Suspension Theory/Chat Thread

[aki]

Have a question about the dynamics between springs/dampers. More about springs.

As a simplification, would it be accurate to say springs determine how much suspension travel there is, while dampers determine how fast the travel is? In addition, in running the springs the softest you can on the track, does that mean you want it the softest you can before it starts hitting the bumpstops? In other words, do you tune the spring rates judging by how often it travels beyond the optimum distance, and try to find that sweet (soft) spot where it doesn't?

But to complicate things, doesn't stiff compression dampening serve as springs in some sense, as it prevents travel when the resistance is high/movement is slow? So would the first step be to work on the springs first to see the extent of travel, then the damper settings to judge the speed of that travel, and to correct the transitionary characteristics (such as rebound dampening)?

Not planning on custom springs/coilovers anytime soon, just wondering how companies (such as Tein or Skunk2) come up with their spring rates. I know it's in part due to the motion ratio, but why have 600lb instead of 400 or 500?

[Highrev1]

Our Mac-Strut front suspension, free-travel is the distance from the bottom of the bump stop to the top of the strut bearing (or camber plate). Most of use lowering springs to lower our cars. Its the easiest thing to use...There are pros and cons to this, but we will go over that in a later article

There are other ways to gain travel such as relocating the hub and spindles higher, Speed tourning cars do this, but I don't think there are too many on this site that will be doing this so we will discuss other options.

What are the disadvantages of lowering a car by shortening the springs?

The primary disadvantage of using shorter springs to lower a car is that the suspension travel is reduced.



This Stolen picture Shows what exactly happens when you lower your car on springs...Obviously this is a basic drawing, but works EXCELLENT for our purposes...The blue circle shows the suspension travel loss when you lower your car.

WHY? Well you reducing the distance between the upper mounting point and top of the Shock or damper. This is forcing the bump stop to be lower to the top of the shock, which in turn reduces travel between the two corelating points, Bump stop and top of the strut housing. The only way around this is to either use a shortened strut housing or to cut the bump stop. There is only one company I know of using shorter struts for our cars

The other alternative is to cut the bump stops..Cutting the bump stops may seem like a bad idea at first, and it really isn't an ideal situation if you are able to maintain the suspension travel through shortend shocks or relocated hubs. The stiffer the spring the less travel is needed since the suspension doesn't compress as much, but alot of our standard lowering springs are only SLIGHTLY higher rate, requiring a trim of the stop. The bump stop is a shock limiter/cushioner, if you have the time and ability you could determine the ideal bumps stop length and spring rate. Too long a bump stop and you add a stop to the travel causing you to ride on the stops and not the springs...Too short a bump stop and when you do fully compress the suspension it will make the change in your ashtray rattle Matching bumpstops to ride height is a key thing to do, this is done with testing an knowledge...Or just get shorter dampers


"Suspension Travel

When you lower a car via lowering springs you are doing a number of things

Shorten the suspension travel (As above) … less travel means the suspension has less movement to absorb the changes in the road surface …ie potholes … you are much more likely to bottom out and hit something important (like an oil-pan).

You MUST have higher spring rates to keep the car within the new suspension travel range. Why … Well the OE springs are designed with a specific spring rates and a specific spring compression (travel) Remember that spring rates are a measure of a springs length compressed by the amount of force(weight) … ie 200 lb/in springs will compress 1” for every 200# applied to them.

If the Stock suspension has for example 6” of total travel, and you lower the car by 2” you have reduced the overall travel by 33% to 4”. So what you say, well the weight of the car has not changed … and if the spring rates are similar to stock, the car will NEED the same amount of suspension travel as OE (6”) but you are only giving it 4” (lowering springs) …so now you will be using all the springs travel AND the car suspension will be spending a lot of time on the bump-stops rather then being supported by the Springs. Related to this, when you shorten the travel you also shorten the distance the shock has to damp the oscillations, so it has to work “harder” with an shorter distance and time.


