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Login to post Forums » Grassroots Motorsports » Intelligent Conversation about twist beam axle sway bar

June 5, 2009 12:43 p.m. adaycj None

I'm looking for ideas and feedback about anti sway bars (ASB) on twist beam axles.

First let me define my definitions, because looking up information across the internet and in reputable publications seems to be hindered by a confusion of terms.

I have a twist beam axle. It is a "c" channel solid axle that flexes (twists) some. There is no torsion bar in it, or attached to it. The car is suspended above the axle by coil springs. Sometimes this is called a torsion axle, which can be very confusing because there are other solid rear axle types that use torsion bars or rods.

I intend to control body roll by adding an ASB in the rear of my Scion xD. My goal is to limit under steer under hard cornering. My secondary goal is to limit body roll at less hard cornering.

Toyota sells a part marketed as an ASB that has no connection to the body of the vehicle. It bolts into the c shape and is simply a beam stiffener. my analysis indicates that this reduces the twisting of the beam, but does little or nothing to limit body roll. It also greatly reduces the twist of the beam, negating much or all of the "independent" action of the rear suspension.

Please post your knowledge of using an ASB on a twist beam rear axle. I am very interested in a universal kit, if one is available. I can find numerous examples of solid rear axles with an ASB, but I have not yet found a OEM twist rear axle with an ASB. I'm am interested to hear about OEM examples.
June 5, 2009 12:51 p.m. iceracer Reader

If it limits the twisting of the beam then it should limit body roll. A stiff rear bar ,even on independent suspension, negates some of the independent action.
I have a 22mm rear bar on my ZX2 so I experience that. I like the handling, so I live with it.
June 5, 2009 1:03 p.m. Per Schroeder Technical Editor/Advertising Director

What Iceracer said. Your car can't lean w/o that rear beam twisting. Reducing twist = reduced roll. That's how the Improved Touring VWs have their rear set up--It's a light and simple way to add roll stiffness.

The VW GTi/GLi beam (A2, some A3s) have an extra welded-in tube that functions like this as well from the factory.
June 5, 2009 1:20 p.m. mw Reader

Your twist beam axle is essentially an anti sway bar. Reinforcing it will do the same as adding a seperate anti sway bar, but is often easier since it does not require additional mounts etc. I cannot see any advantage to adding a seperate ASB if you have the means to stiffen the twist beam effectively.
June 5, 2009 1:42 p.m. jstein77 HalfDork

That's the same way my Sentra is, and it has tons of rear roll stiffness (as evidenced by the three wheel stance in every corner). It certainly works to limit understeer, but can cause oversteer in certain circumstances, particulary on drop throttle conditions.
June 5, 2009 1:52 p.m. fiat22turbo SuperDork

Yep, built my own adjustable rear stiffening plate for my twist beam rear axle. Just needed a flat plate of steel and some exhaust clamps to clamp it to the bottom of the torsion bar. To make it more effective, I'd probably use a leaf from a leaf spring or build a couple of mounts for a torsion bar from a VW or 944/924.
June 5, 2009 6:56 p.m. mad_machine SuperDork

on this same comment (not to thread jack) anybody have any experience with welding two swaybars together to increase the spring rate of the bar?
June 5, 2009 7:08 p.m. erohslc New Reader

AFAIK, you don't gain much (if any) by welding them together vs just running them in parallel (hook the ends together, and use common mount points). With single solid bar, OTH, the stiffness increases as the 4th power of the diameter increase.

Carter
June 5, 2009 9:51 p.m. YaNi Reader

Something like this?

I built my own rear sway bar for my cadavalier. Worked like a champ, but the weak sauce chassis started squeaking and squawking over every bump.
June 6, 2009 9:17 p.m. adaycj New Reader

Dang I wish I could see that picture.
June 7, 2009 8:23 p.m. ae86andkp61 Reader

I have a TRD sway bar on my Toyota Matrix. It is as you describe for the Scion, just bolts into the axle (doesn't attach to body) to increase roll stiffness. I also have the TRD sway bar up front and the handling balance seems to have a bit less understeer and a bit less body roll than I might expect for a cheap FWD econo-wagon. Springs and shocks are stock, tires are sticky street tires in stock size.

With a bit of damp on a long sweeper, plus dropping the throttle at the right moment I can get the tiniest bit of oversteer for a brief moment. I would say it took the handling from boring terminal understeer to almost acceptable for an enthusiast driver.
June 8, 2009 8:36 a.m. jcanracer New Reader

Hey AE86, i also have a Toyota Matrix with similar suspension mods, ( Progress rear sway bar, TRD strut tower bar, but i also threw on the Eibach sportline springs which were worth every penny).

To the OP: Rear sway bars like the one you describe, work well for us matrixes and should work well for you too.
June 8, 2009 11:10 a.m. Ian F HalfDork

Common set-up. Compact, inexpensive and reasonably effective for most driving needs. There are a couple of vendors selling RSB's for VW's that simply bolt into the existing C-channel. Shine seem to be the most popular. From my understanding (I personally don't want to take the ride penalty) - very effective. Oddly enough, Shine does/did not recommend his bar for use with stock springs. Apparently puts additional stress on the bar and C-channel.
June 9, 2009 2:26 p.m. Panzer New Reader

I run a Shine bar on my MKIV VW. While Dick doesn't recommend it be used with stock springs, he recommends that it be used with his own springs which are a hair stiffer than stock. (180f vs 225f and 140 rear vs 180 rear stock vs Shine respectively - all numbers in lb/in).

My car came with an OEM bar in the beam, and I've added the Shine bar on to it. I've run the Shine bar on OEM suspension, Shine suspension, Tein Coilovers (390 f, 360 rear), and Koni yellows with Ground Control sleeves (430 front, 425 rear).

For the poster who didn't want to take the ride penalty, that's the whole point of a sway bar. The proper way is to add spring rate until the car handles the way you want to, but this results in an incredibly stiff car in most cases. With sway bars you can augment the rates of your springs while still being able to roll over a speed bump without compressing your spine.

There is no ride penalty with the internal type bars.

That said, it's lighter and more effective than a bar that goes externally, and doesn't reduce ground clearance. I've never wanted more rear bar either.
June 9, 2009 11:15 p.m. doc_speeder New Reader

With a stiff rear sway, ride quality does not suffer if BOTH WHEELS hit the bump at the same time. One wheel bumps or diagonal bumps will hit harder, as much of the independance of the rear suspension is removed. On an old Mk2 Golf beater I had, I welded a 3" chunk of flat stock to the open side of the twist beam, about 24" long. I had lots of people tell me not to do it, but it was AWESOME. I'd do it again ( and probably will with my current Mk3 project when I get to that point).
June 10, 2009 10:52 a.m. Ian F HalfDork

Panzer wrote: There is no ride penalty with the internal type bars.

I'm sorry, but simple physics says otherwise. If you decrease the ability of the rear wheels to move over bumps independantly, you WILL have a decrease in ride quality. Let's face it, that's basically what an aftermarket RSB does, regardless of design.
June 10, 2009 3:56 p.m. problemaddict Reader

Here is the swaybar on my $1500 Challenge car, Plymouth Sundance.

The endlinks connect to the frame. Seems it may have been stronger if the endlinks connected to the axle and the u-bar connected to the floorpan somehow, but that's how the instructions said to mount it.

I can say that it made a marked improvement in the handling/balance of the car. Lift-throttle-oversteer was easily achieved and made the car easier to drive and much more fun.

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