This will be number five in the study of 1926-27 Improved Models various parts and changes. I thought the last study of the front axles would be simple, It was not, so Im not going to venture a guess where this will go. The only comments I can make is, that there may be, a special radius rod for Improved Models. I have only read about it a couple times, and have been told a few time about them. So all I have to go on is a little bit of "hear say" info. Not a good scientific method, but maybe it will turn up some more info from the forum. I have heard that the angle of the "eye" end of the radius rod is different, because of the lower chassis. ???. I went thru the 18 radius rods I have and compared the ends to each other. I came away from the comparison with a belief "there may be a difference". Of my 18 rods I found 6 with the aprox. 15 degree angel and 12 with the aprox. 7 degree angle. I have shown a pic of the two different angles side by side. I did a crude but pretty accurate drawing of the 2 angles. The angle of the photo, shows the "bottom of radius rod" line above the bottom of the rod, but it is the camera angle. I tried to be as accurate as I could, and be parallel to each surface. The angle difference of the radius rods, appears to be just in the "eye end castings". It may be possible my "castings" are bent, but I had enough radius rods that were in good looking condition, and did not appear to have been abused, to give a decent comparison. As Forrest Gump once said, "that's all I have to say about that" so let the discussions begin .....
Since bending this part of the front radius rod sets the tilt on the axle,it is a moot point.
I just want to mention, that I may be totally wrong on the radius rods being different, but there is just enough info for me to consider it worth a discussion, If Im wrong OK, If Im right OK, or If everyone thinks Im crazy, that's OK also ... (no comments Mike W. ) But this may be part of the reason, there are issues with the steering parts rubbing on the radius rods on lots of 26-27s. ?????
Jack, I agree to a certain extent, and you may be right, but are we "bending" them today to "restore" the angle as "Ford" built it. ???. The difference may just be "an old wive's tale" but in the "studies" I think we need to lay to rest as many as we can ...
If you could teach parts to talk,you could write a lot of books.....
When I looked into this, I found that some rods were slightly shorter.
The '26 cars were lower on the spring, so the radius rod had to have a lesser bend - as above, and also be just a little shorter.
I'll try to find the data. When I fitted a shorter, less bent rod, my castor angle went down to spec.
Chris. When I restored my 27 touring last winter, I had some issues with the tie rod rubbing on the radius rod. I believe at that time is when I was informed of a different radius rod. I also found a different radius rod at that time and solved my rubbing problem. I did not keep any records of what I did then as I had not thought of the studies at that time, but now that you mention it, It may also have been shorter. (I will need to measure my stash of radius rods to see if any are shorter). I just do not remember for sure what I did on my car. I also had to change my drag link to a correct length, and change one spindle arm. The drag links and tie rods are the topic of my next study. Thanks for the input...
Donnie -- Since you found several with each specific angle, it would lead one to believe that they were made both ways, rather than some of them just being bent. I have never heard of a change in that area, but it just may be that no one noticed it before.
The '26-7 spindles were modified to include only about half of the camber of the pre-'26 ones, so maybe the caster was changed at the same time.
On my 26 I had to change the angle from about 4.5 degrees to a calculated/needed 14.5 degrees to get the caster correct.
More discussion of this at:
Here are all my data, set out in this earlier thread:
and here is the main description from it:
Now some basic data. It was a 3-column table, but it won't format here, so I've added commas!
Dimension, My '26, 1923
Ball height, 14.0, 15.63
Axle ht., 11.9, 12.43
Ball-axle, 2.1, 3.2
Radius, 25.98, 25.98
Angle degrees, 4.63, 7.06
The first 2 rows are inches above ground. The axle height is to the radius rod interface. The stub axle positions account for the 0.5" difference in axle height. As you say, the flatter spring changes the height by 1.1" (OK, it should be 1", but after 85 years this is close enough!)
The Radius figure is the dimension from the ball to the mid point of the axle. The perches are 36.5" apart, and the radius rods are (pre-26) 31.75 long. Solve this triangle and you get 25.98". And if you do the sums, you see that this 1.1" changes the caster angle by 2.43 degrees.
So if you take a 1923 car which conforms to the design, and has 5.5 deg caster, then just fit a flatter spring, the caster will go from 5.5 to almost 8 deg. Mine was 9, but again, after 85 years......
It's also important to think about the connection to the spring - which occurs at the spring side of the shackle. This is 3.2" higher than the radius rod interface, and if you look at the table above, that is also exactly the same height as the rear ball on the 1923 car. This means that for small deflections, there is no fore-and-aft strain at the spring connection. Even when the shackle pin is 1.1" higher than the rear ball, as on 1926 cars, there won't be very much.
