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  1. #1
    Super Biker Mtn Mike's Avatar
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    What causes high speed "shimmy"?

    What are the physics behind high-speed bike wobble, shimmy, or whatever you want to call it?

    We were descending today at ~60 mph and my friends bike started to shimmy uncontrollably when he pulled around my draft. Luckily he regained control and didn't eat pavement.

    I've had the same thing at lower speeds when I'm not firmly planted on the seat. We had some debate on what causes the problem. His bike is in great shape with no loose parts and good wheels and tires. My friend believes the shimmy was due to the wind turbulence behind me. Any answers?

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    Senior Member RacerX's Avatar
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    All bikes can do this. There is another thread on this a while ago.
    Pedalling, leaning your leg on the top tube or unweighting the saddle can help.

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    Senior Member Dchiefransom's Avatar
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    Quote Originally Posted by Mtn Mike
    What are the physics behind high-speed bike wobble, shimmy, or whatever you want to call it?

    We were descending today at ~60 mph and my friends bike started to shimmy uncontrollably when he pulled around my draft. Luckily he regained control and didn't eat pavement.

    I've had the same thing at lower speeds when I'm not firmly planted on the seat. We had some debate on what causes the problem. His bike is in great shape with no loose parts and good wheels and tires. My friend believes the shimmy was due to the wind turbulence behind me. Any answers?

    We had a guy go down from this and spent over 2 weeks in a coma. Someone from the club pulled some engineering stuff off the internet and passed it around. Basically it's an harmonic vibration that gets going for a multitude of reasons. Touching a knee to the top tube can help, as well as VERY lightly applying brakes. A guy on our race team stated that he's had to stop completely to get rid of it. Since every bike is not EXACTLY the same, even the same model bike can do it when others won't.

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    Industry Maven Thylacine's Avatar
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    Yeah, there is an harmonic vibration of sorts. Some things that exacerbate the problem though are -

    1) Poor Alignment : Sometimes a badly aligned frame can cause you to make micro corrections to keep you going straight. This is amplified the faster you go.
    2) Incorrect Trail / Front end geometry : bikes that perform well at slow speed will often not perform well when you're going flat out. This can often be negated by checking what your real head tube angle/fork rake is, and adjusting accordingly. Everyone has a different theory on what the best figures are here.
    3) Wheel Tensioning / Detensioning : Now, this one is just a theory of mine, so feel free to shoot it down. Non paired spoke wheels, especially at low spoke counts, have one spoke detensioning to the left, then one detensioning to the right in a repeated cycle as the wheel goes around. Now, what I think contributes to wheel shimmy is that there will be a point where that non balanced tension/detension is able to resonate at a certain frequency/amplitude that will reveal itself in the bike not being able to self centre itself as it is able to at slower than balistic speeds. I don't know if I've explained that well, but that's my theory *heh*.

    There also might be some aerodynamic phenomena that i don't know about, but I haven't looked into it that far so I really can't comment.
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  5. #5
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    Not all bikes shimmy. The reasons that Thylacine mention are the main reasons. But new ultralight bicycles that are welded also have a tendancy to experience this phenomena more than the older heavier lugged frames or touring bikes (comparing both welded and lugged to only mid and high end bikes). I use to race back in the mid 70's till mid 80's and no one I knew, or heard of, ever experience this phenomena and we all were on lugged steel frames. There could be problems with the newer paired spoke rims or low spoke rims that could also cause a problem as Thylacine also noted, except some LBS mechanics do not think this is a theory but could be a fact. When I was racing most guys raced on 32 spokes, some did 28 and some did 36; I ran 36 all the time I was racing.

    You could also get a imbalance wheel and tire combination but those sort of shimmys are harmonic thus only occur at certain speeds (like only between 20 and 25) but will occur at that certain speed all the time until you correct it. This is the same thing that happens to a car tire.

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    Industry Maven Thylacine's Avatar
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    Theres a reason why bikes of that vintage handle so well, Froze It's called "let's put the proper rake on this fork" as opposed to "Ah, 43mm will do. Consumers won't notice"
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  7. #7
    Super Biker Mtn Mike's Avatar
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    Quote Originally Posted by Thylacine
    Yeah, there is an harmonic vibration of sorts. Some things that exacerbate the problem though are -....

    There also might be some aerodynamic phenomena that i don't know about, but I haven't looked into it that far so I really can't comment.
    Excellent, thanks for the responses.

    Theres a reason why bikes of that vintage handle so well, Froze It's called "let's put the proper rake on this fork" as opposed to "Ah, 43mm will do. Consumers won't notice"
    Do high end, custom frames built today perform better in this regard?

