The Stovebolt Site Forums The Shop Area Driveline Caster Angle?

Hello everyone, this is my first post so I apologize if I ramble, do not state the question correctly, or give the needed information for you to help answer my question.

I have a 1959 Chevy Apache 3100 1/2 ton. I completely disassembled the entire truck to the frame. I got the frame and axles powder coated and also purchased new leaf springs and attaching parts. As I'm connecting the new leaf springs to the front axle I have a small plate that sandwiches between the two. It seams to be a small incline. I'm guessing this is what provides the caster angle for my front axle. In my Shop manual it says my truck has a caster angle of 1.5 degrees. I'm not sure if the caster leans towards the front of my truck or to the rear of my truck. I have right now the thicker end of the little plate towards the rear of my truck giving me a caster that leans to the rear of my truck.

Do I have this small plate in backwards?

If I do have this piece mounted backwards, can I just drop the axle and rotate it to correct this issue?

The small plate does have to rest on the axle a specific way. So rotating the axle will also rotate this small plate.
I will also work on uploading photos of this plate between the axle and leaf spring. I'm also new to doing axle and leaf spring work so I apologize if this is a dumb question.

The thick part of the caster shim needs to go toward the front, tilting the axle and the king pins toward the rear at the top. Caster allows the wheels to return to center after a turn by lifting the front of the vehicle slightly as the wheels are turned either direction away from straight ahead. The weight of the vehicle tends to force the wheels back toward center. Think about the fork on a bicycle- - - -the forward slant of the fork pivot does the same thing- - -the weight of the rider makes the bike want to track straight ahead. Zero caster would cause the driver to constantly "hunt" for straight ahead with the steering wheel.

Too much caster will make the vehicle difficult to turn since the driver is lifting the front end with the steering wheel. It will also cause a too-fast return to center after a turn.
Jerry

Hy jeremyA, welcome, in my experience, to achieve positive caster the thick part of the shim is installed towards the rear. Hope that helps.

Nice explanation Jerry. Not being a practitioner, can't tell you how many times I've read manuals or definitions of caster, and still couldn't explain it to you. The half dozen sentences above not only tells us WHAT to do, but also WHY we do it, and tops it off with a simple analogy that we all can relate to. Very professorial and another add to the archive.

Just to provide a counterpoint to Jerry's excellent explanation of why we need positive caster and how it works, some vehicles actually do go the opposite direction and use negative cater. Mostly late '50s to mid '60s Fords, especially the Thunderbird. Given how those cars are setup, with the engine set as far forward as they are, it would be for easier steering effort.

Jerry , the shop manual says the thick end of the shim goes to the rear.

Regards

Can't understand why- - - -that would tilt the kingpins forward, and reduce the caster angle or make it "negative"- - -top ahead of the bottom of the kingpin. Maybe there's some positive caster built into the axle? Could one of the service manual gurus confirm that?
Jerry

Jerry , I've just checked the '37 coupe and the '40 truck and the manuals for both all are as described (thick end to rear).

I do not know about the 1959 shim, but the 47-55 shim has the thick end towards the rear.

http://chevy.oldcarmanualproject.com/shop/1954truck/54ctsm0309.html

I don't know why, but the '55-'59 manuals say nothing about shims at all. I have all 5 handy, just checked them, nothing mentioned.

Thank you guys for all the help. It will be a while before this truck is driving so there is plenty of time to figure it all out.
hopefully this works but as promised here are some pictures.
http://s1378.photobucket.com/user/Jeremy_Happ/media/EF56B4E5-CF2A-49A8-9A1F-F005F603EC93_zpsud46syno.jpg.html?o=2

http://s1378.photobucket.com/user/Jeremy_Happ/media/6620A8B4-88BC-4943-85A8-A01883CFD801_zps7ewa78nv.jpg.html?o=0

http://s1378.photobucket.com/user/Jeremy_Happ/media/B9805384-787C-4E74-B33D-A29D8DDE2187_zpspofjovjn.jpg.html?o=2

I suppose it's possible there's a little positive caster built into the axle, and installing a thicker or thinner shim is used for adjustment. I've been doing and teaching wheel alignment for over 40 years, and it's pretty simple- - -just make the angles do whatever is necessary to make the vehicle steer and handle properly.

