Sway bars. How big is too big?

[Marvelous3]

ELI5.

Stock bars on NA cars were 21mm and turbo's 23mm. There are upgrades available in 25mm and 27mm. From a handling point of view, why would one choose one side bar over the other? If we're trying to prevent body roll, why not just throw a biggest bar on that will fit and send it? Why would I not want a rear bar on my wagon versus having one on my sedan?

 

[740atl]

In my extremely limited understanding of suspension, with properly stiff springs (and of course shocks correctly damped), the need for swaybar thickness diminishes to control body roll. IIRC, too thick of a bar will also cause suspension bind.

In my application, (240, heavy v8 over the front wheels, ipd f/r 25/25 bars), it was generating way too much oversteer in the corners. The advice-fix given to me was to decrease the size or remove entirely the rear bar. This had the effect of increasing the amount of understeer which for my amateur driving ass is way more predictable...Oversteer scares the crap out of me....maybe not others. The other advice I got upon complaining about the oversteer was to simply learn how to drive.. which I took badly at first but with time I see the wisdom in it.

Others will come along and do a much better job of explaining.

 

[shoestring]

In a nutshell, springs are a gross handling adjustment tool, sway bars are more of a fine adjustment tool. What you're looking for is a balance that you're happy with and that's fun for you.

I'm also not a fan of oversteer and I think that to really be able to exploit it you need to be more of a pro driver than anybody I know, and probably anybody you know too. "Learn how to drive" is kind of an asinine comment implying that the only way to drive quickly is to hang the tail around every corner and until you can do that you're not a driver. What understeer really allows you to do is to learn how to be precise and repeatable, and that's the skill you're really trying to develop. I'd rather have a car that has just a hair of understeer in it if I'm going to be in the seat for a couple hours. It's easier to drive, which is less tiring. If you're less tired, you're more aware of what's going on around you, which is better for everybody.

 

[culberro]

Swaybars transfer spring load from one side of the car to the other, which can be great or it can reduce total grip substantially.

They reduce inside tire grip on corners, but they reduce body roll induced camber changes on the tire. If you have a car with crazy toe/camber changes throughout suspension travel, sway bars are generally an upgrade.
If the car does not make wild changes in camber/toe, sway bars will usually make a car have less grip unless you are on perfectly smooth roads.

On a 240 a stiff rear bar reduces rear grip... a lot. On the front they are OK to a point in my experience. A stiff front bar has the benefit of transferring load to the inside rear tire, which can really help a 240 on track if you are traction limited.

 

[Marvelous3]

Swaybars transfer spring load from one side of the car to the other, which can be great or it can reduce total grip substantially.

They reduce inside tire grip on corners, but they reduce body roll induced camber changes on the tire. If you have a car with crazy toe/camber changes throughout suspension travel, sway bars are generally an upgrade.
If the car does not make wild changes in camber/toe, sway bars will usually make a car have less grip unless you are on perfectly smooth roads.

On a 240 a stiff rear bar reduces rear grip... a lot. On the front they are OK to a point in my experience. A stiff front bar has the benefit of transferring load to the inside rear tire, which can really help a 240 on track if you are traction limited.

So, let?s say we had a 240 with standard bne coil overs and stiff springs. 300# front and 200# rear. Compared to say a stock suspension spring set up. You would need less sway bar with the stiffer spring set up versus the stock set up? Gross oversimplification i know.

 

[culberro]

Less stiff bar. The springs are increasing the roll stiffness as well, but don't negatively impact grip.

I ran a big front bar on the drift car with 250# front springs and no rear bar. Lots of rear grip, but a tendency to understeer with the welded rear axle.

 

[dl242gt]

Understeer plows and oversteer is thrills and chills. With the stiff rear bar and 200lb rear springs on the turbo. Oversteer is available but it still feels pretty grippy and I would still have to be pushing it hard to oversteer the car. That is with good tries like those old Potenza S03. With the Michelin Defender roll around tires on there now. There isn't much rear grip and I am very careful.

I like the idea of lowering the rear bar size to get less bind and a bit more grip when the suspension is articulating. Not sure what size to try.

 

[classicswede]

What size bar to use comes down to driver more than anything.

what is important is size balance between front and rear. Same size front and rear is going to tend to put you into oversteer, a little smaller is more neutral handling and a lot smaller is scary.

What is scary is a Nivo equiped car that has been converted normal springs and dampers but not had a rear anti roll bar fitted as part of the conversion. What you end up with when pushing it a little on corners or going around S bends is the rear end will wag and be VERY unpredicatable

If you are comparing the IPD 25mm bars to my 27mm bars the roll stiffness is the same. IPD use solid bar and my bars are tube. The advantage of tube is drastically less weight. The downside is heavy corrosion will weaken them to the point where they they could fail where a solid bar would just loose performance.

 

[Swedbrick]

From personal experience, I really like a smaller bar out back, I ran the ipd 25/25 for a while and then moved back to a 25/19. Especially when I installed the differential locker the difference was noticable, the smaller bar allowed the rear suspension to rotate and work where the bigger rear bar made it really tailhappy.

 

[OVERDRIVE]

Choosing what bar you want is a preference thing, too bad there aren't adjustable 240 bars (that I've seen). I met a guy once who had 25/25's on his wagon, and he said they were a life saver. His reasoning was he said his daughter was driving the car and it did 2 full spins and went off the road on a highway onramp, and the car didn't flip over. I didn't feel like telling him that the swaybars were most likely what caused the car to lose the rear in the first place. Tires also have a big play, a bigger/stickier tire will increase body roll at the limit of grip compared to a skinny all season.

