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Thread: Suspension theory basics

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    rogwick's Avatar
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    Default Suspension theory basics

    I came across an article I thought I would share. Plenty of you guys are posting opinions/ideas in regards to either spings, coilovers, sway bars, shocks or all of the mentioned. I found this article to be quite an interesting read if you're considering or trying to understand suspension basics I hope it helps in any way. Best not get back to me with questions...I didn't write it!! Although I'll help as best I can if they're asked

    **** SUSPENSION THEORY BASICS - by Pierre Dupuy

    When you get used to taking corners much, much faster than you ever thought possible in your car, with absolutely no perceivable body sway, tire squeal, or loss of control, you will find it difficult to drive a car with lower handling capabilities - it gets addictive! The right setup will give you this capability. Some weight & weight distribution, suspension, and handling basics before we get into details of modifications:

    1.) Minimize overall vehicle weight as much as possible. With no other changes, the lower a car's weight, the less weight transfer there is in a corner, and the more cornering force a car will generate. A related issue is front/rear weight distribution - the closer to a 50%/50% distribution this is, the easier it will be to set up the car for maximum cornering. The handling will also be more balanced, predictable, and controllable. One easy way to help distribute weight more evenly is to relocate the battery to the trunk area. There are kits available to do this from a number of suppliers. Be sure you use the best quality, heaviest gauge battery cable possible to avoid increasing electrical resistance and be sure the battery is well-anchored.

    2.) Minimize suspension unsprung weight for best handling. Unsprung weight is what is on the outboard end of the suspension - what is not being "sprung" by the suspension. This includes wheels, tires, brake components (rotors, calipers, drums) suspension arms, spindle, rear axle and associated brake components, and the leaf springs. By reducing unsprung weight you are reducing mass in motion in the suspension. Less weight moving up and down means the suspension can react more quickly to changing road surfaces and keep the tires planted on the ground better. This all equals higher acceleration traction, cornering traction, and overall better handling and road feel.

    3.) Choosing spring rates, other suspension components. Generally, there are two "schools of thought" in setting up a car suspension for cornering / handling.
    a) soft springs/stiff (thicker) anti-sway bars
    b) stiff springs/soft (thinner) anti-sway bars

    Both of the above have some (+) and (-) . The soft spring/stiff anti-sway bar setup, with matched shock absorbers, is, in my opinion, probably the best for an all-around (race and street) car as it gives you a very flat cornering car with a liveable ride on the street. Either approach will require a high performance shock absorber matched to the spring rates you are using. I ended up using the advice of some other SCCA members with 2nd generation camaros and generally went with the Guldstrand Engineering recommended spring rates, anti-sway bars, and shock absorbers on my car. The stiff spring/soft anti-sway bar approach also gives you a flat cornering car but I would recommend this more for a car that will see little if any street use as the ride is extremely harsh and best suited for the smooth surface of a race track. Cars I have seen set up this way have used 700 + lbs/in front springs, 190 to 200 + lbs/in rear springs, a 1" to 1 1/4" fr. anti-sway bar, and often a small or no rear anti-sway bar.

    Balancing handling by adjusting front vs. rear roll stiffness (This applies to the dynamics of really any front engine, rear wheel drive car. I have little experience with front wheel drive cars and hope to never see the day of a front wheel drive camaro!): Note: with the specs of the setup I describe further below (springs, anti-sway bars, shock absorbers, suspension alignment, etc) I ended up with very flat cornering and neutral handling car which needed little adjustment of front vs. rear roll stiffness. Varying tire pressure and changing front anti-sway bar bushing material is about the only change I have had to do to "dial-in" the suspension for different track conditions and for street use.

    TIP: Before you do anything else, try adjusting front vs. rear tire pressure. This is easily done and you would be surprised at what a difference a 2 to 5 PSI change can make. Try adjusting in 2 PSI increments, and record the pressures that work (cold pressures, for repeatability) so you can duplicate them again in the future at different tracks, etc. - be aware that differing air temperature will have an effect on pressures.

