[shivaswrath]

Originally Posted by stevehecht

Could someone go over the pros and cons of installing thicker RSB only vs. RSB+FSB? How do they differ in how they affect the car's response to steering input, cornering forces, etc.? I've got both on my E30 and love it. I've got only RSB on my fwd V6 Honda Accord, and I like that too.

FWIW I won't be tracking the car, just the occasional autoX or ADSS in my F30. And spirited road driving of course.

I'm thinking front would be fine, since we have a tendency to understeer and the larger FSB would offset this.

The larger rear...unsure.

[drob23]

Not sure how close the F80 is to the F30, but is that the anti-roll bar in there (blue)?



Although to be honest, I'm not even sure where the ARB is

[DVC]

Originally Posted by shivaswrath

I'm thinking front would be fine, since we have a tendency to understeer and the larger FSB would offset this.

The larger rear...unsure.

It's actually the other way around... stiffer rear sway bars reduce understeer.

[stevehecht]

Originally Posted by DVC

It's actually the other way around... stiffer rear sway bars reduce understeer.

I'm actually worried about increased oversteer with RSB in snowy/slippery conditions since I have RWD. Winter tires will help, but it definitely would increase the risk of a spin-out on ice or slick snow.

[JMH063]

Originally Posted by shivaswrath

assuming the bottom is before and top is after?

Correct.

[drob23]

Originally Posted by stevehecht

I'm actually worried about increased oversteer with RSB in snowy/slippery conditions since I have RWD. Winter tires will help, but it definitely would increase the risk of a spin-out on ice or slick snow.

The ARB's will primarily effect the handling under load transfer conditions...if you've ever driven a car on the track, there is quite a distinction between this type of driving and putting around town getting groceries. Don't get me wrong, you can hoon your car how you see fit, but the point at which handling changes will not be experienced unless you're putting in hard work

BTW - the snow/ice oversteer situation is almost always due to throttle inputs being too large.

[DVC]

Originally Posted by stevehecht

I'm actually worried about increased oversteer with RSB in snowy/slippery conditions since I have RWD. Winter tires will help, but it definitely would increase the risk of a spin-out on ice or slick snow.

I don't think the improved front grip from decreased chassis roll will in any way affect winter traction. In those conditions there's not enough grip to be had to even get to the point where the increased chassis stiffness even comes into play...

[stevehecht]

Originally Posted by drob23

The ARB's will primarily effect the handling under load transfer conditions...if you've ever driven a car on the track, there is quite a distinction between this type of driving and putting around town getting groceries. Don't get me wrong, you can hoon your car how you see fit, but the point at which handling changes will not be experienced unless you're putting in hard work

BTW - the snow/ice oversteer situation is almost always due to throttle inputs being too large.

It looks like you're saying that the RSB won't even be noticed for improved handling during spirited street driving. I do take the car out for autoX or ADSS a few times a year, but if that's the only time I'll notice the difference then it's probably not worth the expense.

Originally Posted by DVC

I don't think the improved front grip from decreased chassis roll will in any way affect winter traction. In those conditions there's not enough grip to be had to even get to the point where the increased chassis stiffness even comes into play...

As drob said above, those oversteer problems come mainly from inappropriate use of throttle input. Good to know that's it's all about the driver and not the hardware (isn't it always...?).

[drob23]

Originally Posted by stevehecht

It looks like you're saying that the RSB won't even be noticed for improved handling during spirited street driving. I do take the car out for autoX or ADSS a few times a year, but if that's the only time I'll notice the difference then it's probably not worth the expense.

Sorry, I was more speaking to driving on the street getting groceries and the associated safety hazard. I think you would absolutely notice a difference in autoX, pretty much in any situation where you are getting the tires singing

I will say though that keeping the car flat in high speed on-ramps is really nice, gives you a great sense of confidence. So even in a situation where you aren't exactly taking advantage of the more neutral handling, the flat road holding ability is awesome.

Also, I mentioned the change in understeer/oversteer, but also mentioned a big advantage of the bars is not rolling onto the sidewall as easily. Similar to dialing in some extra negative camber. Will help preserve your tire sidewall on track days.

[stevehecht]

Originally Posted by drob23

Sorry, I was more speaking to driving on the street getting groceries and the associated safety hazard. I think you would absolutely notice a difference in autoX, pretty much in any situation where you are getting the tires singing

I will say though that keeping the car flat in high speed on-ramps is really nice, gives you a great sense of confidence. So even in a situation where you aren't exactly taking advantage of the more neutral handling, the flat road holding ability is awesome.

Also, I mentioned the change in understeer/oversteer, but also mentioned a big advantage of the bars is not rolling onto the sidewall as easily. Similar to dialing in some extra negative camber. Will help preserve your tire sidewall on track days.

