Cane Creek damping

[MRT_Xii]

Hello everyone, First of all, this is my first post but I have been reading this forum almost daily for about 2 years now. I must say that this forum has proven to be an excellent source of information for all the questions I've had in the past. I've made a few searches, but perhaps wrong key words lead me nowhere.

Here is my question. For the teams using the Cane Creek double barrel FSAE special, have you found yourself being forced to used really high ride frequency in order to get the wheel rate in a window where the damper will not be over-damped? On 400lbs Sprung Mass car including driver, my calculations show that I get a damping ratio of about 1.1 with a ride frequency of around 2.25Hertz. In order to get below the .8 I need to get the ride frequency up to about 3.5 and more, which I am not completely comfortable doing. (the .8 damping would be low speed compression on the softest setting). As for the formulas, I used the Tech Tips found on the Optimum G website. They usually differ from the formula found in RCVD but I believe it’s a good assumption to say that they are probably right...

I was also told that it is not possible to re-shim the Double Barrels, but that needs to be confirmed. If you guys have contact information of places that would perform maintenance and perhaps help in reducing the damping forces of the damper, this would be greatly appreciated.

Gabriel

[MRT_Xii]

Hello everyone, First of all, this is my first post but I have been reading this forum almost daily for about 2 years now. I must say that this forum has proven to be an excellent source of information for all the questions I've had in the past. I've made a few searches, but perhaps wrong key words lead me nowhere.

Here is my question. For the teams using the Cane Creek double barrel FSAE special, have you found yourself being forced to used really high ride frequency in order to get the wheel rate in a window where the damper will not be over-damped? On 400lbs Sprung Mass car including driver, my calculations show that I get a damping ratio of about 1.1 with a ride frequency of around 2.25Hertz. In order to get below the .8 I need to get the ride frequency up to about 3.5 and more, which I am not completely comfortable doing. (the .8 damping would be low speed compression on the softest setting). As for the formulas, I used the Tech Tips found on the Optimum G website. They usually differ from the formula found in RCVD but I believe it’s a good assumption to say that they are probably right...

I was also told that it is not possible to re-shim the Double Barrels, but that needs to be confirmed. If you guys have contact information of places that would perform maintenance and perhaps help in reducing the damping forces of the damper, this would be greatly appreciated.

Gabriel

[rkraft]

Gabriel,

What you have described is a common issue for many teams. Modifying a Cane Creek is challenging and requires very difficult procedures for properly bleeding the fluid when reassembling the damper. I suggest that you look at the Kaz Technologies damper, which offers much 'softer' curves. For more information, please take a look at our website:

Kaz Technologies FSAE Damper (http://kaztechnologies.com/fsae.html)

If you have any questions, feel free to PM or email me.

[D Collins Jr]

Gabriel,

I believe that the new gen dampers for 2010 are addressing some of these issues. If you haven't already, I would recommend contacting the guys at Motorsports Spares and asking them about the changes to the design for this year. i could be wrong, I've slept since talking to them last.

[MRT_Xii]

Unfortunately, we don't have the money to buy new shocks this year... I'll have to do some black magic to get ours working for us.

By the way, we HAVE the CC Double barrels FSAE special already, they are 3-4 years old and I feel it would be about time to check the nitrogen pressure / change the damping fluid...

I'm sorry if that makes any shock guru die inside, but I'm at least trying to address the issue.

Any ideas?

Gabriel

[Jersey Tom]

Our solution was to use a motion ratio somewhere above 1.0. That way you can run a fairly soft wheel rate while still using the relatively high rates of the springs and damper itself.

[Mike Cook]

I think to get the correct damping, your going to be in the 1.5-2 range for motion ratio.

[MRT_Xii]

Thanks for the reply,

What I don't get is that:
Ks = 4*pie^2*fr^2*msm*MR^2
and
Kw= Ks/MR^2
which follows that

Kw= 4*pie^2*fr^2*msm

which means that Kw DOESN'T depend on motion ratio, only on ride frequency and sprung mass.
Assuming you don't want to increase your sprung mass (Due!), you can only play with natural frequency to increase your Kw because you want to increase Ccr;

Ccr = 2sqrt(Kw*msm)

which only depends on Kw and Msm...

Did I get that right???

