I leave this to the professionals... send your shocks into AccuTune Offroad and be done.

 

I don't know much, but experience and research taught me that 200% step up on the spring rates is way too much. Mine felt like I was hitting the bumpstops. Dropping to a lower step up should help.

Everything is a compromise. I don't know if you can increase free bleed in the piston and hope it chokes off at higher speeds when your most likely using both spring rates? Flutter stack on rebound? Unfortunately springs are position driven and conventional shock valving is velocity driven.

Kevin

 

We need more information. At least spring rates,lengths,travel and position. My guess would be something is missing from the initial setup. But also like mentioned above Ryan at Accutune is the man.

 

Ryan is the man for sure. Unfortunately he stopped tuning help over the phone, and I totally understand why.

I’m using Shim Restackor. The response calculations are much better than my butt dyno. I’ve tried everything from bleed, to flutter stacks, to rate plates but simply can not get a zeta function of 0.7-1 for both rates.

This is more of a question on how do you control two different spring rates with a single velocity sensitive damper.

 

If you want to make it that simple, the answer is that you compromise.

 

There is this, and IMO try for the littlest change in rates you can get. The only way to make a big step up change work Is to use the top spring as a tender that collapses completely at ride height.

 

What about eliminating the step up rate completely? Keep things consistent rate. What issue is the using the step up rate helping you with?

 

That would allow you to get the best rebound response through out the stroke with a non bypass coil over.

The common trend has been to use a step up rate to help control bottoming.... mainly at slow shaft speeds where dampening rates are not significant, such as G-outs (2-5 in/sec).

 

About five inches. 200/300 lb rates. 1250lb sprung corner weight.

 

The 100# split might be getting close to blocking the coil if the transfer nut is set too far down?

Have you tried narrowing the split? That should make the step up rate change more manageable.

 

The stop is currently 1” below the slider

 

I would raise the secondary rings so you have more travel in the initial rate zone and tune for that, let your last 1.5"-2" take that heavier rate to keep you from bottoming out. You might also want to look at some 2" hydraulic bumps if bottoming out is an issue.

Whats your use for the vehicle?

 

You obviously misunderstood the point of this post. This is Is what I’m talking about. The method you are suggesting does not work well for a single velocity sensitive coil over.

As a few people who are smarter than myself suggested dropping the step up rate, or setting the stop so drop travel is the only thing that sees the initial rate.

I’ll have to post some visual graphs. It might explain it better than I can. I’ll run the numbers, 200% step up might be able to work...

I just dug into a WP PDS shock on the rear of my KTM. Interestingly, the progressive rate hits its transition at rider sag or ride height. The soft rate is for drop only. Soft rates are low falling rates.

alright, I've been trying to get up to speed so that I can understand these two posts :laughing: as an ignoramous, it sounds almost like dirtman is saying the same thing that 'people smarter than' wes are saying. set your crossover to a point to where you are only tuning your shock for one spring rate.

the dirt man saying, raise it and others saying to just use the same rate for tender and primary and then others saying to set it so that primary is only on drop (as defined as rebound from ride height?) and tender handles compression from ride height.

either way, attempting to get to where you are only 'tuning' for 1 spring rate across the desired travel.

this quote from the Penske link custom axis tech manual seems to address.

2.2 Tender Spring Crossover Tender spring crossover height is directly related to chassis roll and bottom out forces. Changing the tender spring crossover height is the most significant handling change you can make, using the shock's external adjustments. The crossover height moves the dual spring rate's point of progression in relationship to the shaft travel and motion ratio. Increasing the crossover height decreases tender spring travel, making the main spring crossover sooner in the wheel travel, providing stiffer spring poundage for more spring resistance during chassis roll and bottoming. Decreasing the crossover height increases tender spring travel, making the main spring crossover later in the wheel travel, resulting in less spring poundage for softer spring resistance during chassis roll and bottoming.

Therefore, increasing the tender spring crossover height makes the suspension stiffer. Decreasing the tender crossover height makes the suspension softer. Your suspension should bottom out at least once somewhere on the track not hard enough to knock your feet off the pegs but enough to know that you're using all the available travel.

