sway-bar math
Posted by Iowa999
john vanlandingham John Vanlandingham Elite Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
875 and 575 whats? pound/inch? And how much travel f&R? And you intend to do some rally or grass-o-cross? Where did those springs come from??? John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
mekilljoydammit Mega Moderator Join Date: 09/22/2010 Age: Midlife Crisis Posts: 336 Rally Car: No rally car yet |
In generalities, from shit that I read and interpreted (IE, I'm not an expurt, I just read and think a lot, could well be wrong) it seems like new school gravel vs. old school gravel, they're substituting a lot of the chassis control stuff from springs to dampers. Spring rates keep going down because that keeps the wheels in contact with the gravel, and the dampers are making up for the difference, basically, more or less. And it seems to have a lot of stuff following from that; big bodies with lots of oil, and special valving so that the damping doesn't fall off with temperature changes. I think, unless I'm wrong, not that it's relevant to anything at a North American level, not that I'm likely ever going to be good enough at driving on gravel to need anything more than good sturdy Bilstein based jobbies. Yah, lotta disclaimers - I just wanna make it clear that I'm looking up stuff at that level just for the sake of understanding it (I'm an injurneer, I like understanding things) not because I think I need it. Oh, and the numbers he was quoting - those're corner weights unless I miss my guess, and he's talking 6kg/mm as the spring rate, or 335lb/in for us Amerikanskis. |
Creech Scott Creech Godlike Moderator Location: Jane, MO Join Date: 12/02/2012 Age: Possibly Wise Posts: 415 Rally Car: Audi 90 Quattro (WIP) |
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Crazy night at home and I'm back at work without my notes, but, yes, the 875 and 575 were sprung corner weights in Engrish pounds, but they're both a tad low; it's more like 900# front and 600# rear. The motion ratios above also suffer from my numerical dislexia (which is a trip for my stats students, let me tell ya); the real values are .96 and .84 at static ride height. (And, yes, I do know the difference between dyslexia and mere spoonerisms; back off, please.) At least I got the current planned spring-rates correct, although, consistent with the above, if I can afford tri-gressive valving with hydraulic bumpstops, I'd probably drop to 5k.
As to what the car is being set up for: think duffer without a cage, so third gear on an unpaved road. |
SteveT Steve Thompson Senior Moderator Location: Troy Michigan Join Date: 12/08/2011 Age: Possibly Wise Posts: 13 Rally Car: 2004 Impreza being prepped, going backwards |
Ok, so unless I incorrectly copied things this setup gives the following (neglecting tire rates and bushing torsional rates)
Front Ride Freq - 1.83 Hz Rear Ride Freq - 1.97 Hz Ride Freq Ratio - 1.07 Roll Grad, only springs - 3.26 deg/g assuming 510mm CG and ~20mm RCH at CG Roll Grad, Springs + Bars - 1.62 deg/g These would be reasonable numbers for an on road high end sports car (911, Corvette, etc). Personally I think they are probably a bit high for anything but very well groomed dirt but my off road experience is more in the baja truck realm than smooth dirt road. A lot of it depends on the largest thing you'll hit and the amount of suspension travel and gain in the spring rate (jounce bumper, hydraulic bump stop, bypass shocks etc). Someone here please add some insight if possible. I am not sure what ride frequencies rally cars typically shoot for, although observation of televised WRC suggests to me that they are running 2/3 of the values presented here off road. Of course they have much better spring/shock setups than we do. I think the torsional stiffness of the vehicle with the large bars is going to be unpleasant and would shoot for a bar contribution of 30%, which will yield a roll gradient of about 2.3 deg/g. Too soft for dedicated track work but probably happier on fast dirt stuff, especially if there are a lot of bumps. If anybody wants to volunteer their ride frequencies or similar spring/weight data it would be interesting. Steve |
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john vanlandingham John Vanlandingham Elite Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Was his assumptions of the units correct?
Seems like you don't like to answer direct questions, so hesitant to waste time guessing what "front and rear sprung wts are about 875 and 575" means...but if it is pound/inch I'd say you're already was in excess of what a tarmac rally car would be sprung, way way in excess... But what do I know? John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
Aaron Luptak Aaron Luptak Mega Moderator Location: SLC Join Date: 02/15/2008 Age: Midlife Crisis Posts: 776 Rally Car: Civic... |
reading is fun-duh-mental.
