How do I build some negative caster into this?

I'm building this behind the axle setup and running into a few problems. I put some old bearings between the bones and the perches and got the shackles in the right spot when loaded. Turns out I need a 27" spring. At least I can mock the rest of it up now.

When I built the ch***is I built 8 degrees of static negative caster into the front cross member. So now the spring, shackles, and perches are all in line and cycle bind free. But now my axle sits at 0 to 1 degree positive caster, not what I want. Before I go any further, what do I do with the wishbones to get some negative caster into the axle?

 

You are somehow going to have to "tilt" the axle back about 8 or 9 degrees to get the 7 or 8 degrees of caster you are after. It might be that you have to pie cut the wishbones in some spot to do that, or you might have to lower the rear mounting spot......maybe even a combination of both. It looks to me like you would need to pie cut them ahead of that perch setup you have.

I am also not nuts about using the original perches as you have them there. It might be better to build some gussets and hang new perches on, closer to the bones, that way you can use a wider spring.

Don

I just noticed something in the pictures. Are your bones upside down ? They are made so that one end is further forward which leans the axle back. If you put them on upside down you get caster the wrong way. Hard to tell from the picture but it seems possible to me.

 

Those are new perches (forged) from Speedway holding the spring in place. The ones holding the axle to the bones are just for mock up and will be replaced with later style perch pins.

 

Don is on the right track. I would cut the wishbone where it is welded to the forged front section that attaches to the axle. You can then cut the front end of the wishbone at an angle and reweld it to the forged section. I did this on a Model T tub with a 401 nailhead and it has been on the road for over 3 years without a problem. Good luck and let us know how you make out. Dale

 

Yeah....looks like the bones are upside down. Swap em side to side and flip em over and see what happens.

 

You used '37 through '41 bones and installed them upside down I bet... Flip them over and life will be better. Other wise, piecut them right after the joint with the forging and dial in the POSITIVE caster you need. The crossmember's POSITIVE caster would only effect the out come if the spring was in it's original place above the axle.

 

This picture will show what I mean better than I can explain it. See how the bones are shaped to kick the axle back ? I bet yours are upside down, that is the only thing I can think of that would reverse the caster that much.

Don

 

Well, in the first place you don't want negative caster you want positive caster on an I beam axle.

Positive meaning that if you draw a straight line through the middle of the spindle the top part of the line will be behind the center of the spindle.

In all the years I did front end work and alignements I only saw about two maybe three rear wheel drive rigs run negative caster from the factory.

 

Yep, '37-'41 bones when mounted correctly the weld/seam should be on top.

 

It's possible I have them on upside down but I don't think so. Although I did finish this mock up at about 4am. I'm at work and can't look at the car now. Maybe somebody can see in this pic. They're actually Model A bones.

 

I believe you might be getting your positive and negative mixed-up. Negative caster is like a shopping cart's front wheels. You'd never want any negative caster. Looks like need louvers? might be trying to clarify that for you. Just sayin'...

 

Damn,
Mr48Chev beat me to it...

 

Man, I'm all messed up. Got my casters flip flopped, wishbones might be upside down, must be the heat. ***uming the wishbones are installed correctly I guess I will try dropping the rear ch***is mount down a little lower. If that's not enough I'll pie cut the wishbones at the yoke and tip them back till I get my 7-8 degrees of + caster. Then I'll tack the perches in place on the bones and dis***emble the whole thing to finish weld it. Sound like a plan?

 

Doesn't matter if they are model A bones or later bones, there is a topside and bottom side. Just look at the profile and see if it looks the way the picture I posted looks.

Also, shoot a couple of good pictures of that area and also a side shot showing the full wishbone and where it mounts on the frame. That will help us figure this out for you.

Don

 

Thanks Don and everybody else. I know there's a right and wrong way to mount the wishbone to the axle. Going to feel pretty dumb if I that's my problem.

 

In piecing together the images from your post, then looking at Dons, you might have them flipped. If so, then right-em and see just how low the tierod end of the bone needs to be.

 

Hey, if you think some of us (me) haven't installed the bones then realized they were upside down, you haven't been peaking in our garages.

Just the other night when we had the 3M panel adhesive all laid down I put the first sheet metal floor panel down, and it was upside down ! Shoulda seen me cleaning all the goop off of the topside of my new floor. Dan said he was going to get on me about it, but I was doing such a good job of calling myself a dumb*** that he just left me beat myself up !

Don

 

Think of positive castor like on a chopper motorcycle...front tire out front, with the fork tipped back...just like you know the axles/spindles should be...tipped back.

This shot seems to show that the spring and perch are tipped back, but the pin isn't. Wishbones have to be upside down.

Be happy. Upside down is an easy fix...except in Don's adhesive escapade...lol.

 

If those really are Model A bones, then they are not upside down. Take a look at pic 3 in post 1. You can see the yoke of the bone is placed so that there is more room for the tie rod to go over the bones, just like when A's came from the factory.