 

[Cwazywazy]

It's mostly preference, but if you want some real fun get a set of 25/25 bars and then put those solid poly rear trailing arm bushings in. Get some IPD torque rods with the solid poly bushings to put the cherry on top of your drift missile.

I'm currently running 25/19 I think, on my 244, feels pretty good to me but I really didn't notice a big difference just driving around on the street compared to 21mm front bar.

 

[HiSPL]

Here's the thing. Lots of people on all kinds of cars go for sway bars as their first upgrade. And this works via placebo effect. Yes it dramatically changes the way the car FEELS in a corner, but it most likely didn't make the car corner faster, and in fact most likely reduced your maximum cornering speed.

A car that leans a lot matters very little in the grand scheme of max corner speed. As long as that car can keep the wheels flat on the ground I'd take a car that leans a lot over one that is perfectly flat but lifts the weight off the inside tires.

Race cars get their chassis dynamics to a really good point that is like 98% "there" but then you have to contend with fuel burn causing a shift in weight balance and the tire wear causing a handling mismatch throughout the race. They "fix" these issues with adjustable sway bars that can be increased or decreased front and rear to restore a neutral oversteer/understeer balance. This adjustment is actually "throwing away" grip on that end of the car by causing that end to be stiffer and less able to weight the inside tire. This in turn causes that end of the car to match the other end of the car and keeps handling neutral and easier to drive which equals faster lap times and less driver fatigue.

 

[rb92673]

I don't know much about sway bars, but I do know our endurance race 745 has a big bar in front, no bar in back, 300/200# springs front/rear and race konis all the way around. It pushes in corners and does not have enough camber to wear tires evenly. It is also extremely predictable and most of time when you lose control of the rear end you can get it back in line. I worry that adding a rear bar will make it less able to control when it gets out of line and increase inside rear wheel spin on tight right hand corners.

I am tempted to through a rear bar on to see if it helps with turn in and wheel spin. If it doesn't it's pretty easy to disable.

 

[2manyturbos]

Think of your sway bars as a way of tuning your car's handling. I would throw a wagon rear bar on your endurance 745 and see if you like the result. On Greg Ervin's 740 sedan he ended up with a 25mm front bar and 16/17mm wagon rear bar. It was close to neutral with that combination. It has coil overs all the way around. On my 2019 Mustang GT I run a larger than stock adjustable rear bar to get my car closer to neutral. It has a Torsen differential, therefore, no problem with tire spin. It is also IRS equipped which lowers unsprung weight significantly. IOW, it stays in contact with the surface much better than a live axle car.

 

[rb92673]

Think of your sway bars as a way of tuning your car's handling. I would throw a wagon rear bar on your endurance 745 and see if you like the result. On Greg Ervin's 740 sedan he ended up with a 25mm front bar and 16/17mm wagon rear bar. It was close to neutral with that combination. It has coil overs all the way around. On my 2019 Mustang GT I run a larger than stock adjustable rear bar to get my car closer to neutral. It has a Torsen differential, therefore, no problem with tire spin. It is also IRS equipped which lowers unsprung weight significantly. IOW, it stays in contact with the surface much better than a live axle car.

I will put that on the list to try. I have an open diff, so as soon as the right rear lifts, it spins.

 

[spock345]

It's mostly preference, but if you want some real fun get a set of 25/25 bars and then put those solid poly rear trailing arm bushings in. Get some IPD torque rods with the solid poly bushings to put the cherry on top of your drift missile.

I'm currently running 25/19 I think, on my 244, feels pretty good to me but I really didn't notice a big difference just driving around on the street compared to 21mm front bar.

I did polyurethane bushings in the rear with 25/25 bars for a bit. Don't forget rear overload springs. That thing was really tail happy. Suffice to say I learned my lesson with suspension part selection.

 

[dl242gt]

When I had my Porsche 356 coupe. I tuned the suspension by using a front 19mm anti sway bar and in the rear I installed a 13mm Z bar. Bilstein shocks and wider tires made it very predictable and it got me out of tail wagging situations more than once.

The Z bar acts as both a camber compensator and a rear anti roll bar. The camber compensator stops the jacking effect from the swing axles.

 

[mikep]

In my experience, Z bars give no anti-roll, only limiting droop that could cause bad juju at the crest of an off-camber turn.

ELI5.

Stock bars on NA cars were 21mm and turbo's 23mm. There are upgrades available in 25mm and 27mm. From a handling point of view, why would one choose one side bar over the other? If we're trying to prevent body roll, why not just throw a biggest bar on that will fit and send it? Why would I not want a rear bar on my wagon versus having one on my sedan?

Wheel/tire roll on a strut car (or on a properly designed upper-lower control arm car) is not coupled 1:1 to the body lean. (front of a 2-7-9 series volvo) There is camber gain, so the tire/wheel leans less. To work the best (predictable, etc) the bar should not increase drastically without an increase in spring rate. Otherwise you can lift an inside tire, and the outside tire will lean more without the camber gain from compression. You can also get side-to-side oscillations and non-linear push situations.

On the rear of a solid axle car, you lose a slight amount of camber from tire deflection, then a sudden large camber loss as a wheel lifts. So soft springs with stiff bars are sometimes used when a single larger bar is the only allowable modification according to a rule set, and it CAN inmprove handling overall in some cases listed already (car that pushes badly, car that can't put down power on corner exit)

And as stated, the changes that matter most to least are Springs, Sways, Tire Pressure in order, and that is assuming a somewhat properly set up car in the first place.

What size bar to use comes down to driver more than anything.

what is important is size balance between front and rear. *edit

That is the most important thing in spring and bar selection. Roll Coupling and Balance..