I have already set my u-brakes fine but want not to upgrade pads, hence 2 questions:
- do pads for v-brakes fit mine u-brake bike? I see they are usually with alen bolt while mine are with regular bolt.
- can I use longer pads? like e.g 70mm for my 24" rim? stock pads are 50mm

thanks

Not sure what you mean. post a photo of your brakes. pads usually come in "threaded" and "smooth" posts. the pads must be designed for your brake caliper. most brakes made for the past decade or so use a threaded one, so if yours uses a smooth post, it's probably an older brake. IME, getting the smooth post pads adjusted, and getting them to stay there, is a giant PITA. u-brake calipers are not very expensive so I'd think about replacing the brake if that is the case.

The brake doesn't care what kind of pad is on it. I have used these Kool Stop V - brake pads on my Odyssey EVO U-brake without issue:

Yes, the brake does "care" what kind of pad you put in it. There are two kinds of pad posts and you need the correct one for your brake.
image borrowed from Sheldon Brown



Older u-brakes like Dia-Compe AD990 only work with the "smooth stud" style pads, while most modern brakes use a threaded stud. until OP provides details, no one can make a specific diagnosis.

I was responding to his question about the length of the pad. He already established his were threaded:

When I read "regular bolt," that is still ambiguous. It could be the 10mm but on the bottom of the pad assembly that a smooth post would require, or a nut on a threaded post. OP has not specified which it is.

guys, I was asking about pads only (the rubber part) not pads mount. Besides I have threaded mount as I wrote specifically about bolts, alen and regular.

Nobody answered still if pads length matters...

Thank you for clarifying. The pads are integrated with the posts, which is why I asked. The way you described it could imply smooth or threaded posts and you didn't post a photo. Go look at how pads are fitted on a AD990 for an example.

the pad length does not matter!

Are you doing tricks where you might be moving forwards or backwards? If so, a pad with a really long trailing end designed for mountain bikes might provide inconsistent braking. There are freestyle specific pads that might work better. Look at the difference between the photo posted above and something like Odyssey Slim By Four pads for example. The rubber compound is usually indicated by the color and some pads work better than others depending on the rim's braking surface finish.

I already found that clear pads are usually softer so better braking,
I asked about pads length as if they are not curved enough they could not stick to the rim completely.
Also I guess shorter pads have more pressing force, right?

Depending on the surface of the rim, the stickiness of clear pads might be excessive. They're good for anodized rims but they are LOUD and kinda sketchy on chrome rims. There's no room for modulation on some combinations—the brake is either off or totally locked up. Most people don't want a brake that locks the wheel solid at a touch.

that's my bike. rims are black paint, very smooth. I agree, on/off braking would be no good but when you jump onto narrow ramp to need to brake precisely I guess.

yeah, definitely threaded pads. the old AD990 brakes also use a 10mm nut but it's a totally different system.

you'll probably want clear pads for that rim. the semi-soft red pads might work too. there are ways to minimize it but clear pads usually end up being noisy no matter what you do.