Mountain Bike Reviews Forum banner

1 - 20 of 21 Posts

·
Registered
Joined
·
9 Posts
Discussion Starter · #1 ·
I was riding down the road today as I returned home and took my hands off the handlebars to cruise. When I did, the handlebars developed a vibration that that shook the entire bike side to side and quickly got worse. I grabbed the bars as quickly as I could to stop it and save a crash. After a few more careful experiments, the issue continued each time.

I ride a titanium framed 29er with a 100mm travel fork.

Anyone have any experience with these wobbles or how to fix it?

Webster
 

·
1*14*29*2.1 & 1*1*29*2.4
Joined
·
1,283 Posts
Possibly your frame. Is it off the shelf Ti? Small headtube? Do you have top tube and down tube diameters? I had the same thing with my older steel frame, which was all Ox plat. I was advised later that it's a bit on the light side for a whole frame. The frame rides really well with a rigid fork though, so you could try that combo if interested. When I had my next frame made (Ti by Quiring) I mentioned this specifically, and he suggested bumping up the TT diameter to make it solid. It works a treat. I did the same on my next ti frame, no issues there either.
 

·
Registered
Joined
·
24,083 Posts
Could also be your front wheel, if it's a QR skewer, maybe not tight enough, or the bearings could need adjusting if they're cup n cone or could be worn and need replacing if raced. I had bad bearings in a front wheel and it caused exactly this problem, very offsetting until you figure out why.
 

·
Registered
Joined
·
2,427 Posts
"Tank Slappers" are never fun!

Pedaling can induce them, also sometimes just the vibration from the tires.
I doubt a "stiff" or notchy headset would cause it, more than likely a loose headset would. Also depends on where you were on the saddle, rear suspension (if applicable) compression, front tire pressure, and a myriad of other items can influence a tank slapper.
 

·
Registered
Joined
·
1,801 Posts
It could also be that the headset is preloaded too tightly. When you lean the bike and the fork turns, there will be a delay because it's sticking, then over-compensation to the other side and you got yourself a wobble. So when you check the headset, do not only check that it's tight, but make sure it's not too tight either.
 

·
Professional Crastinator
Joined
·
6,122 Posts
I was riding down the road today as I returned home and took my hands off the handlebars to cruise. When I did, the handlebars developed a vibration that that shook the entire bike side to side and quickly got worse. I grabbed the bars as quickly as I could to stop it and save a crash. After a few more careful experiments, the issue continued each time.

I ride a titanium framed 29er with a 100mm travel fork.

Anyone have any experience with these wobbles or how to fix it?

Webster
I had this on my Niner MCR9 with 490mm rigid fork.

I never figured it out. I could not let go over maybe 18mph. Was running a Maxxis Ignitor 2.1 up front, then a Racing Ralph 2.4 and it was the same wobble. Checked hub, QR, spoke tension, HS, tire pressure, frame alignment, wheel tracking (in-line or not) - no answer.

My only thought was that I never had a brand spankin' new tire on it, and maybe the tires I did have were worn kind of flat in the center, making the edge knobs more pronounced.

I have since bought a new frame, but not for that reason.

If you get to the bottom of it please post the solution.

-F
 

·
Registered
Joined
·
188 Posts
Has the problem always been there or is a new issue? New issues, check the suggestions above. If it has always been there, could be an issue with the for offset, etc.
 

·
psycho cyclo addict
Joined
·
3,016 Posts
I've experienced a "hands free" shimmy / wobble a couple of times on one of my hard tails that I've been riding a lot of miles on since 2010. I was able to correct it by loosely holding the handlebars and getting my 195 lb. carcass off of the saddle.

It is caused by rotating wheels :rolleyes: as Jobst so eloquently NAILS IT here...

Shimmy or Speed Wobble by Jobst Brandt

"Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. This steering action twists the toptube and downtube, storing energy that both limits travel and causes a return swing. Trail (caster) of the fork acts on the wheel to limit these excursions and return them toward center.

