Smart questions
Smart answers
Smart people
Join Eng-Tips Forums

Member Login

Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

tejay10 (Automotive) (OP)
19 Jun 08 8:30
I'm a little confused about the roll gradient equation given by Milliken in Race Car Vehicle Dynamics.  A simplified version is at the bottom of p586, and the complete version including some derivition is found on pages 681-682.  

My question came about when I was correlating some real world data to the output of the equation, and was out by a factor of 10.  Milliken's equation states the result in rad/g, however I believe the acceleration units shouldn't be normalized to g's?  So for example, if I'm working in metric units, the result should be rad/ms-2?

The force due to the lateral acceleration is F=MAy, with M being the unsprung mass, and Ay being the lateral acceleration (actual acceleration, not g's).  This creates a moment about the roll axis which is H distance below the CG: Moment = FH = MAyH.  So, if we have roll stiffness K in Nm/deg, then Moment/K = MAyH/K, so deg/Ay = MH/K.  At no stage is Ay normalized to arrive at this equation.

Have I missed something obvious?
NormPeterson (Structural)
19 Jun 08 12:38
If you're not solving for roll gradient normalized to lateral g's why would you expect your answers to numerically match them?

I hope this isn't what it sounds suspiciously like . . . and I suspect that your answers aren't "out" by exactly 10.0 either.

GregLocock (Automotive)
19 Jun 08 18:39
Check your units


Greg Locock

SIG:Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips.

datstang (Mechanical)
21 Jun 08 7:59
One of the hardest things I've found about the Milliken book is his non-standard use of symbols (at least from my experience).

When Milliken says ay, he means the acceleration due to gravity.

When Milliken says Ay, he means ay/g.

His use of symbols is listed on the inside of the front cover.
datstang (Mechanical)
21 Jun 08 8:28
Edit of previous post:

ay is lateral acceleration, not acceleration due to gravity.

I should never post before my first cup of coffee.
GregLocock (Automotive)
21 Jun 08 9:14
True, the units used in each equation do vary a bit from section to section, but if you read each example properly they are internally consistent, or, at least, I haven't found any significant errors.

Perhaps you can't plug and chug like you can from a textbook. I'm not convinced that is a bad thing.



Greg Locock

SIG:Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips.

datstang (Mechanical)
21 Jun 08 9:45
Agreed, you are forced to question the origin of every term in every equation and check the units.  You have to go through the book with your brain engaged.  I guess that is why I am chewing my way through this book so slowly.
BobM3 (Mechanical)
22 Jun 08 11:40
Just curious (I haven't read the Milliken book and don't know alot about vehicle dynamics), but why do you only look at the unsprung mass when calculating lateral loads?  Are you only interested in frequencies above the 1-2 hertz body natural frequencies?
GregLocock (Automotive)
22 Jun 08 19:29
Grins. Ah that was very subtle Bob.


Greg Locock

SIG:Please see FAQ731-376: Forum Policies for tips on how to make the best use of Eng-Tips.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close