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Front sway bar thread

bostonaudi

Go Kart Champion
Location
Charleston, SC
Car(s)
1995 BMW M3
So, we all pretty much know what a bigger rear bar does for these cars.

What about a bigger front bar? Anyone running both front and rear? Any thoughts or feedback? I've seen comments from APR's drivers that it helps, but does it also induce more wheelspin, or make front plow worse?
 

Dynjo

Ready to race!
Location
Chicagoland
What about a bigger front bar? Anyone running both front and rear? Any thoughts or feedback?

Vehicle dynamics theory says that the balance of the car in steady-state cornering is a function of the RATIO of front stiffness to rear stiffness (F/R). And obviously the roll angle of the vehicle is a function of the TOTAL roll stiffness (F + R).

So, when you increase the front roll bar diameter, you actually do two things: You increase the overall roll stiffness and, probably more importantly, you've changed the ratio of front-to-rear stiffness. In this example, the vehicle would roll less and understeer more.

So, what is the goal? I think we're talking about migrating toward a more race-worthy setup, right? In that case, we have two goals: First, reduce body roll so that the tires stay flat on the pavement, and second, get rid of the understeer.

We've discussed elsewhere how stiffening the rear will reduce understeer. This is because the F/R stiffness ratio is reduced. However, if we then turn around and stiffen the front, we're back into understeer, although we have reduced body roll.

It is quite possible to achieve both goals, though. All that needs be done is to stiffen front and rear, but stiffen rear more than front.

I once went through an interesting exercise with my old car. I figured out how to measure roll stiffness in terms of pounds per degree of body roll. I did this with a couple of different front bars and with all of the position of my adjustable rear bar. I knew from experience which of those combos I liked at the track, so I knew what the F/R ratio wanted to be. Later, I increased the rate on the coil springs dramatically and remeasured roll stiffness F and R. I then adjusted the bars to get back to my "ideal" F/R ratio (now a very different combo of bars and adjustment settings) and voilà!, the car was nicely balanced right off the trailer, except much stiffer overall.

Interestingly, the optimum F/R ratio was very different for autocross and road course. I have another theory on that, but that's a different thread.

In the real world, we've all seen FWD cars lift the inside rear in a corner. This is often the case when the owner installed a stiff rear bar but nothing else. The experts will tell you that a four-wheel car will corner better than a three wheel car. The "three-wheeled" car in this example would benefit by increasing overall stiffness (to keep all four on the ground), but retaining the F/R ratio that was initially achieved by increasing rear bar diameter.

When the APR guys talk about the benefits of a big front bar, I think they are actually discussing the effect of increasing overall roll stiffness.

Whew. Sorry for that tome.
 
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Tpass

Ready to race!
Location
Tree city, Or
Car(s)
08 BMP 6M GTI
I thought I remembered the APR driver saying that it helped keep the inside wheel weighted better as the front is so heavy it overwhelms the out side tire in corners.

subscribed...
 

SimonSaysGTI

Ready to race!
Location
DE
The APR drivers did recommend a FSB. A larger FSB will increase understeer since the front is stiffer, however it will decrease roll which is a good thing. If you go with a large RSB like the 27mm BSH, H&R, or APR then do a medium FSB handling overall should improve, less roll and less understeer should be the end result.
 

Dynjo

Ready to race!
Location
Chicagoland
Referring to the APR-related comments... I remember that, too, but I can't make sense of it. Can't figure out how a bigger front bar would but more weight on the inside tire. It will tend to un-weight the inside tire.

Think about it... The bar is U-shaped. If one of the ends goes up (the outside wheel moves up into the wheel well as the body rolls), then the other end will go up, as well, attempting to lift the inside wheel off the pavement (un-weighting it).
 

bostonaudi

Go Kart Champion
Location
Charleston, SC
Car(s)
1995 BMW M3
Referring to the APR-related comments... I remember that, too, but I can't make sense of it. Can't figure out how a bigger front bar would but more weight on the inside tire. It will tend to un-weight the inside tire.

Think about it... The bar is U-shaped. If one of the ends goes up (the outside wheel moves up into the wheel well as the body rolls), then the other end will go up, as well, attempting to lift the inside wheel off the pavement (un-weighting it).

