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Suspension Ignorance. Please inform.

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  • Suspension Ignorance. Please inform.

    Good evening forum.

    Just doing research for my N15 DE+T street/track build and have some questions regarding suspension modifications i see quite often. I had originally posted simply asking questions and hoping someone else would fill me in on the details but i have since put more effort into my research and learning. Still, if im wrong please advise and correct me.

    Hopefully someone with more knowledge on these things can fill me in but so far i have been able to find (And ill be in some cases directly copying some information ive found in articles online) that a sway bar helps reduce body roll during fast cornering or over bad road surfaces (like most of Sydney). It connects opposite wheels together through short lever arms linked by a torsion spring. This increases the suspension's roll stiffness independent of its spring rate.

    Sway/Roll bars are intended to force each side of the vehicle to raise/lower to similar heights to reduce the sideways roll of the car in corners or big bumps. Without the bar, The wheels can tilt much further apart (Think a rock hopper of 4wd). So in a hard cornering situation, a cars outside wheels are going to drop down low, and the sway bar forces the inside wheels to drop too. As a result, the cars going to get much lower through corners (i guess this is what is meant by hugging the road), where all wheels are closer to the body. After exiting the corner the downforce is going to lessen allowing the ride height to return to where it was prior to the corner, The sway bar will keep the return to the normal ride height at a same or similar rate (so no jelly wobble).

    Please remember that i got alot of this directly from articles i read online and although i do understand what ive included here, its always good to hear a more knowledgable opinion on these things. So a sway bar is usually a torsion spring that resists body roll motions, a U steel bar that connects to the body at two points and the left/right of the suspension. If the left/right wheels move together, the bar rotates about its mounting points. If the wheels move relative to each other, the bar is subjected to torsion and forced to twist. Each end of the bar is connected to an end link through a flexible joint. The sway bar end link connects in turn to a spot near a wheel or axle, transferring forces from a heavily loaded axle to the opposite side.

    Forces are therefore: heavily loaded axle to the connected end link via a bushing to the anti-sway (torsion) bar via a flexible joint to the connected end link on the opposite side of the vehicle to the opposite axle...

    The bar resists the torsion through its stiffness. The stiffness of an anti-roll bar is proportional to the stiffness of the material, the fourth power of its radius (whatever the hell that means), and the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar). Stiffness is also related to the geometry(yay!) of the mounting points and the rigidity of the bar's mounting points. The stiffer the bar, the more force required to move the left and right wheels relative to each other. This increases the amount of force required to make the body roll. Makes sense right? I think im following this.

    In a turn the sprung mass of the vehicle's body produces a lateral force at the centre of gravity proportional to lateral acceleration. Because the centre of gravity is usually not on the roll axis, the lateral force creates a moment about the roll axis that tends to roll the body. (The roll axis is a line that joins the front and rear roll centers). The moment is called the roll couple.

    Roll couple is resisted by the suspension roll stiffness, which is a function of the spring rate of the vehicle's springs and of the anti-roll bars, if any. The use of anti-roll bars allows designers to reduce roll without making the suspension's springs stiffer in the vertical plane, which allows improved body control with less compromise of ride quality. Essentially stiff sway bars means you dont need as hard suspension. Good stuff for getting less roll but not making it painful as a daily.

    One effect of body roll with normal suspension geometry is positive camber of the wheels on the outside of the turn and negative on the inside, which reduces their cornering grip. So more roll means less grip in the twisties and longer lap times at the track.


    Anti-roll bars provide two main functions. The first function is less body roll. The reduction of body roll is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the centre of gravity height and track width. Basically, your cars not going to shift weight faster or differently right? So not going to need to re-learn your car...

    The other function of anti-roll bars is to tune the handling balance of a car. Understeer or oversteer behavior can be tuned out by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front increases the proportion of the total load transfer that the front axle reacts to—and decreases the proportion that the rear axle reacts to. In general, this makes the outer front wheel run at a comparatively higher slip angle, and the outer rear wheel to run at a comparatively lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle has the opposite effect and decreases understeer. Im pretty sure this means that you can tune your cars cornering performance in terms of under/oversteer by adjusting the mounting points of the sway bars... right?

    TO THE DOWN SIDE! Because an sway bar connects wheels on opposite sides of the car, the bar transmits the force of a bump on one wheel to the opposite wheel. On bad (Sydney) roads, sway bars can produce jarring, side-to-side body motions ("waddling"), which increase in severity with the diameter and stiffness of the sway bars. Other suspension techniques can delay or dampen this effect of the connecting bar. Which hopefully ill get to in this soon.

    Excessive roll stiffness, typically achieved by configuring an anti-roll bar too aggressively, can make the inside wheels lift off the ground during hard cornering. This can be used to advantage: many front wheel drive production cars lift a rear wheel when cornering hard in order to overload the opposite wheel, limiting understeer. For anyone thats seen some spirited driving at track days, it also looks pretty damn cool...

    Right so thats sway bars done for now. If you have any comments, corrections or otherwise feel i need to be called an idiot about something please drop a comment. Im taking the dog for a walk but later this evening will be editing and moving onto Strut Bracing and eventually ill get to the control arm bracing like Marty put on his Evo9... just got alot of reading to do before then too.

