Written by: Dan Empfield
Date: Tue Aug 06 2002
I ride a 59cm bike or, if it's a road race bike (as opposed to a tri bike) I'll ride a 60cm bike. If it's a Litespeed tri bike, I'll ride 57cm, and so it goes. How are these bikes measured, and why do I ride different sizes depending on the manufacturer?
Bikes are sized along the seat tube, and that's "D" in the diagram below. Nowadays most bikes are sized "center to center" and most dimensions are figured that way. What this means is, when you look at these lines in the diagram, they all intersect, and they do so "on center." That is, they intersect as if the lines had no thickness. But bike tubes do have thickness, and the difficulty in measuring "center to top"—as was the case in the old days—is that bikes nowadays are made of all kinds of materials and with tubes that come in a wide variety of diameters. A Cannondale, for example, has a much thicker top tube than an older style steel bike, and so a C'dale and a steel De Rosa might have identical seat tubes center to center, but might vary by a centimeter or more in their center to top measurements.
Tri bikes often have dropped top tubes to accommodate shorter head tubes, and when they measure, say, 59cm, they may actually only be 50cm or 52cm center to center (i.e., when the seat tube meets the top tube). In order to make the bike conform to nomenclature with which the buyer is familiar, the bike maker might measure what is sarcastically called "center to air", that is, from the center of the bottom bracket to where the top tube would be if the bike maker hadn't dropped it. If bike makers didn't resort to this semi-foolery, consumers who were used to riding 60cm road bikes (like me) would balk at riding a 52cm tri bike. So bike makers call it a 60cm tri bike, and they just build it to 52cm center to center, and what the consumer doesn't know (or realize) won't hurt him.
The reason I'd ride a 59cm bike in, say, a Yaqui, but 57cm in a Litespeed is that the Yaqui drops its top tube more than a Litespeed, and it has a shorter top tube as well. Therefore, a Litespeed in a 57cm size has the same top tube height and length, and the same head tube length, as the larger Yaqui. In other words, these are the same bikes, geometrically. All you should care about is how the thing fits, not what size it's called. A Kestrel KM40 Airfoil in the large mold size, for example, is considered a 56.5cm bike if you ask Kestrel what numerical size it is. But it's almost precisely the same as a 57cm Litespeed, a 58cm Cervelo, or a 59cm Yaqui or QR, all of which would fit me at 6'2" tall. Anyway, that's how your bike gets its size designation.
You know what the seat tube is, I assume (D), and then there are the other tubes which have lengths that are critical to the geometry of the bike: the chainstay (E); the head tube (G); the top tube (A). The ONLY dimension which is NOT calculated center to center is the length of the head tube (G), which is measured from its top to its bottom.
Line "B" is the "front/center" and is the distance from the BB to the front axle. Line "E" is the chainstay, as is mentioned above, and measures the length of the back half of the bike. Distances B and E will be slightly greater than C if you add them together, because C follows a straight line whereas E and B pass through the bottom bracket.
The part of the bike behind the seat tube is called the rear triangle, and the part of the "mainframe" (the frame without the fork) in front of the seat tube is the front triangle or main triangle, though it is obviously not a triangle, but a rhomboid.
WHAT DIMENSIONS AFFECT HANDLING
The chainstay length is integral to how the bike will handle, and so is the front/center. Generally, it's nice to have your chainstay as long as possible if you want the bike to track nicely through corners. Tri bikes, though, tend to have shorter chainstays. Because these bikes have steeper seat angles (H above); and because you lay a lot of your weight on the front of your bike when you're laid out on your aero bars; and because a tri bike's top tube is short, you'll have an awful lot of weight on the front wheel. A shorter chainstay is going to help even out the rider's weight displacement. Also to be considered is the aerodynamic effects of the chainstay length. If you've got a faired rear wheel as is the case with, say, Cervelo's bikes, you've got to keep the chainstay short in order to suck the rear wheel up behind the seat post.
The bottom bracket drop also affects the bikes handling, in that the lower the drop the better the bike will handle in turns. The downside to this is that you can't pedal through turns without clipping the pedal on the pavement. This isn't going to affect triathletes, who generally aren't going to be pedaling through turns. It is therefore beneficial to have your bottom bracket be as low to the ground as is reasonable, as long as you aren't the type who'll forget what kind of bike you're riding as you enter turns.
