Front Wheel Pulsing

[richardault]

hi BARF'ers, long time member, first time poster, I have a 2019 KTM 790 ADV, low miles (>15K) in top shape. The bike has the stock wheels that came with it, that last servicing had the wheel trued and some shims installed between the rotors and the wheel, which has had negligible impact on the pulsing in the brakes. The pulsing is isolated to the front brakes, I don't feel it in the lever, but the whole bike kind of rhythmically lurches as I come to a stop.

I'm pretty confident the issue is the front hub, a skilled mechanic I work with, through process of elimination and a dial gauge, detected a small hop in the hub. And the pulsing is most pronounced at low speeds, when coming to a stop.

I'd like to ask this community a couple of things:

1. is this the likely culprit?

and if so:

2. Can you recommend someone who works on wheels, ideally in the Bay Area (close to SF or OAK most ideally)

Thanks in advance!

 

[CDONA]

One time I bought a "show" quality brake rotor, polished stainless, that didn't like my brake pads.
The rotor had a hard spot in the surface, that gave a pulse, in use.
Different pads with more sinter, bit into the rotor harder, and made it usable. The polish was gone from the wear surface.

I don't think radial runout, will pulse the brakes, it is the grip/slip that pulses braking surfaces, like a fat spot

 

[richardault]

One time I bought a "show" quality brake rotor, polished stainless, that didn't like my brake pads.
The rotor had a hard spot in the surface, that gave a pulse, in use.
Different pads with more sinter, bit into the rotor harder, and made it usable. The polish was gone from the wear surface.

I don't think radial runout, will pulse the brakes, it is the grip/slip that pulses braking surfaces, like a fat spot

Thank you @CDONA.

The rotors and pads are new, and when they were installed the mechanic I was working with confirmed the "trueness" of the rotor. That being said, I will put the bike on a stand and confirm asap. I'm also pretty confident it isn't the ABS, because I have felt that before in an automobile, that rapid pulsing...

I was anticipating a few answers to that point, that this must be lateral runout or potentially DTV (fat spot) or pad deposits... but now I'm wondering if it isn't one of the above, what else could it be?

 

[tzrider]

I don't think radial runout, will pulse the brakes, it is the grip/slip that pulses braking surfaces, like a fat spot

Radial runout won’t typically cause pulsing, but may indirectly contribute. A disk with runout will whisk against the pads every rotation and over time can create thin spots on the disk.

To see if this is it, I’d use a micrometer in several places around the disk, checking for variations in thickness.

 

[tzrider]

I was anticipating a few answers to that point, that this must be lateral runout or potentially DTV (fat spot) or pad deposits... but now I'm wondering if it isn't one of the above, what else could it be?

Disk surface contamination can produce the sensation. Fork seals doing ok? Could brake fluid or other contaminants have gotten on the disk?

A worn steering head bearing can produce behavior that one might mistake for a pulsing disk. If the pulsing is rhythmic, I wouldn’t suspect head bearings.

 

[ichabodnt650]

I had a new KTM 690 SMC that developed the pulsing you described at 8,000 miles. Spinning the front wheel on a stand, things looked fine. Spinning and applying front brake gently didn't show any sign of pulsing. Even tried loosening the axle pinch bolts, loosening the caliper bolts, and applying brake before re-tightening the caliper bolts. Nothing fixed the problem.

Ordered a replacement Galfer rotor and swapped. The problem disappeared.

If you have new rotors, I might try aligning the caliper as mentioned to see if that might help

 

[ST Guy]

Usually, pulsing as you describe, can be the result of contamination of the rotor surface. This can happen simply from finger oils.

Get yourself a CLEAN rag and a can of brake cleaner and give both sides of the rotors a thorough cleaning. No need to clean the pads. I’ll bet your pulsing disappears.

Please report back after you’ve done this.

 

[Enchanter]

I'm curious why the brake rotor was shimmed. Google tells me that your front brake caliper is a rigidly mounted 4-piston caliper. The pistons will 'self-center' on the brake rotor alleviating the need to shim the brake rotor.

