Greetings, all - and thanks to cwlo for point me to this thread. I'd like to add a few point re: the tuning approach I've come to for optimizing suspensions.
This thread is long but packed with good information, great conversations, and even more importantly, the feedback of E46 M3 owners who tried Flat Ride and uniformly preferred it to the pitch-based setup:
http://www.m3forum.net/m3forum/showthread.php?t=601913
and this thread has some feedback from a few of those driver including a great few posts and on-track videos (by 'bigjae1976') on his M3 track experience with pitch vs. Flat Ride:
http://www.m3forum.net/m3forum/showthread.php?t=611348
Bigjae's post #4:
http://www.m3forum.net/m3forum/showp...95&postcount=4
and post #7
http://www.m3forum.net/m3forum/showp...46&postcount=7
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From a very simple perspective, Flat Ride is the most efficient way to make a vehicle settle quickly when experiencing a ride disturbance (straight-line). The front suspension experiences a bump or dip first, then after crossing the wheelbase, the rear suspension experiences it. To prevent pitching, you'd want the rear to 'catch up' to the front and in numerical terms, that means making the front ride frequency lower than the rear. This has been done widely on passenger and even sports cars for many decades (many BMWs, all Porsche, Corvettes, etc.). The exceptions prove the rule.
Another benefit of Flat Ride tuning is that a higher rear ride frequency than front creates a more rapid initial yaw moment (rotation / turning) of the chassis. So the rear is PUSHING the front into the turn, instead of a stiffer front DRAGGING the rear through the turn. You can feel this in your body - even my last g/f noted from the pax seat that the car felt like it was rotating from the rear. This makes a car more responsive (when combined with other variables like alignment, bump stops, etc.) to driver input, reducing understeer which the factory also designs into every production car and we end up fighting on most aftermarket / track setups.
Strangely, I do see some OEs tuning for pitch these days (and it seems BMW started doing this a few years ago on some M models) which I believe is to give the driver a 'sportier' experience without actually increase performance as much as a Flat Ride-based setup would have. Pitch makes you THINK you're going faster than you are, because your body is moving around more -
but if your body is bouncing, so are the tires ... how is that good for grip, or lap times...?
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In terms of the contribution of springs vs. sways, when you do the roll stiffness calcs, you'll find that essentially ANY strut-based, RWD sports car will have the front bar contribution >50% of the total roll stiffness. This is even at the higher ride frequencies (> 2 Hz) for 'racing' setups. Even at 3 Hz front frequency you're likely under 25% front spring contribution to total roll stiffness vs. 45-50% for the front bar. So no matter whether your ride frequencies are oriented toward pitch or Flat Ride, your front bar is probably TWICE as important for roll control. These specific calcs are for a track-driven E46 M3 but the percentages would likely scale for a 1 Series. I have done some initial calcs for cwlo's setup and also looked at other setups posted in this thread for helpful reference.
I'm not an expert on the nuances of BMW geometry for anti-squat, anti-dive but it seems they design in a reasonable amount of each (at least on my E46) so once you get into the 2.3-2.5 Hz range you're really not gaining as much front anti-dive with VERY stiff front springs as you might be losing in terms of increasing tire contact patch load variation, reducing total grip and stability at the limit.
The winning E36 and E46 NASA ST cars (throughout the season) I've been working with all eventually (through a bit of resistance!) switched to Flat Ride, with frequencies well above 2 Hz, and using a fair amount of aero on wide sticky R-compounds. I've got about a half-dozen drivers I can point to who've felt the improvements when you optimize the suspension around Flat Ride.
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If you're getting into LMP territory then aero grip likely becomes a more dominant factor so ground clearance concerns dictate what springs you'll need to run. I fully accept Flat Ride isn't the answer to everything but on a production-based competition car, I've seen more proof that it's a better foundation to build on vs. pitch. And honestly, saying 'others do it' is not an argument for the correctness, or optimization, of an approach. There may be valid reasons in some situations to use pitch vs. Flat Ride, but those would need to be backed by before-after testing, not argument to authority. There were PLENTY of skeptics in the E46 M3 community before I explained my reasons and they went and tested! I always base my conclusions on real-world testing along with applied theory, and I hope you all do the same. That's the true nature of science, so you're making improvements not just changes.