    Okay so now im back and ive read into the Strut bracing and it seems to follow alot of the principles of the sway bars in eliminating flex. In this case it ties the strut towers together which helps eliminate flexing of the struts independent from one another. This helps eliminate chassis flex in corners which as far as ive been able to read helps keep the car stable in corners. The strut bracing seems to be a good supporting mod for the sway bars as well as coilovers.

    Yeah, i know that one was one hell of alot shorter than the write up on sway bars but it looks like most of the principles of how they worked are the same if not really similar to a sway bar. Stiffer bar means less flex means better cornering performance.
    Last edited by JamesKnowsNothing; 23-09-2019, 07:44 PM.
    Milk, Juice and my Pulsar. All things in 2L

  • #2
    in general, your understanding is going in the right direction. however, it seems you keep mentioning your car being a daily driver. your final statement about stiffer bar/less flex would end up being counter intuitive to a daily, because of the wonderful road conditions you mentioned in sydney. trust me, i know your pain, omaha usa is rated as one of the worst in the nation for road conditions, and my slammed golf feels the pain constantly.

    a friend of mine used to run SCCA auto-cross with his mk1 vw scirocco. his ideal setup (with slicks) was a set of KW coilovers (not cheap) and the heavier-than-mk1 anti-roll bars from a mk2 scirocco. the chassis is basically the same, so the mk2 bars, which are both a little thicker, fit directly in the mk1 bushings, but offer more stiffness. they weren't massive bars, just a tiny bit more than stock, and it worked very well. granted, the slicks allowed him to carry both inside tires on hard cornering, but if he were to slap road tires on it and take it to the shops, it was only moderately more harsh than a stock 16v model of the scirocco.

    my golf, a mk3 5 door, has been upgraded over stock in several ways. in the US, we could only get a 4cyl in the 5-door. mine is a vr6, because i wanted it that way, and i had a rust bucket vr6 gti that was just waiting to get cut apart. as part of the swap, i changed over all of the chassis as well, so i got bigger brakes, 5-lug hubs, wider control arms and rear bar, and bigger anti-roll bars, since the vr6 engine is a couple hundred pounds heavier than the 4cyls. i've left those stock roll bars in, and went with a budget set of coilovers that have average dampening and are probably less than 50% stiffer than the stock vw springs, and the car (with full interior and a big sub in the back) easily handles cornering to beyond the limit of the hankook tires that are on it. the car has a slight negative camber on the rear wheels, and i've adjusted the front to have about 1 degree negative. for a narrower all-season tire, this gives a very comfortable ride, doesnt wear the tires abnormally, and still hugs corners to the point of pushing your body over the high recaro bolsters in the seats, even with a belt. i believe that if i were to strip the car and add strut tower bars, my current setup would probably handle a road-course fairly well.

    that brings us to the strut bars. there are some that are simply for show. basically any of them on ebay that are under a hundred dollars probably dont offer any real structural rigidity. this means that the bars' construction and materials arent solid enough to keep from flexing themselves. a good way to tell the difference at a quick glance is how the bar mounts to the strut. if there's a ring that bolts to the tower, and then that bolts to a bar, those WILL flex. no matter how much you tighten that bolt, itll be a point of rotation when the chassis flexes. keep in mind here that if the forces are strong enough to flex the structure of the car as a whole, itll flex that stupid little bolted joint. i only run bars that are one piece and mount directly to the tops or insides of the towers, without any bolts in between. 3-point, or triangle bars are extra solid, because triangles do not flex, and those bars connect to both strut towers as well as another solid point on the body. they offer only slightly less than what a welded cage would offer in those areas as far as flex and rigidity is concerned.

    camber is the last major piece of the puzzle for the basic track setup. in a perfect world, you'd know what your spring rates were on your suspension, the torsion rates of your anti-roll bars, and the measured center of gravity of your car with it's final track-ready setup. this is done knowing what the curb weight is, and adding that to a formula that takes into account the spring and roll rates. this will give you the "roll center" of the car, and can help you understand just how much the body will roll during a given corner, thus giving you a degree measurement that you can aim for when adjusting the camber of your wheels. as the car tilts, the perfect camber adjustment would place the outside tires (front and rear) at their closest-to-vertical position, thus putting the biggest tread patch on the ground while in the corner. this does have trade-offs however, and increased camber for corners means less flat-road tread touching the ground, which can lessen grip when braking or accelerating. also, corner radius and speed through the apex can change how much the car actually rolls as it dives through the corner, so most drivers will pick a section of track that is important, such as a good passing section, and tune specifically for that. a great way to hear drivers talking about suspension setup for different parts of the track is actually, believe it or not, NASCAR, on some of their mid-sized (mile-and-a-half) ovals. they break the track down into sections and try to tune for each section, then come to a happy medium that can mostly satisfy the whole track. tuning only for turns 1 and 2 (the first big bend) means that turns 3 and 4 (the last big bend before the final straight) could be very tight or loose. look at their car setups for some of the very small tracks, and youll see several degrees of negative camber on the right front tire, and even positive camber for the left front, so that the front end is planted flat when hitting the tightest bends.

    i hope this gives you some food for thought, and maybe gets those gears turning, both literally and figuratively. -cheers
    I will drive no car faster than it can go.

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