Possibly warped rotor? Air in brake line(s)? Bleed the brakes.

Tire defect? Any warping or bulges in tires?

 

I recently experience violent vibration on my fork. Too violent, can't hold a grip on my hood. The road is paved but a little rough. I would surely anticipate the roughness... but not the vibration on my hood. Checked the headset, front hub/bearings, front rotor, break caliper. There's no play or even a tiny wiggle.

What's going on?

My gravel bike is a Velo Orange Pass Hunter with Radiolabs Spork carbon fork.
DT swiss 350 hub
Shimano GR400 caliper/pads.
Shimano RT86 160mm rotor

Never experienced it before.

I probably didn't use the bike for more than 3 months... everything was solid then.

How tall are you? It sounds like what we call "death wobble," and it's terrifying. Large light frames can get into harmonic resonance. I experienced it once descending from Mt Hood Meadows on a rough, paved road and swapped wheels, thinking it was that. The bike was a light alloy Cannondale Synapse, 63cm frame (XXL).

Another possibility is a headset problem, but you'd especially experience that while braking, or a warped rotor but you'd probably hear that 'ping' while not braking.

https://zinncycles.com/front-end-shimmy-or-high-speed-shimmy

One technique I've heard to eliminate it while it's happening is to stop pedaling, try to clamp the top tube with both knees, and brake with both brakes but try to brake with a harder jab to the front (without resulting in an endo, of course).

 

maybe fork flexing, watching bikes in slow motion under hard braking shows the fork flexing back then skipping forward creating a judder, on a mountain bike this requires locking up the front wheel but on a road bike maybe normal braking will do it.

 

If it only happened during braking & stopped while coasting then it's a pad rotor problem = clean both.

 

check your tire pressures. Tubeless? If so, check that there isn't a glob of dried sealant in the front tire

 

I would clean the rotor by scuffing it with an abrasive pad. Realign the caliper. Bed the pads in again. Bedding in burns the oil out of a thin layer of the new pad. This dry layer then works it's way down though the pad with regular use. The oils may have soaked back up from the bike sittting? I don't think the Icetechs are the problem. I find they vibrate much less than other rotors. Maybe new pads. basically a reinstall.

 

Get a stiffer fork. My guess is you don’t notice it just using the rear brake. Either the fork legs or the steerer is flexing.

 

I'd try another rotor. I had this when I put a new 220mm rotor on. Turned the bike into a rocking horse. Rotor was true. I think it had a machining or stamping error and it left a high spot I couldn't measure. Put the 200mm back on and a third rotor and it vanished.

 

Yes... the vibration defenitely goes away when i let go off the brakes..

Yes.. vibration makes the fork flexes back and forth. It's a carbon fork.. i just hope there's no crack or integrity issues. I've rode with it on rocky trails, gravel, mud.. but i don't ride it as bad as MTB.

But I will check the rotor and the pads. Will try to clean it.. then i will see.

Thank you for all the response.

 

But I will check the rotor and the pads

The very first thing I'd do is lift the wheel off of the ground, spin it and watch for rotor runout. If that looks good, grab yourself a micrometre, and work your way around the entire braking track checking for deviation in thickness. If it passes those two checks, throw it away and buy a new rotor and pads.
Edit -check for wear/free play in the wheel bearings too

 

Harmonic resonance, someone else said it. It can be due to bad bearings, but if all bearings check out, it's an interaction between the brake system and the structural support, most likely the fork. Hard to ever know for sure, because changing a rotor, or mount, or fork, isn't a foolproof way to fix it, but you have to somehow "break" that frequency propagation.

I had a carbon fork that was just too flimsy for bigger brakes on downhills. I would use the bike for hill climb training and for that I needed a big rotor for the downhills...but the fork was just too flimsy to support it.

 

Check the easy things first.

Check the brakes, its likely the interaction between pads and rotor. Make sure things are straight and the surfaces are clean and rust free. As the bike has been sitting, it might be time to have the brakes serviced (bleed).

If you're running tubeless, your sealant might have dried up and taken your wheel way out of balance. Consider pulling the old sealant out, and replacing it new stuff. An out of balance wheel might not be a problem at slow speeds but once is spinning fast (like racing down a hill) and it will feel like all hell is braking loose.

Make sure ALL your bolts are torqued to spec. I've had a death wobble on a road bike that was fixed by tightening the front axle to 12Nm. It felt fine when checked by hand, but the torque wrench corrected the issue. Make sure all the bolts are tight as they should be, including the headset, stem, axle, etc... The bike has been sitting and you never know.


Let us know how you and your bike are getting on...

 

Check the rear wheel axle torque and spoke tension. Spec defects get transferred to the front wheel causing wobble.

 

Something to try if none of the above works (simple stuff first).
If you can, roll down the hill to get your speed up, then brake on the flat and see if it's speed related, or incline related. The incline may be taking the trail out of the bike. An angled headset may help.
Read some articles at Rene Herse. They're all about gravel, and get into rim brake vs. disc brakes on rigid forks. They used to favor rim, but now have forks that work with discs. I don't think you're the first one to have this problem.
RH are the experts on gravel bikes, see what they say.

 

Here's another thing to consider. Going down hill for long distances the brakes may be getting hot enough to bring the oils up out of the organic pads and contaminate the brakes that way. I had this on my ebike and had to go to metallic pads. But this can be risky on small lightweight hydraulics. The Icetech rotors do help a lot. Shimano offers metal pads with heatsink for those. For hills I would get those.
L04C-MF

 

There are 2 versions of Icetech rotors. Laser cut Japanese, and stamped Chinese. The Japanese ones are much flatter.

 

contaminated brake pads or incorrect brake pads can also cause this.
my fork with rt86 went wild with anything other than resin pads.

 

I run metallic on RT-86 with no problem. But they have to have ALL the old pad material cleaned off. But they will howl when slightly wet, and quiet down again when fully soaked. It can take forever to bed in metallic pads with Icetechs on a pedal bike.

 

Ping all of the spokes on the front wheel and see if there's any noticeable difference in spoke tension (or maybe even a broken spoke that is still sitting in the wheel pocket).

 

I fitted a Rockshox Judy XPLR fork. Traction on braking is now phenomenal and I can go much faster than the tyres can grip.

 

At Rene Herse they say suspension forks reduce rolling resistance. But they all seem to be running rigid bikes anyway......

 

With this guy's description (and I've had it happen before), I can't imagine it's anything but the resonance issue. The question is why, which is sometimes easily figured out with a loose part/bearing, or sometimes not, because it's the flex of the parts being at the same frequency as the vibration.

 

I've had a similar thing on my gravel bike. It sometimes is triggered when braking on rough surfaces. The frequency is way too high to be a warped rotor or wheel issue.

Even though I came feel any headset play, taking it apart and putting it back together seems to help for a while. I'm tempted to try expoxying the expansion bung into the steerer in case that was loosening slightly.

 

The original description said the bike was previously fine and only showed this problem after sitting for 3 months. Parts don't come loose or become more flexy when sitting idle. Plus he said he checked all the external hardware torque. I still suspect air in the brake line. That can cause pulsating when the brakes are engaged.

 

Try running the headset preload just a little on the tight side. The bottom bearing is always loaded, but some preoad can keep the top one loaded too. If R&R helped that may be the change. Maybe check that Carbon fork for a hidden crack inside the headtube. A growing fracture there could allow the preload to diminish slowly until it breaks.

 

This happened to me doing hill repeats with weight weenie rotors that heated up and warped. Nothing on the bike changed, but I did gain a lot of weight.

 

Any update? If it was fine before, and now it shakes... what changed?

DT