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Old 11-08-2005, 12:36 AM   #1
aakrusen
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Default Benefits of larger sway bars

Can someone explain what happens to the WRX when a larger from sway bar is put on. And when just the larger rear bar is on. And then finally when both bars are on.

I also want to find out when the front and/or rear bar is big enough, meaning like 1mm, 2mm, 3mm, etc larger than stock.

I autocross my car so I'm trying to maximize my handling and the swaybars are just part of my picture.
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Old 11-08-2005, 01:14 AM   #2
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To answer your question directly, you get a +3 or +4 addition in front grip from the camber curve improvement but a -1 in front grip from the "additional weight on the tire" part of the equation that comes from having a larger bar, so overall traction in the front of the vehicle is increased. Of course body roll is decreased, but as a bonus turn-in is quicker.

The car feels like it is understeering less because the limit of grip is a lot higher, while in reality at the limit the car is actually understeering a bit more. Hence why some folks say the car understeers less on a bigger front bar, and then others who haven't tried it bust out the "book and GT4" knowledge and proclaim that a bigger front bar will make the car understeer more.

When you add the front and rear bars together, it's handling nirvana. A large imbalance towards either direction causes the car to feel a little off and thus why I whole-heartedly recommend doing them in pairs without too much difference in bar sizes.

For autox, you want a decent rear and a large front for the best balance and fastest recovery. The largest bar I know of is the 32mm hollow Strano bar (29.6mm solid equivalent) followed closely by Whiteline's 29mm solid bar.

If you're not that hardcore, get the Whiteline 27mm solid bar for the front and a 24mm solid bar for the rear.

The problem with Imprezas is that the wheel rate is horribly low and the camber curves are awful. Increase the wheel rate too much with springs, and one causes a horrible ride, not to mention just a general overload on the tire and skipping.

I've been to "too stiff" and back and I'm currently pursuing a setup with large bars and medium springs (8k/8k.) This will yield best daily-driver and autox combination performance.

(STi can go with a larger differential due to the better diffs, and more specifically the better torque split. 32 or 27mm up front, 24mm in the back for autox.)

For the WRX I'd say 27 or 29mm (or 32 hollow) up front and 22-26 adjustable to setup specifically the amount of tail looseness desired.

There are great autox setups, but none as great as the one that gives the drive the *most* confidence. To heck with what some other driver can achieve with their setup and how great it might be, if it's not confidence inspiring then it isn't the fastest setup for you.

-Biggly

Last edited by DrBiggly; 11-08-2005 at 01:25 AM.
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Old 11-08-2005, 01:26 AM   #3
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For autocross, go with huge ass bars front and rear if your class allows it.
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Old 11-08-2005, 10:03 AM   #4
aakrusen
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Wow Biggly, you know your stuff. Thanks.
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Old 11-08-2005, 10:55 AM   #5
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how does extra weight on the tires = less grip?
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Old 11-08-2005, 04:06 PM   #6
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Quote:
Originally Posted by Lachlan
how does extra weight on the tires = less grip?
I'll let nhluhr answer that as I believe he has read every good suspension book available and while I have a good understanding of the mechanics, he is admittedly more thorough and can go into greater detail and actually use the proper terms.

-Biggly
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Old 11-08-2005, 06:03 PM   #7
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Quote:
Originally Posted by Lachlan
how does extra weight on the tires = less grip?
grip is defined as the ratio between traction and loading. When you increase loading, traction does increase, but not proportionally to the increase in load. The coefficient of friction is not constant for all loading values.

Therefore, if you have a tire with 700lbs loading and it produces a lateral traction of 700lbs, the grip is said to be 1.0. Increaase the loading to 1000 lbs and the traction will not increase to 1000... more like 900, with the grip now becoming 0.9.

Why this is important is the following:
If you have a pair of tires each loaded at 700lbs and a grip of 1.0 in this condition...This gives you 1400lbs of lateral traction.