The astute among you will notice that bottom bracket drop is less in a 650c bike than on a 700c bike. If a bike made with the larger wheels has a BB drop of 7cm, the bike with the smaller wheels might have a drop of, say, 4.5cm. This is because the small-wheeled bike must lessen the drop to offset the fact that the bike's axles sit lower to the ground. In either case the bottom bracket height (off the ground) will remain the same. In my view, the fact that the BB drop is less on the 650c bike does not make it a poorer handling bike. The only things that matter are where the bike's wheels contact the ground and where the rider's center of gravity is in relation to those points.
I'll be writing about the front of the bike in future series segments, in which I'll discuss rake and trail and head angle and how all that affects steering and handling. The only front-end element on which I'll touch now is the front/center dimension. This tends to be short in tri bikes, because the top tube is short and that, by definition, shortens the front/center. As I wrote above, this is a little bit of a problem for tri bikes, because a rider's weight is already sitting over the front of the bike. Best to have a longer front/center, yet without lengthening the top tube. The way you do that is by shallowing out the head angle (J in the diagram above). To keep the steering unaffected you'll compensate by adding rake to the fork (we'll discuss all that later) and this also adds length to the front/center.
Why aren't tri bike front/centers longer? Why don't tri bike makers put shallower head tubes (and forks with correspondingly greater rakes) on their bikes? They probably should, and there are two reasons why many of them don't. One is that there aren't hardly any aero forks currently made with rakes greater than 43mm in 700c and 40mm in 650c. The bigger reason, though, surrounds history and rules and inertia, and the pitiable part of this story is that the rules which prohibited longer front/centers don't apply any longer (but many bike makers haven't figured that out yet, or haven't thought much about it, and inertia causes them to make bikes the way they've been making them).
What I mean is this: When we built the first tri-specific bikes back in the late 80s, there was a maximum allowable front/center dimension of 65cm. That really put a crimp in our ability to make bikes in the larger sizes with the head angles we wanted, so we had to keep the head angles relatively steep (up to 74 degrees). At the same time, we were figuring out that our top tubes needed to be shorter, and they got progressively shorter every year. This substantially shortened up our front/center, and on top of that those ancient front/center rules ceased to exist and we could shallow our head angles even more. But I don't know whether our sport has shallowed them enough to compensate for the percentage of a rider's weight the front end of a tri bike bears. Tri bike head angles of 70 or 71 degrees, with forks that have rakes of 45mm and 48mm, would give us long front/centers, but might return us to the days when we had the right amount of weight on the rear wheel, and a wheelbase that makes more sense.
WHAT DIMENSIONS AFFECT FIT
I either wrote, or dreamed I wrote, an article on this site in which I said that the only tube lengths that really matter when determining fit are the length of the head tube, and the theoretical length of the top tube (I believe the article is here somewhere, I just can't locate it at the moment).
By "theoretical" top tube what I mean is this: In the "old days" bikes were made with tubes that were all straight and level, except for the old "funny bikes" which I won't discuss right now. Nowadays we have Softrides, and "compact geometry", and a variety of designs which make it difficult to measure the "effective" top tube length, if what you want to do is simply figure out how long long your bike should be. With these new-fangled bikes there is no tube across which you can run a tape measure and figure this out. You must pretend for a moment that your bike is built like an old-style bike, and run a straight-edge from the BB to the saddle, imitating a "seat tube." The horizontal distance from this line to the center of the head tube is the "old style" top tube. This horizontal line is the "effective" top tube length.
These two dimensions—the length of the effective top tube and the length of the head tube—are the only dimensions you need when determining your proper anatomical bike size. I'm not saying that these are the only important dimensions. Other dimensions factor when considering how the bike will handle, as I wrote in an earlier section. Plus, other dimensions are important when considering how your body will perform biomechanically (how much power you'll be able to economically generate). But simply from an anatomical perspective, these are the only two measures that are required of the frame (there are other measures which are anatomically important, such as the stem and clip-on extension lengths, but these aren't part of the frame).
For this reason, it's easy for me to determine precisely which frame in a manufacturer's line-up will fit me. If I'm to choose a tri bike with 700c wheels, the effective top tube length will need to be 55.5cm to 56.5cm, and the head tube length must be between 12cm and 13cm (keep in mind that this is for a tri bike, with tri-specific handlebars; for a road race bike with road race handlebars I'll be riding a frame with 59cm of top tube and 17cm or 18cm of head tube). By way of explanation, the reason I say "for a 700c bike" is that if the bike has 650c wheels the head tube must be longer in order to achieve the same handlebar height relative to the ground.
