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  1. #1
    如果你能讀了這個你講中文 genericbikedude's Avatar
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    Handling difference with minimal extension stem?

    My first project bike is now my rain bike. Its a crappy NYCbikes aluminum cyclocross frame with an on-one il pompino fork. It now has fat tires, fenders, radonneur bars, suntour bar ends, dia-compe 287V levers and v-brakes.

    The top tube is FAR too long. I'm only really comfortable on it when standing up significantly far forward of the saddle. This makes me think that a stem with minimal extension will improve matters, but I would imagine that this would significantly alter the handling. How would it change it? With less leverage, would it become more sluggish? I mainly ride in traffic, and I like to bob and weave to avoid potholes and taxicabs, so handling is important. My other bike has track geometry, which suits my city-riding purposes quite well. What will happen with a zero extension stem?

    And who makes one for cheap? Is it a distro item (action, pyramid, dimension, etc) or available online?

  2. #2
    cab horn
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    With a zero extension stem, your bike will be a lot more unstable and twictchy handling wise.

  3. #3
    如果你能讀了這個你講中文 genericbikedude's Avatar
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    Quote Originally Posted by operator
    With a zero extension stem, your bike will be a lot more unstable and twictchy handling wise.
    sooooo, better and more responsive?

    more generally, how does a short stem compare to a low-rake fork, or compare to a sharp headtube angle? is there a difference in type of steering property between these different things, or are they all part of some sort of theoretical steering equation?

  4. #4
    jur
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    A short stem gives you ability to twist the fork quicker. Take a long stem to the limit: the bars have to rotate as well as move to the side you're twisting to, like a rudder. Short stem, only twisting involved. Old skool bikes were all like that. Stem length does not have any big impact on 'twitchyness'.

    Now steep head tube angle will make the bike 'twitchy' - the ride line will tend to be a little unstable. Take head tube angle to the limit - with almost horizontal head tube, you can twist handle bars all you like but little steering will result.
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    Quote Originally Posted by jur
    A short stem gives you ability to twist the fork quicker. Take a long stem to the limit: the bars have to rotate as well as move to the side you're twisting to, like a rudder. Short stem, only twisting involved. Old skool bikes were all like that. Stem length does not have any big impact on 'twitchyness'.

    Now steep head tube angle will make the bike 'twitchy' - the ride line will tend to be a little unstable. Take head tube angle to the limit - with almost horizontal head tube, you can twist handle bars all you like but little steering will result.
    +1. Long stem gives a longer lever arm for more leverage. Easier to move but moves though a longer arc. Opposite for short stem. My understanding is that actual force difference needed between various normal lengths is negligible.

    "Twitchyness" is also influenced by fork rake.

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    A few days ago I picked up a bike ~20 miles away to ride back, and the stem was about 50mm longer than my current relatively short one. The handlebars felt more like a tiller with dramatic side to side motion of the handlebars accompanying any turning. Turning was smoother, but at the same time the bike just didn't react as well.

    There are about a dozen variables when it comes to steering (rake, handlebar type, handlebar width, stem length, position of body in relation to handlebars, tires, etc) but they all make very dramatic differences.

  7. #7
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    The bike fit gurus will tell you one and only one set of frame dimensions and component sizes will truly fit you. That may be true if you are trying for a "perfect" fit but most riders can get away with a great range things to get a "sort of" fit. Try the ultra-short stem and see if it works for you.

    There are some MTB stems so short the handlebar clamp and the quill or stem clamp almost touch. I believe Nitto makes a very short quill-type road stem and I know I've seen some 4 cm stems on the shelf at a couple of LBS.

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    Quote Originally Posted by jur
    Now steep head tube angle will make the bike 'twitchy' - the ride line will tend to be a little unstable. Take head tube angle to the limit - with almost horizontal head tube, you can twist handle bars all you like but little steering will result.
    I'd like to see a bike with an almost horizontal head tube angle. (I think jur meant "almost vertical.")

