Or maybe a dumb builder... Reworking my front crossmember [there were cracks and a nasty plate added] I'm able to raise the center about 1/2". [in the photo I had cut the nasty plate off the top] I'm figuring my over-all rake is going to be 3-4". The initial caster angle on my front x-member was 6.5 Degrees. With the rake the angle will be less. My plan was to raise the rear of the frame 3.5" and set the top of my crossmember to 6.5 > 7 degrees to get the correct caster. The dumb part is, I did this; "With 6.00 x16 and 7.00 x16 tires it gave me about 1-1.25 " drop/rake." But, I didn't record any rake, [if there is any] with the original wheels and tires. So my question is, is the top of a stock model A frame level? My second question is, I was told that 32>34 spindles will lower the car an additional 3/4"compared to stock Model A spindles. I was also told it doesn't. Can anyone say for sure? Thanks in advance, robj
OK this is just my opinion and the way I do it. I set the frame at the height I want when car is done, so IF you want a 2" rake that's where I set the frame height to. Once I have the rake figured out then I figure out tire height front and rear. So let's say 24" tall front and 28" rear. This gives me the height of both front and rear axle center heights, so with 24 and 28 tall tires, half of each tire is 12" and 14" minus 1" for both for tire wall flex. Now I have the center height on both axles (front spindle center). Now that I have axle height for rear and spindle center for front then I mock up the front axle and set caster to front axle (let's say 7°). Now I have an accurate way to set angle of the front crossmember. I can do this several ways, a quick easy way is I take a couple straight edges and clamp to the top of the king pin bosses, front to rear, then I set one across top of front crossmember, again front to rear. Then I just start tilting the crossmember until all 3 straight edges start lining up. If I want to get real picky, once it's roughed in with the straight edges then I go to an angle finder to make sure the angles on top of the king pin bosses match top of crossmember. I never worry about where the car originally sat when I working with a bare frame, only where I want it to set when I'm done... ...
Caster IS NOT SET BY THE CROSSMEMBER! Caster is the angular relationship between the axle and the horizontal plane of the ground. The tilt of the crossmember is set to alleviate some of the leaf spring twist when it cycles through the arc set by the radius arms (wishbones).
The crossmember is set perpendicular to the caster angle of the axle, with the full load of the vehicle set on the suspension. This is the neutral position. As the suspension cycles up and down, the effective caster angle changes, but the crossmember does not move. This puts the plane of where the spring attaches to the perches out-of-plane with the crossmember, deviating more on both compression and rebound. this twists the leaf spring. Due to the nature of the construction of a common leaf spring, this twist is concentrated on the very ends of the main leaf, where it is not supported by other leaves. It is not just twist, either, it is also deflected. In the original design configuration, this is somewhat offset by putting the axle slightly ahead of directly under the crossmember. Leaf springs are a compromise. They work well, but are not the best thing out there.
This is why I always set the angle (caster) of the front axle first then set crossmember as close as I can get to that same angle, also by doing this I have a spring pack that is moving (or as close as possible) to the same angle as the front crossmember thus helping to alleviate the twisting and binding of the front spring and contact between the spring pack and front crossmember. I see this a lot in home builds, then they will try putting pivoting spring perch bolts which help some but doesn't cure the problem... ..
The spindles won’t change the height, but changing to a 33-34 axle will. Find the thread on old Ford axles and look at the shape of an A axle, relatively straight, with the 33-34 axle, some drop. I don’t know how much difference there is, maybe an inch or so.
I understand what caster is, ie. the relationship [tilt?] of the axle to the ground plane and I understand the axle moves in an arc pivoting at the wishbone ball. [This is with my un-spilt wishbone] This obviously puts twist into the spring at the upper and lower ends of its travel. I think the desired position is, let's say 7 degrees castor at that neutral [resting?] spring position. My thought was, at that neutral position, [maybe a resting position?] The factory had 5.5 - 6 degrees "rake" on the crossmember. [I think some recommend 7?] Mine actually measured 6.5 degrees with the frame level and a perpendicular level against the front crossmember representing the "ground" used to measure the existing angle against. If I duplicated that angle, with the frame set up to the rake I expect wouldn't that that put my spring in that same neutral position as the factory designed, as in no twist at rest? And if the spring is in the same positions as the factory had wouldn't the relationship between the spring and the ball end of the [un-split] wishbone remain the same as the factory resulting in the correct caster? Obviously this all depends on my estimate for the drop, front and rear, with the body and engine installed being correct. And, the caster of the axle being correct to begin with. [Note, this is with the banger engine but no fenders.] robj
You would have to change the rake quite a lot to affect the caster to any significant degree. On a 103" wheel base Model A it takes a rear tire diameter increase of 4" to change the caster by around 1.1 degrees. Just for ****s and giggles, that would be about an 18" increase in rear tire diameter to lose 5 degrees of caster......
