How to determine start ride rate?

[Christoffer]

Hi, I would like some help regarding how to determine the start ride rate for our new 07 car. I've found a excel worksheet from UAH which I want to use to determine the spring rate for our cars suspension; unfortunately I haven't been able to determine the start ride rate for our car in the worksheet. What is meant by the start ride rate and how do I determine it?

Excel worksheet from UAH: http://me.unm.edu/~fsae/teams/2003/team_members/simulat...pring_calculator.xls (http://me.unm.edu/%7Efsae/teams/2003/team_members/simulation_team/UNM_FSAE_Spring_calculator.xls)

[B Hise]

this is a good start.

see tech tips:

http://optimumg.com/news_archives.htm

Bryan

[Marshall Grice]

RCVD has a chart that gives recommended ride frequencies vs ride height

[mtg]

Make the roll gradient and ride frequency as the inputs, not as outputs. You want to choose the roll gradient and ride frequency, and then find springs and MR's that fit your desired frequencies.

Around 2Hz ride frequencies and 0.8 deg/g roll gradient are not a bad place to start if you have no idea where to begin.

[Mike Cook]

Matt, witha .8 deg/g roll gradient I think may be a little heavy on the bar side? But anyways, .8-1.5 deg/g and 2-3hz ride freq is a good starting point.

Then you can determine what you need your roll stiffnesses to be and wheel rates. From this, you can determine how much roll stiffness your bar needs to provide in addition to the roll stiffness from your springs.

After you do these calculations you need to determine how you will implement your spring damper assy. Most people use rockers (bell cranks) but direct shocks are not out of the question.

From this point you really need to create a program that you can put your suspension points into and graph your wheel rates and roll stiffnesses vs. travel. You can do this in Matlab, or use a program like Adams. Also I think solid works can figure this stuff out pretty easily. If this is your first time shoot for constant wheel rates and roll stiffnesses through travel.

You used decent grammer. Feel free to ask more questions or use the search bar, a lot of these topics have been covered in more detail.

Mike

[mtg]

With how fast the transients need to be on an FSAE car, I think 0.8 deg/g is a good place to be. The uber sweet method is UWA style Kinetics where you can have 0.5 deg/g and still have a low warp frequency. What an awesome system.

Either way, Christoffer, you now have a couple viewpoints on where to start so you should be able to build a suspension that's half decent straight out of the box.

[js10coastr]

Originally posted by mtg:
With how fast the transients need to be on an FSAE car, I think 0.8 deg/g is a good place to be. The uber sweet method is UWA style Kinetics where you can have 0.5 deg/g and still have a low warp frequency. What an awesome system.


Nah, I don't think they really know what's going on with their dampers. I'm going to try and get some video shots of it though... http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[terra_dactile]

Hi Christoffer,
If you check in Racecar Vehicle Dynamics, Chapter 16 gives pretty much all the info needed in order to calculate your needs for ride and roll rates, which in turn allow you to choose your needs for springs and anti roll bars. From what I have seen it is exactly the same info as mentioned on the optimumg site or during the seminar. The limit of how low your ride rate frequency can go for a FSAE car (standard setup) depends on the travel you have in your system before the cars nose hits the ground, the softer the spring the less they influence negatively the vertical tire load fluctuations of the tire, however this might also make necessary for you to adjust your ride height higher inorder to not hit the ground with the chassis/nose during braking, makeing the CG height rise. If you read Racetech magazine they had a good series on Masterclass suspension design, lots of good information.

Anyways hope some of my Psycho babble was of some use,

Cheers,

Jude Berthault
ETS FSAE 2003-Current
Vehicle Dynamics Leader

[C.Zinke]

With a overall softer setup you will gain more roll angle and also -with common suspension design- more positive camber. So normaly this is the argument, which leads you to a ride frequency not lower than about 1.8Hz.

The reason is, that you can't reduce roll of a soft setup with harder anti roll bars, because you will have very bad damping characteristics during cornering. So the whole thing is a compromise, but in most cases it is not limited by your travel, more by your roll angle or roll damping, if you don't want to have too much positive camber. Nosetravel during braking may be a problem, if your car has no Antidive.

-----
Christopher Zinke
- Head of Vehicle Dynamics & CAE
www.LionsRacing.de (http://www.LionsRacing.de)

[Mike Cook]

I'm not so sure I agree with that. Also, you can dial in more static negative camber and castor and it is probably not the end of the world.

[C.Zinke]

Yes of course, i just wanted to lead the discussion to another direction. Now the task for the team is to find out whats more "the end of the world". Naturally this is depending on the car and the tires. As discussed, there are many more parameters than ride rate vs. static camber for example.

