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
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