    The ideal end state is a neutral handling car which neither understeers or oversteers in a constant turn at a constant speed. A well set up car can be pushed into oversteer by more throttle, and back toward understeer by backing off the throttle - you can "steer" a well-balanced car by the throttle alone.

    1.) If you car understeers too much (the front of the car wants to "push" out of the turn and it takes excess steering effort to keep it in the turn, you have too much front roll stiffness, and possibly the wrong tire pressure (too low). You need to reduce this front roll stiffness. What is happening is that the front tires are not able to maintain enough traction and are pushing (really sliding) out of the turn. The technical term for this lack of cornering traction is "slip angle". The more a tire loses cornering traction, the higher a "slip angle" it is said to have. In this case, the front tires have a higher slip angle than the rear tires. This causes the nose of the car to slide out of the turn and is commonly called "understeer". You are most likely not aware this is happening because there is not enough "slip" to feel, nor enough tire squeal to hear. If you have a good set of high-performance tires, and do not need to upgrade them, follow the advice below to balance handling (assuming all the other suspension parts are installed and chassis work is done). If not, I would recommend you invest in good tires before you do any further suspension tuning. Tires are the only point of contact where all the hard earned $ you invested in suspension and other hardware actually meet the road. Good high-performance will make the single biggest difference in overall car handling and especially cornering (see tire section for tire/wheel size/type recommendations). To adjust the suspension and reduce understeer:

    a.) You can reduce front roll stiffness and adjust tire pressure by:

    - increasing front tire pressure (for street use do not go above the maximum recommended safe pressure-usually posted on sidewall, ask tire manufacturer if you do not know what this is). By increasing tire pressure you are allowing the tire to work as intended, and the full tread width to stay firmly planted on the road for maximum cornering traction. Too low a tire pressure will cause the sidewall of the tire to deform, causing traction loss and understeer. At the extreme opposite end, excessively high tire pressure can distort (bow) the tire tread, causing only a small portion of the center of the tread to contact the road. This too can result in loss of traction and produce understeer.

    - substituting softer anti-sway bar bushings (as discussed in the anti-sway bar part of the suspension section )

    - reducing anti-sway bar stiffness - easier if you have an adjustable anti sway bar, but otherwise you will have to install a thinner bar.

    - reducing front spring rate - install softer springs.

    b.) If you feel the front setup is just about correct, or the above changes are not enough, or you want to experiment with changes and their effect on overall handling, you can also increase rear roll stiffness and adjust tire pressure by doing the opposite of the above to the rear suspension: - decreasing tire pressure (I wouldn't go below about 25 psi for a high-performance street radial or you will really speed up tire wear - ask the tire manufacturer for recommendations on min. safe tire pressure) - installing harder anti-sway bar bushing materials (see discussion under anti-sway bar section) - increasing anti-sway bar stiffness - increasing spring stiffness - can be done of course by swapping in stiffer springs, but you can also add an additional "helper" leaf. A number of auto parts suppliers carry these, to include J.C. Whitney, and they usually give specific info on how much these increase spring rate. This is a relatively easy modification and could provide you that extra measure of rear spring stiffness you need without spending a whole lot more time or money on your "project car".

    Other suspension considerations in balancing handling and cornering:
    - another consideration is the tire size, the tread width particularly. If you have smaller tires up front, your front tire/wheel combination may be too narrow, and you are losing traction in a turn, causing understeer. Consider going to a wider (and even a lower profile) tire. You may have to change to a wider wheel too, matched to the tire tread width. A good rule of is to select a wheel that is as close as possible to the tire's tread width-the width of the contact patch that touches the ground (don't confuse this with section width). The tire manufacturer can get you all these specs. This will allow the tire sidewall to remain vertical to allow the full width of the tread's contact patch to touch the road.