  8. #8
    Senior Member RacerX's Avatar
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    Quote Originally Posted by Thylacine
    Yeah, there is an harmonic vibration of sorts. Some things that exacerbate the problem though are -

    1) Poor Alignment : Sometimes a badly aligned frame can cause you to make micro corrections to keep you going straight. This is amplified the faster you go.
    Wrong.

    Subject: 8.60 Shimmy or Speed Wobble
    From: Jobst Brandt <jbrandt@hpl.hp.com>

    Shimmy is not related to frame alignment or loose bearings as is often
    suggested. Shimmy arises from the dynamics of forward motion and the
    elasticity of the frame, fork, and wheels, and the saddle position.
    Both perfectly aligned bicycles and ones with wheels out of plane to
    one another shimmy nearly equally well. The same is true for bearing
    adjustment. In fact shimmy is more likely with properly adjusted
    bearings than loose ones. The bearing or alignment concept is usually
    offered as a cause of shimmy and each airing perpetuates the idea.

    Shimmy, the lateral oscillation at the head tube, depends primarily on
    the frame and its geometry. The inflation of the tire and the
    gyroscopic effects of the front wheel make it largely speed dependent.
    It cannot be fixed by adjustments because it is inherent to the
    geometry and elasticity of the components. The longer the frame and
    the higher the saddle, the greater the tendency to shimmy, other
    things being equal. Weight distribution also has no effect on shimmy
    although where that weight contacts the frame does.

    In contrast to common knowledge, a well aligned frame shimmies more
    easily than a crooked one because it rides straight and without bias.
    The bias force of a crooked frame impedes shimmy slightly. Because
    many riders never ride no-hands downhill, or at least not in the
    critical speed range, they seldom encounter shimmy. When it occurs
    with the hands on the bars it is unusual and especially disconcerting.
    There is a preferred speed at which shimmy initiates when coasting
    no-hands on a smooth road and it should occur every time when in that
    critical speed range. Although it usually does not initiate at higher
    speed, it can.

    Pedaling or rough road interferes with shimmy on a bicycle that isn't
    highly susceptible. When coasting, laying one leg against the top
    tube is the most common way to inhibit it. Interestingly, compliant
    tread of knobby tires give such high lateral damping that most
    bicycles equipped with knobbies do not shimmy.

    Shimmy is caused by the gyroscopic force of the front wheel that acts
    at 90 degrees to the axis of the steering motion. The wheel steers to
    the left about a vertical axis when it is leaned to the left about a
    horizontal axis. When the wheel leans to the one side, gyroscopic
    force steers it toward that side, however, the steering action
    immediately reverses the lean of the wheel as the tire contact point
    acts on the trail of the fork caster to reverse the steering motion.

    The shimmy oscillates at a rate that the rider's mass on the saddle
    cannot follow, causing the top and down tubes to act as springs that
    store the energy that initiates the return swing. The shimmy will
    stop if the rider unloads the saddle, because the mass of the rider is
    the anchor about which the oscillation operates. Without this anchor
    no energy is stored. The fork and wheels may store some energy,
    although it appears the frame acts as the principal spring.

    Shimmy can also be initiated with the hands firmly on the bars by
    shivering, typically in cold weather. The frequency of human
    shivering is about the same as that of a typical bicycle frame.
    ------------------------------

  9. #9
    Industry Maven Thylacine's Avatar
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    "Shimmy is not related to frame alignment......a well aligned frame shimmies more easy than a crooked one."

    Hrm. Interesting. Okay, how about this. "Research is the quoting of many resourses, plagarism in the quoting of one." I know you love to be devils advocate RacerX, but you've outdone yourself this time. *laugh*

    "Wrong."

    Pluhease

    <edit>
    Actually, the more I read that quote the more riddled with contraditctions it is. Let's cite another example -

    "The longer the frame and the higher the saddle, the greater the tendency to shimmy....Weight distribution also has no effect on shimmy."

    Well, all a longer frame and higher saddle DOES is change your weight distribution.

    I especially love the shivering part at the end - I do believe Mr.Jobst is having a lend of us.
    Last edited by Thylacine; 06-07-04 at 02:17 AM.
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    Senior Member kevmetric's Avatar
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    You need to order yourself a pack of Cone Wrenches, such as
    those from PARK (or at least one...but then, if you buy one, be
    confident you are ordering the proper size wrench), and fix the problem
    yourself in 5 to 10 minutes.

    I know I did, last year.

