I have two related question that I'll dare ask here.

The first is: Why don't people ever take the sway-bar into account when calculating the amount of damping that is required? I have never seen any version of the formula for calculating the initial damping slope that includes the effect of the sway-bar on single-wheel bump.

The second question is: have any of you ever set up a car for low-speed gravel with no sway-bar (connected) at either end, so that you can add (or re-attach) the sway-bars for higher-speed tarmac?

I actually came at these questions in the opposite order. After convincing myself that I could set the car up without sway-bars for gravel and then re-attach rather heavy bars for tarmac, the problem that I foresaw was being under-damped in the latter case. But when I pulled out my notes to see how far under-damped I would be, I found that no formula for initial damping ever included the sway-bars.

Don't ask me, it's one of the great mysteries of life.

Thank goodness I only really care about gravel.

---

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.

My Galant has no sway bar at all.

Removed the damn thing alles zusammen, why carry along dead weight ? I ain't never running on tarmac.

---

Cheers
M.Samli
Phoenix AZ
Gaylant VR4
EVO III GSR (Stolen)


Rallies are no place for traitors

First question - question. What formula are you using to calculate damping required? Is it based on the sprung mass natural frequency and an assumed percent critical damping coefficient or something else? Secondly, the only place I can recall seeing damping related to stabar rate has to do with roll damping, in which case both the front and rear suspensions are resisting roll moment of the car. You have to take into account the roll polar moment of inertia, in this case, whereas inertial effects are ignored for purely vertical movement.

If you are looking at wheel hop damping percentage as it relates to the body first mode, I would suggest calculating a critical damping percentage for the body primary ride using only springs and then evaluating the wheel hop mode damping with the stabar included.

But, it doesn't really matter for 1 wheel bump, as you'll be well out of low speed damping by the time you would care on a rally car anyway. First natural frequency damping of the body and roll damping maybe, during single wheel impact the wheel velocities are too high.

Second question, no. Set the car up like whoever is fastest running your config. Theoretical calcs for this type of stuff are way less useful than real life.

Steve

Thanks for the reply. Yes, I was using the NF & sprung weight approach. And I agree that motions caused by actual bumps will be way beyond the knee, but the bars won't be attached off pavement. What I was worried about is having so much of the total roll resistance (on smooth pavement, with the sway-bars attached) coming from the bars when the bars aren't in the equation. Put another way, I was worried that I would not be able to adjust the damping enough when the car is switched from gravel to tarmac. I guess that I could/should simply calculate the single-wheel contribution of the bars, treat them as a spring in parallel, and see where I'd be.

As to copying what has already been shown to work, I'm having a hard time finding out what others have done. Most seem rather cagey when asked by a total stranger, which I completely understand.

Adjust enough for what? Roll control or single wheel bump damping? You should be able to calculate the roll natural frequency with the bars included and % critical damping. You can probably find some public inertia results for the body, assume some roll center heights and go from there. Someone may even have a nominal fee excel sheet already set up for this. Roll damping percent critical is typically way less than vertical, if I remember correctly, so be careful how you interpret the results.

What roll gradient are you shooting for on the car and what percentage of total roll stiffness do you think will come from the bars?

Quote
Iowa999

As to copying what has already been shown to work, I'm having a hard time finding out what others have done. Most seem rather cagey when asked by a total stranger, which I completely understand.

I don't understand it...and don't understand why, if they know what they are doing, they don't answer things..
I have a strong feeling that many are merely guessing or only opening boxes and installing expensive stuff, and if it's expensive enough, it just has to be good.

---

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 hated, but now look back on an chuckle, going to car shows where you'd see the older guy with a classic car, in decent but not show room shape. Just there with the hood up. Greese on his hands, dirty fingernails, and he'll talk to you all day about what he's done to the car and why. He'll tell you about the old days when no one had any money and they would find stuff cheap in the junkyard and try it out. They made discoveries and improvements with practically nothing. Then you see the slick guy with his leather jacket with the car's model on it, and sunglasses, getting out of a mint late model car. He puts his sign out listing all the modz. Ask him one question about anything on the car and he would have no idea how it improved anything or even how it attaches to the car. He just read what the good mods were online, found a shop and threw money at them. Sad people.

I guess I don't understand the reward of status versus earning. At least the racer uses the stuff and if they don't delude themselves that more expensive means it made it better, they can tell if something is good or not, even if they don't know why.

Tim.

edit: this is a response to Steve

Sorry. That was unclear. It's roll control on tarmac that I'm worried about. And I'm not sure about roll gradient; I'm not even sure where my roll center will be. But, to be honest, we're close to going over head at this point.

