Thucydides

This being new to me I find myself easily confused with the explanations of what a shock and spring are doing as they go about their business. But if you don't mind humoring me a bit perhaps you can tell me whether the following simple analogy makes any sense:

I look at a spring as a mechanical storage device; it absorbs and stores energy as it is compressed, and releases it as it relaxes. It does this very efficiently and in this sense it's like a battery.

I look at a shock as an energy conversion device; it absorbs mechanical energy and converts it to heat energy. It also works with wonderful efficiency and in this sense it's like a light bulb.

In my analogy I'm going to hook up the battery and bulb with the following result:

When the car hits a bump most of the mechanical energy of the wheel traveling upwards is stored in the spring. In this oversimplified model you don't need compression damping at all, so let's pretend it's not there and the bulb remains dark.

As the wheel is traveling upwards the "battery" that is the spring is charged. When the wheel has reached its maximum elevation, mechanical energy input ends as does storage of that energy in the spring; the spring "battery" is fully charged for that bump event.

Since it's not our intent to maintain that store of energy, the energy stored in the spring is immediately transferred to the shock absorber, just as a battery's energy is released to a light bulb. The shock "bulb" glows brightly as the rapid conversion of mechanical to heat energy occurs in rebound.

Ideally, the shock "bulb" finally dims and goes out just as the wheel returns to it's neutral position. The battery is completely discharged, the bulb is extinguished, and everything is again ready to repeat the process at the next bump.

In my analogy the shock needs more rebound damping because that is when it converts the large amount of energy stored in the spring to heat. In compression it passively stands by as the spring does it's own job of storing that energy.

Yeah; of course it's much more complicated than that, and there are excellent reasons to have compression damping, but hopefully my simple analogy sheds some light on why the ratio of compression to rebound damping doesn't have to be 1:1, or any other particular value, other that that dictated by the proper function of the entire system for that particular car, road, and driving circumstances.

Thucydides

So what happens if we remove the spring "battery" and make the shock "bulb" do all of the work during the compression phase. This new system has lots of compression damping, but no rebound damping; in other words, the opposite of the first system.

Imagine the same bump, but this time rather than store the mechanical energy from the bump in the spring "battery" we've decided to convert as much of it as possible to heat energy immediately in the shock "bulb". As the wheel travels upwards the shock "bulb" glows brightly as that mechanical energy is dissipated as heat. As the last of the energy is converted the bulb grows dim and finally extinguishes. In this case, and with no energy remaining to extend the wheel, the car settles at a lower elevation - dare I call it "jacked down"?

This is also consistent with the state of energy (in this case, potential). We started at one energy level with the car at a certain elevation, then we removed energy from the car "system" by converting some of it to heat, and the (potential) energy state is now lower, as is the car.

PhilMD

I'd disagree with that. As spring rates increase, so do ride frequencies and therefore critical damping. Bump damping must go up also.

Phil

hustler

iTrader: (6)

Pics of your trophy case?

PhilMD

http://performance5.com/images/workshop/cad.jpg

Bernie S.

The hardest thing to remember is that the shock has a function, and so does the spring.

When you try to tune one for the other's function it is only a compromise. If you want to do it proper, know what to change and why.

Thucydides

My battery and bulb analogy says otherwise.

But kidding aside, and considering that lots of us don't understand ride frequencies and critical damping very well, could you break it down and explain it like you would to a four year old.

wannafbody

iTrader: (1)

Based on my limited reading, tuners increase bump to give the effect of a stiffer spring and give faster turn in. It's the same concept as increasing tire pressure but there probably is a limit before ride quality suffers.

Thucydides

I agree that stiffer bump damping replicates a stiffer spring, but that's more of a band-aid than the best solution.

y8s

iTrader: (8)

KINEMATICS MOMENT:
spring rate is the resistance to displacement.
damping rate is the resistance to displacement velocity.

so when you increase spring rate, you're saying "i dont want the suspension to compress as much under the same loading conditions"

and when you increase compression damping, you're saying "I don't want the suspension to compress as fast" or conversely, "I want the suspension to compress more SLOWLY"





compression damping rate doesn't change how far your car leans in a turn, but it does determine how long it takes to set in that lean.

Thucydides

This suggests that the more compression damping you have the longer it will take for your suspension to take a set since the rate of movement slows but the total movement doesn't. Yes? I assume the same holds true for rebound damping as well.

Bernie S.

Look at the newer shock technology that is available.

With Penske you have blow-off pistons to bypass more oil on sharp velocity spikes. You have Ohlin's with their DFV technology, also bypassing more oil on sharp/high velocity movements. Hell, you even have Bilstein with their digressive valving.