Not rolling onto the sidewall also increases the contact patch which gives the tires more adherence and the car more stability. (Just quoting you from your post #26 on this thread.)

OK, that's more encouraging about spirited street driving, thanks. Now if we could only do something about the humongous amount of labor involved with installation!

[DVC]

Originally Posted by stevehecht

Not rolling onto the sidewall also increases the contact patch which gives the tires more adherence and the car more stability. (Just quoting you from your post #26 on this thread.)

OK, that's more encouraging about spirited street driving, thanks. Now if we could only do something about the humongous amount of labor involved with installation!

I just got a quote from my favorite local shop - $300 for the install (they are about $100/hr). I can live with that.

[stevehecht]

deleted

[stevehecht]

Originally Posted by DVC

I just got a quote from my favorite local shop - $300 for the install (they are about $100/hr). I can live with that.

And that's installation for front and rear. Sweet! I'm curious as to what TMS will charge if I go there just for the rear.

[DVC]

Originally Posted by drob23

...The converse is you want to have some traction on corner exit, so you'd like to have some roll onto the outside rear wheel. It's a little different since my car is AWD, so it's easier to put the power down than a RWD torquey car like many of your F3x's. But with too big of a rear bar, you won't get any weight transfer onto the rear outside tire and might get premature throttle oversteer...

Somehow I missed this post before...lots of good insight that I agree with, but I'm not sure I follow your thinking on the part quoted above. In terms of traction (i.e. the ability to put power down to the asphalt) on corner exit, why would weight transfer to the outside rear tire be beneficial? We're not talking about cornering grip (which is a different discussion entirely), but rather acceleration traction... weight transfer to the outside tire would, for an open diff RWD car, increase the likelihood that the inside rear tire will spin, which results in immediate power/traction loss. A good LSD of course mitigates the power/traction loss in this situation, but the outside rear tire weight transfer still would not be advantageous for traction.

[stevehecht]

Originally Posted by DVC

Somehow I missed this post before...lots of good insight that I agree with, but I'm not sure I follow your thinking on the part quoted above. In terms of traction (i.e. the ability to put power down to the asphalt) on corner exit, why would weight transfer to the outside rear tire be beneficial? We're not talking about cornering grip (which is a different discussion entirely), but rather acceleration traction... weight transfer to the outside tire would, for an open diff RWD car, increase the likelihood that the inside rear tire will spin, which results in immediate power/traction loss. A good LSD of course mitigates the power/traction loss in this situation, but the outside rear tire weight transfer still would not be advantageous for traction.

drob is saying that you need "some" weight transfer to the rear outside wheel on corner exit because the weight in the rear is biased to the inside when you're in the corner. That's my take on it.

[DVC]

Originally Posted by stevehecht

drob is saying that you need "some" weight transfer to the rear outside wheel on corner exit because the weight in the rear is biased to the inside when you're in the corner. That's my take on it.

The weight in a corner is never biased to the inside - this is the role of anti-sway bars: to fight excessive weight shifting to the outside, and maintain chassis composure.
Some weight transfer to the outside is a good thing (you wouldn't want a completely flat car) to help with cornering grip... I'm just not sure I understand how it would aid in acceleration traction on corner exit.

[drob23]

Originally Posted by DVC

Somehow I missed this post before...lots of good insight that I agree with, but I'm not sure I follow your thinking on the part quoted above. In terms of traction (i.e. the ability to put power down to the asphalt) on corner exit, why would weight transfer to the outside rear tire be beneficial? We're not talking about cornering grip (which is a different discussion entirely), but rather acceleration traction... weight transfer to the outside tire would, for an open diff RWD car, increase the likelihood that the inside rear tire will spin, which results in immediate power/traction loss. A good LSD of course mitigates the power/traction loss in this situation, but the outside rear tire weight transfer still would not be advantageous for traction.

Here's my thinking, keep in mind that this is my opinion and certainly could be wrong. There are also various aspects of the chassis balance that are fighting against each other when cornering on throttle.

With that said, if you don't take an absolute late apex and want to throttle steer the vehicle while still in a corner, I think you'll get tail wagging power oversteer sooner with a stiff rear bar, which is not what you want in terms of maximum acceleration. You need the outside rear to put down the power since that's where the weight will want to go if you're at the limits of the tires.

But this is me keyboard racing, maybe you have better evidence to suggest otherwise? Your point about the LSD is an interesting one, since you will definitely more quickly engage the anti-slip mechanism with weight off the inside wheel. Every time you engage the LSD you will cause chassis understeer since the rear wheels are forced to spin at the same speed (which shouldn't happen while cornering).

[DVC]

Originally Posted by drob23

...if you don't take an absolute late apex and want to throttle steer the vehicle while still in a corner, I think you'll get tail wagging power oversteer sooner with a stiff rear bar, which is not what you want in terms of maximum acceleration. You need the outside rear to put down the power since that's where the weight will want to go if you're at the limits of the tires.

