View Single Post
Old Tue, Jan-05-2016, 02:54:05 AM   #79
ShaikhA
Fat Cat Motorsports
 
Join Date: Sep 2015
Posts: 98
Reputation: 0 ShaikhA is on a distinguished road
Location: Redwood City

United States




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

Hey Andy, thanks for the great info on the OptimumG seminars. I definitely plan to attend one. I've referenced sections of Race Car Vehicle Dynamics by Milliken and Milliken and also used it to help create a lateral load transfer calculation in my new suspension spreadsheet. I plan to make an online version available for BMW enthusiasts as I did for the Miata community. Real-world data acquisition is essential and I will do more of it this Winter and Spring as I make suspension changes/upgrades. That reminds me, got a track day on Sunday at Laguna!

With a friend of mine who's a kickass programmer, we created a Smooth Ride App that runs on a Droid or iPhone and lets you get accelerometer data from your phone to look at vertical as well as lateral g's. If you're curious, you (or anyone reading) can play with it for free (for individual use). I wrote the algorithm for the Ride Score metric and am fairly proud of it. With this app it's easy to compare vehicle to vehicle on the same road features and also examining the impact of suspension changes on the same vehicle. The app has helped me gather data for my E46 330i and we took data on Eric's M3 as well when I was down there.


Absolutely, cars can be set up with many styles and many driving preferences. However, there are some design choices that will improve grip & comfort while other choices that will reduce them. When you create a setup that is high-grip and has reasonable roll stiffness, it's going to be very responsive too. EricSMG's comments point this out (post 60) - I aimed for maximum grip and comfort on his stock sprung/sway setup. From metrics I've developed I had a feeling he would really like it for high-speed use and he does. He'd like a bit stronger low-speed steering feedback and that's very understandable (I aimed on the softer end of the damper forces and it's easy to add a bit more low speed rebound and bump without compromising overall grip or ride quality). It will be a fairly small change so we will remain in a window of 'near perfection' where additional tweaking outside that range would only make the car objectively and measurably worse in a variety of conditions.

===
Two very important reasons it's possible to get an nearly 'ideal' optimized setup are related to:

1) Proper overall proportioning of rebound vs. compression damping at high shock speed; over 10ips (inches per second) and easily up to 30-40 ips even for a street car. This proportioning necessarily involves the rebound-to-compression relationship at mid-speed, around 3-7 ips. The higher the ratio of rebound-to-bump force skews toward rebound, the more you're losing suspension travel as the damper cycles at higher and higher shock velocities, which means you're either feeling bigger amplitude bumps or traveling at higher road speeds or both!

Since we're not driving a late model in NASCAR going to work every day, "jacking down" (much more rebound to compression) is not going to be be our friend - even at high road speeds. The OE E46 M3 dampers and all the aftermarket stuff I've tested suffers from either too much high speed rebound force, or both too much mid and too much high speed. When you experience a more optimally damped setup in terms of proportion, you feel the car resetting back to the zero-point of its travel (static / stationary ride height) no matter how fast you go and how rough the road is.

2) How the damper behaves at low shock velocities, between 0-3 ips. Look at the shock force curves and image the shock cycling between rebound and compression then back again. I put my finger on the screen and trace the curve (this relates strongly to the main idea in my 'How to Handle a Raw Egg' video). Back and forth, back and forth over a small displacement or at low velocity. Is there a near-symmetry in response? or is there a sudden change going in rebound *or* compression? Sudden and sharp rates-of-change in low-speed set up a tendency of constant discomfort (and lower grip). It's the hydro-mechanical equivalent of the expression "the journey of a thousand miles starts with a single step." How the damper is biased at low-speed will establish a certain character and 'feel' that can be very hard to overcome with later force changes. So even though one may feel TREMENDOUS control with more low-speed rebound (the typical solution of most OEs or aftermarket tuners), we've now compromised the overall FUNCTION of the suspension and its potential for keeping the tires in contact with the ground. This means the car is slower, less stable on uneven roads, and less safe. If that kind of behavior is "better" to someone, I don't want to work with them!

