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Discussion Starter · #1 · (Edited)
Here we go. I got a Merlin Proteus in June of last year, and love it... sort of. He's the problem, when I am on desents and start climbing past 35 mph, when free wheeling. I start to get a shimmy that starts in the fork and then cycles through the frame. First time it happened I thought my quick releases had come loose the dang thing was waving so much. It has happened on 4 different decents, with 2 different wheel sets (Protons and Sprint 350's), and 3 different types of tires (Conti force and attack, Michellin Pro Race 2 and Vittoria KXS). On 2 occassions it reached near catostrophic proportions before I managed to get it under control, it scared the crap out of the bunch on one ride that I felt like Moses they parted so fast. Because of the different routes, and equipment, I can only place this on the fork or frame as I never had this problem with my Colnago MxL on the same routes. The Fork is the standard Real Signature HP.

I have also sent three e-mails to ABG asking for guidance and they have not responded to any of them, and I am now half tempted to put the thing on e-bay because of this (I hate crappy customer service). I love the way the bike looks, goes on the flats and up the hill, but I would like to go faster than 30-35 mph when gravity allows.

Now I have read the other threads regarding this issue. Seeing the thoughts on resonace etc. I was curious if a fork not only of greater rake, but greater or less relative stiffness would have at least a delaying effect on when the shimmy begins. I am already planning on trying one of the decents (when whether allows) with at least one knee on the top tube and see what happens since at least two decents I can regularly hit (but have been avoiding since) have me spin out my cassette, and it is only when I am free wheeling that I appear to have the issue, but was curious if a fork change would be a waste of money or not, say to the Real HP Pro or Reynolds Ouzo Pro (gotta have a glossy fork to go with a glossy frame : ) .

Sincerly Mr. Frustrated
 

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Longshot, Well just a stab at things to check. I'm familuer with your fork not your model frame but I think that it may be the same as the litespeed "hidenset" style. Please be sure that your expander plug has some clearance between it and the cap plug, I had trouble at first with the expander sliping a bit when I was tentioning the head set, what that dose is allow the expander to hit the cap and then there is "false" tention. I cleaned everything with compressed air and rubbing alcohol, then it stayed put allowing proper tention prior to setting the stem clamp. The most critical being the lower clamp bolt that is nearest the expander, this must be tightened to the recomended spec for your parts, or after hitting bumps the headset will loosen slightly. To be sure that you are tight enough, hold your finger around the headset cover and the frame, sit on the bike, hold the frount brake and rock your weight front and back with one toe on the ground or curb, keep sneaking up in tention till there is NO movement. Be sure not go the tighter the better, as this will also cause shimmy. Hope you get your problem figuered out. -- Bill
 

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Discussion Starter · #4 ·
bg. said:
Have you had the frame and fork checked for alignment issues?
Not yet. I am going to try the bike first with the LBS owner's fork from his Giant TCR Advanced as thats supposed to be a rather stiff fork. I wanted to try an "easy" fix first because getting the alignment check may mean I have to send the bike out and be without far longer than I would prefer, its my only road bike, but it may well turn out to be a necessity. :(
 

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badge118 said:
I was curious if a fork not only of greater rake, but greater or less relative stiffness would have at least a delaying effect on when the shimmy begins.
I think less rake would provide a more stable ride at speed.
 

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Doesn't increasing the rake decrease the trail? Less trail equals quicker steering. More trail equals slower steering.
 

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It's too bad cyclists are so stupid

Actually rake, fork offset and trail all contribute to stability on a bike. It's too bad we are so stupid that we can't use these terms correctly. First off Rake is not the curve of the fork. Rake in the normal vehicle world is the angle at which the steering axis is inclined as in cars, motorcycles, and scooters, which in reality is the head tube angle expressed from the vertical plane... but not in bicycles where people thought that the curl (offset) at the bottom looked like a garden rake and assumed that's what the term meant. Rake is designed to prevent fork failure from bending. The angle is intended to take the statistically greatest road shocks in compression, making them act axially on the fork blade.

Steering is mainly affected by trail (also known as caster) that should not be so great that is steers the bicycle laterally when pedaling standing, where slide loads from leaning the bicycle act laterally on trail. For this reason, fork offset is used to adjust trail, for any given rake, to a stable straight ahead ride while reducing lean-steer.

Offset is achieved by putting a curl on the end of the fork or by angling the fork from the crown putting the axle an offset distance ahead of the steering axis.

