What is it about a bike that contributes to stability at high speed? For me, high speed is over 40mph. I never went over 40 mph for years but now my new weekend route takes me a long down hill where I could hit 50mph if I wasn't constantly on the brakes. I feel the rush just like you guys do when going fast, but I nudge the brakes at 40.

The bike, a 58cm allez, doesnt feel too stable at that speed, but that maybe coz I'm not accustomed to going that fast. So what contributes to high speed bike stability? I'm guessing it's rider position, tires, wheels and frame, pretty much the whole package. But what is it in the frame that aides stability the most.... head tube ? Would an Allez E5 be more stable?

a bike is a bike(for the most part) when it comes to stability IMO. The only thing that may have an effect is the stiffness, as a stiffer bike wouldn't give the little bit of flex...but I doubt that's even really a factor. Just slowly increase your speed and the stability will come. I used to be unstable doing 30 mph...I can do 40+ now with no issue. Don't know about anything really over 45 b/c there are no hills that long/steep/safe to go bombing down at those speeds.

wheel balance, rake/trail

bicycles are pretty light and don't have the damping that motorcycles have, so riders tend to use their bodies to absorb vibration and resonances at speed (like putting your knees against the top tube on a descent, relaxing your grip on the handlebars etc)

part of it has to do with the handling characteristics of the bike, part to do do with the handling characteristics of the rider.

most road bikes will be inherently stable enough at speed. the rest comes down to comfort/rider handling ability. ride at higher speeds more and more and you'll get used to it. otherwise, if you have access, or otherwise are looking into rollers, they will help sharpen up bike handling skills, too.

Most instability is due to the rider.

Very true.

Confidence at high speed comes with practice and patience.

I agree with umd, it's more about yourself.

If your wheels are out of true you may feel it.

However, I've felt stable on most of my bikes. I live on a nice hill and usually hit about 40mph down it. I've taken it with both my commuter and road bike, and it feels equally stable.

The more relaxed you are the more stable you'll be.

Aside from equipment issues such as:
A bike will track perfectly straight. I've seen plenty of motorcycle races where the rider messes up and crashes. He falls off and the bike continues in a perfect straight line until it hits something.

Same thing on downhills; if you were to jump off the bike when it starts to wobble at 50mph, it will continue perfectly straight by itself. Couple of tips:
The most important part is to relax and let the front-end shake a little when self-centering. If you lock the front-end with stiff-arms and a death-grip, when it hits a bump, it will steer sideways and give you the willies. If you relax the arms, the steering will get knocked to the side and immediately self-center due to the trail. It blips so fast, you hardly recognized what happened until after it's fixed itself.

I've found that some wheel/fork combinations can have oscillations induced more easily than others. You could try a different set of wheels and see if that makes a difference. But, like others have said, it's the rider that starts the shimmy in the first place. Relaxation, avoiding a death grip, even simple things like not getting too cold on a long wet descent can prevent it from happening.

+1....I'm doing nearly 40 on some of the straight sections of our mtn rides with one hand, recorder in the other. Some riders ride the brakes on this descent but after so many times, I have no problems after getting used to the speed and relaxing. Heck, even at the botoom, I show the computer in my video, doing 34. That's like a walk in the park. Relax!

Tandem a different story. We hit 56, no way am I removing a hand from the bars.. But tuck and relax, no problem eventhough the 55+ speed is scary. It's good thing my wife is relaxed on the back as well, helps the bike remain rock solid at that speed. Now that is trust.

Don't coast going down hill, keep pedaling in a smaller gear. I find it helps me feel like I have control over the bike if I keep pedaling, and I don't get all sketchy on a hill. When you get comfortable with that, you can try not pedaling and moving yourself into more aero positions, etc.

Just think about the physics of it. The bike has a tremendous amount of energy pushing it forward; it wants to go forward rather than side to side. Keeping that in mind helps me out quite a bit.

Dont hold on so tight. makes a big difference, stay relaxed and flow with the bike instead of trying to force it.

Bicycles are inherently stable at high speeds. You have two big gyroscopes holding you in position. The trick with any gyroscope is not to hold it too tight. If you hold it too tight, then you end up fighting it. Just have a relaxed but solid grip on your handlebars.

Also let your legs relax as well. The bumps in the road will have a bigger effect at those speeds. Just let the bike float over them under you. If you are stiff, then you end up bouncing over the bumps as if you were attached to your bicycle. Think of it like riding a horse at a gallop. You let the horse do its thing and you enjoy the ride.

a bicycle wont.

don't look at your speed and just let it go as fast as you are comfortable. if you relax and just look ahead, knee on top tube, you won't even realise you are going so fast. i can't tell the difference between 35 and 58 (the max on my computer but i don't know where exactly it happened).

where is this? i've seen in your posts that you ride in my neck of the woods and i can't think of where i can reach 50. my record max was in the south downs.

for a number of reasons. motorcycles have a lower CG, more mass, more damping, and are under power as long as the clutch is engaged.

however, the physics behind both of them are the same. It's just that little things like wind and bumps and unbalanced loads cause a much greater reaction on a bicycle.

On my motorcycles it's really really really really easy to do trackstands at stop signs without much adjustment.

I'd agree that rider and technique are the mst important elements. That said, frame stiffness, and geometry can make a difference. My Giant has a much stiffer front end than my Merlin. Mid 40's on the Merlin and that's fast enough for me.

The Giant however just inspires so much confidence that I find myself pedaling in the high 40's just to see how much faster I can go.

In addition to the above, I'll add the intangible of having confidence that your bike is mechanically sound to handle those speeds. The best thing I ever did was learn how to wrench my bike and do my own maintenance. 40+ mph is no time to have your limit screws set incorrectly and throw your chain e.g., or have something loose because you don't have a routine of checking your own rig.

Either yourself, or get a good mechanic that you trust and keep your bike on a regular maintenance schedule with him. 40+ mph is not like riding the MUP to the store.

yeah, due to the lighter-weight, a bicycle will get tweaked from equilibrium by a falling rider. However, if you've ever hopped off cleanly, like grabbing an overhead branch, or hopping straight up and off the back, the bike will continue in a straight line. Until it slows down enough to lean to one side, in which case it will carve tighter and tighter turn and lean more and more until it lands on the ground. BTW, I don't recommend anyone dismounting at 50mph to test this...

The gyroscopic effect of the wheels on bike stability is minimal. What keeps the bike upright is the continuous "controlled fall" adjustment of the steering by the rider.

No matter what I do to blank it out, I think about my front
tire blowing out, Never happened yet but.....

try riding on rollers at 15kph and 60kph and decide which is more stable

Contributing to high speed are the handling profile of the bike, wheels and length - but as already said the rider is the most important piece in how they place themselves for aerodynamic advantage and stability.

Getting low on the bike will improve the handling at high speed (lower centre of gravity) as well as the aerodynamics. Going really fast (fastest I've been is 92.5km/h) I will crouch low to the bike, have both feet level with each other (ready to lift one into the corner) and grip the top bar with my legs (again, also providing an aero advantage).