This tri bike—56cm by 12.5cm—will fit me perfectly, and it really doesn't matter to me whether the manufacturer calls its size 55cm or a 62cm. I'm not hung up about that. With those two dimensions I know all I need to know, from an anatomical perspective. Of course there are also biomechanical issues, and the seat angle figures into this. If I'm not riding my tri bike in a steep configuration, I can't generate power. I must ride at 79 or 80 degrees or I'm not happy. Therefore, if a manufacturer sends me a bike to test ride that has, say, a 76-degree seat tube angle, then yes, I will want this bike to be longer in the top tube, because I'm going to have to mess around with different seat posts in order to get me back up to 79 or so degrees. Moving the saddle forward is going to shorten the effective top tube length, since I'm "artificially" steepening the seat tube—that is, I'm artificially retrofitting the bike with a seat tube angle the bike maker should've used, had he made the bike correctly for me. So in the case of a 76-degree seat angle tri bike, I'd probably want the top tube to be 57.5cm or 58cm, so that when I steepen the bike my effective top tube length ratchets back down to 55.5cm.
At this point I'll semi-retract something I wrote in a segment above regarding center-to-center versus center-to-top measuring, and I'll start with a quote in a correspondence to me from Cervelo's president Gerard Vroomen:
"As for c-c vs. c-t, I think the different sizes of toptube actually makes it more relevant to measure c-t, since that is a measurement that actually matters (standover, headtube length, etc. are influenced more accurately by the c-t than the c-c. Our Eyre Road and Prodigy have the same standover, because they are measured c-t... I think the majority of bikes today are measured c-t, not c-c, I think the trend is opposite of what you suggest."
I hearken back to what I wrote earlier, which is that head tube lengths must be considered only in their entire lengths, not center to center. I wrote this because the head tube length—more precisely the top of the head tube—determines how high or low you'll be able to put your handlebars (we've all seen, or personally experienced, the phenomenon of the stem with zillions of spacers under it, or the search for the track stem or similar custom stem, with a severe drop, in order to accommodate a bike with a head tube obviously too short or too tall). It does make sense, then, that if one only need consider the overall length of the head tube, and the effective top tube length, yes, it's the center-to-top seat tube length that becomes the relevant measure. While this way of thinking carries no relevance for me, a bike designer (well, former bike designer), it certainly does make sense for a bike rider.
One more thing on this subject, in order to head off the emails from the smarty pantses who read this site. Yes, there technically is a third measure that matters when considering anatomical fit. Up a ways above I wrote about BB drop—the amount the bottom bracket drops below the "wheelbase line" (BB drop is measure "F" in the diagram above). Theoretically, the height of the handlebar is going to be determined not by how high the head tube top is from the ground, but how high it is above the bottom bracket. Variance in BB drop is small, however—there's not much wiggle room in where the framebuilder can place the BB—so when I'm considering how the bike will fit I don't generally consider the BB drop (unless I'm having a frame custom made for me), since I'm assuming the tri-bike designer has placed the BB relatively low to the ground.
Then there are the biomechanical issues to be considered, and one stands out—the seat tube angle. I'm not going to spend much time on this, because we spend so much time on it in our article on tri bike fit, and most of what I want to write about in this series—the real reason I decided to tackle this topic—is to discuss the handling aspects of tri bike geometry. I'm interested in this because we've matured enough as bike designers to know—or we ought to know—how to make a tri bike that fits anatomically and works biomechanically. But we're still struggling with how these bikes ought to handle—and when they handle badly, why that is.
I will mention a couple of things, though, which might not be contained in the tri bike fit article. I was deviated from my "path" quite a few years ago, and bent to market pressures when, as a bike designer, I relaxed the seat angles in bikes I was designing from 80 degrees to 78, and even to 76.5 degrees in the instance of one or two models. After leaving the bike manufacturing biz and spending a lot more time riding bikes as opposed to designing them (it seems in our industry we tend to do one or the other), it became more and more apparent to me that my original ideas about seat angles were correct—at least for me. They were also demonstrated to be correct in scientific studies. Therefore, I shall no longer be shaken in my determination by the slings and arrows of my detractors. I shall stand my ground. And I won't back down. "Hey, baby, there aint no easy way out." But I digress, and besides, I can only excerpt Tom Petty—not print the whole lyric—for fear of getting sued.