Christopher Zinke
- Head of Vehicle Dynamics & CAE
www.LionsRacing.de (http://www.LionsRacing.de)

[sujaykawale]

I'm not one to point out silly things like grammatical and spelling mistakes on the forums, but I've seen a lot of people doing it lately, so I thought i might join in... Mike, it's 'grammar' not 'grammer'...
Haw haw! (Nelson Muntz style)

[mtg]

And another common misnomer:

DAMPER

It's not dampener, dampner, damplener etc. I don't understand why people feel like adding extra letters at will, but it makes them look uneducated.

Whenever people talk about dampeners on their car, I wonder why they are spraying their springs with water.

[sujaykawale]

hahaha... thats ridiculous!
I don't think i've ever seen that before!

[Jersey Tom]

Another thing to keep in mind is the type of track surface you're planning on running on..

[Mike Cook]

Well, spelling and grammar are two different things, and I don't claim to be great at either. But at least my posts are not broken english. I make an attempt to write well. I also make an attempt to bring the tech.


So back on topic.

Chris, why I said I didn't agree with your post is this: Roll damping is related to roll stiffness. How you get roll stiffness- either from bars or springs- really doesn't change your roll damping. So you find out what roll gradient you want, which will dictate a roll stiffness, and from there you decide on damper curves.

If you choose to run small springs and heavy bars, your bump mode may be over damped, as a consequence of what I said above. But this isn't what you said, and thus why I disagree. If I'm off basis here, by all means, someone correct me. I have just been throwing this discussion around in my head for the last couple nights...

[C.Zinke]

This is what i have said... http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Christoffer]

First of all, Thanks for all the replies to my question.

The OptimumG website has already made me much wiser, so thanks for the link "B Hise"�.

PS. Sorry if neither my spelling or grammar is flawless, but so far the EU hasn't demanded that Denmark change its native language to English, and my English probably won't improve much until that happens... sorry guys http://fsae.com/groupee_common/emoticons/icon_wink.gif

Christoffer Laursen
Aalborg University
Denmark

[sujaykawale]

yeah... long live optimum G and their free tutorials!

and i don't think you need to apologise for spelling/grammar!

[Mike Cook]

Originally posted by C.Zinke:
This is what i have said... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Not really http://fsae.com/groupee_common/emoticons/icon_wink.gif

[D J Yates]

As with most things FSAE based, i have my own opinion on the selection of ride rates/frequencies. Having just looked at Optimum G's tech tip 1, and recalling what i read in RCVD, i'm not convinced that picking a recommended ride frequency of ~3Hz is a sensible thing to do for FSAE.

I remember seeing a chart of frequency against amplitude for human tolerance to vibration. Consider the fact that FSAE cars are relatively slow, if a FSAE car and a car twice as fast hit encountered the same series of bumps in the road, the frequency for the FSAE car would be half that of the faster car. Because the vibration is slower, you can allow a larger amplitude (as defined by the previously mentioned chart) - say double - and therefore you need less attenuation, allowing you to run stiffer suspension. Of course, with a stiffer car there are larger shock loadings on the vehicle, so obviously, there is going to be an optimum point of high stiffness and durability. Also, there's a point when increased stiffness is pointless because the tyre stiffness dominates.

Another thing to consider is the coupling of natural frequencies for sprung and unsprung masses. Because FSAE sprung masses are so light and the tyres are so soft, the sprung and unsprung natural frequencies are closer together than in most "normal" cars. I don't know whether it's better to have them close together so that they can both be critically damped, or whether they should be seperated as far as possible so that they have little interaction, but it's something to think about.

Personally, if i were still involved with FSAE and was building another car, i'd do the following:
- set up simple simulations to find optimum spring rate and damping coefficients for combined ride and pitch modes, where the limiting factors are the sprung and unsprung masses, the tyre stiffness and human tolerance to vibration - i.e you're variables are spring stiffness, damping and anti-pitch and you're trying to optimise/minimise the distrubance to the contact patch normal force (critically damped unsprung mass).
- aim for roll center heights (and consequently roll stiffness) that allow for critical damping of sprung and unsprung roll modes, with the spring stiffness and damping you previously determined - i.e. variables are roll stiffness (through anti-roll bars, spring stiffness is fixed) and roll inertia (through roll centre heights), where roll damping is constant.

Hope that's food for thought. Comments?

[Mike Cook]

Good food for thought DJ.

[Christoffer]

Sorry guys must have been have been half brain dead when I wrote my first post, and unfortunately didn't read it carefully enough through now when I later on read your replies. The topic should have read: How to determine start roll rate?

Not that your replies haven't helped me, in spite the fact that I confused you by mistyping, they have very much. Thank you all.

But according to the OptimumG website, Spring & Dampers Tech tips 2, you determine your roll rate from your wheel rate, which you again determine from your spring rate. But the objective with excel worksheet was to end up determining the spring rate?

