The Stovebolt Site Forums The Shop Area Driveline Camber, Toe in/out, and caster on a stock 3600

I've been reading searching through the threads on this subject. My 49' 3600 is completely stock for steering and suspension. I'm running 16" rims with radials. My problem is on a good smooth and flat road it drives and tracks great. But hit some bumps or dips and it wants to wander like heck. It also tends to not want to pull out of sharper curves. I have all new leaf springs shackle bolts/bushings, tie rod ends and kingpins. I also have replaced all lever shocks with gas tube and added a sway bar. I've also adjusted the steering box per factory procedures. At this point I think the front end is about as up tp snuff as I can make it. The outer edge of the tires are wearing prematurely. When I eyeball it from the front the camber looks excessive. The tires are very obviously leaning outwards at the top. I'm contemplating taking it to a heavy truck shop in case the axle needs bending for camber. What I'm not sure of is what would be the best camber, toe in or toe out and caster setting the shop should shoot for? Aligning a 49' 3/4 ton truck is not something they've probably ran into before.

Does the steering wheel return to center normally when driving on smooth roads? Will it track straight if you let go of the wheel? Is it easy to steer?

As for caster, I don't think that's adjustable.

Caster can be changed but it requires changing a shim between the axle and springs.
Sagged springs can also throw caster out of spec.

OP, you say you are running radial tires. Is that on the original wheels? Probably not I'm guessing. So determine the wheel offset as compared with stock. Stock setup should have the imaginary line through the kingpin hit the ground at the center of the tire. On these trucks that will require a positive offset, more so with wider wheels. I have found that a 6 inch, zero offset wheel is acceptable in terms of steering. The problem you describe is one I had running 7-inch zero offset wheels and even worse with 8-inch wheels, even with narrow tires. With original wheels and bias ply tires it steered just fine.

Too much caster will make the truck steer heavy and follow the ruts and tar strips in the road. Caster is usually set with more on one side so the steering will pull toward the crown of the road. I have changed the tapered shims in mine years ago to get it to steer like I wanted but I don't remember the angle of shims I used. I set it to the factory spec and to pull toward the crown. FWIW I have also used them on the rear axle to get the u-joint angles to match. For no vibration they need to be withing 1/2 a degree of each other.

As for camber, radial tires like just about 0 for camber and toe. if the outside of the tire is wearing it usually indicated too much tow-in. Shops used to add toe in to compensate for loose king pins. Then when you put new king pins in there's too much toe in. I use about 1/16" and it might be too much. On bias ply tires outside-shoulder wear usually indicates too much camber but it doesn't affect radials much. As you know, changing camber requires a big-truck shop that can bend axles. I once worked in such a shop and spent too much time at the chiropractor. It was heavy work. Big wrenched and big port-a-powers.

Otto you asked,

Does the steering wheel return to center normally when driving on smooth roads? Not really. I have to steer it back.
Will it track straight if you let go of the wheel? Yes, on a smooth straight road.
Is it easy to steer? It steers easily enough once you get moving

1Ton_Tommy
They are not the original rims. I'll have to check since I put them on years ago but I believe they were 8" which may be contributing to my problem?
Also when setting toe in toe out at what point is that being measured? Across the front edge of the tires? Geometrically the distance you make the measurement in relation away from the centerline of the spindle all comes into play since we are forming a triangle.

Caster angle plays a crucial role in returning the wheels to center - especially on bumpy roads. That's why bicycles are so stable with on;y one front wheel.

Either the steering box is messed up or there is a big problem with the caster angle on your front end.

I can drive at any speed on smooth and bumpy roads and the truck tracks straight. Even on bumpy roads at high speed I cantake my hands off the wheel and it tracks in a straight line with the steering wheel finding its center naturally.

Tommy has given you some good advice. I've never been able to make one side caster be different from the other side in these trucks. That axle is not easy to twist and my experience is using different caster shims just causes the spring to twist more on the side with the higher degree shim once everything is tightened.

Please make certain your steering is aligned so that when your steering wheel is pointing straight ahead your sector gear (inside the steering box) is right in the center of its travel. Actually it should be smack in the center of your ball nut so that the center tooth of the sector gear (there are 5 teeth and the center tooth is number 3) is in the center (which is the high spot for it). This will be the tightest spot...where the number 3 tooth contacts the ball nut. As you turn to the left or right the relationship between those two parts becomes looser. It is very important for the tight spot to correspond with the "driving straight ahead" steering wheel position.

