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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Tracking, Autocrossing, Dragstrip, Driving Techniques > Dialing in suspension for maximum performance



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      01-24-2012, 03:18 PM   #1
turugara
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Dialing in suspension for maximum performance

The question I'm trying to tackle is given a coilover setup that is adjustable with regard to height, rebound, and compression, what is the correct way for an end user to dial in an ideal setup for his/her driving style, car configuration, and track.

I'm not looking for a discussion on specific settings, rather a discussion on the methodology on reaching those settings.

I've looked around for this and there isn't anything specific to our cars. From browsing the intertubes, it seems like this is linked most often.

http://www.bimmerhaus.com/tech/shocktuningTN.html

Koni recommends the following. What do you guys do?

Adjusting The COMPRESSION (Bump) Damping Control (Very Important to do this FIRST!)

Bump damping controls the unsprung weight of the vehicle (wheels, axles, etc.). It controls the upward movement of the suspension as when hitting a bump in the track. It should not be used to control the downward movement of the vehicle when it encounters dips. Also, it should not be used to control roll or bottoming.

Depending on the vehicle, the ideal bump setting can occur at any point within the adjustment range. This setting will be reached when "side-hop" or "walking" in a bumpy turn is minimal and the ride is not uncomfortably harsh. At any point other than this ideal setting, the "side-hopping" condition will be more pronounced and the ride may be too harsh.

STEP 1: Set all four dampers on minimum bump and minimum rebound settings.

STEP 2: Drive one or two laps to get the feel of the car. Note: When driving the car during the bump adjustment phase, disregard body lean or roll and concentrate solely on how the car feels over bumps. Also, try to notice if the car "walks" or "side-hops" on a rough turn.

STEP 3: Increase bump adjustment clockwise 3 clicks on all four dampers. Drive the car one or two laps. Repeat Step 3 until a point is reached where the car starts to feel hard over bumpy surfaces.

STEP 4: Back off the bump adjustment two clicks. The bump control is now set. Note: The back off point will probably be reached sooner on one end of the vehicle than the other. If this occurs, keep increasing the bump on the soft end until it, too, feels hard. Then back it off 2 clicks. The bump control is now set.

Adjusting the REBOUND Damping Control

Once you have found what you feel to be the best bump setting on all four wheels, you are now ready to proceed with adjusting the rebound. The rebound damping controls the transitional roll (lean) as when entering a turn. It does *not* limit the total amount of roll; it *does* limit how *fast* this total roll angle is achieved. How much the vehicle actually leans is determined by other things such as spring rate, sway bars, roll center, ride heights, etc.

It should be noted that too much rebound on either end of the vehicle will cause an initial loss of lateral acceleration (cornering grip) a that end which will cause the vehicle to oversteer or understeer excessively when entering a turn. Too much rebound control in relation to spring rate will cause a condition known as "jacking down." This is a condition where, after hitting a bump and compressing the spring, the damper does not allow the spring to return to a neutral position before the next bump is encountered.

This repeats with each subsequent bump until the car is actually lowered onto the bump stops. Contact with the bump stops causes a drastic increase in roll stiffness. If this condition occurs on the front, the car will understeer; if it occurs on the rear, the car will oversteer.

STEP 1: With rebound set on full soft and the bump control set from your earlier testing, drive the car one of two laps, paying particular attention to how the car rolls when entering a turn.

STEP 2: Increase rebound damping three sweeps (or 3/4 turn) on all four dampers and drive the car one or two laps. Repeat Step 2 until the car enters the turns smoothly (no drastic attitude changes) and without leaning excessively. An increase in the rebound stiffness beyond this point is unnecessary and may result in a loss of cornering power. Note: As with the bump settings, this point will probably be reached at one end of the car before the other.

However, individual drivers may find it desirable to have a car that assumes an oversteering or understeering attitude when entering a turn. This can be easily "dialed-in" using slightly excessive rebound settings at either end.


___________

{Specifically posted in track session as this is a suspension question targeted towards the track user. Not daily driving settings.}

Last edited by turugara; 01-24-2012 at 03:31 PM.
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      01-24-2012, 03:25 PM   #2
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Another great article on adjusting suspension: This is more specific for the E46 m3.

http://www.corsaracecar.co.uk/pdfs/suspensionSetUp.pdf

Also offers a "What to do in case your car does this" scenario.

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Car Under steers in corners

The most common complaint. Typically this means your front compression damping is
too stiff so soften these off a couple of clicks (1/4 turn) then try again. However if the
under steer is accompanied by excessive front body roll then try increasing front
compression damping. You can also try adjusting the rebound damping (harder if the car
is not rolling, softer if it is) but most of the time front end under steer is cured with
compression damping only (does depend on the car).


Car Over Steers in Corners

Normally this means that rear rebound damping is too soft and so needs to be firmed up
but if this is accompanied with excessive body roll then try also stiffening the rear
compression damping as well.


