Lowered trailing arm trucks...and shims

[Ole Greenie]

Hey ya'll,

I've read a few articles about this but I see almost always non-committal about this subject.

I've built two trucks that were dropped a bit. Neither required a C-Notch but I have a bit of info for the forum regarding both.

First, the leaf spring trucks don't necessarily require any longitudinal redirection for the differential...unless you are going low enough to install a C-Notch or high enough to REQUIRE a redirecton. If you lower, make sure you measure the angles and move on (seek assistance from youtube). the difference in angles should be less than 1 degree but more than 0.5 degrees, optimally.

If, however you have a trailing arm truck, particularly a short bed, you will NEED to check the pinion angle. You see, lowering the truck changes the pinion angle of the differential significantly if you lower the truck. Lowering pivots the pinion angle of the differential proportionately commensurate with the amount of drop. You may be able to get away with a 2" drop but you will have to address a 3 or more inch drop because trailing arm drops will rotate the stock rear gear housing.

For a 4" drop, you will need a 3-4 degree shim. if more drop, you will need a bigger shim. Again, this is only for trailing am trucks (AKA: NASCAR coil spring setup rear suspension with panhard bar).

Note: I have a short bed trailing arm truck...year is irrelevant...lowered 4" rear and 3.5 front. Differential rotated upon lowering. Another is a shortbed leaf spring truck, bagged. The coil spring truck needed shims and the leaf spring truck didn't (differential never rotated).

Hope this helps,
John

[rjs53]

Helps a lot! Have been looking for info on this, I'm going with a early classics 4-6 drop so I know pinion angle will need attention. I'm going to get the driveline shortened as this was a long bed. Thanks for the info.

[CC69Rat]

68 Short Fleet. 5/7 drop, trailing arms. 4 deg shims.

You can also flip the front trailing arm brackets upside down and offset about a 4in rear drop. ( shims are much easier )

[RodnRudy]

I've been struggling with this issue also. 68 Fleet with 5 1/5, 7" drop. I raised the trans mount up 1", got it to 3 degrees down. The rear pinion is 1 degree up. This is at simulated ride height, jack stands under rear housing and on outer A arms. The frame sits level front to rear and the drive shaft is 0 degrees. I'm going to go with a 2 degree shim in the rear to bring it up. I hope this remedies the nuisance drone I'm getting.

[dec010974]

where do you buy the shims and what shims are you referring to?i have never heard of this. thanks for the info.

[Rich69shortfleet]

Originally Posted by dec010974

where do you buy the shims and what shims are you referring to?i have never heard of this. thanks for the info.

I'd like to know this too.

[beaterku]

Speedway motors sells them:

https://www.speedwaymotors.com/Rear-...up,294450.html

[RodnRudy]

Originally Posted by dec010974

where do you buy the shims and what shims are you referring to?i have never heard of this. thanks for the info.

Thanks for that Speedway link Beaterku, I almost dropped $65 at CPP for their shims. Placed order from Speedway this morning. Rich69shortfleet, yes thick end of shim goes forward, you want to raise the pinion yoke.

['70_402]

So will any truck that is dropped in the rear need shims??? Is everyone running shims?

[Rich69shortfleet]

I did a 4 inch drop all around about 18 years ago. I've never really had any noticeable driveline vibrations BUT I have replaced the center bearing about 3 times so maybe I have and just haven't noticed it in the driver's seat.

Anyway, just so I am 100% clear, help me understand what I need to do. Speedway is basically in my backyard so getting the shims is easy peasy.

I have a 69 short box 1/2 ton (so, coil springs). When I lowered it with lowering springs I also put in shorter stroke KYB shocks and an adjustable panhard rod and a rear sway bar (and a larger front bar). Given what I've read above I should use 4 degree shims. Right? And I would install the shims with the thick end of the shim facing the front of the truck (to raise the pinion). Right?

Now that I own a 250 lb/ft rattle gun getting those huge u-bolts loose this time will be a heck of a lot easier than they were 18 years ago when I only had hand tools.

I'll be installing new wheels and hit a hard bump recently so I plan to get it re-aligned when this is all done.

[demian5]

3" Spring + 1" block.
So this may be the cause of the bed shake at about 65-75 MPH on my otherwise pretty solid component truck?
You can buy block with the degrees built in from CPP.