Trail is the distance from the intersection of the steering axis and the road to the center of the tire contact (the located vertically beneath the axle). With too little trail or negative trail the bike will handle well at low speeds but will easily develop a dangerous wobble at high speeds. Too much trail and you will have trouble balancing the bike at low speeds and it will handle sluggishly at high speeds.



Take a look carefully at the diagram above. You will plainly see that increasing the fork offset will decrease the trail with the subsequent results of quickening the steering and reducing the offset will do the opposite.
 

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Discussion Starter · #9 ·
WheresWaldo said:

Take a look carefully at the diagram above. You will plainly see that increasing the fork offset will decrease the trail with the subsequent results of quickening the steering and reducing the offset will do the opposite.
There is the contradiction though. While handling may be sped up by additional rake, it can actually help off set the shimmy (at least according to Zinn and others) because it alters the dynamics of the energy that it creating the wave form in the frame. Zinn and other frame builders used to try and clean up high speed shimmy by cold setting steel forks another 3-5 mm, obviously this is not an option with a CF fork. They didn't address alterations in stiffness however, which obviously is not as easily addressed.
 

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In the motorcycle world it is exactly as I wrote, and what is a bicycle, just a motorcycle with a smaller engine. Also you are confusing rake, which cannot be changed on an assembled frame, with fork offset which can be changed by either bending or replacing the fork.

Even Zinn does not fully understand english, some words simply cannot be used interchangeably. He refers to fork offset as rake and offset interchangeably and that is simply incorrect. He also says that you will make a bike more stable by increasing the trail. That is on page 149 of his book on Bicycle Maintenance. The way he suggests increasing trail is to use less fork offset, which he incorrectly calls rake, although he also uses the correct term offset. He also suggest decreasing the head tube angle to increase trail, not really practical or even possible on an already assembled bicycle. I cannot find anything in his writings that says you can eliminate high speed wobble by decreasing trail.

Zinn does say that changing a fork could reduce front end shimmy. Here is a quote from one of his Technical Q&A:
[size=-1]That said, there are exceptions to this when it comes to the subject of front-end shimmy, which is not a steering or stability issue (although it can result in the rider falling off), but rather a resonance frequency issue. In some cases, the interchanging of a fork with few more millimeters of rake can eliminate the shimmy problem in a bike that had a tendency to shake uncontrollably when riding with no hands on the fork with less rake.[/size]
Here he is just making an assumption. If the fork already has substantial offset, 43mm or more, adding more will not help. He correctly calls it a resonance issue. I would contend that fork offset plays a small or even insignificant part. I think you have really gotten to the point when you speak of stiffness as the main issue. I think the Merlin rider should look at forks with the same offset to maintain steering geometry but look at eith stiffer or even more compliant forks as the answer to high speed shimmy.
 

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Discussion Starter · #11 ·
WheresWaldo said:
[/indent]Here he is just making an assumption. If the fork already has substantial offset, 43mm or more, adding more will not help. He correctly calls it a resonance issue. I would contend that fork offset plays a small or even insignificant part. I think you have really gotten to the point when you speak of stiffness as the main issue. I think the Merlin rider should look at forks with the same offset to maintain steering geometry but look at eith stiffer or even more compliant forks as the answer to high speed shimmy.
I wonder as a frame builder though if he is making an assumption or as in another Q/A, he is going by experience, when he notes that he personally has fixed front end shimmies that are a resonance issue by increasing fork off-set as you call it (I wonder why if it is not rake why ALL fork manufacturers referr to the degree of off-set as rake, not doubting you, just confused). He does say that he had to change it by at least 5mm for their to be a difference, and I am not about to try that extreme, I would have to go to a 50mm and thats just crazy.

Also I don't necessarily think you can compare and motorcycle to a bicycle either. The only thing they share is two wheels. The manner in which the materials are used, the geometry of the frame tubes and supporting structure, the way the wheels are designed are all different in regards to weight, density, shape etc. The lack or presence of a motor, the mounts for it which help to counter vibration, the lack or presence of suspension, the constant tension in the drive train from how the motor applies tension to the system, all of these difference while appearing similar on the surface contribute to a different dynamic system with different forces being applied in different directions and at different magnitudes.
 

