Twitchy K5

[wildbilzrydn]

I've got a '72 K5 w/ a 6" BDS lift and 35" MT's installed by the PO. I installed all new tie rod ends and centerlink ends, checked the ball joints and bearings and now the play in the steering is virtually gone. I also put a new steering stablizer on it.
After I did all this I took it to a mechanic in the area whom I've know for about 20 years and had him check the alignment and he told me as far as he could tell the toe-in was correct, the caster was OK but this twitchy feel in the steering was caused by excessive camber.
This truck has little wedges in between the springs and their pads, And I am beginning to see that If I take those out to restore the camber, I'll probably kill the front driveshaft due to excessive pinion/driveshaft angle.
So I'm hoping that someone out there can help me here. I don't really want to pop for the adjustable ball joints. I'd rather try a different set of wedges if I can get away with it or go to 2.5" lift and 32's

Thanks!

 

[69TowRig]

Camber is how the tires tip in or out from the vehicle, not the angle at which the steering axis is inclined (which degree shims on your leafs would adjust). That angle is the caster angle. "Excessive" caster is usually a higher #, but from your description it sounds like you have tipped the pinion up with the shims to make the driveshaft happy, which would have reduced your caster. Once you do that, the truck will not want to "return to center" as quickly, producing a wandering, "twitchy" (in your own words) effect. You probably have close to 0° caster.

If the lift is that big, you may want to cut and rotate your inner Cs on your axle to make the pinion happy and keep the caster in spec. You could also spring for a double-CV driveshaft, remove the shims, and be happy that way too.

 

[wildbilzrydn]

I knew I had the caster/camber thing mixed up. What's a DBL CV driveshaft? Is it a special CV joint where the U joints are now?
I have a 205 case, and lock out hubs so the drive shaft isn't spinning all the time, couldn't I get away without running the shims?

 

[69TowRig]

Double-CV referring to having a double-cardan CV joint at both ends of the shaft, instead of just at the transfer case like you do now. With a double-CV shaft you can run any mis-match of angles that you want between the pinion/shaft/t-case yoke as long as you stay within the operating angles of the joints themselves. Not a cheap option, figure $4-500 unless you use your existing shaft as a donor for the slip-shaft and t-case CV.

On a rear driveshaft, the typical setup is a u-joint at the output of the t-case or transmission and a u-joint at the diff. There is somewhat of an angle on the driveshaft as compared to the yokes, but the angle is about the same at the t-case and at the diff. The driveshaft actually does not spin smoothly, instead it actually oscillates somewhat, speeding up and slowing donw on account of the single u-joints at each end that are running at a small angle. You don't notice this because the u-joints are phased (in sync) and oscillate opposite of each other, producing a smooth RPM at the diff.

The original setup for the front of that K5 was a CV at the t-case and a u-joint at the diff. This works because the pinion shaft and the driveshaft are in a straight line to each other at normal ride height and the angular misalignment occurs at the t-case yoke, where the CV is. Once you raise the truck, the "normal" ride height causes the angle to change from the pinion shaft to the driveshaft, which causes vibration in the u-joint (because there is not a u-joint at the opposite end of the shaft). So, one can use shims to adjust this angle back to stock. Works fine in concept, but in the front now you've found the issue about changing your suspension geometry.

Eventually the kind of vibration developed by not compensating for this issue will destroy u-joints and bearings. When or how the damage will occur I cannot say. Usually it happens at the least opportune moment.

 

[wildbilzrydn]

Sounds like buying a donor D44 and having the tubes turned in it might be alot cheaper. The spring perches and the C\L of the ball joints could remain stock,which would fix my caster but I could rotate the pumpkin to keep the straight line to the transfer case. (to me it sounds like a plan) Do other people do this? I'm sure I'm not the first person to think of that, but I've never heard of it.
Anyhow, thanks for the education. If I lived in Houston, I'd buy you a beer.

 

[69TowRig]

Lots of people do this, it's not glamorous so most don't mention it with their build-ups. Drilling out the plug welds is possible, or re-tubing the axle, neither is going to be much fun. I guess you need to decide what route to take.

I'm moving to Raleigh, NC this week, so if you are ever in town I will take you up on that beer.

If you want some food for though, type "cut and rotate axle housing" in a Google search...

 

[Greg72]

I think your solution is a lot simpler than cutting and turning knuckles....

6" lift and 35's isn't exactly an "exotic" suspension setup. There are plenty of people who run that combo problem-free.


It would help a lot if you could get an anglefinder and measure the true caster on your front axle currently. If you have flat-top knuckles, there should be a nice spot to lay the gauge and measure whether you're postive, negative or zero.

