Suspension Tuning Tips       by Bob Tunnell


Before getting into specifics of suspension tuning, here are some thoughts about suspension tuning drawn from my own experience...

1. When listening to advice, consider the source and the context. Many of the Internet special interest groups are a wonderful source of information. But one of the biggest drawbacks to all the advice you see on SIGs is that the authors are rarely qualified to offer SPECIFIC advice for YOUR car. It's easy to SOUND like an authority... it's rare to actually BE one. When reading their advice, pay particular attention to what make and model of car they own and drive, what modifications have they made that are truly applicable to your car, how experienced they are, what discipline are they tuning for (autocross, road race, street, track, etc.), and how closely their budget resembles yours.

2. Be specific in your analysis. If your car is pushing, pay particular attention to *when* it pushes. Is it on initial turn-in? Is it mid corner? Is it on corner exit? Or is it a combination of all three? For example, if a car pushes in mid corner, but not upon entry or exit, chances are the problem lies with alignment, spring, or bars... not the shocks. So it's critical to properly identify the symptom and isolate the true cause of the problem... the treatment needs to address the root cause.

3. Fix the end of the car that has the problem. If your car isn't turning in properly, don't adjust the rear of the car to try to make the front end work better. Conversely, if the car is extremely tail happy because of low rear grip, decreasing the front grip to compensate may make the car feel balanced, but it will ultimately make you slower. Often in Stock class autocrossing or road racing we're forced into adjusting the "wrong" end of the car because rules limit our ability to adjust properly, but these methods should be considered a last resort.

4. Don't discount driving technique as a big factor. Most drivers don't like to hear this, but it's true. I can't tell you the number of times I've been told by a driver, "My M3 pushes like a pig!" But when I get in the car it seems perfectly balanced to me. I'll ask the driver what his previous car was and invariably it was a Camaro, Mustang, Corvette, or other high horsepower RWD car. M3s were not built to handle the same as pony cars and they cannot be driven quickly with the same technique. Slow sweepers in particular need to be entered slowly, under control, and "carved"... not tail-out Dukes Of Hazard style. And the best part -- changing your driving technique doesn't cost you anything!

5. For the best results stick with one tuner. Professional high performance tuners know more than anyone else about how to make your car fast. We work on making cars faster and handle better day in and day out. We know which products perform best together and which ones don't. By mixing and matching products -- usually in an attempt to get the lowest price -- customers often end up with a car that doesn't respond like any of the advertising claims they've been reading. Put a pot pouri of components on your car and you'll likely end up with a mixed bag of results and wasting your money. Different tuners have their areas of expertise and their advice.... spend a little extra, discuss your needs and budget with a professional tuner, and you'll likely get far more value for your money.

6. "Compromise cars" will not do everything well. I am frequently asked how to set up a car that will be good on the track, but still plenty comfortable to drive on the street. Not everyone can afford to own enough cars to have one for a commuter, one for autocrossing, one for road racing, one for rallying, and another for concourse events. Most of us are forced to live with compromises... one, or maybe two, cars that have to perform a variety of functions. When asking a tuner for advice on how to set up your car, you must first determine in your own mind what compromises you'll be able to live with and be certain to communicate your needs. If winning autocrosses is more important than having a luxurious ride during your 5-minute commute, chances are you'll be happy having your car set up more for performance. But if you drive an hour to work over frost heaves and tar strips, you probably should set up your car more for comfort and leave the WRC Championship for another time. BMWs are amazing cars and can do a lot of things well, but don't expect a tuner to do the impossible.

 

And about the following Shock Tuning Guide in particular...

7. The following guide is for tuning shocks for road racing -- autocross tuning can be very different. Road racing maneuvers are almost always done "in phase," meaning the link between driver input and vehicle response is usually linear, or very close to it. In Autocross we frequently encounter "out of phase" maneuvers, meaning secondary inputs are often necessary before the vehicle has even had time to respond to the initial input... slaloms and high speed offsets are good examples of maneuvers rarely encountered in road racing. The suspension tuning is often one quite differently.

8. Guides like these almost always assume you are able to change all elements of the suspension. The guide below (and most guides like them) are based on the assumption that you have already optimized the spring rates and that you are dealing with a fairly balanced, competent, road car. For most of us we rarely encounter this "perfect" situation. In Showroom Stock road racing or Stock Category autocross, for example, we are faced with preparing a car as it comes from the factory and cannot change spring rates. We often deviate from these basic guidelines to "trick" the car into doing something that would make factory shock engineers cringe. <g>

9. Make the shocks do their job and let the other suspension components do theirs. The primary job of the shocks is to do two things -- affect ride quality and control the rate of weight transfer. Don't ask them to act as springs (unless you're stuck with the dilemmas I presented in #1 or #2). In terms of handling control, shocks do very little in the middle of a corner. Springs and sway bars have a much greater affect on handling in the middle of a corner. By paying particular attention to #4, you'll have a better idea whether the problem lies with your shocks or elsewhere.

 

And one last general guideline to keep in mind...

10. In general, stiffening one end of the car will reduce the mechanical grip on that end. In other words, when you raise the spring rate, increase sway bar size or stiffness, stiffen the bump or rebound of a shock, install firmer bushings, etc. you will reduce the grip on that end and decrease traction. To increase grip you must lower the spring rate, increase the sway bar size of stiffness, soften the shocks, use softer bushings, etc. (Tire pressure is another contributing factor, but that's a discussion for another day.)

 



KONI shock tuning guide
Suggested Adjustment Procedures For Road Racing Use
(from the KONI NA Factory Tuning Guide)


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.


Follow these links for help in adjusting MOTON double-adjustable and triple-adjustable shocks.


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