Why Sway Bars and Torsion Bars?

[autobonrun]

First, mark page 162 of the October 2003 issue of Excellence. It contains a table with the stock sizes of all Anti Roll Bars and Torsion Bars used on 911s. Seems this question always comes up.

However the use of the term Anti Roll bar for Sway Bar got me thinking. I know that shocks dampen the up and down movement of the wheel and to a lesser extent help control body roll. It seems though that the Anti Roll Bars and the Torsion Bars perform the exact same function; eg. reduce body roll through curves. If so, why are both used? I'm sure I'm missing something but I'd like to know what.

Also, after shocks, why do most members on this board, me included, usually increase the size of the torsion bars rather than the Anti Roll Bars during suspension upgrade projects. After what I went through accessing my rear torsion bars, I'm sure the work involved with removing the Anti Roll bar would be substantially less than removing the torsion bars. Couldn't the Anti Roll Bars just be increased and leave the torsion bars stock?

[dotorg]

Torsen bars are linear springs -- they're what hold the car up. The shocks dampen the springiness of the torsen bars so your car doesn't keep right on bouncing after you hit a bump.

Anti-roll bars/sway bars are designed to reduce via a similar torsion motion the independance of the two sides of the suspension.

Bigger torsion bars are the equivalent of putting higher rate springs on a coil-sprung car. Bigger sway bars serve to reduce the independance in the suspension, which reduces body sway, because the compression energy is transmitted in part to the other side of the car.

[porcupine911]

The torsion bars are the springs that support the weight of the chassis.
The sway (anti-roll) bars effectively transfer weight from one side to the other during cornering.

-chris

[911pcars]

If it's an easy DIY job you're looking for, upgrade with larger sway bars or buy stickier tires. However, easy is not always the ideal solution. It's been my experience that larger (not necessarily the largest) torsion bars have more of a positive affect on handling and less of a negative effect on ride quality than if one were to just increase the size of the sway bars and keep the stock torsion bars.

Suspension improvements might be done in this order:

1. shocks (replace worn or upgrade)
2. height adjustment (and F/R alignment)
3. upgrade torsion bars, install sway bars (if none)
4. height adjustment (and F/R alignment)
5. upgrade sway bars
6. corner balance and F/R alignment

Install tires of choice before adjusting height.

MHO,
Sherwood Lee
http://members.rennlist.org/911pcars

[jluetjen]

The best description that I've seen for the different functions of the parts that you mentioned and the affects on handleing was written by Carroll Smith and published in his "...To Win" series as well as the "Engineer in your Pocket". A real brief attempt at answering your question...

As mentioned earlier, T-Bars (ie. springs) hold up the car. Shocks keep it from continuing to bounce in harmony with the frequency of the chassis and the road. Both of these can be used to tune the handling of the car in addition to the basic functions listed.

(Anti-) Sway Bars is really not an accurate description of the function of sway bars. Their function is almost entirely to tune the handling of the car, specifically the front to rear balance (meaning the under/over steer charactoristics of the chassis). They do this by transferring the cars weight when cornering to the other end of the car.

A simple example. Imagine that you have a car with no sway bars. When you corner to the right the weight from the right front will transfer to the left front corner, and the same will occur at the rear. If your car is understeering (so the left front is losing grip because it can't handle the additional weight transfered to it), adding a REAR sway bar will help to aleviate the understeer. With a rear sway bar, a portion of the weight transferred off of the right front will be transferred to the left REAR tire, thus reducing the weight transferred to the left front and as a result reducing the understeer.

Who said anything about sway?

[autobonrun]

Originally posted by 911pcars
If it's an easy DIY job you're looking for, upgrade with larger sway bars or buy stickier tires. However, easy is not always the ideal solution. It's been my experience that larger (not necessarily the largest) torsion bars have more of a positive affect on handling and less of a negative effect on ride quality than if one were to just increase the size of the sway bars and keep the stock torsion bars.

Suspension improvements might be done in this order:

1. shocks (replace worn or upgrade)
2. height adjustment (and F/R alignment)
3. upgrade torsion bars, install sway bars (if none)
4. height adjustment (and F/R alignment)
5. upgrade sway bars
6. corner balance and F/R alignment

Install tires of choice before adjusting height.