Id like to hear from people that actually solved a vibration issue by adding shims.

[CC69Rat]

Originally Posted by demian5

3" Spring + 1" block.
So this may be the cause of the bed shake at about 65-75 MPH on my otherwise pretty solid component truck?
You can buy block with the degrees built in from CPP.

Id like to hear from people that actually solved a vibration issue by adding shims.

I have several times. What you're feeling is due to the harmonic vibration of your drive shaft and joints. The vibration is caused by somewhat of a jump rope effect. As the drive shaft turns, the distance around is more going down than it is going up.. So it shakes a little when the vibrations tries to sync up. Typically you'll feel harmonics at 64mph, depending on your tire sizes.

I usually angle my pinion 1 dag off center, you don't want it zeroed out entirely. I've bu8lt a bunch of these trucks, S10s, etc. Shims will fix the vibrations.

[demian5]

Originally Posted by CC69Rat

I have several times. I've bu8lt a bunch of these trucks, S10s, etc. Shims will fix the vibrations.

Awesome, thank you. This would also explain why I have to spray my center support with white lithium every few months because its getting a little to much force from the pinion being down.

[CastIron]

Here's a video that shows why pinion angles are important:

https://www.youtube.com/watch?v=gmV4qwLfOMY

I'm pretty sure I watched an older version of this made by GM when I was taking automotive classes many years ago.

[CC69Rat]

Here's another couiple videos. First I want to hit on hard acceleration. When you pull away from a red light or whatever, your pinion will rotate UP because of the force of the engine moving the car / truck forward.

https://youtu.be/yKyx_k5JIdg

So, taking that into consideration what I normally do is set my pinion about 1 deg down in relation to the tail shaft. If you have a two piece driveshaft remember you have another set of angles to worry about too. A couple people have mentioned carrier brings, just know you also need to measure the comp flange at both ends and make sure through the whole driveline they cancel each other out within 1 deg or so.

When I say I set mine 1 deg down at the pinion. I'm taking the upward movement into consideration too. I point it down to kind of offset some of that uplift even before it's accelerating. That way if let's say the pinion moves up 2 Deg on a really hard launch.. it's only moving 1 deg past zero if that makes sense ?

Here's another really good video on it. https://youtu.be/wPUgvJsfwmc


What I'd suggest if you're having vibes, run and grab a magnetic angle finder and Home Depot / Lowes or whatever. Measure at all 3 points, your tail shaft (a guy earlier said he had lofted hjis trans tail shaft to try to fix it..

Carrier baring would be 2

Pinion would be 3. Counter balance (shims, or however you chose) the drive shaft rotation angles within 1 deg or so and I promise you'll feel a difference. Prob pick up a few horsepower too.

Hope this helps. I'll post a pic of mine in a few.

[demian5]

Originally Posted by CC69Rat

a guy earlier said he had lofted his trans tail shaft to try to fix it.

But if the engine and trans are in the stock location, the angles should be OK between the trans and second carrier I would think. However I have seen 3/32" and 1/8" shims under trans mounts between trans and mount in the past.

[CC69Rat]

Originally Posted by demian5

But if the engine and trans are in the stock location, the angles should be OK between the trans and second carrier I would think. However I have seen 3/32" and 1/8" shims under trans mounts between trans and mount in the past.

Happens a lot. I've used them in S10s too. With a two piece shaft, you can shim the carrier bearing too. No problem

[Aus69c20]

A lot of helpful info here. I’ll be looking closer at my angles now.

[Rich69shortfleet]

Okay, I got my 4 degree shims for a 4 inch drop. I was looking at my truck as it sits in the driveway. As it sits, the pinion is already pointing up slightly. To help visualize, I jacked up the truck at the bumper to see how it would look when it was at stock height. At that height, the pinion is still pointing slightly upward. So, if I put in the shims the pinion would be angled even higher. My thought is, for whatever reason, my pinion is pointing upward slightly and shimming it would make it WORSE, unless I put the shims in the opposite way. My additional thought is that since I am not experiencing any drivetrain vibration I'm just going to leave it completely alone and return the shims to Speedway.

First picture, stock height. Second picture, lowered static height. In fact, looking at the pictures side by side it doesn't look like the angle really even changes that much from one height to the other.