Bump Stops
-Bump stops are springs … when the suspension runs out of travel it compresses to the point where the car is actually running on the bump stops. Since the bump-stops are actually elastomer springs with a high spring rate, your car “suddenly” goes from one spring rates (springs) to a much higher spring rate (bump-stops) causing the handling to become inconsistent and difficult to control. You car can and will suddenly go from under-steer to over-steer (or vis-versa) mid-corner as you load the suspension up and compresses the springs to the point where you are sitting on the bump-stops. Related to this, you shocks valveing are designed (hopefully) to properly damp or control the oscillations of the springs (based on the spring rate) … when you are running on the much higher rate bump-stops the shock have NO chance in damping them, adding to the instability of the suspension system at the limit (where you need the most stability)


Changing the frequency of the system
-As you increase the spring rates you also increase the frequency at which the suspension operates … The definition of frequency is “cycles per second”. Low spring rates provide long slow motions, high spring rates provide shorter, faster motions. This increase in speed and frequency obviously increases the “work” the shock has to do to damp the motions of the springs. When the car is riding on the bump-stops, the spring rates go up HUGE, thus the work the shock has to do is substantially increased. The Valving of the shock is not only designed for the specific spring rates but also the frequency that the suspension operates at … the valves and orifices are designed to flow a specific volume of fluid at a specific rate … the high frequency motion is beyond the high-speed damping ability of the shock, and as a result it cannot damp these higher rates and the highly increased flow rates wear out the shock WAY faster than normal.

It is my theory that the average lowering spring wears out shocks because it spends so much time on the (high-rate)bump-stops thus speeding up the wear on the shock.


Bottom line … lowering springs do not work well and actually negatively effect suspension performance for the following reasons

-Decrease suspension travel without compensating (with increased spring rates) for this loss in travel

-Allow the suspension to spend WAY more time then it should on the bump-stops … causing instability and wear

-Run beyond the shocks ability to damp the springs rates and motion frequency, and increase the wear on the shocks.


Any changes to the suspension must be systematic ... you need to create a complementary series of components that work well together.

-Spring rates matched to the ride-height and weight transfer charateristic you require
-Bump-stop travel and rates matched to the above
-Shock valving and travel matched to the above

Moose"

[aki]

I'm not sure that copy-and-paste answered my question

In other words, isn't the ideal spring rate at just the point where it doesn't compress to the bumpstops?

[Highrev1]

There is more to take into account....What wheel rates do you want to run...What is the purpose of the car?

[LucasBlack]

Originally Posted by aki

I'm not sure that copy-and-paste answered my question

In other words, isn't the ideal spring rate at just the point where it doesn't compress to the bumpstops?

when i consider suspension tuning/setups/etc., i lean towards regarding the springs as the ability to position the ride height/stature of the car, and selecting spring rates as 1.) to aid in the responsiveness of the car, and 2.) using spring rates as a tool to balance the sensitivity between the front and rear.

i consider dampers as the actual controls of compression and keeping the car off the bump stops. now obviously these two suspension components need to work together. this is where it is necessary to utilize springs that are comparable to the level of dampening the shocks are set at, but there is a range of spring rates possible depending on the shock valving.

with that considered, then we can start putting together the idea of running "soft" springs up front to help with front bite and leaning away from a bouncy/twitchy front end (for us fwd guys).

BUT THEN AGAIN! we can't forget about roll bars, and then of course depending on what size bars you use might require more or less spring rate and/or higher valved dampers to accomodate.....

one thing i always try to keep in mind is that no one suspension component is acting alone.....

[Sirbelch]

Originally Posted by aki

I'm not sure that copy-and-paste answered my question

In other words, isn't the ideal spring rate at just the point where it doesn't compress to the bumpstops?

I would think so. Using the motion ratio of the suspension, you can figure out what spring rates you want to run to get the certian frequency you are after. 1 hz for a street, 1.5 for street/track and 2-2.2 for the track. Then you have to pick a damper based on its motion ratio (if its separate from the spring like our rear suspension) that best utilizes those frequencies. Since you want to keep as much suspension travel as possible, I would think that the valving you want will do just what you said and keep the car off the bump stops.

We need an engineer to explain it better because I paraphrased that from an engineer. Chirs, where you at?