Starting around the 8:00 mark of the first Street Class Tuning video I created I discussed the reason you actually want softer low speed damping for faster roll control. It has to do with the difference between the pitch moment vs. the roll moment (essentially, wheelbase vs. track width). This is predicated on having a spring rates that create Flat Ride. Not every race car using heavy amounts of aero can or wants to use Flat Ride tuning, but the majority of vehicles we're driving are street-based/street-legal without crazy aero so Flat Ride along with generally softer low-speed damping (as the Vette had/has) is very relevant.

===

That's the reason I linked to the Corvette forum thread - curious if you read through that? Both 1) and 2) above were satisfied: the ratio of rebound-to-compression forces were very well-balanced at high shock velocities and also the low-speed ratio and shape were very smooth.

http://www.corvetteforum.com/forums/...s-so-good.html

I was really pleasantly surprised to see them discussing something that backed up my experience tuning suspensions for over ten years now, for all kinds of vehicles. Those factory Corvette shocks were refined on the Nurburgring, plus GM did tuning at a Nationals-style autocross surface (as one poster mentioned). Those damping curves also became be the baseline setting for Dennis Grantz customers' Pen$ke$ - and that they all swore they loved and went faster on. They had rather soft low-speed damping in general, a very smooth S-type curve. The rear compression and rebound were a bit firmer in comparison and this was indicated to provide better axle control and better grip. The OE '04 Z06 Corvette dyno curves have much smoother shapes than you see for typical OE or aftermarket dampers. Those engineers really did a phenomenal job!

If the spring rates increase you certainly need to change the damper forces, but when considering a test like 'how fast if the car on the Nurburgring' I'm not sure what variables you'd change to make the car "better." You can make the car 'different' (increase 'feel' or 'sportiness' which is largely psychological and not measurably better) but not better in an objective sense. I hate being in 'sporty-tuned' cars, because I know there's more shock damping than the suspension needs, or the spring rates are firmer than needed, or the sway bars are too big and set too stiff - so basically, I'm getting abused. I'm not paying a vendor to punish me!

I've build many many non-adjustable setups and have found once I am clear on the customer's needs (tire and wheel choice plus vehicle usage), the setups I build work out great in the vast majority of real-world and race-track situation without any knobs being needed. This is for wet, dry, bumpy or smooth roads, etc. While we can have different tastes in how we want our car to *feel*, what makes a car *work* in terms of maximum grip and sharp response has a pretty narrow window. Outside that window you get a rougher ride and lower grip. Essentially, a lower Quality experience. Subjectivity is not a good reason to make a design choice when there are objective measures that also increase comfort, peace, psychological ease, etc. Someone who enjoys a needlessly stiff setup is deliberately punishing their safety and that of anyone who rides with them. I've seen and done too much to let it go at 'different strokes for different folks.' Not criticizing you per se, but it feels very important for me to point out there are very good reasons for doing things the way I do and why generally fast / comfortable / fun to drive setups, whether OE or aftermarket, all share certain design choices.

I definitely agree that linear is more predictable than progressive and is what I prefer. The way I see it, the primary use of a progressive spring is to eliminate the need for a secondary helper spring in order to keep the main spring in full contact with the spring perches at full droop. However, I'm also appreciating the nature of the BMW OE-style progressive where you get used to a certain driving behavior. It's probably slower from a pure-race standpoint but is great for a dual-purpose car. I actually plan to do some pretty solid testing between a progressive stock-rate setup vs. a higher rate coilover (around 370/700) and perhaps something in-between.

Some springs also have a tapered profile that naturally becomes a progressive spring (like the OE BMW front and rear and all aftermarket lowering springs plus most lower rate coil-over springs). The harder ride you experienced is probably more due to spring rate choice and damper setting than simply a difference between linear or progresive. If you get Flat Ride working for you, the ride is very pleasant even with fairly high rates!
Jump to top ShaikhA is offline   Reply With Quote