Car loses Grip coming out of Corners
Again a common complaint, especially with front engine/rear wheel drive cars like the
M3. Try softening the compression damping and firming up the rebound damping. Also
try lowering the rear tyre pressures.
At the end of this article weve attached an over steer/under steer flow diagram which has
been doing the rounds on the web for ages. Id accredit who wrote it first as its very good
summary but in all honesty weve no idea who wrote it first? If anyone knows then wed
love to credit them.
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      01-24-2012, 03:28 PM   #3
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      01-24-2012, 04:08 PM   #4
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I will post this from Brian Hanchey (AST USA)

Tuning AST Shocks by Brian Hanchey (hancheyb@ast-usa.com)
This is a handy "cliff notes" version of how to setup your car. It is by no means the "end all,
be all" manual to setting up a car for the track, but is a way to get started when you're new
to adjustable shocks. Cars are systems that operate with many variables. Depending on
the car (fwd, awd, rwd) or high horsepower or low horsepower you may have to do
unconventional things to achieve the goals you want. This document outlines some of the
basics to get you on your way.
NOTE: This article assumes your car has quality bushings in all locations. If you
have not checked or replaced all your bushings STOP READING THIS ARTICLE.
You must have properly inspected bushings before beginning to setup a car's
shocks.
The following applies to all AST shocks (4100, 4200, 4300, Sportline 1, etc)
Rebound Adjustment (top knob for most models)
ASTs have detents or clicks when you turn the knob. Usually they have 12 clicks,
sometimes 13. So here is what you do at the track OR autocross, same applies.
AST uses the terms "opened" and "closed" because what you're doing is opening and closing
a bleed valve in the piston assembly. Turn the adjuster to fully closed. This is the same as
screwing a bolt in ("righty tighty") and is labeled "H" on the orange adjuster knob. This
may or may not wind up on an exact click so turn it clockwise until you feel a resistance
then turn it back to the nearest click. This becomes the reference point we'll call "fully
closed". When you discuss setups with AST dealers, use these terms (opened and closed).
Do this for all four shocks/struts. Drive the car on the track and notice if you feel it
understeer at turn-in or oversteer at turn-in. If either of those conditions exist do the
following.
For understeer at turn-in: Turn the front shocks 2 clicks softer or towards open. Drive it
again, and repeat if the condition continues to exist. If you get to fully opened and it still
occurs you'll need to soften the front bar, add more front camber, or check your tire
pressures.
For oversteer at turn-in: Turn the rear shocks 2 clicks softer or towards open. Drive it
again, and repeat if the condition continues to exist. If you get to fully opened and it still
occurs you'll need to stiffen the front bar or check your tire pressures.
Shocks do not affect handling in steady state cornering in perfect conditions. That
is to say they *could* affect handling if you hit a big bump in a corner, but in steady state,
over- or under-steer is caused by spring rate, sway bars, tire pressure, camber, camber of
the turn, toe, etc. So if this happens, don't blame the shocks. Something else is going on.
That helps you rule out one thing!
WARNING: To those that have ever said "my car is so low it is tight!" If your car
understeers, you are most likely "riding on the bumpstops". RAISE YOUR CAR because
going fast at the track looks much cooler than having the car too low and understeering.
Lower is not always the answer in car setup.
For understeer at corner exit: Turn the front shocks 2 clicks towards closed. Drive it
again, and repeat if the condition continues to exist. If you get to fully closed and it still
occurs you'll need to soften the front bar, stiffen the rear, add more front camber, or check
your tire pressures. You might have to experiment with both because it may depend on
many things.
For oversteer at corner exit: Rebound adjustment has little effect on corner exit. You
can disconnect the rear bar if you need to. Alternatively you can close the rebound on the
front shocks too to transfer weight. Also, consider adding more rear toe. We run 1/4" of
rear toe-in to put the power down. Also, on BMWs things like the condition of the RTAB
bushings will change this as well. Make sure they are new or in good shape AND we don't
recommend poly.
These are the basics I use to set a car up. This gets you most of the way there. As stated,
the vehicle dynamics is extremely complex. Depending on your level of car preparation you
might need to consult someone for additional information. Enjoy!
Low Speed Compression Adjustment for AST 4200 and 4300s
Rebound adjustment is the most critical component to car setup. Compression adjustment
gives you that fine tuning ability to change the damper under new conditions such as a new
surface or track. Compression adjustment is a little more tricky than rebound adjustment.
You want to run just enough compression that the tire doesn't "skip" over bumps. On a
perfectly smooth surface this may mean a fully closed setup, while a bumpy surface may
require you to run very little compression. Leave compression fully opened while you
experiment with rebound. Once you're happy with the rebound settings, begin playing with
compression setup.
After rebound adjustment is complete, set all four dampers at 6 clicks from fully closed.
Follow the procedures listed in the above section to determine where this is on the
adjustment range. As you drive notice several things:
1. Do you feel the car skipping over bumps?
1. If yes, decrease compression on the end of the car skipping by two clicks.
Repeat as required.
2. Does the car put power down well on corner exit?
1. If no, increase compression at the rear of the car by two clicks unless the
surface is bumpy. Repeat as required.
3. Does the car porpoise under braking?
1. If yes, increase compression of the front dampers by two clicks. Repeat as
required.
Out of adjustment? Ask us about revalving options.
Brian
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      01-24-2012, 07:23 PM   #5
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In the end it's going to come down to using those guides with whatever knowledge you (or maybe someone else, like an engineer) has, and a heck of a lot of testing.

You need the proper tools, like a pyrometer, camber/castor guage, and some form of data acquisition to be able to document how the changes are affecting the car. I wouldn't expect you to get it "dialed in" your first time out, and probably not even by the tenth time out. There is so much you can fiddle with (provided you have the parts), you'll think you have it right, and then you take the car to a different track and it's not handling quite the same.

it's going to be a long and tedious process! And the reality is most of us don't know enough, and don't have the time/resources to really tune the suspension like race teams do. But - you can find something that works pretty decent after a couple of track days, and then just work on your driving skills, making small adjustments here and there based on the current conditions (weather, temperature, track surface, etc).
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