My brother recently bought a new bike, and he told me I could use his old beat up mountain bike as a base to make a custom bike.

I know how to fix most of it's issues, but it has a twisted front fork which I don't know what to do with. It's rideable, but on an angle that looks weird when looking down at it.

Should I completely replace that bit or is there a fix to it?

Replace it. You are in a world of hurt if the front fork bails on you.

It can be fixed but doing so can create a bit of a challenge. Nobody here is likely to tell you anything that can't be found with a simple Google search for straighten bent fork. You will probably see Shopping results for replacement forks, which can be relatively inexpensive but then you'll be looking for replace fork!

Okay, I think I will replace the fork. Is there a way to make sure the replacement will fit?

The new fork needs to be compatible with the headset, stem, wheel size and frame geometry. In-person assistance is best here (bike co-op available?). If you want to tackle it "yourself" then do some research online.

Measure?

Funny, but the OP needs to know what to measure, thus the referral to online resources.

I think it's sad. The Telegraph reports:
"A few days ago, the owners of Homebase announced they are to close a quarter of UK stores because of the reality, they said, of 'a generation less skilled in DIY projects'."

Public education indoctrinates people against thinking. At least the phones are getting smarter.

What @cny-bikeman said. Bike co-op would be glad to help, and is likely to have a selection of used and inexpensive forks for you to choose from, and you will learn how to do the job yourself if it is anything like the one I work in.

This isn't on the level of asking how to tell what size seat post to get (which I have seen). It's not intuitive what has to be checked on a fork, and it's expensive to make an error.

Maybe. But asking "what will fit" implies a little research which requires a little thought and effort (there's the rub). A little research reveals what needs to be measured.

I know what you're thinking: "Implied questions are way too subtle for this forum."
I wonder if they are way too subtle for this(these) generation(s).

Does the OP perhaps mean that the handlebars are not properly aligned with the fork?

I am aware of your point, which is why I had to repeat and clarify my initial recommendation to look up "replace fork" when the OP asked how to determine if it would fit. We'll have to see what the OP has to say now. Logic and attention span both seem to be lacking in the current generation (or two!).

just buy a New Fork.


Fork blades can be bent in 360 degrees , and each in an opposite direction . and the dropouts twisted,

If you already dont have a grasp on how to measure them to be faced 50mm off the center line

And just right, relative to the fork Crown and the steerer tube.. etc.




(or, show a picture of your shop's alignment table jigs and dial indicators, but then the question wouldn't need asking )

Wondering the same thing here. OP, if the handlebars and fork are simply twisted in relation to each other that is an easy adjustment. Perhaps you should post a photo.

yes I was thinking the same thing. OP try standing on front of the bike facing it with the wheel firmly between your legs and see if you can twist the handlebars straight with your hands. If it does straighten, you also will need to tighten the bolts holding it to be sure it does not slip while riding.