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thicker anti-roll bar ?

Posted by jaffar 
April 30, 2009 09:03AM
hi guys
what should i expect from installing a thicker anti-roll bar (anti-sway bar, stabilizer bar or whatever you want to call it) ?
i know that there were some basic rules, like "thicker bar -> more oversteer on the opposite axle" or something like that.

then we'll get into more details on why i want to change it smiling smiley

thanks

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A physics truck just turned over outside. There's physics everywhere!
April 30, 2009 09:47AM
The larger the sway (a.k.a anti-roll) bar, the less the body will roll at that end of the car. Essentially what the bar does is cause suspension travel at one side of the car to be mirrored on the other side. So when you go around a corner and compress the outside spring, the the bar tries to compress the inside spring also, and the car corners flatter. The larger the bar, the greater this effect.

What also happens while this is going on is that the thicker bar increases weight transfer at that end of the car, reducing grip at that end. Thus the general rule of thumb is that a thicker bar will reduce grip at that end. So a bigger front bar will cause more understeer, or looked at another way, can counter unwanted oversteer. By the same token, a bigger rear bar will create more oversteer by reducing grip at the rear, so it can be used to dial out unwanted understeer. This is why big rear bars are so popular with drivers of front-wheel drive cars, which by their design are greatly susceptible understeer.

OK, so what do you want to do to your E30? smiling smiley

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Dave
'91 325iX
April 30, 2009 03:10PM
From what I have read, larger sway bars also reduce predictability of when you will lose grip.

What I still don't understand about sway bars is, if they reduce grip, why do people use them? being flatter around corners has to help handling somehow, right? I suppose loss of grip for an increase in under/oversteer balance is a good trade off, but then why do people with E30s upgrade both the front and rear bar?


April 30, 2009 03:52PM
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daniel
From what I have read, larger sway bars also reduce predictability of when you will lose grip.
That has not been my experience.
Quote

What I still don't understand about sway bars is, if they reduce grip, why do people use them?
Almost all cars have sway bars, so you're using them whether you've replaced the stock bars or not. I'll assume your question is asking why people use larger bars. Don't think of it as "reducing grip." Think of it as transferring relative grip to one end of the car or the other. For example, I put a big front bar on my track car to help dial out some oversteer, because it could be tricky and tail-happy. The result was not overall loss of grip, but transfer of grip from the front to the rear, so it's less eager to swap ends on me now. Consequently, I'm running faster lap times.

You are correct that being flatter can help handling through the corners, but only to a point. Most stock suspensions are fairly soft, and allow quite a lot of body roll in hard cornering. People upgrade sway bars to flatten out the body roll, but there such a thing as too much of good thing. You could put steel girders in there and have no body roll at all, and the car would handle like poo. There is a happy medium.

It's worth pointing out that you can accomplish the same effect by increasing spring rates. Stiff springs will do the same thing as big sway bars, with the added effect of a stiffer ride. This can be problematic on bumpy public roads, but advantageous on the track. In fact, most track rats that I know prefer to adjust their suspension with stiffer springs rather than larger sway bars. In my own case, I've done both. In fact, I've decided that my car is now too understeery, which I have chosen to fix with stiffer rear springs. It's a bit harsh on the roads now, but it's a dream on the track.

For a street-driven E30, I personally wouldn't bother with bigger sway bars, since public roads don't really allow one to get anywhere near the limit where they would have an effect. If I were autocrossing or tracking it, I might consider it.

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Dave
'91 325iX
April 30, 2009 09:01PM
Thanks Dave G for the info there, and to the rest.

Correct me if I'm wrong about this, but this has been my understanding of sway bars...

Stiffer sway bars increase the amount that one wheels vertical travel will effect the other, correct?
So when in a corner, and the car wants to raise on one wheel, and drop on the other, the sway bar prevents this from happening.
In regular street use, this has the negative effect of feeling the bumps more (I think). Because if the wheels vertical travel is more tied together, it means that a bump felt by only a single wheel, will be felt by the other wheel as well.
While in a racing situation, on a flat no bump track, this isn't a problem. Both wheels should stay on the ground. However if you are on a country road and you get a wash-board style of pavement, the stiffer sway bar causes not just one wheel to bounce off the ground, but cause the other one to lose a lot of traction as it attempts to "raise up" with the other wheel.

I believe this concept was recently felt by experience. I have an e30, another friend has a Suburu Legacy, and the third has a 2001 Bullett mustang. On a bumpy corner, the mustang loses the ability to corner when going over bumps, and feels as though it is "bouncing sideways" off the road. I get this a little bit, but not much, and the legacy loses no perceptible traction.

Please let me know if I am way off base, and this phenomenon is caused by something else :-)
May 04, 2009 02:34AM
Quote
Dave_G
Thus the general rule of thumb is that a thicker bar will reduce grip at that end. So a bigger front bar will cause more understeer, or looked at another way, can counter unwanted oversteer. By the same token, a bigger rear bar will create more oversteer by reducing grip at the rear, so it can be used to dial out unwanted understeer. This is why big rear bars are so popular with drivers of front-wheel drive cars, which by their design are greatly susceptible understeer.

