DeTomaso Mailing List: July 97, Message #42
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Mad Dog, Here is some more info on caster. Caster is the angle between the kingpin (or ball joint axis) and vertical. Positive caster is when the upper ball joint is to the rear of the lower ball joint when viewed from the side. Moter cycles and bikes have a lot of positive caster. Caster creates mechanical trail, which puts the tire foot print behind the axis of the ball joint. So the tire is 'following' its mounting axis. Look at the wheels on your office chair, no matter what direction you move the chair, the wheels follow their mounting point. Caster causes the tire side forces to create a moment (torque) about the ball joint axis, to center the wheel. So when the tire turns, a force is created to center the wheel. Going back to the bike example, with lots of caster the bike wants to go stright, so it is easy (?) to ride with no hands. In engineering terms this is called dynamic stability. A draw back of caster is that it increases steering forces, which is OK for high speed driving. Due to the suspension geometry, as the car turns left, the left front tire will rise and the right front will lower, causing the car to roll, causing oversteer. This is worse with stiffer springs. In theory you should be able to see this on a good alignment rack, but I have not tried. Caster causes favorable steer-camber. In a turn the outside wheel will get more negative camber and the inside wheel will get more positive camber. Again this is due to the geometry, assuming positive caster. Pneumatic trial adds to mechanical trial. Pneumatic trial is when the tire contact patch is behind the centerline of the wheel, due to flex in the tire. In a hard turn the pneumatic trial approaches zero as the tire reaches its limit, reducing the self-centering torque. This can signal to the driver that the tire is near break away. If you want more info on caster or any other suspension topics see the book, "Racecar Vehicle Dynamics" by W.F. Milliken and D.L. Milliken. I hope this doesn't make me sound to much like an engineer. Scott Martin On Mon, 30 Jun 1997, Dennis Antenucci wrote: > S. Martin wrote: > > > > I assume he meant add more caster. The bike you had when you were a kid > > had alot of caster, which is what allowed you to rid with no hands on the > > steering wheel, because it was very stable. > > ***I may be the only guy left who still doesn't get it. . .But I'll ask > anyway. The question still begs to be answered which is, how does added > caster improve high speed stability?> > > > More on aerodynamics: Adding an air damn and adding a way for the > > radiator air to excape are the first steps. The next step is to work on > > the under car aerodynamics. Copy what Le Mans cars and F-1 does, make the > > bottom flat and add a diffuser at the rear. This will drastically reduce > > the drag and add some down force. I was at Gary Halls shop about 8 years > > ago, Mike Cook and him were working on making a flat bottom Pantera. He > > said that was the only way to get above 200 MPH (this was before people > > started using the monster motors). > > > > Make sure your air damn covers as much of the front tires as possible, > > also make sure the rear flares cover as much as possible of the rear > > tires. Tires that are out in the air stream create alot of drag. > >***I've solved the problem with how to vent air from the radiator > (answer: raise and secure front hood 2-3" inches). That mod worked > effectively at the May race. However, I am still researching the air dam > question (of how to get air around and not under the car). I know I'm > gonna sound as if I'm in denial on the full under-tray idea, but let me > ask you and the others more questions. . .How effective will adding the > air dam be (towards reducing air under the car) with out the full > under-tray? And the diffuser? > > >***Scott, thanks for the feedback and support. It's appreciated.Now if I can jus' get the bike to keep from falling over when I take my > hands off the steering wheel! :-> > > Mad Dog > > > > On Sat, 28 Jun 1997, Dennis Antenucci wrote: > > > > > Yo William, > > > Dan (Aero-Man) Jones or Mike Drew or one of the other literate > > > aerodynamics/suspension Guru's on the net should be able to answer your > > > question on added camber(?), (do u mean caster?) increasing high speed > > > stability. . . I don't get it. It's all Greek to me! > > > > > > Mad Dog > > > > ==============================================================================