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Effective rim diameter (ERD) explained

In a different post I explained bicycle wheel building basics. This post deals with only one aspect of those – effective rim diameter (ERD).

Contents:

  1. What is the effective rim diameter (ERD)?
  2. Why measuring ERD is important
  3. How to measure the effective rim diameter (ERD)?
    3.1. Making an ERD measurement tool
    3.2. Measuring ERD
  4. Conclusion


1. What is the effective rim diameter (ERD)?

Effective rim diameter is the maximum distance that spokes need to bridge going from one end of the rim to another (on the opposite side, just like when measuring a circle’s diameter). Deep section double walled rims usually require shorter spokes, since the point where spoke’s nipple is attached to the rim is closer to the rim’s centre – than it is with single walled rims of the same outer diameter.

So all the bicycle rims have two important dimensions:

Important rim dimensions: BSD and ERC - cross section sketch
Important rim dimensions: BSD and ERC – cross section sketch
Picture 1


Picture 2, below, shows how rim wall depth can affect ERD – two rims of the same BSD are depicted, but the one with the deeper section wall has a smaller ERD – i.e. spokes are attached a bit closer to the wheel’s centre:

Two double walled rims with different wall depths
Two double walled rims with different wall depths
Picture 2


2. Why measuring ERD is important

When building a bicycle wheel, it is important to choose spokes of the optimal length. In order to calculate the optimal spoke length, it is important to know the effective rim diameter (ERD), among other things. Because manufacturer provided data is not always correct (perhaps because of a different measuring method), I feel safest when measuring personally.

So this post is primarily intended for my personal use/reference – I will be filling it with measurements as time goes by. I will briefly explain the measurement method, then provide a list that can easily be searched.


3. How to measure the effective rim diameter (ERD)?

Before explaining ERD measurement technique, I’ll first explain what tools I use for measuring it, and how such tools are made – that will make measurement technique easier to understand.


3.1. Making an ERD measurement tool

This is the final goal, and I’ll explain how to get to it:

ERD measuring tool - two identical ones are needed
ERD measuring tool – two identical ones are needed
Picture 3

Take a spoke, then measure and mark a distance along the spoke – starting from the top (where the nipple threads are), measuring a “round” number. I use 20 cm (200 mm) length. For those building small wheels, like 20″ ones, 10 cm is better, 20 cm will be too long for those. You can make two sizes, spokes and nipples aren’t outrageously expensive. Mark the measured length using a pen, or some tape – whichever is convenient for you.

Cut it to a roughly desired length (the part where the spoke’s elbow is will go to the garbage), making sure to err on the longer side. Then use a grinding stone, or a file (which takes more time) and finely get it down to the desired length (to within 1/10 of a millimetre), measuring from time to time in order not to overdo it.

Then screw a nipple onto the spoke, so that the spoke’s end is perfectly aligned with the end of the nipple, in the part where the screwdriver slit is, like shown in the picture 4:

Aligning spoke's end with the nipple's end
Aligning spoke’s end with the nipple’s end
Picture 4


You can put some thread locking glue (like Loctite 243 – Amazon affiliate link) on the threads before screwing the nipple on – I did that, just in case.

After aligning the nipple, pinch it in place so it can’t turn any more, best using some medium sized cutters. Be careful not to overdo it and not to cut it all the way, just make dents that prevent turning. Test by hand if it’s held firmly, then press some more if it still looks flimsy.

Pinching the nipple in place - best done in two places, along the nipple's length
Pinching the nipple in place – best done in two places, along the nipple’s length
Picture 5


If you used thread locker, you could wait another 24 hours for it to cure, but that’s it – you are ready for measuring. 🙂


3.2. Measuring ERD

The procedure is really simple – first place the rim on a flat surface, it will be easier. Then, insert the above described tools into two spoke holes of the rim that are on the opposite sides. Make sure they really are exactly opposite from each other – count the holes between them if needed.

Place the tools so they fit properly, as far as they can go, through the rim holes.

Spoke with a nipple, that is aligned nicely, all the way
Spoke with a nipple, that is aligned nicely, all the way
Picture 6


Now that the “measuring tools” have been inserted on the opposite sides, pull them tight towards each other and measure the distance between their ends:

Measuring bicycle rim ERD
Measuring bicycle ERD – I usually hold the right hand side tool tight with the other hand, instead of holding a camera 🙂
Picture 7
Reading the measurement
Reading the measurement – here it is 129 mm, even though it might not look like it from the picture
Picture 8


Since my tools used here are 200 mm long each, ERD is the measured 129 mm + 400 mm (two 200 mm tools). Which is 529 mm – a decent 26″ rim.


4. Conclusion

I hope the procedure is clearly explained. For those who are more of a “show me” type (and don’t mind tedious explanations in Tarzan English), I have also made a video:

Measuring and calculating optimal spoke length

In a separate post I’ll list ERDs of various rims as I measure them – will link to it once it is finished.

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