.

again, talking about bikes or quads or what-have-you, but that doesn't matter.

so that leads me to wanting to know "in what way?"

In what way does that 'not work' for "single velocity sensitive coil over" ?(emulsion shock?)

 

You obviously misunderstood the point of this post. This is Is what I’m talking about. The method you are suggesting does not work well for a single velocity sensitive coil over.

As a few people who are smarter than myself suggested dropping the step up rate, or setting the stop so drop travel is the only thing that sees the initial rate.

I’ll have to post some visual graphs. It might explain it better than I can. I’ll run the numbers, 200% step up might be able to work...

I just dug into a WP PDS shock on the rear of my KTM. Interestingly, the progressive rate hits its transition at rider sag or ride height. The soft rate is for drop only. Soft rates are low falling rates.

 

I just dug into a WP PDS shock on the rear of my KTM. Interestingly, the progressive rate hits its transition at rider sag or ride height. The soft rate is for drop only. Soft rates are low falling rates.

One thing that's counter intuitive, but can help when thinking about spring how some cars react to spring rate changes.

For a given ride height, a softer spring rate will have more spring force between full extension and ride height than a higher spring rate. This can help the with tracking, forcing the wheels to stay in contact with the ground.

I haven't looked closely at a WP PDS shcok, does it have position sensitive damping in rebound, or just compression? Where does the spring rate transition compare to the damping force transition?

 

You are correct for the most part. The total force at ride height will be the same no matter what the rate is. In example, if your corner weight is 1,000lbs, that’s how much spring force is needed to hold the vehicle at ride height. It does not mater what the rate or the preload is. It’s above and below ride height that changes with spring rate. Stiff springs = heigh rising and falling rates
Soft springs = low raising and falling rates

The PDS shock is a bypass shock but uses two pistons and a needle. It’s similar to the king internal bypass coil overs.

The secondary piston does not come into play until just above the bump stop. It’s not much more than a hydraulic bump stop. The spring rate transition is at ride height.

Looking at the PS6 spring, the adverted rate is 7.1-9 kg/mm (measuring closer to 7.75 - 10.5 kg/mm). That is only a 135% step up rate!

 

visual

I took two screen shots of the response spread sheet. This is a 200/300 lb spring with the stop set 1" above the slider. 12" travel, 5" bump. All I change was the spring rate and adjusted "ride height" for the main spring.

The graph on the bottom and second from far right (zeta vs wheel position) is a good one to look at.

A zeta function of 1 is considered "perfect" rebound dampening, chassis returns exactly to ride height.

A zeta function of .7 is a lighter dampening but will give you the best traction because it will keep the wheels on the ground more. The chassis will raise slightly above ride height when it returns from the bottom of the stroke.

The graph on the bottom left shows the what shaft speed it take to bottom out and what shaft speed you will see as it returns.

For the record, I am not claiming to know the answers. I just think the way our community has been setting up single coil over suspensions should be re evaluated. There are definitely improvements to be had.

Beat95yj pretty much nailed what I am seeing. I wonder how a extremely light combined rate would perform as the axles fall away from the vehicle?
The only other thing I am considering is flipping the springs to achieve a step up ratio of under 200%

 

OK someone help a brotha here. How do I post photos these days?

 

Bump

 

I R Dumb, what is the zeta value? What does it mean in real life? Its hard to make out what the graph is showing in the pic. I'm not familiar with that software. Why is a zeta of 1 perfect?

It is cool to have tools like this, but at the end of the day you need to corroborate software output with your seat of the pants in the vehicle. Have you driven the vehicle with one of these setups and how did it do?

Kevin

 

I R Dumb, what is the zeta value? What does it mean in real life?

Zeta is the "damping ratio". It is never the "dampening ratio". Shocks damp movement. Dampening is what happens with things get a little wet.

The damping ratio describes the amount of damping, that is the amount of energy absorbed by the shock. A zeta = 1 is know as "critically damped". Less than 1 is "under damped". Greater than 1 is "over damped".

