Ride & Roll problem [Archive]

typeh

01-23-2012, 05:10 AM

I worked on ride and roll rates calculations according to RCVD but i faced these difficulties:
1-Weight distribution in our car 60%rear 40% front so 5% as magic number in weight distribution still keep front roll rates less than rear roll rates. will it leads to any problems during testing? i didn't work on 5% i worked with 8% to insure stability.
2-calculated roll rates is higher than desired roll rates so i increased allowable wheel travel to be 43 mm isn't it too much? any 1 mm less will lead to higher calculated rates than desired.
3-in case of maximum deceleration (1.3 g) and maximum lateral acceleration (2.12 g) rear inside wheel will loose load (load will be -12 kg on it). i think case (max.decel. & max. lateral accel.)

i'm working on high frequency 2.8 Hz mean can work on smaller ground clearance , still limited with 43 mm travel?

hope anyone can give me advice and direction can help solve problems

here r some calculations

http://imageshack.us/photo/my-images/507/rollj.jpg/

http://imageshack.us/photo/my-images/580/rates1.jpg/

http://imageshack.us/photo/my-images/850/roll2.jpg/

Alexandria University Motorsports
FSG 2012

typeh

01-23-2012, 05:10 AM

exFSAE

01-23-2012, 08:14 AM

Are your accelerations realistic?

typeh

01-23-2012, 08:16 AM

it's maximum lateral acceleration tire can produce but i think car won't reach this value. anyway it's first year for me and my team so still has no experience

BillCobb

01-23-2012, 10:49 AM

Roll rate distribution includes roll center height factors, or not?

Jay Lawrence

01-23-2012, 07:11 PM

I think what exFSAE was getting at (correct me if i'm wrong) was that you have assumed max decel and max lateral at the same time. This is not really correct. You can have one or the other, or a combination of the 2, but never both.

typeh

01-25-2012, 04:18 AM

Billcobb, yes i include roll center height
front roll centel 64 mm
rear roll center 73 mm
CG above roll center 191 mm

jay lawrence, i did calculations according to the case of max. laterla acceleration + max. decelration
in RCVD calculations have been done based on laterla acceleration only, so u advise me do calculations for this case only not combined?

there is something called tire limit means tire can't produce max. grip laterllay and longtudinal at same time so this case won't happen. i understand this concept right?

Alexandria University Motorsports
FSG 2012

Jay Lawrence

01-25-2012, 03:59 PM

typeh, you are correct - the tire limit means you can't produce both max lat and max long. A simple way to look at this is with the traction circle.

As for your travel of 43mm, this is something that can be tuned with spring rates/pre-load once the thing is built. I'm not sure which frequency you are referring to or whether you have mode separation, but 2.8Hz is on the high side. Perhaps there's something wrong in your calcs.

Adambomb

01-25-2012, 10:59 PM

typeh

01-26-2012, 04:10 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jay Lawrence:
typeh, you are correct - the tire limit means you can't produce both max lat and max long. A simple way to look at this is with the traction circle.

As for your travel of 43mm, this is something that can be tuned with spring rates/pre-load once the thing is built. I'm not sure which frequency you are referring to or whether you have mode separation, but 2.8Hz is on the high side. Perhaps there's something wrong in your calcs. </div></BLOCKQUOTE>

i refered to ride frequency and had higher frequency for faster transient response at corner entry.
all guides refer
2 Hz sedan racecar
3 Hz high down force racecar
my no, in between yes near the 3 but it's itteration after calculation according to lateral acceleration only reached about 30 to 35 mm wheel travel but frequencies still 2.8 and 2.6
what are problem which may face me in case of working with those frequencies?

Alexandria University Motorsports
FSG 2012

BillCobb

01-26-2012, 09:50 AM

Double the front roll center height and drop the rear by 1/2. Watch what happens. Your roll gain will be different and stiffnesses will have to be recalculated, but the learning should be there. Remember, this is a largely rear weight biased vehicle and as such some of the generalizations that you have read about don't apply well. They are intended for fractional variations of conventional architectures. How you produce these SDFs is part of your engineering job.

