Could be a number of things. What is your wheelset? My first suspicion is usually the seam, how is your rim joined? Has it always been this way or did it start after some time of ownership? Have you checked the rim surface for anomalies or buildup? Bulge in sidewall?

 

My hoops are Kinlin XR19's with Sapim Laser spokes. Originally I had these wheels on my 31 year old steel frame, but I had thought it was a loose or bad headset. Recently switched them to my Focus, and it still does it, which means that they've always done it. They are about a year old at this point. I have looked at the rim and haven't noticed any kind of build up on them at all. Sidewalls look good on both sets of tires that have been mounted.

 

turn the wheel around so it rotates the other way, it may help with your problem enough to forget about it.

 

What causes the pulsation? Is it the way that I have the pads set up on the braking surface?

As others have explained it is most likely the rim joint. Try taking a sanding block to the rim sidewall at the joint (on both sides) to smooth out the bump. Another possibility is that the rim took a hit and bulged the sidewall someplace else.

 

Just so it's clear to me, where on the rim exactly is the joint?

 

IDK if it's been mentioned, but sometimes the brake track gets contaminated in a way that causes pulsing. Cleaning the track with a green scotchbrite pad and some soapy water can remove the contamination and restore smooth braking.

 

+1 to looigi's suggestion. We include alcohol pads for an initial cleaning on new builds (grease/oil used in assembly will seep out during shipping), but a clean rim is a happy rim.

 

Thanks all! I didn't have time to take the tire off as of yet, but did flip the wheel around this morning before a quick ride and it did make it better. Still a slight pulse, but better. I'll work on getting the tire off and inspecting the rim a bit more closely this weekend.

 

Did you physically feel and/or for a miss-aligned joint?

Here is my website fix for a bad joint -

Rim Joint Fix.

 

Rim widths often vary around the wheel. If the variation is more than about .1mm, it causes the brakes to pulse noticeably.

It's easy to measure with a set of digital calipers. The tire must be removed. A quick way to check for width variation is to set the calipers at the valve hole then rotate the rim with the calipers in place. You'll see if the rim varies. Then to measure the variation, take measurements at every spoke or every other spoke.

Some rims have non parallel brake tracks. Those are not as easy to measure.