Front only sway bars?

[LiM3y]

I have been musing about getting sway bars put on my car. The front one seems and easy job, but the rear is a PITA. I see that the Dinan suspension kit only has a front sway bar.

Is there any value in just adding a stiffer front sway bar, or will it just unbalance the suspension too much? (I don't know enough about suspension tuning to know the answer of just stiffening the front end). I was thinking of trying it up front for a cheap handling mod, before committing to 4 hours of labour to fit complimentary rear bar (and potentially another 4 hours to put the old one back!)

[Porschefile]

Someone feel free to correct me if I'm wrong here, but I think stiffer front bars cause more understeer and stiffer rear bars cause less understeer (more oversteer). I wouldn't add a front bar only, because I don't want more understeer. Sways are a fine tuning device anyway, they don't make an ill-handling car handle properly or vice versa.

[M3SQRD]

Originally Posted by Porschefile

Someone feel free to correct me if I'm wrong here, but I think stiffer front bars cause more understeer and stiffer rear bars cause less understeer (more oversteer). I wouldn't add a front bar only, because I don't want more understeer. Sways are a fine tuning device anyway, they don't make an ill-handling car handle properly or vice versa.

I have no experience with sway bars on the e9x M3 but the e46 M3 really likes a stiffer front sway bar (relative to the rear) because it greatly improved/controlled the camber curve - stiffer bar actually reduced understeer (obviously there's an upper limit). I believe the e9x has a better camber curve and it this shows up in static camber settings - e46 requires a lot of front camber (more than neg 3-3.5 deg if using r-comps) whereas the e9x does not.

As for the Dinan front bar I think it is designed to work with their spring kit so not sure what effect - over or under steer - it would have on the handling characteristics of a stock e9x suspension?

[LiM3y]

[I have Eibach Pro-kit springs]

[The HACK]

Originally Posted by Porschefile

Someone feel free to correct me if I'm wrong here, but I think stiffer front bars cause more understeer and stiffer rear bars cause less understeer (more oversteer). I wouldn't add a front bar only, because I don't want more understeer. Sways are a fine tuning device anyway, they don't make an ill-handling car handle properly or vice versa.

Typically, in a neutral chassis with double wishbone suspension up front where suspension geometry upon compression is more "predictable," that would be the case.

However, the MacPherson strut design up front actually behaves unpredictably with addition of swaybars up front, especially if the swaybars manage to reduce the "reduced camber" effect when there's too much compression. I've seen cases where additional front stiffness actually ADDED front end grip and alleviated understeer on a BMW. However, that is in combination with a ton of negative camber up front already and a moderately stiff front suspension setup without the ability to independently adjust compression. On a stock-ish suspension, I suspect if you can get up to 2.5-3º of negative camber, adding swaybar up front only may actually IMPROVE the front end grip on an M3.

But not on a car with mostly stock front camber.

[Porschefile]

Originally Posted by The HACK

Typically, in a neutral chassis with double wishbone suspension up front where suspension geometry upon compression is more "predictable," that would be the case.

However, the MacPherson strut design up front actually behaves unpredictably with addition of swaybars up front, especially if the swaybars manage to reduce the "reduced camber" effect when there's too much compression. I've seen cases where additional front stiffness actually ADDED front end grip and alleviated understeer on a BMW. However, that is in combination with a ton of negative camber up front already and a moderately stiff front suspension setup without the ability to independently adjust compression. On a stock-ish suspension, I suspect if you can get up to 2.5-3º of negative camber, adding swaybar up front only may actually IMPROVE the front end grip on an M3.

But not on a car with mostly stock front camber.

Interesting, so on my setup (514/800# and -2.5) a stiffer front bar should decrease understeer?

[The HACK]

Originally Posted by Porschefile

Interesting, so on my setup (514/800# and -2.5) a stiffer front bar should decrease understeer?

COULD, not should. There's really no way to know for sure unless you test.

[Purple Derple]

I posted this in another thread:

Originally Posted by Purple Derple

Anti-roll bars primarily behave like springs on that end of the car. However, they cannot affect the total load that is transfered from side to side. In fact, there is NOTHING you can do to the suspension to affect how much load transfers besides lowering the ride height. So no, a stiffer rear or front for that matter bar wont more lead to a more balanced car necessarily in the sense of evenly distributing weight over all four contact patches. It would simply defy phsyics. So while the car may remain flatter, it still transfers just as much load in the corner.