Jay Lawrence

01-26-2012, 07:42 PM

What he said http://fsae.com/groupee_common/emoticons/icon_smile.gif

typeh

01-28-2012, 01:42 PM

i think i misunderstand something in ride rates will totally change results, ride rate = load cchange/wheel travel

wheel travel is total travel(jounce+rebound)? or just wheel travel for one only (jounce or rebound)only one?
in current results i meantwheel travel is for one side movement only (jounce only or ebound only) so thought will need 40 mm jounce + 40 mm rebound all that in spring stroke i understood right?

Alexandria University Motorsports
FSG 2012

01-28-2012, 07:42 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
i understood right? </div></BLOCKQUOTE>
Typeh,

No, no, no, no, no!

I fear you don't have an iota of understanding of this subject. You are pulling numbers out of the air, plugging them into equations you don't have a clue about, and hoping the results will somehow give you a workable car.

Results like: Roll Gradient = 0.94339623 deg/g!

Since this is clearly just a rough calc, why not;
Estimated Max G force = 2g.
Desired Max Roll Angle = 2deg.
So, Roll Gradient = 2/2 = 1deg/g.

And all the other numbers can be worked out similarly with pencil and paper, so why the spreadsheet?
Do you think the results are somehow better if they come out of a computer?
Are the results really accurate to 8 significant digits(!!!)?
Does a computer maketh an engineer?

So, can you post all your calculations (ie. including the equations, assumed inputs, expected tolerances, etc.), done by hand, here on this thread?

(I could, in less space than this rant. http://fsae.com/groupee_common/emoticons/icon_smile.gif)
~~~o0o~~~

Sadly, the above is unlikely because you don't seem to have the faintest notion what "rate" means.

Have you heard the expression "dy/dx"?
Has your school taught you anything about differential calculus?

I guess not. So I suggest you ask for your tuition fees back.

I further suggest you forget about calculations, and instead build your car by the old-fashioned trial-and-error process (it works, but is slow and expensive).

But, when it is finished, please do not call yourself an engineer. And definitely do not accept any such qualifications from your school. That would be an insult to the name.

Z

BillCobb

01-28-2012, 09:34 PM

Z:
Me oportet propter praeceptum te nocere.

TypeH: Nothing wrong with your spreadsheet approach. Check the units of all your variables for consistency (N, m, rad, sec). Then produce results in N, m, deg, and g. Round your outputs to 1 or 2 fractional numerals. Your target g values look good. The roll gradient probably doesn't include the tire effect (which is quite a bit, actually).

Keep it going...

01-29-2012, 06:02 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by BillCobb:
Z:
Me oportet propter praeceptum te nocere. </div></BLOCKQUOTE>
Bill,

The "hurt" was intended for his teachers and school. There are too many "degree factories" nowadays popping out graduates whose only skill is to use their electronic box of mischief to generate endless lists of 8+ digit numbers (actually, these days they do pretty pictures), but with no real understanding of what they are doing.

The thing that really annoys me is that I have little option but to buy the products these kids design. (I could bore you with the story of the broken suspension balljoint on a Toyota 4WD I recently saw (a design bordering on criminal negligence!), but it's getting late...)
¬¬¬o0o¬¬¬

Typeh,

Keep at it. Your patience and perseverance are admirable.

The one thing I would ask of you is to please take some of your simpler questions to your teachers and ask for (demand?) their help. They are getting paid to teach you these things.

Z

Zac

01-29-2012, 09:32 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The one thing I would ask of you is to please take some of your simpler questions to your teachers and ask for (demand?) their help. They are getting paid to teach you these things. </div></BLOCKQUOTE>

Actually they aren't which is the problem.

RollingCamel

01-29-2012, 10:31 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The one thing I would ask of you is to please take some of your simpler questions to your teachers and ask for (demand?) their help. They are getting paid to teach you these things. </div></BLOCKQUOTE>

Actually they aren't which is the problem. </div></BLOCKQUOTE>

2nd that, plus you'll rarely find a doctor knowing beyond what is given in the curriculum.

Ratna Mitheysh

01-29-2012, 10:59 PM

How did you arrive at 2.12 g lateral acceleration?
Are you making an aero car?

typeh

01-31-2012, 08:49 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
i understood right? </div></BLOCKQUOTE>
Typeh,

No, no, no, no, no!

I fear you don't have an iota of understanding of this subject. You are pulling numbers out of the air, plugging them into equations you don't have a clue about, and hoping the results will somehow give you a workable car.

Results like: Roll Gradient = 0.94339623 deg/g!

Since this is clearly just a rough calc, why not;
Estimated Max G force = 2g.
Desired Max Roll Angle = 2deg.
So, Roll Gradient = 2/2 = 1deg/g.

And all the other numbers can be worked out similarly with pencil and paper, so why the spreadsheet?
Do you think the results are somehow better if they come out of a computer?
Are the results really accurate to 8 significant digits(!!!)?
Does a computer maketh an engineer?

So, can you post all your calculations (ie. including the equations, assumed inputs, expected tolerances, etc.), done by hand, here on this thread?

(I could, in less space than this rant. http://fsae.com/groupee_common/emoticons/icon_smile.gif)
~~~o0o~~~