Now enter a turn where you are transferring 100% of the weight to the outside tire. You now have the outside tire loaded to 1400lbs at a reduced grip ratio of ~0.85 and the inside tire loaded at 0lbs at a grip ratio of, say, 1.1. The total traction now is 1400(0.85) + 0(1.1) = 1190, a decrease in total traction.

The reason a large swaybar increases % of total weight transfer seen by the axle is as follows:
Total weight transfer does not change unless you reduce CG height, decrease lateral cornering force (either slow down, take a wider line, or decrease vehicle weight), or widen the track. Those are the only things that change the total lateral weight transfer. However, realize that the only thing resisting the total weight transfer is the roll stiffness of the front and rear suspensions. If one of them is noodle-soft and the other is very stiff, the car rolls X amount... if both are moderately stiff, the car rolls X amount. The one that is doing the majority of the roll resisting is going to putting the most pressure on its tires so that pair of tires will bear a larger portion of the total weight transfer.

Typically, when vehicle dynamics books mention changing swaybar sizes to affect over/understeer, they are really talking about keeping the same total roll stiffness, but putting more of it on the front or back to achieve the result. If you were to only change one bar, then the change in over/understeer bias is not as clearcut because you are changing the total roll stiffness instead of changing just the bias.

So, if you were to put a smaller rear bar on and increase the front bar the same amount, the result would be more understeer. BUT if you keep the rear bar the same and increase the front bar's size, you will end up with less roll and thus usually more traction. The reason for this is that camber has a much greater effect on grip than does loading... so the tradeoff of increasing tire loading is more-than-compensated-for by the drastic reduction in positive camber during cornering and the end result (on any car, not just an impreza) of just putting on a bigger bar, front or rear, is an increase in total traction.
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Old 11-08-2005, 06:31 PM   #8
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Quote:
Originally Posted by aakrusen
Can someone explain what happens to the WRX when a larger from sway bar is put on. And when just the larger rear bar is on. And then finally when both bars are on.
To address this, lets consider the explanation I just posted above....

1) Increasing just the front bar, the front tires see a larger percent of total weight transfer which increases front tire loading and reduces front tire grip slightly (and also the opposite effects on the rear tires)... BUT you also get a reduction in roll which reduces how positive your camber goes in hard cornering for a large increase in grip. The end result is a good increase in cornering grip with perhaps a very slightly higher bias towards understeer. Now, before a thousand people jump on that last statement, let me explain. I know that people are also putting on big front bars and saying "I got less understeer". I would contend that what their butt-dyno is telling them is not the whole truth. The real story is, the increased grip (from roll reduction) allowed them to go faster in places where they previously had understeer. So yes, perhaps they experience understeer less dramatically than before, but the bias has still shifted towards understeer more... it's just that the limits are now significantly higher so this bias is less apparent. I'd say that the front tires see a moderate gain in grip while the rear tires see a significant gain in grip.

2) Increasing just the rear bar, the rear tires will see a larger percent of the total weight transfer which increases rear tire loading (conversely taking load off the front tires) so you get a slight reduction in rear grip and a slight increase in front grip for a more oversteer-biased setup, but the decrease in lateral roll also increases total grip, just like above. The net change is more grip, more oversteer. I have heard a lot of people say "a rear bar increases oversteer by reducing rear traction". This is not exactly right. What it is doing is taking load off the front tires, thus increasing front traction where it is severely lacking, AND increasing load on the rear tires while also reducing roll. I would say that rear traction is increased marginally overall, while front traction is increased signifcantly.

3) Increasing both bars, you get double the reduction in roll and no real change in bias. This is the way to go, IMHO. You don't change the overall bias of the chassis, but you do vastly increase grip on both front and rear tires. I'd say that both front and rear tires see a significant increase in grip.

