Caster Math

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Model T Ford Forum: Forum 2018: Caster Math
Top of pagePrevious messageNext messageBottom of page Link to this message  By Arthur Babitz on Thursday, March 15, 2018 - 02:30 am:

So I've read the caster is supposed to be 5.5 degrees, and the Ford manual says to establish that by measuring from the top and bottom of the spindle body bushings to a vertical, looking for a 1/4" to 5/16" difference.

If I'm doing my math correct that establishes a 3 degree caster, not 5.5 degrees. The spindle body bushings are 4.75" apart, and arcsin(0.25/4.75) = 3, not 5.5. The "pencil thick" measurement would need to be more than 7/16" to establish a 5.5 degree caster.

I'm going to guess people haven't been doing this wrong for 100 years. Can someone tell me what I'm missing?


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A Bartsch on Thursday, March 15, 2018 - 10:24 am:

Arthur: You are spot on! My measurements and calculations confirm your values of castor being 3.0 to 3.8 degrees from the measurements. So maybe the 5.5 degree was a typo, that 3.5 Deg was the design value. Since very few if any mechanics had an accurate way to measure the angle directly, castor when adjusted, was actually 3 to 3.5 degrees.

Good eye, thanks for sharing, Now to find a NOS perch and measure the angle directly,,,,,. jb


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem S.E. Michigan on Thursday, March 15, 2018 - 10:35 am:

If I recall correctly, for 1926 & 1927 the caster angle was increased. So, the 5.5 degrees is most likely the later spec.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Arthur Babitz on Thursday, March 15, 2018 - 09:32 pm:

Here's the page from the Ford manual. It gives both the 5 1/2 degree number and the 1/4" to 5/16" number, so perhaps James is correct and it's a typo. Sounds like you've all been using the 1/4 space between "B" and "C" instead of trying to get a real 5 1/2 degree caster?

Ford Manual

FYI, my 26-27 front end with pre-1915 above axle wishbone and perches was measuring a caster of -3.5 degrees and spindle arm bushings were very loose. Not surprising the steering was frightening. I'll be swapping out the parts and doing a proper alignment check this weekend.


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A Bartsch on Friday, March 16, 2018 - 04:14 pm:

Arthur: Agreed. The same measurements are given in the Ford Service Book Essentials, but I do not remember the service essentials mentioning the 5 1/2 degree angle. I think most check and adjust by the method shown above, not by direct measurement of the angle which would be difficult.

I would be very interested, as an academic exercise, to measure the actual angle of a NOS spring perch to see what the original factory supplied angle actually was.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Arthur Babitz on Friday, March 16, 2018 - 11:06 pm:

I have a new spring perch on order. I'll try to measure the angle when it arrives.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker, Somerset, England on Saturday, March 17, 2018 - 11:46 am:

The usual reason for having a negative castor angle is that the perches are wrong way round.
You can turn the axle round to fix it, but NOT the spindles as you must have the LH thread on the RIGHT side.
I guess the logic is that if the split pin is lost, you just lose the wheel after the nut falls off. The other way would result in the bearings becoming tighter until they seize and the axle breaks.....and then the wheel falls off!
The later perches have a centre 'dot' on the top. This must be aft of the shackle housing centre line to tilt the axle back.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chris Barker, Somerset, England on Saturday, March 17, 2018 - 11:48 am:

Once the perch is correctly fitted, the castor angle is really controlled by the bends at the ends of the radius rods. The spring will flex to the axle's angle.


Top of pagePrevious messageNext messageBottom of page Link to this message  By James A Bartsch on Saturday, March 17, 2018 - 12:27 pm:

Chris: Correct, particularly after the perch and spring connection is worn even slightly. However, I remain curious as to the angle Arthur will be reporting, jb


Top of pagePrevious messageNext messageBottom of page Link to this message  By Neil Kaminar on Saturday, March 17, 2018 - 01:17 pm:

When I bought my car it had the 1/4 to 5/16 on one side and 0 on the other. I had a hell of a time un twisting the axle. It took a 6 foot pipe wrench (with aluminum plate to protect the axle). I used my floor jack on the end of the pipe wrench and it lifted the front end of the car. The only reason the other side did not change is that I have the after market wishbone reinforcement.

5.5 degrees gives a difference between the top and bottom of 0.455 inch or a little over 7/16.

Increasing the caster will make the car steer better going forward by worse in reverse.


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