Sadly, the above is unlikely because you don't seem to have the faintest notion what "rate" means.

Have you heard the expression "dy/dx"?
Has your school taught you anything about differential calculus?

I guess not. So I suggest you ask for your tuition fees back.

I further suggest you forget about calculations, and instead build your car by the old-fashioned trial-and-error process (it works, but is slow and expensive).

But, when it is finished, please do not call yourself an engineer. And definitely do not accept any such qualifications from your school. That would be an insult to the name.

Z </div></BLOCKQUOTE>

Do you think i wait u advise me do calculations with my hands? this time you don't understand well, at least i did calculations 5 times but this spread to change numbers quicker and more dynamic , don't worry i know how to use my hands well and i rarely trust another thing. another thing, the 8 digits will go to hell when select spring or design it http://fsae.com/groupee_common/emoticons/icon_frown.gif sadly. Can i use CNC to keep this accurate spring i did or maybe laser accuracy help me?
this spread took me time than calculating with hands but now can change numbers quicker.

roll gradient unlucky need negotiation with tires we have maybe u convince them to be 2g and u can check some posts in forum 0.8 to 1.5 deg/g also 2.12/2 doesn't equal 1 http://fsae.com/groupee_common/emoticons/icon_frown.gif

maybe i'm still miss knowledge in some fields but i'm working on it and gain experience but i would like to advise u be more polite when criticize work

--------------------
AUMotorsports
FSG 2012
Design Team Head

typeh

01-31-2012, 08:53 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Sadly, the above is unlikely because you don't seem to have the faintest notion what "rate" means.

Have you heard the expression "dy/dx"?
Has your school taught you anything about differential calculus? </div></BLOCKQUOTE>

I asked question in wrong way but anyway let's forget suspension and cars spring rate(K)= force/displacement
this displacement is in tension or compression not add total displacement for both.

still unlucky won't be able take my fees back from college , government pay it

Kirk Feldkamp

01-31-2012, 02:22 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
Do you think i wait u advise me do calculations with my hands? this time you don't understand well, at least i did calculations 5 times but this spread to change numbers quicker and more dynamic , don't worry i know how to use my hands well and i rarely trust another thing. another thing, the 8 digits will go to hell when select spring or design it http://fsae.com/groupee_common/emoticons/icon_frown.gif sadly. Can i use CNC to keep this accurate spring i did or maybe laser accuracy help me?
this spread took me time than calculating with hands but now can change numbers quicker. </div></BLOCKQUOTE>