Again, I don't see why weighting the outside tire would provide a traction advantage... If anything, I think a greater weight bias to the outside of the car would do more to upset chassis balance, which if anything would detract from being able to rely on both rear tires' traction to apply maximum power at corner exit.
As for tail-wagging oversteer with a stiff rear bar, this I could see being true - but not due to a rear tire traction loss, rather because of the net effect of reduced understeer... which of course can be mitigated by proper steering inputs.

Originally Posted by drob23

...But this is me keyboard racing, maybe you have better evidence to suggest otherwise?

Your version of "keyboard racing" is fine by me... I've been following your posts since joining these boards last Spring, and you always have great information to share and contribute.

Originally Posted by drob23

Your point about the LSD is an interesting one, since you will definitely more quickly engage the anti-slip mechanism with weight off the inside wheel. Every time you engage the LSD you will cause chassis understeer since the rear wheels are forced to spin at the same speed (which shouldn't happen while cornering).

Regarding the LSD, they can certainly cause an increase in understeer - more so in units configured with steeper ramp angles (which result in the pinion shaft forcing the pressure plates harder against the clutch packs) resulting in higher lock-up percentages. Higher lock-up percentages of course mean that the plates are more resistant to slipping, and allowing the half shafts to rotate at different speeds.
The BMW M Performance LSD is configured at 30% accel./9% decel lock-up, which I've found to be a nice balance between improved traction and smooth, progressive unlocking. As a result with this unit, you can definitely feel a hint more initial understeer on the power through a corner, UNTIL you feel the clutch plates slip... and then the understeer dissipates and you feel like the car scoots you through the rest of the corner. (It's a very cool sensation.)

While we're at it, it's worth clarifying a few things about LSDs in general. A traditional clutch pack LSD works fundamentally differently than a helical torque-sensing unit (Torsen, Quaife, etc.) Without belaboring this with a technical description of how a helical unit works, suffice it to say that contrary to popular belief, the "torque sensing" aspect of a helical unit does not really apply to clutch LSDs. Clutch units apply lock-up under acceleration (and typically to a lesser extent - e.g. 1.5 way units - under deceleration). Period. When you get on the power, the pinion shaft presses against the pressure plates, which compress the clutch packs, thus working to lock the half shafts to create that locked differential feel. None of this involves "sensing a slipping wheel" or "transferring" torque to the wheel with grip; I see these descriptions a lot, and they are misleading.
I mention all this to illustrate that with a clutch LSD, you're not "engaging the anti-slip mechanism" in a corner in response to inside rear tire slippage... the anti-slip effect is engaged already if you're on the power. What does happen is the clutches allow a bit of slippage so that the inside rear tire can rotate more slowly, and mitigate understeer. (hence, "limited slip" diff). Sorry for the LSD lecture; I could go on about the deceleration side, but I'll save that for another time

[stevehecht]

I've ordered a RSB for the 328i and will take it to a local suspension shop. They will first eyeball the fit for my 328d and if it looks good will try to complete the installation. Fingers crossed!

I've checked with folks with more knowledge than I have about chassis dynamics and they've agreed with me that the neither the minor weight differential between the two cars or the difference in hp/torque will make the 328i bar inappropriate for the d.

BTW, Tuner Motorsport didn't even have these in stock yet. They had to order it from the H&R warehouse in Washington state.

[DVC]

Originally Posted by stevehecht

I've ordered a RSB for the 328i and will take it to a local suspension shop. They will first eyeball the fit for my 328d and if it looks good will try to complete the installation. Fingers crossed!

I've checked with folks with more knowledge than I have about chassis dynamics and they've agreed with me that the neither the minor weight differential between the two cars or the difference in hp/torque will make the 328i bar inappropriate for the d.

BTW, Tuner Motorsport didn't even have these in stock yet. They had to order it from the H&R warehouse in Washington state.

Cool... mine arrived on Thursday, and I had an afternoon appointment on Friday to install. My shop underestimated the labor though, so they did the rears for me on Friday, and I'm heading back in this week for the fronts.
Loving the improvements already...

[stevehecht]

Originally Posted by DVC

Cool... mine arrived on Thursday, and I had an afternoon appointment on Friday to install. My shop underestimated the labor though, so they did the rears for me on Friday, and I'm heading back in this week for the fronts.
Loving the improvements already...

Please keep us updated on your impressions. You know your sh*t and you know how to write.

What's a reasonable install time for the RSB do you think?

[DVC]

Originally Posted by stevehecht

Please keep us updated on your impressions. You know you sh*t and you know how to write.

What's a reasonable install time for the RSB do you think?

Thanks! And will do... they charged me 3.5 hours for just the rear, which I think was more than fair based on the fact that you need to drop the entire rear subframe to get in there.