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badge118 said:
I wonder as a frame builder though if he is making an assumption or as in another Q/A, he is going by experience, when he notes that he personally has fixed front end shimmies that are a resonance issue by increasing fork off-set as you call it (I wonder why if it is not rake why ALL fork manufacturers referr to the degree of off-set as rake, not doubting you, just confused). He does say that he had to change it by at least 5mm for their to be a difference, and I am not about to try that extreme, I would have to go to a 50mm and thats just crazy.
I am sure we do not have all the facts here. You would be hard pressed to find a fork with 48mm-50mm of offset, unless it was custom built. With that in mind let's just assume that the bike he fixed was probably like my old Klein that has a 38mm fork offset and went to a 43mm offset. Zinn says nothing about fork materials, just that he changed offset, not very scientific for a guy who claims to be an expert (I am not doubting his expertise, just making an observation.) Almost all fork manufacturers use both terms, Rake and Offset, it is just that rake is used incorrectly. The Automotive and motorcycle industries use the terms correctly. A long time ago, before yours and my time, someone said that the curved blades of a fork looked like a garden rake, and now we are stuck with the term rake for the curve (offset).


Also I don't necessarily think you can compare and motorcycle to a bicycle either. The only thing they share is two wheels. The manner in which the materials are used, the geometry of the frame tubes and supporting structure, the way the wheels are designed are all different in regards to weight, density, shape etc. The lack or presence of a motor, the mounts for it which help to counter vibration, the lack or presence of suspension, the constant tension in the drive train from how the motor applies tension to the system, all of these difference while appearing similar on the surface contribute to a different dynamic system with different forces being applied in different directions and at different magnitudes.
Actually go into any custom motorcycle shop and tell them that you want to change the fork offset and what will happen if you do and you should get the same answers as you do from Zinn, Brandt and others. They are amazingly similar, although speeds are quite different. Suspension does play a part, but the effect we are taking about happens in Suspension MTBs also, just to a much lesser extent, as usually MTBs are not ridden at the same speeds as road bikes. That said I have seen it personally on a MTB.

As I mentioned at the end of my last message, you may have suggested the correct approach already. Try a fork out of different materials or different manufacturing process. Resonance is a result of not only shape but material and construction. and changing any one or all three could have profound affect on the high speed shimmy. I have only had one bike that has exhibited high speed shimmy. It was not extreme and would only show up at speeds in excess of 50mph. It was easy to correct with technique and required not changes to the front end geometry. Again, I said it was minor. What the original thread starter suggested seems much more severe. If his Merlin has a 43mm fork offset, then he will not be able to find a replacement. I would suggest he work with Merlin to replace it with another fork of different construction.
 

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Proper English

WheresWaldo said:
Zinn does not fully understand english, some words simply cannot be used interchangeably. He refers to fork offset as rake and offset interchangeably and that is simply incorrect.
While you are technically correct in saying this, you will find that in bicycling, the terms ARE unsed interchangeably. This does cause confusion, and so it is better to talk about fork offset and head tube angle and never mention rake. However, you are off base to criticize Zinn on this, because you will find rake and offset used interchangeably in a wide range of bicycling sources. Just because the terminology is correctly used in motorcycling does not mean that it cannot be incorrectly (but in a standard way) someplace else. You will find this phenomenon repeated in many fields, and to rail against it serves no purpose.

I once had a colleague who insisted on spelling it FACS instead of FAX, because afterall, it was in reference to a facsimile. He was technically correct, but it was a pointless crusade. :)
 

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temoore said:
I have experienced this several times w/ my Giant TCR composite. One time was really bad. The following article is by an ex-frame builder.
http://davesbikeblog.blogspot.com/2005/11/high-speed-shimmy.html

I have found that on descents that will approach 40MPH, I grip the top tube between my legs, and try to get the weight forward. Seems to help.
Temoore this is exactly what I do, and the article you pointed to says basically the opposite that Zinn says. DaveM says not enough trail will cause high speed wobble. I think that both Moulton and Zinn fixed their high speed wobble problems but drew the wrong conclusions as to why.

In the case of Zinn he increased fork offset, reducing trail and he says that fixed the wobble, Moultan says you decrease the offset, increasing trail and that fixes the wobble. They can't both be right, can they! In both cases the only known variable is that they changed the fork. I really believe it was just a coincidence that the offset changed, and the real reason the problem was solved was that the new fork they both used had a different resonant frequency that the original. So to attibute it to fork offset and not explaining how else the forks differed is misleading. I would also assume that in each case Zinn and Moulton used what they had around to solve the problem. If Zinn had a fork with a very small offset already, then he had to go larger, decreasing trail. If Moulton already had a significant amount of offset, then he had to go smaller, increasing the amount of trail. In both cases they claim to have fixed the high speed wobble problem and then made blanket generalizations that they solved some deep mystery of the universe.