(emphasis mine - I'd make it blink if I could) so actual weight of the corner of the car being held up by the spring/suspension, right?  KF7RWG 
http://www.utahrallygroup.com |
SteveT Steve Thompson Senior Moderator Location: Troy Michigan Join Date: 12/08/2011 Age: Possibly Wise Posts: 13 Rally Car: 2004 Impreza being prepped, going backwards |
No magic or Evo X data. You stated 50% of your roll stiffness would come from bars somewhere earlier in the thread so I assumed you had some idea of their overall effect. This means doubling the roll stiffness from springs alone. We could calculate the bar effect based on geometry when you get it. |
John - sorry that that wasn't clearer. When I give weights, such as the front and rear sprung weights, I do this in English (pounds), but when I give spring-rates, I do this in metric.
Aaron - yes, those were sprung weights, so they were the weight (or mass, if you prefer) of the part of the car that is held up by the springs. It's what matters to calculating ride frequency and desired damping and such. With that said, I have been wondering for some time whether ignoring the inertia of the unsprung weight (i.e., wheel, tire, brakes, and knuckle) when you hit a really nasty bump is leaving a lot out of what matters. Mostly of the math and texts are designed for road-racing. Steve - now I get it, but those were "paper napkin" values that I posted, so they could be off by a lot. When I have accurate values for the current and possible sway-bars, I'll post them. But, as of now, it seems that even 5k might be too much spring without bars, so the bars for tarmac might end up ridiculous, so this whole idea might need to be scrapped. The current springs are 3.5k front and rear, so maybe I'll just disconnect the current bars and take the car out this weekend. On paper, that seems too low, but I never actually tried it. And I've only ever set up double-wishbone before and struts seem to behave differently (at least, to me) in ways that the math doesn't warn you about. |
Just to be clear about the unsprung weight issue....
When we replaced the OE Brembos with Wilwoods (mostly to get 15" wheels on the car), we also dropped 15# per corner. FYI: the change in braking distance on tarmac was hardly affected at all; I really hate Bremobs. This drop in unsprung weight doesn't show up in many places in the math, but it clearly made a difference, all else being equal. On washboard gravel, for example, I had much more lateral grip. This is why I have started to question if using the advice and math from standard texts (again, mostly written by road-racers) is wise. |
john vanlandingham John Vanlandingham Elite Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
But the question is which metric measure. kg/mm? N/mm?
While I've been a machinist almost strictly metric since '73 I do still use English on some things just because either that's what the dimesnion is like wheel bolt patters---4 1/4" when it's 4 on 4 1/4", or 4 on 100 when its 4 on 100 and then when it is clearer like rod and main clearances I'll use 0.0015" to 0.002"---a lot of that because that is what the trade HERE uses so no point in confusing the crank grinder.. Springs I maintain the English again for the sake of clarity because that is what the bulk of the springs I will encounter are listed and what the scales i will encounter are....ie I weigh a car the scale says 2725 lbs, I look in a catalog and they say 275 lb/in.... If I do use metric then I use the standard metric notations so it's clear so 0,15 I know is 0,15mm or .006".. It's fun enough making educated guesses at spring and damper rates without uncertainty of what numbers mean... And really, realistically I am always confused why, when a person has a mass produced car why the desire to fret and play with the whole process of figuring out everything from some theoretical point of view to arrive at an answer that a) has already been figured out 10s of thousands of times before and b) may leave you with an "answer' for which there is no parts---at which point an educated guess as to what of what is available is the best compromise.. particularly when one's job is not to be a car suspension development "injur-near" , and the task is to go down some hunk of road real fast.. Put another way, since "the job" is "to drive down some hunk of road fast" with a Nowigotcha GXSTR, and the Nowigotcha is a popular vehicle, it seems we can arrive at an answer quicker by searching for the best Nowigotcha GXSTR driver and ask "Yo doooood, whatcha using for springs? Any idea of what the damping rates are?" as a starting point. Assuming of course that one does desire to "drive down a hunk of road real fast"... In other words PRECEDENT in the specific vehicle and use, then fine tune from there..... not "zero in on some theoretical choice out of infinity", then start guessing from there. And judging from the results I see of the "I'll calculate the fawk outta everything myself and then start spending money" it seems that guys invariably end up with outrageous wrong---always too high---spring rates, and limp dick type damping that they haven't any idea what it is... So, I have to ask: have you contacted any truly fast drivers of the car you're thinking of and asked? John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