[Highrev1]

I would alsways prefer to tune with springs and dampers rather than Sway bars...There kinda like big Band-Aid's....

[LucasBlack]

Originally Posted by Highrev1

I would alsways prefer to tune with springs and dampers rather than Sway bars...There kinda like big Band-Aid's....

touche

[Sirbelch]

But swaybars let you run softer springs for those bumpy tracks.

[Borbor]

Depends entirely if the bumps you see are mostly of the one wheel nature or two wheeled nature

[aki]

Originally Posted by Highrev1

There is more to take into account....What wheel rates do you want to run...What is the purpose of the car?

Well as far as future aspirations (I'm putting a freeze on spending $$$ on my car until the end of the year though, spent too much already), the purpose is quite frankly for it to be fun, not to beat out cars on the track or autox.

Definition of fun = rear end that gets loose on initial turn in, stays neutral, and has mild oversteer that can be modulated with the gas pedal. Oh, and doesn't break my back from bumpiness. Simply put, tossable in corners.

I guess specifically I want to get rid of the wall of understeer I feel at turn-in.
It feels like it pushes very hard initially--but then through the corner and out the apex it's pretty neutral and rotates nicely. Possibilities I suppose would be softer front sway, but maybe also stiffer springs to the rear? I think the springs for the Mugen suspension is actually stiffer up front compared to rear, and even then the spring rates are softer than OEM. I suppose ideally you'd want to have the wheel rate closer to the front, but dang that'd mean for 200# fronts you'd need 600# rears? I'm guessing that'll wear out the rear dampers in quick order.

This is all just theorizing for now though, since like I said I'm done spending money on my car for now. After getting wheels powdercoated for 2nd time, dismounting/remounting and balancing, I wanna law low :)

[Borbor]

I can almost guarantee if you run that setup, you'll complain about exit understeer.

Unless your driving ability matches those of the premiere touring car drivers, I can see that setup leading to what I described above.

[mbringle]

Originally Posted by aki

I guess specifically I want to get rid of the wall of understeer I feel at turn-in.

Try this:
Oh hay, understeer!

[LucasBlack]

Originally Posted by mbringle

Try this:
Oh hay, understeer!

if only it were that easy

[Borbor]

Actually, it IS that easy.

The hardest part is having a driver that can tell what the car is doing at each phase of the corner AND be able to determine if it really is the car, or if it's just driver error.

I personally won't adjust the compression of the damper to tune the chassis behaviour mid-corner, and there are a few things in that flow chart that doesn't agree with how I've been taught to do things, but overall that's not a bad place to start.

Just the question "Does the front/rear roll excessively" alone is enough headache for most amateur drivers

[LucasBlack]

i wasn't denying the fact that it could be a decent place to start, but as far as being able to associate that chart to any and every driving condition on-track is kinda silly....


...or am i completely in the wrong here?

[Borbor]

From my personal experience last year, it's almost always the same procedure after the car comes off the track.

So is it silly to associate that chart with a bunch of different tracks? Not really :)

[aki]

Thx mrbringle for the link, saw it before but couldn't find it. +1

I think I messed up terminology--which wouldn't be surprising since I always butcher the words. I feel the understeer when I'm pointing my car toward the apex, which I guess is technically mid-corner?

[Highrev1]

I say reduce the effectivness of your FSB, the two sides are probably to tightly sprung together...From what I gather this is getting back on the throttle right?

[aki]

Originally Posted by Highrev1

I say reduce the effectivness of your FSB, the two sides are probably to tightly sprung together...From what I gather this is getting back on the throttle right?

That's what I was getting the feeling of, that it seemed to prevent too much roll (hence my toying with the idea of the slightly softer Si Sedan FSB). It's at the point where I let off the brakes and turn into the apex, so it's before getting on the throttle. When applying the throttle through the apex it feels great.

I haven't pushed my car in awhile though, ironically because the limit is higher and because I'm able to go a lot faster, esp with the tire upgrade to RE070. On an open road (where I can crash and hit a tree) even without any other cars I'm getting hesitant to push it. And today I had to replace one of the tires since the tire bead was bad according to Wheel Works, so one of the tires is a RE960 pole position instead.