Hope this sheds some light on the reason for the SAS 'death wobble:mad: . It is a bit long.

It can be caused by several factors - low tire PSi on one side, loose suspension components, bent parts, etc. But by far the most common reason is excessive positive caster.
Caster is defined at" the tilt of the steering axis toward the front or rear of a vehicle compared to true verticle"
Positive caster is prefered on most vehicles because it improves straight ahead stability, steering return, cornering ease.
However too much positive caster can cause shimmy at hiway speeds, especially with wide tires.

From every source I can locate, the normal caster range is 1/2 to 2 degrees positive. I cannot find a specific for an SAS p/u, but a 91 FJ-80 manual calls for 1.5 degrees.

As a reference I took some measurements on my 84 p/u.
All measurements are center to center
Height from floor to front spring bolt - 25.5"
Height from floor to front shackle upper blolt - 25.5"
*So on a stock SAS truck (not coverted from ex-IFS) the bolts are on the same horizontal plane*
Stock shackle - center to center ~ 3.25"
Extended shackle ~ 6.50"
Redrilled (like photo with red circles ~ 5.50"
Front spring ~ 46". This can vary +/- and inch or so.
The arc (lift) of the spring is not a factor ( or at least very minimal) due to the attach point is at the center and the spring is a constant arc.
The outcome of all this
*With a 46" spring mounted at an OEM location and using a stock (3.25) shackle, the angle between the horizontal bolt plane and the plane the front bolt/lower shackel is ~ 4.0 degress

*With a 46" spring mounted at an OEM location and using a redrilled extended (5.50) shackle the angle between the horizontal bolt plane and the plane the front bolt/lower shackel is ~ 6.8 degress

*With a 46" spring mounted at an OEM location and using an extended (6.50) shackle, the angle between the horizontal bolt plane and the plane the front bolt/lower shackel is ~ 8.2 degress.

If you are running and SAS conversion, you would need to take into consideration if the front / rear bolts are on the same horizontal plane.

All other factors remaining the same ,(built in caster in the axle, spring pad, etc) by going to the longer shackle you have increased positive caster by ~ 4 degrees.

To solve this you can.
Remove the longer shacke (and kill the flex).
Lower the front mount (kill clearance / approach angle)
Install a 4 degree shim.

Hope this helps :D
BC

Sources: ICAR 2000 suspension class, Toyota FJ-80 manual, 46" long steel bar, combo square, Inogon angle gauge.

ok, what can ya say, umm, MY EYES!!!!!

every rig i have set up to date, i try to hit about 7 degrees of caster(positive) this makes the rig stable, steering return to center pretty good, and no death wobble... In my personal opinion, toyota messed up when they built the trucks as far as the steering, NOT enough caster.. One thing jeep did a good job of was in there steering, they are usualy around 7 from the get go,,

I set them up at 6 degrees and ahve no problems. My truck right now is a little closer to 10, and still no problems. Factory gm D44's came with 6 degrees.

just for ease of comparison, could you guys measure the angle of the pinion flange? thanx
greg

Originally posted by 4x4runner
just for ease of comparison, could you guys measure the angle of the pinion flange? thanx
greg
That would work for stock axles, My knuckles are turned so I was back to measuring castor and there is no pinion flange on Dana's. You're high steer or crossover arm should be close enough. I measure mine on the flat portion of the knuckle

I know that most SAS rigs have issues with the pinion being at a closer-to-horizontal angle than a stock SA rig, which tells me that the majority of SA conversions increase the caster somewhat.

Like Crash said, I'm thinking that if I can get my Caster angle down to about 8 degrees or less of positive caster, I should be okay. Right now I'm around 12 degrees, which is what's causing the death wobble.

And anyone who suggests that the tires are out of balance or it simply needs a steering stabilizer has never experienced true (OH SHIT WE'RE GONNA DIE!) death wobble. It can *begin* the process by getting the shimmy started, but once the front suspension reaches it's resonant frequency and begins oscillating, things get spooky *fast*. There's no way you'd mistake death wobble *completely* for imbalanced tires if you've experienced it.

Someone posted that the factory spec for Toys was 2.5 degrees positive. Another said 1.5. So I've gathered that Toys have a very similar situation to stock Scouts - which have ZERO caster facotry spec. :eek:

It's important also that your trunion (knuckle) bearings are set correctly. When I rebuilt my front axle prior to the SAS, I went for the high end of the factory specs on the bearing preload, and both of mine ended up being 12-13 lbs when torqued to spec. Facotry spec is 6-13 pounds preload.

After I drill the shackle (will actually be 5" eye-to-eye) and raise the rear of the truck up/lower the front as per Zags recommendation, I'll se what has happened. If the wobble is still there, it's on it's way to an alignment shop.

I haven't taken it to one yet, simply because I know WHAT the problem is - excessive positive caster. The alignment shop will be useful in telling me exactly HOW MUCH caster is present. And I don't want to throw $$$ into that yet - I'm going to be fiddling with the front spring packs and the shackles at both ends, so that's going to change.