7.00x16 is 30.9" dia. 6.00x16 is 28.3" dia. Difference in diameter is 2.6" Only the radius affects rake. So difference using these tires would give you around 1.3" of rake. (You say you measured it at 1-1.25") My guess.... With the same size tires front and rear the frame would have been damn near close enough to level. Now, if you are going to run 4" of rake we should be able to work out what crossmember angle to have. If 2" of rake change on a wheelbase of 103" removes 1.1 degrees of caster, you will lose 2.2 degrees (approx.) with a 4" rake change. 6.5 degrees + 2.2 degrees = 8.7 degrees........ Let's call it 9 degrees for crossmember angle and shim if required. Check the math for yourself. No responsibility taken. @anthony myrick , you were close man, real close. Well done.
Best way to do it is to mock up the actual parts you actually intend to use. No fudging, or you might get a different result. Just like you could using somebody else’s measurements.
Again as in my first post, I always set up frame height first when doing any type of suspension setup. Comes back to the old saying measure twice-cut once.... same idea, mock up, take measurements, tack weld and measure again.... ..
I guess what I will be doing is set the frame up with my anticipated rake then tack the top of the crossmember at the desired 7 degrees. Then if necessary changes can be made. Thanks to all, robj
After more thought, I tacked the top of the front crossmember in at 9 degrees. I just got my axle back a few days ago, dropped and stretched 3". I put the axle in and attached the wishbone. Obviously the wishbone location being fixed pulled the axle back to its original location. [regardless of the angle of the crossmember, probably putting twist in the end of the leaves but I really wanted to just check the caster. ] I first put a digital level on the top of the kingpin lug/mount and got 4.0 deg. on the drivers side and 6.5 deg. on the p***enger side. Obviously cause for concern. Trying for more accuracy I turned down some tubing to a snug fit in the kingpin hole and measured again. Drivers side 3.5 deg. P***. side 5.75 deg. My estimate is 4.5 -5" of rake so my caster will need to be more than 6 degrees. [I won't really know for sure until I mock up with the engine/ trans in place, the body on and the correct size tires on the ground] When I get to a final number it looks like there are some choices on how to get to the correct caster angle. 1. bend the axle to the correct caster [determined with the car mocked up, [weighted and on the ground] 2. lengthen the wishbone 3. split the wishbone [and if that were the choice would a hairpin type give more adjustability? Also, I'm debating on at this point bringing my axle [the 3.5 side] to at least the stock spec. [6 deg.] myself. I have a rosebud tip for my torch but I think I may need a bigger one. My thought was turn a piece of bar stock to fit in the kingpin hole long enough for decent leverage, heat and tweak. Have a decent vice bolted to a 4'x4' x 3/8" plate on my bench so I think the vice will handle it. Or, if bending the axle to achieve the desired caster is the way to go, do I not even worry about it until after mock-up, then bend the axle to the correct caster. And at that point adjust the crossmember? Sorry if I'm appearing obtuse, I've never been here before. Thanks, robj
As an indication of how this is making me nuts, I'm replying to myself. Anyway with spring twist and possible binding in mind, I disconnected the rear of the wishbone, Then moved it up and down until I had achieved what felt like a neutral position and blocked it there. I rechecked the caster on the more accurate side, [p***.] and got 9 deg., which, if my estimate of the final rake is accurate, probably puts me at 6 deg. caster. [note; at this point the frame is leveled] All well and good, but, when looking at the radius ball, it's forward about 3/8 to 1/2". If that's raised and pulled into position, the spring no longer feels like it's in a neutral position. So do I reduce the angle of the top of the crossmember to a more stock position? [which I would think would put the radius ball farther back where it needs to be] Then put all of the caster I need into the ends of the axle? My brain keeps telling me the top of the crossmember should be at 6 deg. [to the ground plane] taking into account the rake. But, that puts me back to where the ball is too far forward. Is the spring supposed to be perpendicular to the ground? It "seems like" if the crossmember mount angle was reduced, [to put the radius ball where it needs to be, the top of the spring would be leaning forward, [due to the rake] What the heck am I missing here? robj
Although going to split wishbones with adjustments at the axle looks like the easiest way to build in caster adjustability, I re-read my initial question in what became by build thread and looks like the general consensus is you're better off not splitting the axle. Hot Rods Model A Rake and Mechanical Brakes, etc.. Build thread?