OK, so what do you want to do to your E30? smiling smiley

actually, this is about my e36, but i suppose it's similar.
my problem has a long history, so i'll try to resume it here

- when i bought the car, it was completely unbalanced and strange. different tire size front/rear, different season tires front/rear, worn bushings, the works. the car understeered horribly.
- after fixing lots of things, understeer is still there, and i really hate it. even with all stock parts replaced with new ones, new tires, stiffer shocks etc.
- finally, i found a LSD, in good shape, too. all i can say now is - WOW ! my car is finally what i expect: it oversteerrs like my old e30 used to smiling smiley it's very pleasant (even a bit too much), but now i have to learn to drive this car again.

so, now i'm interested in fine tuning everything. i changed the front arm bushings (front end is identical with the e30) to solid excentric bushings from the M3, so now there's no play any more. there's no comfort either, but at least everything is predictable now.

but... something is still wrong. when i corner hard, the car leans way too much. especially on left turns (which are also a lot easier to throw the rear around). i really hate that, i feel like i'm going to roll over ! i really don't see any reason why it would lean more on a left turn than on a right turn (since i also add a lot of weight on the left side...).

since you say thicker front bar would also reduce oversteer, it might finally give me the balance i want.

Edit: just to be clear: i don't track my car (probably mainly because there's no track around smiling smiley ) and i don't mean to install fancy racing parts in it. i was just thinking of changing the sway bar with one from a 325, which is just a bit thicker.

--
A physics truck just turned over outside. There's physics everywhere!



Edited 1 time(s). Last edit at 05/04/2009 02:39AM by jaffar.
May 04, 2009 03:26AM
you could use the 325 bar, or find even thicker bars from aftermarket places. when it comes down to it, though, the thing that will reduce body roll the most is stiffer springs.


May 04, 2009 03:41AM
maybe stiffer springs will also keep some confort for city driving smiling smiley

but... if i change the springs, shouldn't i change all 4 of them ? and if i also change the rears, then i will reduce the effect of transferring grip to the rear ? smiling smiley

--
A physics truck just turned over outside. There's physics everywhere!
May 04, 2009 07:48AM
The springs-vs.-bar debate for controlling body roll is a religious debate that I will try to avoid. Both have their adherents, and both approaches are effective. There are differences, however. A bigger front bar will control body roll, and cause the car to understeer more, but it will not appreciably affect the spring rate as you go over bumps. Stiffer front springs will also control body roll, and cause the car to understeer more, but it will also create a stiffer ride as you go over bumps. With properly-matched dampers, that stiffness does not necessarily need to translate to a harsher ride, but it usually does.

Generally speaking, the stiffer bar approach is favored by drivers of street cars who are looking to flatten out body roll without affecting the ride quality. Since that's what jaffar seems to want, that's the approach I would recommend here.

Quote
jaffar
but... if i change the springs, shouldn't i change all 4 of them ?
If you do change the springs, you do not need to change all four of them. People change just the fronts or just the rears all the time as a way of balancing the car. (I just put stiffer rear springs in my track car this weekend.) So you can put in stiffer front springs as an alternative to a thicker front bar, but it will likely give you a harder ride as a side effect.

__________
Dave
'91 325iX
June 01, 2009 07:22AM
ok - this is weird. i made a long(ish) drive during the weekend, including lots of twisty roads. i had someone drive behind me (i hope the movie turned up ok, didnt see it yet) and they told me that my rear-left tire seems not inflated enough. i checked and it was at 2.0 bar (i run with 2.2 front and 2.3 rear - so -0.3 difference from the right). i fixed it and ... WOW ! now the car doesn't roll so much at all, and left-hand roll is exactly the same as right-hand roll !! it was a pleasure to drive on the way back.

can someobody please enlighten me ?

thanks

--
A physics truck just turned over outside. There's physics everywhere!
June 01, 2009 06:37PM
Quote
jaffar
ok - this is weird. i made a long(ish) drive during the weekend, including lots of twisty roads. i had someone drive behind me (i hope the movie turned up ok, didnt see it yet) and they told me that my rear-left tire seems not inflated enough. i checked and it was at 2.0 bar (i run with 2.2 front and 2.3 rear - so -0.3 difference from the right). i fixed it and ... WOW ! now the car doesn't roll so much at all, and left-hand roll is exactly the same as right-hand roll !! it was a pleasure to drive on the way back.

can someobody please enlighten me ?

thanks

i think you answered your own question. running with low tire pressure will reduce the precision and responsiveness of that particular corner of the car.


