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BMR Sway bar kit installed on GXP (w/calculations)

11K views 39 replies 22 participants last post by  JonRobbinSS 
#1 · (Edited)
The BMR bars are installed and ready to go.. I was working on calculating the % increase of the rear bar over the stock GXP bar and came up with some interesting numbers.


GT rear bar diameter is 18mm
GXP rear bar diameter is 20mm
Given that the two bars are both solid and of the same design the GXP rear bar would be 52% stiffer than the GT bar.

That along with BMR's published increases of their bar over the GT rear bar I got the following
hole 1 - 26% stiffer than GXP rear bar
hole 2 - 74% stiffer than GXP rear bar
hole 3 - 154% stiffer than GXP rear bar

Given the front bars are the same for both cars at
hole 1 - 87% stiffer
hole 2 - 137% stiffer

Then the fact that I would like decreased body roll with just a little more rear bar bias then my current stock setup leads me to think that full stiff on both bars might be the best bet F137% / R154% (17% stiffer bias to the rear than stock GXP). This would seem to be the only choice to get close to what I am looking for.

All the other choices stiffen the front bar more than the rear bar increasing bias to the front (increasing under-steer). If this ends up being too stiff a set-up, the only choice would be to go back to the stock front bar with the BMR rear set to hole 1 for a 26% increase to the rear but that will not do much to decrease body roll.. We will see how it goes on the course this Sunday.
 
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#6 ·
I'll be installing the BMR sways soon. I'm a GT but will have FE3's and Eibach lowering springs. Will this be a good starting point? Full soft(hole 1)/Full stiff(hole 2)?
 
#8 · (Edited)
Today I setup a low speed skid pan in an unused parking lot and went to work testing the different settings I was considering.

I used Harry's Laptimer on an iTouch with a 5Hz external GPS to record lateral acceleration and speed data to help quantify the different settings.

My plan for testing was for the following combinations. Test 1 and 4 were the same to help find any errors with the test over time(tire temp, track cleaning and the like)
test 1; F-hard/R-hard
test 2; F-soft/R-hard
test 3: F-soft/R-medium
test 4; F-hard/R-hard

I then downloaded the CSV file from the data logger for all the runs and played with the results in excel - the highest peak speeds and accelerations were found on tests 1 and 4 with test 2 following very close behind (really close), test 3 was a bit farther down.

So it looks like full hard is the way to go on the rear bar and it is up to your driving style and comfort as to how you set the front. I think what is happening in the front is on the hard setting the bar is reducing the body roll enough that the outside tire is retaining more negative camber and that is working to get you around the turn quickly, on soft more weight is transferring to that outside tire giving it more traction.

I am going to go with the front on full hard. Everything being equal - keeping that outside tire's negative camber by reducing body role will hopefully save a little wear on those outside shoulders, and might allow me to lower my tire pressures on the track for even more traction.

I am looking forward to seeing how it does on the autocross track tomorrow.
 
#19 ·
what is happening in the front is on the hard setting the bar is reducing the body roll enough that the outside tire is retaining more negative camber and that is working to get you around the turn quickly, on soft less weight is transferring to that outside tire giving more traction across the pair.
Fixed that for you ;)

To paraphrase late great Carroll Smith in Tune to Win, "any increase in lateral load transfer between two tires decreases the total traction of the pair". That's because traction increases with tire load, but not linearly. For example if you have 1000 lbs of load on a tire and you add 500 more, you don't get 1500 lbs of traction, you get 1250 (strictly for example's sake). So if you have two tires each with 1000 lbs of load, you have 2000 lbs of available traction at that end of the car. If you take 500 off one tire and put it on the other, now you have 1500 and 500 lbs of load, but only 1250 and 500 lbs of traction, for a total of 1750 lbs at that end of the car, which is 250 lbs less than you started with.

And that's why, assuming a properly set up car in the first place, you lose traction at the end of the car you stiffen. Or, in your case, you keep traction at the end of the car you don't stiffen.

What complicates all of it is camber control, among other things, which you already know. If you're losing camber on the outside tire due to body roll, then reducing body roll by adding stiffness nets you more traction than you lose to the increased load transfer.
 