Let me quote for you. I know it is poor form to use such a big block quote, but he demonstrates the concept better than I can. Carl Lopez writes in Going Faster!: Mastering the Art of Race Driving, "To Illustrate how spring rates affect the cornering balance of the car, let's go to an extreme and see what happens...the front springs and shocks [are] removed and replaced by solid bars, essentially making the front suspension solid...whe the car corners and the load transfers, all of the... load will be absorbed by the outside front tire (assuming an infitnitely rigid chassis), since the front end resists all the rolling motion of the car. The outside rear tire doesn't contribute to any of this and consequently retains 100% of its original grip. The outside front, because of all this extra load, suffesrs a loss in CF relative to the rear and as a result operates at a higher slip angle than the rear tire will. The car goes into understeer--in this case, probably pretty gross understeer--as the outside front is hopelessly overtaxed." He goes on to show a similar example for the rear, which to this thread is most relevant; "Now, lets put the shock/spring unit back on the front of the car and make the rear suspension solid. Under cornering...all the load transfer goes to the outside rear tire, causing the rear end to operate at higher slip angles--the definition of oversteer. What you're seeing here is the world's stiffest spring, one with a spring rate of a million pounds per inch, or so. Nobody would really do this to a poor car, but it helps to illustrate the principle. All other things equal, a stiffer spring will increas the resistance to roll and absorb more of the load transfer at the end of the car on which it is installed." So, "At the rear, a stiffer spring increases the tendency toward oversteer, while a softer one goes in the opposite direction, decreasing the oversteer tendency and increasing the tendency toward understeer."

Since we know anti-roll bars behave just like springs that resist lateral motion only, we can substitue the spring concept in that passage with the anti-roll bar concept to understand its behavior. Because of this behavior, anti roll bars are used to tune front to back handling balance, just like you thought. But of course there is much nuance and dark art to suspension tuning. Spring rates and damper rates are used primarily while anti-roll bars are used for more fine tuning. For some more thoughts about it, check out this thread started by the pro driver, racing coach and and author Ross Bentley, http://www.m3post.com/forums/showthread.php?t=759078 I also reccomend his books. They're totally mental and are applicable to more than just race driving. I was reallysurprised to see him post on the forum actually and encourage everyone to pick his brain--I know I will.

Likewise, given that you cannot change the total load transferred, a flatter car is not necessarily faster, depending on the circuit. Indeed, take a track that does not have transition corners like MR Laguna Seca for example. A relatively compliant suspension is faster at the track because compliance = grip at the cost of transitional weight management given that the car does not bottom out and that the suspension geometry doesn't overly roll into challenges. But managing transitional weight is not the priority when the track has 1 transitional corner and it is low speed and leads onto a downhill section like at Laguna Seca. So it is not rewarding to tune the car to a relatively unimportant corner.

The the HACK said, dynamic camber change is an issue with macpherson strut design. The basic effect is that as the spring compresses when you load that corner, the wheel itself goes through a camber change, first gaining camber, and then losing it as the spring goes deeper into compression. This rate of camber change can be plotted on a graph where you see can see the camber change for a given length of compression and that is called the dynamic camber curve. Peep this vid.



I have no idea what the dynamic camber curve looks like for the e9x m3, but I would guess it is among the best out there since bmw, and m division really do know how to get the macpherson strut working for them. I believe it is for these reasons: 1 the longer control arms on the m3 broaden the curve so that marginal the rate of camber change is lessened at every point. If you think about it geometrically, an arc that is had wider radius has less change in degrees for a given distance than a short one. 2. the m3 has higher control arm joints with the chassis than other bmws that suffer this problem. Higher control arm joints mean a more acute angle between the A arm and the strut, putting the majority of the suspension's travel in the camber gain part of the curve. Camber gain is better than camber loss, but it can still hurt the contact patch nevertheless. That said, no proper tube chassis race car would have this design.

Ultimately, like the HACK said, the only way to know what works is by testing it, and what works with one tire will not necessarily work with another, especially if they're very different in grip.

[bigjae1976]

I've got the dinan stage II with GC camber plates at -3.0 with square 275s. My E90 does not understeer at all unless I get totally retarded when turning in. In fact, its a little too much on the oversteer side. I'm going to try -2.5 at some point in the future and see if it makes the car more neutral.

Maybe I'm used to my 330 but it feels like I'm always working the keep the back end in the back.

[M3SQRD]

Originally Posted by bigjae1976

I've got the dinan stage II with GC camber plates at -3.0 with square 275s. My E90 does not understeer at all unless I get totally retarded when turning in. In fact, its a little too much on the oversteer side. I'm going to try -2.5 at some point in the future and see if it makes the car more neutral.

Maybe I'm used to my 330 but it feels like I'm always working the keep the back end in the back.

So did your car have the over-steering characteristics/feel before you installed the front-only sway bar?

[bigjae1976]

Originally Posted by M3SQRD

So did your car have the over-steering characteristics/feel before you installed the front-only sway bar?

Not sure if it was the sway bar, square tires, camber plates or a combination of all of the above.

[lutfy]

Originally Posted by bigjae1976

I've got the dinan stage II with GC camber plates at -3.0 with square 275s. My E90 does not understeer at all unless I get totally retarded when turning in. In fact, its a little too much on the oversteer side. I'm going to try -2.5 at some point in the future and see if it makes the car more neutral.

Maybe I'm used to my 330 but it feels like I'm always working the keep the back end in the back.