Gabriel

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MRT_Xii:
Thanks for the reply,

What I don't get is that:
Ks = 4*pie^2*fr^2*msm*MR^2
and
Kw= Ks/MR^2
which follows that

Kw= 4*pie^2*fr^2*msm

which means that Kw DOESN'T depend on motion ratio, only on ride frequency and sprung mass.
Assuming you don't want to increase your sprung mass (Due!), you can only play with natural frequency to increase your Kw because you want to increase Ccr;

Ccr = 2sqrt(Kw*msm)

which only depends on Kw and Msm...

Did I get that right???

Gabriel </div></BLOCKQUOTE>

Something seems goofy in there...really MR is nothing more than the lever arm between the wheel and the shock, so necessarily increasing the MR will get you a lower Kw, which will result in a lower natural frequency. Might want to re-evaluate those equations.

We were looking at running the CC's a couple years ago, ran into the same issue in that damping was always too high. The folks we talked to said that the only way they could get it even close to as low as we wanted would be to take out ALL the shims...we declined. Going with a higher motion ratio would solve that problem, although you would naturally either need to design for more suspension travel or provide some sort of stop to limit shock travel.

Those Kaz shocks look pretty hot...money is tight this year, I'm beginning to consider what crimes we will have to commit to afford them this year http://fsae.com/groupee_common/emoticons/icon_biggrin.gif.

[MRT_Xii]

Adambomb, I understand that changing the motion ratio will lower your natural frequency AND Kw given Ks is the same. This is just playing around with the equations and isolating the variable you want to play with. My point is that if you do increase Ccr, you will invariably increase your natural frequency, which you can only do up to a certain point where your car starts bumping around.

I'll repeat that I took all the equations on optimum G tech tips... If something is wrong there, I don't know who I can trust. Can you be more precise and point out what is wrong maybe?

[Adambomb]

First off I'd like to say it sounds like you're definitely going in the right direction with this. I would like to help you out by finding the problem myself, but unfortunately I just plain don't have the time to scour the equations. I've seen (and used!) those tech tips, they are good. Good chance there's just something that got "lost in translation" there or something, that's the danger in just taking someone else's equations.

It sounds like you have a good enough concept of it, why not just do a free-body diagram of the system and do a basic engineering dynamics analysis on it? Yes it's more work and will take some time, but then you can know for yourself what is right.

I remember fighting these calculations myself...for some reason it took WAAYYY longer than I thought it really should have (but then again I am partially dyslexic, which does not help the issue http://fsae.com/groupee_common/emoticons/icon_wink.gif ), but in the end I learned much more about it.

[dazz]

Just a quick note on rebuilding dampers. I haven't tried this before but have heard that some of the Downhill MTB guys get around the bleeding issues on the shocks by rebuilding them completely submerged. If you have all parts and tools absolutely spotless (as should be the case anyway), you could even keep the excess fluid for future rebuilds. If anyone has tried this technique, how did it work out?

Now that I think about it, it may have been the method for rebuilding MX steering dampers that I have described. Can't see why it wouldn't work for shocks as well though.

[cmeissen]

I haven't looked into this in detail, but I beleive the problem you are having is that you have neglected that fr, the natural frequency, is also a function of the motion ratio.

fr = 1/(2pi)*sqrt(kw/msm)

where

kw = ks/MR^2

Like was previously suggested, I would develop these equations on your own, so that you understand the theory and see where everything comes from.

[Jersey Tom]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Cook:
I think to get the correct damping, your going to be in the 1.5-2 range for motion ratio. </div></BLOCKQUOTE>

Indeed. We used 1.7.

[jkaz]

There are several disadvantages, all related to the reduction of stroke by reducing the motion ratio. With a motion ratio of 1:1, the range of shock displacement on a FSAE car is approx. 1 inch, with typical velocities up to 5 in/sec. This is already a small amount of displacement, and relatively low velocities for a shock. With these small displacements, you have a small volume of oil moving through the valving. As you reduce the motion ratio, you reduce the displacement, thus reducing oil volume moving through the valves. Even worse, you reduce the velocities of the shock. This reduction in velocities and displacement means you are only using the low speed or bleed portion of the valving. This is not a very effect use of the shock, and, in this low velocity range, the adjusters in any shock are not very effective. So, what you want to do is use motion ratios of 1:1 or greater to get the best performance from your damper. If not, you will have a hard time making effective use of the dampers.

