What causes wheel migration to the axle head?

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Vitamin K
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What causes wheel migration to the axle head?

Post by Vitamin K »

So, by now, most derby car builders are familiar with the wheel migration test: Namely that if canted rear axles are properly aligned, the wheels should migrate towards the head of the axle when the car is rolled forward or back.

The question that I have is: What causes this? What's the physical principle at play that would cause the wheel to move away from the body of the car, and why would toe-in or toe-out negate this effect?

Help me out, physics people. :D
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whodathunkit
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Re: What causes wheel migration to the axle head?

Post by whodathunkit »

VK,
I'd say the weight on the car has a lot to with with the wheel migration as the car is moveing forward or backwards in the angled axel.
As the car rolls forward toe-in would drive them out.. and toe-out would drive them in.

In a rev roll.. the toe-in( faceing the front of the car ) would drive the wheels in because of the toe-out.

Even with good alignment (no matter the toe!)
If you take the weight off the canted rears wheels.. in a forward or backwards roll the wheels will drive back in to the car body.

You can see this when wheel drop out from under the car in the stoping pit.
What type of automobile can be spelled the same forwards & backwards?
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Stan Pope
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Re: What causes wheel migration to the axle head?

Post by Stan Pope »

I understand the reason for your question ... the wheel has slightly outward force due to the axle slope, BUT the wheel has two bits of friction pushing it to stay where it is! The bore/axle friction is probably inconsequential, but the wheel/track friction should substantially exceed the force due to axle slope! So, how can the wheel drift out?

I think that the answer is not that the axle slope overcomes the wheel/track friction. Rather, the axle slope force minutely steers the wheel, which can happen because the wheel is rolling and its small contact patch and small inertia can easily be overpowered.
Stan
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Vitamin K
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Re: What causes wheel migration to the axle head?

Post by Vitamin K »

I think what's got me curious is determining how toe-in or toe-out for a forward rolling wheel on a negatively canted axle either influence the movement of the wheel away from or towards the body of the car. Conventional Pinewood Wisdom tells us that if the wheel has neither toe-in, nor toe-out, it should migrate to the head in both directions. What I want to know is how accurate the wheel migration test is in really determining the wheel's alignment, and to do so I need to unpack the mechanism behind why the wheel moves as it does.
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Re: What causes wheel migration to the axle head?

Post by Stan Pope »

I think you will find that the "null" widens as the camber becomes more negative. If true, it means that accuracy degrades as the camber becomes more negative.

If my "model" (from my first response) is correct, then the more negative camber increases the "to the nail head" steering force, and overcomes the inclination to "follow the toe" to move to the body, even as the toe angle increases from zero.
Stan
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LightninBoy
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Re: What causes wheel migration to the axle head?

Post by LightninBoy »

Vitamin K wrote:I think what's got me curious is determining how toe-in or toe-out for a forward rolling wheel on a negatively canted axle either influence the movement of the wheel away from or towards the body of the car. Conventional Pinewood Wisdom tells us that if the wheel has neither toe-in, nor toe-out, it should migrate to the head in both directions. What I want to know is how accurate the wheel migration test is in really determining the wheel's alignment, and to do so I need to unpack the mechanism behind why the wheel moves as it does.
It's an approximation only. But a good one.
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davet
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Re: What causes wheel migration to the axle head?

Post by davet »

whodathunkit wrote:VK,
I'd say the weight on the car has a lot to with with the wheel migration as the car is moveing forward or backwards in the angled axel.
As the car rolls forward toe-in would drive them out.. and toe-out would drive them in.

In a rev roll.. the toe-in( faceing the front of the car ) would drive the wheels in because of the toe-out.

Even with good alignment (no matter the toe!)
If you take the weight off the canted rears wheels.. in a forward or backwards roll the wheels will drive back in to the car body.

You can see this when wheel drop out from under the car in the stoping pit.
I believe my experience has been the opposite. A car rolling forward with toe-in will have wheels migrate to the body. The outside edge of the wheel is now getting the pressure exerted on it as it rolls forward which pushes the wheel in.
If the wheel has toe-out, the inside edge is now being pushed by friction and wants to force the wheel out.
Take your DFW and run the mark at 9:00 and then at 3:00 and you'll see it work.
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sporty
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Re: What causes wheel migration to the axle head?

Post by sporty »

A real world.
A car properly aligned.

The rears do not move. In or out.

But that's so hard and darn near impossible. So we have them migrate out some. And I mean some. Just because they migrate out. Does not mean they are good or aligned.

Axle head friction / wheel hub.

That's why track. Timer testing is a factor over board testing.

The test. Timed runs. After about 8 to 12 runs. Track time drops. If more so than usual. Can be a indicator to look there. Not just at axle or wheel bore prep. Or your burnishing. Application process.

Oh. The tiny areas. We must not over look.
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davet
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Re: What causes wheel migration to the axle head?

Post by davet »

If you could get a wheel to ride between the head and the body (perfect world) it could in no way maintain that down a track. Once a slight track imperfection, wind turbulance from neighboring car or the transition itself cause the car to deviate even slightly, one wheel would now move to either the body or the head and the opposing rear would move the opposite direction. Once they have moved to their new location, the very method used to keep it off both the head and the body in the first place would now force the wheels to stay at their new location. The new location would be one wheel against the head and one wheel against the body. Both have differnt co-efficients of friction so the car would now be rear steering.
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