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05-04-2018, 05:26 PM | #1 |
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Effective spring rates and the necessity of a Front Sway Bar
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OK, so starting to read up on suspension tuning a little more and finding a good balance for spring ratios front to rear. I'm currently running 392lb front and 673 rear, stock rear bar and M3/select sphericals in control arms. Recently switched from 27mm H&R front bar to a completely deleted front bar as a trial. The removal of the front bar seems to have increased front grip and shifted the balance of the car towards neutral rather than understeer. A little more body roll and I haven't encountered push mid corner since the switch. Is there a way to calculate or ballpark effective spring rates given arm/bushing/ball replacements, springs, and sways? How should I be thinking about this? Based on how the car drives and corners now, while I'm not sure a deleted front sway is the way to go, the car definitely had too much front bar as it was. fe1rx berns Ginger_Extract houtan BimmerAg bbnks2
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05-04-2018, 06:26 PM | #2 |
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Thanks for the consideration, I'm honored. Interestingly, I stumbled upon this thread by chance, tagging me didn't give any sort of notification. Just an FYI if others are slow to respond.
Continuing on...You're on the right track. Understeer is slow, and neutral/oversteer-bias is fast, allowing a decently skilled driver more options. I ran no bar, and thought the handling balance was fun, but it was ultimately too loose for the spring rates I am using (336/700), and reduced high speed cornering stability. Then again, I had a 15mm 335xi OEM rear bar in place during that testing. With no bars all together, I could see the "standard" 400ish/700ish spring rate stagger working well with the right tire setup. With those details laid out, I will say this: I matched my best lap times to the tenth at the time with no bar after just one session of learning, and I definitely had more left in it. Confirming my thought that oversteer is faster than understeer. For now, I am using a modified OEM 135i hollow front bar, having taken out all of the binding characteristics it comes with from the facory. That is paired to a 18mm solid UUC adjustable rear bar on the soft setting. The balance seems quite nice, and fast. Need more track time to see how much faster the car is at a variety of tracks, but initial impressions are very positive. The car now feels like a rear-wheel drive should, and offered substantially faster cornering speeds since the car is no longer grinding it's front tires to death with understeer. I can't definitively answer your technical questions, I am a backyard engineer, not a real one. That said, there are multiple ways to achieve a fast lap time, and a nicely balanced car. Everyone throwing massive bars and monster spring rate at their cars may be doing too much, too fast. Incremental steps and experimentation are the only way to truly dial the car in correctly.
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05-14-2018, 10:27 PM | #3 |
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Good thread and great post.
Reading here the explanation re: McP struts and why adding a stiffer front sway bar to a stock suspension BMW makes sense: https://oppositelock.kinja.com/why-y...bar-1733925447 As stated in that article though.... "All of this does come with an asterisk though. Making the front end of your car too stiff will exacerbate understeer issues, and reduce ultimate traction. Your suspension should only be as stiff as you have tire grip to match. If the car is too stiff at either end, and the tires aren’t up to snuff, the car will “skate” over bumps and lose grip. Don’t just throw parts at the car without considering how it will affect the car’s dynamics. If your spring rates are already sky high, but you want better wheel control, consider a softer spring with a bigger bar. Conversely, if you feel like you’re compromising grip on rougher pavement, consider a softer bar with a heavier duty spring rate. For E82/E90 folks’ reference, this car has a race weight of 3400lbs, and uses 255 section width street tires. I am using a 26mm sway bar with 400lb front springs and 700lb rears. With this combination, [Jake Stumph Racing] would not recommend any additional front spring rate or bar without a serious uptick in front tire grip." So it really depends on the overall set up one is running and what the goals are. I'm just using mine as a DD currently but with coilovers and 336/672 springs, RSFB inserts, diff lockdown brace, M3 arms and mild camber increase front and rear the car handles 10x better vs. stock. Calm and composed over bumps and uneven pavement. Understeer is reduced and the car is better balanced. If/when I get to tracking it I plan to add camber plates and see how the balance feels on track. I'll add a bar if and when I get to feeling like I need it.
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05-16-2018, 03:25 PM | #4 |
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Just stumbled on this thread. As you probably already know, I am no expert and kind of just go based on trying things. I currently have the e93 M3 front sway, stock rear sway, with TC kline 350fr, 600r, 2.2 degrees front camber, 10 degrees caster, sumitomo htr iii 235/275.
Ride wise, my car is awesome over pretty much all road conditions. But I don't know if the front sway bar improved my handling. It doesn't seem like it did. If anything, my car may understeer even more. The tires are the stickiest though, so I am going to leave everything the way it is and put on some better tires to see what happens. Will be glad to post back once that happens. hopefully that helps.
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05-16-2018, 03:52 PM | #5 |
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Great replies guys.
Been driving with the FSB delete for a while now, definitely less understeer than before. But, the car is balanced a little towards oversteer now. Car is possibly 5-10mph faster mid corner, no measurements, though I do have to work a bit harder. I'm somehow more comfortable with the way the front end feels like this too, though it is a bit less stable over 100mph, which I don't like. You were pretty spot on Jake. Whats the smallest/lightest front bar we can get? Anyone know? Other option being going back to a stock size front bar, increasing the rear by a tiny bit. Secondary objective is to never put heavy parts on the car again.
