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  1. #1
    Member Loyal's Avatar
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    Mountain Bike Frame Geometry

    Hey All . . .

    I have been riding the pavement for a couple of years and now want to try off-road. I'm interested in XC. I'm doing my research . . . hardtail vs full, 29 vs 26 and new vs used.

    My question is about frame geometry. A friend mentioned that if I buy used make sure I don't get a downhill bike as the frame geometry is different than a XC. I have not had a chance to ask him to explain further. Can someone help me out.

    What are the visual differences in frames? Is there a difference? What to I look for to ensure that I am looking at a XC bike and not a downhill?

    Tks :-)
    Zuki

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    What bike is it? find out, then look at the manufactures website, they will have the intended use on it.

    In very broad terms for XC vs DH, normally XC will have upto 100/120mm front travel, hardtail or FS, the bikes will be light weight, DH minimum 160mm always full suspension normally a lot heavier than XC, AM falls in the middle with 120-160mm front, and same rear.

    The gemoetry will reflect the amount of travel that the suspension has.

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    I don't have any particular bike in mind yet, I still scanning the advertisements. Good idea though . . . check the manufacturer's web site.

    I like your broad stroke explanation . . . that will get me started. I'm not familiar with the abbreviation AM though??

    I thought geometry also had something to do with the angle of the steering post . . . or am I mixing up bike technology?? :-)
    Zuki

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    AM - All Mountain or trailbike

    For the angles, this relates to the steerer tube and seat tube, the effective top tube & seatstays all play a part.

  5. #5
    Member igotfatlaces's Avatar
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    AM stands for "All Mountain" which is riding up and down a mountain. AM is whats between XC and DH because DH is just for going down and its a lot harder to go up. XC is basically trail riding. Best shown in 2 pictures so here you go.

    Here is a downhill bike. If you notice how tilted back it all its, this is so it keeps your center of gravity as flat as possible when flying down hill. Also DH bikes are always full suspension and like jimc said, will have long travel in the suspension.


    Here is a full suspension XC bike, see how its more leaned forward to the DH bike? Keeps your center of gravity for a flat trail. XCs are also hard tail a lot of the time.

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    Quote Originally Posted by Loyal View Post
    I like your broad stroke explanation . . . that will get me started. I'm not familiar with the abbreviation AM though??
    "All Mountain". A compromise between a twitchy, harsh race-oriented XC bike and a squishy, porky downhill bike, meant for general-purpose use. The downside is, they are more expensive than most entry-level riders want to spend

    I thought geometry also had something to do with the angle of the steering post . . . or am I mixing up bike technology?? :-)
    That's called the "head tube angle", if I understand you correctly, and is one of many figures. A steeper head tube (closer to 90-degrees relative to the ground) makes the bike handle more quickly, but decreases stability. XC bikes typically have steep head tube angles, and downhill bikes have slack head tube angles. The most important thing is the fit of the bike, and that is dominated by the "effective top tube length", which measures the distance between the saddle and the handlebars. The best way to figure out whether or not a bike fits you is to ride it. Ride a bunch of different bikes, and you'll find one that just feels right. Most shops selling more expensive bikes will customize the fit by swapping out the stem or seatpost as appropriate.

    More than you ever want to know about the subject is here, courtesy of Sheldon Brown, may he pedal on in Valhalla forever.

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    For all the different types of riding, there is a "sweet spot" that makes it not only most efficient (fastest WITH lesser effort), but the most enjoyable. Geometry affects that directly.

    The first item affected is steering; a 'steeper' head angle makes for faster steering, which can put you on your face going down a steep hill. But for flat areas, pavement, etc., it can be a good thing. Seat angle is next, and that directly affects where YOU are in relation to the pedals; it can be harder to pedal if you're too far 'behind' the pedals themselves (and inefficient if you're too far forward). Like Little Red Riding Hood, somewhere in the middle is 'just right'.

    Measurements are also part of geometry; too short of a top tube (from the seatpost to the stem) can make you feel cramped, and make it harder to handle the bike. Too long of a top tube can actually injure you, from being too stretched out. A shorter wheelbase handles and turns faster, a longer one is more stable in the rough stuff.

    There's a fair amount to it, that's why 'fit experts' can charge what they do. But they are best left to competitive riders; the average rider just has to hit 'comfortable'.

    Example: for me, the distance from my all-the-way-down pedal to the top of my saddle is very nearly a meter; my ideal top tube/stem combo is 3/4's of that. I like my handlebar less than 1 inch below saddle height, and the 'kneecap drop' just behind the pedal axle. A handlebar less than 26 inches wide makes me feel cramped. My favorite bike's headtube angle was about 68 degrees, not real steep (70+ is considered 'XC aggressive').

    They're getting closer, but with 29er's, it's still coming together, not as established; I couldn't tell you what my 29er fit numbers would be. (Not worried about it, either, a 29er is not in my future....)

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