Quote:
Originally Posted by Glider
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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Hi!
Hey, I'm going to chime in here on both of your initial questions.
*The answer to the first question will be geometry-based......while the answer to the second will be yet another anecdotal and opinion-derived one (with plenty of reference and data points, however) --- to add to all the input here on this thread.
To question 1:
I read this whole thread, and I don't think it has been brought up (or corrected),
but I do believe you have your calculation *backward*.
Raising the rear of the van will *decrease* positive caster, not increase it.
To start this explanation off best, I should point and and clarify for the point of discussion that modern cars and trucks (and motorcycles....and bicycles, etc) have what is referred to as
positive caster, NOT negative caster. And positive caster is measured in the number of degrees that the imaginary line through the upper and lower steering ball joints is tipped *rearward* at the top (relative to a dead-vertical axis).)
This was the simplest/easiest-to-digest diagram of caster angle (positive and negative) that I could find on the interwebz this afternoon:
It's easier to see when looking at that diagram how raising the rear of the van would actually make that initial caster angle begin to "close down" and become *less* of an effective positive caster angle (as it approached straight up and down vertical.....)
(Taking this further --- if you raised the rear of the van enough, eventually you could arrive at ZERO caster, which would not be an optimal arrangement for anything other than perhaps a shopping cart....)
At least that's how I read the tea leaves.
To question 2:
I've been through the wringer trying to get my recently-lifted 2WD van (6" Action Van lift, running 285/70R17 BFG KO2's) to drive properly. The steering has been super "squirrely", as you say. So I've gone through perhaps 3 separate alignment attempts, and on each attempt they've boosted the caster angle (which is done on the 2WD front ends using different eccentric sleeves that press into the upper ball-joint cradling location for the steering knuckle.)
We started out at under 3 degrees positive caster per side. Van was super darty-feeling...wanted to head its wheels nervously left-right when driving over even what seemed like perfectly-smooth pavement.
Next we upped it to somewhere between 3.5 and 4 (which the alignment shop techs declared was totally within Ford specs, as others have noted) -- and the van still drove identically sketchy.
So I did some digging --- and found the numerous RV forum discussions that others have found, where they swear that you won't see decent driving dynamics out of the wandering Ford front ends until you get up well over 5 or 5.5 degrees of positive caster. And they recommended the particular caster-adjustment sleeves (again, only for 2WD or TTB 4WD front ends, but the caster numbers should apply).
Additionally, I did some research with the guys that have experience with these lifted E-series vans....I called Larry at Action Van, and Jeremy at WeldTec and both guys seemed pretty consistent in their assertion that you really need around 6 degrees or more to make these drive right. (Echoing what Agile Off-Road specs their TTB front ends at, as well: 6 degrees.) Interestingly, Larry at Action-Van said "You can't have too much caster angle!" and said that even getting up near 7 degrees or more wouldn't do anything other than start to decrease turning radius.
(
Incidentally --- Mercedes is famous for spec'ing TONS of caster on their sedan's front end suspensions. My '06 Dodge Magnum SRT8 has a copy of the Mercedes suspension, and has
OEM caster specs that are around 10-11 degrees! High caster = high straight-line stability for high speed cruising. Autobahn geometry, in other words. Anyway --- back to the Ford vans....)
So---
I was sold on it at this point. Went ahead and picked up a set of those high-caster-angle-increasing adjustable sleeves and had the alignment shop bolt them in.
We got caster WAY up. I think somewhere between 6 and 7 degrees or so. And you can DEFINITELY feel the difference.
Steering effort is a bit more, for sure, and the tires contact the suspension a bit sooner in tight turns than they would have otherwise (since they "lay over" as they turn, with the higher caster angle....(to picture what I'm talking about, think about what the front wheel on a crazy extreme chopper motorcycle (with perhaps 70 degrees of positive caster head-tube angle!) does when it turns, it basically just flops sideways) --- but I'm way happier.
Overall result: The van now tracks WAY better. It's not perfect, to be sure --- and I think some of the wander that's present now is due to the worn-out rear leaf springs (that have a helper spring sandwiched into them, to no avail) -- and what I think are worn-out rear shocks. It feels as if the rear of the van sways left/right and I'm sure that loads/unloads the tires weirdly and contributes to some steering nervousness. (*That's on deck to be corrected soon with some brand-new Fox 2.0 shocks all around, and custom long-travel rear leaf springs that Jeremy at Weld-Tec is crafting.)
ONE more thing:
A really good old-school truck alignment guy once told me that one of the biggest contributors to straight-line stability is the tires you run, as well. Tread patterns that have a lot of straight-line grooves, front-to-rear, like Semi trucks run, are going to track straightest. And the more randomized the pattern of your tire tread gets, the less the van will naturally "run true" as it rolls down the highway at speed. The knobbiest of tires are going to be squirrel-iest. So that's a factor again as well. Bad-ass-capable off-road tires won't be the most bad-ass-tracking on-road tires. But that's kind of common sense in retrospect.
The whole thing is a crazy symphony of parts and pieces that needs coordinating.....orchestrating....to really deliver a top-notch driving experience. Fine-tuning.......
So anyway, there ya go.....yet another lengthy caster ramble...
...hopefully it provides more data points for your overall algorithm.
Thirsty
Silver Streak
Vandiesel II
Linear Mode
