hustler

iTrader: (6)

Everyone knows the flow chart that tells you to adjust the front/rear of the side that slips or grips, I want to take it a step further so here are a few questions I've had over the years that I'd like to get clarification on:

If I have 50/50 cross weights, how do I adjust for the F/R weight ratio for neutral handling?
How do I know when I need to adjust spring rate?
How do I know when I need to adjust sway bar stiffness?
How do I know when I need to adjust the rebound ****?
If I feel the front of the car rolling, and the rear flat, is that necessarily a bad thing?
Why does my car feel planted going over curbs or big 1" dropped grates in the road, yet slightly "pogo" over undulations in the road? I'm thinking this new valve may be in order (blue line, from AST's Facebook):

hustler

iTrader: (6)

lot's of views, no answers, not surprised.

Gotpsi?

iTrader: (5)

What are your spring rates? and what sway bars do you have right now?

BenR

iTrader: (2)

You'd probably get a better more in depth and accurate answer from first reading past issues of Mark Ortiz's Chassis Newsletter, then posting a specific question directly to him.


http://www.eviltwinmotorsports.com/?page_id=204



In response to your last question.


The Mark Ortiz Automotive
CHASSIS NEWSLETTER
PRESENTED FREE OF CHARGE
AS A SERVICE TO THE
MOTORSPORTS COMMUNITY
August 2000

WELCOME
This newsletter is a free service intended to benefit racers and enthusiasts by offering answers to
chassis questions. Selected questions will be presented, at my discretion. Readers are invited to
submit questions by mail to: 155 Wankel Dr., Kannapolis, NC 28083; by phone at 704-933-8876;
by e-mail to: markortiz@vnet.net
Mark Ortiz
“THE SHOCK DYNO LIES!”
How come if I dyno a Bilstein shock (Roehrig dyno, using their software), and then duplicate the
graph exactly on an Ohlins, the shock feels completely different to the driver? I mean different as in
no side bite, and no forward bite – but the driver says the car doesn’t feel any stiffer.
Also, if I vary the gas pressure, the dyno displays the same graph, but the driver can feel a difference
in the shock. Why?
At what point in rebound/compression valving split will jacking occur? We valved an Ohlins to be
exactly like our Bilsteins, and the Ohlins ratcheted down in the turn until it bottomed out and almost
scared the driver to death.
How do I match the shocks to the track surface for maximum compliance and therefore maximum
forward bite? The tires of the winning car in the 100 lap day show seemed to just roll over the
washboard surface, and on all the other cars you could see the tires bouncing up and down.
The tracks get rough, and because the roughness is caused by the same type of vehicle, the roughness
always has the same look. I would think that since the pattern of 3” to 4” holes and bumps is always
similar, the “perfect” spring frequency and shock valving could be found. How do I test for this?
The shock dyno doesn’t lie, but it doesn’t tell the whole truth.
First of all, I believe your dyno cycles the shock through a 2” stroke, normally near mid-travel,
at 100 cycles/min (1.67 Hz). This gives a peak velocity at mid-stroke of just over 10 in/sec. A
rough dirt track may give you shaft speeds above that.
Your dyno only generates simple harmonic motion. Other companies make more expensive
dynos that can generate approximately square-wave (very high acceleration) motion, or can
reproduce on-track motions recorded by electronic data acquisition. These modes of testing
have value because shocks are sensitive to acceleration as well as velocity.
Your software can be programmed to show you various outputs. Many people look only at one end of the stroke, most often the extended end (rebound valving closing, compression valving
opening). It helps to look at both ends. A plot showing both ends of the stroke, force versus
absolute velocity, will have two points or noses at the left side, and four traces. I have seen
instances where looking at the full stroke showed me acceleration sensitivities that I never
would have known about if I had only looked at the extended end of the stroke.
The usual thing the program does with the gas force is to re-zero the load readout after
stopping momentarily at mid-stroke and reading the gas force. However, the program should
tell you, as a numerical readout, what that gas force is. It will vary with the pressure you put in
at build, and also with the volume of the gas, which you control by varying the floating
piston’s position at build or the oil volume you pour in.
You don’t always get the same trace with different gas pressures, even with re-zeroing. More
gas pressure actually increases rebound damping force (with gas force omitted), due to reduced
nucleate boiling (incipient cavitation) on the downstream side of the piston. This effect is
greatest at high velocity, with a stiff rebound stack, low gas pressures, and hot oil. The effect is
least – sometimes unnoticeable – at low velocity, with a soft rebound stack, high gas pressures,
and cooler oil.
Valving split is sometimes expressed in terms of control ratio – the ratio between rebound and
compression damping, at a particular shaft speed. As a rule of thumb, a control ratio of 1.3 to
2.5 is pretty normal; <1.3 is somewhat bump-stiff; >4.0 is likely to jack down. Jacking is also
promoted by stiffer dampers, softer springs, or a bumpier track.
Tuning for a particular disturbance frequency is mainly a matter of making sure your natural
frequencies don’t match the excitation frequency. Since your unsprung masses and tires are
similar to the other cars’, this means using spring rates that don’t match theirs, or using stiffer
damping. Sometimes it helps to stiffen just rebound, but if you’re jacking down to the bump
stops you may be too far down that path now. Soft damping gives better roadholding, except
when the bumps excite the system at one of its natural frequencies. Stiff damping raises natural
frequencies, and also makes the system less frequency-sensitive.
You can also raise natural frequency by using somewhat stiffer springs than your competitors.
In the days of cart-sprung cars with primitive dampers, this was a major reason people sprung
race cars stiffly. Another approach is to run substantially softer instead. With everybody so soft
nowadays, that can be difficult, but if you add a sway bar and good bump rubbers it can work.