Do not bend the axle for caster. Just don’t. If you need to adjust, it’s very common to re-slice the wishbone weld right behind the axle, remove a small piece of pie cut bit, close the gap and reweld. Of course this will require precise measurements, testing those measurements, and expert welding. The length issue might be cured a smidge by heating and bending the rear yoke a bit tighter, essentially lengthening the yoke. From a U shape to more of a V shape.
Thanks Alchemy. 1. Ok, but don't I need to bend the one side of the axle so both sides are the same? I'm getting 5.75 and 3.5 degrees. [obviously the 3.5 deg. side] 2. I just watched a Youtube on pie cutting the wishbone last night and did think that was a potential solution to the height issue at the rear of the wishbone. Obviously the "precise measurement" part is paramount. 3. Good Idea. Didn't think of that. I was stuck at lengthening the wishbone. It's such a small amount that may work fine. Another thought. Is it that terrible of an idea to split the wishbones? What are the downsides? Seems something like this would give all the adjustability one would need. Thanks for taking the time to reply. It means a lot. robj
BEFORE I did any twisting on the axle for caster I would take the axle out and clamp it in a vise right in the middle. Then I would take caster readings on both ends. I want the axle totally free of anything that might help put it into or hold it in a bind. I want "free" axle readings as a starting point then move on from there. I also want the ends to be close to axle center as far as relationship side to side. Example: IF WE HAVE way 5° on one side 9° caster on the other side, I will take a reading as accurately as I can to axle center caster reading. Then I would take the end furthest from axle center and move that side. I want the axle as straight from one end to the other side AND keep good caster readings from end to end on the axle itself. This way I know my spring arch is following the caster arc. Then once that's done I would figure out spring position to get the caster I want. Then move to cutting and pie-ing the bones to keep that caster there. The shop I worked in for almost 40 years had the tooling for bending caster end to end on these old axles on the cars, done it dozens of times over the years. When it closed a couple yrs ago I don't know what happened to the tooling, would have loved to have bought it all though... ...
Yep. Take a reading with no pressure on it, then bend so every hole in the axle is the same plane. Who did your axle dropping? Just so we can know who would send out a crooked axle.
"Example: IF WE HAVE way 5° on one side 9° caster on the other side, I will take a reading as accurately as I can to axle center caster reading. Then I would take the end furthest from axle center and move that side." So with the axle clamped in the vice in the center, the axle should be plumb [in the center section] correct? This: "axle center caster reading" was confusing me a little. In the vise the axle should be plumb then take the caster reading at each end. Correct? rj
Well I'm not 100% sure I would like to say.. It was a well known guy, but not Nostalgia Sid. He reiterated he would correct any issues but my thinking is why not get it right the first time?
Maybe give him a call to see if he wants to repair it for you? Including shipping costs, of course. All four holes should be on the same plane. Always. Four degrees difference is embarr***ingly wrong. You could probably see that with your bare eyes.
@tricyclerob it doesn't matter how plump the axle is in the vise. All we want to know is relationship end to end. So all we need is the caster readings from each end to compare to each other. Let's say with axle plump we have 4° on one end, 7° on the other, that's 3° difference. Now say the axle isn't plump, now we read 5° on one end and 8° on the other, we still have 3° difference. All we care about is that 3° difference. So the axle in the vise angle isn't all that important, it's that same 3° difference that IS important. Now on the axle center angle, yes I just want to take the angle finder and put it up against the upper and lower axle flange close to the center of the axle as I can and read the caster angle there, just get it close to the vise as you can and take your reading, take a reading off of each side of the vise on the axle if that makes you feel better. All we are trying to do is get some kind of number off of the middle (or close to middle) of the axle and each end. The numbers don't have to be exact as installed in the car, we just want the readings to read what the relationship from end to end of the axle is. Hope that makes sense and helps you out ! ....
The Jalopy Journal
