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Monkey Wrangler
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Discussion Starter · #1 ·
After 39 years of riding all kinds of bicycles, I had something totally unexpected and scary as **** happen to me on a gravel ride yesterday.

My new gravel bike really flies on the down hills. We have a massive hill in my area that you lose/gain 353 feet in 1.2 miles. It is a great hill to climb when training and always scary going down.

I am almost to the bottom and things are going well. My computer said I was going 38 mph on the gravel when I went over a few bumps in the road that were caused by traffic turning into an alfalfa field. After hitting the bumps, my handle bars began to violently whip side to side causing my rear wheel to follow suit. For what seemed to be an eternity (but only lasted probably 10-15 seconds) I held on waiting to crash on to the ground. It was all I could do just to hang on.

I have crashed numerous times and even if the yard sale takes a millisecond, it seems the brain always has time to think "Oh Crap" before you hit the ground. Waiting for this to happen over five seconds was torture!

I managed to hang on and slowed down enough that the wobble finally started to cease. I pulled over and took a look at my bike (after cleaning out my shorts!).

My front and rear wheels both are in perfect true vertically and laterally. Both tires were seated correctly, my headset was not loose...I am at a loss for what caused this. Anyone else ever have this happen?
 

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My cousin back in 1977 got up to an estimated 45mph on a bmx bike before the death wobble. 2 weeks in hospital with a brain injury and now he has something like cte all these years later and is in bad shape mentally.


Haven't there been some dampers..maybe Hope used to make a damper.. I think..
 

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After hitting the bumps, my handle bars began to violently whip side to side causing my rear wheel to follow suit. For what seemed to be an eternity (but only lasted probably 10-15 seconds) I held on waiting to crash on to the ground. It was all I could do just to hang on.
In bold is 100% rider created instability. Head shake like this stays isolated to the steering unless the rider gets ridged. The rider essentially connects what would be isolated to the steerer and transfers it to the frame.

Head shake itself is for the most part caused by the rider being tense as well. Hitting bumps and head shake starting is another classic example of the rider being to tense. Our bikes are designed to be stable and the front end will always find the most stable path (assuming no mechanical issues and decent traction). Hitting the bumps on a loose surface will start tossing the front tire around a little. As the wheel tries to correct itself a tense rider will tend to push it past this point of correction causing a feedback loop. Wheel tries to correct, tense rider pushes it past, wheel again tried to correct, rider pushes it past the stable position the geometry of the bike is trying to find.

More aggressive geometry can make that shake happen easier but shouldn't be more than a few minor shakes if the rider is relaxed.

It can be really tough on bikes since you don't have a fuel tank to squeeze with your legs. For the most part it's fine to rest some weight on the handlebars but if you start gripping tight and your arms go ridged you'll definitely run into problems.
 

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psycho cyclo addict
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That is scary stuff... haven't experienced it at high speeds but I have at relatively low speed as Fajita Dave describes.

One of my 29er's (with more aggressive geometry) has exhibited this behavior a few times over the past 10 years when riding with no hands on slight inclines or descents (aka- rider was clamped down on the saddle for balance).

I've always tended to hover just above the saddle on bombing descents to prevent my body from absorbing all of the chatter from what the bike is rolling over.
 

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Monkey Wrangler
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Discussion Starter · #11 ·
My fork is on correctly.

Tires are Maxxis Ramblers set up tubeless, 700x40.

I guess I caused this to happen. I was in my drops and off the brakes just letting the bike go. After I hit the perpendicular ruts in the road the wobble started. I have descended this hill hundreds of times on my gravel and mountain bikes and never had anything like this happen. Inexperience on my part.
 

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My fork is on correctly.

Tires are Maxxis Ramblers set up tubeless, 700x40.

I guess I caused this to happen. I was in my drops and off the brakes just letting the bike go. After I hit the perpendicular ruts in the road the wobble started. I have descended this hill hundreds of times on my gravel and mountain bikes and never had anything like this happen. Inexperience on my part.
Just re-read my post. Didn't mean to make it sound so much like your fault haha!

On an unpredictable loose surface like gravel and braking bumps it can definitely cause head shake even if you do everything right. Being tense will only make it worse. No idea what kind of tires and pressure you're running but you might want to try dropping the front pressure a little. It will let the tire absorb any abnormalities and break the oscillation. Of course don't go to low or you'll end up with the front tire trying to wander around like you're riding a flat tire.

On a sport bike I've had headshake exiting bumpy corners heavy on the throttle making the front end light. A moment of lost traction will cause the steering to turn and sometimes the bumps cause it to bounce back and forth a bit. On a dirt bike I've had it happen in pretty much every situation including braking hard through braking bumps. I kept chasing a suspension setup to fix it until I finally figured out it was because of my dead grip! Also my tire pressures were a bit to high. Never had a problem after solving the rider problem and dialing in my tire pressure.

Bottom line that headshake is nothing more than the front wheel trying to find a stable position. If you or the terrain/traction does anything to interfere the steering will continue to shake until the front tire finds stability. Try your best to let it do it's thing and go straight. If you need to brake only use the rear (assuming you can).
 

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Great description above regarding death wobble. I rode a lot of track supermoto and would sometimes get wobble in long straights if I hit a small abnormality in surface. A lot of mine was tuned out by adding more weight to my front end (sliding forks up thru the triples) and going with a different offset triple to stabilize at speed.

So, the expensive option would be get a different offset fork. Cheapest option is moving body position forward or just slowing down on your downhill lol 38mph is fast!
 

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Cane Creek Viscoset adds a bit of damping specifically to address the speed wobble problem.

I had it once on my touring bike on a fairly mellow greenway ride. It was really odd, but haven't had it since (and I've bombed some totally sketchy gravel on that bike, too). I'm glad I was going fairly slowly when it happened to me. I was able to stay loose and slow down until it subsided. The path was poorer quality, but I think what set the wobble off in my case was a stiff crosswind.
 

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Wow....doesn't sound like fun.... had my Maxxis Ramblers up to 47 on a downhill in a race Saturday with no issues. ( note that I was NOT looking at my computer at the time...saw the speed afterwards ) :)
 

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Great description above regarding death wobble. I rode a lot of track supermoto and would sometimes get wobble in long straights if I hit a small abnormality in surface. A lot of mine was tuned out by adding more weight to my front end (sliding forks up thru the triples) and going with a different offset triple to stabilize at speed.

So, the expensive option would be get a different offset fork. Cheapest option is moving body position forward or just slowing down on your downhill lol 38mph is fast!
Agree, try a fork with less offset. Your Gravel bike may be designed with a low trail figure and will be twitchy. Less offset will give you a higher trail figure and a more stable response. I aim for 75-80mm trail for high speed situations on gravel. 38mph is fun!!!!

Eric
 
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