I have been an amateur Model T mechanic for 50 years now. I've become proficient at some things and chosen to ignore others. When I restored my 1915 Runabout in 1985 it developed a front end shimmy. I read what I could and applied some of the remedies that were suggested. None seemed to help and I learned how to drive out of the shimmy. I ignored the caster as it was set at the factory and the cures seemed to be bending the axle. I feared I might make it worse. This Summer I added a lower wishbone thinking that might cure it. It became worse. A simple look told me it had plenty of caster. Upon checking the angle I found it was over 12 degrees, not 5 1/2 as it should be. The lower radius rods threaded into a yoke with pipe threads. On doing some calculations I found I needed to re-thread the rods to make them 1/2 inch shorter.
I just took a test drive through a park with several speed bumps and nary a shake or wobble. Conquering a problem can make you feel great. That's part of what I like about Model T's.
Everything you need to know about checking caster is covered in paragraphs 147 & 148 of the Model T Ford Service manual. Many aspects of the Model T seem intimidating but are really quite simple. The Forum addresses all these things somewhere or other. There are many things that can cause shimmy and caster is only one and often overlooked.
Nice of you there!
That is the fun, attending to the right job and getting it fixed.
These guys too were probably going to do the same, but the fellow on the right can't really set that caster using a drive shaft, he needs the proper axle bending tool!
And his mate on the left, needs the correct headlamp post bending iron, not that ball pein hammer
After reading Richards post, i decided to check the one thing that i had never checked since i bought my car....HOLY CASTER BATMAN!!! I have about 11/16" on both sides instead of the 1/4" its supposed to be. I find it hard to believe its bent this bad, and this evenly on both sides, but who knows what has been done over the years. Maybe something has been installed incorrectly, and i just don't see it. Can anyone here see anything glaringly wrong? I always thought that it drove pretty well, but this being my first T, what do i know..but i bet it would drive a lot better with the proper caster.
Too much positive castor can also be caused by sagging of the chassis near the crankcase ears. Before bending the axle check this area of the frame and straighten if necessary.
Yes, I do see a problem (unless you've accounted for it).
Its not enough to make sure your square is horizontal, you must also make sure the car is setting perfectly level. If (for example) the rear wheels sit lower than the front, that would increase the caster and vice versa.
Just a thought.
Thanks Norm, I'll check that, but I'm pretty sure my frame is straight...Bud, good point, its sitting in my driveway which is pretty level, but i can see how that could throw off the measurements..whats the best way to take this measurement, put a level on the frame?
John,I have a question about your car on a different subject. It looks like you have flat washers under your rim lugs. Is this a good way to fill the gap and tighten the lug nuts? I'm asking because I just put my front wheels together and have that gap and am a little concerned about how tight to make the nuts. Excuse my ignorance on this but these are the first T wheels that I have assembled.
You might call it bending the axle to set castor but really all you are doing is bending the radius rod to get it right, an easy job to do.
Justin, i added the washers in some places to true up the rim/ fellow..probably not the best idea, but it works for me until i can get some new ones.
Thanks Frank, not sure how bad it really is now to be honest.
I have never heard of "too much positive caster" being a shimmy issue
The greater the caster(positive)the effort required to steer the car is increased however the "return to centre" after cornering is easier. Obviously, there is a limit to compromise between stability and ease of driving. Positive caster within spec or greater shouldn't cause shimmy in my book. Negative caster of course is bad and you then have the "shopping trolley front wheel shimmy" going on. Additionally, true wheel wobble caused by out of balance wheels generally happens at a greater speed and is repeatable but shimmy after a bump in the road was general steering looseness in our case. Every car appears to have its own cause and cure. Very frustrating indeed!
Just my memory from a time in the wheel alignment department of a Ford Dealership in the 60's! Alan in West Australia
Wow, something is funky with my pics, sometimes there's 2, sometimes 3 or 5 if you refresh the page. Did anyone get a look at my spring perches?..I could never understand what the consensus was about whether the boss should be inboard or outboard, and does that effect your caster?..Mine are inboard.