What I know for sure is that I seem to drive best (on tarmac) with a ton of rebound, probably because I'm not smooth and always late. (Due to some activities in the 1970s, my brain has a low and fixed maximum speed; I don't exceed it when grip is also low, but I do when grip is high.) My previous car was set up for tarmac only and was getting about 1/4 of the total roll stiffness from the bar. If I really spring the new car soft and use huge bars for tarmac, I could be getting more than half of the roll stiffness from bars. That's what made me ask the question about taking the bars into account when thinking about the low-speed damping: on the previous car, ignoring the bars was probably still within the margin of error. I doubt that this will be true this time.

But maybe this is moot. Since I seem to respond best to lots of rebound, and the bar acts against extension of the inside suspension, for all I know, I'll need much less rebound damping, not more, when huge bars are attached. This is the issue that I can find nothing on in any book, so I asked about it here.

If the answer is that "you can't find anything on it in any book because it's a complete non-issue," then please tell me that (bluntly, so it gets through). If the answer is "this is completely empriical, stop masturbating with your spreadsheets and drive the bloody car," then that would be fine, too.



Edited 1 time(s). Last edit at 01/16/2013 12:23PM by Iowa999.

to Tron -

There is a monthly car show in the lot right in front of where my daughter takes ballet. It's 90% older guys and 100% Amurican Muscle; the most "exotic" being a Viper RTS. For shits and giggles, I entered my car. After the basic, which included a sneer when I said "Mitsubishi," the rest of the sign-up went something like this:

guy at table: "mods?"

me: "skid-plates, Wilwoods, Braids, and a tune"

guy: "sponsors?"

me: "I dropped out of AA"

guy: "best ET and trap?"

me: "I'd have to say the tiny alien in Liquid Sky and the Hav-a-hart classic"

guy: "in the face or stomach?"

me: "sorry?"

guy: "where do you want me to hit you?"

Quote
Iowa999
If the answer is "this is completely empriical, stop masturbating with your spreadsheets and drive the bloody car," then that would be fine, too.

Steve is a really sharp guy, actually works and pays the rent doing suspension and set up related junk-n-stuff...

But you may have the real answer right there.

It may be---after wasting tons of time, that it's "this (or that) may be TRUE, but totally immaterial and an oinions peels thickness of %..
And it is nearly certain that none of us are so good, so sensitive, that a fraction of a % is meaningful..

---

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.

If you think about it, the sway bar effects are going to vary with what you're doing; they're doing jack during longitudinal weight transfer, and more than jack during lateral stuff. Theories and calculations for damper settings to account for crap without sways often get close, but empirical testing's necessary as a sanity check at the very least. For that matter, if you look at the big expensive rally dampers (EXE-TC, Reiger, etc) from all accounts it seems like they're doing things with damping that goes way against the conventional critical damping theory... which probably isn't applicable here, but is an interesting datapoint IMO. Anyway, now, with sways thrown in, it seems like you'd have to get even more empirical... if you adjust damping to take into effect the full possible effects of the sway, you may well end up overdamped for the acceleration and braking phases of things, and the tradeoff there is probably going to vary with driving styles.

J. Toby - You can assume your front roll center is probably approx 0-10mm and the rear is maybe 35mm for general calculation's sake. I don't have any direct experience with an Evo so this is a guess. CG you can figure about 480-525mm.

That being said I wouldn't be too concerned about getting the spreadsheets 100% correct. Personally, and I would more than welcome other opinions, I would aim for less than 50% of my roll stiffness from bars. If you look at data from a number of road vehicles you'll typically see 30-50% from stabars, and you are correct, the vertical/roll damping starts to become a problem when more than 50% is from the bar. This is empirical, for the most part, and you could build some models and screw around but with the stuff we do it's not worth the time.

I assumed, btw, you're chaging springs for tarmac. But, rereading your earlier posts it looks like you might not. If you feel comfortable, what ride frequencies and wheel rates are you looking at running? PM me if you don't want it to be public. If you are too low on road you'll still be looking at having heave and pitch problems. And when you say on road, you mean public roads, not track right?

Quote
mekilljoydammit
For that matter, if you look at the big expensive rally dampers (EXE-TC, Reiger, etc) from all accounts it seems like they're doing things with damping that goes way against the conventional critical damping theory... .

What are they doing for damping?
And which?
old school gravel at 210mm travel?
old school tarmac at 155mm travel?
New school gravel with 300-330mm?
New school tarmac with 210mm travel?

---

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'm not changing springs ... at least, that's not the plan. I know this is crazy, but it's an experiment of sorts. Unless and until it is proven not to work, I want to keep going. That's why I'll end up with so much roll stiffness from bars on tarmac.

I am far from secretive. Rather, to paraphrase Shrek, it's getting me to shut up that's the trick. From memory, front and rear sprung wts are about 875 and 575. Motion ratios are .94 and .86. Springs would be 6k at both ends. I can't recall the sway-bar dimensions from memory. I'll check when I get home.