A slightly higher rear frequency doesn't by itself make the car undriveable - the factory tuned it that way on most BMWs to begin with. It's how you take the other variables into account for high-performance / competition situations that helps balance the vehicle. The damper tuning really matters, along with the damper gas pressure (esp. for the monotubes many of us prefer), then sway bars, bump stops, alignment, ride height, tire pressures, etc.
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This variable doesn't get nearly enough attention: how your dampers are dynamically influencing understeer/oversteer. Many aftermarket (and a lot of OE) dampers use linear rebound vs. digressive compression which allows the suspension to 'jack down' over a series of chassis movements. This usually results in bump stop contact up front and will cause terminal understeer. Or, if you have more rebound bias in the rear, or insufficient droop travel, you could get rear bump stop engagement / inside wheel lift from the jacking down and oversteer. Sometimes you'll get both understeer and oversteer in the same corner - this happens far more often than many realize. I've focused on providing very neutral damping up front and a *slight* rebound bias in the rear. This both helps counter-act the factory-designed methods for understeer (esp. less front camber gain vs. rear in bump) and also lets a competent driver safely and quickly drive a car to its limit when the environment and reason allow for it.
It's very important to realize that how the dampers are tuned will influence the 'zero point' of the suspension, the effective ride height while driving. Rally cars uses damping to generally bias the suspension upward, NASCAR biases downward - where should we be? It depends on your driving style and roads, but an excess of either compression or rebound (or gas pressure) will cause unintended effects that will make a car more unstable and harder to keep on the edge, preventing you from getting the maximum possible grip from the tires.
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I believe any mechanical engineer will agree that one key to maximum tire adhesion to the road is minimizing the variation of the tire's contact patch load. You want consistent, and predictable, changes in tire loading. The driver's behavior influences this greatly, but a suspension that has pitch (higher front frequency vs. rear) will by its nature induce MORE contact patch variation and reduce grip.
I appreciate this observation by gmx:
Quote:
I have noticed since the E36 generation with "professional" race cars the driver applies such input and the the front end of the car visibly oscillates and even skips along the surface. I see this a lot on American race-prepped E36s but I do not know anything about the condition of rubber, track, ie. surface type etc. The tyre, roll-centre, surface type are key variables regardless of suspension, specifically front/rear spring rate coupling. I've assumed they don't know wtf they're doing and have slammed the front for more static camber while ruining or not correcting the roll-centre. Who knows. Or maybe crap tyres, brash inputs etc - a lot can cause that.
Anyway, I NEVER see such things on raeder motorsport (Manthey Racing), schirmer cars lapping the ring. You can see this slight behavior in my own car at 0:38 in the first video.
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It was this kind of pogoing / oscillating that I observed even with overly-stiff autocross cars (before I got more involved in track driving/tuning) that made me question the wisdom of the higher front vs. rear ride frequency approach. E46 M3 track driver bigjae1976 observed that in his comments above.
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The M3forum.net thread I started (with the spreadsheet I developed for them, and am willing to also create/share with this community) has well over a dozen instances of E46 M3 owners switching their spring rates to achieve Flat Ride and reporting benefits to grip, composure, stability, and lap times.
There's more than one variable at play - just getting Flat Ride in your spring rate choice won't bring peace on Earth. But I've found and consider it the foundation. I didn't invent 'big bars, soft springs' - but I understand the pros vs. cons pretty well. The approach is use I called Ride Harmony (3 principles) and Race Synergy (3 additional elements) and it seems to work very well for street, track, autocross, etc. I'll hat tip Dennis Grant/Far North Racing and take some credit for providing empirical and analytic tools to show how effective Flat Ride is when applied as part of a whole suspension tuning philosophy.