Optimum caster angle?

Bassman/NZ

Bassman/NZ

Well-known member
What is the opinion of the Barners on an optimum caster angle? I've done a search and got wildly varying opinions, so I'd like to consolidate into one thread. Vehicle is AV8, 4" dropped axle, split bones, and the steering is quite heavy. I was aiming for 7 degrees when I built it, but the final result is about 10 degrees, no wonder the steering is heavy. Changing the angle is going to be quite a bit of work, so I want to get it right. Should I shoot for my original idea of 7 degrees?
 
This is one of the areas where different people have different preferences, not sure there is a one-size fits all answer. 7 is a little on the strong side, but if it works for you then that is what counts, I believe 2 1/2 to 5 degrees is more in the normal range. Changing the caster on split bones is not that difficult if you can weld or know someone who can. Cut the radius rod almost all the way through, move it to where you want, tack weld it to hold it in place and then weld it up. Only takes a few minutes to do, just need to decide where to make the cut to get the angle you want, near the front original welds to the forging is a good location.

Note: Did some looking, 6 to 7 seems to be popular with a lot of rodders. Hot Rod article suggest 6 to 8.
 
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My AV8 is set at 6-1/2 degrees and steers great. Bias ply tires grab the road abit, but there is no wandering or hard steer.
 
Here are the specifications from the operators manual for my 47. Range is 4.5-9° so your 7° looks better than 10°.

AlignmentSpecs0001.jpg
 
For some reason Ford seemed to move the caster specs all over the place for different years, interesting. 4 1/2 to 9 shows you there is a wide range!
 
You don't say what tires you are running. Width and type (bias, radial, original etc). This will make a big difference.
Radials will track better so you will need less caster for them.
The less you can run and still not have any wandering or shimmying, the easier it will steer.
Like JSeery said, it has a lot to do with what you prefer.

If you like the way the car handles with a lot of caster but prefer a light feel in the steering wheel, power steering can be installed on vintage cars that does not show.

There is more to this than caster angle also.
Steering ratio can be adjusted to ease steering effort.
Without power steering, enough caster to keep wandering to a minimum can sometimes be compensated for by slowing the ratio.
With a dropped axle, ackerman is also affected sometimes due to spindle arm location.
 
Sorry, tires are 5.50x16 bias. I don't have wandering, just a heavy steer. I have a VW bus box with 40 ford wheel. I think reducing the caster will improve the problem. I can't alter the angle of the bones because of clearance with tierod and steering rod, so will remove the bones and cut and pivot the yoke that the perch goes through.
 
Why remove them, easy to do in place? Less than a one hour job. Also consider that off the axle/car it is somewhat of a guess. On the car you can get them dead on. I wouldn't cut the radius rods right at the forging weld because the forging extends inside the rod a short distance. It would be better to cut the radius rods back an inch or two from the original welds.
 
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I'm wondering what you mean by "heavy" steering. If it's just hard to steer it may be a dry king pin. I just fought that on my last road trip for 1,300 miles. Diagnosed it when I got home by jacking the front axle up to get the tires off the ground and try steering. It was mighty stiff. When I got the drag link and tie rod off of the right wheel I could hardly turn it. Might address your problem. Of course, if the steering is easy off the ground it's probably not your problem. It's at least worth a quick test to see.
 
Old Henry said:
I'm wondering what you mean by "heavy" steering. If it's just hard to steer it may be a dry king pin. I just fought that on my last road trip for 1,300 miles. Diagnosed it when I got home by jacking the front axle up to get the tires off the ground and try steering. It was mighty stiff. When I got the drag link and tie rod off of the right wheel I could hardly turn it. Might address your problem. Of course, if the steering is easy off the ground it's probably not your problem. It's at least worth a quick test to see.
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Thanks Old Henry, the steering is very easy when the front is jacked up.
 
JSeery said:
Why remove them, easy to do in place? Less than a one hour job. Also consider that off the axle/car it is somewhat of a guess. On the car you can get them dead on. I wouldn't cut the radius rods right at the forging weld because the forging extends inside the rod a short distance. It would be better to cut the radius rods back an inch or two from the original welds.
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Thanks JSeery.
 
Bear with me here.
How are the split bones mounted to the chassis?? Bungs and tie rod ends??
If mounted that way , would not the caster angle be changed if the tie rods were turned in?? In effect shortening the length of the wishbone in an arc to change caster angle??

Or am I all wrong?? I have thick skin so enlighten me please.
Paul in CT
 
1931 flamingo said:
Bear with me here.
How are the split bones mounted to the chassis?? Bungs and tie rod ends??
If mounted that way , would not the caster angle be changed if the tie rods were turned in?? In effect shortening the length of the wishbone in an arc to change caster angle??

Or am I all wrong?? I have thick skin so enlighten me please.
Paul in CT
Click to expand...
The effect of turning the rod ends in is just to pull the axle backwards which will bind up the spring shackles. The front only moves about 1 1/2" full travel, so the arc will be minimal. The rod ends need to be raised up toward the frame to reduce the caster, but in my case that's not really practical. I'm actually not after the how, just trying to find the how much. This seems to be the old how long is a piece of string story.
Think I'll shoot for 6 degrees, as that's halfway on Fords chart.
 
Bassman/NZ said:
The effect of turning the rod ends in is just to pull the axle backwards which will bind up the spring shackles. The front only moves about 1 1/2" full travel, so the arc will be minimal. The rod ends need to be raised up toward the frame to reduce the caster, but in my case that's not really practical. I'm actually not after the how, just trying to find the how much. This seems to be the old how long is a piece of string story.
Think I'll shoot for 6 degrees, as that's halfway on Fords chart.
Click to expand...

6 degrees worked great on last a-v8 I built. pitman arm length will make a difference too.
 
Bassman/NZ said:
Think I'll shoot for 6 degrees, as that's halfway on Fords chart.
Click to expand...

Although there is a pretty wide range of actual caster between 4½ and 9 degrees, the more critical is that, whatever angle the caster ends up at, there can be no more than ½° difference between the two sides.

Good luck. Let us know how it all comes out.
 
I know how to change the caster but how do you determine what it actually is ?

OK...don't laugh at my silly question....I'm just the "lock & key guy"

Thanks
 
It is the angle of the king pins to the ground. If the king pin bores are parallel to the spring purch bores you can check it there.
 
I have a tool for checking caster/camber. It is a piece of round stock, same size as king pin , about 4 inches longer with a flat machined in the center of the excess length. I use a magnetic angle gauge on the flat. To the rear it will tell you caster, to the side camber.
 
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