    As for jur's claim that a steep head tube angle means instability - not true. Steep head tube angle plus short-rake fork makes for a good amount of trail and great stability. True track bikes, as the original poster very well knows, are excellent, predictable bikes with respect to handling; note that he likes his steep-angled track bike for maneuvering in city traffic. It's the short wheelbase, not the steep angles, that give such bikes a reputation for "twitchiness." After you ride a steep-angled true track bike for a while, any road bike is going to feel pretty vague in the handling department.

    Having a very long or very short stem doesn't affect steering per se - at anything above walking speed, you turn by leaning, not by steering with the handlebars. The length of the stem affects only the rider's weight distribution on the bike. An ultralong stem can mean that the rider has too much weight over the front wheel, with a correspondingly higher potential for doing an endo when braking at speed or downhill. At the other extreme, plenty of bikes have ultrashort stems. Think of the traditional Raleigh three-speed bike - the stem is as short as it can be.
    Last edited by Trakhak; 04-23-06 at 05:44 AM.

  9. #9
    Senior Member AnthonyG's Avatar
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    I'm effectively riding with a 45 mm stem (butterfly/trekking bars) and I don't have any stability problems with it. You will notice the change at first but you will quickly become accustomed to it and you won't notice it anymore.

    Regards, Anthony

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    With a short stem you will notice a difference when you are riding out of the saddle as in climbing or sprinting. Ideally your weight should be directly over the front hub and therefore over the point where the front wheel contacts the road. If your weight is behind this point the bike will feel dead and sluggish. The “wheelbarrow effect” as I always call it.

    As the bike moves from side to side as it will do to some degree when out of the saddle, with your weight directly above the point of contact you are moving in a vertical plane and the front wheel stays in that plane. With your weight behind the point of contact you are swinging from side to side in an arc causing the front wheel to steer first one way then the other; not holding a straight line. At the same time the gyroscopic action of the spinning wheel is trying to keep you on a straight line. So the two actions are fighting each other; hence the bike feels sluggish.

    To demonstrate this effect to yourself; hold a pen or ruler on a table top at 90 degrees and move from side to side keeping the point of the pen in one spot; you are moving in one plane. Now hold the pen at an angle of 45 degrees and move from side to side and you will see that you swing in an arc.
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    Quote Originally Posted by Dave Moulton
    With a short stem you will notice a difference when you are riding out of the saddle as in climbing or sprinting.
    Granted.

    Ideally your weight should be directly over the front hub and therefore over the point where the front wheel contacts the road.[snip]
    I do not see the logic behind this.
    It would help to know if by "your weight" Dave means "center of mass".
    If so, that would be rather difficult. If not center of mass, what is meant by "weight" in this context?

    The two latter paragraphs seem sensible to me however.

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    Quote Originally Posted by tvphobic
    I do not see the logic behind this.
    It would help to know if by "your weight" Dave means "center of mass".
    If so, that would be rather difficult. If not center of mass, what is meant by "weight" in this context?
    You took me literally when I said “Weight” over the front hub; I try to explain things in simple terms without getting too technical. Of course to have the center of mass over the front hub the rider would have to stand on the handlebars, and you are right the center of mass is always going to be behind the front hub.

    There is a shift in weight forward when a rider gets out of the saddle to climb or sprint, and perhaps what I should have said was the rider’s hands should be as near as possible over the front hub. What I aim for is to place the center of the foremost part of the handlebars over the front hub. (See the attached picture.)

    This means with the hands on the drops they are slightly behind, and with the hands on the brake hoods they are slightly ahead of wheel center; a good compromise. Slight variations either way are not that important, but remember the original question in this thread was “What would be the effect of an extremely short stem.”

    Rider position and comfort would take precedent over this, but if a person is trying to decide between two different sizes in a compact frame or top tube lengths; choose the one that is going to place the handlebars in the position I suggested above.
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    I think we are getting far to involved with this. The OP has salvaged a low cost bike he wants to use for utility purposes and is looking for a cheap way to make it managable, not ideal.