I know that you like to argue (and I do, too), but... [remaining deleted for self-preservation]. Of course it was g/mm. The car weighs 3400#. Did you really think that I was planning on putting 28500 pounds/inch springs on the car? Because that's what they'd be if the "5k" I mentioned was in N/mm. Maybe I don't get out enough, but I thought it was absolutely standard to say "5k" when you mean "5Kg/mm." But it's your sandbox, so I will be painfully explicit from now on. As to whether I have contacted fast folks with Xs, the answer is that I have tried. As I mentioned above, they do not seem open to sharing the details. They just say "get DMS" or "get HotBits" or "stay the fuck away from K-Sport Gravels; they are shit" and the conversation ends. Maybe I'll head down to 100AW in a month and climb under their cars. Will you volunteer to act as my distraction if I ask really nicely? Edited 2 time(s). Last edit at 01/18/2013 11:05AM by Iowa999. |
SteveT Steve Thompson Senior Moderator Location: Troy Michigan Join Date: 12/08/2011 Age: Possibly Wise Posts: 13 Rally Car: 2004 Impreza being prepped, going backwards |
To tell you the truth I've never heard of springs being rated as 5k or 6k, but I tend to work with OE spring people and not the NOPI cool kids. I had no idea what it meant either. Anyway I give Toby credit for actually trying to calculate things before setting up his car. Asking NASIOC, or whatever the Evo equivalent is, would be stupid as they would have you riding on poly bump stops with 700lbs/in springs and a 34mm rear bar. The problem is I'm not sure too many people have arrived at a good answer through calculation, as opposed to trial and error. I am a big proponent of a mixture of both. Just slapping on whoever's kit and thinking your car is going to be fast is doable, and often a good starting point, but not necessarily the most complete way to make a car quick. You CAN put really stiff springs on a car, tune it for the street by underdamping the crap out of it and call it good. Lots of manufacturers do it, much less aftermarket suppliers. But it won't work for gravel. As far as damping calculations as they relate to the unsprung is concerned I would reference Olley's book and his treatment on calculating first order wheel hop. It's essentially the same as sprung mass frequency and damping %. The natural frequency of the unsprung is controlled by the tire vertical rate (assume 375 N/mm for your tire, high stiffness for a road car) and wheel rate in parallel, and the weight of the unsprung mass. This will give you an unsprung natural frequency that you can relate to the ratio between your sprung mass natural frequency and critical damping percentage value. Critical damping varies linearly with natural frequency and mass (2 x omega_nat x mass), meaning that the ratio of your unsprung and sprung masses needs to be similar to the frequency ratio for the same % critical damping. I doubt you'll care for the car on road without bars. Gravel may be ok. |
john vanlandingham John Vanlandingham Elite Moderator Location: Ford Asylum, Sleezattle, WA Join Date: 12/20/2005 Age: Fossilized Posts: 14,152 Rally Car: Saab 96 V4 |
Don't go and get all butt-hurt---if that's the tone I'm hearing... Maybe you don't get out enough...I have NEVER heard "5 kay" , maybe I don't...I did explain nicely why a common language is helpful when seeking help....that's why I learned Swedish and French and German, to make it easier on those I was asking for help... When I asked if you had contacted anybody fast, I really didn't mean anybody in North America, especially anybody in USA....look further afield, look northwards. Search piccies of gravel Evobitichis and find team contact and write a note. Most normally adjusted people in the rest of the world DO share info, it is primarily among poseurs/amateurs in USA that I see this "secretiveness" phobia, and it usually turns out they actually don't know a thing... John Vanlandingham Sleezattle, WA, USA Vive le Prole-le-ralliat www.rallyrace.net/jvab CALL +1 206 431-9696 Remember! Pacific Standard Time is 3 hours behind Eastern Standard Time. |
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