From that measurement, you can decide which direction the housing needs to rotate. My guess is that you will need to rotate the housing to move the pinion closer to the ground. You can use steel shims to do this safely...it's cheap, and will help you deterimine if this is ACTUALLY your problem, without spending a ton of money. Figure out how many degrees off from "stock" you are, and buy a shim that will correct for the difference...I run a 6* shim with my front leafs and that's perfect for my combination.

Ultimately, you may create more angularity in the front driveshaft by adding the shims. While you're trying to isolate the problem, I'd just pull that shaft out completely. Honestly, the shaft should never be spinning on the street anyway so it SHOULDN'T matter.... but simpler is better when you're looking for the real problem.


Good luck.

 

[wildbilzrydn]

Greg, I was kinda waiting for you to weigh in on this, because I remember you telling me about one of your setups and how nice it drove with 1/2 ton axles and a swaybar. How much spring lift do you run? What's your current caster? How much deviation from "straight line" between the pinion and case can I get away with? (I wouldn't think it would have to be perfect)
An angle finder is about the only tool I don't own, But i'll get one this weekend. My service manual says caster should be 4* positive (?, if i'm wrong it's because I can't remember fwd. vs rev. but it said 4*) so the closer I get to that the better I am right? Im guessing my shims are like ten degrees. fwd refers to top of the centerline of the ball joints being tilted to the front correct?
You say that I need a shim to get back to "stock". Wouldn't I just take the shims out for that? Spring flat against the perch?

Thanks for the help. I owe you a beer too.

 

[Greg72]

OK, first of all let's get straightened out on the terminology:

Caster is the imaginary line drawn between the upper and lower balljoints (or kingpins) and is measured in degrees. If the front of your truck is facing the left side of this page (looking at the drivers side) then:

/ would represent "positive" caster

\ would represent "negative" caster

| would be zero caster.


You want positive caster, and 4 - 6 degrees is probably what the factory manual will show as a "stock" specification for your truck. Even with a lift, you'd like to still aim for that number.

SO....looking at those crude diagrams, if you have negative caster now, and you rotate the pumkpin (with shims)...then you should be able to see how the pinion will start angling downward.

The reason I wanted you to have an angle finder (and a cheap $10 will work fine) is that the top surface of the knuckle (if it's flat) will be exactly 90* off from the caster number you want to find. That will make the math easy.

My own truck isn't really a good example because of some weirdness caused by the custom springs, but I'll tell you the story to at least show you the problem solving process.....

My (4" Lift) front springs were made 1" longer than stock, and the spring pin was offset 1" further forward over stock. That meant that my axle was sitting further up the "smile" of the spring, and was actally rotating the pinion DOWN automatically. My D60 steering arms are nice and flat, so I was able to get a measurement easily....but it showed 10* of positive caster! I only wanted 4* of positive caster which meant that I needed to rotate my pinion UP (not down, like most people) to get rid of the excess positive caster. I added a 6* shim under each spring (and also had to use some spherical washers on the side with the cast-in perch to avoid putting 6* of "tweak" in the U-Bolt plate bolts.

Anyway. The result was that I got to 4* postive caster and the truck ended up driving better afterwards.


Conceptually, that's what you want to achieve....but again, the steps you will need to take are going to depend on your own caster measurements.

 

[wildbilzrydn]

Thanks,
My shims measure 5*20' (trig'd out) ,375" h x 4" long. So if thats all I know and my spring pads are 4* positive stock, than that means i'm running 1* neg. That's gotta be at least some of the problem, Huh?
Thanks again for the help. I'll measure the angle this weekend, and if it isn't raining/snowing too hard on me, I'll pop those shims out and see if it makes an improvement in the way it drives. Then If it does, I'll see about running it a little in 4-hi and see how much vibe I get out of the front shaft.

 

[Greg72]

Thanks,
My shims measure 5*20' (trig'd out) ,375" h x 4" long. So if thats all I know and my spring pads are 4* positive stock, than that means i'm running 1* neg. That's gotta be at least some of the problem, Huh?
Thanks again for the help. I'll measure the angle this weekend, and if it isn't raining/snowing too hard on me, I'll pop those shims out and see if it makes an improvement in the way it drives. Then If it does, I'll see about running it a little in 4-hi and see how much vibe I get out of the front shaft.

You can't really assume that your pads are at 4*... I'm not convinced that you'd have 4* caster with the pads sitting perfectly level. GM may have set them up differently based on the front spring they used.

The only true measurement of caster (accounting for your lift springs, shims and anything else) is the one you take directly at the knuckle on level ground with the wheels pointed straight ahead.

If you're off by a LOT, you could almost flip the shims around (or as you said, remove them) as a quick way to change your numbers by a few degrees....