Help needed with front brake ‘grabbing’ rim

[bencook]

Hi all. I’ve had this bike for around a year and have never got to the bottom of why the front brake grabs the front wheel once per turn. What I mean is, when I apply the front brake a get a small jolt each wheel revolution.

Bike is pictured. Wheels are Fri Aria. Things I’ve tried:
- checking wheel is true (it’s good)
- checking rim at sticky point for a bulge or deformation (nothing obvious, and I did try sanding it gently)
- removing brake pads, sanding down, refitting with them ‘toed in’ slightly
- moving brake pads up/down so they hit different part of the braking surface on the rim
- turning the wheel around in the fork
- cleaning the rim as best I can

And it still grabs! It’s seems to be by the join in the rim. Should I just go in a bit harder with the sanding? Any other ideas?

Thanks in advance.

 

[Lucidone]

Can you feel this only when ridden, front wheel lifted or both?

Nice bike!

 

[jim haseltine]

If it's anything like the rims I've just discarded it could be caused by distortion near the joint. I lapped smooth a small step at the joints with fine emery paper laid on a piece of plate glass but that made little difference so I laid a steel straight edge on the rims which highlighted slight narrowing of the rim width just beyond either end of joint sleeve. Concluded that brake effect was reduced for a split second at the narrow point and what I was feeling was the return of full effect at the joint. It's didn't feel at all good, giving an impression that under really heavy braking the front wheel would lock.

 

[bencook]

Can you feel this only when ridden, front wheel lifted or both?

Nice bike!

Thanks!

I just tried it lifting the front wheel, and yep I can feel it that way too.

 

[bencook]

If it's anything like the rims I've just discarded it could be caused by distortion near the joint. I lapped smooth a small step at the joints with fine emery paper laid on a piece of plate glass but that made little difference so I laid a steel straight edge on the rims which highlighted slight narrowing of the rim width just beyond either end of joint sleeve. Concluded that brake effect was reduced for a split second at the narrow point and what I was feeling was the return of full effect at the joint. It's didn't feel at all good, giving an impression that under really heavy braking the front wheel would lock.

Good idea, I’ll get the tyre off and put a steel rule over the grabby bit.

 

[fettler]

Try turning the wheel round, it helps sometimes but depends on the join.

 

[pigman]

Try turning the wheel round, it helps sometimes but depends on the join.

Yeh, I've done this and it helped to the point that I could live with it

 

[doctor-bond]

If the rim is really true, then think tyres. Maybe the pads are too near the tyre; as you put on the brakes they go inwards and slightly upwards and could be hitting an imperfection in the tyre. If so, bringing the pads down a tad in the slots might work.