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When I apply front disk brakes I see slight bend in the rim and wheel. Is this normal or I have a low quality parts.

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    That really doesn't sound right, even for low-quality parts. It sounds more like something is wrong, like maybe the axle isn't tight enough. Can you explain better what you mean by "bend?" Flex to the side? It would be best if you can include a short video, or at least a picture. Regardless, get this maintained before you ride it again! – DavidW Dec 17 '20 at 15:58
  • Welcome to the cycling corner of SE. This doesn't sound normal, can you organise a couple of photos to show the difference? Might need 3 or 4 hands. – Criggie Dec 17 '20 at 19:44
  • Are the QR properly tightened? In the intended way? – Carel Dec 19 '20 at 18:56
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Of course wheels bend somewhat when braking force is applied. Otherwise they would need to be infinitely stiff, or at least so stiff it would make it stupidly heavy.

“Somewhat” means, they bend so little that you can't possibly see it, though you might be able to feel it. Trials riders certainly do, and it's one reason why they're still mostly holding out with rim brakes, unlike all other MTB disciplines.

If it does bend visibly, then probably something is very wrong indeed. Low spoke tension is the most likely reason, as the other answers have said. Properly trueing a wheel is a bit tricky, but at least you should check if any spokes are snapped yourself or at least at way too low tension (you can feel it when “plucking”/twisting a spoke; it should feel almost solid, only slightly elastic but certainly not wobbly).

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No, not normal at all. You should have a bike shop look at it for a safety check if you are new to the sport and bike.

One thing that can cause a wheel to bend is poor spoke tension. A good bike shop should be able to fix that for you pretty quickly.

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Are you talking about small bends in the rim, maybe a few mm, that are always there, but you notice them when you are braking? In other words, when you spin the wheel, do you notice that the rim appears to wobble between the brake pads, but the wheel does not actually wobble on its axle, and the wheel does not change shape? I'm talking about the problem where the rim is slightly bent by a few millimeters right or left of center in some places, but not up or down.

If that is the case, then you have a wheel that is not "true", which is a very common issue. If the rim is bent far enough that braking is not as effective, then you should get the problem fixed. It is usually possible to adjust the tension of the spokes to bring the rim closer to the ideal shape; this process is called truing the wheel. There is lots of advice on the internet about how to true a wheel. Here is one such video from YouTube.

Lower-quality bicycle wheels need more truing than higher-quality wheels, and may even come from the factory or bicycle shop needing to be trued, but in my experience all bicycle wheels eventually need to be trued. Rear wheels with gears on the side have more tension in the spokes on one side than the other, which is not the case for front wheels, so rear wheels generally need truing more often than front wheels.

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    Additionally, disk brake wheels can be ridden when they're out of true to the point a rim brake bike would suffer from brake-rub. – Criggie Dec 17 '20 at 22:32
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    Just as an FYI fixing spoke tension will also true a wheel. – Tude Productions Dec 17 '20 at 23:30
  • With a disc brake (OP said), a bent rim won't affect braking. That's only a problem for rim brakes. – Tim Dec 18 '20 at 16:35
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Another possibility is that the front bearings are loose and are being twisted to one side when braking.

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