  11. #11
    Senior Member RacerX's Avatar
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    Quote Originally Posted by Thylacine

    I especially love the shivering part at the end - I do believe Mr.Jobst is having a lend of us.
    Tom Kellogg says about the same thing about the rider but YOUR opinion is right and everyone else is laughable.



    Back to the REAL topic, shimmy happens and you have to experiment to find out how to correct it or accept it and control it.

  12. #12
    Industry Maven Thylacine's Avatar
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    I was laughing at Jobst's contradictions specifically, I don't recall laughing at 'everyone else'. And anyway, "Shimmy caused by shivering" isn't funny? What's next? "Dressing to the left helps balance the left/right weight balance inequity due to the drivetrain being on the right?"

    But yeah, you're right - many factors do come into account and "you have to experiment to find out how to correct it or accept it and control it." - RacerX
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  13. #13
    Tiocfáidh ár Lá jfmckenna's Avatar
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    Quote Originally Posted by Mtn Mike
    What are the physics behind high-speed bike wobble, shimmy, or whatever you want to call it?

    We were descending today at ~60 mph and my friends bike started to shimmy uncontrollably when he pulled around my draft. Luckily he regained control and didn't eat pavement.

    I've had the same thing at lower speeds when I'm not firmly planted on the seat. We had some debate on what causes the problem. His bike is in great shape with no loose parts and good wheels and tires. My friend believes the shimmy was due to the wind turbulence behind me. Any answers?
    60MPH :O Thats cookin. I shimmied on my old steel touring bike at 51MPH and it was quite a horifing experience. I read later that standing up on the pedals will make it go away or clenching the TT w/ your knees. My bike was a large 62cm steel lugged frameset and intuitively I think that was the problem. I cannot explain the physics but it seems that a larger frame will have more spring in it and it's the springyness that causes the shimmy. I also had a rack on the back which when I took of it did not shimy anymore, mind you I was also scared so I never went 51 again but came close w/ no problems. I now have a smaller AL/Carbon frame and at 52mph it is solid as a rock.

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    Maybe you're all drunk and it's the spins!

    Just Kidding, of course. I, too, have experienced this- although NOT at 60 MPH.

    I attributed it to the old bike I was riding and the fact it might not be completely straight...I bought it used so I figure it might be a tad out of whack.

    This thread makes my observation obsolete. Thanks for enlightening me...and giving me some ideas how to "fix" it next time it happens.

    PJ

  15. #15
    Super Biker Mtn Mike's Avatar
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    Quote Originally Posted by pjbaz
    Maybe you're all drunk and it's the spins!

    PJ
    heres to that.

    Some have commented that shimmy can be stopped by STANDING off the saddle...this is interesting because I have experienced shimmy at lower speeds (~35mph) that occured while I was out of the saddle decending. The shimmy was rectified by my SITTING down in the saddle. Go figure. I guess it just shows that it's a complicated phenomina.

    Also, regarding shivering causing it, I experienced this several weeks ago, but I didn't attribute it to the same physics; I mean, duh, if the RIDER is shaking, of course the bike will shake. In any case, neither episode of mine was as crazy as my friends 60mph shimmy. What an eye opener for him

  16. #16
    Chairman of the Bored catatonic's Avatar
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    I've had shimmy on an mtb with knobbies going 40 down a paved hill.

    My theory is its the tire and the pavement not meshing together well. maybe the pavement is too rough for the tire at that given speed, or vice versa, this might cause an intermittant grip level issue, causing minor twitches in the front end of the bike...this possibly causes a reduction of grip in the back from the back end trying to trail the front, which is swaying tot he left and right rapdily in very short distances....causing back end float as well. thus, shimmy.

    I know in sports cars once the front end starts loosing grip and getting sloppy, the back often follows suit, so this might also apply to cycling.

  17. #17
    Senior Member JavaMan's Avatar
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    It is tempting to attribute shimmy to bicycle frame geometry or material. However, bicycles don't have a monopoly on shimmy or "speed wobble". I rode a motorcycle for 10 years and can tell you motorcycles do it, too. It happened to me in a turn at 75 mph once. The next time you see motorcycles racing on a track, look closely as they go into a tight turn. Look at the front tires as they brake hard and lean into the turn - you can see the wobble. On a motorcycle, you can accelerate out of it, but is this an option when descending on a bicycle?

    On a bicycle, you can often stop the wobble by grasping the top tube tightly with your knees. My bike has a shimmy at about 25 mph when descending with no hands. If I touch the top tube with one leg it goes away. I was descending at 45+ mph 2 weeks ago (holding on with both hands!) and had a very slight shimmy, even though my knees were on the top tube. When I let off the brakes, it went away. When I re-applied the brakes, it came back. I was too chicken to go faster than that, so I endured the wobble.

