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merlin speed wobbles(12 posts)

merlin speed wobblespslater
Jul 8, 2003 9:03 AM
I recently built a new merlin magia in XL size. Full DA with Velomax Orion Comp wheels. On about the 4th ride, I got over 40 mph and was rewarded with a terrifying speed wobble. I had the headset adjusted and changed to a conventional 32 spoke wheelset that I have put more than 9k miles on with no problems. First ride after the changes the speed wobble returned. Since then I have heard of or talked to several others that have reported similar stories. Anyone out there with stories like this? Merlin swears there is nothing wrong with the bike. Why do all other 3Al-2.5V Ti frames weigh a full pound more than this one?
If all your components are in working order ...DY
Jul 8, 2003 9:34 AM
It is most likely your frame. If the bike fits you correctly and you are not doing anything wierd descending, it is most likely your frame.

I belong to another group of frame builders. This topic has come up every so often. The scientific reason that is usually blamed (when components are ruled out) is that the top tube develops a type of harmonic vibration. A combination of events can set the top tube vibrating and funny enough the more in tune your bike is the MORE likely it is to happen. A little slop here and there is said to cancel out some of this frequency and vibration. If your bike is in really great tune it will really show off a weakness or design problem with your top tube.

When frames are welded, there is a considerable about of heat is used. This effects physical properties of the tubing. The tubing wants to move and bend in response to this heat. Frames are made in jigs. These jigs are used to help control this movement. If a joint was overheated on one side during construction, but the tube was not "allowed" to move during that time because of the jig, that tube/joint may have internal stresses. In certain circumstances (like when you are going down hill at 40 moh) these internal stress are exposed. The tube may want to move to the position it wanted to during welding or a slight weakness from the overheated spot may allow the tube to move when it shouldn't.

Because frames are so thin and lightweight now, I believe a small error in welding/overheating can make a difference between two identical bikes. One wobbles, one doesn't.

I have heard of it happening to all types of metal bikes. I have also had this happen. When I got about 35 mph it would happen. First time it happened I thought I had a flat and the tire was about to come off the rim. Nothing I could do about it but squeeze my knees on the tube to help lessen the vibration.

Eventually I got the bike company to exchange the frame.

Hope you get to the bottom of your wobble. Do some more searches on the subject and you'll find more info.
Don't look now, but your stresses are exposedKerry Irons
Jul 8, 2003 4:59 PM
Internal stresses are exposed? What kind of gibberish is this? Speed wobble is a function of many things, but once things are properly adjusted, then it is a frame that is not stiff enough in the top/head/down tube junction. The fact that the tubes were held in a jig during welding/brazing is meaningless. This is like saying that a cold set frame (stresses induced) is more likely to wobble. Nonsense.
I see that the resident "Metallurgist" speaks.....DY
Jul 8, 2003 9:01 PM
Talk about gibberish.......I'm sorry that you don't like my wording, but I was trying to make the concept of frame induced speed wobble easy to understand.

My experience:

(A) Heated metal wants to "move" (bend, distort, twist).
(B) Unevenly heated/overheated metal can have uneven strength characteristics (more or less strength on one side of the tube than another).
(C) A frame that was unevenly heated or overheated can have a "weak" area. This weak area can allow the frame/tube/joint to flex to that side easier than the other side.

Yes I am making broad statements and using general terms for the sake of simplicity.

If the joint strength is uneven or weak on one side, it can behave in an unexpected way when something triggers it (high-speed, vibrations, rough road, etc.). That being said, it doesn't mean that having a weak/uneven joint would cause the bike to do anything weird. In 99% of the cases it probably doesn't, but in 1% it could if the correct conditions occurred.

Of course jigging a frame does not cause it to wobble, but over "jigging it" (forcing it to stay jigged) and over heating it can cause "stress" from the metal wanting to naturally move. That is why good "Joints" which you mistakenly call "Junctions" are carefully heated and welded in a pattern to minimize uneven heating and the build up of dynamic stress in the joint.

This would all be nonsense if we were talking about 1980 Schwinn Varsities but we aren't. We are talking about ultra light, thin walled frames and yes when this machine is pushed to its edge.....its design or construction faults are "exposed" for lack of a better word.

Notice how I use the words (if, could, may, can, might) in both my messages. I am just pointing out reasonable possibilities. One explanation in which it is not the fault of the rider or the components...it could be the way the frame was made. There are many other possibilities of why a bike wobbles. When it is not the components or the rider it is probably the frame and if the frame was designed correctly (correct tubing, correct geometry) it then points to ....construction. This is just my experience.

Oh and one last thing... I never said anything about cold setting. I always talked about heating. Manipulating metal by heating it rather than when it is cold is metallurgically different, but you would already know that.
An engineering view pointoddsos
Jul 9, 2003 4:21 AM
From my understanding of materials the frequency a structure is prone to vibrate at is relatively unaffected by any locked in stresses resulting from welding or cold working. Neither of these operations change the stiffness of the frame and so will not affect the frequency of vibration (unless you start everything working in the plastic region. For a bike frame this would be a very bad thing).