Both Zinn and Moulton do agree on one thing. they both elude to the frame being the real problem and suggest that the head angle is to blame. Since you cannot change the head angle the only real things you can do is to get another frame, or change the fork. In either case Merlin should step up and take responsibility for this issue.

My apologies to badge118, somewhere along the line I forgot he was the originator of this thread. I still stand by my statements from earlier. Call Merlin, explain the extreme concern you have for your safety riding their bike and your sincere desire that they help you or your lawyer fix it. I would think that would get them to respond. I would also get your local Merlin dealer involved. Despite what Merlin says or doesn't say, it should be a warranty issue.
 

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Kerry Irons said:
While you are technically correct in saying this, you will find that in bicycling, the terms ARE unsed interchangeably. This does cause confusion, and so it is better to talk about fork offset and head tube angle and never mention rake. However, you are off base to criticize Zinn on this, because you will find rake and offset used interchangeably in a wide range of bicycling sources. Just because the terminology is correctly used in motorcycling does not mean that it cannot be incorrectly (but in a standard way) someplace else. You will find this phenomenon repeated in many fields, and to rail against it serves no purpose.

I once had a colleague who insisted on spelling it FACS instead of FAX, because afterall, it was in reference to a facsimile. He was technically correct, but it was a pointless crusade. :)
Kerry, I am not trying to rail Zinn and I did mention that he does use both terms usually the sentence is structured like this, "blah blah blah fork rake (offset) blah blah blah." I understand that many publications use these terms interchangeably. I am not on a crusade to change the world, just trying to use the correct terminology myself and when I have to explain it to others to avoid the confusion that fork rake denotes. In any case if I sounded like I disrespected Zinn in any way I apologize. I may not agree with everyones conclusions but that doesn't mean I don't respect their knowledge/experience.
 

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Just to be clear, the reason I believe trail is just a small part of high speed shimmy is that in most cases bicycle frame geometry hasn't changed. and from one manufacturer to another there is very little difference in the actual measurements. Looking at the trail numbers you will see that many bicycles have the same trail figures. So why do two identical bikes react so differently for different riders. Some of it has to do with experience and technique. Temoore highlighted a good technique to avoid high speed shimmy. It could be riding position. If too much weight is on the back of the bicycle that could lead to high speed wobble, try moving your body forward on descents and getting into a tucked position with your thighs pressed against the top tube. More than likely it is slight differences in manufacturing or having components near their tolerance limits that make one shake and the next one completely stable. Changing fork offset really means changing forks, and there is no way for you to tell if everything else is exactly equal between those two forks. Merlin designed that frame to have a balance between agility and stability, changing offset may upset the design criteria for that bicycle. Try to get them to send your dealer a new fork, at their expense and have the shop install it at Merlins expense. That is probably really all that is needed. I am interested to hear how this turns out for you.

One last thing I would like to add. I usually just lurk on this board. For the most part discusions usually degrade into useless drivel. In this case I thing we have had a refreshing exchange of ideas and for that I thank everyone.
 

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Trails connection to shimmy, and long offset forks

WheresWaldo said:
Just to be clear, the reason I believe trail is just a small part of high speed shimmy is that in most cases bicycle frame geometry hasn't changed.
I think if you examine the shimmy phenomonon closely, you'll see that trail is inextricably linked to shimmy. As you say, shimmy is a resonance of the frame/fork system. In this resonance, the frame/fork act as a spring, flexing laterally/torsonally back and forth. The trail acts as the moment arm causing the front wheel to steer back and forth as the frame flexes back and forth. Without the trail, there would be no moment arm on which the lateral/torsional "spring" (frame/fork) to act on to produce a steering motion.

On the other hand, as you say, the differences in offset between different forks is usually minor, so a few millimeters change in offset only changes trail by a few millimeters, which may or may not change the tendency to shimmy.

WheresWaldo said:
I am sure we do not have all the facts here. You would be hard pressed to find a fork with 48mm-50mm of offset, unless it was custom built..
Forks with 50mm of offset are not uncommon at all - there are probably hundreds of thousands or maybe millions of them on the roads today. Here are a few common ones you can readily buy off the shelf:

Tange steel road fork
Dimension fork
Dimension hybrid fork

While road forks might have an average of about 43mm of offset, the range of available offsets is roughly 30mm - 55mm.
 