In real life, an under damped system will oscillate (bounce in a suspension application) until it settles out (decays). This is known as a decaying oscillation. Imagine taking the shocks off a car and driving over a speed bump with one axle. With the shocks removed there is still some damping due to friction in the joints or the rubber bushing (even tires), but it is a small amount relative to the shock. The car will bounce as it goes over the bump, but will eventually stop.

An over damped system never oscillates, but will take a long time to respond. Over damping on rebound can cause the shock to become fully compressed and you can run out of suspension travel while driving, usually fast. Over damping on compression causes a rough ride.

Critical damping is the border between the two. There is no oscillation or overshoot, but the system is slower to respond than the under damped case.

Wikipedia has a nice graph that shows the different damping ratio responses (search "damping ratio"). The Wikipedia article references a "second order response". This is just a way to model (write a formula to describe) the response of a system. It is a very common method using a second order polynomial also known as a quadratic equation. Step response for all kinds of system can be modeled this way from suspension to electronics.

The reason a damping ratio of about 0.7 is targeted in many applications is because this produces almost no decaying oscillation with a slight overshoot (this is the chassis raising slightly above ride height) that is tolerable because of the speed of the response. A faster response would cause too much oscillation and a slower response would just be slower. You want the system to response as fast as possible without too much oscillation. Zeta = 0.7 usually gets you there. This is what helps keep the tire in contact with the ground.

I think using the modeling program is a great approach. Much better than butt dyno. It might feel "good", but maybe it could be "great" and you never know that because the butt dyno says it's "good" and you stop there. Butt dyno are rarely well calibrated in multivariable systems. There is a learning curve, however, and most people don't have the patience or the background to take a more analytical approach.

OP, if you have some experience with Shim ReStackor start a thread about it and what was required to get started tuning your rig with it. The learning curve for modeling suspensions is steep. When people hear terms like "second order response" or "frequency domain" eyes start to glaze over. No one needs to be a system response expert to tune suspension, but the theory is intimidating and when you start reading about it either people don't fully understand it or they understand it really well and their explanations aren't accessible to most people.

The program only seems to be mentioned in a handful of Pirate threads, and non really explain how to use it. It doesn't cost that much relative to what people spend on coil overs and springs and could save many hours wasted tuning shocks and attempting to asses the changes.

 

The funny thing is I had been tuning by feel for years. I also took all the advice given to me off this forum. Never really satisfied with the results I bought the program, which is not expensive.

After I spent the time learning the program and taking all of the tedious measurements, it basically told me I was ass backwards. I was running #35 rebound stack with two bleed holes. To get the numbers right, I would have to go to a #90 stack with three bleed holes. I laughed and almost reassembled my shock like it was. I stopped myself though, because I would be real asshole to spend time and money on something and not try it out. Sure enough, it was a significant improvement. I don’t think this program is spot on, but it’s much closer than most butt dyno’s. It easy to get way off track with tuning.

If your not good at math, just think of the zeta function as a target. Their website explains it all of your curious.

 

AJM8127 Thank for the correction and thank you for that excellent explanation. Much better than I could have done!

The program is a very steep learning curve, it also takes a lot of patients. I think I played with this program for 2 years before I got it to work. If you miss one measurement, or it is incorrect the program crashes and it does not highlight the problem area.

It is for that reason that I wanted reevaluate spring selection for single coil overs. A guy should be able to look at a valving chart and get his suspension in the ball park, but he is screwed from the start if the springs he picked wont allow it.

Shim Restackor isn't my program, so I feel weird talking about something I hardly have a grip on. Maybe Ill reach out to the owner.

Going back to spring selection for non bypass set ups. I'm thinking we should suggest three options:

1) No step up rate and and zero preload. This would give you the most plush and accurate feel.

2) 150% step up or less. This is a compromise on response but it is still in the acceptable range. The benefit is more bottoming control at slow shaft speeds.

3) Fully collapsed tender at ride height. I think this might work for some, but I have no experience with it. I am thinking this would be a better slow speed set up? Someone please chime in if they have experience.

 

Wes, wasn't trying to correct anything you've said. This has been a great conversation overall.

Shim Restackor isn't my program, so I feel weird talking about something I hardly have a grip on. Maybe Ill reach out to the owner.

