Spring rates!

[-francisco-]

Originally posted by BilletB:
Haha, nice. I may possibly know the authors of that piece. =] Fun to see it posted here! If you look hard enough you can find 150 lb springs. What is the ID and free length you need for those shocks? We run the Cane Creeks and 150's are out there for them.

Thanks. But yeah, last year pirate4x4's article on suspension design was what really taught me how to draw a linkage suspension system, well it only gave me the basics, but it was all i needed, first for mountainbike design and now for Formula Student. Going to the second year in college, now i could have easily figured out how to do it with a bit of applied physics, but still, the complete lack of information on the matter at car suspension books is patethic.

[-francisco-]

[quote]Originally posted by BilletB:

Technically, wheel rate doesn't include the tire. It's the rate at the wheel center. Ride rate then accounts for the rate of the tire. You really need to calculate and know both.

And it would be

ride rate = (wheelrate*tirerate)/(wheelrate+tirerate)

It makes sence, but then what do you use for natural frequency calculation? wheel rate or ride rate? I entered the team this year and my job was to develop a new pushrod system for last year's car (we'll be on class 1-200) keeping the overall A-arms geometry and thwe same tyres, so i didn't really looked in tyre performance.

Also what formula are you guys using for Fn? I'm using:

Fn = sqrt ((386.088 * WR ) / CSW ) / 2pi

But i've seen guys using diferent formulae/convertion factors in forums and such, so I'm really not sure what is the correct one.

As for springs we just got a local spring shop to wound custom springs for us.

[BilletB]

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 BilletB:

And it would be

ride rate = (wheelrate*tirerate)/(wheelrate+tirerate)

My bad, should of checked it before posting. Don't know how i mixed up ride rate and wheel rate. But I'm pretty sure you subtract the wheel rate from the tire rate when calculating the ride rate. </div></BLOCKQUOTE>

Your ride rate is your wheel rate and tire rate as springs in series. How do you find the equivalent spring rate of springs in series?

(1/k_eq)=(1/k_1)+(1/k_2)+...+(1/k_i)

For just two springs in series it reduces to the product of the rates divided by the sum of the rates.

[BilletB]

Originally posted by -francisco-:
It makes sence, but then what do you use for natural frequency calculation? wheel rate or ride rate? I entered the team this year and my job was to develop a new pushrod system for last year's car (we'll be on class 1-200) keeping the overall A-arms geometry and thwe same tyres, so i didn't really looked in tyre performance.

Also what formula are you guys using for Fn? I'm using:

Fn = sqrt ((386.088 * WR ) / CSW ) / 2pi

But i've seen guys using diferent formulae/convertion factors in forums and such, so I'm really not sure what is the correct one.

As for springs we just got a local spring shop to wound custom springs for us.

Thanks for saying that about the pirate article. That's really what we were going for.

As you said, physics, statics, and dynamics are the fundamentals of what we're dealing with. I came up with that natural frequency equation for that article because it was simpler to just say that for the non-engineers. You are an engineer. For you, fn=sqrt(k/m)/2pi. Then, you can derive your own equations for shortcuts if you wish, but don't use the shortcut equation until you understand and derive it. You should also understand where sqrt(k/m) comes from. Think equations of motion and solutions to those equations. That is how you model and help design suspensions.

Do you use the nat freq of wheel or ride rate (I call these suspension frequency and ride frequency respectively)? Which do you care about more? Well, those are questions for YOU to answer. I can tell you that you damn well better know both. The best suspension designers can blend some logic and art with their science.

[Mike Macie]

Originally posted by BilletB:
<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 BilletB:

And it would be

ride rate = (wheelrate*tirerate)/(wheelrate+tirerate)

My bad, should of checked it before posting. Don't know how i mixed up ride rate and wheel rate. But I'm pretty sure you subtract the wheel rate from the tire rate when calculating the ride rate. </div></BLOCKQUOTE>

Your ride rate is your wheel rate and tire rate as springs in series. How do you find the equivalent spring rate of springs in series?

(1/k_eq)=(1/k_1)+(1/k_2)+...+(1/k_i)

For just two springs in series it reduces to the product of the rates divided by the sum of the rates. </div></BLOCKQUOTE>


Again i mixed up ride rate and wheel rate. You would subtract when calculating the wheel center rate using the ride and tire rate. I'm used to calculating the desired ride rate and working my way back.

[-francisco-]

Originally posted by BilletB:
Thanks for saying that about the pirate article. That's really what we were going for.

I came up with that natural frequency equation for that article because it was simpler to just say that for the non-engineers. You are an engineer. For you, fn=sqrt(k/m)/2pi.

So, you know the guys that wrote the article or were you the one who wrote it?!
The Fn formulae always get me in trouble because of working with the imperial system and its wicked conversions factors from mass to weight and such. But know i ave everything straight.

One question, i did all the maths for our suspension with all the wheel travel visualized in Solidworks, but everyone is bashing me cause i ordered 215, 225 and 250 lbs springs to play with, and we haven't tested the car yet, susp everyone is saying the susp is gonna be too soft.

We have a car with 200kg -444lbs sprung mass, about 50-50 distribution front to back, the push rod is at 40º working in a plane. and the rocker's MR is 1,1:1.

The general Motion ratio is about 1,18:1. Down on paper i have ride frequencies between 3 and 3.4hz (MR is adjustable). I told them that's what most people run on their cars.

Any advice?

[Adambomb]

I'd say 3-3.4 is a bit higher than I'd prefer for an FSAE car; surely not too soft.

I hear you on everybody saying the suspension is going to be too soft. I've heard that a lot over the last couple years. The only advice I have on that is that most of those naysayers (including the engineers), quite honestly, don't have a damned clue what they're talking about, and I can almost guarantee haven't looked at the problem with the level of depth presented here.

Most of them have gained all their knowledge from movies like "The Fast and the Furious," and will swear up and down that the "best" suspension for performance is the stiffest. Not true at all.

Sorry, got off on a bit of a rant there, but yeah, you're in the ballpark at least. Ride rate is a bit high for my tastes (I've seen some really good publicly posted papers from Optimum G on the subject), but other than that the numbers sound reasonable.

[flavorPacket]

Instead of guessing on what is 'too soft' or 'too stiff', why don't you evaluate the effects of these parameters on tire load variation, force transmissibility to the sprung mass, etc? Or even better, just test both setups?

[Yellow Ranger]

force transmissibility? now we're just making up words

We tested a few different ranges last year, and kept going stiffer and stiffer cause we were trying to get our tires temps up (scrub wasn't bad but can't remember the figures). The problem was, we were used to running the old harder compound tires and when we got the new hoosiers, they were like butter. So now we're stepping down in the freqs, but hey, thats why you test these things... oh, and were were near the 3 and 3.5 range.

what tires are you planning on running?

[flavorPacket]

force transmissibility = transmission of force

transmissibility is not limited to examining displacement, though that's the most common. Force and displacement transmissibility are only the same for 1 DOF linear systems. If you think your car is one of those...