Is start roll rate mentioned in the excel worksheet from UAH, a totally different thing than the roll rate they calculate in Spring & Dampers Tech tips 2 from OptimumG?

Christoffer
Aalborg University
Denmark

Sorry for the mistyping. http://fsae.com/groupee_common/emoticons/icon_frown.gif

[mtg]

You want to pick your roll gradient first. Then do the equations starting on page 2 in order:
- calculate the roll gradient due to the ride springs (gives you an overall idea of how much roll resistance is being resisted by the ride springs)
- calculate the front roll rate due to the front ride springs
- calculate the rear roll rate due to the rear ride springs
- Now you calculate the total roll rate needed by your anti roll bars to increase the roll gradient to your desired value

[terra_dactile]

Hi Christoffer,
I believe your confusing what you set at the begining which is how much deg of role you want per G of lateral acceleration. The role rate calculated in the optimum G tech tips or in RCVD is in Nm/deg (more of a stiffness given by the suspension springs and roll bars during role).
The value of degrees of role per G is assumed to vary between 1.0 and 1.8 deg/G for low down force cars, the guy from UMR mentioned 0.8deg/G which is what is recommended for high down force cars(there car has well tested Wings), who knows what is the perfect value. However you can adjust this value by changing the stiffness of the anti roll bars during testing, if you see that the car understeers or oversteers to much.

Your spread sheet shows that you have read the section in RCVD, read the first few pages and the assumtions are listed to answer the question you have asked, Chapter 16 page584 (table 16.1 Typical roll gradients), also check the paragraphs in suspension techtip 2 off the OptimumG site, the deg/G gradient is mentioned their as well.

Jude Berthault
ETS FSAE 2003-Current
Vehicle Dynamics Leader

[mtg]

Originally posted by terra_dactile:
the guy from UMR

Hilarious, there is a reason I put my name in my signature. Anyway, "the guy from UMR" is also "the guy that used to work at OptimumG and spent a long time writing tech tips".

Wings aren't going to affect roll gradient as much as ride frequency. So, I still think that if you have no idea where to start (and it doesn't matter if you have wings or not), 0.8 deg/g is a good place to begin.

[ben]

0.8deg/G always worked well for me. That was Bruntingthorpe where the kart track is relatively smooth. I guess the Silverdome was bumpier but the new place looks pretty smooth too.

Ben

[skillet]

does anyone modify there roll rates to improve transient response times? or do you accomplish this with damping alone?

[flavorPacket]

I'd think dampers and distribution should get you where you need to be...

[terra_dactile]

Matt Giaraffa,
Please excuse the "guy from UMR comment no disrespect intended" I wrote the reply at 12:45 in the morning,I am fully aware that your worked at OptimumG, it was in your title for the last few years.
The point why I thought 0.8 deg/G was for winged cars has to do with the fact that in the Spring and damper techtips , it is seperated between 0.2-0.7 for higher downforce cars and then mentions 1-1.8deg/G for low downforce cars. In 2004 we had 1.0 deg/G and found that althoug the car reacted quick it was hard to recover from oversteer. In 2005 we used 1.5 deg/G and the car is so much easier to drive, you can lose the rear and recover with ease.

I am aware that this is most likely related to the fact we went from a 510lb R6 powered beast, to a 365 lb Wr450 powered vehicle. The inertia of our car decreased significantly.

Obviously I am nowhere near your level of understanding, please enlighten me on why OptimumG seperated role gradient depending on downforce (which directly linked to wings usually).I have trouble to comprehend the difference between what I mentioned, my understanding was that with downforce setup you could not run a soft setup in roll or you would end up having a very slow response to driver inputs.

Sincerely,

Jude Berthault
ETS FSAE 2003-Current
Vehicle Dynamics Leader

[mtg]

Jude,

No worries, I didn't take offense. The chart in the tech tip about roll gradient has RCVD as a source obviously- the recommended roll gradients are not a function of downforce, but the target audience was professional racing where cars with lots of downforce also end up having stiffer suspension all around.

Purely from a suspension design standpoint, I would not make roll gradient a function of aero (unless you were in F1 or something)

In your example, a car with 1.5 deg/g is going to be slower responding than your older car with 1.0 deg/g- that alone will make it easier to catch when it loses traction. However, it will be slower responding, that's the tradeoff.

So, basically, I could have worded that section a little better so it was easier to understand.

[Mike Cook]

Originally posted by mtg:


In your example, a car with 1.5 deg/g is going to be slower responding than your older car with 1.0 deg/g- that alone will make it easier to catch when it loses traction. However, it will be slower responding, that's the tradeoff.



However, as he stated, inertia decreased which should also decrease response time.

Maybe the inertia decrease had a much lesser affect than the roll gradient? I'll have to add to this later when I have a book in front of me.