Even if you aren't an expert with the program, it would be interesting to read the details of your application of the program. If anything it would be a helpful stating point for others in the future (like myself).

My experience professionally with second order response is focused on electronics, but that is the beauty of this approach. It is generalized enough that it can be used for many types of systems. A spring-mass-damper system is commonly used in teaching second order response because it is more easily visualized and understood than something like electronic filter response or power supply transient response.

Provience, I think for simplified suspension response it doesn't matter how many wheels the vehicle has. Obviously the program wouldn't be able to model body roll effects if it assumes two axles and one wheel per axle. I am not even sure it even considers two axles. Most of what I've seen from Shim ReStackor relates to a single wheel only. Often times it is useful to simplify a complex system into a less complex model for analysis. Breaking apart complicated problems in to solvable pieces is what engineering is all about.

I think in the end it is going to be difficult to fine tune a damper with a fixed rate for a suspension with a variable spring rate. There is going to need to be a compromise.

The two options seem to be using a single spring rate and a hydro bump, or multiple rates that are either fairly close or only come into play for the last few inches of travel. Perhaps if you limit the travel for the heavy spring rate until just the end the rebound is less important there.

I assume if you are running out of travel you want the damper to extend faster (less damping) than if you are in the ride height range. If that is the case, you'd tune the valving for the main rate, and then when the step up occurs in the last couple inches of travel the decrease in damping force is OK because you want to shock to extend more rapidly here. If you limit this range to just the last few inches i.e. not at ride height, then it might not be a problem. I am assuming we are talking about go fast applications where you have several inches of up travel.

 

Their website is very educational

http://www.shimrestackor.com/

 

Their website is very educational

Shim ReStackor, Finally software to tune a shim stack

question for ya.

the language they use on their website seems to be directed to motocross dudes. do their numbers, or rules of thumb, need to be viewed differently when used on a much heavier (both sprung and unsprung) rig?

I appreciate the approach they've taken to putting a bunch of math into a much handier format :smokin:

 

The program is absolutely geared towards motorcross bikes suspensions but almost all of what you read will apply to your truck. Dirtbike suspensions and what they see is not much different. As long as you use the correct and accurate information, it will work.

 

sweet.

for my own sake, and others who may find this thread?, do you mind if I put up a couple of links?

https://www.crawlpedia.com/shock_valving.htm

https://www.crawlpedia.com/shock_tuning.htm

crawlpedia has some good articles, but I HATE this

Without getting too advanced,

I would like the encyclopedia to get too advanced :laughing:

https://www.ohlinsusa.com/files/files/Inside_TT44_Manual_p1.pdf

this is a good tech manual recommended from the following

Reviews of Shock Books Paul Haney claims he is putting out an in depth shock book, but the only thing I could find from him post 1999 was one dedicated to tires. granted, everything in life revolves around tires, but my only hesitation would be that it is more of a different focus and not as detailed on shock tuning, or rather it would be a side thought like most other books that talk about them.

 

https://www.pirate4x4.com/forum/des...-racing/573430-shock-valving-info-any-good-write-ups-what-all-numbers-mean.html

from that thread, is this link

http://www.penskeshocks.co.uk/downloads/CustomAxisTechManual.pdf

another thing to read. I'm still working my way through the TT44 manual and didn't want to lose these :homer: I apologize for being a few (10?) decades behind the curve here :rasta:

This helps make "the program" start to make sense for use as a starting point, and then something to play with during tuning to sort of mimic having a dyno. as in, if you make a change, log it into the formula and see if it matches what you are feeling/seeing.

as opposed to starting off too XYZ and then going too far ABC to box in what you want. with lots of install, test, rebuild, test, etc.


https://bustedknuckleoffroad.com/Shim-Stacks-Standard-Shim-Stack.htm

so that ties in to this then, busted knuckle has got some "shim stack" charts...not dyno stuff, just lists of what shim diameter and thickness they put in a stack.

am I correct in saying that this would be sort of 'industry standard' for somebody to buy a prebuilt stack? Like, call them up and say " I want a #30 stack " and if that is too little damping, call them back and say " send my a #80 stack " ?

dang I need to get my pile together so that I can pull my shocks apart

 

Tony I like what you are bring to this conversation. I think bump stops can be another issue, and it really depends on what you use as they come in many different forms. Some are more of a spring, some are more of a dampener.

