I’ve noticed that sometimes the attachment point of a bone is the middle of the frame and other time there’s a bracket that hangs and is attached there. What’s the difference? Is it the front crossmember, flat vs. model A?
Cowl steering , personal preference and spring perch width goes into it as points of the mounting. Drop in axle may play a part in plan as well.
Esthetics, some prefer the cleaner look without a bracket below the frame. To set proper caster with a high mounted split wishbone will typically necessitate a pie cut in the ‘bones.
This is the way it should be done. I was scolded on here (and rightfully so) for assuming I could just put the bones wherever they laid. This helped to explain it in very plain terms.
Technically, the correct design is like Henry intended it to be, a single pivot point for both bones, on the centerline of the chassis. Split bones with anchor points on or below the side rails behaves like a very stiff anti-roll bar, resulting in the front end absorbing most of the car's roll couple. This puts considerable stress on the bones/axle assembly, and causes an under steering characteristic when cornering vigorously. On the other hand, it's traditional.
Both post right above this are right. But the way OP was made,sounds like a small miss of understanding,that you do need to mod the shape of bones so they hold axle close to 6* to 7* when the rear end is mounted. The rear point the bone is mounted should be for a side steer car,as close to outer end of pitman arm as you can,in keeping with the drawing shown{ so that really is part of your planning ahead for good engineering.
Planning is the main point. I have seen some use a wide spring perch axle and then try to compensate for tire rubbing by modifying the bones .
30 something years ago, when I first joined Downeast Street Rods the safety talk was of split wishbones with a [newly available] tube axle, it didn't flex like the I beam and broke some perch pins... I sectioned the '34 tranny saddle down to 1" thick to let my C4 clear and ran an unsplit wishbone...
Most of the cars that have the rear attachment point of the front split bones in the frame also have a suicide front or a frame sweep. You can pie cut the bones to get caster, but for looks you don't want them running up or downhill much. That's hard to do if you bring the back mounting points up and don't do an equivalent change on the front.
Because when you split the radius rods, the axle becomes a big anti-roll bar (sway bar in common terms). The I-beam cross section of the factory Ford axle has little resistance to twist. The tube axle is a VERY high resistance to twist. A tube axle can use wishbone (unsplit radius rods) because the twist factor is removed with the single rear mounting point. Back to the OP's question, it has been answered by previous posts. But to add a little, the reason for the diagrams showing the (mostly) identical arcs of the two components is to minimize bump steer. That is the root reason why the mounting point issues, once that is done in the right location, the radius rod can be pie cut as needed to get the right amount of caster.
Original frame model A has the wishbone mount to a single point on the bottom of the transmission. From 1932 to 1948 the wishbone mounted to a single ball socket in the crossmember. Lets say youre building an av8 with a 3sp and a flatty. You can use the 32 k member and retain the wishbone mounting point. You can leave the wishbone unsplit though now your model A wishbone is too short. Now lets say you have a huge nailhead with a powerglide behind it youd like to run. At that point youd probably be looking at splitting the wishbone to clear your transmission and bell housing etc.
By suicide you mean spring behind axle? Is there an image I can look for to see what pie cut bones look like?
A suicide front end has the spring mount on the front of the front crossmember and the frame horns cut off at the crossmember. Usually a custom built crossmember. The spring can be in front, on top, or behind axle. The suicide term comes from the fact that if anything breaks there is no frame above the axle to catch anything so your frame is on the ground. A pie cut is a narrow "v" cut so you can bend the front axle attachment yoke to correct the caster. Some front crossmember have built in caster, but after you move everything around it may not be enough, or you need to raise the rear attachment point. Search both terms and you will find lots of pictures.
I use split bones all the time, got a roadster pickup that handles awesome, I use a heim joint at the rear mouting point, I cut the wishbone right behind the perch pin to adjust for caster, it is a place not noticed when seen from the side. No binding or handling issues if done right.
Oh ok I get it now, pie cutting the bones allow the bone to reach the frame. Without a pie cut, you could bolt it to the frame but it wouldn’t connect to the axle properly.
Not exactly right, but I understand what you are saying. Your method will enable to get the right caster while mounting the radius rods to the frame sidewall. Proper caster is one of the things needed to be right for good road manners. But your method does not, or might just get lucky, make for correct bumpsteer. Review the diagrams with the intersecting curves of the radius rods and the drag link. You want these curves to be as much the same as you can to minimize bumpsteer. Just to go off on a slight tangent, the bumpsteer issue is why cowl steering creates so many problems. The two curves are way out of alignment in cowl steering.
Gotcha, I missed the part about the mounting point, how low or how high on the frame To get the right arch. Right? I definitely see why cowl steering creates problems. but if I did cross steering then the bones need to be Parallel to the floor? Like the photos I posted. and get the drag link mounting points even with the bones mounting point of the frame?
Cross steering you can do as you plan. The crossover takes away the bumpsteer issues for most part, assuming you have drag link approx parallel to the axle beam. Bumpsteer is issue when drag link is parallel to the frame. Sent from dumb operator on a smart phone
It has nothing to do with the radius rods being parallel with the floor but rather the caster angle at the spindles. If you don't pie cut the radius rods, you achieve this by how far down you mount your pivot point at the back end. In the pictures you provided on post #18, that pivot point is in the center of the frame rail. Without the pie cut, you will have about zero caster. Before you do the pie cut or mount the back end of the radius rods, mock it up with the tires you intend on using and the rake you want to achieve and tack weld it together and measure your caster angle.
Caster is KING !.... If you decide to let aesthetics rule your mounting point,... And you decide to pie cut the wrist on your bones. Make sure you reinforce the welded area as much as possible. There is a lot of force and shock that transfers through them. So its a pretty big deal.
Back to your question of why the end gets flipped when pie cutting. I remembered reading this in an old magazine article where Mike Key did it for steering arm clearance.
makes sense... and also don’t you think a parallel bone and correct caster compliment each other? My thought is, and according to the illustration mentioned earlier, the mounting point on the frame effect the suspension arch. I.E. if mounting point is lower than parallel to the road then the travel is tilted backwards. And vise versa. But if mounted in the center or parallel to the floor and caster is correct then arch travel should be close to vertical. I’m not trying to be stubborn, and I agree caster is very important.
The crossmember does set the caster,but the split bones can end up needing 8 inch frame mounts because the ends are so low.By pie sectioning the bones near the perch pins the rear comes up so you only need 2 inch brackets which improves ground clearance,and looks.
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