But Crash, thanks for helping out with you caster angle experiences. Coyote, thanks for the stock measurements - they may come in useful... But Newbie, next time click REPLY, not POST. :flipoff2: :flipoff2: :flipoff2:

Whoever said steering stabilizer :smokin: they jsut hide problems, instead of fixing one. I'm not running one on my rig with all of the crap I have and 37's. Build your shit right and you won't need it. Dr Evil, once you get the castor right, everyhthing should be fine.

Hmmmmmmmm looks like the NEW guy thought he was going to tell us "SOMETHING WE DIDN'T ALLREADY KNOW":flipoff2: :flipoff2: :flipoff2: :flipoff2: :flipoff2: We are not stupid, want to tell us where the light goes when you turn them off now, does it just keep going away from the rig or what:rolleyes: :flipoff2: :flipoff2: :flipoff2:

Easy, Darren. Most of us already know that, true. But this way, since all newbies run searches before posting, we shouldn't see any more posts on "Why do I have death wobble without shocks, and running un-balanced 14.50 Boggers on square wheels?" - Right? :rolleyes: :rolleyes: :rolleyes:

Originally posted by Dr. Evil
Easy, Darren. Most of us already know that, true. But this way, since all newbies run searches before posting, we shouldn't see any more posts on "Why do I have death wobble without shocks, and running un-balanced 14.50 Boggers on square wheels?" - Right? :rolleyes: :rolleyes: :rolleyes:

yup, just giving him the tradional pirate welcome:flipoff2: :flipoff2: :flipoff2: :flipoff2: :flipoff2: :flipoff2: if he can't take it, then he will start crying as a reply;)

That would be the 3rd or 4th one you've cracked this week, wouldn't it?

Originally posted by Dr. Evil
That would be the 3rd or 4th one you've cracked this week, wouldn't it? :question: :question: :question: :confused: :confused: :confused:

:crybaby2: :crybaby2: :crybaby2: :crybaby2: :crybaby2:
Maybe toy94:rainbow: x4 was rite - my newbie feelings are hurt:flipoff2:
NOT

Point is that by doing some measuring you can get a idea of what total caster is. In a conveted SAS (ex IFS), if the forward spring mount is higher that the rear upper shackle bolt, this will add to the total castor.

Take the front spring point measurement(a) - the lower shackle bolt measurement (b) = total spring angle differnece. (also known as the RISE)

Take the spring length (also know as RUN)

rise
------- = Tan of the castor angle.
run

Since it had been years since I looked a a tangent table here are some quick numbers:

Tan A = .0175 = 1 degrees
.0350 = 2 degrees
.0700 = 4 degrees
.1050 = 6 degrees
.1405 = 8 degrees
.1765 = 10 degrees

(Any math guru feel free to correct/make simpler)
The idea of this entire post is to try and provide a useable way to measure some things and help determine the castor and solve the problem. Using a tape measue and some math may save you having to spend $$$ at an alignment shop to determine the same thing.

:flipoff2: the rest ya - no hurt newbie feelings here

Originally posted by BlueCoyote3
:crybaby2: :crybaby2: :crybaby2: :crybaby2: :crybaby2:
Maybe toy94:rainbow: x4 was rite - my newbie feelings are hurt:flipoff2:
NOT

Point is that by doing some measuring you can get a idea of what total caster is. In a conveted SAS (ex IFS), if the forward spring mount is higher that the rear upper shackle bolt, this will add to the total castor.

Take the front spring point measurement(a) - the lower shackle bolt measurement (b) = total spring angle differnece. (also known as the RISE)

Take the spring length (also know as RUN)

rise
------- = Tan of the castor angle.
run

Since it had been years since I looked a a tangent table here are some quick numbers:

Tan A = .0175 = 1 degrees
.0350 = 2 degrees
.0700 = 4 degrees
.1050 = 6 degrees
.1405 = 8 degrees
.1765 = 10 degrees

(Any math guru feel free to correct/make simpler)
The idea of this entire post is to try and provide a useable way to measure some things and help determine the castor and solve the problem. Using a tape measue and some math may save you having to spend $$$ at an alignment shop to determine the same thing.

:flipoff2: the rest ya - no hurt newbie feelings here


cool, u r not another toy94x4 afterall....:flipoff2:

Originally posted by SeaBass44



cool, u r not another toy94x4 afterall....:flipoff2:

have patience, another will come along soon enough:flipoff2:

I agree. I set my '86 4runner at 7 degrees as well. I had a few people say that I was running too much, and that it should be approximately 1 degree, but I had problems with handling until I set it to 7.

CAZ

Good to know, CAZ. I'm going to do the shackles and pull a spring leaf or two out of the front springs this weekend, along with throwing in some bump-stops.

I'll shoot for 7-degrees after I get all that done. I might be pretty close, seeing as how I'm at about 10 degrees now, and my suspension fiddling should give me around 2 degrees less caster.

Just for a comparison, I'll post what DRT12's castor is before and after we get the shackle problem solved. He's supposed to come by around 8 tomorrow morning.