Sway bars, why not just stiffer springs.

[911st]

I am trying to understand why we have sway bars on our cars instead of just stiffer springs.

For example, if the spring rate in the back is say 250 lbs per inch. As say we run a sway bar that has no effective rate at all when stopping or going over a speed bump but, it adds say 100# of effective spring rate to a cornering load.

Why not just run a 350 lb spring rate in the back w not sway bar.

What advantage dose this give us?

It would seem the 350# springs only would have the same corning ability and also reduce front weight shift.

Is there something we are getting with changes in toe or camber with a spring / sway bar set up?

I can see some advantage if the sway bar is adjustable for tuning the dynamics in a corner.

Maybe it would be better to run a stiff spring only set up in the front to maintain less squat and neg camber gain under braking and use a spring/ sway bar combo only in the back for tunning. Of even a larger spring and smaller rear sway bar.

Any guesses or knowledge out there?

I heard of one local instructor that found he was faster disconnecting his rear sway bar totally on a narrow body car w race suspension???

[RWebb]

you need to think about the reaction force -- think about what each is attached to

try this experiment - place a small log in front of only one wheel and run over it w & w/o sway [anti-roll] bars

now, repeat with both wheels going over the log [or into a linear pot hole]

it will be immediately obvious when you do the 2nd comparison

[sww914]

The advantage of swaybars is that you can control body roll and still maintain supple enough suspension to soak up some bumps.
I drive a 914 and I can't run a rear sway bar at all. I tried and it was like my rear tires were greased, after the first practice session I disconnected it and all was well.

[Dueller]

The chances are high that you have experienced the effects of body roll during the course of your normal driving. It happens during almost every turn when one side of the car lifts, causing the entire vehicle to "lean" toward the outside of the turn.

The cause of body roll is simple physics: an object in motion tends to stay in motion until acted upon by an outside force. So in practical terms, as you drive ahead in a straight line, you allow 2300-2600 pounds of vehicle, fluids, and passengers to build momentum in a straight line. Suddenly, through input at the steering wheel, you tell everything to change direction. But even though the front tires may change direction, thanks to the mechanical advantages of the steering system, the momentum of the vehicle, fluids, and passengers continues in the original direction. The tires are the only element capable of generating an outside force that can act against this momentum and change its direction.

At this point, one of two scenarios is most likely to occur. If enough momentum exists in the original direction, and the tires lack enough grip to act against the original forward energy, then the vehicle will slide out of the turn as the tires loose traction. However, if the tires have enough grip at the road surface, then instead of sliding, the vehicle’s traction at the road surface will overwhelm the original forward momentum and act upon the original forces to induce a change of direction. Hence, a cornering maneuver.

But what happens to that energy? Even though we may have had enough grip to hang-on through the turn, we know that the momentum of the vehicle mass will continue in the original direction. The result is a weight transfer toward the new outside edge of the vehicle – the same direction as the original forward momentum. If enough energy is behind the weight transfer, then this energy will cause the outside suspension (in this case, the spring and strut assembly) to compress while the other side lifts and extends. An engineer-type likes to describe this by saying that one side moves into "jounce" while the other moves into "rebound." The rest of us call it "lean" or "body roll."

Why is Body Roll a Bad Thing?

We often hear that preventing body roll is "so important" that we all must rush out and buy "this" product or "that" product in order to prevent it. And many enthusiasts have consequently accepted that body roll is therefore "bad." But what exactly does body roll do to negatively affect vehicle handling?

For starters, it disrupts the driver. This is probably the effect that most drivers can see and feel during their own driving experiences. And while this is not the most important negative effect of body roll, it is true that the car does not drive itself – no matter how many aftermarket parts you install. So keeping the driver settled, focused, and able to concentrate on the task of driving is a foremost priority for spirited vehicle handling.

However, the most often misunderstood effect of body roll upon vehicle handling is the effect of body roll upon camber - and the effect of camber changes upon tire traction. Put simply, the larger the contact patch of the tire, the more traction exists against the road surface, holding all else constant. But when the vehicle begins to "lean" or "roll" to one side, the tires are also forced to "lean" or "roll" to one side. This can be described as a "camber change" in which the outside tire experiences increased positive camber (rolls to the outside edge of the tire) and the inside tire experiences increased negative camber (rolls to the inside edge of the tire.) So a tire that originally enjoyed a complete and flat contact patch prior to body roll must operate only on the tire edge during body roll.

The resulting loss of traction can allow the tires to more easily give away to the forces of weight transfer to the outside edge of the vehicle. When this happens, the vehicle slides sideways – which is generally a "bad" thing.

