Motion Ratios, Interpreting Shock Dyno Graphs, and picking shocks

[sbrenaman]

Ok, first let me point out that our team does not have any reference materials regarding the advanced stuff. We have a car tuning for dummies book, but that's it. Next year we will be buying Milliken/Milliken's book, probably along with that brake book that everyone talks about.

Anyway, on to the questions. So I am doing the suspension bellcranks/geometry/damper/springs/pushrods this year, and I am stuck on calculating the required spring rate given the natural frequency (2hz) I picked for the "Ride Frequency" and how I need to get the shocks valved. I am extensively using http://www.optimumg.com/Optimu...Others/TechTips.html (http://www.optimumg.com/OptimumGWebSite/Others/TechTips.html) OptimumG's tech-tips site due to the fact that we don't have any reference material or upperclassmen that know about this type of thing.

1. We have a motion ratio of 1:1. We're using the Penske 7800's and they've got plenty of travel. So given the motion ratios, ride frequency, and the approximate sprung weight per corner (w/200lb driver, given 650lb car), the required spring rate is 294lb/in. This seems high, considering I see teams running a motion ratio of about 1.6"wheel travel/1"shock travel (1.6:1) and they have lower spring rates than us!? Am I calculating something incorrectly? I know we're heavy, but wow.

2. Onto shock valving. I calculated the damping force required given the damping ratio of 0.8 (once again, from OptimumG's site), and critical damping force of 460lbf/(in/s), to be ~368lbf/(in/s). So how do I get that translated into 'what we need' our shocks to be valved to? OptimumK's site mentions what the slope of the initial valving should be, but I'm getting something rediculous like 14,000lbf/in.

Any help appreciated at this point.

Thanks
Scott

[sbrenaman]

Ok, first let me point out that our team does not have any reference materials regarding the advanced stuff. We have a car tuning for dummies book, but that's it. Next year we will be buying Milliken/Milliken's book, probably along with that brake book that everyone talks about.

Anyway, on to the questions. So I am doing the suspension bellcranks/geometry/damper/springs/pushrods this year, and I am stuck on calculating the required spring rate given the natural frequency (2hz) I picked for the "Ride Frequency" and how I need to get the shocks valved. I am extensively using http://www.optimumg.com/Optimu...Others/TechTips.html (http://www.optimumg.com/OptimumGWebSite/Others/TechTips.html) OptimumG's tech-tips site due to the fact that we don't have any reference material or upperclassmen that know about this type of thing.

1. We have a motion ratio of 1:1. We're using the Penske 7800's and they've got plenty of travel. So given the motion ratios, ride frequency, and the approximate sprung weight per corner (w/200lb driver, given 650lb car), the required spring rate is 294lb/in. This seems high, considering I see teams running a motion ratio of about 1.6"wheel travel/1"shock travel (1.6:1) and they have lower spring rates than us!? Am I calculating something incorrectly? I know we're heavy, but wow.

2. Onto shock valving. I calculated the damping force required given the damping ratio of 0.8 (once again, from OptimumG's site), and critical damping force of 460lbf/(in/s), to be ~368lbf/(in/s). So how do I get that translated into 'what we need' our shocks to be valved to? OptimumK's site mentions what the slope of the initial valving should be, but I'm getting something rediculous like 14,000lbf/in.

Any help appreciated at this point.

Thanks
Scott

[Mike Macie]

Well something is wrong with your spring calcs

Ks = 4*pi^2*fr^2*sprungmass*mr^2
so according to your numbers
Ks = 4 pi^2 *2Hz^2 * 73.7kg * 1^2
So Ks is approx 11638 N/meter = 66 lbs/in

What you did wrong in your calcs is that you used the total vehicle mass instead of using the mass on that corner. I just divided the total by 4 but it will depend on your weight distribution. I assume you made the same mistake in your critical damping calcs

[sbrenaman]

Wait a minute... What is the difference between lb-m and lb-f? What is the conversion? Aren't the SI units of mass slugs?

[sbrenaman]

Nevermind, I thought I was going crazy for a second there. There was an extra *4 in my spreadsheet, like you said http://fsae.com/groupee_common/emoticons/icon_wink.gif. I didn't forget to divide, I think I spaced and multiplied by 4 twice.

So now that I've straightened the spring rate selection out, I can increase the ride frequency to >2hz and I have a bit more freedom with motion ratios, but now that I know my required damping force, how do I apply that to the necessary shock valving? Necessary force is 6559 N/(m/s) according to my math (4 * pi * (damping ratio=0.8) * (ride freq=2hz) * (sprung weight=326kg) ).

Thanks

[billywight]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Wait a minute... What is the difference between lb-m and lb-f? What is the conversion? Aren't the SI units of mass slugs? </div></BLOCKQUOTE>

The Si unit of mass is the kg. Forget the slug*rankine*BTU/Bushell*Therm nonsense system, convert to metric, solve, then convert back. You'll save yourself a lot of time and error in the long run...

[sbrenaman]

Haha yeah. Its seemingly easier to do it all in metric and convert at the very end. I just finished my core requirements and finally have junior status. I think a lot of my headaches would have been non-issues if I was further along in ME school before I started tackling the suspension. But I guess on the other hand, I will be just that much better next year and the year after!

[J. Vinella]

Scott,

I would recommend stopping by The Shock-Shop in Portland.

http://www.shock-shop.com/Home-Misc/contact_shock.htm

Chris Billings is a super nice guy and I am sure will help you out with tuning your 7800s. We ran 7800s for years. We worked with him a few years back and have been happy with our dampers and knowledge ever since.