I got an email just this morning asking me whether seat angles ought to get shallower as bike sizes get larger. This is a long-time trend in road bike design. It's customary for a road bike model to have, say, 74 degrees of seat angle in a 50cm size, yet 72.5 degrees at 62cm. I don't think a rider's height has any bearing on seat angle. I think morphology and physiology do. I believe that the more you are an "endurance" person—the less capable you are of ballistic muscle contractions, and the better you are at just going and going—the more you'll want to spin a higher cadence and the steeper your seat angle should be in order to more easily accommodate that. I can't sprint on a bike to save my life, but I can pedal at a high rate for a long time, that is, I can generate quite a few watts over the period of 10 or 20 minutes, but I can't raise that wattage very much when asked to do so for 10 seconds. I therefore must spin, and must get myself over the cranks to do so effectively. I therefore could very easily ride a 74- or even 75-degree road race bike, even though I'm 6'2" tall.
Carrying this theme forward, I suppose one could make the argument that since I like riding a steep road bike angle, that's why I like a steep tri bike, and there should be an accommodation in tri bike geometry for those who would ride a shallower road bike. In other words, I tend to add 5 or 6 degrees to my road bike seat angle to achieve my proper tri bike seat angle, therefore, a person who road races at 72 degrees might therefore ride a tri bike with 77 or 78 degrees of seat angle. Perhaps so. I suspect, though, that our sport tends to have a lot more "runner" types like me in it, and also our training tends to make us into "runner" types, even if we didn't start life genetically predisposed that way. Furthermore, our riding style—tactically, as triathletes—is to ride "quietly." Since we have to run off the bike, we must conserve, while riding fast. The bike segment of a triathlon is not unlike a contest to see who can whisper the loudest. Such a riding style requires that we achieve our power output without great neuromuscular cost (that article is coming). This means that even if we were to ride a straight time trial most effectively at 77 degrees of seat angle, we might still be better off riding a tri bke at 79 degrees.
In summing up, just realize this: The bike—any bike—has a plethora of measurements associated with it, but if you can break them down into those that matter anatomically, those that matter biomechanically, and those that control how the bike will handle, then you'll be able to get your arms around the concepts of frame design and decide easily when a frame will fit you properly.
STEERING AND FRONT-END GEOMETRY
Bikes have “trail” and without it you’d be falling off them often. "Trail" causes the wheel of a bike to self-center, in the manner of a shopping cart's caster wheel. It's trail that keeps the bike upright.
There is a fancy formula for generating trail, but it's easier to understand if you look at a diagram. In order to have "trail" the front wheel’s contact patch—the place where the rubber literally meets the road—must fall behind the point where the bike's steering axis would (theoretically) meet the road. The further the contact patch falls behind the steering axis, the more trail there is and the more likely the bike will want to go in a straight line. The degree of the bike's determination to steer in a straight line equates to it's "sluggishness" as well as it's "stability" and "predictability," all words which represent different sides of the same coin.
Keep in mind that the effect of adding rake to a fork is counter-intuitive: adding rake decreases trail and makes the bike handle faster, livelier, and perhaps scarier. You've got to shallow the head angle to counteract the effect of increasing a fork's rake.
There are many places on the web where one can read the explanation above, but there is more to the story if you’re riding a tri bike. First, there is the analysis of how a bike is steered, and a subset of that, which is how a tri bike is steered. You don’t steer a bike—any bike—by turning in the direction you want to go. You first countersteer, that is, you turn the wheel in the opposite direction. You may not realize this until you’re next to a curb, or next to another rider, from which you must veer away. You then realize you have no room to maneuver, and that’s because the first maneuver is to veer away from the desired direction in order to get your center of gravity between the bike and the direction you want to go. Then you can lean into the turn.
The reason this is important to know is that a tri bike can be a big ship to turn (when you're in the aero position) if it’s a slow handler. It takes longer to countersteer a tri bike when your elbows are in the cups, and that’s why you can’t have a bike under you that handles sluggishly. Trail is good, but too much trail means the bike won’t want to turn at all. I once rode a tri bike that had a fork with no rake, and the bike had a ton of trail. While riding it down a straight road in the aero position I tried to go in a straight line but ended up making side-to-side "waves." My goal was to keep their amplitude as small as possible. Not a good way to ride.
