More spring, or sway bar?

[Bill Miller]

Something in the spherical bearing thread made me think of this. That, and I thought a nice technical discussion would be a good change of pace!

I'm interested as to what people's thoughts are about using a higher spring rate, or a sway bar to control body roll / transition.

I know on an A1 VW, the last thing you want up front is a sway bar, so we use all spring. But, on the back, it's not uncommon to see people running multiple bars. I'm curious as to what others do on their cars, and why they went that route.

[joeg]

With FWD, you are stuck with going that way, although with a good diff. you can reduce the front wheel spring rate somewhat.

[Matt Rowe]

Bill,

My experience started in formula cars (where you start out with RWD and a more optimized geometry) but so far nothing from FWD cars has changed my mind. My attitude is still you use spring rate to control body roll, anti-roll bars are there only as a fine tuning tool. There some wonderful theories behind this, such as the idea that a anti-roll bar resists roll at the expense of taking more load off the inside tire. Or sway bars are only indirectly dampened as opposed to directly damped coilover springs, although that one is fairly minor.

Naturally the anti-roll bar effect is going to seem more pronounced in FWD where a loaded outside tire is going to force the inside wheel off the ground providing even greater chance of wheelspin. Which would mean that soft springs and a big sway should cause even more wheel spin problems as the larger bar will take even more weight off the inside wheel. Yes a good diff will help, but as long as you're not burning up the outside tire due to the cornering load the heavier spring is a better trade off. A sitffer spring up front will keep the roll to a minimum while not detracting from the load on the inside wheel.

I think the reason the rear is treated differently in so many FWD cars is the prevailing theory that the rears are only used to keep the back end from scraping the ground. Therefore springs or sway bars in the rear seem interchangeable when you're not worried about putting power down. The only time you might see issues is hard braking under high lateral load, but how often does that happen? ;) But based on the earlier discussion on SB's, it is important to remember that the rear end does effect the roll rate of the front end. So there might be some interesting ideas of using a large rear bar to control front roll, provided the stiffer front springs are not possible/practical/ideal.

One last thought, the early Dodge's are similar to some of the VW stuff in that you can get some camber gain in the front end, but the back typically used a live axle setup which means any rear body roll costs you contact patch. In theory you would think that keeping the back end of the car flat would be ideal, as long as you don't go to far and throw off the balance of the car. But then you are back to gaining contact patch at the expense of losing load on the inside wheel. Everything is a trade off.

Damn it, when is the first test day up here? :(

[Bill Miller]

What's interesting, is that there seem to be different schools of thought, for different FWD cars. A lot of the VW guys run higher rates in the front, than in the rear (I ran 650F/400R), but I know the Honda camp tends to go the other way, w/ some really high rear rates. I've heard of rear rates in the 800 - 1100 # range! :119: :119:

Nice thing about using bars on the rear, is that it's usually a pretty easy deal to take one off, if you need to soften the car for cold/wet conditions. Believe me, an optimized dry setup on an A1 VW is hella scary in the wet!!! You'd swear you were driving a RWD car.

[Matt Rowe]

I have to agree with the wet issue. And a great example of why FWD is always the greatest in the wet. With the rate springs I run in the rain is not the friend all of those RWD guys think it is. :D

On the Honda spring rate thing, one thing to keep in mind is the spring rate isn't the whole story. You have to look at the location of the spring and consider mechanical advantage. I've been caught out before wondering about somebody's quoted spring rates until I realized the springs had something like a 1:2 mechanical advantage.

[RSTPerformance]

Back before the IT guys were running acuras and when the Speedvision/RealTime Acuras ran Lime Rock, I loved the way the inside rear tire was in the air the entire way around "big Bend." I thought, hey I need to do that... well a couple years passed and then we tried a set-up... we knew that the acuras were running 1100-1200+ spring rates in the rear, we backed it down and decided to go with 800lb in the rear... The car was loose, but ok on smooth tracks such as NHIS, It was undriveable at Lime Rock with the bumps. All in all we have ditched that effort, but we may try something at NHIS this year.

