No,. My point is that it is difficult and possibly inaccurate to measure caster directly (esp. in a repair shop paid by the job). So the difference in camber at different steering angles is used to calculate the caster angle.
Tony I think that your method could be valid in theory, but would be very difficult to do accurately.
From my notes on this subject:
How to measure castor:
Place each of the front wheels on a glossy magazine (Road and Track or Car & Driver). This enables the wheels to turn easily. You’re going to need to measure the camber change over a 30° total steered angle, 15° each way from straight ahead.
Mark a line at a 15° angle from straight ahead on the floor from the centerline of each front wheel.
Steer a front wheel to align with one of the 15° marks, measure and make a note of the camber.
Now steer the wheel to the other 15° mark and measure the camber again. (One reading will be positive, the other negative.)
Add the two figures and then double the result and that figure is the castor angle.
For the 15° I made a couple of large protractors:
BTW - I think ‘castor’ = UK, ‘caster’ = US. Either way, shopping carts (trolleys) would be difficult to push w/o it.
Hi John The slots are cut vertical to the length of the angle iron/aluminum, not horizontal. This allows you to drop in the bracket at the end of the tape. It’s 29 inches only because the guy that showed me this method made it 29, In reality it’s the distance between the cuts (27" if you make them 1" in from each end) which is important, and approximates the diameter of a 185/15 tire, which is actually 25.6" more or less as I recall.
George, as I mentioned in my note to Andrew the castor is actually 1/2 of the total camber change over the 30 degree you use, or the 40 degree (my instrument requires) swing. I used to know the theory behind this but don’t currently recall, but it’s clear from the graduations on my instrument.
Thanks again Terry. Now it all makes sense.
So just to clarify for those of us filing the information is it double the result or half the result that is the castor angle?
Nope ignore this - I just tested this on one of my E’s on the hoist, over 20 degrees swing each side as required by the gauge instructions - the camber change was 1 1/2 degrees over the total swing and the castor readout was 2 1/4 degrees positive, so no apparent relationship to each other.
The instructions in the attached article are the same as the ones that came with my camber gauge. My gauge looks like the one Andrew shows in his attachment.
Forgot to attach the link.
Just did it on my OTS which is sitting on the floor at a normal ride height. For .875 camber change, castor was 1.5 degrees positive. Ratio between castor to camber on the two cars was 1.5 on the hoist car and 1.75 on the floor, which can be accounted for by the ability to accurately read bubbles… Can only see that the gauge would be wrong if the principle it’s based on is wrong which I doubt. It’s a very simple piece of equipment. It’s not supposed to matter if you swing the wheels 30 or 40 or more degrees.All measuring devices are supposed to give a more accurate reading the greater the swing. My conclusion is that I’m happy to rely on the gauge. Castor is simply self centering, and consequently steering effort. If you don’t like either than just change it Just remember changes to castor affect toe, but not camber in any meaningful way.
I saw that article - I am pretty sure that is a TR3 in the photo. An odd choice as the TR3 had zero castor (and was not adjustable).
thanks…my problems arose when I rebuilt the IFS on my 420G, and noted the castor shims were all over the place, and very different for both sides. I just put them back where they came from
Once rebuilt and installed, I took it to an alignment shop. Their incompetence was unbelievable
I lack both flat ground, (dont know why a builder would lay a concrete slab that isnt level),
and and a camber/castor gauge. (it is possible to level the cars up, using wood or ramps)
I do suspect it wont be easy to establish the centreline of the the loaded joints with sufficient accuracy, but I am going to give the stated method a try and see if I can detect a measurable difference altering the shim packs.
I will eventually report back as to whether I was able to succesfully measure a change or not
The above method is from an old time car & truck guy
Andrew, your rims appear to be in very good shape, but if not true, I believe could induce error. Most cars have scrub radius of other than 0, I believe it is positive on an E type. This will cause the tire to slightly rotate when the steering is turned. That turning will introduce additional angle if the rim is not true, and it will vary depending where the “untrueness” is positioned.
Paul thanks for your kind offer of these. Don’t see how I would use them.
Hi, does seem logical, so I had a look, but brake backing disc is in the way,
are you referring to the measurement 7.38cm ? (x2)
The early 3.8’s don’t have a backing plate for the disc - a fact I didn’t consider when I suggested all guys use “my” method. The flats you use as reference areas are along the vertical dimension lines at the left and right extremes of the second drawing.
As I suggested, you do need a fabricated bracket to worm around the “stuff” that is in the way, but it’s well worth it. A minor problem is the possibility that you need left and right adapters depending on how you make the adapters and how your brake shields are made. The brake shields may make this method difficult - I don’t know as I don’t have them.
thanks for the reply…I still like the method, but the brake shield make it tough. I would need to undo the bolts to even get a steel rule in (those bolt centres are also in line)
interestingly, looking at my diagram, if the outer edge of the legs of that support the lower ball joint are parallel, which looks likely, (have to measure when i get a chance)
I am able to place an electronic angle finder against this flat and get a reading
As the OP on this thread, wanted to circle back with what I found. As @ Aristides and others had pointed out, the problem was in the turn plates I had made. I was using two slippery plastic tiles with a bolt in the center which was not allowing the wheels to move in an Arc. I then tried various combinations of plastic bags, oiled tiles and shinny magazines which improved things but I still found more variation in the readings than seemed acceptable. So I bit the bullet and bought a set of ball bearing turn plates allowing free movement in all directions (https://www.amazon.com/dp/B0758Z612B/ref=psdc_15709141_t1_B00XNTO8G2). This improved the repeatability of the readings considerably. As a side note and as @ inlinesix pointed out, I also found that after making a change to either camber or caster that it was necessary to drive the car a couple of miles to gauge the effect of the change. Simply measure / adjust / re-measure did not give me an accurate reading. This made it a fairly lengthy and iterative affair but the outcome was -1deg camber and +2deg caster on each wheel and 1/8" toe-in. The car feels and drives much better as a result. Many thanks again for the suggestions and help with this.
Hi Andrew. Glad to hear it worked out for you. That’s a terrific price for those plates. Don’t know if I mentioned it but the pro shops have hooks to pull the car down to it’s ride height when it sits on the plates. It might well be that if you have a set of turnbuckles to use in place of the shocks you could pull the car down to normal/equal side to side ride height. Did you try that?
Needles to say though that for all of these measurements to make sense, you must be sure that your wheels are absolutely straight…
Those plates look real nice BTW.
And this is my version if someone is interested…