Finding a good combination is mainly cut-and-try at the track. Electronic data acquisition can
be a big help. I have an associate who specializes in that. His name is John Chapman. He’s in
Charlotte at 704-549-1309, e-mail jchap56756@aol.com. For shock dyno and build service, I
recommend Scott Munksgard at Munksgard Technical Services in Concord, NC at 704-782-
2611, e-mail MTSdyno@aol.com. He custom-builds Bilstein, Ohlins, Penske, and Carrera
shocks, and sells AFCO and Pro shocks and Afcoil and Hypercoil springs.

mgeoffriau

iTrader: (7)

Sounds like you need an intense 1 on 1 session with sjmarcy.


EDIT:

And he told me he wanted to take a good look at your underpinnings.

Gotpsi?

iTrader: (5)

Suspension tuning is endless, you cant just put 2+2 together and get 4 when it comes to suspension. You may want to change something at every different track, or with changing weather for example. I know there are some very knowledgeable people on here but hopefully someone is close to you and can drive your car at the track and let you know whats up, it sounds like you may need more rebound to stop the "Pogo"but your description is to simple for me to determine that adn it could be something else.

O sweet just saw that this was my 1,000th post!

hustler

iTrader: (6)

I have a log for him.

hustler

iTrader: (6)

700/400 RB front/ FM rear on the middle setting, about to go up to the stiffest setting. My corner balance is 200lb fat on the front left, and I don't know how bad that really is.

My car felt dialed in at TWS and HHR, which are two tracks on opposite ends of the spectrum; TWS is fast and I never go lower than 4th gear, HHR is tight ant twisty, I spend lots of time in 3rd. However at MSR-H, before the swaybar link broke, the car felt like a mess in "Sugar & Spice" but nowhere else on the track. I'm trying to avoid this when I hit MSR-C in a couple weeks.

hustler

iTrader: (6)

Awesome, thanks for that link.

sjmarcy

Has it been established that racecar setup can be fully encapsulated in one sentence with small words?

BenR

iTrader: (2)

You are adorable.

hustler

iTrader: (6)

STFU with your horse-**** or it will be another 3-months before you figure out that you're on global ignore because you're so accustomed to being ignored by all people in general, especially women.

golftdibrad

I'll add something here in case you didn't already know.... you cannot adjust this except my moving weight around the car. This surprises a lot of people....

hustler

iTrader: (6)

I get that, but do I adjust that balance with the spring or the sway?

golftdibrad

A little of both. You want to run the softest spring possible while maintaining compliance. I prefer to use spring rates to balance the car in the big steps and sway bars for the finer adjustments.

spoolin2bars

iTrader: (1)

by pogo do you mean it's trying to "buck" you out of your seat? the car is still planted but it's jerking you in your seat? or are you saying after the undulation the car still rocks once or twice afterwards? if it's the second part, that means the shocks are set too soft and are not controlling the motions of the springs. go up a click or 2 until it stops. if it's the first part, well that sounds like my shocks. probably could use a little low speed valving adjustment. (as in re-valve) that's the advantage xida's have over any other shock in the world for a miata. motons and penske's are great. but they are designed for race teams with shocks dyno's and dedicated "shock builders" to set them up for each track they visit. in my case i can deal with it because the only place that has "undulations" like that is the straight between 2-3 and 8-9 at harris hill road. the car's tires aren't loosing contact with the track, but if it wasn't for my harness my butt would leave the seat.
last thing, if you get serious with this, take a notebook with you to every track event. note the air temp, track temp, conditions, etc... what shock settings, tire pressure, sway bar settings you ran. also how the car felt, notes about the track features, lines through certain corners, etc.
i'm no pro, but this is stuff i've learned over the years. basically you want the car to turn how want (good turn in, little to zero understeer) and still feel planted which instills confidence, especially on fast corners (little to zero oversteer) and that allows you to find the limit without drama. if setup right, you should be able to adjust your line through a corner at the limit with the gas pedal. and if it's really awesome, with little if any correction with the steering wheel. if any of this doesn't sound right right, i'm sure emilio can help or explain it better.

spoolin2bars

iTrader: (1)

sorry, been at work. lotta ?'s.

hustler

iTrader: (6)

Thanks.
I need to stiffen up rebound damping on the "drive home from the track" settings at 4/2 from full soft.
I keep a notebook, now I need a pyrometer to add the right data to the mix.
I need to experiment more, I still have not cranked the rebound valving to "see what happens" yet.
The last track day was a waste due to the broken swaybar, I want to be sure I have my cards set-up right for the next race. The previous day at HHR I thought the car was wonderfull and had no complaints...this thread stems from my inability to hit Sugar and Spice hard.

I guess I should try the tightest sway-bar setting and see what it does, then go back to playing with valve settings. I need a damn test day rather than a race day.

hustler

iTrader: (6)

...and the corner with all the weight is the corner that doesn't steer, correct? If my front left is heavy, could that explain why steady-state right-turns are a story of miserable understeer?

golftdibrad

All other things being equal I think so, but there are alot of other things at play like alignment and stuff too. I really don't have that much experience beyond basic setup.