Techniques for measuring caster

[jhm]

Hello all....I have a bit of a theoretical question regarding the various common techniques for measuring front caster.  Yes, I've searched extensively (across the internet) but haven't found an answer to my specific questions.

 

One of the most common techniques has one measuring the delta camber as the front wheels are turned from right to left....in either a 40 degree arc (20 "in" and 20 "out") or a 30 degree arc (15 in and 15 out).  One takes the delta camber measurement and multiplies it by 1.5 (if using a 40 degree sweep) or 2.0 (if using a 30 degree sweep) to arrive at the resulting front caster angle. 

 

My first question is why is a multiplication factor necessary at all, and what is the supporting math that dictates a factor of 1.5 or 2.0?

 

My second question is if one uses a 60 degree sweep (i.e. 30 in and 30 out), is the multiplication factor 1.0?

 

Please understand that I'm not taking issue with the technique or the math methodology used to arrive at the result....merely trying to understand the underlying math or engineering principles that were used to define the methodology.

 

Yes, I'm aware of the large variety of caster gauges; but haven't sprung for an expensive combo gauge yet.  For the time being, I'm using a simple digital angle finder with DIY homemade turnplates for the front wheels.

 

Thanks in advance for anyone who can help shed some illumination on this for me!

[ZHoob2004]

I found this paper while searching. There's a lot of garbage, but page 3 has a section on the right-hand side that might be helpful. 

 

http://www.nacomm07.ammindia.org/Contents/papers/NaCoMM-2007-106.pdf

 

It seems that ideally you would be able to turn the wheel 90 degrees in each direction, putting the wheel directly behind and in front of the ball joints, allowing you to simply subtract the two camber angles and leave only the caster angle. All the multiplication is doing is calculating that same difference from a smaller angle.

 

After further review of the above paper, I've found it mostly only applies to their special tool that they invented. This paper appears to cover the actual math of caster calculation, and shows that the aforementioned multipliers are, in fact, an approximation.

 

https://disco3.co.uk/gallery/albums/userpics/24543/steering-geometry-and-caster-measurement[1].pdf

 

According to this, caster can be calculated as (180/pi)*(camber1-camber2)/(toe1-toe2) or (180*camber difference)/(pi*swept angle). This would mean with a 30 degree sweep, your multiplier should actually be 1.91, and 1.43 for a 40 degree sweep.

 

By this logic, you would use a multiplier of 0.95 for a 60 degree sweep, very close to your inferred 1.0.

Edited April 10, 2019 by ZHoob2004

[SleeperZ]

Wouldn't it be as simple as leveling the car and measuring the distance of the wheel center from a plumb line centered on the top of the strut?  It's simple math given those measurements.

[jhm]

Thanks for the replies.  I'll take a look at the link ZHoob posted.  Again, this is a bit of an exercise in theory. 

 

Not sure exactly how well the plumb-bob method would work on a lowered car, with coilovers and adjustable camber plates....since camber and caster need to be measured with the wheels mounted on the car, the car sitting on the wheels and the suspension loaded (static loading).  But who knows -- maybe it would work with sufficient accuracy to be meaningful.

 

I agree that there are many ways to measure caster....I've just never understood the math/engineering behind the technique that many car builders seem to use.  Hence my original questions...