To feel the gyroscopic forces involved in bicycle shimmy, take a front wheel, holding it by its axle in both hands, and give it a spin. Manually steering it from side to side generates strong tilting forces always at right angles to the input. These forces sustain shimmy and are the motions one uses to make quick steering maneuvers while riding no-hands, shifting the hips laterally while firmly seated. The same effect as when wheeling a bicycle wile holding it only by the saddle. "
 

·
EAT MORE GRIME
(ノಠ益ಠ)ノ彡┻━┻
Joined
·
6,224 Posts
wow never had MTB shimmy

once had high speed road shimmy just like Jobst Brandt describes, due to the
a) speed above 60mph and b) geometry of my road bike at the time. the cure
was lock my inner arms against my drops and reduce the wobble.

never had it on an mtb, check all bearings for adjustment, sealed bearings if loose need replacement. may need to check frame on a jig if you cannot find the root cause
 

·
Professional Crastinator
Joined
·
6,122 Posts
I've experienced a "hands free" shimmy / wobble a couple of times on one of my hard tails that I've been riding a lot of miles on since 2010. I was able to correct it by loosely holding the handlebars and getting my 195 lb. carcass off of the saddle.

It is caused by rotating wheels :rolleyes: as Jobst so eloquently NAILS IT here...

Shimmy or Speed Wobble by Jobst Brandt

"Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. This steering action twists the toptube and downtube, storing energy that both limits travel and causes a return swing. Trail (caster) of the fork acts on the wheel to limit these excursions and return them toward center.

To feel the gyroscopic forces involved in bicycle shimmy, take a front wheel, holding it by its axle in both hands, and give it a spin. Manually steering it from side to side generates strong tilting forces always at right angles to the input. These forces sustain shimmy and are the motions one uses to make quick steering maneuvers while riding no-hands, shifting the hips laterally while firmly seated. The same effect as when wheeling a bicycle wile holding it only by the saddle. "
That sounds very plausible. I bet it happens much less frequently on aluminum frames, and much more frequently on steel and Ti frames - esp. "springy" XC frames.

-F
 

·
psycho cyclo addict
Joined
·
3,016 Posts
That sounds very plausible. I bet it happens much less frequently on aluminum frames, and much more frequently on steel and Ti frames - esp. "springy" XC frames.

-F
As luck or fate would have it, the frame I've experienced shimmies on is aluminum. I have two other aluminum (one is a 2-year newer model of the same hard tail frame) and one steel 29er and had no issues on them.

There is some speculation about thinner frame tubes (not the case on mine) or a larger / more stretched out frame/setup (which is true in my case) being part of the equation. Others suggest the wheel set may be a factor- I've run that front wheel on all of my bikes (plus the complete wheel set on two other bikes) and had no issues.

I had a car with a bad pitman arm bushing that would cause the front end to shimmy under certain conditions. Only way to disrupt it was to hit the brakes (until I replaced the bushing after the novelty of freaking out my friends wore off :thumbsup:).
 

·
Registered
Joined
·
1,286 Posts
I've never experienced it on an mtb, but I have on my roadie going flat out downhill around a turn, the whole bike turned into a wet noodle, scary s**t. I upgraded a stiffer wheelset after that. It's much better, but not to mtb standards.
 

·
Registered
Joined
·
58 Posts
I had that happen on the street going down hill. As my speed increased the front end would shake more. I checked tire pressure and it was at 25 psi. Next time I went up to 35 psi and did not have a problem on the same hill.

What was your tire pressure when the shaking happened? Try increasing it by 10 psi and try it again.
 

·
1*14*29*2.1 & 1*1*29*2.4
Joined
·
1,283 Posts
I've experienced a "hands free" shimmy / wobble a couple of times on one of my hard tails that I've been riding a lot of miles on since 2010. I was able to correct it by loosely holding the handlebars and getting my 195 lb. carcass off of the saddle.

It is caused by rotating wheels :rolleyes: as Jobst so eloquently NAILS IT here...

Shimmy or Speed Wobble by Jobst Brandt

"Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. This steering action twists the toptube and downtube, storing energy that both limits travel and causes a return swing. Trail (caster) of the fork acts on the wheel to limit these excursions and return them toward center.

To feel the gyroscopic forces involved in bicycle shimmy, take a front wheel, holding it by its axle in both hands, and give it a spin. Manually steering it from side to side generates strong tilting forces always at right angles to the input. These forces sustain shimmy and are the motions one uses to make quick steering maneuvers while riding no-hands, shifting the hips laterally while firmly seated. The same effect as when wheeling a bicycle wile holding it only by the saddle. "
not sure I buy that explanation really. I would have thought it was a resonant frequency thing. Much like balance of a car wheel having a spot at particular speeds where it wobbles. When the wheel speed and frame resonance match up there are problems. Much the same as brake shudder. No doubt there are gyroscopic forces but that is constant and I would imagine progressively increasing, not oscillating. In any event, it doesn't matter if the solution is to stiffen the frame. Either way there is a solution that requires attention to frame design.
 
1 - 20 of 21 Posts
Top