some basic suspension stuff

[scottzg]

This little primer will discuss various options and what actual changes are taking place, albeit with too many generalities and without enough detail.

First thing to understand is that ‘improving the handling’ could mean several things; making the suspension stiffer and more sporty, making the controls more precise and limiting bushing deflection, and altering the balance of the car to encourage it to behave differently to inputs.

A sporty ride is usually accomplished by changing shocks (or in our case, struts, same thing) and springs in for stiffer ones. Oddly, going to a stiffer strut is going to provide a bigger improvement in actual handling prowess, especially as the vehicle’s speeds increase and the wheel movements get larger. Most oem struts are designed to be most effective at low speeds and provide soft ride with the springs they’re matched to, so finding a more stiffly valved shock designed for oem springs can be a worthwhile exchange. Keep in mind that stiffer springs tend to prefer less compression and more rebound from a shock, so while a single adjustable shock is an improvement, it still might not be the ultimate solution. It will provide a degree of tunability that a fixed damper won’t allow. Of course, if you don’t want to be fiddling with the settings, a shock/spring set might be the most sensible solution.

Springs are the most popular modification made to a car’s suspension. Aftermarket springs can be of a different length, stiffness, or of variable stiffness. A longer spring will be compressed on the strut even before the weight of the car is on it. This makes the spring feel stiffer than it is, especially before weight is transferred to it from cornering. Variable rate springs use wider and narrower coils, or taller and shorter coils to make the spring rate change through its travel. An effort by the designer to make the spring suit a given shock, or provide improved ride quality while still resisting body roll. There’s just no simple formula to apply to how a spring will ride or make the car look. Lowering the car looks great, but it has some trade offs. The main one is that generally it moves the roll center away from the car’s center of gravity, actually increasing roll. Subjectively, this isn’t noticed, since the person is sitting lower and doesn’t feel the sway as much. The effect is somewhat curbed by the stiffer springs’ resistance to roll, as well. Another problem is that the struts gain less and less negative camber as they are compressed. The first inch of travel that is lost lowering is the inch with the most negative camber gain. This will be discussed later. The last problem is that the stiffer suspension reacts more harshly over bumps, meaning a rougher ride and bumps in a corner upset the chassis more. On the positive side, the stiffer springs do reduce suspension movement, keeping the suspension in the ‘sweet spot,’ better. The stiffer suspension also allows the weight of the car to transfer faster, so the driver doesn’t have to wait as long for the car to react before he can make his next move. This makes for a nimble feeling and gives the driver more space to correct from bad inputs, although it gives less time to react. Once lowering springs are fitted, be sure to get an alignment, as the lowering affect camber and toe.

Swaybars, or anti-roll bars do the same things the coil springs do; they reduce body roll and (anti-roll bars are really just torsion springs) increase the speed which the chassis reacts to inputs. Word on the street is that swaybars are twice as effective at limiting roll as the springs are. They’re cheap and super easy to swap, so a casual racer could have a couple lying around that they swap for changing conditions to increase a car’s tendency to oversteer/understeer. The adjustable ones are pretty cool too. Of course they have some drawbacks as well; big roll bars can pick up the unweighted inside tires, they unsettle the opposite wheel when one wheel hits a bump, and since their spring rate is dependant on weight transfer, it’s impossible to have them perfectly damped all the time. Basically, they suck off road.

A car that reacts precisely to the driver’s inputs is much easier to drive quickly, even if its limits are low. A communicative car involves the driver, or whatever. Simply put, taking out the slop in the suspension will, well, make the car less sloppy. First off, even on an old car, if the ‘controlX feels like crap’ something is busted or worn out. Fix that first before trying to modify other parts to conceal it. Steering racks moving in their mounts and improperly bled brakes are classic examples of this. The best thing, once sure that the car is in good repair, is to get some good tires with a nice stiff sidewall. Having the stickiest rubber isn’t that important, although large tread blocks tend to provide better feedback. Polyurethane steering rack bushings are wonderful for steering feel, and fresh brake fluid can make a profound difference for braking feel. High performance brake pads, however, usually feel like crap unless they’re being used for their intended purpose. If they’re not getting hot, they’re not worth sticking on. Never cooked your brakes? Leave them alone. Driveline bushings reduce the lag between driveline movements and wheel movement, with the tradeoff of some nvh. Polyurethane bushings should be avoided anywhere where there is movement inside the bushing, for example, on lower control arms or C style endlinks. The urethane does not twist like the rubber does, and so the bolt will wear the urethane loose over time. Stiffer rubber (measured by the durometer) is a better solution. Stiffer rubber bushings in the suspension arms reduces dynamic toe and camber change due to load, making for an easier car to place. NVH is a trade off, of course.