    Tom

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    Quote Originally Posted by Thylacine
    Yeah, there is an harmonic vibration of sorts. Some things that exacerbate the problem though are -

    1) Poor Alignment : Sometimes a badly aligned frame can cause you to make micro corrections to keep you going straight. This is amplified the faster you go.
    2) Incorrect Trail / Front end geometry : bikes that perform well at slow speed will often not perform well when you're going flat out. This can often be negated by checking what your real head tube angle/fork rake is, and adjusting accordingly. Everyone has a different theory on what the best figures are here.
    3) Wheel Tensioning / Detensioning : Now, this one is just a theory of mine, so feel free to shoot it down. Non paired spoke wheels, especially at low spoke counts, have one spoke detensioning to the left, then one detensioning to the right in a repeated cycle as the wheel goes around. Now, what I think contributes to wheel shimmy is that there will be a point where that non balanced tension/detension is able to resonate at a certain frequency/amplitude that will reveal itself in the bike not being able to self centre itself as it is able to at slower than balistic speeds. I don't know if I've explained that well, but that's my theory *heh*.

    There also might be some aerodynamic phenomena that i don't know about, but I haven't looked into it that far so I really can't comment.
    Good post. Like to read about good info like this.

  19. #19
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    All objects have a natural resonance frequency...

  20. #20
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    Shimmy is usually caused by not having enough trail. To explain trail for those who don’t know: If you draw a line through the center of your head tube and therefore the steering column, that straight line will reach the ground at a point (Point A.) ahead of the point where the wheel contacts the ground (Point B.)

    I always built my bikes with at least 2 ½ inches of trail. Trail is common to all wheeled vehicles, cars and even a shopping cart will have it. If you make the head angle steeper it means less trail because you move point A closer to point B. Also if you increase the fork rake you make for less trail; in this case point B moves closer to point A. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero.

    Trail keeps the bike going in a straight line, and also assists a two wheeled vehicle in its self steering abilities. As you lean to the left, point A moves to the left and the wheel Pivoting on point B will turn to the left. The gyroscopic action of the spinning wheel also plays a big role in self steering, but this is another subject and I only mention it because the heavier the spinning wheel, the more it keeps straight. Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail.

    What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.

    Large frames are more prone to shimmy for two reasons. Large frames are taller and also should be proportionately longer, but there is a school of thought that believes a race bike should have a short wheelbase, so the builder makes the head angle steeper to shorten the wheelbase, but in doing so lessens the amount of trail. Large frames also tend to have shallower seat angles to accommodate the rider’s longer legs therefore the riders weight is more over the rear wheel.

    Any vehicle that has its weight towards the rear is less stable; ask anyone who has driven an old VW bus in a cross wind. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. Finally if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first very gently and only apply the front brake after you have come out of the shimmy .
    History, photos and tech articles on my website. Also check "Dave's Bike Blog."

  21. #21
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    Quote Originally Posted by Dave Moulton
    Shimmy is usually caused by not having enough trail. To explain trail for those who don’t know: If you draw a line through the center of your head tube and therefore the steering column, that straight line will reach the ground at a point (Point A.) ahead of the point where the wheel contacts the ground (Point B.)

    I always built my bikes with at least 2 ½ inches of trail. Trail is common to all wheeled vehicles, cars and even a shopping cart will have it. If you make the head angle steeper it means less trail because you move point A closer to point B. Also if you increase the fork rake you make for less trail; in this case point B moves closer to point A. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero.

    Trail keeps the bike going in a straight line, and also assists a two wheeled vehicle in its self steering abilities. As you lean to the left, point A moves to the left and the wheel Pivoting on point B will turn to the left. The gyroscopic action of the spinning wheel also plays a big role in self steering, but this is another subject and I only mention it because the heavier the spinning wheel, the more it keeps straight. Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail.

    What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.

    Large frames are more prone to shimmy for two reasons. Large frames are taller and also should be proportionately longer, but there is a school of thought that believes a race bike should have a short wheelbase, so the builder makes the head angle steeper to shorten the wheelbase, but in doing so lessens the amount of trail. Large frames also tend to have shallower seat angles to accommodate the rider’s longer legs therefore the riders weight is more over the rear wheel.

    Any vehicle that has its weight towards the rear is less stable; ask anyone who has driven an old VW bus in a cross wind. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. Finally if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first very gently and only apply the front brake after you have come out of the shimmy .
    Top Notch post.