As speed wobble depends on exciting a natural frequency of the frame your best option is to change the natural frequency. As a rider hurtling down a hill you can do this by changing your contact points on the bike. If the wobble starts when standing try sitting down or touching the top tube with your leg to damp the vibration. Unfortunately if the conditions are right to excite the wobble there is nothing you can do though.

Chris
Read more carefully, or at least don't misinterpretKerry Irons
Jul 10, 2003 5:18 PM
I never called a frame joint a junction. Don't try that "debating" technique. I specifically talked about lack of stiffness in the top tube/head tube/down tube "junction." IOW, the combination of these three tubes. Review Lennard Zinn's article on this subject. He builds large frames and has done a lot of work on this. His conclusion is that you need to have adequate stiffness this combination to eliminate speed wobble.
On the same track.........DY
Jul 11, 2003 10:35 PM
Don't you think we are arguing about the same thing?

We both agree that the frame is flexing........

I just offered a possible explanation of why. The poster said the frame company said it was not the frame. I was pointing out how it could be the frame. Not enough front end stiffness because of lack of tube stiffness. Possibly because of poor welding.

I'll be welding some frame joints this weekend. I could rig up a test (kind of like golf shaft truing) where a weight is attached to the end of the tube and the tube is rotated through 360 deg. Then the amount of deflection is measured and graphed. In a perfect world the graph would be a circle. I'll guarantee you that if one side is welded too hot then the tube will bend easier to that side and the graph will have an oval shape. Enough to cause a speed wobble or to get the ball rolling? .....possibly if everything is just right.
Why is is so clear it's the frame?wolfereeno
Jul 12, 2003 1:25 PM
and not the fork or wheels resonating?

Frame flex alone doesn't seem to explain how I have the same exact thing happen on a stiff carbon 1 piece kestrel frame.
Unless you weigh 130/140 pounds, the fault...........MR_GRUMPY
Jul 8, 2003 10:11 AM
is probably your own. The lighter the frame, the easier it is to start a speed wobble.
Super light bikes, need super light riders.
Agree with DY & Mr_GrumpyRickC5
Jul 9, 2003 5:56 AM
a) Super light frames need light riders. As a bonafide clydesdale, I can attest to having several bikes over the years that developed high-speed wobbles. Good components & proper set-up. Could have been caused by my weight or ?. All the wobbly bikes went "down the road" many years ago, and the two I have today (Litespeed Arenberg & Trek 5200) do not wobble at speed.

b) I also feel that DY has a valid point in that the heat of welding can exacerbate stresses that may exist in the tubing and would also agree that two bikes built by the same builder, in the same size, and on the same day, may still exhibit different riding characteristics. Notice I say "may", not "will". No two things created by hand are ever EXACTLY alike. As one frame builder told me years ago, "Some bikes just don't ride". I fully believe he knew what he was talking about.

Try to get Merlin to work with you in determining the possible causes. If you get too assertive (or aggressive), they will get defensive (human nature) and you won't get much out of them. Good luck.
sounds like hooey to mewolfereeno
Jul 9, 2003 3:56 PM
I'm not a clydesdale and ride a Kestrel 200sci. It wobbles every now and then too.

I think we do at least all agree that the wobbles are due to harmonic resonance somewhere. Whether it's the wheels, fork, frame, tires, my balls, hasn't been proven by anyone's arguments yet for my money.

I know wobbles happen to me when I'm going too fast for the condition of the road. Something always realized too late. When it happens I apply the brakes lightly. At 40+ mph they don't do much fast enough. So I apply them harder. 23mm tires pumped to 125 are bouncing across the rough spots. Your not going to stop while 1/2 inch off the ground. Getting nervous, my hands grip the bars tighter. All my muscles tense up. You can't even see straight due to the road chatter. You wobble, the road gets smother and you eventually slow (or I guess you could wipe out and die).

On a smooth road where I can see the run off at the bottom and know I can just relax and zoom, there's never a problem. For me I think there's a bit psychological counterpart. So lately I just try to relax and give in to the speed.

Some advice I was given in another thread about wobbles was also mentioned above. Try to do something that changes the equation. Stand out of the saddle. Put knee(s) against the top tube. Relax. let go of the handlebars. Pull emergency chute. Laugh about it with friends over beer.

Enjoy
re: merlin speed wobblesBirddog
Jul 14, 2003 6:47 AM
There are several factors at work here, but based on my own experience,I'm convinced that YOU are a large part of the problem. Try setting up the wobble or shimmy in as controlled a way as possible, and when it just starts to shimmy, loosen your grip on the bars and see if it doesn't let up. If it does, as it did with me, then your own "death grip" is a large part of the problem. Try pushing down hard on one pedal, this may help too.
Birddog