Rear Sway Bar Question

For those more knowledgeable than myself, can you think of a reason that on a car that exhibits understeer in stock form and has a front anti-sway bar, that the general rule of thumb would not include adding a rear anti-sway bar? In this particular case, this trend seems to be independent of spring rates or dampening, and in fact usually results in using a larger front anti-sway bar.
I know you will want more information, but I want to see what you guys come up with before adding any more detail. I'm just trying to understand a trend I've come across that seems to go against what I would normally do to correct understeer.

A simple rule of thumb is make changes to the end that is working. In the case of a car that under steers you would either stiffen the rear springs or add or stiffen the rear sway bar.

In reply to jimbbski:

Yes, I know. That's why I'm asking, because the common thinking on this particular car is to do the opposite of this with regards to sway bars.

Are you talking about the Mustang in your avatar? If that's the case, you add a bigger swaybar to the front because the suspension doesn't have enough (if any) camber gain in bump. So if your camber was set at zero, as soon as the suspension compresses, you start getting into positive camber.

In this situation, setting some static negative camber, and then trying to keep as much of it as possible by preventing roll (with a bigger swaybar) will reduce the understeer.

bravenrace wrote: For those more knowledgeable than myself, can you think of a reason that on a car that exhibits understeer in stock form and has a front anti-sway bar, that the general rule of thumb would not include adding a rear anti-sway bar?

Lots of reasons... look at most RWD stock-class autocross cars.

Roll control up front is more effective, and it doesn't screw up the car's ability to plant the power.

In reply to ProDarwin:

I'm not really asking about overall roll control, but more about balance. But wouldn't adding a larger front sway bar increase understeer instead of reduce it?

My rule of thumb is generally to add a sway bar to the end that ISN'T providing power. It has worked well for me in three builds now.

bravenrace wrote: But wouldn't adding a larger front sway bar increase understeer instead of reduce it?

Maybe if the car was already close to balanced. Most stock cars have tons of front-roll, and understeer is induced by horrible camber-gain. A bigger front sway-bar can keep this under control.

bravenrace wrote: But wouldn't adding a larger front sway bar increase understeer instead of reduce it?

Only if the rest of the system is working correctly. It's possible that increasing the front roll resistance is acting as an 'adhesive bandage' for an even larger shortcoming...Or maybe the common thinking on the chassis is simply wrong, being derived from one specific set of conditions but then becoming accepted and applied universally.

You guys are kind of getting ahead of the things a little here.
Forget stock or modified, or if the rest of the system is working well or not. Just answer this simple question - If a car is understeering significantly, why would you increase the size of the front sway bar in an attempt to decrease the understeer?

In reply to bravenrace:

I would not

As I understand it, if you are in a vacuum and know nothing about a car except that it understeers, adding roll stiffness at the rear would be the suggested move.

But the question isn't "what would you normally do?" The question being asked is "why would you add more front bar?" The answer to that is "because it's common for cars to roll badly enough that reducing roll by any means will increase front grip and improve balance" possibly with a side order of "assuming front weight bias and rear drive just because that's pretty common, you'd wreak more havoc on overall grip and forward traction by trying to do it at the rear."

People's getting ahead of the question by noting excessive body roll is because that's exactly what's indicated by the question's parameters.

I suspect my favorite answer would be "add more both", but that's also outside the scope of the question.

In other words, you can't just ignore whether the rest of the system is working. If it were balanced, you could fine tune with F/R bars' relationship. If it's way off, different rules apply, which is what raised the whole question in the first place.

I have this same question. A 2nd gen RX7 stock will understeer. The "hot setup" is to run a HUGE front swaybar and take off the rear bar.

I know with our car, when we stiffened the front, it did help turn in and eliminated understeer.

The car has a strut front end and I know the camber curves are bad and therefore if you can stop the front arms from moving, you eliminate the camber curve.

That is the only reason I see this working. (bad camber curves).

BTW, I would rather work on the end of the car that is NOT working properly. If you mess with the other end, you are reducing your overall ultimate grip. Having said that, the front end stiffness helped in my case.

Rob R.

In reply to aussiesmg:

Stiffer springs all round?

The whole situation seems to describe one of the specific areas where we're after a band-aid. A bigger front bar can be an improvement on a car which really needs a bunch of stuff.

I'm so glad we don't normally have to throw parts at cars in a complete vacuum...

In reply to ransom:

I understand and agree that in reality you can't ignore the rest of the system, but the reason I'm asking the question the way I am is because with this particular car, it's very common to add a larger front bar and not have a rear bar at all, regardless of what the rest of the system consists of. And to clarify, this is on a car where you have the ability to change anything you want. Not talking autocross classes or anything like that.

