Big rear bar = oversteer right?

[Appleseed]

I heard that if you have a bigger and/or stiffer bar out back that it will help promote oversteer. Is there any truth to this?

[Craftsman]

Yes, this is true... This is due to the suspension flexing less in the rear. The bar will resist suspension from roll by transfering the the displacement to the side of the turn...

For example, when you take a left turn, the car will roll to the left, and the right suspension will compress and the left suspension will expand. The roll bar evens out the amount of compression and expansion between them and cuts down on a alot of the roll that is experienced.

When the bar is thicker, the displacement between them is held more constant because the deflection of the roll bar is less. With thin bars, the bar deflects more so the suspension is allowed to displace more.

I'll draw or find pictures if you need a better visual understanding.

[Appleseed]

No pics nessesary. Bear in mind that this isn't an IRS rearend. It a traditional Gm triangulated 4 link. I assume that the same physics still apply.

[malcolm]

yeah... more or less.

simplified version: the stiffer the bar, in relation to the front, will cause more oversteer.

ok, it's theory time boys and girls... this may be bs, but it's my mind at work.

perhaps when you add a stiffer bar to the rear, when you corner, it will shift weight to the front. Since the body will still roll a bit because the front has a soft sway bar in the front by comparison, the front will still want to roll. When the body does roll, the rear bar wants to resist that force, so it tries to lift up the inside rear. Since the front is still soft, the body rolls on an axis that goes from the inside front wheel to the outside rear wheel.

Say, on a given car you are cornering with equal (or well matched) sway bars, the outside tires get 100 lbs of additional weight transferred to them because of body-roll (and therefore the inside tires would have 100 lbs less, respectively). When you add a stiffer bar on the rear, it changes that. Because the car wants to rotate on that inside-front-to-outside-rear axis, more weight is transferred to the outside front. It would be more like the outside rear gets 80 lbs more, the outside front 120 lbs more, the inside front 80 lbs less, and the inside rear 120 lbs less.

The same would therefore apply to a stiffer front bar, in that weight would transfer to the rear and thus cause understeer.


does this seem to make sense? just one of those ideas I thought up... might be right.

[Craftsman]

Malcolm, Let's all just thank God that you speak English and I choose to use Engineerish... People understand you...

[CrazyHawaiian]

Yes.

[SaintDrift]

So you are basically just getting more weight transfer where the bar is. Interesting. I always had a hardtime understanding this. I suppose switching to stiffer bushings on your swaybar would also give the same effect. Some guy was making harder bushings for my car and I had a hardtime understanding how they promoted understeer. Thanks for the info.

[Craftsman]

It's like getting stiffer springs and higher dampening. It prevents your geometry from flexing...

Basically an Anti-Roll bar is a lateral spring that goes from side to side rather than up or down... (I just had and ephiphany... MUhahahhaa)

[malcolm]

haha... a sway bar just resists a difference in compression between either side of the suspension. If the left side gets compressed, the sway bar wants to compress the right side. No lateral forces here...

if my theory is correct, then you get more weight transfer *away* from the stiffer bar. ie: stiff rear bar? then you get more weight transfer to the front, and it encourages oversteer.

[SaintDrift]

So this weight transfer isn't lateral. I assumed it was since you were losing grip where the bar was. Instead it's weight transfer going down on the road, which means you are getting more grip. I think I'm going to stop worrying about the whys. I'm not a damn engineer and I don't want to be. Big bar up front makes my car hells lot easier to drive. That's all I need to know.

[malcolm]

yeah... that's my theory anyway. Seems logical. could be wrong though. anyway....

sway bars resist lateral weight transfer, that's a fact. I think it transfers it into longitudinal weight transfer, but that's just a thought.

[Craftsman]

I don't think it would... If you do a free body diagram on the bar and the forces that it sees, it's prolly just a spring.. I'm not sure though. It could transfer weight forward by resisting displacement in the suspension from side to side... I dunno...

Ok enough geek type talk for me... Malcolm knows exactly what I'm talking about....

[SidewaysGts]

General rule of thumb:

Stiffer front in any respect promotes under steer

Stiffer rear - OverSteer.
___

Ill try to sum up what i think malcom meant and what i think/agree happens.

Basicly oversteer is when the tires have too much traction up front and cause the car to tuck in, understeer being just the opposite where they dont have enough traction to redirect the front of the car.

By making the back end stiffer, you allow less weight to travel to the back and keep it in the front (thus more traction; oversteer) Doing the opposite, making the front stiff- allows the weight to shift to the rear easier effectively giving you less traction up front, understeer.
___

On a side note i personally find understeer a bit better for drifting and oversteer is easier to learn on. Keep in mind with oversteer getting heavy on the throttle just increases the angle of the drift greatly and increasing your chances to spin out. Understeer will resist that turn in and u need to be heavier on the throttle to increase the angle. Also weight shifts back easier helping with exit speed i think... oh well just my 2 cents.

[captobvious]

Originally posted by malcolm
yeah... more or less.

simplified version: the stiffer the bar, in relation to the front, will cause more oversteer.

ok, it's theory time boys and girls... this may be bs, but it's my mind at work.

perhaps when you add a stiffer bar to the rear, when you corner, it will shift weight to the front. Since the body will still roll a bit because the front has a soft sway bar in the front by comparison, the front will still want to roll. When the body does roll, the rear bar wants to resist that force, so it tries to lift up the inside rear. Since the front is still soft, the body rolls on an axis that goes from the inside front wheel to the outside rear wheel.

Say, on a given car you are cornering with equal (or well matched) sway bars, the outside tires get 100 lbs of additional weight transferred to them because of body-roll (and therefore the inside tires would have 100 lbs less, respectively). When you add a stiffer bar on the rear, it changes that. Because the car wants to rotate on that inside-front-to-outside-rear axis, more weight is transferred to the outside front. It would be more like the outside rear gets 80 lbs more, the outside front 120 lbs more, the inside front 80 lbs less, and the inside rear 120 lbs less.

The same would therefore apply to a stiffer front bar, in that weight would transfer to the rear and thus cause understeer.


does this seem to make sense? just one of those ideas I thought up... might be right.

damn, are you like a physics buff or something, i think i got smarter just by reading that

[Craftsman]

I think I better draw pictures... Gimme a few minutes. I'll show you my elite MS Paint skills...

EDIT: This is harder than I thought....

EDIT: Here are your pictures that I promised..