MHO,
Sherwood Lee
http://members.rennlist.org/911pcars

I've done the first 4 steps in just the order you mention. I think before I go to step 5, I'll get a couple track days to see just how the current setup handles at speed. From the other posts, my take is that the torsion bars are more akin in function to springs and that torsion bars and sway bars perform similar but different functions. I'm thinking the torsion bar reacts after the body has transferred weight. The sway bars limit body weight transfer in the first place.

But assuming I stiffen the front and rear sway bars by the same percentage, I will induce more understeer by reducing the amount of weight transfer to the wheel doing the steering. If after doing time at the track, my current setup still oversteers, I'll increase the front sway bar more than the rear. This should transfer more of the weight to the rear wheel away from the front, inducing more understeer. I think I understand it now. I assume if I do change sway bars, I should go with adjustable Weltmeisters?


thanks

[jazzbass]

Suspension improvements might be done in this order:

1. shocks (replace worn or upgrade)
2. height adjustment (and F/R alignment)
3. upgrade torsion bars, install sway bars (if none)
4. height adjustment (and F/R alignment)
5. upgrade sway bars
6. corner balance and F/R alignment

Where would you put "pain in the ass bushing replacement" in this list (if at all)?

[masraum]

Actually if they are old and worn then they should be in the number 1 or 2 slot since the rest are not accurate without decent bushings.

[dd74]

I know guys whose torsion bars are so large, particularly in the rear, they opt to not run sway bars at all. If anything, this relieves them of some weight.

One consideration is, of course, the roads you drive on. In L.A., on fast freeway jaunts, I can imagine the ruts and gaps bouncing a thick-torsioned car all over the place. That is unless it has the weight to necessitate bigger torsions, such as a larger engine; a 3.6 for example.

[jluetjen]

autobonrun;
Before you throw more money at the car, could you explain in more detail about what you mean by "oversteers".
* Does it do it on corner entry/middle/exit?
* Does it do it when braking/trailing throttle/neutral throttle/on the gas? When are you picking up the throttle?
* What speed corners - high/medium/low?
* Is the pavement rough or smooth?

Where would you put "pain in the ass bushing replacement" in this list (if at all)?

-- # 1 or # 2 since if your bushings are bad, the rest of the adjustments will be worthless.

I will induce more understeer by reducing the amount of weight transfer to the wheel doing the steering.

Actually Autobonrun, your logic is wrong but your predicted outcome is right. Stiffening the front sway bar will generally increase understeer BECAUSE YOU ARE increasing the weight transfer to the wheel doing the steering. In most cases, tires lose traction because you are asking them to do too much, not too little. Increasing the weight transfer to a tire is asking it to do more. The trick is to get all 4 tires working to their maximum to achieve maximum performance.

[304065]

And now, The Global "Why?" for torsion bars. . . to be vintage-racing eligible.

Torsion bars are cool, they put the weight down low, but are difficult to adjust at the track, when you need ease of adjustability because you don't have TIME to screw around figuring out the geometric formula for ride height. I always wondered why Porsche went to coil-springs for their race cars in the EARLY 70s, this is why.

Also, I saw a guy with lightweight 911R clone that made me drool like a St. Bernard, he said the reason he built it with Torsion bars was he needed to remain vintage-legal. He could get the weight down even lower (into the 1700# range) by going with lightweight aluminum 935-style pieces. . . for a modern Super Class car there's no alternative to coil-overs.

[1fastredsc]

With sways you don't need adjustables front and back as long as one of them are adjustable. This way you can tweek the steering characteristics of the car depending on whether you like some over or understeering. BTW, to all the suspensions guru's, since i'd like to keep a street feel to my suspension, do bigger sways make a noticable improvement?

[autobonrun]

John, where I noticed oversteer on my car was at the entry to a high speed corner just after hard braking just as I get back on the throttle. I'd like to see more push at this point. Again, before I touch anything else, I need to put it back on the track as I have not tested the previous changes I've made in a track environment.

Typically the track surface has been smooth. I'm somewhat confused when you state stiffening the front sway bar increases weight transfer to the wheel doing the steering thus increasing understeer. I was under the impression that the stiffer front sway bar would help prevent weight transfer to the outside wheel, which in a turn, is the one doing the steering. By placing less downforce on that wheel, wouldn't it provide less steering? Likewise, I thought if I increased the rear sway bar diameter, I would tranfer less weight to the rear wheel and that weight would instead have to go to the front wheel which would increase oversteer. This is why I thought if I used a slightly higher percentage increase on the front over the rear, I would get less body roll in total (since both diameters are being increased), yet still reduce the amount of oversteer.