In reply to ransom:

With my RX5.0 and the Mustang I have no rear sway bars, much higher spring rates front to rear and huge front sway bars.

I like this set up as it turns in fantastically and I can turn on the rear end as both are LSD stick axles.

However the OP didn't ask that or give enough info to answer that way.

Unless its a mid mounted engine the reasoning is beyond me with the amount of information provided

Let me change my answer a bit...

When my 2nd gen RX7 first hit the track, we tried everything we could to help it out for that weekend.

My train of thought was much like yours.

We grabbed some spring rubbers and shoved them in the rear springs. We set the alignment to 1/8" toe out in the rear. We drilled new holes in the rear swaybar to shorten teh moment arm.

It pushed worse. And it had this sort of unstable feeling on turn in. No good.

We then went with the accepted way of making the car handle. We stiffened the front. We set the rear back to stock.

Car is now balanced and dances nicely.

I'd agree, more roll stiffness is needed, ideally front and rear, but a rear sway is not necessarily the best choice for doing so.

On my Volvo 240, I had 25mm sway bars front and rear. On soft springs, it wasn't terrible, but I'd hike the inside rear tire and lose the ability to put down the power. Moving to a larger 28mm front bar, stiffer springs front and rear, balance is improved but I still lose out on cornering finesse when the inside rear lifts. I took away the rear sway and the car is much more controllable. There might be a little more roll in the rear, but that pales in comparison to the ability to use the throttle to manage the car in turns.

If I knew nothing else:

Car plows, do something to make the front softer (smaller front swaybar, softer springs) OR do something to make the back harder (stiffer rear springs, adding a rear bar, or swapping for a larger rear bar).

Here comes the disclaimer: That is all IF the front springs aren't already too soft, the front bar too small, or poor alignment settings (insufficient tire contact in the front) that is causing the issues

However, most of my knowledge comes from reading books. I haven't been to an autosports event in almost half a decade.

Why the "car" secrecy though? We're all friends, you can tell us

Okay, let's get into more detail. This is about first gen Mustangs. When I bought mine, the suspension was stock except for the addition of Koni shocks. This is what I did:

• Dropped the upper control arm 1-1/8" to improve camber gain.
• Replaced all the suspension bushings with hard rubber.
• Installed rollerize spring perches and idler arm.
• Installed 650lb/in springs in the front and 180lb/in in the rear.
• Installed a larger front sway bar and installed a rear sway bar where there wasn't one previously.

I did this based on past experience, which while significant was not with that particular car.
Later I found that hardly anyone else was using a rear sway bar, and that included cars ranging from basically stock to cars that were more like mine.
Now, my car handles really well, so I've never been tempted to remove the rear bar, but I'm planning some other changes in the rear where I'm going to start having clearance issues.
Knowing that the universally accepted setup on these cars does not include a rear bar, I'm not thinking about removing it, but am hesitant to do it for two reasons. One is like I said, it handles really well now. Two is that I don't like making changes without fully understanding why and what the effect will be. Since this change goes against my prior experience, I need to ask those that have experience I don't.

One more thing to keep in mind. Typical street cars like mine tend to run much softer springs all around. 400-500 in the front and 120-150 in the rear. So Bill, my front springs are if anything too stiff.

I dunno, it looks like you put some thought into your suspension and you say your car handles "really well" now, so I wouldn't change a thing.

I guess if curiosity got the best of me I would remove the rear swaybar, do some spirited driving and see what my butt told me Depending on what modifications would warrant removal of the rear bar (for clearance) it may be worth it.

Sorry for the generic answer. I'm afraid my lack of experience is beginning to show.

Good luck!

-Bill

what is interesting with the stang is the stick rear axle. The wheels do not gain or lose camber towards each other or to the road, but will do the body of the car when it leans.

It would seem to me that traction in the rear is not the issue, but the lack of traction in the front. If you can keep the front wheels from gaining (or losing) too much camber when turning, you will gain traction and keep it from plowing

I'm very curious to see what you learn here. I've been very curious about the tradeoffs in terms of gaining sufficient rear roll stiffness via bar vs spring; I have the impression that the bar is supposed to cause more problems with putting down power with the inside wheel, but for a given level of roll and roll stiffness, the weights across the wheels would seem to be fixed, and then you're just getting increased wheel rate... (?)

Mad - My car has very limited front suspension travel, as the springs are really more suited to racing than street. I run a healthy amount of negative camber static camber in the front, but I don't think it changes much in corners. Ransom - I forgot to mention the trac-loc rear. No real problems with the inside tire hooking up. One behavior I have noticed is that sometimes the rear will hop in a corner, and not because of a bump. It seems to do it most when powering out of a corner. It almost seems like it should be breaking loose,but instead hops and then grips again. That may change when the new engine is installed, which will have double the power of the current one.