maddy13

I need help. I'm 5'1. I can mob down a hill on my 44cm Bianchi San Jose. Descending on my 44cm Amira scares the hell out of me. Things start to wobble and shake. I've had two bike fits. Is it me? Do tiny bikes just inherently have bad geometry?

PepeM

You might need to balance your wheels.

merlinextraligh

High speed wobbles are the result of harmonic vibrations. Some combinations of rider, bike, speed, position, hit exactly the right resonance to shake.

Assuming everything is adjusted correctly and in good working order (most notably the headset) there are things you can do to avoid, or correct, the high speed wobs.

1.) loosen up, flex elbows, grip the bars lightly. This is counter intuitive because your natural reaction to the bike shaking underneath you is to tighten up. But if you alter the biration being transmitted you can avoid or cancel the wobs;

2) if you start vibrating, clamp one, or both knees to the top tube. This dampens the vibration, and almost always stop the wobs.

3) Alter your position. Slide up or back on the saddle, lower your body by bending your elbows. Changing your position can get you out the frequency that's causing the wobs.

4) Descend in the drops. Not necessarily a direct response to the wobs, but descending in the drops gives you a lower center of gravity, more control, and better platform from which to react to problems, including the wobs.

5) Alter speed. The wobs take the right combination of bike, rider, position and speed. Slowing down, or speeding up will alter those conditions and stop the wobs. Sometimes a bike that wobs at 30 to 34 mph, doesn't wob above 35mph. So pedaling through it can be an option.

indyfabz

+1. Serious cyclists balance their wheels.

maddy13

I will look into balancing my wheels. Descending in the drops is not easy (wrists have to cock backwards), but I definitely stabilize with a knee.

I don't have this problem with my San Jose, or when I rent bikes that are too big. Do any other tiny people have stability issues descending on their road bikes?

bonz50

you're missing the sarcasm there.

merlinextraligh

I think its actually more of problem for larger people on larger frames.

Frames that flex a lot tend to be more prone to the high speed wobs. All else equal larger frame sizes with larger riders flex more ( less true today where additional stiffness can be dialed in to larger frames in the CF design).

Its just a function of hitting the right harmonics, so its happening to you because all the conditions happen line up just right.

And if your wrists have to cock back in an unnatural position for you to descend in the drops, your position on the bike isn't right, even with two fittings.

Dan333SP

Came to this thread looking for this response; not disappointed.

HTupolev

What?

nycphotography

If you can't ride and descend in the drops then you need to see a fitter because something, somewhere, ain't right.

maddy13

Proportionally femurs and torso are both long. Seatpost is set back, which negates any top tube benefit of my long torso. The combo scrunches me up in the drops. Not what should be causing downhills to wobble though.

You guys joke about wheel balancing, and I definitely defer to your knowledge, but some buddies with Rolf Vigors had to add little weights to their wheels. It seemed to help them.

topflightpro

It sounds like you may not have enough weight over the front of the bike.

Women tend to carry more weight in their hips and have a lower center of gravity compared to men. (I'm assuming you are woman based on your username and the fact you ride such a small Amira.) You mention that you have your saddle pretty far back. Combine those issues, and you've now got all your weight over your rear wheel and little on the front. That could allow for the harmonic vibrations to cause the front end of the bike to shimmy more than others.

You don't experience this as much on the Bianchi for a couple of reasons. One, the geometries of the two bikes are different. But two, the San Jose has a much heavier fork. I'd guess you also probably have much heavier wheels too.

Next time you go down hill on the Amira, try sliding forward on the saddle some and put more weight over your bars and see if that helps.

Also, there are three excellent reasons for down hilling in the drops:
1. You get better leverage on the brakes.
2. You put your hands behind the bars. This means you are less likely to lose grip should you hit a bump, plus you can push back against the bar should you start braking hard and your body start to go forward over the bars.
3. It helps get your weight over the front wheel.

rgconner

Dynabeads!

maddy13

Thanks so much. All of this makes perfect sense. I'm going to experiment with moving weight onto my bars on downhills. The ST angle is 76 degrees, which means that my seat has to be positioned way back to compensate, moving most of my weight to the back of the bike.

Dan333SP

What kind of weights did they use?

mpath

Let's see a pic of the bike in question.

HTupolev

Inherently? No. 44cm frame with 700c wheels? Typically.

The basic issue is that the front wheel is too big for the bike. Without making weird frameset design choices, you end up with a problematic amount of toe strike.
To some extent this is solved by using a slack head angle to hold the wheel farther from the bottom bracket, but this results in terrible handling (way too much trail) unless the fork is given a ton of rake. And since the framebuilders don't want to make more fork variants, they can only slacken the head angle so much before it would go from "stupid" to "comically unacceptable."
The next tactic is to simply move the bottom bracket backwards. This can be done with or without lengthening the top tube, but is usually done more or less without, resulting in the steep seat tube angle. Ultimately you ended up with "a long top tube" anyway because your saddle is moved back on the rails.

You might be able to "fix" it by throwing KOPS out the window and moving your saddle forward (and probably slightly up), but note that this'll make the bike more aggressive.

Or you could get a smaller-wheeled bicycle.

wheelreason

Sounds like you are having weight distribution issues. You should be balanced between your wheels (55/45 ish) without scrunching or reaching. Your drops should be where you are most comfortable bombing down at speed. The feeling is probably accentuated by the fact that you are used to the handling of the San Jose.