With the 1/2 ton trucks I normally use 3 degrees positive shims. Makes driving in town a bit harder but settles things over 50mph nicely and that is where your risk lies. 2.5 degrees is okay. When you get to 2 degrees your truck will steer easier but at higher speed it will have a tendency to wander more and below 2 degrees positive it will wander around like a drunk billy goat...especially if you hit a bump or something. This is called bump steer and if you lack enough caster it will take a while to recover as the vehicle at that point is trying to steer itself. And most of the time doing a good job of it. Your sway bar will help but it will not cure an incorrect caster setting in my experience. One other thing worth mentioning...if you have removed leaves from your springs hoping for a smoother ride, it will affect things negatively.

For what it is worth I agree that your wheels (and maybe also tires) are too wide.

Another thing or two...and if you have new leaf springs, etc this should not be the case but if your rear end sits higher than your front end it will by definition reduce your caster angle. I'd get a front end shop to check that angle to see what it is so you'll know.

And also, check to see how your drag link sits. In a perfect world (and the AD trucks sadly didn't all exist in one) the drag link should be parallel to the ground. If it sits lower or higher (not usually the case), the arc of the circle works against you. Years ago I made a diagram showing this. Basically the end of your drag link would make a circle in its up/down travel. Parallel to the ground the arc of that circle is at the "flattest" point for the movement of the drag link end (which will exacerbate bump steer). As you can imagine the further down in the "arc" the end of your drag link end sits, the smaller the amount of movement is needed for it to jerk the control arm further. I hope this helps. Please let us know if you have any questions.

The entirety of your issues are very likely due to your non-stock tires and wheels. Been there done that, not going back there.
These trucks need skinny tires close to the original tire height. Too much variation from this will cause your kind of problems and then some.

This is all great information and is giving me a lot to check on my truck. I did measure the camber today and it's 2 degrees positive on the drivers side and 1 degree positive on the passenger side. I will need to measure the caster shims to see it's present angle. At this point I have to no reason to think they aren't the original shims but I will measure. I will need to pull a wheel off and determine the rim width and offset. I did have to put 1/4" wheel spacers to clear the tie rod ends when I changed the rims. I can't find my receipts for the rims so I will have to measure it out.

Jon G I understand what you are saying about the drag link. Question is if it's not fairly parallel to the ground how can one fix that? The truck does sit level and I replaced the front springs with the correct set meant for this truck with the same number of leaves as the old ones.

What I'm thinking is I've screwed up the steering geometry in my efforts to modernize the rims and tires. I'm not opposed to replacing rims/tires to a more appropriate sizing.

Can you all help me understand the old tire size designations. According to the shop manual stock would have been 15-6 or 7.00-17-6 How do those number translate into a modern tire size designation? 15-6 = 15" rim x 6" width?

The spacers change both the offset and the backspace. It decreases the backspace measurement and moves the wheel offset outboard (toward positive offset).

There is no direct translation from bias ply sizes to Metric sizes.

Go to Coker Tire's website. They have (or at least had) a chart with tire dimensions of metric and bias ply so you can compare and find a metric size with similar tread width, section width, and diameter of the bias size you wish to duplicate.

Don't buy Coker tires, just use their chart as a resource.

https://www.cokertire.com/tires/700-17-firestone-blackwall.html

As long as your drag link is relatively parallel, that's going to be about all you can do. GM knew about this. In fact if you look at the shop manual/service manual, you'll see in the pictures the drag link slopes downward from the pitman arm to the control arm. Safety governs this situation...it is important for the ball to be securely attached so trying to make spacers to raise that ball won't help. You could lower your springs by using shorter shackle pieces, but only by about 1/2 inch to 3/4 inch. Using a longer pitman arm would alter the steering quite badly and dangerously so that isn't an option.

Every one of these trucks was built with camber setting of 1 +/- 1/2 degree, so I would check this measurement again. Both sides should be pretty close to the same measurement and I'd be very happy with 1 degree on the nose. As you know, bending the axle is the only way to adjust this. Any good axle shop could do that.

As for caster, yes...please check the shims. Highly likely they are stock, but you never know. As I said before 2.5 to 3 degrees will be ideal. Anything less than 2 degrees won't be as ideal. But in fairness, these trucks were not designed for today's highway speeds. I know...the speedometer says either 80 mph or 90 mph as a top speed. No engineer at GM expected these trucks to be driven at 80 mph. Nothing about that front end and braking system contemplates that and it didn't when they were brand new. Years ago there was a memo on the GM Heritage site...it was what they sent to dealerships...encouraging them to advise buyers to not exceed 55 mph. Good luck and thanks again for keeping us informed.

BZTGUY, I'm betting your problem is wheel offset, though,it could also be a combination of alignment and misassembly.

