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Old Sat, Jan-02-2016, 05:32:20 AM   #77
ShaikhA
Fat Cat Motorsports
 
Join Date: Sep 2015
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Location: Redwood City

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Default Re: Shock dynos, Fat Cat Motorsports and custom valved Bilstiens

I understand the disappointment, but keep in mind the leaf spring-based Vettes have some issues with mid-corner stability (from what I've read) and the low motion ratios (around 0.6-0.7) require very stiff springs which conduct a lot of force into the body. I am think (but am guessing without enough evidence) that the '04 Z06 suspension described and praised in that link may have been more of an aberration in that later models might have tended toward the stiffer-than-needed / 'sporty' direction.

I'm not sure any OEM has put everything all together but I think BMW has done a damn good job in general in the chassis / engine / handling balance area. I think there's almost always some opportunity to improve upon what the factory did, though it seems the E36 damper tuning was far closer to 'perfection'.

I wonder if we get enough evidence together about what a number of enthusiasts sign off on as a 'great suspension' that BMW might take that into account when planning an optional suspension package? I don't know enough about how the newest models have been tuned to say if that's not already happening, although my sense and on-road observations suggest there may be more 'feel' than is needed.

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Originally Posted by twentyseven View Post
That Z06 thread was depressing because a GM car gets 'perfect' dampers stock and yet the BMW's have to go through this process!

I agree that the fastest setup is the way to go, I just also want the car to also have a fun personality on the street where lap times are irrelevant. Compliance is paramount but I also don't want it to feel like a Cadillac per se... one of my favorite things about changing suspension is making an M3 behave more like one would expect it should (Felt more Porsche-like? :P jk) But I think we can all agree that for some reason, BMW just didn't get the suspension right on this car and did an amazing platform a huge injustice. That's why your tuning philosophies really resonate here with us! Last year I borrowed a buddies' 330i and it was so much nicer on the street I wondered why I wasted my money on an M3...
I want that ideal balance, too. I really think the less 'optimal' E46 M3 (and the non-M also suffers, trust me) damper tuning came from a desire to a) slow people down given the speed potential of the E46 M3 and b) to make them feel the newer chassis was more of a 'race car' than the E36 it replaced. It's more aggressive, but I don't think the E46M3 had the kind of grip-plus-ride-quality that the E36M3 had. BUT - we can achieve that now and do even better than the factory would have done with features like Ripple Reducer and KBO.

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I was intrigued by the presence of the droop spring on the non-M E46 front struts. That deserves a separate post but as I've mentioned before, I did use droop stops previously on a stock-sprung Miata and recently on my 330i's front struts (and rears also), also with stock springs. This is a tuning trick that does effectively lower a car's center of gravity (the inside wheel doesn't extend as easily since the droop spring acts to resist the rebound motion). The M3 front struts don't have a droop spring but the 330i Sport suspension does. So... why didn't BMW use that droop feature on the M3 as well? There's a reasonable amount of droop available although from experimenting with my stock springs, I would prefer to only use a droop limiter on a lowering spring since you end up having to compress the spring a bit farther as the shaft is not extending as far. For those who haven't seen the droop spring, skip to about 11:30.

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From the one street-going Porsche I've tuned so far (we've done a 911 autocross XP car and I'm gearing up for a '06 Porsche Cayman in Jan/Feb), the OE 993 M030 (Euro spec) suspension was pretty firm, and had too much high speed rebound IMO. The owner agreed as they said our retuned setup (with stiffer springs as well) rode better and handled better than the factory Porsche setup. I was really glad to hear that (my butt dyno already told me it was better but the customer has to be happy too!).



But to its credit, the damping forces changed very smoothly, even if the rebound was too much for US roads. The smooth 'S' type curve shown above plus a clear digressive blow-off would feel more comfortable and predictable than a damper with very sharp edges (sudden downward ramp of low speed rebound, or no blow-off of high speed like the OE E46 BMW suspension has plus the aftermarket options). A year or so ago I made a video 'How to Handle a Raw Egg (Isolate and Control)' that discusses this idea.


Quote:
Eric and I have long lambasted the common choices of spring rates around here, and once we saw your frequency video it clicked. The factory F/R delta is usually ignored, largely due to TRACK tuning tips which are perpetually spread on forums by misinformed HPDE heroes. Lots of people run heavy front springs to maintain the camber curve yet only increase the rear by 50 or 100lb, and don't even get me started how they f**k with their antiroll bars...
I'm really stoked you guys have been working in parallel - in the Miata community there is too much reliance on the HPDE or track day heroes, even people who win championship and tout that as proof their suspension design is superior They can't let the bump stops engage (their very short non-linear stops) without that 'end of the car not working' Yes, stiff sways especially on our strut cars and up front really show the reality of 'sways do affect ride quality & grip'.

Quote:
FYI PSS10 spring rates are 340/565 (progressive) so they do compliment your ride frequency formula more than most. The balance is easily noticeable once educated what to look for. I don't know the H&R coilover rates as they aren't published but i'd imagine they are similar. In my experience with E36's, the H&R race were always too short/low for their soft rate and were always paired with Bilsteins so they basically rode on bump stops everywhere. Can't say for certain about the E46 variants you have as I've no experience with them but that has been the general complaint about them in the BMW world.

Am selling my car with the PSS10's soon, but might buy the one off my buddy with the H&R coilovers next year if I'm able. In that event they will be removed and sent to you!
The 340/565 gives nominal rate of 1.90 / 1.84 Hz, so it's not Flat Ride but not highly on the pitch side. If the rear increases rate faster (usually the case based on what I've seen on spring rate data from Vorshlag) then during compression you probably get closer to Flat Ride. I would drop the front or increase the rear - probably drop the front for a street tune or increase rear for a track tune. It's a good baseline though to work from. Would definitely be happy to work with you!
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