My caster mod (with pictures)

[SCOTI]

Originally Posted by gringoloco

Research has led me to believe they gain caster via the uppers, which would pull the wheel back further.

My eyes could be deceiving me but look @ the distance for each end of the shaft on the blue CPP arm. Notice it's not centered (more front side overhang). Also, notice where the coil seats? It's further back vs where the OE arms spring pocket is.

The unmodified side on crakarjax's OE arms set-up is centered (top of pic). The side w/the modified cross shaft is no longer centered (more front side overhang).....

[picklito]

Originally Posted by crakarjax

Should be some basic geometry if you know the distance between the upper and lower balljoints, and the current angle.

Assuming 9" between ball joints and a currently vertical spindle,

3.14 * (4.5^2) = 64 (radius of a circle)

360 * (64 * .25) = 1.4 degrees (360 degrees in a circle times the fraction of the circle that is 1/4")

So 1" should net about 5.6*, 3/4" would be around 4.2* gain

Looks like you calculated the area of a 9" circle. I'd do the calculation by 'assuming' that we're swinging the lower ball joint forward around the circumference of a 9" circle with center at the (fixed) upper ball joint.

Circumference of a 9" circle would be: 3.14 * diameter = 28.26"

degrees per inch moved (around that circle) would be 360 / 28.26 = 12.7

This ignores the cosine error, of course, making my calculations a little larger than we'd actually get. So, compared to a vertical spindle...

moving 1.0" equals 12.7 degrees caster
moving .75" equals 9.5 degrees
moving .5" equals 6.35, etc....

[BMERDOC]

Uhm yeah, just goes to show how bad I am with Geometry. Picklito, are you sure about those numbers? Using my truck as an example again, my LF has 2.7 degrees of positive caster. According to your numbers just moving the index .5" would make my caster 9.05. That couldn't be right. Or am I reading it wrong and total caster would be about 6.35?

[picklito]

No, you're right that you'd have 2.7 + 6.35 degrees of caster. You're also right that nobody is getting that much caster increase. The math is right, the numbers are right, but the model it's based on is poor and doesn't really represent what actually happens in the truck...

We're assuming the lower ball joint is moving around the circumference of 9" circle. It actually moves straight forward. And we'd need to use the center-to-center distance between ball joint pivots. If that's (just guessing) 12" rather than 9", then the circumference is 37.68" rather than 28.26" and moving the control arm forward by .5" produces 4.77 degrees of additional caster rather than 6.35. Then add in the (small but real) cosine error and you'd get slightly less. So,

2.7 + 4.77 = 7.47 degrees of caster in your case before removing the cosine error. That's probably closer?

[picklito]

Crakarjax,
In this photo of your drilled hole, how far is the center of the hole from the end of the shaft? It looks FAR! In light of the recent discussion about all the different hole locations we're finding... did you end up farther than 2"?

[crakarjax]

Originally Posted by picklito

Crakarjax,
In this photo of your drilled hole, how far is the center of the hole from the end of the shaft? It looks FAR! In light of the recent discussion about all the different hole locations we're finding... did you end up farther than 2"?

Good catch on my math, not sure why I was using the formula for area! But I was just trying to illustrate how it might be calculated and is simplified, idk the actual spindle distance or anything.

If I recall, the center of that hole is at 2.25"

[picklito]

Originally Posted by crakarjax

If I recall, the center of that hole is at 2.25"

So your caster is gonna be very good! But the reason I asked is that being over 2" is probably why you had to clearance so much compared to some of the other guys.

[SCOTI]

Originally Posted by picklito

So your caster is gonna be very good! But the reason I asked is that being over 2" is probably why you had to clearance so much compared to some of the other guys.

x2.

[picklito]

SCOTI,
You mentioned previously that you didn't have to trim a whole bunch. Do you remember where you put the hole from the end?

[SCOTI]

Originally Posted by picklito

SCOTI,
You mentioned previously that you didn't have to trim a whole bunch. Do you remember where you put the hole from the end?

http://67-72chevytrucks.com/vboard/s...d.php?t=487363

2". See post #273 & 274 in the above thread for dimensions w/pics.....

[crakarjax]

Originally Posted by picklito

So your caster is gonna be very good! But the reason I asked is that being over 2" is probably why you had to clearance so much compared to some of the other guys.

Well, originally I thought I needed to go .75" from my replacement shaft holes and it seemed like a lot to clearance -- then I realized they were already .25" from stock so I only moved .5" which should give me significantly less to shave. I haven't clearanced yet but I'll take before/after pics to see exactly how much you'd need to trim for 2.25" out.

[picklito]

Originally Posted by SCOTI

http://67-72chevytrucks.com/vboard/s...d.php?t=487363

2". See post #273 & 274 in the above thread for dimensions w/pics.....

Doh! I had just read that the other day. Thanks.

[picklito]

Mine arrived today. Moog K6147. Haven't yet measured the one in the truck.

[solidaxel]

Waiting to see if I order them !

