Measuring PCD holes

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How would I measure the PCD of the holes in this wheel.?

'Friend of a friend' wants some spacers making. And friend said to friend, I have a friend who will make you those.! He is aware you can actually buy them but they have slots and holes all over them to make them universal. These I am told he does not want.

So I have the spare wheel which I am told is off a 1994 C class Merc. But what is the best way to measure the PCD.?

Cheers, Nick

Whilst you're revising for your geometry-test, Nick

Look here: **LINK**

http://www.wheel-size.com/size/mercedes/c-class/

MichaelG.

112 pcd, c/o Google

Google is your friend. 5 holes on 112mm PCD

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Cheers guys

But if that data was not available on the net what would be the method for calculating such.

Thanks again, Nick

If you really feel the need to measure them:

• Make two tight fitting plugs, with centres, to fit the holes.
• Choose any two holes, and measure the chordal distance between their centres
• Repeat for another pair of holes
• Learn about 'perpendicular bisectors'
• Apply your new-found knowledge.

MichaelG.

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I don't. ........... I trust the information provided by Merc.

I was thinking more for items I may come across in the future that did not have such information readily available.

Cheers again, Nick

Here is an alternative method. Start as though you were measuring the diameter of one of the holes with digital calipers. With the calipers in the hole press the zero button. (So the diameter of a hole will be subtracted from the following readings.) Insert the calipers into two adjacent hole. Note the reading. The reading displayed will be the distance between the centre of the two holes. We know the angle between the holes is 72 degrees. (360/5) So a right angle triangle with the right angle half way between the holes with one side half the distance between the holes an the hypotenuse between the centre of one hole and the wheel centre will be formed with an angle of 36 degrees and the third side (The side from the centre of the wheel and half way between the two holes.) So the radius of the holes (The hypotenuse) is the half the distance between the holes divided by sine 36 degrees. So the PCD is twice this value.

Edit . I've just realised this is the same method that Michael has suggested.

Les.

Edited By Les Jones 1 on 27/11/2016 22:27:53

For five holes, measure the centre distance between two adjacent holes and divide that number by 1.175.

Martin.

Footnote:

The elegance of the 'perpendicular bisector' geometrical solution is that it also works for any three randomly positioned points on a circle.

MichaelG.

three randomly positioned points on a circle.

Yes, but simultaneous equations might be needed if one does not know the subtended angles? Only need two points if the subtended angle is known.

FYI the formula for equi spaced holes is:

PCD = chord divided by sin(180/number of holes) in this case

= chord/sin(180/5)

= chord/sin 36

As suggested above, tight fitting plugs in two adjacent holes, measure across, minus one diameter and that is the chord.

You can buy a little gadget to measure wheel bolt/stud PCD or as others have said measure distance between two holes and then put that into one of the online PCD calculators.

You need to know what you are doing though as I had a client who bought a set of custom made wheels with an enlarged offset for his Bristol and he got the PCD wrong so had to scrap the wheels. I did bore one out to check the correct PCD to my measurements not his.

Edited By JasonB on 28/11/2016 07:59:45

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No, No , No

It's a geometrical solution ... documented by Euclid, and used [approximated to about fourteen decimal places] by many CAD packages.

MichaelG.

Measure with calipers:

PCD = (diameter of centre hole) + (diameter of one wheel nut hole) + 2*(distance between centre and wheel nut hole.)

The same will work for the vent holes though they will be harder to measure.

I used the perpendicular bisector method to find out the details of a Woodruff key slot on a worn leadscrew for someone last week. Length of slot and max depth values gave a diameter of 1/2".

Martin

Yes, yes, yes. Even Euclid would have used simultaneous equations, if the three points were truly random. Need two equations to solve with two unknowns. Not actually difficult, as a mathemetician, but just necessary.

'All' Euclid did was to give the unknowns arbitrary values and develop a formula from x, y, z, etc algabreically and prove it geometrically. The proof meaning that it worked for any measured values you might choose. He was a great mathemetician of his day, but whether he actually worked them all out himself, as the first to ever do that, is questionable. Some were doubtless simple (or complex) proofs of other's already practical knowledge. He actually documented all the proofs as a record at that time. He did get one or more wrong, mind?

This problem is made simple by knowing the number of points around the PCD. The answer can be documented to as many decimal places as you wish, but Euclid didn't work in decimals, did he? Halves, quarters eighths, etc would have been his units for values less than units still? But Euclid's Theorems did not require units to be proofs. Inserting values into theorems is still required to solve the problem, to arrive at a definitive answer for any set of data points - to however many significant figures you care to work to.

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Which is why I used the phrase "documented by"

... and mentioned the geometrical solution [using compasses and straight-edge] not a mathematical proof.

MichaelG.

Quickly looking at the three random hole problem suggests to me three "easy" ways of solving the problem: Draw it out, use trigonometry and final co-ordinate geometry. They all use the 'perpendicular bisector'. I personally would use the co-ordinate geometry route if only out of elegance.

JA

Edited By JA on 28/11/2016 11:51:12

On some vehicles, the central bore is an accurate fit on the wheel hub - but not always. In which case, the critical / functional surfaces to dimension from are the conical faces that the wheelnuts act against, not the clearance holes themselves. The dimensions (and roundness) of the clearance holes that the studs poke through is probably not controlled with any great accuracy as these steel wheels are designed to be punched / drawn in high volume, resulting in a fair bit of distortion in that area.

You may be able to balance out any radial positional error but I suspect you'd be best to measure the hole positions (and position the wheel for boring) by working from the conical faces. Placing a conical reference plug in each hole to measure from might be a good place to start.

Murray