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Old 01-17-2013, 02:42 PM   #16
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Originally Posted by grippgoat View Post
protc3,

I'm having trouble relating what you've said to my own experience, observations, and layman's understanding of physics.

I get the shopping cart analogy, but shopping carts have casters on the front wheel, and you steer them from the handlebar at the back. Because they're casters, the front wheels don't really generate any lateral grip, they just align to roll freely. If shopping carts had steering wheels, they'd handle best with the weight in the middle of the cart, no?

I've always thought of it being that, when the car is at speed on the track (that's what really matters) the point around which it rotates is defined by the CG location, and the lateral grip being generated by the tires. Of course, the lateral grip being generated by the tires is dependent on a whole boatload of things.

A car rotates because steering input causes the front wheels generate lateral force, which creates a yaw moment around the CG, which causes the rear tires to develop a slip angle and generate a lateral force to keep the car from spinning out.

The further back the CG is, the stronger the yaw moment the front wheels can generate is, and the more lateral force the rear need to generate to keep the car from spinning out. I guess when I think of it that way, your first explanation makes a bit more sense.

But I still don't get the statement "A car will rotate around the centerpoint of the rear driveline." At my track, the fastest cars (with the fastest drivers) are always visibly rotating around a point just a bit forward of the center of the car. It's especially noticeable on a 1/12 scale with a rollover antenna. It looks like the antenna is following a line around the track, and the car is just pivoting around it.

-Mike
All front wheel steering cars will rotate around the center point of the rear driveline. All wheel steering would change this but when you have fixed rear wheels, the rear would literally have to slide around the centerline of the car as it would no longer be the pivot point.

My example I used with the shopping cart was explaing how any car will rotate that utilizes a front wheel steering system. The front of a shopping cart indeed does use casters but the only difference is that there is no restriction or ackerman needed as they find the path of least resistance. This is why correct ackerman is so important as you can easily lose rotation by having the inner and out wheel fight themselves going through a corner. Hope this helps.
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Last edited by protc3; 01-17-2013 at 03:02 PM.
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Old 01-17-2013, 03:38 PM   #17
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Think of it this way...if the car were rotating around the CG
I didn't mean to say that it rotates around the CG, and I think I might be wrong about creating a yaw around the CG on turn-in. But the CG plays an important role in determining both the grip available to the tires, but also the forces that grip is working against to get the car through the corner.

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then that would be the fixed point and the front of the car would pivot in and the rear would pivot out.
That fixed point of rotation is what you see on the track with a really well-sorted car, though, even if it's not the CG.

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If that were happening you would see 2 things...first the rear tires would lose grip and secondly the front and rear tires would travel in closer to the same arc the closer the CG got to the center point of the car.
See Adrian's post.

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But this does not happen as the rear tires always travel in a tighter arc than the front.
That's the definition of understeer. So surely it's not always the case, and is in fact not the way you want the car to work.

All that being said, the initial yaw momentum generated on turn-in does have a large effect on the way the car rotates through the rest of the corner, especially on tighter tracks.

-Mike
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Old 01-17-2013, 03:51 PM   #18
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Jay and I are saying the same thing two different ways.

Ask he said the center of the rear axle will always follow the arc described by the front wheel angles if you have 100% traction and no tire slip.





However, all tires slip to some degree and in our cars we set them so in a smooth and predictable way the center of the rear axle will travel outside of the arc the front wheels are traveling mid corner. This is what we describe as rotation.
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Old 01-17-2013, 04:13 PM   #19
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This is a great website explaining the basics of all of this with animations so it makes sense.

http://www.racecartuner.com/03/207.html

Prepare to waste about an hour checking out all the tabs...lol!

Good video about tire slip angles.

http://youtu.be/JxGrwu0wqv8
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Old 01-17-2013, 04:14 PM   #20
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Quote:
Originally Posted by grippgoat View Post
I didn't mean to say that it rotates around the CG, and I think I might be wrong about creating a yaw around the CG on turn-in. But the CG plays an important role in determining both the grip available to the tires, but also the forces that grip is working against to get the car through the corner.
Right...where the CG is determines how much weight is on each tire. Since tires need some weight in order to have grip.

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Originally Posted by grippgoat View Post
That fixed point of rotation is what you see on the track with a really well-sorted car, though, even if it's not the CG.
Not exactly...I ran into this when I played iRacing and I thought their physics was messed up. It was a very long and complex answer that I got that I can't replicate but in summary it had to do with there is an optical illusion that takes place when viewing from an overhead that causes the car your looking at to appear to be pivoting around the centerpoint of the car.



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Originally Posted by grippgoat View Post
See Adrian's post.
Adrian's post and mine basically say the same thing but his is much more technical.



Quote:
Originally Posted by grippgoat View Post
That's the definition of understeer. So surely it's not always the case, and is in fact not the way you want the car to work.
Actually it isn't. Even in a well handling car the rear tires are going to have a smaller radius arc than the front tires. The only way to have front and rear travel in the same arc is with 4 wheel steering.

Understeer is when the front tires are turned in at a sharper angle than the car is turning at.

Quote:
Originally Posted by grippgoat View Post
All that being said, the initial yaw momentum generated on turn-in does have a large effect on the way the car rotates through the rest of the corner, especially on tighter tracks.

-Mike
Right...and moving the cg forward and back affects the yaw momentum. Or more accurately because we are moving in a arc the direction is constantly changing so you could refer to it as the change in momentum or acceleration. Moving the CG forward is going to make the front more difficult to change it's momentum when rotating making a more lazy feel to the steering because it is going to resist the change in direction more. Just like that old physics experiment with the pole with weights on the end. When you hold it in the middle and rotate it it will get easier to turn the closer to the center the weights are moved. The same holds true if you hold the rod at 1 end and use a weight at the other and try to turn it. The closer the weight gets to your hand the easier it is to turn the rod. This is much more like what we see in a car cornering where your hand would be the rear tires and the weight would be the CG.
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Old 01-17-2013, 04:58 PM   #21
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The following diagram shows the arcs the tires will travel in given no slip. As you can see the red arcs of the rear tires are well inside that of the green arcs of the front tires. Slip at the tire is going to move these slightly but for a well handling car it should not be much. If the rear is moving in the same arc as the front then it is slipping a lot and the car will be quite loose. This is why if your making a turn around a corner at low speed...if you turn right at the corner your rear wheels will hop the curb.
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Old 01-17-2013, 06:03 PM   #22
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I think we're all on the same page. I'm just questioning the real value of understanding the no-slip condition, because no-slip is slow.

-Mike
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