I just found out about an old fwd drag trick where you preload the front sway bar to help spin both tires equally without an LSD and it got me thinking.

My question is, how does this affect handling? I do understand the whole concept of stiffer rear sway / rear shocks / rear tire psi vs. front in terms of over and understeer but I don't fully understand how preload affects the way that a car handles. Any info/input about swaybar preload?

 

For circle track use (or I suppose road courses with all the major turns occurring in one direction) you could preload the swaybar to produce more anti-roll stiffness in one direction. Essentially, by changing end link length, you can have the bar already in a state of twist while the car is at rest.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hatch_R &raquo;</TD></TR><TR><TD CLASS="quote">I just found out about an old fwd drag trick where you preload the front sway bar to help spin both tires equally without an LSD and it got me thinking.

My question is, how does this affect handling? </TD></TR></TABLE>

Not well, unless you're racing on an oval track as TunerNOOb suggests. Pre-loading an ARB will cause the car to steer and handle differently in left vs right hand corners, probably making it great in one direction but relatively crap in the other...

As to wheelpsin off the line...

My understanding was that pre-loading the ARB was RWD drag trick...?

A FWD car with an 'east-west' engine configuration doesn't suffer from the same problems associated with torque loadings causing one of the driving wheels to unload under hard acceleration, as <U>do</U> cars with a 'north-south' engine configuration (and even worse with a live rear axle as well).

With a car that has a north-south engine that has a clockwise crankshaft rotation (typically RWD) and the differential attached directly to the chassis itself (i.e. some form of independant rear suspension), the engine torque will lift the left side of the chassis (i.e. attempt to rotate the chassis mass around the crankshaft axis in the opposite direction to crank rotation) and unload both left side wheels.

An otherwise similar car but fitted with a live rear axle (where the differential is not attached to the chassis but to the rear unsprung mass) will also attempt to rotate the chassis around the crank axis, however there is an opposite torque reaction at the live axle that causes the right side rear wheel to unload despite the torque reaction through the chassis.

Engine torque will cause this car to load the right side front wheel / unload the left side front wheel and also unload the left side rear spring and load the right rear spring, however, the loadings at the rear wheels themselves will be opposite to the loadings at the rear springs, i.e. the right rear wheel will will become unloaded and the left rear more loaded (despite the opposite loadings seen at the springs).

An east-west engined car (whether FWD or RWD) doesn't see lateral weight transfers due to engine torque loadings, but rather sees longitudinal weight transfers that occur equally on both the left and right sides of the chassis, so neither of the driving wheels is unloaded or loaded by torque loadings at the expense of the other.

Any tendency for a given FWD car to spin one wheel more than the other won't be due to any changes in driven wheel loading caused by torque loadings through the chassis, but will be due to the basic static weight distribution on the two driving wheels, i.e. if one wheel is statically lighter then it will tend to spin more easily.

The correct way to address this problem would be by redistributiiing static weight to give equal weight on the driven wheels, or by 'corner-weighting' with spring seat height adjustments to give equal driven wheel weights (so long as the rear wheels don't become too 'unbalanced' as a result).




Modified by johnlear at 4:55 PM 5/6/2008

 

I already understood the circle track thing but I'm wondering what it will do in terms of under and oversteer (for example, getting a stiffer rear sway will produce more oversteer - how is this affected if you proload the front sway bar, equally on both sides?). And will it affect anything else? I mean equal preload on both sides of the swaybar, not preload to one side.

And no, we were talking about FWD cars since they can tend to spin just one wheel when launching.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hatch_R &raquo;</TD></TR><TR><TD CLASS="quote">I already understood the circle track thing but I'm wondering what it will do in terms of under and oversteer (for example, getting a stiffer rear sway will produce more oversteer - how is this affected if you proload the front sway bar, equally on both sides?). And will it affect anything else? I mean equal preload on both sides of the swaybar, not preload to one side.

And no, we were talking about FWD cars since they can tend to spin just one wheel when launching.</TD></TR></TABLE>

You can't preload both sides of a swaybar (to use the term how I think you intend).

When you apply preload to one side, you're simultaneously applying "negative preload" (I just invented that term) on the opposite side. Meaning, one side will have the bar trying to lift the tire (fight the spring), with the opposite side pushing it down (assist the spring).

Er, I'm not so good at explaining things. A swaybar is attached to both sides of the suspension, and pivots about the chassis. If you lengthen one endlink, you'll have to twist the bar to attach both endlinks (assuming a level car). That twist, is the preload. If you then lengthen the other endlink (to try and preload it as well), you'll undo what you previously did, and have an unloaded bar.

 

it will put more spring force to one side than the other.

this is NOT a good thing for handling when you have to turn left and right.

any more thought on this topic is worthless.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by TunerN00b &raquo;</TD></TR><TR><TD CLASS="quote">

You can't preload both sides of a swaybar (to use the term how I think you intend).

When you apply preload to one side, you're simultaneously applying "negative preload" (I just invented that term) on the opposite side. Meaning, one side will have the bar trying to lift the tire (fight the spring), with the opposite side pushing it down (assist the spring).

Er, I'm not so good at explaining things. A swaybar is attached to both sides of the suspension, and pivots about the chassis. If you lengthen one endlink, you'll have to twist the bar to attach both endlinks (assuming a level car). That twist, is the preload. If you then lengthen the other endlink (to try and preload it as well), you'll undo what you previously did, and have an unloaded bar.</TD></TR></TABLE>

It won't be unloaded though because it's still attached where it mounts at the bushings. BTW, I'm talking about the front sway so it's a different situation from the rear. If you were to lengthen the endlinks in the front, both equally, then wouldn't it preload both sides equally?



Since the endlinks are forcing the ends of the swaybar up and the swaybar is trying to straighten itself back out at all times, even when the car is not moving (equally on both sides), doesn't that mean it's equally preloaded? I'm wondering if this would affect handling and how.

 

The sway bar wouldn't bend as you drew in the 2nd pic. It would just rotate inside its bushings, and you would end up with 0 preload and longer endlinks.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Hatch_R &raquo;</TD></TR><TR><TD CLASS="quote">It won't be unloaded though because it's still attached where it mounts at the bushings. BTW, I'm talking about the front sway so it's a different situation from the rear. If you were to lengthen the endlinks in the front, both equally, then wouldn't it preload both sides equally?

Since the endlinks are forcing the ends of the swaybar up and the swaybar is trying to straighten itself back out at all times, even when the car is not moving (equally on both sides), doesn't that mean it's equally preloaded? I'm wondering if this would affect handling and how.</TD></TR></TABLE>
That's not how a sway bar works. It is only effective under roll, not under pitch. You can't equally preload both sides, because the entire bar just twists in the bushings. The bar only resists twisting forces when each end is twisting in an opposing direction.

 

Ah haha good point, I didn't even think of something so simple as the bar just rotating in the bushings, I kept thinking of where the bar mounts as fixed points without considering that the bar could rotate. I must be thinking of something wrong because the guy who told me about this front sway preload has been in the Honda scene since the early 90s and has a pretty fast drag car so I would think he knows what he's talking about.