How to Prevent Body Roll

By definition, body roll only occurs when one side of the suspension is compressed (moves into "jounce") while the other extends (moves into "rebound.") Therefore, we can limit body roll by making it harder for the driver side and passenger side suspension to move in opposite directions.

One fairly obvious method to achieve this is through the use of stiffer springs. After all, a stiffer spring will compress less than a softer spring when subjected to an equal amount of force. And less compression of the suspension on the outside edge will result in less body roll. However, stiffer springs require the use of stronger dampers (struts) and have an immediate and substantial effect on ride quality. So even though handling is improved, they may not be the easiest or most cost-effective way to achieve the objective of reducing body roll.

For many enthusiasts, the use of anti-sway bars – also known as "anti-roll" bars, "roll" bars or "sway" bars – provides a more cost-effective reduction in body roll with minimal negative impacts upon ride quality.

How an Anti-Sway Bar Works

Put simply, an anti-sway bar is a "U-shaped" metal bar that connects to both wheels on opposite sides of the car at the ends and connects to the chassis in the middle. Essentially, the ends of the bar are connected to the wheels while the center of the bar is connected to the body of the car.

In order for body roll to occur, the suspension on the outside edge of the car must compress while the suspension on the inside edge simultaneously extends. However, since the anti-sway bar is attached to both wheels, such movement is only possible if the metal bar is allowed to twist. (One side of the bar must twist upward while the other twists downward.) So the bar’s "torsional stiffness" – or resistance to twist – determines that bar’s ability to reduce body roll. Less twisting of the bar results in less movement into jounce and rebound by the opposite ends of the suspension – which results in less body roll.

[911st]

In put appreciated but still not getting it. A sway bar is just another spring that only is effective in one direction. This should make dampener design more difficult as it has a variable number of spring rates to deal with depending if it is a bump on one wheel or both of the same wheels hooked up to the sway bar.

Seems a spring only system using a stiffer spring equal to both the spring and sway bar spring system, would have some good advantages. First being less forward pitch and resultant increases in the neg camber of both front wheels which reduces braking effectiveness. And again, requiring a shock to be a comprise setting.

One though I had is it allows for a lower spring rate to absorb bumps and a higher rate to are st roll.

However, that would only be a true statement if most bumps were to hit both wheels attached to a sway bar at the same time.

Part of what has triggered my question is recalling a statement in Bruce Anderson's 911 Performance Handbook about his preference to add rate to the torsion bars instead of the sway bars and that is upsets the car less.

His actual words were:

"Larger torsion bars will make the 911 much more predictable and fun to drive but without as much degradation of ride quality as would be experienced with stiffer sway bars. Stiffer sway bars tend to make for a choppy ride over potholes and the like: whereas stiffer torsion bars just firm up the overall ride, but not to an objectionable degree."

Again, why is adding spring rate using a sway bar spring better that just adding the spring rate to the main spring?

I can see the on a monster truck where it has to make big jumps and needs to be able to have big soft springs to absorb up the landing and then is expected to make a flat track turn without much sway.

For us that is a prescription for a bigger than needed weight shift under braking.

[Bill Verburg]

The difference between springs and sways

springs only act on one corner at a time so can deal with road irregularities independently(theoretically in practice the body acts like a sway bar) of each other, sways connect both sides transferring energy from one side to the other

if you never turned you wouldn't need sways, but since you do, they pull down on one side while pulling up on the other, this limits camber change and transfers apparent weight from the outside wheel to the inside wheel.

I disagree w/ the comment that heavy sways degrade ride more than springs,

When I set my 993 up I went w/ heavy sways a relatively light springs because the local track surface was so terrible, since it's repaved heavier springs w/ the same(adjustable) sways are the plan.

on a 993 M030 sport springs are 150-200 fr 225-275 rr progressives w/ 22/20 non adjustable sways

stock RS is 246/457 w/ 23/20 adj bars

last years JIC 392/560 w/ 25/22 adj bars, for a rough track

next years JIC 448/672 w/ 23/20 adj bars, for a smooth track

[911st]

Thanks for the help getting this through my thick skull.

I love hearing the spring rates at the different levels. Than is almost Porn to me.

So it sound like it is the pulling down on the inside of the car that is where the benefit hides.

I see and understand softer springs w stiffer sways if there are dips in a track that effect both sides mostly at the same time. If instead the track has holes or random undulations that do not mostly effect both wheels at an end, it would not be any better strategy that just running a softer total suspension no mater if w springs or sways.

So if we start with car A that has 200 lb springs w a 100 lb sway bar and car B with no sway and a 300 lb spring.