#9 ·
Great test data!
Probably close enough that it will depend on tires, shocks, springs, camber and track conditions.
I never went full hard on the front, but I did like full hard rear better than medium.
I am putting a set of eibachs and new fe3's on soon... May try full hard at some point down the road after the Michelin SS's are broken in.
My camber is at -1 front and back an the wear is more on the inside edge, so I can't go much more without tearing up my new $$ rubber.

Let us know how you do at the autoX!
 
#10 · (Edited)
Well after a great day on a good fun course, I am happy with both bars on full stiff. The car is really neutral. At the end of the video (link below) you can hear the announcer commenting on a slide during my downshift (kept a pretty neutral throttle), you notice no slide within the car or any counter steering input required. The slide was a very controlled four wheel slide, felt good.

On the run after this one I pushed past the limits of adhesion in a few different spots to see what would happen - during braking I got under-steer, on the power I got over-steer, and on a neutral throttle the handling was neutral. Couldn't ask for much more.

Link to post with video of best run
 
#15 ·
Well after a great day on a good fun course, I am happy with both bars on full stiff. The car is really neutral. At the end of the video (link below) you can hear the announcer commenting on a slide during my downshift (kept a pretty neutral throttle), you notice no slide within the car or any counter steering input required. The slide was a very controlled four wheel slide, felt good.

On the run after this one I pushed past the limits of adhesion in a few different spots to see what would happen - during braking I got under-steer, on the power I got over-steer, and on a neutral throttle the handling was neutral. Couldn't ask for much more.

Link to post with video of best run
I also found an error in my first post's calculations.. The rear bar sizes are 18mm and 20mm - that changes the rest of the numbers since that increase is a 52% stiffer bar. I will work it out and make the corrections shortly.

Done - The info has been corrected.
Thanks for posting up the video and giving us some very thorough feedback! :)
 
#11 · (Edited)
I also found an error in my first post's calculations.. The rear bar sizes are 18mm and 20mm - that changes the rest of the numbers since that increase is a 52% stiffer bar. I will work it out and make the corrections shortly.

Done - The info has been corrected.
 
#20 ·
MFE, I see what you are getting at and its a good point..

But I stand by my first statement, if you assume the rear bar fixed at full stiff and the two choices are stiff and less stiff for the front bar it is more total traction across the pair on soft due to larger lateral weight shift with more weight to the outside tire and the increased weight caused by that load transfer will give the outside tire more traction when compared to the stiffer setup (you pointed it out you loose traction as you stiffen that bar). The outside tire is doing the lion's share of the turning work - Here is where your quote comes in.. if you transfer weight off the outside tire to the tire not doing as much work with a stiffer bar you loose total traction at that end.

And like we both pointed out camber control issues then rise up and make the whole issue into something else because both set ups end up with the same total traction and you end up making the decision based on tire wear patterns and car feel at the limit.

This stuff is complicated, hard to get your mind around and even the experts have differing opinions on why things work and others don't. And why after all the thinking is done you go out and test and decided what feels good for the car and your driving style.
 
#21 · (Edited)
But it's not more weight shift to the outside tire on a softer setting, that's the whole point. With a stiffer bar, you're not moving load to the unloaded tire, you're moving it the other way, from the unloaded tire to the loaded tire. I guess you could visualize it this way: The stiffer the bar, the more the bar is trying to lift the inside tire as the outside tire's suspension is compressed. That's load being transferred from the inside (less loaded) to the outside (more loaded) tire. I think you're visualizing body roll as if the roll itself is applying weight to the outside wheels, but that's not the case. FWIW this isn't me pulling it out my ass, it's physics as determined by vehicle dynamics experts long ago. :driving::us_flag:
 
#23 · (Edited)
After a year of thinking about what MFE was saying above and a lot more testing and data acquisition on the autocross tracks I have found the ultimate cornering grip so far using the King HD springs is to use the stock front sway and the BMR rear sway set on its softest setting.

The BMR front sway was a little too stiff for the HD springs even on its softer setting and did reduce the available lateral grip of the car

I did enjoy normal street driving on the BMRs both set to full stiff but the on-track performance in the turns suffered. Full soft for both BMRs was better but going back to the stock front bar was the best combo I could find for getting this beast around the turns quickly.
 
#30 ·
After a year of thinking about what MFE was saying above and a lot more further testing and data acquisition on the autocross tracks I have found the ultimate cornering grip so far using the King HD springs is to use the stock front sway and the BMR rear sway set on its softest setting.

