-stock weights from vorshlag site
-0.96 front 0.73 rear motion ratios
-200lbs unsprung weight front/rear
-11k front 10k rear

the above provides 2.32hz front and 2.48hz rear, which based on what you guys have kindly shared would seem to be "favorable" math. am i missing anything?

any weight loss on the car will most likely be from the drivers seat back, nothing really over the front end.


what i managed to learn:
-as you lower weight the frequency of that end of the car goes up, subtracting the weight for the spare tire/jack removal bumps the rear up over 2.5hz.

heel2toe

iTrader: (41)

What effect does having mismatched sway bars have when thrown into the equation? Let me be more specific. I recall reading that with the stock setup 50% of the roll resistance comes from the springs and the other 50% comes from the bars.

Now say I wanted to run more bar. If I were to go with a larger bar in the rear that say now equates to 60% roll resistance would I need the same up front?

I'm think Im having trouble effectively conveying my thoughts

Basically, if I have more bar in the rear than up front can I make up for it with stiffer spring rates in the front?

I'd like to go through the math but I have some reading to do before I get there so Im just thinking out loud right now.

Dallas J

iTrader: (1)

Those numbers came from me, but there was an error in one of my cells (some how missed squaring the MR in one part). The correct numbers are 50% front and 60% rear roll rate are from the bar.

That is, % rate = (Bar wheel rate) / (Bar wheel rate + Spring wheel rate).

Ive have 2 front bars and 3 rear bars now and my feeling is that ~0-10% more rear is good, too much more is not and makes for a twitchy car. Less and my car will rotate under heavy throttle but I dont get mid corner rotation. Also a little lift or tapping the brake wont help the front to tuck into a turn like it does with 10% more.

Though, the only way to know what you have is to do some measuring (I have a thread on that somewhere here).

heel2toe

iTrader: (41)

Dallas- thank you for your response. Im headed out for the weekend shortly but would love to read up and play around with the numbers. I have my corner weights as of the end of last year and while things have changed a little, it should be good enough to get close.

I have a 25mm Perric RSB currently on full soft and I am using the stock front bar. However my front bar has been drilled so it's on the stiffer setting and I picked up an adjustable FSB bracket from Rick@ CDR. Im trying to get away with not swapping my FSB and simply leveraging the bracket as well as my new mounting hole on the bar.

I know my spring rates are meaningless without my corner weights but I have 10k on the front and 650#(~11.7k) on the rear.

My car is a pig, stupid SSL, but given that I still DD the car I dont want to go that much higher with my spring rates so I'd prefer to make up for it with bars.

griceiv

iTrader: (2)

my guess is that not one of us here can feel a difference as a result of changing the ratio of roll stiffness contribution from bar vs from spring, for a fixed roll stiffness magnitude and distribution.

the only effect changing the ratio of spring/bar roll stiffness is that it changes your roll damping. With that change being from massively over damped to slightly less over damped. Definitely not a primary handling performance adjustment on the scale we're talking about.