Yes, there IS something funky with your pictures. I kept refreshing until a perch came up. As best I could tell your left one looks to be correct. Haven't seen the right one. The caster is IN the perch, put them on wrong (left one on the right and right on the left) and your car will be almost undrivable: very "darty" at any speed faster than a walk. Please check your right perch as I once saw a T with both perches being of the same side! Not good.
The perch boss should always be towards the back, never the front.
Thanks Bill, i couldn't tell if the pic problem was just on my end. And yes both perches are inbound which is good news. Still not sure if i need to do some bending to bring my caster up to specs or not.
To answer your question about the washers behind the rim lugs; they do not belong there. Contrary to popular belief, it is not necessary to tighten the lug nuts until that gap is closed. What's important to keep in mind, is that the rim seats against the angled flange of the wheel felloe. Tightening the lugs nuts sufficiently enough to draw up the rim onto that flange is all you need to do. Yes, they must be "tight", but they need not be tightened enough to remove the gap. Putting washers in the gap makes the lug nuts tighten up nicely, but may also cause the rim to not seat against the angled felloe flange and therefore not be supported by the wheel as designed.
If you have a rim that fits over the felloe such that with very little or no effort, the lugs touch the felloe with no gap, you have one of two situations to look for.
1). The wheel at point in its existence, was rolled around with no rim/tire on it. This bends the angled flange out flat, or at least at a lesser angle, and the rim will never seat correctly.
2). The rim has been over tightened in the past and rim lugs are bent inwards.
The argument to this is always, "What keeps the lugs nights tight?". The answer is, the beveled end of the lug nut fits snugly into the tapered hole of the rim lug and effectively "locks" the nut in place. Never ever had one come loose.
Thanks for the clarification and I understand now why John is using them until he can get better rims. Both of mine have equal gaps on all and I just snugged them up but not to where the lugs pulled the gap shut so I guess that I'm okay. Yes, I did wonder if they were at risk of coming loose but the lock in the taper makes good sense. Thank you.
That is an interesting comment by Alan George Long. I should mention that by correcting the angle I noticed that the second spring leaf moved to the rear. It had been interfering with the shackle movement before the adjustment. That may be what ultimately resolved the shimmy problem. New shackles, King pins and all bushings were replaced during the restoration in 1985 so no play should have caused the shimmy.
Bottom line, I am very pleased with the result.
Alan George Long is absolutely correct. on my Model T race car #4, using the John Noonan method of measuring, my castor is 11/16 inch. Been running that since the restoration was completed in 1967. It could not track better on straight roads, at any speed. Driving it to the Centennial in 2008 from CA. I was so bored going across Nevada on hwy. 50 (2 lane rd. No Traffic) that I would straddle the center of the road at about 55 mph, take my hands off the steering wheel and see how long I could go without touching it. The record was 16 seconds! The down side is, in the mountains with lots of turns it takes more strength to make all of those twisty turns. The upside makes it all worth it, and although most of you are not going to experience this, on the race track I feel like I have full control. When I complete a turn it wants to spring right back to center. I love it!
Ed aka #4
Ed & Alan,
My speedster also has a lot of caster. haven't measured it, but it's much more than stock. In my case, it was a side effect of using a dropped axle. Yes, it's hard to steer, (so I use 5:1 gears), but boy does it track beautifully.
On a tour a few years ago, my car started to shimmy. It had not done so since I rebuilt the front axle. I found that the spindle arm on the driver's side was loose and I tightened up the nut which stopped the shimmy. Many little things can cause such a problem. All it takes is for something loose to move which sets up a vibration back and forth. The shimmy will usually stop as speed is increased or the vehicle is stopped. So check for looseness of all the parts in the front axle.
Well after prematurely freaking out that my caster was more than an 1/2" out of specs, and a big problem, i have come to the conclusion that it isn't. After checking everything i could, i can't find anything wrong other than me being somewhat anal about specs. I drove it tonight after work, and it tracks perfectly and turns with no problem...I appreciate Alan, Ed, and Jerry's comments who figured this out before i did. If it works, i say "let it be"..
I'm glad to see these comments. Thanks for shedding some light on the subject. I see that I had jumped to a conclusion and glad to better know what is going on up front. There are many mysteries to the Model T and a wealth of information here.