Caster/toe

[47silver]

redid my suspension and have read many articles on how to adjust the caster.
In the porsche technical specifications it says to: "caster results from total camber difference at 20 degree left turning radius and 20 degree right turning radius, times 1.5.

does that mean say left you measure 86 degree change and right you measure 87 degree changet that your caster is 87-86 = 1 *1.5 = 1.5 degree?

Also on the toe there is a note that reads "toe in pressed with 15 KP".

does that mean we have to use some type of clamp to pull the wheels together with 15 KP of force? What is a KP?

thanks

[47silver]

KP is latin for kilopond, pond = weight = 9.80 newtons or ~2.20246 lbf
so 15kp ~ 33 foot pounds.
So when doing toe how would you put 33 footpounds across the wheels?

[gled49]

Caster is a measurement of camber @ 20* in and out.
We had a spring loaded rod in the old days that was installed at the front of the tires to push out the front tires, to mimic road forces against the front tires.

[47silver]

Gled
thanks but what does the document mean by "times 1.5?"
do we take the difference and multiply by 1.5?
seems to be what they mean.
and the springload pushes out on the tires? glad you let me know that as i was thinking that it would be a clamp pulling in the front of the tires.
something like a spring loaded curtain rod or something similar

['78 SC]

Pay attention to the sign of the camber measurements. For example, if you measure -3* camber when steering one way and +1* camber for the same wheel when steering the other way, then the change in camber is 4*. In that case, 1.5 x 4* = 6* caster.

[47silver]

I think I have become a moron.

Caster
Turned the wheel to the right (20 degrees) put on a digital scale on the right wheel and measured 87.3 degrees (after making sure it was 90 degrees to the ground.
Turned wheel to the left (20 degrees) and measured 88.3 degrees. The difference is 1 degree? So camber is 1*1.5 or 1.5 degrees?
Does not seem right.
Should it be 91.7 - 87.3 = 4.4 degrees? Then times 1.5? = 6.6 degrees of camber?
Thanks

[47silver]

Ah reread your post thanks

[WP0ZZZ]

Originally Posted by 47silver

so 15kp ~ 33 foot pounds.
So when doing toe how would you put 33 footpounds across the wheels?

Just to clarify, a kilopond is a force unit and therefore can not be equivalent to foot-pounds. 1 kp is a force equal to the weight of 1 kg under standard gravity, i.e. a kgf.

The kilopond is no longer used in metric (SI).

[jjeffries]

47Silver,

I've become something of an obsessive about home alignments. I'm also not a great mathematical thinker, doing better with visuals than sums. When I aligned my 82 SC in 2018, I didn't think about caster because, in stock form, it's not easily varied. But with my current rehab project, an 87 924S, caster is variable and I'd had everything apart, so I needed to make sure it was set properly afterwards. I bought a Longacre gauge and enjoyed using it.*

Just FYI; there are a number of ways to do this properly.

John

*I got to use the gauge again a couple of weeks later when I installed aftermarket rear upper control arms in my wife's 09 Honda CRV, to correct an excessive rear negative camber problem common to that model.

[stownsen914]

You want to measure the difference in camber for a given wheel when turned 20 degrees left vs. when turned 20 degrees to the right. Since camber changes based on how much caster you have, caster can be inferred by this test.

Is it camber you are measuring to come up with the 87, 88, 91 degrees measurements? It's hard to tell from the way it's described.

I tried to upload the instruction manual from a Smart Camber tool, but can't seem to upload a file? The manual describes caster measurement pretty clearly. Here's a copy paste of the instructions.

Measuring Caster
Caster is calculated by measuring the camber angle in two different
positions and then putting those measurements into a simple formula.
To measure caster, do the following, but be sure to take your time the first
few times you do it:
1. Start with the left front wheel.
2. Place the layout template sheet (included) on the floor and use chalk (or
paint for permanent marking) to scribe lines that are about a foot longer
than the template. Then remove the template and connect the lines as
on the template.
3. Roll the car until the front wheel is on the centerline (CL) axis and
parallel to the X axis.
4. Turn the steering wheel until the left front wheel is parallel to the Z
axis.
5. Use a stiff piece of cardboard approximately 7”x18” as a “plane”
reference template (flat sheet metal is better), carefully put the plane on
the Z axis line and carefully lean the template against the tire. If it
touches the tire evenly, then the wheel is turned 20°.
6. Measure the camber angle in this position and note.
7. Steer the wheel back through the center-steer
position and parallel to the Y axis. Repeat the
plane template alignment method.
8. Measure the camber angle in this position
and note.
9. Calculate the caster angle using one of the following examples:
Note: 1. Example No. 2 is the most common.
2. What you are trying to achieve is camber difference from
turning left and right.
1. If both measurements are negative then
Subtract the small number -3 *
from the larger number -1
——
Equals 2
Multiply 2 by 1.5 = 3
The wheel has 3° of caster.
2. One negative and one positive, then
Add the numbers together -3 *
+1 ——
Equals 4
Multiply 4 by 1.5 = 6
The wheel has 6° of caster.
3. If both are positive, then
Subtract the smaller number +3 *
from the larger number +1 ——
Equals 2
Multiply 2 by 1.5 = 3
The wheel has 3° of caster.
10.Make whatever adjustments are required and remeasure until you have
the angle you want.
11.Repeat the same steps on the right front wheel.
12.Reset the toe (can be done with SmartRacing Products-SmartStrings).
Any changes to the caster or camber angles will have changed the toe
settings.

[dannobee]

For checking caster, in the shop we'd use turn plates but every modern alignment machine will tell you when you've turned the wheels 20 degrees (or 10 or 5, depending on the machine), then will compute caster. Obviously we don't have that luxury here.

