bike geometry..what?

Discussion in 'Mountain Bikes' started by ride, Nov 2, 2003.

  1. ride

    ride New Member

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    Can any1 tell me what this bike geometry is about? A longer top tube does what? Head tube angle, why? shorter chainstay's.. so?

    I know the difference between road and mountain bikes.. I have set up my bike accordingly to how I ride but I dont know what makes what difference.. I am trying to buy a new bike but all this bike talking is getting me confused..

    any links to article's net , very much appreciated!

    Ride ...On!
     
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  2. Hecubus

    Hecubus New Member

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    Top tube length should be based on the proper sizing of the bike
    for your body. The remaining difference in top tube length is then adjusted with the stem length. Depending on the purpose of the bike this configuration can be considerably different. For example in a XC bike it is normally recomended that after you get the proper top tube length for your size the stem length should be somewhere in the 100-130mm range. For downhill bike stems tend to be much shorter to allow the rider to easily drop farther behind the saddle and keep the weight behind the rear wheel.

    Headtube angles will affect how the bike steers. XC bikes for example have very steep head angles (around 71 degrees) which give them very fast steering and manuverability in tight spots. The trade off is that fast steering makes the bike more unstable at high speed. Trailbikes or "all purpose" mountain bikes have more relaxed head angles around 70 to 69 degrees which is a nice compromise allowing the bike to still steer fast enough but provide some improved high speed performance. More aggressive bikes such as downhill bikes have extremely relaxed head angles which can be anywhere between 69-65 degrees. This will give them maximum high speed stability because the steering is very slow and predictable. The difference is that the more relaxed the head angle gets the sloppier the steering becomes and the front wheels tends to flop around more when turning. Bikes with slack head angles tend to sort of lean into a turn rather than actually turning sharply. It also affects climbing since a slacker angle will put the riders weight farther behind the front wheel. This means the front end will tend to wheelie during climbs and climbing overall is made more difficult.
    Shorter chainstays will make the bike accelerate faster, climb better, and turn quicker. The drawback is that shorter chainstays means a shorter wheelbase causing the bike to be more twitchy at high speed and going downhill. A longer wheelbase will give a more stable ride.
     
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