donour

iTrader: (6)

1) Buy a car without a trunk liner (RS) .

2) I don't adjust my shocks. My street setting is my race setting. The rebound is set at ~70% critical. That never changes. It is best for ride quality and handling.

d

goofygrin

iTrader: (5)

donour, here in Dallas we're lucky enough to have 3-4 surface types/conditions, so you HAVE to adjust your settings to fit each surface.

Slicker/bumpier = softer setting
Grippier = harder setting

donour

iTrader: (6)

It sounds like your damping curves are not sufficiently digressive. If I soften my koni 8611s the ride gets _worse_ over bumps.

d

EDIT: FWIW, I use the same setting for topeka that I do for mineral wells.

CoreyR

iTrader: (5)

Right in the rules:

13.5.F

A hole may be added to an interior body panel to provide access
to the adjustment mechanism on an allowed adjustable shock
absorber. The hole may serve no other purpose, and may not be
added through either the exterior bodywork or a strut bar. Interior
panels are defined to be those pieces which cover the interior
of the vehicle and are accessible from inside the vehicle.
They do not include structural panels, such as wheel wells or
inner fenders, which may also be accessible from inside the car
but which actually form part of the body of the vehicle.

Corey #89 STU

kekek

iTrader: (22)

Don't forget that anyone can protest any car in ProSolo.

spool_sample

iTrader: (3)

Agreed. I noticed the same thing with the 8611s on my 240, and set them up the same way you did as a result - street and auto-x settings were one and the same.

I've always believed that with adjustable shocks, the knobs are there to tune the shocks for spring rate, motion ratio, etc., rather than surface conditions, as long as the shock was digressive or had separate high-speed rebound/bump adjustments. When they're right, they're right, as the high-speed damping doesn't change much (8611) or it can be altered (3-way or 4-way shocks).

With linear shocks (read: crap) that only have one or two damping adjustments, you have to find a compromise between low-speed and high-speed settings, and it's almost impossible to find the "sweet spot" between the two. As a result, you usually end up with a car that has good response and handling but crashes over bumps, and vise versa. Usually, the high-speed settings change too much with each click/turn/whatever, and as a result, you will feel that you need to adjust your shocks for different kinds of surfaces.

I'm not sure if any of that is technically correct (I'm not an engineer), but it makes sense to me when I look at a shock dyno graph. For example, I found this one for the Ohlins DFV (Miata version, fronts):



See how there are much greater changes in rebound at the low end of the plot, creating a sort of "balloon"? That is digressive valving at work, and it's a good thing.

delongedoug

iTrader: (20)

1) I'm good.

2) I'll have to see if it's even worth fiddling with or if I can just take the track setting on the roads.

Good find. Thanks!

goofygrin

iTrader: (5)

I have very stiff coilovers (14/14kg, likely too stiff) and on street tires the tire is overwhelmed before the strut is. Since the new clutch is putting me in BSP, it's a moot point anyway since I'll be going to hoosiers/kuhmos anyway.

But I'm sure my cheap coilovers aren't as good as ones costing 2-3 times as much.

I'm also still fiddling with the settings. I've been riding on the track setting for a month now and it's way too rough for daily driving, however is was pretty much perfect for the track days I had in December. Sure it handles great, but potholes, expansion joints, etc. all just are too much for comfort.

ETA: I've been to Mineral Wells but not Topeka, but I understand Topeka to be smooth/slick/sandy. I found MW to be pretty rough (lol I know), but very dusty and "slick" so I actually got faster times with a softer setup that allowed the weight to transfer a bit more onto the wheels.

After 15-16 90 second + runs in one day at MW my Star Specs sounded like Jeep tires all the way home. They were nice and pock marked up Here's a video from the next day (2 day event): http://www.youtube.com/watch?v=23YcO0JHyPU

delongedoug

iTrader: (20)

Why wouldn't you just change to softer settings for the street? Isn't it just a few clicks and 15 seconds of time?

Disclaimer: I don't have my coilovers yet, so if there's something I'm ignorant to, I plead the fif.

goofygrin

iTrader: (5)

I forgot to, then I've been lazy. Doing the rears isn't a big deal (just open the trunk). The fronts are inverted, so I have to reach under a dirty car. I wanted to also get more of a feel for how the car reacts when setup this way. I can't drive at 9/10th (or even 5/10ths really) on the street, but the car definitely handles differently with different settings.

My next step is to raise it back up a bit and level it out to see what that does for me (I've got a bit of forward rake going on).

donour

iTrader: (6)

What damper are you using? Dyno plots?

delongedoug: It is not nearly as simple as making the shock 'stiffer' or 'softer'. Shock absorber valving design is fairly complicated. Changes to the damping curve don't effect just one thing. Changes that cause oversteer on corner entry, can can understeer on exit. Body motions and road irregularites cause different speed inputs into spring/damper system. I wasn't kidding best my best handling setting also provides the best ride quality. FYI: my springrate is above 12kg/mm.

d

hancheyb

A digressive piston means there is less relative force generated at higher shaft speeds than lower shaft speeds. Linear pistons build force as a direct relation to the velocity of the shaft (a straight line on the graph). The change between the each click has nothing to do with the valving design. But I think that's what you were saying.