When you look at the total spring to damper ratio you want to stay in the 1 to .07 range, bump stops included. When my slider hits the stop collar, my wheels are almost at a .7 zeta, but my chassis is down at a .06. This is fine for G outs on a straight away, but I do not enjoy it in a turn.

This brings me to a difference in bike vs truck tuning. A rider that stands most of the time is easier to tune for, because you can focus on the wheel response (the chassis floats under the rider). If you are a seated rider or you are tuning for a truck/buggy, Then you need to fit the chassis response in that range as well.

Remember all that crap you heard about how you should keep your sprung/un sprung mass at a reasonable ratio? Well this is where it bites you.

 

When you look at the total spring to damper ratio you want to stay in the 1 to .07 range, bump stops included. When my slider hits the stop collar, my wheels are almost at a .7 zeta, but my chassis is down at a .06. This is fine for G outs on a straight away, but I do not enjoy it in a turn.

looking at their website, it seems like they are aiming for the chassis zeta to be .7 instead of the wheel Zeta of .7

which makes sense if you consider the thing that is being attempted to be damped is the chassis, by way of the tire.

the ground is firm, the tire is oscillating in response to the ground, firmly. the shock and the chassis are both mobile, so while we can't do anything to damp the tire in relation to the ground (unless we have a dozer) we can damp the chassis oscillation. so if you chassis zeta is going below your ideal, the what would it take to raise that?

 

Ok I’ll try and consolidate this, this is my statement...

If you plan to use a non bypass coil over, you will get better results with a spring step up rate of 150% or under.

Nothing is wrong with where you set your transition.

 

Nothing is wrong with where you set your transition.

except that, while you still won't be able to get 'ideal' valving for one rate or the other, you can shift your transition so that the spring you valve for is doing most of the work, and the other that isn't ideal is just at one extreme or the other.

i.e. compromise

right?

 

[/quote]so if you chassis zeta is going below your ideal, the what would it take to raise that?[/QUOTE]

More rebound damping. The problem is if you valve for one spring rate, it will not be in the ballpark for a spring rate that is 250% away.

See statement above

 

so if you chassis zeta is going below your ideal, the what would it take to raise that?[/QUOTE]

More rebound damping. The problem is if you valve for one spring rate, it will not be in the ballpark for a spring rate that is 250% away.

See statement above[/QUOTE]

alright, so that makes sense. but I've got to ask, what or where are you getting your "standard spring selection" that says you want/need/should have such a difference between your spring rates?

I am under the impression that the 'standard guidance' is to minimize the difference between the rates.

damn, I did a whole bunch of reading over the last couple days just to be able to understand your comments. :laughing:

 

http://www.kaztechnologies.com/wp-c...5/06/A-Guide-To-Your-Dampers-Chapter-from-FSAE-Book-by-Jim-Kasprzak-Updated.pdf

link from above.

 

from the kaztechnologies link

In the best case, dampers are “The frosting on the cake.” If everything else in the suspension design and component selection is correct they simply damp the vibrations at resonant frequencies, control the rate of weight transfer and enhance vehicle comfort and performance. In the worst case dampers “Hold the cake together!” They are asked to compensate for structural deficiencies, control heave, pitch or roll imbalances, and compensate for design deficiencies like lack of suspension travel.

that quote is a nice introduction

Most text books state the proper damping ratios are 0.2-0.3. This is appropriate for passenger cars, but not enough for FSAE and other race vehicles with higher spring and tire rates, and thus, higher natural frequencies. As a rule of thumb, a FSAE car will require a damping ratio of 0.5-0.7 to control the heave, pitch and roll resonances of the sprung mass, and a damping ratio of 0.3-0.5 to control the unsprung mass.

should these numbers, 0.3,0.5,0.7 be taken as the same Zeta number being referenced in shim restacker?

