Suspension: When to adjust what?
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 knob? 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): http://a2.sphotos.ak.fbcdn.net/hphot..._4138903_n.jpg |
lot's of views, no answers, not surprised.
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What are your spring rates? and what sway bars do you have right now?
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Originally Posted by hustler
(Post 692975)
lot's of views, no answers, not surprised.
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. |
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. |
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! |
Originally Posted by mgeoffriau
(Post 692982)
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. |
Originally Posted by Gotpsi?
(Post 692979)
What are your spring rates? and what sway bars do you have right now?
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. |
Originally Posted by BenR
(Post 692980)
:hustler:
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 |
Has it been established that racecar setup can be fully encapsulated in one sentence with small words?
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Originally Posted by sjmarcy
(Post 693008)
Has it been established that racecar setup can be fully encapsulated in one sentence with small words?
You are adorable. |
Originally Posted by sjmarcy
(Post 693008)
Has it been established that racecar setup can be fully encapsulated in one sentence with small words?
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Originally Posted by hustler
(Post 692959)
If I have 50/50 cross weights, how do I adjust for the F/R weight ratio for neutral handling?
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Originally Posted by golftdibrad
(Post 693037)
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....
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Originally Posted by hustler
(Post 693049)
I get that, but do I adjust that balance with the spring or the sway?
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Originally Posted by hustler
(Post 692959)
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? combination of sway bars, springs, and shock settings. remember cross weights don't mean 50/50 weight frt. to rr. they are corner to corner and help make the car steer the same when turning left/right How do I know when I need to adjust spring rate? well, your proboably not gonna be switching out springs, and miata world already have this figured out somewhat, you're probably good already unless you picked out some wacko rates. How do I know when I need to adjust sway bar stiffness? basically if you adjust the shocks for an over or understeer condition and can't get it done, check into the sway bar. once set you generally don't need to mess with them. maybe if it was real slippery wet conditions you might need to loosen them. How do I know when I need to adjust the rebound knob? at different tracks you might need to adjust a little. is the frt. end understeering? some track surfaces are different and might require a small adjusment. is it a big track with lots of high speed sweepers? you might need to stiffen the frt. and or stiffen/loosen the rear for a little more high speed stability. If I feel the front of the car rolling, and the rear flat, is that necessarily a bad thing? bad thing? no. as long as it is handling the way you want it's all good. that's all your trying to do with the adjustments. get it to where it feels comfortable, easy to control, and does what you want. 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): http://a2.sphotos.ak.fbcdn.net/hphot..._4138903_n.jpg 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. |
Originally Posted by hustler
(Post 692975)
lot's of views, no answers, not surprised.
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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. |
Originally Posted by golftdibrad
(Post 693037)
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....
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Originally Posted by hustler
(Post 693083)
...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?
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