Then we put it in a full g turn and say 300lbs in dynamic weight are shifted off the inside suspension and added to the one side's of the car what happens?

The outside of both cars would compress about the same 1".

As to the inside things change.

Car B with only one spring would have the inside raise about 1" with the shift in weight in a corner.

Car A with spring & sway would be transferring 100 lbs in the downward direction against the 300 lb loss in weight. Thus it would only rise about 2/3".

Shifting to a 100 lb spring with 200 lb sway would keep the outside compression at 1" but the inside would only be about 1/3".

I think I get it now.

[911st]

As to ride quality, a sway bar transfers a bump to two wheels, a spring keeps the bump mostly at that wheel.

This must be what Bruce Anderson was talking about.

[911st]

Bill, what is the spring rate of a stock 993 please?

[tcar]

Originally Posted by sww914

I drive a 914 and I can't run a rear sway bar at all. I tried and it was like my rear tires were greased, after the first practice session I disconnected it and all was well.

Maybe you didn't put a heavier bar in the front when you put the bar on the rear.

Rear bar increases grip on the front, making the rear loose (oversteer).

Front bar does the opposite. Increases rear grip - understeer.

A bar on the back also needs a bar or bigger bar on the front or it can be very unstable.

914's especially, with their low polar moment (mid-engine) are really sensitive.

Please excuse me if I'm preaching to the choir, but many people don't realize that.
I, a long time ago, thought that to increase rear adhesion I needed a heavier rear bar. Oops.

Discovered that you need to balance the F & R.

[sww914]

You're right, I could have put a heavier bar in the front. I only have a 19mm non adjustable bar up there. A larger adjustable would cost more than the $40.00 I spent on the 19mm one and I'm already the fast guy in my class. Typical friggin 914 owner, cheap as can be, but it is fun to beat cars worth 10-20 times as much as mine.

[911st]

914

What size front torsion bars and rear springs are you running?

What is the typical weight distribution of a 914?

The polar movement thing is a cool concept I had not thought about.

If I was starting over I would get a 914. Rust free 2.0's seem to be getting near SC prices though.

Would be fun to start w a 914 in the local TT club and build track records working up the classes.

[gestalt1]

i actually posted a similar question a while ago. basically i was thinking of not having a rear sway and using larger than what is normal rear torsion bars to compensate on my swb 911. i would then invest in a good adjustable front sway and use the front sway adjustments to balance the under/over steer attitude of the car. this is how the swift db formula cars are set up, basically the sway is used for tuning more than adding spring rate in corners. my goal was to have a better ride on rough roads and saving money on a rear sway. i'm not sure how this will work but i'm going to give it a try. the problem is that walking around any paddock all the production based races have sways on the front and rear - there must be a reason.

[Chuck Moreland]

As Randy and others point out, springs operate on the change in position of a single tire. Sways operate on the difference between two tires.

Each behaves differently for controlling roll, squat, dive, and ride quality over a variety of bump types.

But perhaps the biggest difference is ease-of-use as a tuning aid. Sway bars and springs can both be used to change the understeer / oversteer balance.

But adjustable sways are far easier to adjust.

New-school thinking about sways and springs is to run fairly stiff springs and fairly soft sway bars. The sways are primarily there as trim to adjust under/oversteer.

Old-school was to run softer springs, stiffer sways on the theory that softer springs allowed tires to better follow the road/track surface, sways controlled roll. There are several limitations, flaws with this theory.

[Wil Ferch]

I think people grubbed over the basic answer already given here......for a suspension to work it needs to follow the road irregularities. This means soft(ish) springs, typically. Anti-roll bars do just that, limit the amount of body roll in corners such that the more ideal tire contact patch can be maintained ( not leaned way over) while cornering.

Up to a point.... stiffer springs do the same thing, and also add front anti-dive and rear anti-squat resistance. But go too far and tires skip over irregularities, and compromise the functionality of a "suspension".

That *should* sum it up in a nutshell. Of course, the various nuances will make this thread explode to 50 pages or more, but I think the fundamentals are within this explanation, and as given by others already.

[sww914]

Originally Posted by 911st

914

What size front torsion bars and rear springs are you running? 22mm & 200lbs

What is the typical weight distribution of a 914? It's so different on different levels of modification, mine is 60% rear

The polar movement thing is a cool concept I had not thought about. fun till you spin, you're a top!

If I was starting over I would get a 914. Rust free 2.0's seem to be getting near SC prices though. Quite. I saw a '73 2.0 this morning for $15000.00 Rust free almost doesn't exist at least around the battery area, it's either been repaired or it will be.

Would be fun to start w a 914 in the local TT club and build track records working up the classes.