The BMR front sway was a little too stiff for the HD springs even on its softer setting and did reduce the available lateral grip of the car

I did enjoy normal street driving on the BMRs both set to full stiff but the on-track performance in the turns suffered. Full soft for both BMRs was better but going back to the stock front bar was the best combo I could find for getting this beast around the turns quickly.
This makes sense to me since springs are always fighting the sway bar. With stiffer springs, you can downsize the sway bar stiffness and have better traction distribution since the stiffer springs will help keep the inside tire planted (or so I think).

Side Note... while anti-roll bars reduce the amount of roll, thus helping to maintain camber and CG heights, they increase the rate at which load transfers from the inside wheel to the outside wheel in a turn, lessening the usage of the inside tire. They're a great tuning tool to help balance a car, but stiffening a bar actually removes total available grip from the axle that is working in order to balance it with the one that is not. Often times, camber, toe, and/or tire pressure can be used instead to help improve the grip on the axle that isn't working as an alternative, resulting in greater total available grip.
..
Good find. I think many people are unaware of this. Body roll is uncomfortable for the driver, but is not the bane of good handling by itself. If the CG of the car is so high that the body roll causes a large lateral movement of the CG, then accepting the sway bar's penalty of traction reduction on the inside tire may be worth it. It just depends on how much CG movement/forces the sway bar saves vs the traction reduction on the inside tire. I tried to do the math one time, got lost, and gave up.

Another way to think about it...a sway bar might make a car roll less, but it may be slower through the turns. Conversely, going with stiffer lowering springs and softer sway bar may result in some body roll AND might be faster through the turns.

Point being, "Zero Body Roll" does not necessarily equal "Faster".
 
#24 ·
Interesting. It is so hard for me to get out of the fwd mindset I've been in for years, I've always ran a super stiff rear in autocross to rotate the fwd car (some ran super soft but the rear would break loose in two steps), in a powerful rwd car you want a soft(er) rear for on throttle bite, but ......auuugggghhhh need more thinking!
 
#25 ·
make sure you keep a eye on the nuts that hold the D-bushings for the front bar , the pass side came loose on mine , no idea how long they were on the car i bought it beginning of nov 13 , thursday i got a little bit of a odd noise , then friday leaving work the front end felt broke , but was still driving normal , felt ok saturday , then crossing a street and hitting the gutter(was very shallow) it felt like it hit a wall and went over it , checked it today and found those nuts loose , either use new nuts and/or blue locktite on them

nice info and thank you for the info !
 
#27 ·
Also another point to mention is you don't have to technically run both links on the same hole by having them on different holes can halve the desired affect of body roll correction!

So if you were to run hole 3 and hole 2 you would get half of what you would with both on hole 3!
 
#28 ·
I thought this at one point but after extensive study and a few suspension gurus beating me up with diagrams and force charts I will now wholly agree it is not a good idea.

On this style bar you should not run it on two different holes left and right. The problem is you are changing the lever arm length so you end up with asymmetric forces working on the left and right of the car.

The only way this works is if you have a method that keeps the same length lever arm while allowing for adjustable flex - like the adjustable blade style lever arms found on the truly top end adjustable bars.
 
#29 ·
From Dan Tiley - the engineer for the Richard Petty Race Team..

Dan Tiley said:
posted 11-19-2009 02:41 PM
So here's my 2 cents on the subject. I do this stuff for a living on a NASCAR team as a Vehicle Dynamics Engineer, but by no means claim to be the next Carroll Smith.

Think of the suspension inputs to the anti-roll bar in two ways... Ride Motion and Roll Motion.

In Ride Motion, when both wheels travel equal amounts, the side with the shorter arm length (ie. rearward most hole on the rear bar), will twist the bar a greater amount than the side with the longer arm length. As a result, the bar will actually induce roll towards the side with the longer (softer) arm length with a pure vertical "Ride Motion" input.

In Roll Motion, assuming that the left and right travels are equal and opposite, meaning for roll in a right hand turn, the left side moves into jounce 1" and the right side moves toward rebound 1", then the torque build in the bar is exactly the same if you run the left side soft and right side stiff, or vice versa. Since the length of arm to each side is different, however, the side with the shorter arm is resisted with a greater vertical force than the long arm side. Think of it as putting a slightly higher rate spring on the short side and slightly softer spring on the long side.