Geez. This is exactly the problem Z is getting at. You're failing to see the "bigger picture." When you focus on things like 8 decimal place accuracy in your magical spreadsheet, you completely miss the fact that that level of detail is completely useless for something like this. Does it matter if it's 0.94339623 deg/g or 0.94339 deg/g or even 0.94 deg/g? For a beginning analysis, you could probably do with 0.9 and move on to move important things like what that number actually means for your car! Z is trying to help you see that you need to come up with a better understanding of what the equations you're trying to use actually mean before you just start blindly plugging in numbers.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
maybe i'm still miss knowledge in some fields but i'm working on it and gain experience but i would like to advise u be more polite when criticize work

still unlucky won't be able take my fees back from college , government pay it
</div></BLOCKQUOTE>

You're not going to get a lot of help around here by first asking for someone to do your design for you, and then tell them that they're being a jerk when they get exasperated that you're being difficult. Hopefully this is simply a language issue, because your government should ask YOU for the money back if you're not seeing what the problem is! Z has been uncharacteristically patient with you... you should read some of his older posts! Haha.

typeh

01-31-2012, 05:13 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kirk Feldkamp:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
Do you think i wait u advise me do calculations with my hands? this time you don't understand well, at least i did calculations 5 times but this spread to change numbers quicker and more dynamic , don't worry i know how to use my hands well and i rarely trust another thing. another thing, the 8 digits will go to hell when select spring or design it http://fsae.com/groupee_common/emoticons/icon_frown.gif sadly. Can i use CNC to keep this accurate spring i did or maybe laser accuracy help me?
this spread took me time than calculating with hands but now can change numbers quicker. </div></BLOCKQUOTE>

Geez. This is exactly the problem Z is getting at. You're failing to see the "bigger picture." When you focus on things like 8 decimal place accuracy in your magical spreadsheet, you completely miss the fact that that level of detail is completely useless for something like this. Does it matter if it's 0.94339623 deg/g or 0.94339 deg/g or even 0.94 deg/g? For a beginning analysis, you could probably do with 0.9 and move on to move important things like what that number actually means for your car! Z is trying to help you see that you need to come up with a better understanding of what the equations you're trying to use actually mean before you just start blindly plugging in numbers.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
maybe i'm still miss knowledge in some fields but i'm working on it and gain experience but i would like to advise u be more polite when criticize work

still unlucky won't be able take my fees back from college , government pay it
</div></BLOCKQUOTE>

You're not going to get a lot of help around here by first asking for someone to do your design for you, and then tell them that they're being a jerk when they get exasperated that you're being difficult. Hopefully this is simply a language issue, because your government should ask YOU for the money back if you're not seeing what the problem is! Z has been uncharacteristically patient with you... you should read some of his older posts! Haha. </div></BLOCKQUOTE>

i didn't see any help or advice u can see previous posts and compare with mr.Z attacking post.
i'm not asking someone do designs to me u can see all my posts i show my work and ask advise and admitted i asked stupid question didn't need all that from mr.Z and he only came at the stupid question don't know hwy he didn't help before

Kirk Feldkamp

01-31-2012, 07:31 PM

In that case, if you're looking for some advice... it would be a very good idea to find someone on your team or someone else (friend, family, teacher, etc?) that can help you with the English in your posts. It's somewhat difficult to understand your writing, which leads me to believe that you may also be misunderstanding what people are typing to you. There are other people on this forum from Egypt that I'm sure would be willing to help you out with improving your communications. I certainly couldn't do what you're trying to do on here in Egyptian with my knowledge of Egyptian (non-existant), so I commend you for trying. A little more effort in this area might really help generate more positive and helpful responses!

-Kirk

02-03-2012, 05:49 PM

Typeh,

1. No pain, no gain. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~~~~~~~~~~~~~~

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
spring rate(K)= force/displacement
this displacement is in tension or compression not add total displacement for both. </div></BLOCKQUOTE>
2. Your "force/displacement" is a ratio of two finite quantities. Therefore, it is NOT a "rate"!

A "rate", in the sense used here, is a ratio of two infinitesimal quantities (ie. it is the ratio of "the ghosts of departed quantities", which here is "df/dx").