    2.) If you car oversteers too much (the front of the car wants to steer into the turn too much, you have too much rear roll stiffness.and may also need to adjust tire pressure What is happening here is that the rear tires have a higher "slip angle" than the front tires, causing the tail of the car to slide out of the turn and aim the nose too much into the turn. Try increasing rear tire press / reducing front tire pressure, or some combination, and adjust roll stiffness as detailed below:

    a.) To decrease rear roll stiffness: - installing softer anti-sway bar bushing materials (see discussion under anti-sway bar part of suspension section) - decrease anti-sway bar stiffness - decrease spring stiffness

    b.) You can also increase front roll stiffness to further correct oversteer by: - substituting harder anti-sway bar bushings (as discussed in the anti-sway bar section ) - increasing front anti-sway bar stiffness (install new anti-sway bar) - increasing front spring rate - install stiffer springs.

    Handling - Objectives

    Handling comes down to one thing: Traction, and not just traction during cornering.A good handling car will be predictable and easy to control under cornering, braking, acceleration, on all sorts of roads, and in all sorts of conditions. As with anything, this means you need to compromise or balance the chassis for the majority of conditions the car will be operated under.....
    Street vs. Racing - Ride vs. comfort.

    Understeer vs. Oversteer



    Most cars are designed to understeer, which is when the car approaches a corner it wants to keep going straight. If you enter a turn too fast the front end will slide and turning the steering wheel is useless, so you'll have to slow down and turn into the slide. This is also known as "push".

    Oversteer is the opposite, where the rear end wants to slide out from under the car while the car continues to turn into the curve. This is known as "Loose".

    Most cars are designed to understeer so a normal driver can instinctively control the car. In a race situation, oversteer is prefered because a good driver can control the car easier and corner faster.

    To reduce understeer you add oversteer to the car by:

    * Increasing front tire and wheel size
    * stiffen the rear springs
    * increase front tire pressure.
    * increase rear stabilizer bar.

    Summary: You want to set the car up to oversteer. I repeat this because if you run out and spend $$$$ on a larger front swaybar, you're actually adding more understeer to the car, which you are (supposed to be) trying to get rid of.

    Wheels and Tires



    The only thing attaching your car to the road surface is the tire. Therefore, the tires are the key to a good handling chassis. A poor driver with good tires can outrun a good driver with poor tires anyday.

    Again, compromise is necessary due to the conditions the car will be driven under.

    Shocks



    Shocks dampen the actions of the springs.
    For the ultimate in tunability, you want adjustable shocks. These are expensive and if you don't know what you're doing, a waste of money. Since you're reading this they may be beneficial. Street shocks are designed to provide most of the dampening force on the rebound, i.e. when the tire is going back down towards the road after it hits a bump. This makes for a smooth ride. Performance shocks usually dampen at a 50/50 rate, rebound vs. compression. This improves handling at the cost of a firmer ride. Nitrogen gas shocks are the prefered way to go. Adjustable shocks allow you to tune for compression or rebound, depending on the road conditions.

    Swaybars and Springs





    Swaybars or stabilizer bars twist when a car leans during a turn.

    If your car doesn't have a rear swaybar, PUT ONE ON. This will be the most significant handling improvement you can make for little money.

    By switching to polyurethane, you effectively make your bar think it's 20-25% larger in diameter due to reduced deflection which rubber mounts. This will also speed up the bars reaction time.

    Springs hold the car up. They'll also rattle your teeth if they're too stiff. Use swaybars to TUNE your ride while maintaining softer springs if you need to drive on the road. Shortening springs lowers the car, which lowers the center of gravity. If you go too far, your car will bottom out either on the road or on the suspension, so pay attention.

    Other Tips
    Weight distribution - balance. Less weight = less effort to accelerate/decellerate.
    Last edited by rogwick; 16-03-2011 at 12:17 PM.

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