My dollar to speed ratio is almost unbeatable. I'm as fast around Willow Springs as a well driven Caman on DOT race tires for quite a bit less money. Also about the same as an RSA. With a $4000-$5000.00 car. It's not the nicest car out there.

[Bill Verburg]

Originally Posted by 911st

Bill, what is the spring rate of a stock 993 please?

not sure, I never used them but it''s probably something like the 92-94 C2/C4 169 189, at least in front, the back is likely to be different

[911st]

I am concluding:

Springs operate on one wheel, sways on two wheels.

Sways do effectively reduce lean in corners and more so that an equal increase in spring rate.

A "spring only" car would sway a bit more in corners with the same effective total spring rate but have less fore aft shift under accel and braking. This should keep front neg camber gain lower with a stop and could benifit braking.

A spring only system would make it easyer to more acuratly match the shock to the spring as the spring rate variabliity is not exadurated by the zero to 100% rate contibition. Not just to the one but two wheels.

Sway bars are the best/ease of method for tuning front to rear reactions.

Hearing that more modern cars are taking a bias toward increased spring rates instead of sway rates makes some sense to me.

Some aditional data/discussion points I find interesting:

I made a calculation assuming the tire contacts patches were circles using tread width specs for a car running 225/245 Toyo R888's .

SC Carrera 911's look to have about 20% more tire in the rear to support about 50% more weight .

Thus, the rear tires are the limiting factory in cornering. We look to need max rear tire hold. We can sacrefice some of the front hold as it has much in reserve but how much?

A stiffer front will sacrfice some front tire contact to insure the rear tires stay on the ground more.

Tuning a 911 looks to be mostly a case of tuning how much front traction we are willing to give up. To much and the front will lose fron contact patch and slide, to little and it will hold and the the rear will slid first.

Get is ballanced and the car will have a ballanced slide.

Want to run 225's front and rear instead of 205/225's because they are cheep or you can not get 245 in that tire. This should not only help braking but may also help the rear if the front is further stiffened letting the rear tires work to 100% if there potental.

Sway bars reduce the spring push back of the inside wheel so we really benifit from a sway on the front toward stiffing the front in a faster prograssive manner.

Again, on a SC Carrera it looks like we have about 20% more rear tire patch area supporting 50% more of the cars weight.

These factors help me understand why we run so much more relitive front sway and spring rates.

Making some more assumptions, if we can get 100% traction on the outside rear wheel and 50% traction on the inside rear wheel. Taking the weight distribuition into account, it may only take a front tire patch size about equal to one rear tire and still hold front traction.

Seen pictures of the fast 911's cornering on three wheels?

On an early car with equal tire sizes we may easly have one front tire contact to sacrafice. On a flaired car we may need some traction from that inside front tire to.

Dose my free association add up or make any sense?

Sorry for the spelling, my spell check died.

[911st]

Originally Posted by sww914

My dollar to speed ratio is almost unbeatable. I'm as fast around Willow Springs as a well driven Caman on DOT race tires for quite a bit less money. Also about the same as an RSA. With a $4000-$5000.00 car. It's not the nicest car out there.

Got my attention.

So what kind of specs or build dose a $5k 914 look like to spank the $60k Camans?

2.0 w carbs & cam, SC brakes, and striped to the bone?

[sww914]

Sorta, I built almost every inch myself with a couple hours help here & there from friends and I bought most of the parts back when synchros were still $15.00 instead of $75.00-$90.00. I used leftover paint, I couldn't even come up with enough of one color to do the whole car so it's 2 colors, I traded an old spraygun for the F. 912 suspension and calipers, I paid $100.00 for some slightly mismatched wheels, etc., etc., just go down the line, I almost never paid full price for anything and I still spent about 8K all together. As we know, what you spent on it has no relation to it's actual value.
It was a caged 2.0 sideshifter with a fuel cell for $800.00 to start. I did the brake and 5 lug conversion and flared it right off. I went through all the systems to make sure that everything was OK and I still toasted the engine within 5 events. I bought another blown up T-4 for the cam, carbs, light flywheel, crank, and some other parts, got the heads ported with 40mm & 50mm valves and rebuilt for $300.00 from a machine shop that I had referred a lot of work to and I bought some 96mm P's & C's. I built a 2056 with 10.5:1 compression and open exhaust, re-geared my tranny and added springs and torsion bars as well as Bilstein sports all around. I run 205/50 and 225/50 Hoosiers DOT tires. Smaller tires are cheaper. Everything else has been either safety related or oil pressure related for the most part.
Sorry to hijack your thread but you asked for it.