Torsion Beam Stiffeners and Autocross Classification

Hey all,

I'm a distant autocross admirer, hoping to be involved once I have space in my schedule and budget for it. While looking up suspension upgrades for a car I don't intend to autocross, I came across an odd part, and it got me thinking about how it might factor in to autocross classing if it were used.

The part in question is Autoexe's Torsion Beam Stiffener for the 2019+ Mazda 3. I have no idea what kind of effect it actually has on handling, but let's say it operates as it purports to. Would a car using this part, or another one like it, be able to remain in its Street class? I know that the addition or replacement of a rear sway bar, which would theoretically have a similar effect, is allowed. Would this be equivalent to messing with a rear sway, adding a member brace, or something else entirely?

Thanks in advance for your patience with a question from a spectator.

Not exactly sure where it would land, but definitely not street class legal.

I am trying to figure out what that even does.  Is it supposed to block the suspension bushings from moving laterally?  That would eliminate the engineered-in kinematics.  Basically the same effect as giving the rear suspension static toe-out, as it appears to block the lateral motion that steers the axle to the outside of the turn with lateral load.  (This is necessary to counteract beam deflection, VW has been doing it on their beam axles since the beginning)

I do not see that as changing roll stiffness in any way.

Bolting on a rear bar into most of the older VW's was just a massive bar bolted to either end of the torsion beam. Wouldnt it be classified as a rear bar as its bolting a piece between two points on the rear axle?

This isn't stiffening the beam but the mounting points. If you put stiff bushings in place you have a pretty decent lever arm and just these mounts controlling lateral axle movement. No idea if the mounts flex enough for it to matter but the theory is there 

as i see it, that's effectively chassis bracing from a classing standpoint and is a non starter for street class, i think it bumps the car to SP?

if it was stiffening the beam it would be a sway bar.

kevinatfms said:

Bolting on a rear bar into most of the older VW's was just a massive bar bolted to either end of the torsion beam. Wouldnt it be classified as a rear bar as its bolting a piece between two points on the rear axle?

The wheel end of the beam? If it influences suspension geometry then it's not legal.

In reply to dps214 :

The middle of the beam, turns it from a C channel into a tube.  I used a piece of 3" by 36" plate and welded it in.

It was hilarious, could lift the inside wheel by jerking the steering while turning at the Taco Bell drive-thru.  Did not appreciably alter handling, though, so it came back off.

spacecadet (Forum Supporter) said:

as i see it, that's effectively chassis bracing from a classing standpoint and is a non starter for street class, i think it bumps the car to SP?

if it was stiffening the beam it would be a sway bar.

This is my take as well.

I made a 3/4" simple bar with brackets that bolt into existing holes in my 2010 Hyundai Accent.  I have a length of 7/8" bar to make another.

dps214 said:

kevinatfms said:

Bolting on a rear bar into most of the older VW's was just a massive bar bolted to either end of the torsion beam. Wouldnt it be classified as a rear bar as its bolting a piece between two points on the rear axle?

The wheel end of the beam? If it influences suspension geometry then it's not legal.

On the Fiesta ST for example there are two types of bars - on that bolts into the beam center and the type that bolts to the spring pockets.

I would assume the first type that bolts into the main portion of the beam would be considered a sway bar as its effectively accomplishing the same thing as the bar that bolts into the spring pockets of the torsion beam just at a reduced rate?

Both bars stiffen the rear torsion beam. Shouldn't both be considered anti-sway bars? Thus effectively placing the car into street class?

Just finding the semantics in the rule book and how its interpreted.

SkinnyG (Forum Supporter) said:

I made a 3/4" simple bar with brackets that bolt into existing holes in my 2010 Hyundai Accent.  I have a length of 7/8" bar to make another.

 

So this would be considered a sway bar correct?

Yes.

In essence, it's stiffening the torsion beam.

I don't see the point of reinforcing the mounts, unless they are a known weakness (tearing out??).

kevinatfms said:

SkinnyG (Forum Supporter) said:

I made a 3/4" simple bar with brackets that bolt into existing holes in my 2010 Hyundai Accent.  I have a length of 7/8" bar to make another.

 

So this would be considered a sway bar correct?

Functionally, yes. But within the SCCA street class rules, it's probably at least in a gray area. The fact that it physically attaches to the "arms" of the torsion beam means it's having at least some slight impact on suspension geometry as the beam moves, which isn't allowed.

That's a shame.

In reply to SkinnyG (Forum Supporter) :

So.... you ready to move in from this project?