Yet tri bikes are scary when the handling is too skittish, mostly because a triathlete’s weight is positioned over the front of the bike when he’s in the aero position. Even when he’s not laid out on the tri bars the weight is still forward by virtue of a tri bike’s short top tube and steep seat angle. The trick is to both shallow the head angle and increase the rake, in which case the trail will remain roughly the same, but the bike’s front/center (and therefore its wheelbase) will increase dramatically. This will help in getting the weight displaced over the bike more evenly.
You might wonder, then, why tri bikes have head angles and fork rakes roughly similar to road race bikes. That’s just because tri bike makers haven’t (in my view) figured out yet that they have the ability to make a much better handling bike by abandoning traditional modes of front-end bike design. In their defense, it is also the case that aero fork makers haven’t yet made forks with 48mm and 50mm of fork rake. In the fork makers' defense, however, they haven’t yet been asked by the bike makers to make aero forks with this much rake.
Just the same, expect to eventually see tri bikes with 70 and 71 degree head angles, paired with forks that have upwards of 50mm of rake. These bikes would handle well, and be much better on turns and descents. In fact, they’d probably have about the same amount of wheelbase as standard road race bikes, and ought to handle fairly similarly. They'd also offer a plusher ride.
I’ve been messing around with tri bikes that have forks with a lot of rake, and I find that while they’re a little twitchier than forks with less rake (all other things equal) they handle surprisingly well on technical descents. This is for two reasons, I think. First, arc'ing turns with a longer wheelbase feels better. Second, when I’m descending a curvy road, my hands are on the ends of the pursuit bars, and my “tiller” is longer than if I was riding with my weight on the armrests. In other words, every twitch with my elbows, when they're in the armrests, affects the steering quite a bit, because my axis of movement is close to the bike's steering axis. When my hands are on the pursuit bars, the same amount of left or right movement only effects the steering half as much (or even less than that) because the point where I initiate the movement is so far from the bike's turning axis. The point is, at higher speeds and on curvy roads, so long as I’m out of the aero position it seems that the longer wheelbase more than compensates for the smaller amount of trail. (But that’s just my personal observation, and should be taken as only that).
There might, however, be something to the issue of weight displacement and fork rake. One of America's best known custom bike makers, Roland Della Santa of Reno, Nevada (maker of most of Greg LeMond's bikes throughout much of his racing career), said this to me: "When I make a touring bike that'll have panniers on the front, I make a shallow front-end, like 70 or 71 degrees, but I put even more rake than normal. It seems like the weight of the panniers helps the bike track in a straight line."
What can the triathlete take away from all this? First, that a tri bike ought not to simply point itself in a straight line, it needs to have a little sprightliness. Second, that by virtue of its short chainstay, short top tube, steep seat angle, and weight bearing aero bars—all of which force the rider's weight forward and over the front wheel—bike makers ought to compensate by lengthening their front/center dimensions, with fork-makers aiding the process by making aero forks to match. We haven't yet seen that to that to any great degree.
THE GEOMETRY OF 650C VS 700C
The president of Cervelo Cycles, Gerard Vroomen, reads Slowtwitch—or at least those parts of Slowtwitch that concern matters of fact—and it's good for us that he does. He's caught me in a couple of errors of fact, and the beauty of writing a series (as I'm now doing) is that I have the luxury of righting a wrong written above. Unless I go back and change it (which I haven't yet done as of this writing) I write that, "...a 650c-wheel bike ought to have a little more rake or a slightly shallower head angle to account for the difference in wheel radius." Actually, it ought to have a little less rake to make up for the difference in wheel radius.
Gerard also writes to me that, while I'm complaining that tri bike wheelbases needing to be longer so that they descend and corner more like road race bikes, it ought to be noted that tri bikes have wheelbases about the same as their similarly sized road race counterparts. He's right. That's because of their steeper seat angles. Yes, tri bikes have shorter chainstays (generally) and tri bikes have short top tubes. But their steeper seat angles push those top tubes forward, creating larger front/center dimensions. Indeed, I remember back when there were rules that mandated that front/centers could be no longer than 65cm (both USCF and USAT had such rules). This really put a crimp in our style as bike designers, and at 57cm and up we had to keep steepening our head tubes and lessening our rakes so that our bikes would be legal for racing (thank goodness that rule no longer exists).