In reality, the loads for any typical turn on a road course will induce a combination of ride and roll motion so that the left and right sides will not have equal and opposite jounce and rebound travels. Aerodynamic downforce and roadway camber both induce vertical travel (Ride Motion) in the jounce direction, which means that the car will be subjected to unequal suspension roll forces based on left and right hand turns. Non-typical turns that are significantly off camber (ie. Turn 3 at Grattan) or on the downside of a hill crest (ie. VIR Turn 9) can put the suspension in rebound, which will reverse the Ride Motion effect of bar assymetry, reversing some of the roll effect.

Most people neutralize their bars with driver's weight at the static condition, and "jounce" and "rebound" travels are relative to that static position.


TileySpeed guide to Front & Rear Sway Bar Assymetry (for "typical" turns)

Front Bars (in order from Max to Min understeer on right hand turns)
Left Short, Right Short: Maximum front roll stiffness. Max understeer on lefts and rights
Left Short, Right Long: More understeer on rights than lefts
Left Long, Right Short: More understeer on lefts than rights
Left Long, Right Long: Minimum front roll stiffness, Minimum understeer effect on lefts and rights
Rear Bars (in order from Max to Min oversteer on right hand turns)
Left Short, Right Short: Maximum rear roll stiffness, Max oversteer on lefts and rights
Left Long, Right Short: More oversteer on lefts than rights
Left Short, Right Long: More oversteer on rights than lefts
Left Long, Right Long: Minimum rear roll stiffness. Min oversteer effect on lefts and rights

Side Note... while anti-roll bars reduce the amount of roll, thus helping to maintain camber and CG heights, they increase the rate at which load transfers from the inside wheel to the outside wheel in a turn, lessening the usage of the inside tire. They're a great tuning tool to help balance a car, but stiffening a bar actually removes total available grip from the axle that is working in order to balance it with the one that is not. Often times, camber, toe, and/or tire pressure can be used instead to help improve the grip on the axle that isn't working as an alternative, resulting in greater total available grip.

So much for 2 cents.... this turned into a book!

[ 11-19-2009, 08:21 PM: Message edited by: Dan Tiley ]

--------------------
Dan Tiley
 
#31 ·
yup on the track large bars soft springs , or stiff springs and small dia bars , or no bar/s

on the street its easier to run stiff and large dia bars
 
#32 ·
Great thread with tons of info. Thanks for sharing!
 
#36 ·
Thats the way I am going to go as well. Some Eibach springs with these BMR bars set to stiff should give me the handling I am looking for without sacrificing much ride quality.
 
#34 · (Edited)
A previous experience with what swaybars does for you... I autocrossed my '98 firebird for years, and had a chance to have a very experienced autox driver compare my ride with his z-28. My car was modified only with Koni SA shocks set mid-range front and almost full soft in the rear. His car had the same shocks set the same but he also had a stiffer front swaybar. We both had identical tires.

He drove both of our cars, and got nearly identical times. He noted that my car felt like it had slightly more power (I think my motor was an early build freak LS1 with a few vette parts from the factory since it dyno'd at 320hp rwhp bone stock when it had 120k miles on it) and my car also had higher ultimate grip, but the factory swaybars in my car made it slightly harder to drive due to less precise steering responses and longer time for the car to settle into a turn. His car was significantly easier to drive but had marginally less total grip.

In the end, he was very slightly faster driving his own car (.1sec over a 58 second course) and although I was very inexperienced at the time, I was within .2 seconds of his time when driving my own car after riding along with him and copying his driving style. In the end, we were both better in the cars we were used to driving, and while the setup differences resulted in noticeable handling differences on the track, both cars were essentially identical in overall track performance.

So, unless you're good enough that your track times really do depend on that last bit of grip, the lesson is that for most people its best to set up the car so it "feels good", and then just learn how to drive it fast that way. And if later its not good enough, fiddle with it. But initially for most drivers, making the car more predictable and drivable will probably improve track times more than chasing the last percent of ultimate grip.
 
#35 ·
I'm very happy with the front set to soft and the rear set to the middle hole on my BMR sways. With Pedders HD springs and GSR Struts/shocks and staggered wheels, it feels very balanced around corners. If I want the rear out, I just increase the throttle, and have a nice drift around almost any corner.
 
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