Brake Question - pulsing

bikerboy337 · #1 02-28-2012, 07:51 AM

So I'm trying to figure out whats going on... have a fairly new set of wheels that i just started using again. Problem I'm having is a pulsing as i brake. I've swapped brake pads, cleaned them and all is good with teh brakes/pads. I've inspected the rims, no noticable bumps, hops, out of true, etc. I haven't put it in a truing stand, but i've spun it with a tool just off the rim and there are no wobbles/hops that I can identify...

any idea why i might be getting a pulsing when i brake? its my front wheel by the way...

any thoughts would be appreciated. Going to drop off at my LBS this afternoon... but wanted to see if anyone had any ideas...

thanks...

AngryScientist · #2 02-28-2012, 07:53 AM

it almost has to be some irregularity in the rim. slight concavity or something you might not see without a truing stand can definitely cause braking pulsations.

christian · #3 02-28-2012, 08:22 AM

Can you put it on your other bike to try to isolate whether it's the brakes or the wheel or something else? Do that and report back.

bikerboy337 · #4 02-28-2012, 08:25 AM

Agreed... as i was typing the original questions this morning... i realized that I haven't swapped wheels between frames... so that will be my next step... i just took a flat edge screwdriver and held it just off the rim and spun... no noticable wobble, dips, etc that i could notice

i'll swap the wheels and report back...

fun stuff...

Thanks!

keevon · #5 02-28-2012, 08:34 AM

Did you toe-in your brake pads? I can usually eliminate pulsating this way.

A credit card positioned under the back 2-3mm of the brake pad seems to be just about right.

yashcha · #6 02-28-2012, 08:35 AM

Is it a Kinlin rim?

I just took apart my wheel built with a kinlin 300 rim because it pulsed so badly that the whole front end would shake during braking. I have heard of many others complaining about this issue as well. The problem with my rim is that there are several sections where the rim width meaures narrower than the rest of the rim.

My solution was to replace the rim with a Velocity A23 .

MattTuck · #7 02-28-2012, 08:36 AM

Quote:

Originally Posted by christian

Can you put it on your other bike to try to isolate whether it's the brakes or the wheel or something else? Do that and report back.

I like this... sounds very scientific.

Joachim · #8 02-28-2012, 08:41 AM

Not sure what headset you have on there, but sometimes a loose headset can cause pulsing when braking.

christian · #9 02-28-2012, 08:49 AM

Quote:

Originally Posted by Joachim

Not sure what headset you have on there, but sometimes a loose headset can cause pulsing when braking.

You are a scientist, yet you are jumping, nay, leaping, nay bounding madly to conclusions. I think this is a likely scenario, too, but let's get there methodically!

Joachim · #10 02-28-2012, 08:52 AM

Quote:

Originally Posted by christian

You are a scientist, yet you are jumping, nay, leaping, nay bounding madly to conclusions. I think this is a likely scenario, too, but let's get there methodically!

What can I say, I had a cup of coffee and threw all reason out the window during a time of complete and utter madness.

bargainguy · #11 02-28-2012, 11:04 AM

Rim weld that isn't finished properly. I once spent hours chasing my tail (truing, repacking hub etc.) until I figured it out.

Don

bikerboy337 · #12 02-28-2012, 11:44 AM

Thanks for all the thoughts folks... these are Campy Zondas by the way...

i'll test a few things out tongiht before I take it to the LBS tomorrow....

I'll update once I figure it out...

Definately not the headset.. have a CK headset, plenty tight... I had that thought too though...

We'll see!

Ken Robb · #13 02-28-2012, 12:30 PM

Make SURE there are no contaminants any where on the rim. Something you can't see or even feel with your fingers can cause a variation in coefficient of friction and lead to pulsing brakes. I use aerosol brake cleaner for automobile brakes to be sure I get everything off.
I'm assuming you mean the brakes seem to grab and release a little while you maintain steady pressure. If you get pulsation through the brake levers it must be a mechanical problem. I wonder if there can be a weak or thin place in the rim that "gives" a little under pressure from the pads.

mister · #14 02-28-2012, 01:56 PM

brake pads are probably worn enough to cause this chatter
replace them or try toeing them in like previously mentioned

bikerboy337 · #15 02-28-2012, 02:02 PM

THIS... is one of my other thoughts... i'm going to be cleaning the rims tonight as well... making sure they're nice and clean... there is a good bit of brake pad on the rim from a set of campy pads i had on this bike... swapped to kool stop as i like em better, but i'll clean off the residue thats left...

Quote:

Originally Posted by Ken Robb

Make SURE there are no contaminants any where on the rim. Something you can't see or even feel with your fingers can cause a variation in coefficient of friction and lead to pulsing brakes. I use aerosol brake cleaner for automobile brakes to be sure I get everything off.
I'm assuming you mean the brakes seem to grab and release a little while you maintain steady pressure. If you get pulsation through the brake levers it must be a mechanical problem. I wonder if there can be a weak or thin place in the rim that "gives" a little under pressure from the pads.