In a funny aside to that, back about ten or twelve years ago—when Kestrel came out with its original KM40 Airfoil—Kestrel's bike was geometrically based (roughly) on our 57cm Superform (which they'd purchased from us for the purposes of evaluation). Our bike had a 64.9cm front/center, and the KM40 had 65.2cm (their bike was probably exactly the same as ours in front/center, but their EMS fork had a little extra rake). At that point, their bike was illegal for racing, and it was a bit late for them to change the mold. Fortunately, that silly rule was abolished and the KM40 became legal, and remains one of triathlon's most popular models. I still remember, though, the look on the face of their (then) sales manager Rick Vosper (a very good guy) when I informed him at Interbike—as the KM40 was being introduced to the world— of the dimensional issue. But I digress.
Though Gerard is right—a road bike that would fit me (6'2") would have roughly 100cm of wheelbase, and tri bikes as they're currently built would probably have 100cm to 101cm in my size—I'd still like to see a longer wheelbase in my tri bike. I'm currently testing a bike with 103cm of wheelbase, and 105cm might be even better for me.
As for wheelsize, there's not much that separates 650c and 700c if you build the bikes to accommodate the difference. As previously stated, trail needs to be accounted for by giving a 650c-wheeled bike a shallower head angle or a fork with a little less rake.
There are two differences I perceive between riding two otherwise identical bikes—identical except for their wheel size difference—and it might very well just be my imagination in both cases. It seems to me, though, that when you climb with a 650c-wheeled bike every pedal thrust gets you up the hill a slight bit easier (since the wheel is lighter) but inbetween the pedal strokes the bike correspondingly slows down a bit more than its larger wheeled cousin (the downside of the wheel carrying less inertia). This seems especially true when out of the saddle, and this makes a 650c-wheel bike feel a little herky-jerky.
The second difference is that it seems like a 650c-wheeled bike is a slight bit more prone to oversteering; that is, when the corner is tight and the turn is tight, the arc the wheel makes is slightly more pronounced. It's not problematic, it's just slightly different. It has not been my experience that a 650c-wheel bike is less able to execute a tight or a fast turn, and in fact may be better able to do so. The only way I can imagine that might be tested is to test bikes of both wheel sizes to "failure" while executing a turn, and my enthusiasm for science doesn't extend quite that far.
When a customer brings his bike into his LBS and complains of speed wobble (death wobble, front-end shimmy) often the typical bike shop mechanic immediately starts farting around with the headset adjustment. There is no adjustment problem, nor are the wheel bearings loose, and in fact it is most likely that nothing is wrong at all.
Speed wobble is just an inherent "flaw" of bicycles. Bicycle frames are amazingly stiff and strong in a vertical plane, but horizontally they are not. They are like ladders, and when you stand on a ladder it is strong, but when you and a friend stand a distance apart, each of you grasping one end of the ladder, and you twist, the weakness of the ladder is apparent.
In that same way, when a bicycle is moving in a line straight ahead, and gyroscopic effects of the wheels weigh on a unit as elastic as a bicycle frame—with its fork and wheels—you might get shimmy.
Shimmy occurs at a certain speed, depending on the bike and the rider. I have heard it said that every bicycle is prone, depending on its geometry, materials, and rider position, a particular resonant frequency, at which the bicycle oscillates. All that occurs when a given speed is reached on a descent.
The shimmy will stop if the rider unloads the saddle, because the mass of the rider is the anchor about which the oscillation operates. It may also be helpful to place the inside of your leg against the top tube on a descent when you experience a shimmy.
Certain elements will make the bicycle more prone to oscillation. All other things equal, those elements that render the bike more elastic will aid in its ability to shimmy. Therefore, a longer head tube will make the bike more likely to shimmy. Lighter, thinner tubes will make the bike more prone to shimmy. I suspect that shimmy might be more prominent now than in days past because bikes are so light, and their components somewhat more susceptible to lateral flex.
Shimmy is more likely in cold conditions, or when a rider is nervous and grips hard on the bars, because the vibration of a shivering rider is roughly similar to that of a frame's shimmy frequency, and this can help bring on the shimmy.
Therefore, while speed wobble is disconcerting, if you can adopt the habit of unweighting the saddle during a long, straight descent any shimmy problem should be largely abated.
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