I am in the same boat. Stock suspension w/GC plates -1.8 back, -2.8 front camber. Before the car would plow and I would hammer the gas at track out (slower speeds at steady state allowed for full throttle exits).

With increased grip, the rear is traction limited at track out. Thats the only place I see track out. Instead of reducing camber (front grip to compensate or negate to rear grip), my suggestion would be to run wider rubber at the back. I have a square setup now (275 Toyo R Comps) but will get a pair of 295 for the back.

We ran a staggered setup in the GT4 Vantage N24 and the car was much balanced.

Lutfy

[bigjae1976]

Originally Posted by lutfy

I am in the same boat. Stock suspension w/GC plates -1.8 back, -2.8 front camber. Before the car would plow and I would hammer the gas at track out (slower speeds at steady state allowed for full throttle exits).

With increased grip, the rear is traction limited at track out. Thats the only place I see track out. Instead of reducing camber (front grip to compensate or negate to rear grip), my suggestion would be to run wider rubber at the back. I have a square setup now (275 Toyo R Comps) but will get a pair of 295 for the back.

We ran a staggered setup in the GT4 Vantage N24 and the car was much balanced.

Lutfy

Thanks. I was planning on trying a staggered setup in the future. Yeah, it can be a handful at times but it goes through slower corners like a much lighter car.

[Protocav]

Start by adding a 5mm spacer to the rear. If you get wider rear tires, you will just induce push with the front. It will solve the throttle oversteer, but you will probably loose the crispness on turn-in. It sounds like the car is well balanced but just needs a tiny bit of rear grip. Try lowering your rear tire pressure 1-2 psi as well.

Some might say to be more patient on exit, but we all know that isn't fun at all

[bigjae1976]

Originally Posted by Protocav

Start by adding a 5mm spacer to the rear. If you get wider rear tires, you will just induce push with the front. It will solve the throttle oversteer, but you will probably loose the crispness on turn-in. It sounds like the car is well balanced but just needs a tiny bit of rear grip. Try lowering your rear tire pressure 1-2 psi as well.

Some might say to be more patient on exit, but we all know that isn't fun at all

Oversteer is way more fun than understeer! I don't track my M3 much so I haven't played with pressures yet. I just notice that I have to be a lot more mindful of what my rear end is doing in the M3 vs the 330. The 330 behaves much better.

[GTM_Challenge]

What are your toe settings at for the rear?

[GTM_Challenge]

When diagnosing handling issues, it's important to note the subtle nuances of where and when the car is understeering or oversteering. You wouldn't fix an oversteer on entry issue the same way you would fix an oversteer on exit issue. Try and keep notes of where the car is doing something you dont like, what your settings are at, tire pressures, and if you can - look on video and see what your steering angle is at. All this information helps diagnose a handling issue much easier.

[bigjae1976]

I used the dinan Stage III specs (front and rear) but with more front camber. I ran 39psi hot on street tires. It tends to oversteer on exit. I can throw away one session were my rears were 47psi...definitely could tell it the tire pressures were too high while I was driving it.

If I come off the track and all of the pressures are equal on all 4 corners then I can say the car does lean towards oversteer. Not a huge issue though, I just keep my foot in the throttle and DRIFT BABY! I am asian so that is totally normal.

[lenzm3edan]

so is it worth adding the front only as long as you have lowering springs?

[GTM_Challenge]

Originally Posted by lenzm3edan

so is it worth adding the front only as long as you have lowering springs?

From a general suspension set up standpoint, no - not unless you're getting oversteer on track.

As mentioned before, a front sway will usually cause the car to understeer more - something you dont want if the car is set up to push from the factory anyways.

However, as seen in the thread, some people are reporting that a front sway can cure an understeer issue. I've not experience this myself. Your results my vary. My general suspension setup knowledge leads me to say that a front bar would cure an understeer issue, but i've never tried to fix one with a front bar - always a rear bar or alignment setting.

[bigjae1976]

If you drive the car on the track and have understeer issues, I think it is a step in the right direction.

[bigjae1976]

Originally Posted by GTM_Challenge

From a general suspension set up standpoint, no - not unless you're getting oversteer on track.

As mentioned before, a front sway will usually cause the car to understeer more - something you dont want if the car is set up to push from the factory anyways.

However, as seen in the thread, some people are reporting that a front sway can cure an understeer issue. I've not experience this myself. Your results my vary. My general suspension setup knowledge leads me to say that a front bar would cure an understeer issue, but i've never tried to fix one with a front bar - always a rear bar or alignment setting.

Typically in an E46 (suspension design is not that different, right?), most run the biggest front bar possible and a smallish rear bar. Some even skip using a rear bar..all in the name to banish understeer. Sheesh, Turner sells a 40mm front bar. If the normal thinking were to hold true, wouldn't a 40mm bar render your steering wheel useless?

I think the main difference with the E9X is that you don't have to do nearly as much to get the car close to neutral.