My thoughts exactly, which is why I fear messing with my stock front bar. I think stiffer front springs might be better before a bigger front bar, but as said, if both front and rear are upgraded but in proportion. I do have an LSD so perhaps worries of unloading inside tire are unfounded.
 

ducsNdubs

Ready to race!
Location
pittsburgh
i run both neuspeed front and rear sways. as discussed above it does help reduce the body roll in the front of the car greatly and keeps the car much more flat. It eliminates the 3-wheel condition completely. I have the rear set to full stiff and the front to medium. I also have the dampening in the rear of the car set stiff and the front a little less. The car feels good on turn in and mid corner but after apex when trying to ease back into the throttle i still get a ton of inside wheel spin and understeer. I know i need an lsd but to get one installed is so costly.

any other tweaks i could do to remedy this. I was also thinking of camber plates up front and the walk anti lift kit. Anyone with some good advice?
 

Frank Rizzo

Trailingthrottleoversteer
Location
Via Mulholland
I have them front and rear...but I have an R32 so it's different dynamic than a GTI.

They are fantastic for the track but a disadvantage at Auto-X.

.
 

mocohead

Ready to race!
Location
Sunny CA
I have the Hotchkis 27mm front on medium and the Autoech 24mm rear set on firm. My experience so far is one auto-x and a few hundred miles on the street.

Interestingly I have found that turn-in is more crisp and the car feels like it has less entry and mid-corner understeer. During constant acceleration corners I can feel the car rotating just a tiny bit, which was my goal. It tricycles less than before and generally behaves better. Power understeer at the exit is still an issue but seems to be tamed a touch.

On the street the car feels really well planted and very stable. The exception to that is on rough corners, where the stiff FSB tends to cause the front of the car to be less settled.

It's going to be a while before I get to a track with this setup unfortunately.
 

xSabretoothx

Fast w/ training wheels
Location
Raleigh, NC
Car(s)
2008 GTI
They are fantastic for the track but a disadvantage at Auto-X.

.

Exactly, depends on what you are looking for. I have a 28mm solid on the front set on soft, and a 27mm solid set on hard on the rear. This car handles remarkably on the track, but doesn't pivot as well at the auto-x. Regardless, for daily driving it is amazing!
 

bostonaudi

Go Kart Champion
Location
Charleston, SC
Car(s)
1995 BMW M3
I have the Hotchkis 27mm front on medium and the Autoech 24mm rear set on firm. My experience so far is one auto-x and a few hundred miles on the street.

Interestingly I have found that turn-in is more crisp and the car feels like it has less entry and mid-corner understeer. During constant acceleration corners I can feel the car rotating just a tiny bit, which was my goal. It tricycles less than before and generally behaves better. Power understeer at the exit is still an issue but seems to be tamed a touch.

On the street the car feels really well planted and very stable. The exception to that is on rough corners, where the stiff FSB tends to cause the front of the car to be less settled.

It's going to be a while before I get to a track with this setup unfortunately.

Well, I ordered a Hotchkis F/R 27mm sway bar kit a few weeks ago, and finally got the front bar on yesterday. A bit of a pain (subframe has to be dropped), but I got the same results you describe above, so well worth the hassle of installing. Turn in is definitely more crisp, with both bars installed and both at middle setting the car feels super planted on corners now. I have to drive it more, but so far seems like total win. I haven't noted any traction issues but I also have an LSD so power will go to outside tire if inside spins anyhow.

Also, the bushings are quiet, much more quiet then the Autotech rear bar I used to run. They come with grease fittings, and the bushings have channels to distribute the grease. Ride quality doesn't seem affected either. The Hotchkis bars are same as APR, but priced lower. An excellent kit. First time I've ever been pleased with polyurethane!
 

madrestaur

Having Fun driving my VW.
Location
Springfield, MA, USA
Car(s)
MKV '06
24 rsb and 28 fsb

So, we all pretty much know what a bigger rear bar does for these cars.

What about a bigger front bar? Anyone running both front and rear? Any thoughts or feedback? I've seen comments from APR's drivers that it helps, but does it also induce more wheelspin, or make front plow worse?

I tell you what these have had a dramatic affect on the corning / handling, especially on HP track events.
I had just the rear in for New Hampshire, but it was not right, still fast, I had the front installed and at the glen wow, super.

Daily driving it is very flat on the turns.
 
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