BTW, 7/8 Degrees is a wives tale from the past. Look up Ford specs in an old repair manual. The typical V8 Ford I beam ran at 4.5 to 9, and the midpoint is 6-3/4, so someone chose the midpoint back when, and everybody copied him, as gospel.



Put up a side profile pic from down lower, to show what is going on there, to be off that much on caster..bone angle, bone rear mount height, etc

 

I just took this pic of a model A wishbone. You can see that on the A bones, there is nowhere near as much lean on the perch bolt centerlines, compared to 32, or 33/34 bones.



If the OP has his bones heading downhill as they go towards the front end, then you will end up with no caster. Put up a side pic of the build.


Also, I don't get why the crossmember was set to 7/8 degrees where the spring sits? This is not a spring-above-axle setup, so that is not right.

 

Well, he wants the spring and axle to swing in pretty much the same arc, which is why he set the spring angled back like that......I did the same thing on my cars.

Don

 

While you're at it, check your turning radius. The bones look like they are mounted way out there. Maybe it's just the angle of the pic.

 

I'm lost with that statement, Don. What does the axle caster setting have to do with angle of spring mount on that type of suspension?


Spring above axle, then Yes, you do set the spring mount the same as the desired caster of the I beam...or use adj perches.

 

I'm at work till 7am so I can't go check anything right now. Is that true, Model A wishbones don't have as much + caster built into the yokes as the later bones do? They're definitely Model A, I cut the yoke off for the Model A/33-34 hybrid wishbone that's under my AV8 roadster. These were left over and I figured I'd use them. As soon as I get home I'll post some better pics. Thanks again fellas.

 

I think Don is suggesting to mount the spring such that it travels in the same arc as the bone in suspension travel, to avoid binding. Makes sense....unless the perch is designed to rotate on the bone? Given the amount of suspension travel, is this an issue? Or do the bushings absorb the potential bind?

 

Heres a few things I'll mention. Not saying your wrong.

On a suicide setup like this, I like to use 35 - 48 front bones. They are alot thicker in the tube section. Model A's are pretty thin.

Its nice with a setup like this to not weld the perch to the bone. If its not dead nuts you can get some shackle bind that might cause problems and a ****ty ride. Take those perches to your machinist buddy and have him cut the 5/8" threads a few more inches up so you can bolt them thru the bone using a crush tube.

I would set the rear of bones where they look good on the frame, and then piecut the front of the bone to get the correct caster.


Good lookin Roadster!! Ive been watchin your build.

Rob

 

Step 1. already noted Positive caster tips axle to rear of vehicle, negative to front.

Step 2. Cross member angle will have no bearing on the caster when using swivel spring perch mounts. Rigid spring perch mounting places undue stress on the mount as the axle pivots and suspension spring twists to compensate.

Step 3. Wishbone Caster angle is determined by the rear most mounting point of the wishbone measured at the axle face from vertical with the vehicle at ride (rake)height front to rear.

Step 4. Before determining final wishbone rear mount position the steering arm drop and Tie rod clearance to wishbone must be considered especially using dropped axles.

Step 5. 4-6 degrees is sufficient caster angle, the lesser being better approach.

Step 6. Should necessary caster be determined by excessively long rear mount bracket to frame, the mount may be shorted up by pie cutting the wishbone tube at a point where the tube to the perch mount casting join and removing a determined angle of material from the tube side and re welding the perch mount to the tube.

Step 7. In some instances based upon radius rod design additional tie rod clearance to wishbone and caster angle can be achieved by splitting the perch mount to radius rod tube mount weld, swapping the perch mount side to side and flipping them upside down. Thus giving more off set to the top side or bottom side of wishbone for tie rod clearance.

 

On the subject of "pie cutting" wishbones, this is the method I prefer. After deciding how much more caster is needed, I cut from the bottom up, leaving about 1/4" of material to hold it in place. I heat and bend it the desired amount and tack weld it to hold. At this point you can install it to check caster, if it is good then weld it up. It gives me a means keep everything aligned as I proceed. This example is a 32 that has not been split.

 

I'm surprised nobody has mentioned that the caster will also be affected by how the car will sit.

When you said you built in 8 degrees into the crossmember, how was the frame set up? If you have the frame sitting level and put in the 8 degrees, then you put on big & little tires, what is the rake going to be?

I see it quite often that nobody thinks about this when setting up a suspension. Lets just say you get everything together and the car has a 3 degree rake, your crossmember now only has 5 degrees.

I always plan tire size and rake angle that the customer wants, then dial in the desired caster. I like to run no more than 5 - 6 degrees, anymore than that and you risk the dreaded "death wobble" caused by weird spots in the road.

Keep in mind, more caster means better straight line stability, but also increases steering effort.

Don't do anything permanent until the whole plan is in place.

 

You don't have any very clear pictures of the perch mount area of these 'bones. BUT one shot seems to show a very large hole drilled right through the center of the "joint" between the sheet metal tube and the forged axle fork. I'm not a lover of using any Model A radius rods due to their spindly construction, 35-48 are definitely a better pick for this purpose in any non-stock usage.