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Mark McM said:
I think if you examine the shimmy phenomonon closely, you'll see that trail is inextricably linked to shimmy. As you say, shimmy is a resonance of the frame/fork system. In this resonance, the frame/fork act as a spring, flexing laterally/torsonally back and forth. The trail acts as the moment arm causing the front wheel to steer back and forth as the frame flexes back and forth. Without the trail, there would be no moment arm on which the lateral/torsional "spring" (frame/fork) to act on to produce a steering motion.
Did you really mean to say trail in the above description. I think you meant fork offset acts as the moment arm. And you are basically saying the same thing that Moulton suggest that reducing offset, lengthening trail will eliminate the high speed wobble. I think it might but I also think that people give too much credit to fork offset as the culprit for high speed wobble. Look if you really want a bike to only go straight and stable, then use a fork with negative offset, to picture this look at the front wheels of a shopping cart, these wheels are offset from the steering axis and the offset is behind the axis, unlike a bicycle where the offset is ahead of the steering axis. In the end, there are really not a lot of choices to make a significant difference in the trail when it comes to carbon forks.

Forks with 50mm of offset are not uncommon at all - there are probably hundreds of thousands or maybe millions of them on the roads today. Here are a few common ones you can readily buy off the shelf:

Tange steel road fork
Dimension fork
Dimension hybrid fork

While road forks might have an average of about 43mm of offset, the range of available offsets is roughly 30mm - 55mm.
While these forks may be available for purchase. I would not put any of them on the Merlin. As you have accurately pointed out steel forks come in all sizes of offset. I should have stated plainly that I was only looking at carbon forks. In the case of carbon the choices are much more limited, not because of the material, but because of market demand. Still with all of these comments, how do we help badge118? I still think his solution is to replace the fork with one having the same offset but constructed differently. I do not know what fork Merlin uses on their bicycles, but perhaps switching to a Look, Columbus, Reynolds, Easton or Profile Design fork will allow him to retain all of the good steering geometry characteristics and eliminate the high speed wobble.
 

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Zinn and Moulton could be looking at the exact same bike and both be correct, especially if the replacement forks had the same resonant frequency. It's the system (fork, frame, bars, and rider position) that create a shimmy. If we set up a bike with an adjustable fork offset (say, by hinging the crown in some manner) and it assumed a shimmy for a certain rider, either increasing or decreasing could correct it, by changing the harmonic input.

Trail keeps a bike stable precisely because of lean-steer. As the weight of the rider moves off-axis and leans the bike, the trail forces the wheel to turn in that direction, bringing the contact point back under the dynamic center of gravity and restoring balance. Too much trail, and the bike becomes hard to maneuver. Way too much, and the bike begins to "wobble", as it constantly overcorrects. Too little, and the steering becomes too quick, allowing a "shimmy" to occur as there is too little damping force. Either (or neither) could be the guilty party for a given rider and bike.

I'd be interested to hear what a framebuilder (hell, an engineer/designer of any vehicle, for that matter) would have to say of your assertion that the head tube angle is chosen based on statistically greatest road shocks. I'd love to see any sort of concurrence to that notion. It'd also be interesting to explain the variance in steering angles with that view in mind. To my understanding, head angle is based more on desired input force, or steering sensitivity, in its interplay with rake/offset. For example, a designer looking for a more "stable" bike will slacken the head tube angle and decrease the offset to achieve roughly the same trail for handling, but that will require/tolerate a firmer hand at the bars. Check out a copy of "Bicycling Science" (David G. Wilson and I forget his co-author) for full explanations.

Little known secret of good riders of both bikes and motorcycles: we steer by shifting weight, and maintain balance by "steering", except for at very low speeds. The fastest way to execute a turn is to "bump" the handlebars away from the desired direction, then release the pressure and allow them to turn. This moves the bike out from under the rider's CG, effectively creating a lean in the desired direction. This happens much faster than we can "lean" that way, which typically only moves the upper body one way, and the bike in the opposite direction of that desired for efficient steering. Actually, I lied when I called this a little known secret. This method is automatic for all cyclists. The secret part is that most aren't actively aware of it, and so don't use it to best effect.

FWIW, "fork rake" is perfectly correct. All language is local, and simply because two groups use a particular word differently does not invalidate either one. (Motor)cyclists have certain terminology, (bi)cyclists have another. Talk to an American auto mechanic about a boot, and he'll look for something wrapping the axles/cv joints. Mention the same boot on the continent, and they'll look to the aft luggage compartment. Tell either one of them that they're wrong, and you'll be wearing a wrench in a very uncomfortable location. All anyone will be able to say about the incident is "too bad he was so stupid." :)
 
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