Do it yourself, iPhone caster measurement (1 Viewer)

[edwjmcgrath]

Ok, I'm waiting to hear what is wrong with what I've just done...

First of all, if you didn't know, the iPhone has a very accurate-repeatable (to a 10th of a degree) accelerometer built into it. There are several apps that measure angles using this. I use one called clinometer. I calibrated the phone and then started measuring.

For background, my rig has longer shackles (5" pin to pin), and some kind of after market spring lift that gives somewhere around a 4" lift total. I have a bit of wander at highway speed so I thought I'd look into the caster. There are no shims on the springs so I expected to find that I have negative caster.

I pulled the rig front first into my garage, cleaned the floor and measured the slope of the floor at the wheels using the phone and a 4' straight edge on the floor. One measured +0.5 degrees and the other side +0.7. So I've got about a half degree slope going down toward the door which would make sense to carry any water out of the garage. Then I cleaned the crud off the knuckle spindle where it attaches to the knuckle. I measured using the machined flat on the knuckle spindle on both sides getting -4.5 and -5.8 respectively (the phone is lower toward the front of the vehicle). I repeated these measurements 3 times and got the same results.

This would say that I have -5.0 and -6.5 caster. I know this isn't rocket science, so my thoughts were to get a pair of 6 degree shims and mount them fat side forward.

 

[FJsnoozer]

I hadn't even thought to use my phone since I hadn't gotten to that point yet. I would be weary of your ground measurement however. The only real accurate way to measure thy slope is by shooting a grade.

Everything else sounds good.

 

[misterpepper]

Everything sounds good so long as you are keeping track of which direction the iphone is facing when taking measurements and adjusting accordingly. The closer you get the iphone to parallel with the centerline of the vehicle the more accurate your measurements will be. I'm leery of the fact that the two sides are 1.5 degrees different -- that seems like a lot to me. It sounds like it would almost be better to get two sets of shims (or buy the shims individually if that's possible) to try to even things out a little.

 

[Fast Eddy]

I'm leery of the fact that the two sides are 1.5 degrees different -- that seems like a lot to me.

Yea, if it's not cut & turned (badly), they should be parallel. Run a straight-edge across the shackles and see if they're at the same angle.

 

[misterpepper]

Another thing you should do is check your tire pressure and make sure everything is aired up how you want it as it could affect the measurements you are getting.

 

[misterpepper]

I think i might know what is causing the difference between the sides. It has to do with the calibration of the iphone and which direction it is facing. Try this: calibrate the iphone to zero and then turn it 180 degrees in place and see if it still registers zero. I think something is off with the calibration surface so that when you are measuring both sides (and turn the iphone around on one of the sides to make it easier to read), you effectively double the error in level of the calibrating surface, causing it to read as a large difference between the two sides. Either try to find a surface to calibrate on that reads the same from both sides (facing east vs. facing west) or take all of your measurements with the iphone facing the same direction.

 

[FJsnoozer]

sounds like frame flex

see if you can measure frame angle on each side from a flat point. unless the axle is twisted in the 3rd some how (I dont know if that is possible on these trucks). It would be interesting to see where the difference is coming from.

 

[FJsnoozer]

i agree in using the same side of your phone. when I flip mine, it is .5 different and then the bottons on the side come into plan and affect that as well. Your phone case if you are using one could very well throw it off by that much. Try it with no cover using the same side.

 

[edwjmcgrath]

Ok. I just inflated the tires a couple of days ago all to 35. I had been doing all of the measurements without the case and being careful to use the side without the buttons. I calibrated the level before I did all of this. BUT...I just pulled out an 8 foot straight edge and redid everything and it makes more sense now.
The floor has too many undulations in it to use a short straight edge. Also I went out for a ride and parked in a different spot so I get different numbers.

Now I get -5.2 driver and -4.7 passenger side. With the 8 foot straight edge going from front wheel to rear wheel I get .3 driver and .5 passenger. Since this slope increases the caster, I have to add it to the numbers I measured so I now get 5.5 driver and 5.3 passenger. I still think that 6 degrees makes sense.

 

[FJsnoozer]

Thats what I was thinking... I was going to reccomend a 12 foot straight edge for the best acurracy, but of course, the wheel sitting in one of those dps can throw off that.

Looks like we have a new method,

 

[edwjmcgrath]

Purely in the interest of science mind you, I found a local shop that specializes in alignments on 4wd and non-stock suspensions. I'm going to take it there on Monday and have them measure it up and do the caster shims (and toe-in). I'll let you know what their measurements are.

 

[Fast Eddy]

sounds like frame flex

Frame flex can't explain a caster difference between the two sides of a solid axle.

 

[FJsnoozer]

Frame flex can't explain a caster difference between the two sides of a solid axle.

I was 1 for 2. Sue me.

 

[edwjmcgrath]

A little follow-up. I took it to the shop and they measured 4.5 degrees. So my measurement was off by 1 degree, but not bad. It seems to me that Pinhead said at one point that that flat was level when the alignment was correct at +1. (If I understood that correctly.) So that could explain the difference.
They did not have 6 degree shims so I ordered them. But they did correct the 1/2" of toe out that I had. What a difference! I took it out on the highway and got it up to 100km/hr without white knuckles. Can't wait to see how it runs with the correct caster.

 

[NikP]

I used my iPhone app also to determine what shims I needed on front and back. Works great. I ended up in the neighborhood of 6 degrees for shims. Like you I had a longer shackles and and lift springs.

 

[edwjmcgrath]

Shims on the rear?

 

[FF LC FREAK]

Shims on the rear?

To get your pinion and t/c output parallel. Lift springs and/or longer shackles can change this relationship and cause driveline vibrations.