I was trying to take a pic today of the sunset, going down hill at 30 mph or something like that, and with no hands on the bars, the front of my bike started to wobble back and forth pretty good. my body was stabilizing the rear wheel, and i went nowhere, but that wobble was kind of surprising, and I never noticed it before.

I'm a 6' - 6'1" guy on a 56 cm caad9, 15.5 cm head tube, with a slammed -17 deg 140 mm stem. pretty short wheel base.

Does this wobble happen because of the short wheel base?

get some weight over the front wheel leaning back with no hands at 50kph is asking for trouble.

Pressing your knee against the top tube often helps stabilize the bike, but the sunset pic is best made from a stop. Having a spectacular pic to show from your hospital bed or during your funeral retrospective is not great compensation.

140 stem= clown bike. The wobble is designed in for the amusement of spectators. Be sure your AGVA dues are current, for insurance.

Not wise.

High speeds are fun. Riding no-handed is fine. Combining the two is a recipe for medical bills.

It's your bike's way of telling you to stop being stupid.

received

now, is anyone going to answer the question?

It happened because there was not sufficient weight on the front of the bike for the speed you were traveling.

It's caused by a combination of factors and not specific to short wheel bases alone, although a short wheel base can exacerbate it.

https://en.wikipedia.org/wiki/Speed_wobble

Or read this...

https://www.sheldonbrown.com/brandt/shimmy.html

I have had Speed Wobbles going 54 mph. It ended well, Still had to go to the Hospital to remove my saddle from my bum. It was very unnerving to have your front end dancing all over the place at that speed or any speed for the matter of fact.

i.e. hands not on the handlebar.

Dan

The wobble happened because your hands weren't on the bars. Maybe if you installed low riding panniers on your front wheel it might slow the wobble down and increase wind resistance so you can't descend as quickly.

I've seen wobble cured by tightening the front Q/R and another time by adjusting the headset. These were folks I was riding with, I've not experienced it first hand. It appears there may be multiple possible causes.

https://www.sheldonbrown.com/brandt/shimmy.html

Yea, but you're not going to find a better backdrop during the ceremony. It'd have to be closed casket though. You win some you lose some I guess...

I tried the same descent with my cross/gravel bike, no wobble without hands. wider wheelbase, stiffer frame, wheel is wider, so more lateral stiffness all around, and steering is generally better. I'm guessing it's the wheel/tire that's the difference. the wheels on the caad9 are basic aluminum box rims with 25 mm tires, which isn't really the best handling combination.

also, if you want to be funny or slick, do a little better than making jokes about hospital bills and caskets, or better yet, go read a book. i know it's the internet, but you don't gotta be a prick.

It's the frame not the wheels

Is a particular style of frame geometry better for no hands riding?

I have read from a guy who has extensive experience with this type of stuff say that it could be as simple as tire swap. even a bump in the tire, or slight untrue wheels can cause oscillations.

I think my caad9 is plenty stiff for a road bike. I mean sure the caadx is even stiffer, but I feel like the bigger difference between the bikes are the wheels. the cross wheels i have on the caadx are just solid. they're wider, so tires don't flex as much. the spokes are stiffer too i think. on the other hand, my caad9 has traditional double walled alu box rims. when i corner in a race on them, I can feel the flex. If I were to guess, I could make the wobble go away if I switch to proper carbon clinchers, which i won't do because they're expensive AF.

it could be the frame. they have slightly different geometries.. maybe my friend will let me borrow his wheels for a day to put on the caad9 to find out.

I had this issue with a front mudflap. I cured it by not taking my hands off the handlebars.

I was discussing it one day with my dad who is an audax rider who said he was having the same issue.
For us we solved it by removing the wide mudflap and replacing it with a slim one - the issue was that the wind was catching the flap on each side and blowing the wheel back the other way, at which point it would rinse and repeat.

IMO no hands on bars certainly a factor but I wonder if the weight shift alone was a big factor. It would be interesting if you could find a friend with gorilla arms to to sit up like that while keeping hands on the bars. I don't think 30 mph is *that* fast, so I won't chastise you.

I have a bike with steep seat tube and short chain stays that is by far my most "squirrelly" bike - I can't ride it 5 feet without hands but haven't had a shimmy in any high speed descents whereas another of my bikes has had that occasional issue which I counter by a knee on top tube and a lighter touch on the bars.

I've answered my phone, pee'd, and attempted to work a knot out of my shoulders at that speed. I'm not going to lecture you.

I've only had the speed wobble on two bikes. Both times I had my hands firmly on the bar and both times the only cure was to slow way down.

Once was on a Felt B2 with Hed 3 trispoke front wheel with a side wind. This only happened once, the wind must have been at the perfect angle.

The other was the one that really upset me. On my "dream bike". Brand new Litespeed Vortex with all the fancy shi(f)t. I couldn't correct it at speed and had to inch my way down the descents. I bought it to race the crap out of it. It was utterly useless unless the race had no real descents and ended on top of a hill. I could have raced a $1500 Motobecane faster than that $8000 POS.

In my case, I think it was too many lightweight components, each with different flex properties. All these differences begin fighting with each other at certain speeds. A little heavier bike with good material properties is worth it to me. I'm currently on a Columbus steel frame with Columbus steel fork. It descends like you're on rails, smooth and fast rails.

I sort of think the bike is too small for you? I'm 5'8", 5'9" after a good night sleep, and rode 56cm most of my life due to availability but usually went 55cm. So if you're 6'2 on the same frame as I'd be comfortable (if a little elongated) on, it seems like it has to be too big for you. Do you have access to a friends 58-59cm to preview? Unless your Caad9 is dented, you can almost certainly sell the frame on ebay for no loss on the frame swap.

^^ you mean frame might be too small? it's possible. riders my height usually go for 58, not 56. But the bikes built these days all tend to have tall head tubes, which puts me more upright than I'd like. I ride competitively, and my ideal position is to have the back parallel with the ground. To do that, i need to go a size smaller, and compensate the reach with a longer stem. not everyone does this, but if I were to pick between getting lower (while remaining reasonably comfortable), and spending $1000's on aero equipment, I'll choose the free option. besides, I ride a 56 on the caadx as well. as I mentioned, the difference in geometry between the two bikes (one that wobbles and one that doesn't) is pretty small. idk. I'm not too worried about it at this point. i don't normally take my hands off the bars like that anyway.

stiffer top tube in the next frame .. alignment spot on..

As long as it only happens with your hands off the bars, I'm with you, I wouldn't worry.

BTW, the steel bike I'm constantly raving about is semicompact, 53 standover, 56 toptube, as I've said so many times and will say again, is the most stable bike I've ridden. So would a 55 seattube and 58 toptube bike be a worthwhile investment? Dunno, and those won't be easy to test drive.