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05-16-2018, 05:51 PM | #6 |
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I say try the stock bar. It's likely as weak as you are going to get.
And I don't think Jake is on this thread. I was quoting him from the article.
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05-16-2018, 09:03 PM | #8 |
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Is the 135 and 128i bar the same? Thought Msport suspension got a bigger bar.
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05-16-2018, 10:44 PM | #9 |
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I think they are the same diameter but have different part no's. Probably the tapering is different.
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05-17-2018, 07:11 AM | #10 |
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I'll look into it then. BMW has made the same part with different numbers in the past, but its not typical.
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05-17-2018, 08:05 AM | #11 | |
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Non MSport 128 has softer front bar? Real OEM shows two different front bars for with or without MSport suspension: http://www.realoem.com/bmw/enUS/show...diagId=31_0696 Last edited by MightyMouseTech; 05-17-2018 at 08:12 AM.. |
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05-17-2018, 12:46 PM | #12 | |
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Through trial and error I've found my current setup to be the best and most balanced I've had: Front: Wheels: 17x9 ET25 Tires: 255 Z224 Spring: 6k Sway: stock (modified bushings to prevent bind) Ride height: 5.25" Control arms: M3 Alignment: -2.6* camber and 0* toe Rear: Wheels: 17x9 ET38 (Rear quarter panels are rolled and pulled) Tires: 275 Z224 Spring: 16k Sway: E92 M3 (modified bushings to prevent bind) Ride height: 5.5" Control arms: M3 guide rod and toe arm with aftermarket spherical wishbone (used M3 hubs but kept the stock lower camber arms) Alignment: -2* camber and .15* toe-in ***Note*** I found that even with 16k rear springs I was hitting the rear bump stops which was exaggerating the over-steer a bit. Moving to the bigger rear bar, and adding a bit of pre-load to the rear springs, helped regain some bump travel. The car handles well now (once I get heat into these tires). As for your other questions... From what I understand, the "spring rate," or "damping impact," of bushings are important to know for a few reasons such as: 1) How does the bushing compliance affect the dynamic movement of the suspension under load (bump steer) 2) How does the bushing compliance affect the effective wheel rate (relevant for BINDING bushings) 3) How does the bushing compliance affect "load shock" as in... how quickly does load transfer through the suspension to the tires Binding bushings, like the ones found in the suspension arms, contribute a significant amount to the effective wheel rate. A stock GTI has spring rate of 175/230 lb/in, but actual wheel rates were measured to be ~1000 lb/in on a K&C machine. Where does the 800 lb/in. come from? Binding sway bars and control arms... You would be taking a complete stab in the dark if you tried to guess how much "friction" there is in the stock 135i/128i suspension... You would need to get the car on a set of scales and start plotting suspension movement under varying loads in 1/8" increments to reverse engineer the actual effective wheel rate. Moving to M3 controls arms, or adjustable arms with spherical bushings, can actually work to reduce the effective wheel rate since they remove some BIND from the suspension. They also significantly increase the shock load to the tires since load is transmitted more abruptly. This is generally felt as being a very positive thing for translating driver inputs to steering angle (agility) but can be detrimental to putting power down without addressing the tires. Speaking of tires, tires can actually be a source of significant sway rate... the suspension can be rock solid but the car may still roll an inch from sidewall deflection alone... time for stiffer sidewall racing tires! The suspension geometry also changes less as it is loaded due to the decrease in bushing compliance. Dampers become a much more significant part of the overall suspension and its ability to control load (most aftermarket BMW dampers seem to be lacking in this area). Again, it would be a complete shot in the dark to try to guess how much of an affect there is without doing a lot of math. Calculating sway bar rates can be done, but, it wouldn't be worth the time to do accurately with our sway bars since they have so many curves, bends, hollow structure, and taper. Last edited by bbnks2; 05-17-2018 at 01:54 PM.. |
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05-18-2018, 10:35 AM | #13 |
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Great thread! Unfortunately I'm still on stock suspension unlike the other more hardcore track guys in here, but like I've mentioned before, in my personal experience, the stiffer H&R front sway bar has increased understeer pretty significantly. There is likely a point (somewhere between the stock and H&R bar) where the reduced camber loss from a stiffer front bar increases front grip and decreases understeer, but the H&R bar is way past that point.
There is a small local track that does monthly time attack events that I'd like to do some testing at next time I make it out. The event is structured so that each car gets three heats of timed laps, and there's usually at least an hour between heats. My plan for next time is to run each heat with a different setup: H&R bar, stock M-sport bar, and no front bar. Lap times will obviously tell part of the story about which setup is best, but with only a few laps to get used to each setup I'll try to collect as much additional data as possible. The track has a pretty long-sweeping 180° turn that will hopefully provide some good data on steady-state cornering g's with each setup. I'll be sure to post the results in here as soon as I get them! P.S. Love nerding out about suspension with y'all. Great discussion!