PS: We havn't noticed much of a difference in the Audi with or without a the rear bar (obviosly it just isn't big enough ;) )

I would think that a correctly set up car would have a bar on the front and the back to "fine tune" the roll...

Raymond

[Jim Susko]

Swaybars should be used as a tuning tool, not as a major suspension component. On a car that has the ride springs properly damped and the roll center height properly located, adding a large sway bar gives you extra roll stiffness that is entirely undamped. No matter what shock settings you use, you will be either over-or under-damped in ride or roll. So the general principle is to use bars sparingly.

Large bars are used on road vehicles when a matched, soft ride is desired on both ends but there is a large difference in axle weights which would overload the heavy axle in roll and cause loss of grip at that end, i.e., over- or understeer. A swaybar is then used at the other end to increase weight transfer to the outside wheel and balance the car out. But everything done on a road car is a compromise and grip is often sacrificed for ride. So although big bars may be commonplace they are rarely desirable from a performance standpoint.

On street cars turned into race cars with limited allowances for suspension modifications like IT, lowering can result in a major loss of roll stiffness, especially on a strut type suspension. To get the ride and roll rates back into balance, a bar must be used. But damping will still be compromised to some degree. On a car designed specifically for racing where the rules do not force a major compromise in design, the ideal is to use only a light swaybar at one end to allow fine tuning only.

[lateapex911]

Veeeery interesting that, esp coming from an ex GM chassis engineer!

[Bill Miller]

Swaybars should be used as a tuning tool, not as a major suspension component. On a car that has the ride springs properly damped and the roll center height properly located, adding a large sway bar gives you extra roll stiffness that is entirely undamped. No matter what shock settings you use, you will be either over-or under-damped in ride or roll. So the general principle is to use bars sparingly.

Large bars are used on road vehicles when a matched, soft ride is desired on both ends but there is a large difference in axle weights which would overload the heavy axle in roll and cause loss of grip at that end, i.e., over- or understeer. A swaybar is then used at the other end to increase weight transfer to the outside wheel and balance the car out. But everything done on a road car is a compromise and grip is often sacrificed for ride. So although big bars may be commonplace they are rarely desirable from a performance standpoint.

On street cars turned into race cars with limited allowances for suspension modifications like IT, lowering can result in a major loss of roll stiffness, especially on a strut type suspension. To get the ride and roll rates back into balance, a bar must be used. But damping will still be compromised to some degree. On a car designed specifically for racing where the rules do not force a major compromise in design, the ideal is to use only a light swaybar at one end to allow fine tuning only.
[/b]

Thanks Jim, that's exactly the kind of information I was looking for! :happy204: :023:

[racer14itc]

Hmmm, maybe my practice of using no swaybars at all on the GP car isn't so far fetched at all. Just enough rear spring to get the car to do what I want. The rear axle on the A1 VW is really a sway bar with about 75 lb/in rate. :eclipsee_steering:

A long time ago (12 years ago??), Bill Sulouff sent me an e-mail and mentioned that the back of an A1 car can't tell the difference between springs and swaybars. I still have that e-mail in my notes, Bill! You were on to something there, Bill! :024:

MC

[eMKay]

Hmmm, maybe my practice of using no swaybars at all on the GP car isn't so far fetched at all. Just enough rear spring to get the car to do what I want. The rear axle on the A1 VW is really a sway bar with about 75 lb/in rate. :eclipsee_steering:

A long time ago (12 years ago??), Bill Sulouff sent me an e-mail and mentioned that the back of an A1 car can't tell the difference between springs and swaybars. I still have that e-mail in my notes, Bill! You were on to something there, Bill! :024:

MC
[/b]

Doesn't the rear bar on our cars help keep everything stable though? Meaning it reduces toe changes under load, so if the thought about the A1 can't tell the difference between springs and swaybars, isn't it better to run the thickest bar you can on our cars? Mine is 450lb front with no bar and 350lb rear with a neuspeed bar. Haven't had it on the track yet so I have no idea how it handles

[Knestis]

On the MkIII Golf, we ended the year with 500# springs in front (w/o any bar) and 700# springs in back, with the Shine "racing" bar added to the rear beam with the GTI integral bar. Each time we made a change, I thought, "WOW! Now it's not pushing" but then, after getting used to it, I came to the conclusion that it was still safer than it was fast.