OK. So now we have a 1923 car with a '26 spring, or we have a 1926 car with a 1923 radius rod. The geometry will be the same. There will be almost 8 deg of caster.
If you mentally disconnect the spring for a moment, you can consider the effect of bending the radius rod end lugs to bring the top of the axle forward so that you have 5.5 deg of caster. But now, the shackle will be out of alignment - the perch side will be about 3/16" forward of the spring side.
I selected about 12 wishbones from Neil Tuckett's collection (pile) (all post 1921, with the tapered nut seats), and found that 3 of them were slightly shorter than the other 9. Long ones measure 31.75" along the rod (hole to ball); shorter ones are about 31.5". When you solve the triangle again, the radius of these is 25.67 - about 0.3" less. I have a photo of 2 short and 2 long side by side.
I also measured the angles of the lugs.
I have 4 wishbones here, 2 'long' and 2 'short'. I measured the angles (both lugs of each wishbone) as follows.
I placed the wishbone on a flat floor with the ball supported on a small screw jack. I put a spirit level on a lug (the surface which meets the axle), with the level pointing fore-and-aft. I adjusted the jack height to get the bubble in the middle. The lug angle is given by the difference in height of the ball and the lug surface, divided by the radius (25.9") x 57.3 degrees. My car's 'long' wishbone, and a similar spare have angles of about 13 degrees; the better short one (which is now on my car), was 11.
If you have 13 degree lugs and a 1923 car, the caster should work out at 13 - 7.06, which is just under 6 degrees. With 11 degree lugs and the 1926 car, the caster should be 11 - 4.63 which is 6.3 degrees. When I fitted the latter, it actually measures about 5.5. Close enough for a Model T - and much better than 9! In practice, when you attach the spring, there is bound to be some 'strain' in some direction, and the radius rods give a little to accommodate this.
I conclude that 21-25 cars should have a wishbone with 31.75" rod lengths, and (5.5 + 7.06) = 12.56 degree lugs. And a 1926 car should have 31.5" rod length, and lugs at (5.5 + 4.63) = 10.13 degree lugs.
These cannot be precise measurements - a lot can happen in 90 years, and factors such as the ride height at the rear of the car (and therefore its weight) will have an effect. But changing this rod on my '26 Coupe has increased the clearance between the steering cross link (drag-link?) and the RH radius rod from under 1/8" to about 1/2". And it's now close to spec.
I am interested in the effect on shimmy. I have suffered this in the past, at about 12mph. It was clearly a problem when the cars were new, and it wasn't understood then. It's obviously an instability - like a clarinet reed vibrating, or the motorcyclist's 'tank slapper' effect, and just getting rid of wear and play does NOT fix it. The only reliable fix is to add some friction. It can be a steering damper, but in my case, just 'nipping up' the spindle bolts did the trick (not to make the steering stiff, but just perceptible). Now, part of the instability mechanism is the restoring or self-centring force. I have now reduced this, which might improve matters, or it might just change the speed at which it occurs. However, I'm not going to slacken off my spindle bolts to find out!
You mention the effects of pot-holes. As you say, a rearwards impact on a wheel will probably make a temporary increase in caster angle - the top of the perch will probably bend the spring back rather than the bottom compress the radius rod. The 2.43 degrees caster angle change for 1.1" axle height change will be fairly constant (and applicable to earlier cars too). But I suspect that the greater effects of a pothole are on the steering. I have never understand why the drag link outer attachment is below the track rod. It makes the drag link very non-horizontal; when the RH wheel drops, it will pull the track rod left and apply a right steering input. So bump steer is very noticeable. It's just as well we don't go faster!.
Anyway. Enough for now.
Chris, Thanks for a very good explanation, It explains why I needed a shorter wishbone. My tie rod had zero clearance and had worn half way through the top of the wishbone. I could see in "my mind" why just bending a rod that is too long does not solve the problem. I was good at geometry and algebra back in school, a loooooooooooooooooong time ago, but now I do not remember how to "prove" things out. Jim, thanks for your input and photos, It appear we all came up with the same aprox conclusions using different methods. You input is greatly appreciated. This type of thing is another reason for the "studies", I have read many posts and talked to lots of people thru the years about 26-27 steering parts issues (I did not know the answer, but I knew something was wrong). There has always been a problem with the parts being "mixed" and things not working right. So with the conclusions above, it appears there is a shorter radius rod, with different angle ends, to use on the 26-27 Improved Models. and to answer Jack's comment, "we are trying to make the parts talk"