One thing we do need to keep in mind- - - -major changes to steering and/or suspension components often have unintended consequences where handling and safety are concerned. A lot of research and engineering goes into designing a suspension system, and a lot of the guys who tinker with a vehicle to make it please them cosmetically have little if any understanding of the physics that makes it steer, stop and handle properly.
Jerry

Putting the thick part to the rear will increase the positive caster.

How? Is the axle mounted under the springs? Positive caster has the top of the kingpin tilted toward the rear. Maybe I'm thinking about a Ford front axle!
Jerry

Think about how the front wheels of a shopping cart work to stay straight going down the aisle.

Placing the top of the king pin slightly forward of the lower makes the wheels want to return to center.

At least that was the way it was explained to me in front end alignment classes back in the dark ages.

Yes, but that's "negative caster". What's the caster specification for a Stovebolt truck?
Jerry

OK- - - -just found a pic of a 53 truck suspension- - - -the axle is UNDER the spring, so the thick part of the shim will go to the rear for positive caster. Sorry about the brain fart!

http://image.hotrod.com/f/10690132+cr1/0807clt_01_z%2b1953_chevy_truck_front_end_suspension_rebuild%2bsuspension_system.jpg

Jerry

Sorry, my feeble mind is messed up. Benn couped up too long.

Here is a link to a 55 Chevy 1/2 ton pickup. Maybe it will work. Not much help though. No specification is given in the chart for a 1/2 ton truck that I can see.

http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm0314.htm

GMC 47-54 lists 2° for half ton and 2°30' for 3/4 ton. I would expect that to be the same for Chev.

I just found the specs for 55-59. It lists 1° +or- 1/2 a degree. Springs lose caster as they weaken. The second series had better springs and probably lost less caster.

I am curious why they didn't just engineer the front axle at the correct castor angle and omit the angled shims. I few pennies per truck adds up to some serious cash when you are talking about millions of trucks. It sure would have saved me a lot of frustration trying to find where I left those buggers every time I take a front axle apart.
Carl

The shims are there to compensate for different loading of the bed, like maybe a utility bed, or a truck that runs with the bed half full of tools. etc. most of the time. If the axle were designed to one fixed caster setting, it would only be right when running no-load, something only the anal-retentive restorers do with their Cowboy Cadillacs these days. Remember we're working with vehicles that were intended to work for a living 50-60 years ago. Shimming an axle for caster correction was a routine procedure when doing a wheel alignment on a working truck back then. Shims come in all sorts of thicknesses for adjusting the caster.
Jerry

Alright, Crenwelge . . . you just brought my brain to a stop.
Why does weakening springs increase caster? In my mind, with the axle mounted below the springs, and the spring shackle at the front of the frame, the springs weakening - flattening, yes? - the shackle pivots forward, and, pivoting on the rear spring hanger, the axle "tilts" toward the rear, which is "increased" caster, isn't it?

Of course, the very fact that I think this is so should be proof positive it works the opposite way . . .

-Michael

So, if we tend to run our trucks empty, would we benefit from thicker or thinner castor shims? Same question for fully loaded trucks. Were the ones installed from the factory for half loaded trucks?
Carl

About 2 degrees to 2 1/2 degrees positive caster should be sufficient, just enough to get a predictable return to center after a turn, and a stable straight-ahead track on a level road with an average drainage crown. On independent suspension systems, it's common to put in about 1/2 degree "caster stagger" where the left wheel is closer to zero caster than the right by a fraction of a degree. This compensates for drainage crown and allows a vehicle to track more or less "hands off" by leading slightly to left and "climb" the drainage crown. A straight axle would have to be twisted to get different caster side to side, something that I haven't seen done regularly since the late 1950's. I have seen an occasional bending of the end of an axle up or down to correct a camber problem, but putting a twist into the end of the axle is a lot more difficult than an up or down camber correction.
Jerry

Since the old pickups were pinned in the rear and shackled in front, the axle tends to roll forward as the springs sag, the caster angle is reduced. Older trucks were designed for very little weight to be transferred to the steering axle. Without power steering, heavy axles are difficult to turn and 50 years ago there was always a fear of a steering axle tire blowing out.

