Who runs a rear sway bar?

[mwood]

How many have gone to no rear bar on their autocross cars? Why or why not?

I've just bumped my rear spring rates from 350# (which isn't really accurate, as one tested at 314# and the other at 336#) to 375# Hyperco's. I'm going to experiment with the bar disconnected, first, but seems (back of envelope) that wheel rate is going to be close to the old springs with the bar at a softish setting. If I could get rid of the bar and hardware, it should help inside wheelspin and also drop a few pounds (I'm about 12# over, with 7-10# of it being excess fuel). Only thing, I've never driven the car on fresh tires, which I now have mounted up and ready to go, so I'm likely to be off on set up, in any case.

Anyhow, curious to hear thoughts...

[Neil_Roberts]

New tires will completely transform the car, so you probably won't notice a 7% rear spring rate change.

On a smooth surface, springs and bars are interchangeable for cornering-only performance. Stiffer springs and softer bars will reduce suspension travel and camber changes due to braking and acceleration. A car with no bars has truly independent suspension, so one-wheel bumps hardly affect the car at all. However, I prefer to keep my bars connected because they provide an easy way to adjust the cornering balance between runs, or even during a run with cockpit adjustable blade angles.

Rear toe in is a powerful adjustment to change the magnitudes of inside wheelspin and power oversteer. More rear toe in will reduce inside wheelspin and increase power oversteer, and vice versa. More toe optimizes the car for a tighter turn radius. Formula SAE cars have successfully run as much as 1/2" total toe in due to the extremely tight course designs.

[Brian]

I like to run road course cars with open diff's and medium power with no rear bar, just to maximize grip (not just power-on traction, but over-all grip). You can adjust balance with an adjustable front bar, if you want to. You can adjust balance with ride height, springs, etc. I guess in my simple mind an ARB is fundamentally something to reduce grip, not add grip - but it's obviously more complicated than that.

Brian

[mwizard]

A rear sway bar allows for quick suspension adjustment based on different surface conditions and tire wear. I tightened my rear bar 2x at the last event to reduce push and move my car toward neutral handling on asphalt. Before that it was fine on grippy concrete. I have no problem with inside wheel spin. Chris stiffened his rear bar at the last event and took 1.4 sec off his time.
Mark

[chrisw52]

Mike,

I ran the boondoggle with a new set of stickers. My first run was good, but as the tires warmed up, so did the understeer. I tried pressure adjustments on my second run, which didn't help as much as I hoped it would. A quick adjustment on the rear bar almost eliminated the understeer on my third run.

I would say leave it

[mwood]

Plan is to leave it, for the time being, and experiment by disconnecting a link, if indicated.

Neil, given that the Hyperco's are probably much truer to the stated rate than the cheapo springs replaced, I picked up closer to 15% on spring rate. I would think that would have a pretty noticeable effect?

[Neil_Roberts]

Neil, given that the Hyperco's are probably much truer to the stated rate than the cheapo springs replaced, I picked up closer to 15% on spring rate. I would think that would have a pretty noticeable effect?

If your setup is really good now, you may notice it. Race cars are much more responsive to small adustments when the setup is "in the window". However, the spring change will be minor compared to the difference that new tires make.

[TedV]

one note on using huge amounts of toe on a FF, it might be fine for smooth pavement but taken from personal experience on very bumpy courses, the car will change directions just like bumpsteer when you know you have measured for bumpsteer and don't show any bumpsteer on the gage. Car will be consistent in that corner or bump but will not be consistent around the course. On my CM, it didn't work for me.

I have experimented with and without the rear bar. My car on old tires is more predictable without it. But it has an old club ford flexible rear chassis. All I can say is try stuff, see what the car and tires want.

[chrisw52]

one note on using huge amounts of toe on a FF, it might be fine for smooth pavement but taken from personal experience on very bumpy courses, the car will change directions just like bumpsteer when you know you have measured for bumpsteer and don't show any bumpsteer on the gage. Car will be consistent in that corner or bump but will not be consistent around the course. On my CM, it didn't work for me.

I have experimented with and without the rear bar. My car on old tires is more predictable without it. But it has an old club ford flexible rear chassis. All I can say is try stuff, see what the car and tires want.

Ted,

what kind of car do you have?

[TedV]

Royale RP31

[ewcmr2]

Rear toe in is a powerful adjustment to change the magnitudes of inside wheelspin and power oversteer. More rear toe in will reduce inside wheelspin and increase power oversteer, and vice versa. More toe optimizes the car for a tighter turn radius.

Really? This seems counter intuititive.
We have an event at El Toro in a couple of weeks...

[Neil_Roberts]

Really? This seems counter intuititive.

The key to understanding rear toe tuning is the friction circle. More rear toe in reduces the slip angle on the inside rear tire relative to the outside rear, so it has more of its longitudinal grip available to combat inside wheelspin. The downside is that more rear toe in also reduces the total cornering grip available from the pair of rear tires, so more toe in increases power oversteer. Especially with an open diff, toe is the most effective adjustment available to tune the compromise of inside wheelspin to power oversteer.

The reason that the toe setting is only optimum for one turn radius is simple geometry. The inside rear is in a different place than the outside rear relative to the turn center, so it operates at a different slip angle. The geometry of a car in a corner produces the effect of front toe in and rear toe out. The usual front toe out and rear toe in compensates for that. However, the amount of effective toe changes with the turn radius. So, your toe settings are only optimum for one turn radius. There is an illustration of this effect on page 134 of Think Fast.

We have an event at El Toro in a couple of weeks...

I'd love to come out and play, but my new job situation does not allow any play time in the schedule. Hopefully in the fall.

[mwood]

Just as an update...

Neil, you were right on the money, new tires transformed the car. How much of it is due to the other changes I made (besides the rear springs, I also widened the front track 1" and the rear .5"), I can't say. The car was very good when run two weeks ago. Only thing, that was on a surface (Marina) which is known to promote push in cars that are normally neutral. But, seeing as it is the only site I'm running the next couple/few events, I think I'll leave well enough alone. The car was good enough for TToD at AAS and something like 2nd on pax (not that I really care about pax, but with guys like McKee, Ellam, Frasier, Cox etc in other classes, at least it is some type of benchmark).