I am starting a new post to answer questions on dampers. Please feel free to ask questions.

Jim Kasprzak
Kaz Technologies
jkaz@kaztechnologies.com

[skidpadguy]

Did you guys ever directly compare the damper curves of the CC DB and the new Penske Damper.

If I compare the fully open damper curves of the CC (found on http://www.motorsportsspares.c...adjustment_sweep.jpg (http://www.motorsportsspares.com/images/db1_adjustment_sweep.jpg)) and the softest Penske valving 4 ( found on http://kaztechnologies.com/Pen...e%202009-09-22.pdf), (http://kaztechnologies.com/Penske%207800%20Series%20Technical%20Info%20Brochu re%202009-09-22.pdf),) to my understanding the CC DB offers lower damping characteristics...

Penske (valving 4): around 400N @ 150mm/s
CC DB: around 40 lbs @ 6in/sec= around 178N @ 157 mm/s

Penske provides a +- 15 tuning at the valves for this setting, but to me is still much "harder"

[Luniz]

@ skidpadguy: you might have got your numbers wrong there, one inch equals 25.4mm so 4in/sec should be 101.6mm/sec

[jkaz]

Several points on comparing the CC DB damping curves with the Penske 7800 curves:

1. The data is not recorded the same way. The CC DB curve is a compression open/rebound closed curve, while the Penske uses peak velocity forces.

2. The forces shown for the Penske are the Stock valvings at mid adjuster settings. The advantage of the Penske is that you can valve to shock to obtain your desired control forces.

Jim Kasprzak
Kaz Technologies
jkaz@kaztechnologies.com

[MRT_Xii]

Would you guys mind sharing what natural frequency and spring stiffness you run?

[skidpadguy]

sorry, I fixed the wrong numbers.

jkaz, could you please point out the difference of the two measuring techniques a little more? What consequences do you see?

to 2.: as far as I understood your brochure, there are 4 different basic valvings/shim kits available, as can be seen in the brochure. Additionally it is also possible to adjust the shape of the curve, based on the 4 "basic valvings".
Did I get anything wrong?

[rkraft]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by skidpadguy:
Did you guys ever directly compare the damper curves of the CC DB and the new Penske Damper.

If I compare the fully open damper curves of the CC (found on http://www.motorsportsspares.c...adjustment_sweep.jpg (http://www.motorsportsspares.com/images/db1_adjustment_sweep.jpg)) and the softest Penske valving 4 ( found on http://kaztechnologies.com/Pen...e%202009-09-22.pdf), (http://kaztechnologies.com/Penske%207800%20Series%20Technical%20Info%20Brochu re%202009-09-22.pdf),) to my understanding the CC DB offers lower damping characteristics...

Penske (valving 4): around 400N @ 150mm/s
CC DB: around 40 lbs @ 6in/sec= around 178N @ 157 mm/s

Penske provides a +- 15 tuning at the valves for this setting, but to me is still much "harder" </div></BLOCKQUOTE>

Yes, both products' performance characteristics were evaluated on the dyno and on track.

The difference here comes with what the curves look like and how they were produced, as jkaz pointed out. It is incorrect to compare curves by picking one point off of a random dyno plot and comparing it to another point at the same velocity on another random plot.

Ignoring this for now, please notice that the Penske has a digressive compression curve at soft settings, where the Cane Creek is very close to linear. If you want to run a digressive curve using a Cane Creek, you have to cut down your motion ratio (i.e reduce damper resolution) because the digressive curves are only available with much higher forces when you use the adjusters. This has many negative effects, both in terms of damper performance and general suspension performance.

[jkaz]

This reply is to skidpadguy.

The data displayed for the CC DB shock is taken from half of a cycle of a 10 in/sec peak velocity sine sweep input curve. What is displayed is the compression acceleration/rebound deceleration portion of the curve. This is what is called a compression open/rebound closed curve.

The data we display for the Penske 7800 shock uses the forces from the peak velocities from a series of sine sweep curves. We use this format because we think it is easier to compare multiple curves on the same plot.

There are multiple ways to display shock performance data. Each one has its own benefits. However, if you want to compare performance of dampers you want to use the same test and display data using the same format.

