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  #16 (permalink)  
Old 09-02-2012, 09:29 AM
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Brian Martin,Freelance adviser
 
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In that photo I don't see anywhere near 7 degrees, so either you changed it from the photo or it's just an odd angle and I can't see it. But 7 degrees is quite a bit and you WILL see that just looking at an axle in a car.

The ackerman angle is a basic principle that when you round a corner the inside tire is following a tighter radius than the outside tire.

Like at a track meet, the inside runner has to have their starting point back from the outside running because they are going to be running on a shorter lane, right?



The inside lanes are noticeably shorter than the outside lanes.


Just as the track when your car goes around a corner, the inside tires are taking a shorter route. The rear end differential allows the inside tire to roll slower than the outside. If you had a locked rear end the tires would be squealing skidding on the ground fighting against each other as you went around a corner right? The front is doing the same thing on your car. The not only is the inside tire not turned sharper than the outside but it is literally not turning as much! Your left tire is is turning LESS than your right tire when you make a left turn!

This can make a car do all kinds of odd things as you are going in a straight line as your wheels are wanting to to in different directions every so slightly but that is what is happening with just a little turn of your steering wheel.




There are tons of cars out there with the setup as you have so it "works" for some people, maybe with wider tires it is less noticeable? It may "work" but it is NOT correct, it is a horrible mistake when we talk about steering geometry, it is WRONG.

On the caster issue, most old cars have very little caster, 1-3 degrees. The more caster the more it likes high speed in a straight line. The more caster, the less it "likes" to make turns. As an example a high end BMW seven series which is made for those high speed autobahns will be 6 or 7 degrees while a little BMW 3 series will have only 3-4 because it is more of a "canyon carver" kinda car.

So the more faster the straighter it should go and the less rolling resistance. A land speed record kinda car will have as much as 18 degrees. I remember the old rails use to run so much caster the tires would flop over side to side instead of turn! But you don't see that anymore so they must have learned that is just isn't worth the loss of control for the little less rolling resistance they gave up with less caster.

How are you measuring the angles?

Brian
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Old 09-02-2012, 10:22 AM
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castor and kingpin

Castor angle and king pin inclination have to work together. bend a paper clip into a T shape, to simulate kingpin and spindle at 0 castor . rotate the spindle end, it stays the same height as the wheel turns. (Some heavy equipment geometry) Now tilt it back to simulate castor with a 90 * kinpin inclination. as you turn the spindle end moves closer to the ground in one direction and farther away the other direction, as the spindle tries to move closer to the ground it raises the vehicle, the weight tries to move the wheel back straight ahead and the inertia can move the wheel past straight and it will lower that side, The Front end can start to "dance". now bend the paper clip to simulate king pin inclination, at 0* castor. as you turn either direction the end of the spindle tries to move closer to the ground, raising the vehicle, vehicle weight tries to return the wheels to straight ahead whether you turn left or right. Now with your king pin inclination add in some castor, as you turn you will get differents amounts of lift depending which way you turn , with a rigid chassis the weight on that side will help point the wheels back straight. as you add more castor you can reach a point where you will get a lot of lift in one direction and Drop in the other direction. that tire will try to "float" no directional stability when turned one direction for that wheel.. Some of the 70's drag cars had new spindles made with around 15 degrees king pin inclination so they could run a lot more castor before reaching the "float point". I the early 60's I tried to explain what was wrong with the local engine builder's Studebaker Bonneville racer front end geometry. as I left I said you will get that thing upside down !!. Two weeks later He ran 5 MPH over the record , then on his back up run it started to "dance" rolled several times, The hemi stayed under the hood but the bellhousing broke apart and the 4 speed went out the backwindow breaking his arm on it's way thru.
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  #18 (permalink)  
Old 09-02-2012, 10:25 AM
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Brian Martin,Freelance adviser
 
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Quote:
Originally Posted by timothale View Post
Castor angle and king pin inclination have to work together. bend a paper clip into a T shape, to simulate kingpin and spindle at 0 castor . rotate the spindle end, it stays the same height as the wheel turns. (Some heavy equipment geometry) Now tilt it back to simulate castor with a 90 * kinpin inclination. as you turn the spindle end moves closer to the ground in one direction and farther away the other direction, as the spindle tries to move closer to the ground it raises the vehicle, the weight tries to move the wheel back straight ahead and the inertia can move the wheel past straight and it will lower that side, The Front end can start to "dance". now bend the paper clip to simulate king pin inclination, at 0* castor. as you turn either direction the end of the spindle tries to move closer to the ground, raising the vehicle, vehicle weight tries to return the wheels to straight ahead whether you turn left or right. Now with your king pin inclination add in some castor, as you turn you will get differents amounts of lift depending which way you turn , with a rigid chassis the weight on that side will help point the wheels back straight. as you add more castor you can reach a point where you will get a lot of lift in one direction and Drop in the other direction. that tire will try to "float" no directional stability when turned one direction for that wheel.. Some of the 70's drag cars had new spindles made with around 15 degrees king pin inclination so they could run a lot more castor before reaching the "float point". I the early 60's I tried to explain what was wrong with the local engine builder's Studebaker Bonneville racer front end geometry. as I left I said you will get that thing upside down !!. Two weeks later He ran 5 MPH over the record , then on his back up run it started to "dance" rolled several times, The hemi stayed under the hood but the bellhousing broke apart and the 4 speed went out the backwindow breaking his arm on it's way thru.
VERY interesting point and well explained.

Brian
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Old 09-02-2012, 10:35 AM
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bruce info

Bruce Cambern as I noted can take 3 or 4 hours explaining the New Geometry and setups he uses on his Cobra for different tracks, but doesn't like traveling all over the country any more, stays mostly in Calif races any more.
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