Why is my wheel going out of true after I mount and inflate the tire?

Question

I’m wrapping up my very first wheel build (rear wheel), and I noticed that the wheel is going out of true after I installed the tire. Before installing the tire, I had the wheel trued pretty well (within one millimeter in all directions). With the tire on, it’s now got a few millimeters of lateral runout. What could be causing this?
The rim is a 29” 28h Stans Arch MK3. The spokes and hub are generic, although I will mention that the spokes are significantly too short. I am replacing the rim, which cracked after a period of riding MTB with chronically low tire pressure (and thus many rim strikes.) The new rim is shallower than the old one, so the ERD is bigger. The tires are set up tubeless. I don’t presently have a tension meter, but I can tell you that the spoke tension is pretty even (squeeze method) and is comparable to the other three wheels I own, maybe a bit lower. I don’t want to overdo the tension, especially on a MTB which will see jumps and drops. I will borrow my friend’s meter and update the question.
Side question: why are the spokes not equally screwed into the nipples? Of course, there might be discrepancy between the drive- and non-drive sides, but on the same side, there’s a millimeter or two of difference. The wheel is true (without a tire), so why is there a difference?
Update 1: Deflating the tire brings the wheel slightly back into true. Dismounting the beads brought it most of the way back, although it is still wigglier than I remember trueing it to. It’s only lateral true that is affected; radial has been pretty much perfect the whole time.

More photos without the cassette:

juhist · Accepted Answer

why are the spokes not equally screwed into the nipples? Of course, there might be discrepancy between the drive- and non-drive sides, but on the same side, there’s a millimeter or two of difference.

That's a sign spoke tensions are not equal.
The fact that spoke tensions are not equal is a sign spoke tensions are low.
Increase your spoke tensions to the required level and equalize them.
Besides, how did you see that spokes are not equally screwed? Do you see the threads below the spoke ends at the rim? If you do, the spokes are too short. The only way that in a well-built wheel you can observe if spokes are equally screwed is to look at the spoke threaded end inside the rim.

Before installing the tire, I had the wheel trued pretty well (within one millimeter in all directions). With the tire on, it’s now got a few millimeters of lateral runout. What could be causing this?

Tire inflation pressure affects spoke tension. The more inflation you have, the less spoke tension you have.
To ensure this is not a problem, the tension of spokes before inflation needs to be as high as possible.

TheWheelMan · Answer

Good job on your first build!
It does really just sound like you have low spoke tension. Usually, i tighten the spokes until no threads are visible on any spokes as a starting point. If your spokes are too short (not threaded into the nipple all the way into the rim) the nipples will prematurely break, especially on a mtb.
Inflating the tire will squeeze the rim inward radially ever so slightly, lessening the spoke tension, but that's usually at high pressure. If you're experiencing this at 30-50psi, you're probably still way to loose.
Some other things to consider: Did you mix up the drive and nondrive side spokes? Did you use boiled linseed oil or thread locker on the nipples? Are your hub bearings properly adjusted? Are you properly tightening your quick release skewers?
Did you stress your wheels while building them? It's really not an optional thing that you do to check after you're done but an integral part of the wheel building process.
Once you get your wheel laterally and radially true initially, lay it down and gently step on the spokes with one foot until you feel them give a bit, rotating the wheel until you've stressed all the spokes. Flip it over and repeat.
You can also push down on opposing sides of the rim gently with your hands until you feel the wheel flex slightly. Rotate the wheel to stress all the spokes, flip and repeat.
Now the wheel is most likely out of true, especially if your spoke tensions are too low. True the wheel again by only adding tension. Check the dish and stress it again.
If it doesn't stay true after stressing, it's not going to stay true while riding. Repeat the process until the wheel stays true. Don't forget to keep checking the dish each time.
I usually have to stress a wheel at least three times because I start with relatively low spoke tension. When it finally holds true, i check the tensions with a meter and they are usually spot on. Before i declare victory, I check the egg again and make sure that it's round.
While +/-1.0mm isn't too bad if you're eyeballing it, a truing wheel with travel gauges (+/-0.05mm) is a really useful thing to have if you intend to work on your own wheels in the future. I true my wheels to +/-0.5mm or less, sometimes in the neighborhood of +/-0.2mm on a new, high quality rim.
Good luck getting it straightened out and happy trails!

Why is my wheel going out of true after I mount and inflate the tire?

2 Answers

Add your own answers!

Ask a Question