Up until 20 years ago, there were a lot of experienced alignment shops around and it was more cost effective to oursource alignments. The experienced shops closed and tire shops got computerized alignment equipment run by kids with all kinds of certifications but no common sense. And our tire wear became a real problem.

It was then that I decided to buy old fashioned Wheel a Matic equipment. http://www.wheel-a-matic.com/page11.html

We do the alignments with the tractors empty. Doing our own alignments and using nothing but premium Michelen tires and a few Goodyear singles we have drastically improved our tire wear.

s

Speaking of caster...I've been told that if you go from bias tires to radials, the caster needs to be adjusted (not sure how much, which way or why).

Anyone have info on this?

Back when we made the switch from bias to radials on the steering axle, we changed caster. Its been so long that I can't remember how much or which direction. But all of the trade magazines of the day had articles on the subject. But I receive about a dozen trade magazines a month, and needless to say I didn't keep any of them. Bias tires do not react as violently as radials when you hit a drop off such as dropping off on to the improved shoulder. Changing the caster eliminated a lot of this. But for many years, we ran bias on the steer and radials on the drive and trailer just for this reason. When we made the switch from duals to super singles, it just about eliminated the violent reaction of a drop off.

Further to the radials change.

I have been modestly content to be bouncing down the road , skipping over the high spots , anything to keep riding in my truck.

Then I put radials on ..........

What a difference, it goes where I point it , is light in the steering , real light , but thats cool , I gotta keep busy to stay in a straight line , but nothin like the awful crossplys.

Then I put the camper on the deck (610 kg loaded). OK now it's a challange to steer straight, got a bit of windage going on , truck is down in the back.
Made a rear sway bar , added overload springs , got the rims widened and big tyres fitted , things are getting better each time , still a little vague in the directional control.

Found and fitted some thicker castor wedges.

So after a wheel aline check I have gone from 1 1/2 deg to 3 deg castor.
BEAUTIFUL steering control

The camper comes off this week , the bum will be up in the air again , report to follow

As a general rule, radials need a bit more caster than bias ply tires because the sidewalls are a little more flexible. Some highway cruisers like big Mercedes cars, etc, run the caster up to 7-8 degrees or more to assist straight-line stability. At that point, tires start to experience some scrub wear in tight turns because of the camber change they experience as they turn away from the straight ahead position.
Jerry

Yes Jerry , I have one of those old Mercedes . castor is huge ,

So the camper is off the truck and the 3deg castor is very noticable, much quicker self centering and more steering wheel effort to turn . Heavy at slow (car park) speed . Not undriveable but .....

I may change to a 2deg when I get the time.

Dusty, I have a friend who lives near Auckland- - - -are you anywhere close? His family owned a big winery there, and now he grows orchids for export and crews on an offshore sport fishing boat. He wants me to visit there sometime soon and build a couple of engines for him.
Jerry

Auckland is not far as the crow flys Jerry , about 1000kms . LOL .
I have driven there but was put off by the large population. If you view google maps , you will see Canterbury is in the middle of the mainland (south island) and we are very happy that the aucklanders all live in auckland population about 2 million, total population of the south about 800,000.

I really like our quieter country roads.

I hear the hunting is better on the south island, also! That's my other vice besides stovebolts- - -building custom hunting rifles.
Jerry

I smell a Stovebolt road trip a brewing. I'm in!
Carl

P.S. Hey Sweet! Wake up man, there's some serious off-topic violatin a goin on in here.

Dusty, thats pretty good information about your caster and the fact that you can tell the difference with such little change to the ride hight. How much do you think the rear of the truck raise or lowered with camper?

Joe

Joe , the truck sits down nearly 3 " with the camper on and there is a little lift at the front too. Not much, maybe 1/2". So it goes from nose down to a bit nose up.

Makes you wonder how many trucks are running with messed up caster. I know mine felt terrible with just a inch or so drop of the bumper hight.

Joe

I know mine is sitting a little low, but I still need to get shims for it. It did not have shims in it when I got it, and I do have the HD spring option, which makes it sit up in the rear a bit more than that the standard truck.

Isn't checking and correcting caster part of every wheel alignment? Are our guys just making a SWAG about alignment, or ignoring it altogether?
Jerry