This is an important difference. If you don't understand this difference, then you will have many problems later.
~~~~~~~~~~~~~~~~~~

3. When posting, start a new thought with a capital letter (press <shift> key and <letter>, and end with a full-stop <.> ). This is called a "sentence".

Separate groups of thoughts by pressing the <Enter> key twice. These are called "paragraphs".

The white space is free. It helps communication, so use it!
~~~~~~~~~~~~~~~~~~

Z

Pete Marsh

02-03-2012, 08:20 PM

I think typeh is actually asking for the displacements on each side of the car when in roll, one in compression, the other "tension" = less compression than before. Or, really, the actual forced based center of rotation in roll.

Is this what you want typeh?

If so, I'm afraid this is not a fixed point, and is actually quite difficult to plot it's position as the chassis rolls. It is easier to get an estimate of a final value using traditional kinematic roll center analysis, it will be wrong, but still very useful, and more than likley all you need to build a good 800 point car.

Pete

typeh

02-04-2012, 06:14 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Pete Marsh:
I think typeh is actually asking for the displacements on each side of the car when in roll, one in compression, the other "tension" = less compression than before. Or, really, the actual forced based center of rotation in roll.

Is this what you want typeh?

If so, I'm afraid this is not a fixed point, and is actually quite difficult to plot it's position as the chassis rolls. It is easier to get an estimate of a final value using traditional kinematic roll center analysis, it will be wrong, but still very useful, and more than likley all you need to build a good 800 point car.

Pete </div></BLOCKQUOTE>
I hope this time i can express well.
My question was: in this relation
Ride rate(K)=load transferred /wheel movement
wheel movement here represent total movement or movement in one direction only.
1- total wheel movement jounce + rebound
2- wheel movement in one of them only

i see the right answer is choice no.2

I hope my communication skills are better in this post http://fsae.com/groupee_common/emoticons/icon_smile.gif .

AUMototrsports
FSG 2012

02-06-2012, 05:45 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
I hope my communication skills are better in this post http://fsae.com/groupee_common/emoticons/icon_smile.gif .
</div></BLOCKQUOTE>
Much better! Like night and day.

I will answer your question tomorrow (long day, need sleep).

Z

02-06-2012, 09:20 PM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by typeh:
My question was: in this relation
Ride rate(K)=load transferred /wheel movement
wheel movement here represent total movement or movement in one direction only.
1- total wheel movement jounce + rebound
2- wheel movement in one of them only
</div></BLOCKQUOTE>
Typeh,

A "spring rate" is the ratio of the small increment of force associated with a small increment of displacement, with both force and displacement being in the same direction. (Mathematically these are "infinitesimal" increments, but for engineering purposes we can just say "small".)

As such, it doesn't matter if you consider bounce/jounce, droop/rebound, or both together. As long as the direction of force and displacement INCREMENTS are consistent the answer will be the same (ie. -dF/-dx = +dF/+dx).

What is important is that the increments should be small. It seems you want to use "total movement" as the displacement (either jounce, rebound, or both), which is plain wrong. This is why I said you seem to be completely missing the point regarding "rates".

Consider this Force-Displacement graph:

^Force
9...................o.o.............o.
8...............o.........o.........o.
7............o...............o......o.
6..........o...................o....o.
5........o.......................o..o.
4.......o.........................oo..
3......o..............................
2.....o...............................
1...o.................................
oo..1...2...3...4...5...6...7...8...9.
------------Displacement>-------------

Near the origin the rate is low. Around displacement=2 the rate becomes higher (ie. steeper slope). At about disp=5 the rate becomes zero. It is then negative to about disp=8.5. Thereafter it approaches infinite positive rate.

There is nothing unusual about this sort of curve. It is typical of "over-centre" linkages, like a "toggle-lock", or some of the newfangled archery bows with their pulley mechanisms. The important point is that it is meaningless to say that this curve has "some rate = some-force/some-displacement".

A car's suspension has a different shaped curve (a bit like above but without the middle bit). But, again, it is meaningless to suggest that it has "some-rate = some-force/some-displacement".

Z