Bistein has some great material on their site. Here's one of their manuals. http://www.bilstein.com/valvingmanual_1to10.pdf. (See page 3) Roehrig Engineering has good info as well.

In sports car/road racing applications you rarely see speeds over 5 inches per second so you want to see those higher forces at the slower speeds. Most turns generate under 2 inches per second. So those Ohlins generate almost no force in a turn when fully opened (purple and blue traces) and a good amount in the upper settings. The first 3 clicks provide almost no damping force at these piston speeds.

I'm working on a white paper on this mainly around sub $1000 kits and how their 40 click adjustments do almost nothing along with why they brag about having seperate ride height and pre load adjusters. We were able to dyno some kits this week and we shot some video and documented all of it. It was a learning experience for sure.

We try to rent Mineral Wells once a year to do testing. Depending on how much of the site the organizer uses depends on if your Topeka setup would work at MW. There are parts of the site that have huge bumps and we have to turn down compression to nothing or just skip across the lot. But there was a Pro a few years ago that used 1/2 of the site and ran North/South staying away from the really bumpy parts. When you criss cross East/West there are really big seams in the pavement that you don't notice walking the course. The BMW club runs 1.3 mile long Solo 1 courses with 100+ mph straight aways. Those courses find every bump, grass clump, and gravel infested spot on the site. They make non BMW classes run first to clean it up.

Fair

Hmm.... I don't know how to put this any other way - this is more of that Dennis Grant internet shock 70% critical damping theory, run amok. Its one part of the puzzle that is used to develop OEM type suspensions that someone took out of a textbook (and somewhat out of context), but it doesn't take into account a LOT of other variables that come into play in a racing environment. Its one of the nuttiest things floating around the internet right now, with regard to shocks.

Honestly, this "70% theory" is not what racers use to tune shocks by, nor should they. A lot of pro race teams will end up with damping that is 80, 90, 100% critically damped, or even more. There are conditions when this makes a lot more sense (and provides lower lap times - the ultimate metric) than this magic "70% idea". In an autocross scenario, from run to run, you may be changing one end of the car from 60% to 90% - to suit the conditions, the driver's preference, etc. There is no one setup that works for every driver, too.

You will find adjustable shocks on just about every race car in the world, from autocrossers up to Formula 1, and top level race engineers won't lock the shocks into one setting and leave them there for every track... much less between the widely different conditions you'll see in a dual use street/track or a street/autocross car. Heck, the difference in spring rate between a race tire and street tire can be considerable - and most track or autocross racers have different street and race tires. How do you adjust for this in the critical damping theory? You'd need a knob to turn to get back to that golden 70%, right? But again - this theory doesn't take the spring rate of the tire into account - among other things.

The knobs are there to be used - to fine tune the damping forces to adjust for changing track conditions (rain, surface smoothness, etc), changing setups (spring rate/bar/tire/etc), driver preference, ride quality (street use), and more. I can take you for a ride in a car with adjustable shocks and make them ride GREAT... then change a few settings and make it handle better (ie: definitely faster), but maybe a little less comfortable on the street (Low Speed Compression adjustments make the biggest change to ride quality). There is no magic setting that can be perfectly optimized for street comfort and competitive track times- everything comes down to a compromise. Now a GOOD damper can handle both street and track use very well, but that's because good dampers tend to have a wider range of usable shock damping forces - that can be adjusted with those pesky knobs.

This "70% critical damping" shock tuning idea does not work well except in a very narrow window of OEM shock design conditions. Its a theory that is overused and should largely be ignored in a racing environment. I know this won't go over well here, as this is the current Holy Grail that so many folks on the internet seem to live and die by, but real shock development largely ignores it.

Fair

I hear ya... the Mineral Wells site is massive, and one of the bumpiest sites I've ever raced on. We usually run very low compression damping there to deal with the big bumps and rough patches. The Topeka site (the Heartland Park RV pad - here's to hoping we never go there again!) is one of the smoothest sites in the country. We always make several shock changes when going between these two surface extremes. MW is the more difficult site to setup for, of course.

http://www.youtube.com/watch?v=tLulXGmdjSw - that is Hanchey driving our STU prepped E36 M3 at Mineral Wells on a 1.4 mile "autocross" run. LOTS of big big bumps to deal with, as you can see.


The same car a couple of months later running at Topeka. Butter smooth surface. Different setup.

Cheers,

donour

iTrader: (6)

Actually. I got that number from from Dixon's book and running things in simulation myself.

d

EDIT: It hardly matters though. The point was about how much you need to change the setting for ride quality. My curves go down to around 30-35% critical above ~4 in/s. There's a lot of literature out there that shows why this a sweet spot for passenger cars, which is what we are talking about here.

EDIT2: Do we really want to spend this thread deriving the damping curve from first principles?