[Robznob11]

Fyi, hit me up if your looking for a set of 4* shims, I built a 9" and welded in the 4*'s

[CC69Rat]

Originally Posted by Rich69shortfleet

Okay, I got my 4 degree shims for a 4 inch drop. I was looking at my truck as it sits in the driveway. As it sits, the pinion is already pointing up slightly. To help visualize, I jacked up the truck at the bumper to see how it would look when it was at stock height. At that height, the pinion is still pointing slightly upward. So, if I put in the shims the pinion would be angled even higher. My thought is, for whatever reason, my pinion is pointing upward slightly and shimming it would make it WORSE, unless I put the shims in the opposite way. My additional thought is that since I am not experiencing any drivetrain vibration I'm just going to leave it completely alone and return the shims to Speedway.

First picture, stock height. Second picture, lowered static height. In fact, looking at the pictures side by side it doesn't look like the angle really even changes that much from one height to the other.

I agree. It looks like someone may have adjusted yours at some point in the past. Not all trucks will need them. That's why I had suggested an angle finder first.. to figure out exactly what you need. 3-4 deg shims for 4-5" drop is about normal. SWB, LWB. History of your truck. Mileage, etc can change things. Ex if it's hauled a trailer all it's life.. things like that

Make sure you measure the pinion angle at ride height.

[Rich69shortfleet]

The truck was bone stock when I got it. I've lowered it. All I did to the control arms was to add poly bushings and I put in some lowering springs and a rear sway bar. There are no shims in it. I did replace the u-bolts when I installed a set of G60 rear helper springs and had to get the half-leaf touch plates into the axle/control arms/spring plates/shock mounts sandwich but nothing in that mix changed the control arm to axle angle.

However, now that I think about it, the rear axle is not original to the truck. I got it from Manes years ago as it is an original truck 12-bolt posi. It was easier to just swap rear ends than swap the guts into my existing housing. Maybe this axle had a different angle built into it's control arm mounts for whatever reason. Do coil spring and leaf spring axles interchange? If so, do leaf spring rears have a different mount angle than coil spring rears?

[68 TT]

Originally Posted by Rich69shortfleet

..... Do coil spring and leaf spring axles interchange? If so, do leaf spring rears have a different mount angle than coil spring rears?

I know really old thread but nobody answered so I figured maybe it would help someone out if I did.

Trailing arm housings the mounts are on the bottom side of the axle tubes and at a 15-degree angle to the center line. The trailing arms align at a 30-degree angle. They also have mount holes in them for the 3-1/2" x 3/4" U-bolts to pass through to keep them captive. This will make the mounts easily identifiable from the more common car axles with bottom mounted leaf spring perches as the leaf spring perches do not have holes in the outboard sides, only the 1/2" to 5/8" hole in the middle for the leaf spring alignment pin.

Truck leaf spring housings for 67-72 and 73-87(91) have the leaf spring perches on the top side of the axle tubes as the housing is on the under side of the leaf springs. This is unless someone has done a flip kit equivalent and cut off the perches and installed new perches on the bottom side to flip the housing to the top of the springs to lower the truck in the same fashion as a flip kit. Leaf spring perches are installed onto the housing perpendicular to the axle center line.

Different perches being installed is also commonly done to housings from different model years to make them fit our trucks too so you may have to identify the housing type and model year by width and ring gear size and axle spline count.

I used a square body 4x4 housing in my 69 stepside and just installed a set of trailing arm mounts and panhard bar mount onto it, so just seeing the trailing arm mounts on a housing doesn't mean it is a 67-72 part. I also found a set of 2wd square body 5-lug axles and brakes for mine too so counting the wheel studs isn't always a model year giveaway either.

The square body 2wd and 4x4 axles are the same width so my truck can be either 6-lug or 5-lug in just a couple hours of work. I wasn't sure what wheels I wanted to run so the flexibility was a nice option and not having custom axles to get there was nice too. A little extra width but I like how it evens out the front to rear fender lip clearance a little compared to the stock housing.

The 88-98 body style 4x4 housing width fits nicely and evens out the wheel offset front to rear relative to the fender lip even more than the square body housing does so it is a fairly common swap. The perches are in the wrong place to just bolt in though. It requires new perches to be installed for it to work. In the 88-98 body style they went to a different housing width for 2wd that is narrower than the 4x4 housing so there are variations in width to watch out for.