The following is all about oversteer and understeer, and the common ways to tune for both. First off, a car will exhibit different characteristics at different speeds and radius turns, due (among other things) to power, lift, and suspension geometry. Some modifications affect oversteer/understeer all the time, like swaybars, and some only under acceleration or braking, like brake valving.
Really, the balance of the car should not be tampered with by someone who cannot comfortably and safely explore the character of the car before and after the modification. A inexperienced commuter driving an oversteering beast is only waiting to get bitten, and is probably not aware of what they’ve done. That said, most cars are set up to understeer endlessly and will never allow one to explore the balance. To get a legacy to exhibit some lift off oversteer while still being fairly toothless, a 18mm swaybar or -1.5 degrees of camber in the front with 0 toe all around will do the trick. The alignment won’t affect tire wear significantly. Both together is what I run on the street, and it’s proven to be easy to live with and fun, although I’ve had to catch the rear end several times when the trunk is loaded.
The basic idea is to give one end of the car more grip than the other; a car with more grip in the rear at any given moment will understeer, more grip in the front will oversteer. The driver can transfer the weight of the car around and control the balance this way. Stiffening one end of the car will generally reduce grip. Stiffer springs, shocks, swaybars all do this. The exception is that if the parts keep the wheel in better contact with the road, the grip actually increases. An example of this is the front swaybar- a big front sway will cause understeer with jerky inputs, as the weight transfers quickly and overloads the tire, but once the wheel is weighted, the bar keeps the wheel in the sweet spot of its travel.
Stock cars are designed with about 10% more braking to the front than rear, regardless of a sporting intent. Once the car is spinning under braking, the effectiveness of the brakes is significantly reduced. You want a car that understeers under braking. A car with sport tires and/or properly damped suspension is going to be able to stop better, transferring more weight to the front; most modified cars already have more oversteerish brakes than the manufacturer intended. Be aware.

This was written off the top of my head while I was stuck with my laptop at a bus terminal. Consider the source. Comments and corrections are appreciated.

[Yukonart]

Good ole Scott. . . always watching out for the LCO folk. . .

[tris91ricer]

Omg wow. I think my head hurts, but I wanna go for a drive.

[IronMonkeyL255]

That's a good write-up.

Thanks for taking the time to do it (as it obviously took some time).

[vrg3]

Yes, thank you, Scott.

I have one question/comment (so far): You say sway bars do the same things coil springs do. But that's not quite true, right? Sway bars do act in parallel with the coil springs when they're acting, but they are inert when both wheels they're attached to are doing the same thing, right?

So bigger swaybars can stiffen the car's response to roll while leaving its response to pitch unchanged, allowing you to do interesting things with weight transfer during acceleration and braking.

Is that correct?

[Brat4by4]

You are correct Vikash. I think he was referring to what the sway bar does in a turn, though.

If I were to take out my STi suspension. It would still go flat around the corners, but dive and lift like a power boat (like it did before).

[scottzg]

If I were to take out my STi suspension. It would still go flat around the corners, but dive and lift like a power boat (like it did before).

other way around. Swaybars dont affect dive or lift, just side to side movement.



should also mention that the swaybars are directly affected by the rate of the coil springs. If the coils keep the car from leaning, the swaybar is going to effectively have less spring rate.

[greg donovan]

If I were to take out my STi suspension. It would still go flat around the corners, but dive and lift like a power boat (like it did before).

other way around. Swaybars dont affect dive or lift, just side to side movement.

i think he meant if he takes off the struts and springs but left the bigger/stiffer swaybars.

[scottzg]

If I were to take out my STi suspension. It would still go flat around the corners, but dive and lift like a power boat (like it did before).

other way around. Swaybars dont affect dive or lift, just side to side movement.

i think he meant if he takes off the struts and springs but left the bigger/stiffer swaybars.

blarg, im only semi-literate

[vrg3]

should also mention that the swaybars are directly affected by the rate of the coil springs. If the coils keep the car from leaning, the swaybar is going to effectively have less spring rate.

In other words, the sway bar will have the same spring rate, but it just won't make as big a difference due to the car's overall higher resistance to roll, right?

The swaybar also doesnt hold the car up.

Holding the car up involves holding both sides up though.

[greg donovan]

other way around. Swaybars dont affect dive or lift, just side to side movement.

i think he meant if he takes off the struts and springs but left the bigger/stiffer swaybars.

blarg, im only semi-literate

well you are literate enough to write a great post about suspension.

i had to read what he wrote a couple times to make sure i was reading it right as well.

[scottzg]

thank you all for all the nice words