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Open Roads Forum  >  Tech Issues

 > Estimating Caster Using a Digital Camera

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Harvard

51.6N 114.7W

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Posted: 01/17/15 08:26pm Link  |  Quote  |  Print  |  Notify Moderator

This is a method estimating the front end caster using a digital camera and using Windows Paint to estimate camber angles from the picture.

Normally one would take 2 pictures from each wheel, one full left and a second full right. For this approximation we will assume both wheels are identical and simply use one camber picture from each wheel when the steering wheel is cranked full right.

NOTES ABOUT THE CAMERA SETUP:
1. Camera lens should be in line with the outside side of the wheel.
2. Camera should be mounted on top of a stand (I use a bin that sits upside down on the ground) that sits on the ground such that the camera is parallel to the vehicle. The vehicle needs to be parked on a flat surface BUT being level is not a requirement.

This first picture is taken in line with the outside of the drivers side wheel and measures a camber of NEGATIVE 2.5 Degrees.
[image]

This second picture is taken in line with the outside of the passengers wheel and measures a camber of PLUS 3.5 Degrees.
[image]

This picture is taken with the camera resting on the drivers side fender and captures the drivers side wheel to be cranked about 30 Degrees to the right.
[image]

This picture is taken with the camera resting on the passengers side fender and captures the passengers side wheel to be cranked about 33 Degrees to the right.
[image]

The estimated CASTER (K) is found as:
K = ( 180 / pi ) * ( ( C1 - C2 ) / ( T1 - T2 ) )

Where:
C1 is the +CAMBER when the DS wheel is FULL LEFT or the PS wheel is FULL RIGHT.
C2 is the -CAMBER when the DS wheel is FULL RIGHT or the PS wheel is FULL LEFT.
T1 is the +TURN ANGLE when DS wheel is FULL LEFT or the PS wheel is FULL RIGHT.
T2 is the -TURN ANGLE when DS wheel is FULL RIGHT or the PS wheel is FULL LEFT.


In this case:

K = (180 / pi) * ((+3.5 - (-2.5)) / (+33 - (-30)) )
K = 57.32 * (+6.0 / +63) = +5.5 Degrees

* This post was edited 01/20/15 07:54pm by Harvard *





ScottG

Bothell Wa.

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Posted: 01/17/15 08:47pm Link  |  Quote  |  Print  |  Notify Moderator

Actually that would be camber, not caster.


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Harvard

51.6N 114.7W

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Posted: 01/17/15 08:56pm Link  |  Quote  |  Print  |  Notify Moderator

ScottG wrote:

Actually that would be camber, not caster.


Caster is derived by taking two camber readings, one (+camber) is from full left and the second (-camber) is full right. Then the two camber readings are used in the caster formula as shown.

ScottG

Bothell Wa.

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Posted: 01/17/15 09:09pm Link  |  Quote  |  Print  |  Notify Moderator

Caster angle.
You can have all the camber angle in the world and no castor.
Or you can have way too much camber on truck suspensions without effecting caster much. That is what happens when the suspension components are badly worn - you can see it by the way the wheels tilt inward badly.
What you are measuring is purely camber:
Camber angle
Also, you can't really tell much with the wheels turned because camber changes - especially if ball joints are worn.

Harvard

51.6N 114.7W

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Posted: 01/17/15 09:14pm Link  |  Quote  |  Print  |  Notify Moderator

Quote: "you can't really tell much with the wheels turned because camber changes"

Yes, the camber changes because of the caster angle. [emoticon] By taking a camber reading at full left and another camber angle at full right we can derive the caster angle.

Harvard

51.6N 114.7W

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Posted: 01/17/15 09:20pm Link  |  Quote  |  Print  |  Notify Moderator

See formula (8) in this Reference

ScottG

Bothell Wa.

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Posted: 01/17/15 09:23pm Link  |  Quote  |  Print  |  Notify Moderator

Your link actually supports what I'm saying.
But there's no shop in the country that would measure angles in that manner. Your numbers are inaccurate because your turning the wheels and trying to extrapolate straight ahead angles.
We'll have to agree to disagree but your front end is either worn or is sagging badly.

VintageRacer

Tatamagouche Nova Scotia

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Posted: 01/18/15 05:39am Link  |  Quote  |  Print  |  Notify Moderator

That is exactly how, in principle, caster is always measured in a shop. I would use turn plates (two pieces of plywood with some grease or a plastic bag between them), I would turn 20 degrees each way rather than full lock, and I use a digital protractor or a camber gauge to measure the camber, but that is in essence exactly how caster is measured. Race shops all over the country do that all the time. Astounding precision in the measurement technique isn't required, because as long as you do it the same way for all the measurements the errors tend to cancel out. For years I measured camber on my race cars with a level and a tape measure and a little math.

Brian


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j-d

Sunny Florida USA

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Posted: 01/18/15 06:18am Link  |  Quote  |  Print  |  Notify Moderator

I think we're on different sentences on the same page. Of the three primary angles, Caster/Camber/Toe, Toe is the only one that can be set independently. At least without modifying the geometry that's built into the chassis. Changing Caster to get more with be limited by acceptable Camber.

I visited John at the iconic Henderson's Line Up in Oregon. He worked in the shop 25 years, doing loads of alignments. Said "Caster is the Change in Camber as you Steer."


If God's Your Co-Pilot Move Over, jd
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JaxDad

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Posted: 01/18/15 06:20am Link  |  Quote  |  Print  |  Notify Moderator

Harvard wrote:

Quote: "you can't really tell much with the wheels turned because camber changes"

Yes, the camber changes because of the caster angle. [emoticon] By taking a camber reading at full left and another camber angle at full right we can derive the caster angle.


Seems like a valid way of doing it to me. I think my western friend has it right.

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