That also seemed to be the consensus of drivers who shared the car at enduros.

In stock form (SSC), the Golf was a barnstormer in the rain at Roebling. I finally got to run it in the rain in IT trim at a VIR NASA race, and it was AWESOME in that configuration too - with the general set-up described above, albeit with the damping dialed back to full min.

That would seem to support the theory that it was still pushing in dry conditions, since it finally seemed truly tail-happy in the wet.

For this seasong, I'm adding a 1.25" OD, .025"-wall, tubular, adjustable bar to the rear end, on top of what we ran last year. The integral bar in the new rear beam may be smaller (remind me to check, Cameron since I know you are reading this), but the theory is that the old set-up is now our new over/understeer baseline, and we'll have nothing but room to go more over from here. We can drop a link in the wet and be right back to a known package.

One thing that I guess I don't understand, though: Wouldn't motion controlled by bars be damped by the struts/shocks, to the extent that the independent vertical displacement of the wheels would be controlled? Sure - the ARB "spring" itself is not damped and nor, strictly speaking, is the relative displacement that it resists but the wheels still have to move up and down...?

K

[Greg Amy]

You know, I always get a giggle whenever someone declares "this is the way it oughta be", like it's a settled fact. Frankly, I've seen it all in production based cars: soft springs, hard springs; soft swaybars, hard swaybars; hard springs/soft bars, and vice versa. Hell, today the "settled fact" with World Challenge is hard EVERYTHING!!

The true and only "fact" is...there is no fact. No one - and that means NO ONE - has come up with "The Absolute Truth" in the "best" design for production-based cars (or even open cars). No one; no, not even him. Every time someone declares the problem 'resolved', a team or competititor comes that proves them wrong. No one can declare "this is the way it is" as a blanket statement for all cars, like it's a settled rule of physics, no more than someone can declare the prettiest color in the world is blue (despite the inarguable fact that it is.)

Bottom line: you gotta try. Even though it works great for "him", it may not work great for you. Just because it works on "that car" doesn't mean it's gonna work on yours. Sure, it may be a nice starting point (only the fool would ignore someone's else's experience), but if you think it's the end-all-be-all, you're in for a surprise: the next guy's gonna come along and prove you wrong.

SO, try it and see how it works FOR YOU.

Greg Amy
Kakashi Racing - and named that for a particular reason...

[racer14itc]

Kirk,

Since the ARB doesn't have any effect when the rear of the car is vertically displaced (both wheels at the same time), if you valve the shocks to control both the springs and ARB in roll they'll be too stiff (over damped) under braking or on acceleration.

If your shocks are properly valved for the vertical displacement (braking or on acceleration), then the ARB will not be damped properly (underdamped) in roll.

That is why for years I've used springs only in the rear, I can valve the shocks properly to the spring/unsprung/sprung weight without having to compromise so much for a huge ARB. It seems to work for me. Another benefit is that the inside rear wheel isn't lifted off the track so quickly on corner entry, which can make the car twitchy as rear grip is reduced very quickly. I tried to run a HUGE rear bar and soft springs at one time on the GP car...I didn't like it at all. But that's just me.

MC

[Conover]

It seems that when the rear inside lifts it is at the point when wieght transfer has already shifted the grip away from that wheel, whether it is on the ground or not it isn't going to stabalize the car.
When your running a massive rear bar it may be that your getting closer to a live axle type situation, where the spring rate, and dampening for that matter, is effectively doubled because both sides are moving together. so 200# on each side nets a 400# spring rate.

[Knestis]

Good point, Mark. I neglect the knock-on influence on fore-aft transfer damping. Thanks! Oddly, were I working on something with more power, I don't think i would have made that mistake. I'm kind of lulled into disregarding that issue with a low-power, high-center-of-mass car. If I were a harder-on-the-brakes guy, it might have been more on my mind, too.

K