front-end geometry

Courtney Weeks · Joined: 12 Sep 2001 Posts: 3 Location: Boulder CO

Seeing someone else post a homework problem prompted me as well...I'm doing a physics paper on the front-end geometry, and the only aspect about which I can find little information is castor. I was wondering if anyone can give a good explanation using equations to explain how and why it works...Thanks in advance guys.
Court

Brian Kay · Posted: Thu Sep 13, 2001 6:03 am Post subject: front-end geometry

Courtney,

Definately post this question in the "Chassis and Handling" forum. Some real chassis nut-jobs in there. Have been amazed at the content available.

Wish I had such cool projects way back when I was in school!

Brian K - T2 RaceKarts

Pete Muller · Posted: Thu Sep 13, 2001 7:36 am Post subject: front-end geometry

quote:
Originally posted by Courtney Weeks:
.... the only aspect about which I can find little information is castor. I was wondering if anyone can give a good explanation using equations to explain how and why it works.

It's a plant that grows about 6 to 15 feet tall... and the fruit or "bean" is used to make an oil many of us mix with gasoline.

Sorry Courtney, couldn't resist.

Doug Welch · Posted: Thu Sep 13, 2001 8:22 am Post subject: front-end geometry

Two places to look, this site has a good article on it, look on the home page. another site to try is below The Physics of Racing Its called the physics of racing series.
If the link doesn't work, its because I orginally put a comma in at the end. I edited it out, but my browser hasn't figured out that it is not there. Delete the comma and it should go.
Doug

[ September 13, 2001: Message edited by: Doug Welch ]

[ September 13, 2001: Message edited by: Doug Welch ]

Courtney Weeks · Joined: 12 Sep 2001 Posts: 3 Location: Boulder CO

Thanks so much for all your help guys! And I gotta say, I was wondering how long it would make for someone to make the oil joke...not too long at all, I suppose! Thanks again,
Court

John Denman · Posted: Fri Sep 14, 2001 1:00 pm Post subject: front-end geometry

Castor alone would be an incomplete on a physics paper...good thing I am not grading!

The following is a summary on some things you may want to eventually consider;

X-Axis runs from left to right
Y-Axis runs from back to front
Z-Axis runs from ground up

Related and affecting castor would be;

Scrub Radius - this is the moment arm length from the fulcrum point on the ground to the center of the tire contact patch on the ground. Since the kingpin is tilted, extend this line in its natural direction to the ground to find point "A". Center of contact patch of tire is point "B". Changes to caster and camber will change Scrub Radius length. (X-Axis)

This is important as it determines the leverage or mechanical advantage of the caster change in the Z-Axis.

Caster -I will let you research this.

Camber -I will let you research this.

Virtual Swing Axle Length (extra credit) -Tony figured this out, but few others (including Trackmagic) have. Its easier to understand looking at an Indycar with suspension.

On the left front suspension take the top wishbone and draw a line from the outer ball joint to the shear plate rod end, and extend the well line past the right wheel. Do the same for the bottom wishbone, and at some point these two should intersect. (Its possible they may be dead parallel, and thats OK too.) If they aren't parallel (and almost never are) these two lines will intersect. Do the same for the right front.

Note the length between the intersecting points on the X-Axis for the left and right side suspension. Thats the Front Virtual Swing Axle Length.

Repeat the exercise for the rear.

The Front and Rear VSAL should be the same. Now look at the Z-Axis relationship. There is a proportion that tends to be higher on the end that has a lower true Center of Gravity.

Now for the magic. On a suspended car as the suspension rises or droops the VSAL changes. Same with a kart only it occurs as the chassis flexes. Under braking the front drops and the rear rises. The X-Axis change of VSAL will also change the moment angles.

In changing these moment angles it will affect the caster. A really good Suspension Engineer can use subtle changes in VSAL to dial in a superior handling chassis that will remain very drivable in changing track conditions.

Although a kart has no suspension the chassis does flex. How and where it flexes is a very exact science.

Carrol Smith wrote a book called Tune to Win. It covers all these topics.

Barry Hastings · Posted: Fri Sep 14, 2001 3:13 pm Post subject: front-end geometry

I must reread my Tune to Win, and find the thing you are talking about with the Tony. I remember reading it, but must have for gotten it when converting the knologe to karts. I like what Carroll Smith writes, reccamed it to anyone. I agree with him on lots of points, one of wich proved to myself. Run what works even if you dont like it, and if it isnt theroretical best. Run what works.