As far as the standard valve codes available for the Penske shock, there are 4 standard compression and 4 standard rebound codes available. You can choose any compression with any rebound code.

There are shim kits, an optional piston and the required tools available to create whatever whatever damping characteristics you desire from the damper. We will also develop a custom curve if you desire.

Jim Kasprzak
Kaz Technologies
jkaz@kaztechnologies.com

[skidpadguy]

Jim,

thanks for the reply and the explanation. I understand the two measurement techniques, but are the results absolutely non comparable? Even if I allow a discrepancy of 50 percent, the CC DB still doesn´t seem to provide much higher damping.

I don´t want to discuss what is "good or bad" (yet), I just want to point out that comparing the two dampers the CC DB doesn´t appear to be overdamped.
This also corresponds to my personal experience with the CC DB, our car is definitely underdamped with fully open CC DB (motion ratio close to one, no "very high" natural frequency for a SAE car)

[jkaz]

skidpadguy,

It is true that if you fully soften the adjusters on the CC DB, the damping is softer than the forces we have chosen for standard code 1 compression and standard code 1 rebound.

But as you point out, a CC BD damper at full soft does not provide enough damping. Thus a FSAE car at this setting is underdamped.

If you want less damping than code 1 compression and code 1 rebound in the Penske damper, you can revalve the damper to obtain the damping you want.

[Vittorio]

Jim,

I'm not sure if I have understood the difference between the two methods of measurements.
The first method is clear.
If I am right the second takes for each point x_i (X axis, velocity) the peak force measured during the sine sweep cycle with a peak velocity x_i.
What is the difference between the two methods?
If I am correct it is a more or less slight difference (usually not desired, isn't it?) increasing with hysteresis and displacement sensitivity (the influence of the position of the shaft, right?) of the shock and decreasing with a better repeatability.

[mtg]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MRT_Xii:
Thanks for the reply,

What I don't get is that:
Ks = 4*pie^2*fr^2*msm*MR^2
and
Kw= Ks/MR^2
which follows that

Kw= 4*pie^2*fr^2*msm

which means that Kw DOESN'T depend on motion ratio, only on ride frequency and sprung mass.

Gabriel </div></BLOCKQUOTE>

Gabriel, you are making an incorrect assumption that you can substitute Ks into the Kw equation and cancel out the motion ratio. It doesn't work that way. The Ks equation tells you what spring rate to use once you pick a frequency, and then the Kw equation tells you what the wheel rate is with the calculated spring rate.

As others have pointed out, Kw definitely does depend on motion ratio.

[MRT_Xii]

Thanks for the reply mtg,
Let me reformulate the point that I was trying to make.
People say that you can increase your Motion ratio to increase your Ccr for a given Ks. I understand that point. But it seems to me that it will also increase your natural frequency which you can only do up to a certain point.

I was just trying to get a feel of what kind of natural frequency people with this damper were using, as I do not feel comfortable designing with a 3.5-4Hz natural frequency.

We have all taken math classes and are well experienced with algebra... I just don't want anyone to waste their time replying to my dumb question by stating the obvious.

Thank you!
Gabriel

[Adambomb]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by MRT_Xii:
I was just trying to get a feel of what kind of natural frequency people with this damper were using, as I do not feel comfortable designing with a 3.5-4Hz natural frequency.
</div></BLOCKQUOTE>

We used to run 3.5-4 Hz a few years ago. Not exactly sure why, to be honest. The car might as well have not had any suspension at all, because it didn't even move. Needless to say the car was very skittish. Also hammered flat spots into the rocker bearings.

[Adambomb]

Oh wait, I think I might know why we ran that ride frequency: we were running Risse's that year. And, you know, if they don't move, they can't leak! http://fsae.com/groupee_common/emoticons/icon_wink.gif Sadly, somehow they still leaked...

[STRETCH]

Our 2009 car ran Cane Creek DBs. We ran ride frequency of just over 3Hz front and rear without any issues, (in fact we finished 2nd overall in germany). Not sure we could go stiffer cos of the bumpy FS tracks we have, but on a nice circuit stiffer would be quicker.
Motion ratio wise I stuck with using the full stroke of the damper (MR = 1) rather than worrying a lot about velocities, although ideally you would valve the damper to make best use of the high/low speed tuning option. Unless you wanna take the unit apart though, you cannot have it all!