I had a very similar issue on my S1000rr, I scrubbed the rotors with scotchbright, no change. I used a brake rotor hone with no change. I sent them out to be machined (one one place in the country does motorcycle brake rotors), and they were great for a couple thousand miles, then right back to the pulsing. I finally replaced the rotors and they've been perfect for many more thousand miles.

 

[ctwo]

Usually, pulsing as you describe, can be the result of contamination of the rotor surface. This can happen simply from finger oils.

Get yourself a CLEAN rag and a can of brake cleaner and give both sides of the rotors a thorough cleaning. No need to clean the pads. I’ll bet your pulsing disappears.

Please report back after you’ve done this.

Someone on this very forum told me the equivalent of brake clean is practically satin's juice and should not be used on brake parts. Should use a tooth brush and dish soap.

 

[CDONA]

I have a pulse, right front on my truck, intermittent.
I hit the cold brakes lightly I can feel it, a bit more pressure it quits,
rotors get hot, no pulse.
I think some rotors are made with hard spots that resist brake pad friction/wear,
mistakes in the metallurgy .

 

[augustiron]

This is a known problem on the KTM 790 adv.
Most replace the rotors with aftermarket.
There is a lot of discussion of it on KTM adventure specific forums.
Some of the threads might be older as I don't think it is as prevalent on the 890s of the last few years
There is also a front brake recall for certain years.
A quick Google search Will give you all kinds of leads to follow.

 

[ST Guy]

Someone on this very forum told me the equivalent of brake clean is practically satin's juice and should not be used on brake parts. Should use a tooth brush and dish soap.

Wrong.

OP should get a can of some brake cleaner, spray on a clean rag to get it wet, and then clean the rotors well. Any brand will work. Just get it from an automotive store.

It’s worked perfectly for me me every time I tried it. And it worked on my Toyota 4Runner as well.

And one time after I drove through a large puddle of water, with oils and other contaminants in it, the pulsing returned because of the contaminants. I simply pulled out my can of cleaner and a clean rag, cleaned the rotors again, and the pulsing was gone.

It’s a contamination problem that needs to be cleaned up. And any brake cleaner is fine for the job.

Just don’t spray the stuff directly onto the rotors. Doing that may get the spray on parts you don’t want it on. Remember, it’s a solvent and depending on the type of paint, it may damage it.

 

[ctwo]

Wrong.

I'm not wrong - someone actually said that...

Here's an instructional video from an authoritive looking source - I mean, they even put their name on the can...

It's dangerous stuff in the hands of an amateure...

 

[ST Guy]

You said that someone said brake cleaner is the equivalent of “satan’s juice” which implies it’s bad. It’s not.

And on a car, you need to remove the wheel to do the job with generally no exposed painted surfaces. On a bike you don’t need to remove the wheel which leaves painted surfaces exposed to the spray.

 

[ctwo]

You said that someone said brake cleaner is the equivalent of “satan’s juice” which implies it’s bad. It’s not.

That's not really what I said, but the point was it's not to be used to clean brake parts.

 

[ST Guy]

Why not? Give me a good reason.

 

[ST Guy]

And you just posted two videos of a cleaner doing exactly what you say is bad.

 

[ctwo]

And you just posted two videos of a cleaner doing exactly what you say is bad.

That's why I said it's dangerous in the wrong hands, or something like that.

I won't be able to find the thread now, but the referenced way of how to do it properly is on the Dave Moss Tuning channel.

Somehow, I suspect wiping the rotors off with a chemically dampened cloth is going to fix the problem.

Oh, I did find the video through...

 

[ST Guy]

That is a video on cleaning calipers, not cleaning rotors.

 

[ST Guy]

Also, if the pulsing is felt thru your braking hand (or foot) only, it’s a rotor that is warped or has some similar kind of defect.

If it can’t be felt by your hand or foot and only through your body, like through your ass or arms, it’s a brake contamination problem, most likely.