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Old 04-22-2017, 12:24 AM   #1
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Fun With Caster

Clearly, I don't have enough going on to keep me occupied...

I was just contemplating the impact, on caster, of raising or lowering the rear of the van. Here's what I got:

The change in caster should be the arctangent of:
[the change in height at the rear axle/the wheelbase]

(This is because the tangent of an angle in a right triangle is the length of the side opposite the angle--in this case, the change in height--divided by the length of the side adjacent to the angle--in this case, the wheelbase.)


For example, for an increase of 1" at the rear axle, and a wheelbase of 138":

ARCTAN(1/138) = .415 degrees

Consequently, if I have my head on straight, then raising the rear of a van by 1" should increase the caster by .415 degrees, or nearly half a degree.

Similarly:
.5" height increase would increase caster by .208 degrees
.75" height increase would increase caster by .311 degrees
1" height increase would increase caster by .415 degrees
1.25" height increase would increase caster by .519 degrees
1.5" height increase would increase caster by .623 degrees
1.75" height increase would increase caster by .727 degrees
2" height increase would increase caster by .830 degrees

Which means that loading down the van and dropping the rear by similar amounts would cause the caster to decrease in the same manner.

Now, for the wiser heads in the group:
1. Do I have this right?
2. How much does it matter?

Thanks!
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Old 04-22-2017, 05:57 AM   #2
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Hmmm. I think you are the wiser heads in this group.
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Old 04-22-2017, 07:15 AM   #3
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Can only make this observation:

If- you determine the ArcTan calculated to three Significant Figures (.415); Then- you are disallowed the use of 'Nearly' when evaluating that number!?
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Old 04-22-2017, 09:12 AM   #4
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Quote:
Originally Posted by EmMay View Post
If- you determine the ArcTan calculated to three Significant Figures (.415); Then- you are disallowed the use of 'Nearly' when evaluating that number!?
EmMay, I humbly bow to your impeccable logic.
-Bianca
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Old 04-22-2017, 10:33 AM   #5
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Now, doesn't the caster angle effect the steering and handling of the rig, predominantly at higher speeds?
If all those numbers calculated end up less than a full degree, won't that fall into a range that is considered a 'norm' for the rig?
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Old 04-22-2017, 10:36 AM   #6
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Quote:
Originally Posted by EmMay View Post
Now, doesn't the caster angle effect the steering and handling of the rig, predominantly at higher speeds?
If all those numbers calculated end up less than a full degree, won't that fall into a range that is considered a 'norm' for the rig?
My questions, as well. Which is why I'm hoping one of the wiser heads will chime in.
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Old 04-22-2017, 11:54 AM   #7
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Quote:
Originally Posted by 86Scotty View Post
Hmmm. I think you are the wiser heads in this group.
In my case, I think a more appropriate diagnosis would be, "chick with a van, a couple of end wrenches, and an overactive imagination."
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Old 04-22-2017, 11:55 AM   #8
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Caster matters. A radius arm suspension goes through changing caster values depending on where it is at in the suspension travel. I don't think it is anything to worry about as long as you're within a range of values that would be considered acceptable.

From 2005 to 2016, Ford specified 1.7deg to 2.7deg caster at ride height. In 2017 they upped that value over 3deg. Personally, I don't think that is enough, especially when you lift the vehicle (and in most cases...have more suspension travel). I like to start with a higher value at ride height, 3.5-5.5deg. That way I'm pretty comfortable with the range of caster I'm likely to encounter during normal highway driving conditions. Only in extreme cases will you be at full droop (lowest value of caster in the travel), and you'll likely be at lower speeds when/if it happens.

I know that my van drove like absolute crap with 1.5deg caster (how it came from Quadvan). My van with my new suspension drove like a dream with 3.5deg caster... now I have an even more updated setup, modified radius arms and ~5deg of caster at ride height. Not a huge amount of difference in how it drives but now I know that my caster values won't go too low.
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Old 04-22-2017, 12:15 PM   #9
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^^^Thank you, MG, that's really helpful. Somebody said something to me once about increasing caster--that if it goes too high, it can cause wheel chatter. What are your thought and observations on this question?
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Old 04-22-2017, 01:33 PM   #10
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That's also why Ford pays for an first post-delivery alignment for motorhomes. By the time 6,000 pounds of box is added to the E450, then the owner loads up another 2,000 pounds of "stuff" and water, the rear end can sit many inches lower than original, and the front axle is often off-loaded, because motorhomes are so tail heavy. The worst ones end up with almost zero caster until they get re-aligned. But of course, most Ford dealer still only "bring it within spec" but the good ones know the max-caster trick.
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