[crakarjax]

Originally Posted by picklito

Mine arrived today. Moog K6147. Haven't yet measured the one in the truck.

Interesting, my stock shafts are 1.5", replacements are at 1.75", and so I drilled my holes at 2.25". Are most stock shafts drilled at 1.25"?

[crakarjax]

So, I trimmed my arms with 2.25" to center holes and didn't have to take all that much off. I did not trim my crossmember at all. (see pics)

I have a question that is sort of unrelated to this caster mod though (sorry to be off topic) -- if you look at the pics of my lower arm with the bumpstop on (pic 3) vs removed (4th pic), it looks like there's no way the shock could be mounted at that much droop because the shaft would need to go through the crossmember. Am I right? Obviously this is affected by moving the control arm forward...

I removed the stop because I read that was necessary when bagging to get max drop. I'm planning to bag with plates and I can't see ever dropping that much just due to the thickness of whatever bag I use. How much clearance between the lower arm and the crossmember can I expect when aired out with that kind of setup?

[picklito]

Originally Posted by crakarjax

Mine are TRW 13168A made in USA

Are these the ones at 1.75"? Now I'm surprised that my Moog's are at 1.25". Oh well, doesn't really matter since I'd be drilling anyway. However... for stock replacement parts... what the heck is going on here? I've never seen so much variance in other parts over way too many years. Can you imagine if a non-hobbyist had to have just one side rebuilt for some reason? He could have alignment issues forever and I GUARANTEE YOU that the average shop tech wouldn't be able to find it!

Overall, thanks to the awesome people on 69-72 we've uncovered a lot of good info. Pretty clear now why some are having so much interference. If a guy happened to have 1.75" shafts and tried to do the "3/4-inch mod" he'd have problems. Sounds like, regardless of where we start, 2 to 2.25" is where we want to land (with the real goal of about 7 degrees of caster).

Awesome post and pics, above, by the way. Super helpful. Unfortunately, I'm still weeks away, so I won't be able to post what my stock shafts have for quite a while. Looking at the truck, though, I'm gonna like having the wheel farther forward, despite how small that will be.

If I were a better welder, I think I'd cut the lower arm and re-weld the ball joint mount farther forward. That seems like a better overall mod. I'd like to keep the spring as straight as possible.

[crakarjax]

Yes those are the ones at 1.75" from the factory.

This pic shows how my shock mount points right at the crossmember when down all the way (right side). I'll probably run plates and cups though so aired out would likely clear (left side) assuming that sandwiching 3" of lumber in there is a good gauge.

[SCOTI]

When you shift the lower arm forward, the shock isn't going to align w/it's original position. You'll need to get things mocked up & either space the lower mount back and/or make a new upper mount.

[Johnny_Lamebridge]

This is just my opinion, having done this mod on my truck...

It seems to me that the amount of caster increase is really hit & miss when using control arms with the steel threaded bushings. A couple revolutions of the control arm shafts will make quite a difference plus or minus depending on which way you turn. I believe that to get a consistent caster change, you have to be careful to "centre" the shafts in the threaded bushings. Wouldn't the rubber bushing style be much more consistent? My .02

[chevyrestoguy]

Originally Posted by SCOTI

When you shift the lower arm forward, the shock isn't going to align w/it's original position. You'll need to get things mocked up & either space the lower mount back and/or make a new upper mount.

This was a possible concern of mine as well, but I just put my new shocks (KYB Exel-G) on a few days ago and everything lined up nicely. The saving grace is that our upper mounts are a stud and not a double tab. The stud allowed the lower shock eyelet to rotate slightly forward (I only moved my LCA .75"), and the lower bolt went right in with no issues. It fit just like stock.

It might become an issue if you go more than .75". Maybe?

[picklito]

What Johnny said.

I haven't done my front end rebuild yet, still collecting parts. But knowing how they go together I think he makes a very important point. That, and the variance we've seen on the cross shafts that we're pulling out of these trucks. We just have to be careful verifying our starting point and how we put it back together if we want a good caster mod.

[BMERDOC]

Originally Posted by chevyrestoguy

This was a possible concern of mine as well, but I just put my new shocks (KYB Exel-G) on a few days ago and everything lined up nicely. The saving grace is that our upper mounts are a stud and not a double tab. The stud allowed the lower shock eyelet to rotate slightly forward (I only moved my LCA .75"), and the lower bolt went right in with no issues. It fit just like stock.

It might become an issue if you go more than .75". Maybe?

One of the questions is ".75" from what point?" Seems the factory arms have the indexing hole 1.5" from the end and aftermarket shafts are drilled at 1.25". From what I remember your shafts were factory so your index location should be at 2.25" from the shaft end correct?

[crakarjax]

You're right that it's possible to have the shaft off-center, but it's not that hard to ensure that each bushing is threaded an equal number of revolutions per side to keep it centered.

[Domer]

Sorry for the long delay, been out to sea. Here are my stock measurements, and then my new ones.