[porcupine911]

Originally posted by autobonrun
John, where I noticed oversteer on my car was at the entry to a high speed corner just after hard braking just as I get back on the throttle. I'd like to see more push at this point...

to change the behavior during a transitional manouver such as turn-in (versus during a steady-state sweeper), you should make changes to the damper (shock) settings. to reduce turn-in oversteer you could try backing off the rebound on the rears.

fyi, we run a little rear toe-in on our autox miata to be able to get on the throttle early and hard.

-chris

[masraum]

I know guys whose torsion bars are so large, particularly in the rear, they opt to not run sway bars at all. If anything, this relieves them of some weight.

Hmm, I'm far from an expert, but that doesn't seem that smart. They perform two different jobs. If a race car has both then I would think you would want both on a street car.

Just a side, but related note, When Ruf built the yellowbird ctr they used stock size torsion bars but different sways and that was good for over 200 mph.

[dickster]

autobonrun,

i think you got it.

i bought 22/31 t/bars, but was advised that 21/31 would be a more lively setup!

increasing the front diameters induces understeer because less weight is transfered to the outside front wheel.

[jluetjen]

Hmm, I'm far from an expert, but that doesn't seem that smart. They perform two different jobs. If a race car has both then I would think you would want both on a street car.

To every rule there is an exception. No less of a designer then Gordon Murrey prefers to designs his race cars WITHOUT rear sway bars. Others like to use them. You can draw your own conclusions.

John, where I noticed oversteer on my car was at the entry to a high speed corner just after hard braking just as I get back on the throttle. I'd like to see more push at this point.

I agree with porcupine911 that this sounds like a shock setting issue, not a sway bar issue. Depending on the speed and your rules, aerodynamic downforce (or rather reduction in lift) at the back might also be helpful.

I'm somewhat confused when you state stiffening the front sway bar increases weight transfer to the wheel doing the steering thus increasing understeer. I was under the impression...

You are correct that if a tire is too lightly loaded it won't get good grip. The reality is that this occurs in very few if any cars. The SCCA Spec Racer which is seriously overtired may be one rare example.

If you look at a tire's traction profile based on the weight on the tire, you will find that it is basically an arch shaped curve, just like a HP curve. Up to a certain point more weight will increase traction, beyond which it will drop off again. The peakyness of the curve as well as the absolute level depends on all of the factors involved in tire construction as well as the suspension geometry. If you are driving a car at it's limit, then the limiting factor will be the tire which has reached it's peak peformance at which further force (either weight, accelerating or braking) will reduce the available corning force. Once you've reached that point the only way to corner harder is to either increase the available corning forces by reducing the accelerating or braking forces by changing the driving style or reducing the weight on the tire. The only way to reduce the weight on that wheel be transferring it to another tire. The way to do that is to stiffen up the opposite end of the car in roll (by stiffing the sway bar at the other end) which then transfers some weight to one of the other less highly stressed tires.

Does that help?

[jluetjen]

PS autobonrun; Another cheap option that might help you car point into the corners better on corner entry is to dial out some of the front toe-in (assuming that you have some) or even try about 1/8 of an inch of toe out. It will make the car wander and maybe be a bit twitchy at speed, but you'll be surprised how much faster the transistion to the corner will be.

[dotorg]

Keep in mind that a race car has far less need for a anti-sway bar than a street car. One of the modes in which it enables weight to transfer from corner to corner and front to back is via the stiffness of the car in general. A car will a full cage is substantially stiffer than a "street" car, and has FAR less need for them.

Thats why a lot of race cars don't use them, they're just not needed, at least to the extent you'd otherwise. A combination of race springs that are transferring most of the energy to the chassis instead of being absorbed in the suspension travel, and a stiff chassis removes the need for them.

[jluetjen]

Uh dotorg;
I'm not sure if I agree with you. If you are running really stiff springs and/or sway bars you ABSOLUTELY need a stiff chassis and cage. Otherwise the chassis flex will absorb any of the weight transfer that you are trying to accomplish with the bars and springs. The result is a car that doesn't respond to spring and/or sway bar changes. Not to mention that the chassis is not dampened like the suspension is which can create all sorts of strange movements resulting in the car feeling like it is "walking" sideways in a corner. I once raced a Crossle S2000 which had this problem and it was sort of weird the way it moved around on the banking.