    Summary: Fit a very short stem and learn to live with it. It isn't dangerous and you will adapt quickly.

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    Quote Originally Posted by HillRider
    I think we are getting far to involved with this. The OP has salvaged a low cost bike he wants to use for utility purposes and is looking for a cheap way to make it managable, not ideal.

    Summary: Fit a very short stem and learn to live with it. It isn't dangerous and you will adapt quickly.
    I agree but some people come on this forum to learn something, and others like me who have a little experience try to pass on that knowledge.
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    Quote Originally Posted by HillRider
    I think we are getting far to involved with this. The OP has salvaged a low cost bike he wants to use for utility purposes and is looking for a cheap way to make it managable, not ideal.
    That's just what happens within threads; they are easily sidetracked. If I had started a separate thread to ask Dave about his terminology I doubt he would have seen it. BTW thanks for the clarification Dave.

    Also, I have run some stems as short as 80 mm on frames with 60cm top tubes with no serious handling problems, ie twitchyness. However my ideal is ~57cm with up to a 140 mm stem. I am sure I would find a short wheelbase frame with a really short stem rather quick turning.

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    Dave; Thanks for the information. That is one of the best explanations I have seen & read. Please allow a bit more on this thread.

    If I understand your post correctly, a rider should select a stem length that puts the rider's hands over the front axle when their hands are in their normal position. A rider who rides in the drops will have a slightly longer stem when compared to the rider who is on the hoods or on the bars. I assume that the frame angles & rake have a bit to do with this also. Ie. various 57cm frames would have different stem lengths (because of frame angle, top tube length, rake, etc.) for the same given rider. Correct.

    What do areo or tri bars do to this formula? Should the stem length shorten if the rider spends much time on them?

    Thanks again. Bob

  17. #17
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    Quote Originally Posted by Bob S.
    If I understand your post correctly, a rider should select a stem length that puts the rider's hands over the front axle when their hands are in their normal position. A rider who rides in the drops will have a slightly longer stem when compared to the rider who is on the hoods or on the bars. I assume that the frame angles & rake have a bit to do with this also. Ie. various 57cm frames would have different stem lengths (because of frame angle, top tube length, rake, etc.) for the same given rider. Correct.

    What do areo or tri bars do to this formula? Should the stem length shorten if the rider spends much time on them?
    No, you choose a stem that gives you a good riding position; this takes precedent over all else. If this places your hands directly over the front hub then this would be ideal; if you are slightly off from this you are probably not even going to notice the difference so no big deal. And remember I was saying the effect of this was only in the way the bike feels when riding out of the saddle, as in climbing or sprinting, which is only a small part of your overall riding experience. Aero bars do not come into this picture because you are not riding out of the saddle on aero bars.

    A history lesson that is relevant to this subject.

    Back in the 1950s when I started riding, frames had very shallow seat angles, long top tubes and very short stems. Fork rakes were as long as 3 inches which put the front wheel way out in front of the bars. These bikes felt awful when climbing out of the saddle, sluggish and with a tendency to weave all over the place. There were those who advocated that real bike riders sat down to climb.

    By the 1960s early 1970s framebuilders (Started mainly by the Italians and everyone followed suit.) discovered if you made the head angle steeper and fork rakes shorter, thereby bringing the front wheel nearer the bars, the bike felt more lively and was better in a sprint. Head angles got to be as steep as 75 and 76 degrees making for some very twitchy, bordering on dangerous bikes.

    Seat angles remained shallower than the head angle, and top tubes were still fairly long. I refused to go with this trend, and found I could achieve the same lively feel by keeping the head angle at 73, making the top tube shorter and using a longer stem. Thus moving the bars over the front wheel and putting the rider’s mass in the center of the frame, making for a better handling bike. This is how most bikes are designed today.
    History, photos and tech articles on my website. Also check "Dave's Bike Blog."

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