With modern technology, I'd use jeffries kind of wheel attachment but put secure a smart phone on top of the Longacre gauge and set the smart phone to compass (yes, N, S, E, W compass) degrees. Move the wheel (road wheel, not steering wheel) by hand 20 degrees each way, carefully watching for your 20.000 degree change, then note the camber reading, then compute and record caster. Remember, you must have the brakes applied during caster checks otherwise the wheel will turn and throw off your readings. Use a prop rod or stick and wedge against the seat cushion to keep the brakes applied during caster checks.

[47silver]

Thank you all.
Ill pick up a gauge. Is longacre the goto gauge?
This is the wheel spreader tool.
Specialty Products 99918 Wheel Spreader Pressure Bar SP99918

['78 SC]

Originally Posted by 47silver

I think I have become a moron.

Caster
Turned the wheel to the right (20 degrees) put on a digital scale on the right wheel and measured 87.3 degrees (after making sure it was 90 degrees to the ground.
Turned wheel to the left (20 degrees) and measured 88.3 degrees. The difference is 1 degree? So camber is 1*1.5 or 1.5 degrees?
Does not seem right.
Should it be 91.7 - 87.3 = 4.4 degrees? Then times 1.5? = 6.6 degrees of camber?
Thanks

I think you're getting to the right value, but the numbers look funny because you're using values straight off the level, so 90* is actually 0* camber. I also use a digital level, but convert the readings to camber (including sign, +/-) before using them because I'm interested in the camber.

Camber is expressed as degrees the road wheel is tilted from vertical; tilted toward the vehicle (leaning in) is negative camber and tilted away (leaning out) is positive camber. So, 88.5 on the level is +1.5* camber or -1.5* depending on which way the wheel is leaning.

If your example above is for the right wheel, you may be measuring +1.7* when turning right (90.0 - 87.3 = +1.7) and -2.7* turning left (88.3 - 90.0 = -2.7). Difference is -4.4 (-2.7 - (+1.7) = -4.4). Then times 1.5 gives -6.6* caster. If you're getting negative camber in both cases (i.e. the wheel always leans in at the top), the difference is only -1.0* and the caster is only -1.5* (not likely).

How heavy is your steering? It should have a strong centering action if caster is -6.6*. Small changes in caster, say one degree, will make a big difference.

[dannobee]

wheel spreader tool isn't necessary. I *might* have seen one in the corner of a shop somewhere in my 30+ years, but nobody (and I do mean nobody) uses it.

Verify that all steering components are tight and that the wheel bearings aren't loose.

If you look at the later Workshop Manuals, there is no reference to "pressing" the front wheels.

[dannobee]

Longacre tool is common and works, but I'm sure that any of the others work just as well with a skilled operator.

One caveat though, if you use jeffries tool, the correct way to use it is to jack the wheels off the ground, install the tool and camber gauge, check the camber reading, then turn the wheel 180 degrees and verify that the camber reading is exactly the same. You're checking the runout of the hub and wheel edge. If there's a difference, find out why. Once you're good there, put the tool and gauge back to normal, then put the car back on the ground, settle the suspension (F&R), then use your brake pedal depressor to lock the brakes on. THEN you can check caster and camber.

[47silver]

The 75 tech manual says to measure toe pressed with 15 KP. See above. On line conversion sites say 15 kp is = 33 psi?
If is not necessary then that is great.
Reading about it mercedes requires it. Some youtubes show toe without and with the device. My suspension is all new. I have a lazer toe measuring tool. Just want to do it correctly.

[47silver]

I rechecked and my math says 6.1 degree on left and 5.9 on right. Specs say 6 +/- 5 minutes so need to adjust a bit.

[stownsen914]

Personally I wouldn't bother trying to correct .1 of a degree caster. That's probably within margin of error for a home setup anyway.

[dannobee]

Here's the deal with caster. We'd use it as an element to keep the car from pulling OR when combined with checking "toe out on turns," to identify a bent unibody or suspension parts. * Funky camber and caster readings, or wildly varying left to right readings would cause us to look for crashed/bent parts.

From experience, 1 degree of caster split is worth 0.5 degree of camber split, with respect to "pulling" or "leading" to one side. Since the vast majority of roads in the US are crowned for water drainage, and since we drive on the right side of the road, without a split, the car will "pull" to the right. 0.3-0.4 degrees of camber difference is required to correct for that, i.e., +0.2 over the preferred spec on the LF and -0.2 under the preferred spec on the RF. So if the camber spec was 0 degrees, we'd set it up +0.1 LH, -0.2 RH, caster the same. You could also set it up with camber 0.0 degrees on both sides and caster 5.8 L side and 6.5 R side to make up for the road crown. The car will pull or lead to the side with more positive camber or least positive caster.

If the customer was picky and wanted everything within "factory specs", we could set it up that way just as easily but explain the road crown issue and that it WILL pull to the right.

* The later Workshop Manual alludes to this by giving the spec of when turning the inside wheel to 20 degrees, the outside wheel should be at 15 degrees, with a tolerance of 1/2 degree or 30 minutes. If more or less, there is something bent on the car.

Now, if we're talking racing on a track, what we would do for a setup is entirely different, along with using tire temperatures to reset camber and toe. If you're whipping out the camber gauges and toe strings at the track, something is definitely wrong.

Oh, and first things first, before you start on this endeavor at home, be VERY sure that the floor is FLAT and LEVEL. Use shims under the tires if needed to get it there.

[jjeffries]

Originally Posted by 47silver

I rechecked and my math says 6.1 degree on left and 5.9 on right. Specs say 6 +/- 5 minutes so need to adjust a bit.

I (an amateur) wouldn’t be trying to improve on that.

John