Lift kit questions on 87 K-5

[Big Katz]

Hello All,
I'm new to lift kits and am curious about my problems. I purchased an 87 K-5 Blazer with a 4" lift kit (spring in front, blocks in the rear), 33"x12.5"x15" radial tires, and a 3/4" transfer case drop. It has bad driveline vibration between 35 mph to 45 mph and bump steers pretty bad. I put in a new front driveline and new rear slip yoke and u-joints but still have the same vibration. It also had a 6 degree shim on the front end that was made out of aluminum that was broken. I removed the shim and put it back together. This helped a little bit with the bump steer put it is still pretty bad, but did nothing for the vibration. I am wondering if there is a yoke angle I should shoot for and shim accordingly or should it just have the matching angles on both ends of the driveline? As far as the bump steer is that just something that is inherent of a lift? I have checked all of the ball joint and tie rods and the toe in on the front end. Everything is tight and there is 0 toe in. I will try the tires on one of my other trucks later this week and see if it is tire related. Any ideas or info is appreciated.

[Da67goatman]

The vibration is most likely the driveshafts being out of balance.  And the bump steer comes with bigger tires that's why they make steering stabilizers.  And your alignment could be off causing bump steer.

[Psycho71]

I'm experiencing bumpsteer on my K20 burb, with a freshly rebuilt front end and new steering stabilizer. I set the toe in at 1/8", and it helped a LOT. The bumpsteer was horrible at 0 toe-in. But it didn't completely fix the problem. I have been advised now to check my caster angle. I'm suspecting, from seeing it while I was doing the other work, that my caster is more negative than it should be and will need some angled shims to correct. The tires do not lean out when at full lock near as much as they did before I lifted it, which means the caster angle changed when i lifted the back more than the front. I hope this is my issue, and hope my experience might give you some food for thought on yours.

Also, look at your steering arm and see if it's level, or at an extreme angle. It should be level, and I'm told that an angle will cause a certain amount of bumpsteer too. Mine is at a slight angle, so I might have some issues to address there too. but I'm gonna attack it one thing at a time. Looking at raise steering arm options now as well.

[Grim 82]

Raised steering arm or Z-draglink could help the bump steer if it's a geometry issue, and proper wheel alignment as stated. On a short wheel base like a blazer, your vibration could be driveshaft angle. Are you only having the vibration in 2wd? I ask because then the broken shim in the front and whatever you have going on up there is irrelevant to the vibration. Sounds like 2 seperate problems, neither of which should be too tough or expensive to get squared away.

[Big Katz]

I forgot to mention that I do have a raised drag link. The pitman arm and drag link are almost the same height. With the pitman arm being slightly higher (1/2") than the drag link. I think I will try and toe in the front end 1/8" and see how it handles then. I have been searching the internet about the optimum drive line angles with little luck. I have a driveline shop down the road that I deal with alot. I'll have to pick his brain later today and see what he has to say. It vibrates worse in 4x4, but is still bad is 2 wheel. I don't believe the issue is a balance one. I used to build drivelines and can tell when they are in the lathe if they are too far of on the balance. These drivelines ran smooth with the lathe at max rpm.

[Irish_Alley]

Could be a tire. But do you keep the hubs locked?

[Big Katz]

I think I have the problems figured out for the most part. The bump steer is in the tires. I towed the front end in 1/8th inch and it is acceptable steering now. I might play with it some more and bring it in a little bit more and see how it does. The vibration is a combo of two things. The front drive axles are starting loose the u-joints. and the bushings in the transfercase rear driveline slip yoke is Very sloppy. I will get these replaced next week and try her again.
For future reference, I talked to the driveline guy down the road and he said the best you can really do with the short drivlines on blazers is try and set the u-joint angle as close to each other as possible with the pinion angle being a couple of degrees more than the tranny to account for the axle wrap under load.
Irish Alley, Yes I do keep the hubs locked in year round. The rig is on sand beaches all summer and will be plowing all winter.
Thanks everybody for your input. I'll let you know how it works out.

[Irish_Alley]

If they are your turning the front u joints as you said and since they are bad they will vibrate.