urbanknight

But I assume the "bump" would be via a hand on the front guy's back, right?


Right.

Blossom

Yes there is a mass term in the equation, but in my experience aerodynamics is more important. I'm a small guy (125 lbs) and I used to get dropped on descents all the time. Eventually I bought a new bike and I was very picky about the fit and am now able to get into a much more aero position. Descents are much better now.

Straight descent? Put your hands right next to the stem and get your chin as low as it will go.

kudude

the point is that the mass dependence should be something weaker, like m^(2/3) in the drag term

YMCA

You've obviously never experienced a high-speed wobble (I did a couple times at 50mph+).
I always keep my hands in the drops at anything over 35mph.

Flatballer

You've obviously never learned to grip the top tube with your knees.

MDcatV

dude ... YMCA is one of a hand full of bf.net posters who while I might not agree with all of their philosophy, I would not question their knowledge or skill when it comes to things bicycling or racing related.

Flatballer

That's fine, but that doesn't change the fact that speed wobble can be mitigated by gripping the top tube with your knees. This is coming from watching my Pro and Cat1/2 teammates descend in the mountains. They don't seem real worried about speed wobble. One of them even descends sitting on the top tube on straights. The only time I've had it was when I was just starting and was death gripping the bars and didn't know to grip the top tube.

Just because someone has won races doesn't mean they know everything.

ridethecliche

While this is true, I'd say the same for CDR and he's repeatedly talked about descending as described earlier.

I think it has to do with fit and such (weight distribution). And of course luck.

Blossom

Small changes in the weight distribution and speed can cause a resonance feedback. It occasionally happened to me with my old bike and gripping the top tube with the knees always stopped the wobble. Never happens with my new bike.

If your bike gets the wobblies, don't try the method I described earlier. I would, however, seriously rethink that particular bike/rider combination if you live in an area with hills.

mike868y

I have the exact same problem! I'm not going to argue the physics of it, but I'm 120 lbs and get killled in my local A ride on descents. Luckily, I make it up on the climbs

Phantoj

The drag term is "f" in the diagram...

I can't stand snarky physics posts, the kind where the poster tosses out an equation or diagram without explaining how it is applied. It's supposed to be a sarcastic burn, implying that the application of the physics is so trivially easy that it doesn't need to be described. Anyone with half a brain could get it. Ironically, most of the time it seems like the equation being (mis)applied is not the correct one or is incomplete.

Grumpy McTrumpy

it's similar to headshake in motorcycling (like a mini tankslapper). On a motorcycle the best thing to do is not fight it. I've never had more than a very slight headshake on the bicycle even at speeds up to 50mph. I just relax and let it ease off.


Here's a headshake taken to the Nth degree..

https://www.youtube.com/watch?v=LZ1srcQMa_0

carpediemracing 

If my bike starts to shake at all, or feels like it, I stop trying to go faster. My (early edition Giants) limited me to about 45 mph. I am too much of a wimp to push a wiggly bike fast, and I haven't had a compelling reason to do so.

Having said that, if my bike feels good, I'll let'er go.

I've tucked hands next to stem, knees holding top tube, with full confidence, up to and over 60 mph (Specialized M2 S-works frame, box section front wheel, 440 rear, and 100% confidence in my equipment). I've regularly hit 50+ mph, although I've struggled to hit that speed in the last couple years, dunno why. I've even burned my chin on my front tire when I hit some "waves" on a descent at full tilt (where pavement rolls up and down slightly but looks smooth, something you only feel at high speed). Had a nickel sized scab on the bottom of my chin after that ride.

I have a feeling that if I actually had to go fast on a random bike on a random day (like I got a wiggly bike from neutral support and I was in contention, or if my team sponsor gave me a wiggly bike), I'd be more leery. If that was the case I think holding the drops would be better, simply because it puts your hands in a place where it's easier to feel any wiggliness (also easier to control wiggliness if it starts). Definitely the conservative place to be. Second most conservative anyway, the first being to not go that fast (like me).

Not ever being a good enough rider to get a free or partially free frame or even a neutral support bike, I am glad to say that I've never been in this situation. YMCA may not have been in the same position because he was (is?) actually a good racer, not just an experienced rider.

Regarding weight - I hit my top ever speed when I was 30 lbs lighter than I am now (140 vs 170). When I started pedaling, I was going 64 mph (and I quickly stopped and re-tucked). I regularly hit 50-55 mph coasting when I was 103-112 lbs (college days). Nowadays I accelerate faster on descents, but I seem to be slower top-speed-wise, probably because I don't do a frantic sprint before I tuck, and my sprint is 6-8 mph slower than it used to be. Now it's a more leisurely acceleration then a tuck.

cdr

umd

IIRC YMCA is or was a cat 1.

EvilGoodGuy

Problem fixed. This time, there were more pros out on the A ride. It was easy to find a good wheel to follow on the descent. I grabbed a wheel then followed it. The pace was much faster too so there were far fewer riders over the top of the climbs.