aboss3

I would like to get some help with downhill riding technique. I've noticed that when I'm descending and my speed gets over 23-24mph, my bike starts vibrating and shaking. Basically it gets to the point where it becomes very unstable. I tend to slow down then in fear of causing a crash and let other riders pass by.
I usually ride along the PCH or Burbank mountains in greater LA area, where the road conditions are good.

My bike is Aluminum-framed Specialized Allez with Easton Circuit wheelset.

clink83

Is your stem flipper up high?

aboss3

No, not at all.

climber7

your bike won't vibrate and shake from lack of technique, at least not at that speed.

are your wheels true? is your headset/steerer tight? i don't really know what would cause vibration at only 24 mph, but i think it would have to be some kind of assembly/frame/wheel problem. otherwise, that's just weird.

Nerull

Speed wobble can start around that speed, so it's possible it is related to technique.

Too much of your weight on the handlebars, plus a death grip will do it. Make sure your saddle position is correct, relax a bit, and if a wobble starts touch your knees to the top tube, which should stop it.

therhodeo

How tight of a death grip do you have? Most guys I know who have trouble downhill go all white knuckle. I love descending and I just have my hands resting on the bars not really holding them tight.

aboss3

Thanks for all the help! Well I'll try to shift my weight more backwards to ease up on the handlebars. I was wondering if it may be due to the aluminum frame.

dspaff088

speed wobble at 24 mph? there is no way thats technique, Id take it to a shop or someone knowledgeable and have them look it over for the aforementioned issues

wrr1020

24 mph is pretty slow so i don't think you should be experiencing wobble as bad as you say it is. Has nothing to do with being aluminum, i've descended at 45-50 mph on an aluminum Trek with no wobble at all. Something might be loose on your bike or an out of true wheel, take it to an lbs and have them look at it. I also second loosening your grip on your bars if your holding on tight.

popeye

Relax your grip, let the bike find its way, knees against the top tube and eyes way down the road.

EventServices

Reeeelax everything. Generally, the bike wants to go straight.

Get the bike checked out for peace of mind more than anything else.

mustang1

i had shimmy (speed wobble) on my 2006 A1-frame'd Allez at about that speed (and also another at about 40mph).
OP: i haven't read the rest of this thread, but in case no one mentioned it, rest one knee gently against the top tube if you're coasting.

redtires

It's been mentioned a couple of times...clamp your knees on the top tube, it works. You can do a "check-up" on your bike right at home by making sure the headset is adjusted correctly and the wheels are true. Honestly, if this were the case, you would notice it at any speed. The headset especially just by hitting the front brake, you'll feel it "clunk". It is however possible that it is too tight. This being said, I actually don't think it's a mechanical issue. Lot's of bikes/riders have this issue, and it can manifest for one rider and not another on identical bikes based on bike set-up (positioning), rider weight, even tires can have make a difference. What's occurring is a "harmonic imbalance" at a specific "frequency" (i.e. vibration through the bike from the road). Clamping your knees on the top tube disrupts the frequency wave through the bike. I suspect you just have the "perfect storm" for this to occur and a few small changes may solve it. If you have another set of wheels, I would try that first (or if you can borrow a set for a ride) if you still have the shimmy, try lowering your stem a bit and +1 on making sure your saddle is properly positioned, especially fore/aft as that can have an impact on weight distribution. Also, what kind of fork are you running? Stiff frame, light wheels and a soft fork can ruin a ride rather quickly as well.

urbanknight

In addition to shifting your weight back and relaxing your grip, make sure your elbows are slightly bent (not locked)

JTGraphics

All the above is good advice but at only 24 mph I'd have to think something is not right with the bike it's self alignment of wheels bearings not adjusted correctly wheels, wheels true? and or headset or even a slightly bent fork.
Have these all checked out and ride that hill again.
I descent all the time @ 47-50 mph and I can go any were on my bike forward or back sit down low or sit up and wobble or even a trace of it never comes into play.
But someone also mentioned the same frame having some issues so it could just be a design issue with that model in which case you most likely can do nothing about but to try to cut down on some of it but @ 24 mph is kind of questionable that would drive me crazy and it would be gone and replaced.
Good luck and hope you find a cure!

Shimagnolo

What size is your frame?

Dan The Man

Putting a lot of weight on the handlebars at the right angle can definitely induce a wobble at low speeds. Basically you are changing how the bike responds. And if you are actively trying to control the bike by trying to resist the handlebars you can end up making it worse because your corrections are always out of phase and actually end up adding to the wobble, kind of like the Tacoma Narrows.

JamieElenbaas

I can attest that weight distribution and frame fit can cause wobbles at very low speeds.

I ride my BMC close to 50 mph without a bobble.

Just week before last, I was visiting a friend and borrowed his slightly too small for me cf Trek for a ride in the hills around Bend OR. There was no computer onboard, but I'd swear that I was going under 30 MPH when the first the wobbles came on. I did the 'relax the arms and grip the top tube with my knees' maneuver and the bike settled down enough for me to apply some brakes and slow down.

Being conscious of the issue, I really kept my speed down after that. Nevertheless, when a d-bag in a big pick up truck buzzed me on a decent, the side force from his wake started the wobbles again. I'm pretty sure I was going under 25 mph at the time. Pretty exciting as the road curved to the left and the bike didn't much want to turn until I was into the sandy pavement at the side of the road. Wobbles and a drift in the sand! It was more luck than bike handling skills that got me out of that one without going down.

After that, I tiptoed back to my buddies house.

When I told my him about it, he informed me that he doesn't have any 'death wobble' problems on that bike. I know that this guy has no problem going over 50 in descents, so it really was just a bad combination between me and his bike.

The worst part about the story is that I was riding with my friend's wife who climbs like a mountain goat. We're fine together on the flats, but the only way I can keep up with her in hilly country is to bomb the descents. Since I couldn't do that on this ride, she thoroughly kicked my ass. I don't know what felt worse; getting terrified on the descents, or having my ass handed to me by a girl

coasting 

loose grip. knee against top tube. this works for me. i have never had a wobble but i still put a knee against the top tube just in case. it's no effort once you get used to doing it. i've been up to 58 mph.

Shimagnolo

High-speed shimmy is an interaction between the bike and the rider.
The less trail the bike has, the easier it is to trigger it.
90% of frame mfgrs stupidly put steeper head tube angles on their tall frames, which reduces trail, making them more prone to shimmy.
(You can also put some blame on UCI for their front-center limits.)
Excellent articles on the topic:

https://davesbikeblog.squarespace.com...e-visited.html

https://velonews.competitor.com/2010/...-poorly_121162

TromboneAl

coasting 

that guy took his hands off and let it wobble like crazy! he must have enormous balls.

duderdude1

He caused it to start wobbling by smacking it. LOL

gazelle5333

at 24 mph I would agree with those above who said to take it to a shop and get it looked at...I pull 25+ when I'm really hammering the flats and no problem. I don't feel anything by way of wobble until almost 45 mph and then that's mainly nerves...nothing that would make me slow down

Dan The Man

Keep in mind that riding on a downward slop changes the bike handling, generally making it twitchier.