Seat tube angle and saddle fore/aft position questions
While doing some online research about getting a good fit on a mountain bike I came across some information about setting saddle fore/aft position. With the crank arm parallel to the ground your knee should be directly over the pedal spindle.
Maybe I’m missing something. If your seat is adjusted so that your knee is over the pedal spindle, wouldn’t that position negate any difference in seat tube angle? For example, on a bike with a 71 degree seat tube, the seat may be on the front of the rails to keep your knee over the pedal, but on a bike with a 69 degree seat tube the saddle may be on the rear of the rails to keep the knee over the pedal, effectively causing both bikes to have the same virtual seat tube angle.
What are the advantages/disadvantages to different seat tube angles, if any? Or should I not fret over it and just ride?
The seat tube angle determines the location of the seat relative to the cranks as the seat is raised or lowered. Most bikes of a given style will have similar seat tube angles. A cruiser bike for example might be designed to place the seat further back and lower so that the rider can touch the ground with both feet while seated.
The seat tube angle also provides for riders with different leg lengths. A rider with longer legs will set the seat higher. Since a taller rider will normally have a longer thigh length, the seat needs to move back as it is raised. That is accomplished by the seat tube angle (plus fore-aft adjustment if needed). Once the seat is locked in place the seat tube angle has no effect on performance for a given bike.
Last edited by billee; 05-17-2009 at 08:35 AM.
What you're looking at is called KOPS (Knee Over Pedal Spindle) and is a good starting point for fit When I fit people I use it as a gauge and reference not a goal. Mostly this is useful for road folks that are in the same position for hours at a time. With all the up and down on MTBs it doesn't matter as much.
Also other riding characteristics and needs will outweigh standard fit guidelines. Most DH/AM riders will want to be further back than endurance XC riders as and example. Also particular physical limitations or injuries will also dictate saddle movement. If you have ITB problems a fit a little lower and more forward may be important.
Don't sweat the angle too much unless you know exactly what fit you need.