  22. #22
    Industry Maven Thylacine's Avatar
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    And here's me thinking this was a new thread! Imagine my shock when I read something I wrote back in June and I still actually believe it!

    Anyway, I ride a sloping top tubed 62cm equivalent steel road bike, and I've never experienced 'shimmy' even at 55mph. In fact, my bike feels like it's standing still at high speed - it's very stable and intuitive. I attribute this largely to the geometry of the whole package - 48mm raked fork with a 73deg head angle ( 51.9 trail ), and a 71.5deg seat angle, 425mm stays and a long wheelbase.

    This current obsession with short and steep bikes sure doesn't help - especially when most manufacturers spec a 45mm raked fork with a myriad of head angles throughout a model range....some bikes will handle okay while others will feel like *****. Welcome to the world of 'Mass production' I guess.
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    Quote Originally Posted by Thylacine
    And here's me thinking this was a new thread! Imagine my shock when I read something I wrote back in June and I still actually believe it!

    Anyway, I ride a sloping top tubed 62cm equivalent steel road bike, and I've never experienced 'shimmy' even at 55mph. In fact, my bike feels like it's standing still at high speed - it's very stable and intuitive. I attribute this largely to the geometry of the whole package - 48mm raked fork with a 73deg head angle ( 51.9 trail ), and a 71.5deg seat angle, 425mm stays and a long wheelbase.

    This current obsession with short and steep bikes sure doesn't help - especially when most manufacturers spec a 45mm raked fork with a myriad of head angles throughout a model range....some bikes will handle okay while others will feel like *****. Welcome to the world of 'Mass production' I guess.
    What frame is that?Sounds pretty nice.

  24. #24
    ex frame builder
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    Quote Originally Posted by Thylacine
    Anyway, I ride a sloping top tubed 62cm equivalent steel road bike, and I've never experienced 'shimmy' even at 55mph. In fact, my bike feels like it's standing still at high speed - it's very stable and intuitive. I attribute this largely to the geometry of the whole package - 48mm raked fork with a 73deg head angle ( 51.9 trail ), and a 71.5deg seat angle, 425mm stays and a long wheelbase.

    This current obsession with short and steep bikes sure doesn't help - especially when most manufacturers spec a 45mm raked fork with a myriad of head angles throughout a model range....some bikes will handle okay while others will feel like *****. Welcome to the world of 'Mass production' I guess.
    You are right when you say it is the whole package; also has a lot to do with weight distribution and that can be affected by the individual rider’s build. A rider with a lot of muscle in his arms, shoulders, and chest will have more weight forward whereas a rider with a slim upper body but a muscular butt and legs will have their weight further back.

    A good production bike is one that will handle right whoever rides it. I am amazed when you speak of frames with a myriad of head angles; is this really the case? The head angle and the correct fork rake for that angle is critical to the way that bike handles. It has been established for almost a hundred years now that the ideal head angle for a road bike is 73 degrees. You can vary one degree from this; 74 degrees with less fork rake for a criterium bike; 72 degrees with more fork rake for a touring bike or a very small frame where this is the only way to make the front end measurement long enough.

    Track bikes are a whole different animal. On a road bike you lean into a corner and the bike steers itself around that corner. On a banked track you do not lean the bike if you want to change direction you have to steer it; so steeper angles up to 76 degrees are called for.

    Fashion should only dictate how a bike looks; the current trend for sloping top tubes and straight forks is okay; it shouldn’t affect the way the bike handles if the rest of the geometry is right. In the mid 1970s the fashion was 76 degree head angles. I refused to go with this trend and stuck with what I knew worked; 73 degrees with 1 3/8 in. (35mm.) fork rake. I had great success because my bikes handled better than anything else on the market. Also all my customers were people who raced and cared more about how the bike rode and handled than fashion.

    By the time I came to the US in 1979, the fashion had gone back to 73 degree head angles (As it always has, and always will do.) This was a good thing because it seems here in the US fashion does matter to a lot of people.
    History, photos and tech articles on my website. Also check "Dave's Bike Blog."

  25. #25
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    This brings up a question: who designs most of the big-name bikes nowdays, like Trek and Cannondale? Are they engineers that have a degree and no cycling experience, or are they cyclists or former frame-makers with a knowledge of the past history of the bicycle and perhaps no degree?
    I know back in the mid-80's Specialized wanted Dave Tesch to make and design frames for them, but he didn't. They eventually bought out the name.
    You say the hill's too steep to climb...you say you'd like to see me try...-Pink Floyd

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