Each one of these shock companies use different ways of decreasing shock damping on the the compression spike. Why? So it doesn't affect the chassis. When you put more compression into a shock you end up unsettling the chassis because one tire hit a bump.

hustler

iTrader: (6)

I never would have realized this without your thread. Thanks.

y8s

iTrader: (8)

it's the "Speed bump at 50mph is smoother" design.

at that speed, the force of the bump is so intense that it overcomes the spring rate and the only thing acting on the wheel is the damping force. how fast your wheel can get out of the way of the bump is what determines the feel as you drive over it.

too high and it launches the chassis up in the air. too low and the wheel ricochets off and breaks traction.

but that compression value at very high speed (50mph bumps) is not the same that is responsible for how your car reacts to changes in direction or acceleration (+ or -).

Bernie S.

With high speed compression, the wheel has to accelerate through the low speed compression first. With more low speed compression, the wheel may never see the high speed end of the compression.

So, when you use low speed compression to tune a car you end up unsettling the chassis when you do hit a bump because your suspension is already being over damped on the low speed end. Adding more velocity is only making things worse.

Another problem with compression tuning is that it will work for some turns and not the others. This is simply because not every turn is the same speed. In relation, not every turn is the same shock velocity.

Why do you want a velocity dependant tuning tool, especially on the street?

It sounds so much easier to tune with the shock doing one thing, the spring doing the other. Then you match the shock to the spring, the spring to the car.

AD70

Some of the confusion is due to not separating two jobs the damper has to do.
Roll control (low speed) is one thing, the other is control over bumps.
That is why good dampers have low and high adjustments.
Low speed control is for roll (turns) high speeds control is for bumps.
Dampers dont control the amount of weight transfer, they control the RATE of the weight transfer.

wannafbody

iTrader: (1)

Don't you use swaybars and springrate to tune for roll? Springs function to support the vehicle. Shocks function to control the spring.

mgeoffriau

iTrader: (7)

Dammit Bernie...stop teasing us with these hypothetical questions and scatter shot explanations...I want you to write a 100 page explanation on shocks, starting with basic theory and ending with real world application from your own experiences. Keith Tanner has done like 3 of these Miata books now...contact his publisher and work something out. Or ask Corky Bell or Matt Kramer about their books. It's absolutely worth it. I'd pay $30 for it.

PhilMD

OK, let's look a ride frequencies first. Take a soft rate spring and put a weight on one end and fix the other end to something rigid. Now pull on the weight and you'll notice the spring will naturally extend and compress; this is it's natural frequency. A softer spring will have a lower natural frequency, stiffer will be higher. Ride frequencies are the natural undamped frequency of the body in ride, but you need to understand that both mass and the motion ratio of the suspension are required for calculating this. Also important is we're interested in the ride frequency, not just the raw spring rate; a 350lb/in spring on a NB will have a different ride frequency on a NC mainly due to the change in motion ratio.

When you add damping to a spring, you allow the spring to reach steady state rather than eternally vibrate at its natural frequency. Damping does not effect the steady state position, rather the time in which it takes to get there. Less than critical damping and you have the fastest response time but overshoot, more than critical damping you have a slow sluggish response.

Bernie's made some comments on compression tuning and I'd add:

There are 3 main things to look at with compression and rebound dyno traces. The trace is split into 3 different sections:

Low speed rate
Knee position
High speed rate

There are schools of thought that low speed compression tuning should be controlling weight transfer, but if you build a damper to have critical damping to control a couple of thousand lbs of car in moment of inertia you're going to screw the ride. Rather I use low speed compression to control the ride frequency and add some vertical loading to the tire. Too little damping here and you end up with a bouncy ride as the spring wants to keep vibrating, too much and you get the horrible boulevard jerk on smooth surfaces where you're bouncing off the tire sidewall.

The knee point determines where the damping changes to ride, and this is where the main shims open or 'blow-offs' in the damper. The high speed rate is much more digressive than the low speed rate.

The actual stiffness, or numbers, is not nearly as important as the high speed rate. If you look at a Bilstien R-Package damper dyno trace you'll notice the high speed rate is almost flat. If we remember that the damper controls the rate to steady state, you're going to need more damping over a large hit than a smaller one (the damping available is based on the velocity of the piston), but the Bilstein is not offering that. You could add stiffness to the high speed and keep the same rate, but it wouldn't change the outcome; you'd have the right damping at one point and everywhere else you'd either have too much or too little damping.

Phil

PhilMD

Generally speaking, springs and sways have different functions even though that act together in roll. You should be using springs for your chosen ride frequency and look to have a higher frequency at the rear than the front. This allows the car to move as one over bumps and undulations to prevent the car from pitching (the front will hit the bump before the rear, so the rear needs to 'catch-up').

Sways are used for roll moment distribution. You should also match the sway rate with the spring rate if you want to have correct damping in roll. Slapping on a stiff sway will make you underdamped in roll. You'll get quicker response, a pointy car, but you'll also get overshoot of steady state and nasty contact patch loadings.

Once you've done the spring/sway stuff, you can then go on to design damper rates.

P