Oh...he is also one of the main Suspension Design Judges at Formula SAE - California so picking his brain is always worth it. You are lucky to be so close.

Cheers,

[sbrenaman]

We have been in contact, however he is hesitant to do too much analysis for us :P We bought our Penske's from him. Super nice guy, I recommend him!

[J. Vinella]

He will certainly not give you the answer, but will help you get your shim stack and bleed valve dialed in to get the force velocity curve you desire. This tuning process takes a great deal of time if you have never done it before.

[sbrenaman]

Indeed. But I'm lost as to how to determine the curve I need.

[exFSAE]

Damping rate seems high. 37+ lb*s/in at the wheel? I think I used ~8 with a stiffer spring setup. Is there a square root in there?

Any event. If you know the rate you want that defines at least a basic (linear) "curve" no?

Give yourself plenty of adjustment range to go above or below that. Ideally I think you'll want the high speed to be pretty digressive...

Other thing to consider-

When I wanted a 8 lb*s/in rate at the wheel, there was no way I could get that from any damper I looked at without revalving it or going with super low viscosity oil and whatever. More time and more pain in the ass.

Damper I was primarily interested in (Cane Creek) at that point had a minimum rate of about 24 lb*s/in, so we just designed a ~1.7 MR. That way we didn't have to rework the damper at all, and goin with a 300-350 lb spring gave us the wheel rate we wanted.

Worked out well.

[Mike Macie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sbrenaman:
Necessary force is 6559 N/(m/s) according to my math (4 * pi * (damping ratio=0.8) * (ride freq=2hz) * (sprung weight=326kg) ).

Thanks </div></BLOCKQUOTE>

You are still using the total sprung mass in your calcs. You need to use the sprung mass supported by EACH damper.

[sbrenaman]

Oh there we go. Thanks so much! So the initial slope of my graph needs to be 1640 N/(m/s), or 368 Lbf/(in/s). So y=368x for 0 &lt; x &lt; Xf? That seems a bit high, considering the slope of the off-the-shelf Penske's is 14x (lbf vs in/s).

[Mike Macie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sbrenaman:
Oh there we go. Thanks so much! So the initial slope of my graph needs to be 1640 N/(m/s), or 368 Lbf/(in/s). So y=368x for 0 &lt; x &lt; Xf? That seems a bit high, considering the slope of the off-the-shelf Penske's is 14x (lbf vs in/s). </div></BLOCKQUOTE>

You should take more math classes. Your conversion to lbs/in is incorrect.

[sbrenaman]

No, I should have paid attention better in Physics :P These unit conversions are killing me. 368 was lb/(m/s). Once I converted to lb/(in/s), I am at ~10.3 lb/(in/s). This has been extremely helpful, thank you so much guys!

[BilletB]

SI units blow. For something like this do EVERYTHING in pound force (lbf), inches (in), and seconds (s). You will never go wrong and everything is cake. Also, if you're like me you have intuition about 10 lbs, 15 lbs, and so on. You also know what 3 inches and 8 inches is. Fractions of meters and Newtons mean absolutely nothing to me. Also, I am awesome at screwing up an SI answer by some factor of 10 because it's freaking everywhere and impossible to pick out when you screw up. Also, there's no need for millions of prefixes and the standard unit to be some crazy small realistic value (Pascal). I know going from feet to inches requires a 12, miles to feet requires a 5280, etc. etc. I can easily find those in my calcs to see if I did the right conversion.

So for doing these kinds of calcs in English units use lbf, s, in.

Force -&gt; lbf
Distance -&gt; inches
g -&gt; 32.2*12 in/s^2
mass -&gt; lbf*s^2/in

You'll never go wrong, you'll easily find mistakes, you'll have intuition every step of the way, and you won't be a sucker that can only work in SI units and whines about lbm and lbf. Wait 'til they get a problem in the real world with kgf and go into vapor lock. Haha.

=]. Just having fun with a small thing that irritates me to no end.

[exFSAE]

+1 to Imperial units.

Though that's mostly from working in a machine shop for a couple years. When 99% of the tools , fasteners, and dials are in inches, you quickly start to dislike Metric prints.

You also get F good at fractions http://fsae.com/groupee_common/emoticons/icon_smile.gif

[sbrenaman]

This is frustrating, because I come from an workplace whose products are mostly for foreign (metric) companies. Also, my autocross car is an Integra, so I've gotten real used to metric. Oh well, I just need to adjust to going back and forth often.

[J.R.]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Mike Macie:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by sbrenaman:
Oh there we go. Thanks so much! So the initial slope of my graph needs to be 1640 N/(m/s), or 368 Lbf/(in/s). So y=368x for 0 &lt; x &lt; Xf? That seems a bit high, considering the slope of the off-the-shelf Penske's is 14x (lbf vs in/s). </div></BLOCKQUOTE>

You should take more math classes. Your conversion to lbs/in is incorrect. </div></BLOCKQUOTE>

So do you, units are never plural, they are lb. http://fsae.com/groupee_common/emoticons/icon_smile.gif Stop by the shop next week and see the car, we'd appreciate the feedback.

[Mike Macie]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by J.R.:
So do you, units are never plural, they are lb. http://fsae.com/groupee_common/emoticons/icon_smile.gif Stop by the shop next week and see the car, we'd appreciate the feedback. </div></BLOCKQUOTE>

I think it's more English related. I'm not the greatest speaker/writer but who cares, words do not design/build cars. http://fsae.com/groupee_common/emoticons/icon_cool.gif

[sbrenaman]

Touche, J.R.