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05-18-2018, 11:02 AM | #14 | |
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I've considered 3D modeling the suspension like teams that build SAE and stock cars do, but it just isn't worth the time I'd have to invest to measure everything out and research. Lowering a car can have a pretty significant impact on the "camber curve" and you can easily end up with MUCH WORSE camber recovery and MUCH MORE toe-out than a stock car if you go too low. |
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05-18-2018, 11:13 AM | #15 | |
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How low is too low? It seems like most people here only have moderate drops (1-1.5") that still keep the suspension in a good part of the camber curve. The amount of lowering also seems pretty heavily restricted by tire fitment too.
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05-23-2018, 03:06 PM | #16 | |
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read this: https://www.caranddriver.com/feature...-still-feature The article kind of sucks in that car & driver correlates some of the findings to incorrect causes, but the data is there to draw your own conclusions. The GTI sits .6" lower than a base model car and it sees camber recovery fall from 18% to 15% up front... funny thing is the lower ride height actually had the opposite effect in the rear of the car which saw a positive change to the camber curve from 19% recovery to 20%. More understeer oriented and more bump steer. |
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05-23-2018, 07:50 PM | #17 | |
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Right?
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05-24-2018, 08:34 AM | #18 | |
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Personally, I think the phenomenon is completely over exaggerated and taken a bit OUT OF CONTEXT. I think what people DO GAIN/FEEL from a stiffer front sway is better transitional steering response. This placebo affect feels great on the street but does not mean the cars balance has moved away from under-steer AT ALL. Spring rate and sway bar variances from F:R have always been, and will always be, the most influential aspect over the cars steady state handling characteristics. Yes, camber loss is a real thing, but now consider both axles lose camber in a corner. The car is not necessarily going to UNDERSTEER like crazy just because the suspension compresses. And, tires don't magically lose all their grip because the wheel is in 2* of positive camber at max suspension compression. This is especially true for those that have dialed in 2*+ of static camber to offset the loss. Now consider that tires also see significant deflection, and the contact patch even increases from load regardless of how much or how little camber you have. The tires are literally pressed into the track surface. Fe1rx measured a Direzza star spec II tire to have almost 1" of compression at 1600lb load (approximate load the outside tire sees in a corner). Tires are not rigid like you see in illustrations that show the outside tire instantly "tipping" into positive camber as the car corners and loses camber. The wheel might move into positive camber but that does not mean the contact patch on the outside tire is following the exact same path... sidewall compliance! Also, in order for a sway bar upgrade to have ANY affect on camber loss, it has to be stiff enough to REDUCE COMPRESSION TRAVEL. At that point of stiffness you are hand down no doubt about it going to see MORE steady state under-steer unless you also upgrade the rear sway bar or increase your rear springs rate by 3-4x. I'll say it again, I've yet to see anyone ever actually see a decrease in under-steer by stiffening the front sway bar. You can review some of the technical analysis fe1rx performed here: https://www.1addicts.com/forums/showthread.php?t=956039 Last edited by bNks334; 05-24-2018 at 11:01 AM.. |
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05-24-2018, 12:47 PM | #19 | |
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However, fe1rx did measure the effective wheel rates a long time ago and found the following: Up until the bump stop engagement points, it appears that fe1rx found the rear control arm bushings to be providing roughly 40lb/in of additional roll resistance. You can then see an additional spike in effective wheel rate beyond 1" of bump travel in the rear. That could be due in part to the bump stop engagement as well as the exponential increase in the torsional resistance of the binding control arm bushings... it would be interesting to see the results of this test overlayed with the test performed with M3 control arms in place. Oh and he also measured the effective rate of the stock sway to be 209lb/in and the rar bar at a measly 15 lb/in... (I am really glad I re-read that thread): Last edited by bbnks2; 05-24-2018 at 01:49 PM.. |
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05-24-2018, 01:31 PM | #20 | |
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Sounds like the rear bar doesn't contribute much. And yeah, looks like the exponential rate increase is from the bump, which makes sense. Let me know if I'm interpreting that incorrectly or so poorly that I'm not even wrong.
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05-24-2018, 01:41 PM | #21 | |
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you can use the measurement of the stock bar (209 lb/in effective wheel rate) to deduce the effective wheel rate of an H&R 27mm bar (54 % stiffer than stock). The H&R bar would provide 209*1.54 = 322 lb/in of effective wheel rate. Last edited by bbnks2; 05-24-2018 at 02:02 PM.. |
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05-24-2018, 02:23 PM | #22 | |
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The more I look at it I think trying to get a non-sport 128i front bar, and trying to find some toe arms with bushings in them (opposed to the sphericals that are in there now) might do it. I also need to look at my ride height again, currently set to 1/4" rake, think I want to reduce that some, reversing that would be better. I'd remove the rear bar too, but I feel like the bars do confer some additional stability. I have no interest in increasing the coil spring rate any further. Only question is how to remove bind from the 128i front bar bushings.
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