So, if I had to choose between 1) or 2), which would I choose? The decision would be based on a couple things, but lets say we are talking about a WRX sedan. The front is very heavy so front tire loading is already a problem... The torque bias is 50/50 so you have further load on the front tires. I would definitely choose the rear bar over the front bar if I was forced to choose only one. However, many autocrossers are limited to changing just the front bar, in which case, definitely do it. Any increase in overall grip you can get is an increase worth getting.
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Old 11-08-2005, 06:45 PM   #9
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That should be a sticky!!
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Old 11-08-2005, 06:57 PM   #10
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Would you recommend sway bars before Strut bars?

im driving an 02 WRX
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Old 11-08-2005, 07:15 PM   #11
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Thank you "nhluhr" and "DrBiggly" for the information. It all reinforces my desire to add the Whitline 22 mm front bar now to complement the Whiteline adjustable rear [20-24 mm]...and with brackets and endlinks, of course...and sign up with the Tar Heel Sports Car Club to pursue autox in 2006.

Again, thanks for taking the time to educate folks on "what-is-what" with Scoobys and handling.

Toivo
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Old 11-08-2005, 07:21 PM   #12
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Get the 24mm!
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Old 11-08-2005, 07:26 PM   #13
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Thanks for the info, tuskenraider. 22 is only a smidge over the stock 19 mm, so 24 mm fixed up front will give me full useage of the rear range without pulling a Porsche 911 polar inertia surprise event.
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Old 11-08-2005, 08:12 PM   #14
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This is good stuff. +1 for sticky.

So bigger bars on the front and rear are basically bumping up the limits of grip? So this gives the appearance of a neutral car when in reality the limits are probably farther than where I'll be taking the car?

How about the advantage/disadvantage of the hollow bar vs the solid bar? Does the weight of the bar play a big enough role to buy the hollow over the solid?
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Old 11-08-2005, 08:22 PM   #15
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Quote:
Originally Posted by jc8587
Would you recommend sway bars before Strut bars?

im driving an 02 WRX
Absolutely - strut bars really don't do too much on our cars which are pretty struturally sound already. One could make the argument for a rear strut bar in a wagon. Sway bars first though
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Old 11-08-2005, 08:50 PM   #16
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Quote:
Originally Posted by BlkWRXWag
struturally
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Old 11-08-2005, 09:02 PM   #17
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Ooops - there's a freudian slip if ever I saw one
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Old 11-08-2005, 11:15 PM   #18
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What autox class are you targeting? What other suspension mods do you have?
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Old 11-09-2005, 02:51 AM   #19
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WOW... fantastic post! very helpful info!
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Old 11-09-2005, 11:31 PM   #20
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Quote:
Absolutely - strut bars really don't do too much on our cars which are pretty struturally sound already. One could make the argument for a rear strut bar in a wagon. Sway bars first though

thanks I think ill be getting some Cuscos, rear adj.
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Old 11-09-2005, 11:54 PM   #21
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This is good stuff. +1 for sticky.

So bigger bars on the front and rear are basically bumping up the limits of grip? So this gives the appearance of a neutral car when in reality the limits are probably farther than where I'll be taking the car?

How about the advantage/disadvantage of the hollow bar vs the solid bar? Does the weight of the bar play a big enough role to buy the hollow over the solid?
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Old 11-10-2005, 11:15 AM   #22
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wouldn't a hollow bar be easier to flex?
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Old 11-10-2005, 12:44 PM   #23
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Yes but it depends on the wall thickness, also it is lighter as well

Easiest way to explain is this. Take one straw, easy to flex right? Now take around 10 of them and bunch them up, getting harder and harder to flex right?
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Old 11-10-2005, 04:28 PM   #24
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What if you don't autocross. Would one notice a difference in going with a larger swaybar, or tower strut bars, with every day driving?
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Old 11-10-2005, 05:07 PM   #25
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Quote:
Originally Posted by subie4life
What if you don't autocross. Would one notice a difference in going with a larger swaybar, or tower strut bars, with every day driving?
It depends if you're a "spirited" driver who likes driving in the "twisties"
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