For example: If the drag link in not assembled correctly or has weak springs it will not be the correct length. So if you adjust the steering box with the wheels straight ahead rather than with the pitman arm disconnected you may not be on the high point when you make the adjustment. In which case, the steering gear will bind when it goes past the high point and not return to center automatically. Also, if the tapered caster shims were installed backwards there would be negative caster rather than positive and the steering would not return to straight very well if at all. It would also hunt all over the road and require much steering correction. The spec for caster is + 2-1/2 degrees plus or minus 1/2. Camber is +1 degree plus or minus 1/2. With radial tires camber can be reduced along with toe in.

Caster impacts another dimension known as trail, which is the distance the king pin line leads or trails the contact patch center. The more trail, the heavier the steering will feel. Trail is directly affected by caster and indirectly by tire rolled radius. As tire rolled radius increases, the distance between the kin pin contact point and a vertical line dropped to the center of the contact patch will increase. So larger diameter tires will make steering feel heavier with no change in caster. Thus, even though the big trucks with 20" wheels have only 1/4 degree more caster they have a great deal more trail and of course will have heavier feeling steering. With zero caster there will be no change in trail with larger diameter tires. The commonly used 235/85X16 tire at a measured 15.5" RR is a great deal larger than the original 15 inch tires supplied on 3/4 tons with specified rolled radius of 14.1". So you can see that the trail will have increased with the larger tire making for heavier steering.

I stated incorrectly in my last post that the king-pin line goes to the center of the tire. That is not the case. It will go slightly inboard of outboard of center (depending on the design) so the wheel/tire assembly will roll slightly, rather than scrubbing, when the steering wheel is turned greatly reducing steering effort at low speed.This is a function of king-pin inclination and is set at the factory when the steering knuckle is forged and machined. The intersect point is also affected by wheel offset. If a spindle is bent that point will move with the degree of the bend.

The further the intersect point is from the tire's contact-patch center, the more steering effort is affected by caster. If it's too far away you simply can't hold a straight line on a rough road with no power steering. There was a design used on heavy trucks that I'm familiar with called center-point steering. This put the contact point of the king pin line exactly at the center of the tire's contact patch and used hubs that mounted the wheel further inboard. In the days before power steering on heavy trucks (1970s and before) this was used to reduce driver fatigue and was not affect by caster angle. I did not like it as the steering required too much correction.

If you still have the original wheel/tire combo, you might try putting them on to see if the problem goes away. Failing that figure out what the offset is on your wheels. Remember, offset is not the same as backspacing and if you have wide wheels the proper offset will not give you enough backspacing to clear the tie-rod ends. With 17 inch wheels on a 1-ton the tie rod ends isn't such a problem but it is with 16" wheels.

I have looked high and low for alloy wheels with positive offset and I don't think the aftermarket supplies them. I did find 6", zero offset alloy trailer wheels and have used those on my 1-ton and it steers fine. I cannot use 25mm positive offset Chevrolet steel wheels because of the tie rod end issue. I tried.

Bztguy,

Offset = distance from the mounting surface of the wheel to the centerline of the wheel. Farther outboard is (+) offset, inboard is (-) offset. Centerline directly over mounting surface is (0) offset.

Backspace = distance from inboard edge of the rim to the mounting surface of the wheel.

Incorrect offset can also damage spindles and wheel bearings since it determines where the weight of the vehicle is carried on the axle.

I can't thank you all enough. I dug into it last night. Pulled a caster shim out and it is the factory required angle. I pulled a wheel off and measured it to be 16x6 with a 1/2" positive offset. I had put in 1/4" wheel spacers to give me more breathing room between the tie rod end and the rim but I removed them and I still have a solid 1/4" clearance from tie rod end to the rim. Now for the grand finale.......and probably the main culprit. I had 1/4" of toe out! I dialed her in to 1/16" toe in. It drives like a entirely different truck now. Steering is lighter and more relaxed and it doesn't dart like a jack rabbit over every bump and dip in the road. Before I had to two hand drive the thing and prepare for it veering every time I hit bumps or a roll in the road. Last night I drove it relaxed with one hand on some smooth and bumpy roads. Across railroad tracks. She tracked straight and clean through it all and never darted. Thanks!!!!

Good to hear the problem is solved and thanks for letting us know.

Hey that's awesome bzt.

I also find my truck to be a pleasure to drive at any speed. I think a lot of people don't take the time to set their antique vehicles up correctly and just assume that they handle poorly. Then they waste a lot of money trying to make a 75 year old truck operate like a modern truck with Mustang II front ends, disc brakes, etc.

Oh I agree. I had thoughts of doing a front end conversion at times thinking that was the only way this truck was going to pleasant to drive. But the cost and my desire to keep things as stock as possible stopped me from taking that route.

Otto: To clarify, Positive offset moves the mounting surface outboard relative to the centerline and will move the wheel centerline inboard and vise versa. See Les Schwab website for illustration.