Before considering how much damping you need for the car, you must design the suspension correctly to make the best use of the damper, and then properly specify the damper for your suspension design. Of course you want the lightest damper that has the maximum stroke and requires the least amount of space. But one must consider many factors when packaging the damper into the suspension system. These include required wheel travel, jounce bumper travel, desired wheel rates, strength requirements and packaging constraints. Probably the most important is motion ratio.

now I'm just picking out some stuff :rasta:

There are several reasons for using high motion ratios. The first is higher motion ratios require lower spring rates for the same wheel rates. Lower spring rates are also lighter, and result in less spring and shock friction as well as lower component loads. The other reason is greater damper travel and higher shock velocities. Since dampers perform better at higher velocities, and the wheel displacements are quite small on a FSAE car, higher motion ratios produce better shock performance. Figure 13 shows a chart comparing travels, velocities and spring preload for various motion ratios.

higher shaft velocities mean we are spending more time dealing with the shim pack, the thing that is easiest to tune and with numerous finer adjustment opportunities.

this follows with the common phrase of "1:1 motion ratios are easier to tune than 2:1 or other"

What are the “right” ride frequencies? The ones that make YOUR car go the fastest! Don’t get hung up on ride frequency numbers. They are for reference. Of course it is useful to calculate the front and rear ride frequency to determine initial ride balance and starting spring rates. But ultimate spring rates and damping ratios that make the car go fast are much more important than the theoretical numbers.

Note that at a zero damping ratio, or no damping, the vehicle will continue to resonate. This is only true if there is no friction. At a damping ratio of 1, or critical damping, the vehicle does not resonate at all. This condition is overdamped and will result in increased tire force variation and loss of tire adhesion. At a damping ratio of 0.2 the vehicle exhibits motion, but the motion damps in 1 ½ to 2 cycles. This level of damping lets the vehicle move, but controls the sprung mass. ..

...

What the above examples illustrate is there is no one correct damping ratio. Damping ratio is just a number, like spring rate, ride frequency and tire pressure. The “correct” damping ratio is the one that makes your race car go fast.

alright, so this guy in his application is talking about very low damping with the 1.5-2 oscillation goal, but the numbers and theory are still scalable

The low speed ratios control the low frequency motions of the sprung mass (heave, pitch and roll), as well as the rate of weight transfer in transient maneuvers (braking, turning and acceleration). The high speed ratios damp the high frequency inputs and keep the suspension in control over bumps.

this is the big consideration where body roll is concerned, and it depends greatly on application and overall suspension design. I am personally not convinced that body roll should be eliminated or even minimized at all times. it should not create a rollover situation, unless your suspension is built that way, but some is not bad and from a chassis/handling point, it seems to be more of a driver 'comfort/confidence' issue rather than a grip or rollover issue. i'm aware i'm likely in the minority here and full race stuff is going to be pushing the limits harder than trail or daily driver stuff.

so, issues with roll/lean control would be focused on the bypass ports, if you've got the ports closed, then stiffening up the initial opening of the shim stack.

The other ratio to consider is the ratio of compression to rebound damping. Most text books recommend a ratio of three to 1, rebound to compression. This is the typical ratio seen on most passenger vehicles. This is due to the fact that the sprung mass is significantly heavier than the unsprung mass, and primarily controlled by the rebound, while the initial movements of the unsprung mass are controlled by the compression.

and this reinforces the notion that unsprung mass is derogatory, but only if you are concerned with things like performance, fuel economy, speed, handling and power. IFS/IRS cars have a natural advantage by design, but at the same time it isn't always the greatest combination if you are building a moon buggy/rock comp buggy. application is important

Once each of the dampers is performing properly, you want to make sure that the damping forces match for a pair of dampers. In other words, the damping forces for the left front and right front should match at the same setting. The same should be true for the left and right rear...
...
If the damping forces for a pair of dampers do not match at the same settings, you can try tuning the adjustments to get them to match. If this is the case, make sure you record the differences. Also, be aware this may not be the same through the range of adjustments.

this is a point that is likely highly overlooked due to most folks not having access to a shock dyno to test the stuff and the difficulty in seeing the individual corner vs the reaction of the whole end , matching the shock to the corner so that you are getting similar damping for the various corner weights