Mike T.'s Wheelbuilding

 

Measuring ERD - Effective Rim Diameter -

I have two methods of measuring ERD here. 

ERD is the most important measurement to input into a spoke calculator.  The definition of ERD is this -

"The distance, in millimeters, between the end of one spoke, in a finished wheel, to the end of the other spoke, that is diametrically opposite, in the wheel." 

Some rim makers and marketers list ERD on their website; some even on the rim decal itself, but all wheelbuilders have to learn how to measure their own ERD so that problems are not caused by incorrect information.  Do not accept anyone's figures for ERD!

As we need to gauge where the ends of the spokes will be in the finished wheel (that isn't spoked yet!) we have to use creative ways to get that measurement.

Where should the end of the spoke be in a finished wheel?  -  I like the ends of the spokes in my wheels to be at the bottom of the nipple slot.  This gives me a leeway of 1mm each way before I start running into problems.

Check the photo below.  This spoke, barely sticking out of the top of the nipple, is bottomed out.  The nipple will not screw down any further.  The end of the spoke should be at the bottom of the slot which gives us a 1mm leeway each way - from the top of the head to 1mm below the slot.  1mm shorter than the slot still leaves some of the spoke going up into the nipple head.  If the end of the spoke goes below the head, and ino the neck, this leaves the neck of the nipple the weakest part of the whole wheel and the nipple head will probably snap off.

If you can see threads below the nipple or above the nipple, you're going to have problems with the wheel.

To measure ERD, we need some very simple tools but we must work to a fine degree of accuracy as spoke lengths are critical to +/-1mm (plus or minus 1mm).

How to measure ERD.  Method 1.

The tools needed for this method are -

Always check to see if the Spoke length calculator of your choice shows you where to measure to for ERD.  Usually, it's to the bottom of the nipple slot.

  1. Work on a clean flat surface like a table or a carpet.  Lay the rim flat.
  2. Choose two spoke holes directly across from each other (be careful here as it's easy to be one hole out!  Count empty holes if necessary).
  3. Insert a spoke into one of the holes and screw the nipple onto it a few threads (it doesn't matter how many).  Have the nipple washer installed if you're going to be using it.  
            See Photo 2.
  4. Insert the other spoke into the hole opposite and install the washer and nipple.  See Photo 3.
  5. Link the two j-bends with the elastic band.  See Photo 4.
  6. Make sure the nipples are seated inside the rim.  It's easy for them to get hung up.  See Photo 5.
  7. Measure VERY carefully between the end of one nipple and the end of the other with the tape measure,.  Do this 2-4x to make sure you're doing it perfectly.  Butt the measuring tape end-piece up to the nipple and make sure it stays there.  This step is VERY important and accuracy is needed here.  Have a helper hold the tape end if needed.  Write this number down on the paper.  I use a 1 meter ruler instead of the tape measure.  See Photos 6&6b, 7&7b.  
  8. Unhook the elastic band off the spokes and remove the nipples and washers.
  9. Measure a nipple - from the bottom of its slot to the end of the nipple.  On this 12mm nipple, it's 11mm in my Photo 8&10.  A vernier caliper is best (make sure the jaw fits to the bottom of the slot) but the tape measure can be used.  See Photos 9&10.  Write down the lengths of TWO nipples.  Do the addition.  Write down the total.  See Photo 11.
  10. Repeat the whole evolution again at 90 degrees to the first measurement as the untensioned rim could be oval.  If there is a difference,  average the readings.

That's it.  You now have a perfect ERD (if you have measured carefully!)  and the ends of the spokes in the finished wheel should be right at the bottom of the nipple slot, if all other measurements are correct - this gives you a leeway of +/-1mm for slight imperfections in your measuring and spoke calculator workings behind the scene.  See Photo 12.

 

 

 

 

 

 

 

 

 

 

 


The ideal place for the end of the spoke in the finished wheel is at the bottom of the slot. 
See how little leeway you have with the +/-1mm measurement!

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How to measure ERD.  Method 2.  The Musson Method (Roger Musson of Wheelpro wheelbuilding e-book fame).

For this method we're going to use a pair of dummy spokes.  You're going to have to make them and I'm going to show you how, so that you get them perfect.
Roger makes his dummy spokes differently than I do and his method is in his e-bbok.

The tools needed ERD measuring and spoke making are -

 Making the ultimate Dummy Measuring Spokes -  the Mike T. method.

Here's how to make the ultimate micro-adjustable dummy spokes.

*Grab your two old spokes, the two nipples, the sidecutters, your mm tape or ruler and a Sharpie marker.
*Measure from the threaded end of the spokes and mark them at 19.5cm (trust me on this length).
*Cut the j-bend end off at your marker line.
*Grind or file the cut spoke ends square.
*Screw them into the nipples - 4 twists of the thumb is fine (no accuracy needed here!).
*Crimp the nipples onto the spokes with the sidecutters.  See Photo 1 below.
*Using the ultra-accurate "2 ruler" method (see Photo 2) align the nipple slot exactly to the start of the ruler.  Keep the two rulers in place for the next three steps.  See photo 2.
*Check to see if the cut end is exactly at 20cm (200mm).  Use one eye open/one eye closed to check this perfectly.  See photo 3.
*Adjust the spoke as needed by gripping the spoke in a vise or with pliers or the cutters and adjust the nipple with the spoke wrench.
*Re-check and re-adjust as needed.  Accuracy counts here.  You now have perfect 200mm micro-adjust measuring spokes - adjustable for changing atmospheric conditions!

  

 

Making Dummy Measuring Spokes -  the Roger Musson method.

You need two spokes and two nipples.  Use loc-tite or some form of glue on the spoke threads.  Cut and file the spokes exactly 200mm long.  Screw the spokes into the nipples exactly to the bottom of the nipple slot.  Be accurate.  You now have two 200mm ERD measuring spokes.

 

Measuring the ERD.  The Musson "No Math" Method -

See the Method 2 tools below.  Photo 4.

  1. Work on a clean flat surface like a table or a carpet.  Lay the rim flat.
  2. Choose two spoke holes directly across from each other (be careful here as it's easy to be one hole out!  Count empty holes if necessary.  Apply marker tape to mark the holes!).
  3. Insert a dummy spoke into one of the holes.  Have a nipple washer installed if you're going to be using it.   See Photo 5.
  4. Insert the other dummy spoke into the hole opposite.  Have a nipple washer installed if you're going to be using it.  See Photo 5.
  5. Make sure the nipples are seated properly.  It's easy for them to get hung up inside the rim.  
  6. Keep the dummy spokes pulled inwards gently.  Point them at each other.  Get them parallel to the table top.  I rest my spokes on pencils.  See Photo 7. These are all very important steps for accuracy.
  7. Measure VERY carefully between the end of one dummy spoke and the end of the other with Roger's downloadable and printable measure or a steel ruler.  Measure FROM THE 40cm or 400mm LINE!  This automatically adds the two 20cm dummy spokes to the equation (no math needed, where mistakes can be made).  See Photos 6, 7 & 8.
    Do this 2-4x to make sure you're doing it perfectly. Use the one eye open/one eye closed method.  This step is VERY important and accuracy is needed here.  Have a helper hold the spokes if needed.  Write this number down on paper.  This is your ERD. 
  8. Repeat the whole evolution again at 90 degrees to the first measurement as the untensioned rim could be oval.  If there is a difference,  average the readings.

 

 

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