Singlespeed & Fixed Gear - Fixed Gear Fork stiffs up when straight?

Every time I put my handle bars straight it stiffs up and loosens when I turn it sideway

It happened after I rode on a group of rocks while going fast and then hit a curb straight on

you have ruined your headset bearing races. you probably had a loose headset when you hit the curb, which pitted the race and now the bearings get tight when they fall into those pits. or, you warped your head tube and it binds. i'd guess it's the first.

or, it's an old headset with a center notch to aid in keeping the wheel straight while not in use, and you just never noticed it until now. but i'd assume it's not an old headset.

either way, new headset and face/ream the headtube.

you ****ed your headset, or it has no correlation to the accident, and you just have indexed steering due to the barrings digging holes in your headset.... either way, you need a new something....

you have ruined your headset bearing races. you probably had a loose headset when you hit the curb, which pitted the race and now the bearings get tight when they fall into those pits. or, you warped your head tube and it binds. i'd guess it's the first.

or, it's an old headset with a center notch to aid in keeping the wheel straight while not in use, and you just never noticed it until now. but i'd assume it's not an old headset.

either way, new headset and face/ream the headtube.
Thanks, Its a new headset I bought at bike store

recently installed?

recently installed?
I wouldn't say recently

well either they installed it wrong or when you hit the curb the impact was too great for the headset to withstand the forces.

you didn't screw any of the bolts on or off on the stem did you?

either way, a new headset is the least of your problems.

A bent fork (at the steerer/crown) can also make for wonky handling, but probably hosed headset.

and just so everyone is clear, NONE of this has to do with the fact it's a fixed gear.