Wheelset Question

There may be no need to replace your wheels - wheels are expensive.

Before dropping money on the problem, start off by checking the spoke tension. Spokes can break because they're too tight or too loose, and loose is far more common than too tight.

If you tap the spokes lightly with a hard plastic or a metal item, you get a sound. Very approximately you want a sound that is lower than a guitar-string TING and higher than a dull thud

Another gauging method is to squeeze two crossing spokes together with your hands. They should deflect with some effort, not be floppy while not being bow-taut.
You can calibrate your fingers approximately by checking the spoke tension on a bunch of other bikes.

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Yes, you could put 29" MTB wheels in place assuming the OLD of the axle is the same, and the brakes match (likely disk brakes on both) and they were affordable.

Both use "622mm" tyres, when using the ETRTO measurement system. However the rim's internal width will suit a range of tyre sizes.

I have a gravel bike and the rear wheel spokes keep breaking.

There can be a number of reasons why wheel spokes are breaking:

• You have a high weight. The more the weight on the wheel, the greater the likelihood of spokes breaking
• The spokes are all from the same set and have a manufacturing defect or are of poor quality
• The spokes are straight gauge instead of being butted (ideally triple butted)
• There are too few spokes -- 36 should be preferable for 622mm (28") wheels, 32 might be acceptable on 559mm (26") wheels especially if the wheels have low pressure tires
• The spokes have not been stress relieved during building the wheel
• The spoke tensions are not equal and high
• The hub has a particularly high asymmetry -- most likely today on 130mm OLD 11-speed road bike rear hubs

First, assess the situation. Fixing an existing wheel (by stress relieving all spokes, replacing the ones that broke, and tightening them to a high and equal tension and truing the wheel) takes about 2 hours. If your time is valuable, you might want to use it effectively and build a high quality wheel from the scratch (takes about 3-4 hours) rather than trying to fix a low quality wheel such as one having poor spokes or too few spokes. But most existing wheels with enough many spokes can be fixed to reduce the rate of spoke breakages.

If you want to build a new high quality wheel, my advice is 36 triple butted stainless steel spokes, a rim having double eyelets and a hub having cup and cone bearings. You should tension the spokes to an equal and high tension, stress relieve them, and take into account spoke windup when turning the nipples with the spoke key.

If you want to fix the existing wheel, the way to go is to grasp four spokes all going in the same direction and squeeze tightly with your hands -- so tightly that it's practically impossible to do this properly without thick gloves or else your hands will hurt. Some spokes may break during this and if they break they need to be replaced. After all quadruplets (4 spokes each) have been stress relieved, you may begin to increase spoke tension to an acceptable level, equalize the spoke tensions and true and center the wheel. Each increase of spoke tension should be followed by a new round of stress relieving. If an otherwise true rim becomes slightly untrue after stress relieving, or if every event of truing the rim causes the rim to become untrue in some other location, it's a sign that the spoke tensions are too high. Back off the tension of all spokes and try again with a slightly lower spoke tension. A spoke tensiometer helps to achieve the correct tension: most rims have a specification of 1200 Newtons (about 120 kilograms of force).

The reason stress relieving works is that spokes are formed from a wire bent to a coil plastically (elastically bending it would be a major hazard as it could explosively unravel). The wire needs to be straightened to a straight line by bending it back plastically, and then the hook at the end of the spoke must be bent. Those bendings cause parts of the material to reach yield stress. A component with oscillating load at the yield stress doesn't last a long time; instead, it will fatigue. By grasping the spokes, the stress is increased above that of yield stress, relieving the stress and thus the spoke no longer operates at the yield stress. For a new wheel, the spokes won't break but for a wheel with history of spoke breakages, it's likely some existing spokes will break during this stress relieving. The ones that didn't break will be strengthened by stress relieving and will subsequently last a long time.

For more information, you can read the book The Bicycle Wheel written by Jobst Brandt. Unfortunately, Jobst Brandt is no longer among the living, but his book can still be purchased.