June 01, 2009 08:06PM
The difference between 2.0 and 2.3 bars is 29 vs. 33 psi. That may be enough to make a small difference, but not the WOW! dramatic difference that he's describing. Also, no one can see that small a difference in tire pressures just by looking at the tire, certainly not while driving behind the car. You can't see an underinflated tire until it gets a lot lower than 2.0 bar/29 psi. I think there's something else going on, but we don't have enough information to say what.

__________
Dave
'91 325iX
June 02, 2009 02:14AM
Quote
Dave_G
The difference between 2.0 and 2.3 bars is 29 vs. 33 psi. That may be enough to make a small difference, but not the WOW! dramatic difference that he's describing. Also, no one can see that small a difference in tire pressures just by looking at the tire, certainly not while driving behind the car. You can't see an underinflated tire until it gets a lot lower than 2.0 bar/29 psi. I think there's something else going on, but we don't have enough information to say what.

it is a difference of over 10%. to me that is a significant difference. i think you would notice the varying tire pressures more than you think.


June 02, 2009 03:28AM
when you walk around the car, the difference is not obvious. but when you drive behind, it becomes :undecided:

--
A physics truck just turned over outside. There's physics everywhere!
June 02, 2009 03:56AM
Quote
jaffar
when you walk around the car, the difference is not obvious. but when you drive behind, it becomes :undecided:

i agree that you probably couldnt tell just by looking, but, under the force of a cornering vehicle, it is conceivable that someone could notice a difference in how the tire deforms as you go around corners.


June 02, 2009 04:10AM
Quote
daniel
Quote
Dave_G
The difference between 2.0 and 2.3 bars is 29 vs. 33 psi. That may be enough to make a small difference, but not the WOW! dramatic difference that he's describing. Also, no one can see that small a difference in tire pressures just by looking at the tire, certainly not while driving behind the car. You can't see an underinflated tire until it gets a lot lower than 2.0 bar/29 psi. I think there's something else going on, but we don't have enough information to say what.

it is a difference of over 10%. to me that is a significant difference. i think you would notice the varying tire pressures more than you think.

Which does not correspond to a 10% difference in form, or a 10% difference in feel.
I do not think in our cars that you can see a 29 vs 33 psi difference. I don't start noticing it until I'm down around 26 or so.

A car in motion is going to have the tires look more inflated (if driving in a straight line) if anything, as the centripetal force* should be acting to pull the rubber out, thus causing the tire to look normal. Perhaps in a turn a deflated tire would be more noticeable, but I dunno... 29vs 33 isn't much at that amount of psi.

It's a mystery smiling smiley My bests would be on an inaccurate initial measurment of pressure. If they tires were at 24 or something, than yeah, you would feel that and see it with ease, no expertise required.

~Tyler


*I'm aware it isn't a real force. But I'm not talking to physics majors for the most part, so I'm going to relate to it as one smiling smiley
June 02, 2009 10:04AM
Quote
Earendil
Quote
daniel
Quote
Dave_G
The difference between 2.0 and 2.3 bars is 29 vs. 33 psi. That may be enough to make a small difference, but not the WOW! dramatic difference that he's describing. Also, no one can see that small a difference in tire pressures just by looking at the tire, certainly not while driving behind the car. You can't see an underinflated tire until it gets a lot lower than 2.0 bar/29 psi. I think there's something else going on, but we don't have enough information to say what.

it is a difference of over 10%. to me that is a significant difference. i think you would notice the varying tire pressures more than you think.

Which does not correspond to a 10% difference in form, or a 10% difference in feel.
I do not think in our cars that you can see a 29 vs 33 psi difference. I don't start noticing it until I'm down around 26 or so.
you're right. it doesn't necessarily correspond to that, but right now you are riding on a stock suspension with stock sized wheels and tires. there is a reason why autocrossers and racers fine tune their tire pressures in as small of increments as possible... because they can tell a difference, and it influences the dynamics of the vehicle.
Quote

A car in motion is going to have the tires look more inflated (if driving in a straight line) if anything, as the centripetal force* should be acting to pull the rubber out, thus causing the tire to look normal. Perhaps in a turn a deflated tire would be more noticeable, but I dunno... 29vs 33 isn't much at that amount of psi.
i agree, which is why i said "under the force of a cornering vehicle".
Quote

It's a mystery smiling smiley My bests would be on an inaccurate initial measurment of pressure. If they tires were at 24 or something, than yeah, you would feel that and see it with ease, no expertise required.

~Tyler


*I'm aware it isn't a real force. But I'm not talking to physics majors for the most part, so I'm going to relate to it as one smiling smiley

depending on when he checked the tire pressure (hot or cold tires, what the ambient temp was), the tire pressures could have read differently. but it sounds like he checked them all at the same time, so they would still have similar differences between them.

call me crazy, but there have been many times when, before checking my tire pressures and adding air (if it is getting hotter outside), i can predict which corner (if any) has the least air in it, even if its just by 1 or 2 psi. maybe it is because i have been driving on grippier tires?


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