Spoke length calc – am I missing something?

Hi
New rim going on to existing hub and needs new spokes. I can’t accurately measure the dimensions of the hub without taking dismantling the wheel which I can’t do for a few weeks. Can I calculate the length of the new spokes as below, or am I missing something?

Current rim ERD is 589mm
New rim ERD is 601mm

Old spoke length
Drive side – 279mm
Non-drive side – 281mm

rim ERD difference is 12mm
so add 6mm onto length of spokes
New spoke length
Drive side – 285mm
Non-drive side – 287mm

So 286mm should work for both sides. Is it this simple or should I take into account that the increased spoke length would be the slope of a triangle and therefore slightly more than the vertical?

cheers
Rich

You may be missing a few things. Unless the wheel is radially the spokes run on a tangent.

Dishing will also have an impact.

You should be able to find the measurements for most hubs online and it is always worth double checking that these are close with a ruler or calipers.

I would take extra care over all your calculations and measurements if you want to use an in-between spoke length. It is important they reach a sufficient depth into the nipple.

I’ve done similar and it’s worked out fine, less than 6mm spoke difference in my case but the method is valid unless the difference is 26″ to 29″, its always worth checking ERD with Parks two spoke method, not all rims have it written on, and you should never trust the ERD stated in the description on CRC/Wiggle website.
Getting the hub dimensions yourself with digital calipers is easy enough if you cant find them online.

If you can take your own cassette off you can use digital calipers to measure flange diameter, distance between flanges, then rest the wheel on its endcaps on a table and measure distance from table to flange with a ruler, if it’s a QR hub then use a board with a hole in it, you can use these measurements to calculate the flange to centre measurement. Simple.

What hub? Some spoke calc sites will have the data even if the manufacturer doesn’t.

As long as the new ERD is correct I’d say your logic is sound (captain). Tell us what the model of hub and rim is, how many spokes, and how many cross. Someone may have done similar or the same. If you know these you can try a few online spoke calculators to check, and compare with your own calculations.

Your methodology is close enough IME.

You can also measure or very closely estimate the hub dimensions without even taking it out of the bike, if you have verynears.

Cheers all – It’s a Bontrager Line comp 30 wheel with dt ex511 new rim, so pretty sure the erd will be accurate. The hub is the Bontrager one. Trek help can’t give me the sizing. 29er 28 hole. Rim is dented tyre is buckled but is useable for the time being and I want to get a last few months use out of it during the winter before a rebuild. I can’t get callipers behind the rotor so will have to take that off and the cassette to accurately measure? Not sure I can be bothered – not a big job but feels like doing something for nothing really if I can get the length another way. Looking to order the spokes in readiness for new year rebuild.

Hi, I have just found this thread. I am currently thinking about the same rim change – from bontrager line comp 30 to Dt Ex511. Wanted to check if you actually changed the rims and if so then how did the 286 spoke length work out for you?
Because when I use the Dt spoke calculator with the Ex511 rim and Bontrager hub dimensions (all available on Trek site) then spoke length comes out at 283.
Thanks!

Just to close thread if needed for future ref.
286mm spokes worked fine on bontrager line comp 30 rear hub with dt Swiss ex511 rim.