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Becoming a Mountain Biker



6. The right size?

Of course, the quality of components on your bike is important, but not as important as the right size of the frame. It is even more difficult to determine the right size than a beginner can imagine, since there is quite a confusion in this area.

How is the size indicated?
The size of a bike or better the size of a frame (of the bike) is usually given as the length of the seat tube in inches (in). The seat tube is a part of the frame between bottom bracket (BB), where the pedaling axle is, and the seat post. And here comes the confusion - manufacturers measure the seat tube in two different ways. It can be either "center to center" (from the center of the BB to the point, where the top tube is welded to the set tube), or "center to top" (from the center of the BB to the top of the seat tube). There is usually one size difference between these measurements, meaning: 17 center to center = 18 center to top. The size of a bike is usually indicated via a sticker on the seat tube.


How to check the size?
Stand over the bike and you should have 7 to 10 cm clearance between your crotch and the top tube. This is your approximate size.

And why do manufacturers indicate the size of their bikes by the length of the seat tube? Because it is the easiest to try when buying. All other dimensions of the frame are related to the seat tube then, e.g. seat angle, top tube length, etc. Unfortunately people are not always in standard proportions and it can happen that two bikers of the same height ride bikes of different size - one can have longer legs than the other. This clearly illustrates the fact that sizing according to the seat tube length is really just approximate.


The length of the bike
Very important, if not even the most important, is also the length of the bike, i.e. frame. And it can be measured in many ways as well. The exact length of the top tube is not the same as the length of the frame itself, since mountain bikes have a sloping top tube. So the term effective top tube is used to indicate the horizontal length of the frame, which is simply horizontal distance from the middle of the head tube to the middle of the seat tube or seat post (see the picture on the side).


How to check the length?
Sit on the bike and position yourself as if you were riding - lean forward with hands slightly bend at the elbows. Look in direction of the front hub, and if it is hidden behind the handlebar then the length of the bike is more or less correct. If the hub is visible completely and is in front of the handlebar, closer to the rear wheel, then the bike could be too long and the size of the bike too big. And if the hub is visible on the other side of the handlebar then the bike could be too short and the size of the bike too small. Of course it is best to do this check while riding for real.


You can adjust the length of the bike by moving the saddle fore and aft to a certain degree, since the saddle position is also related to bottom bracket and pedaling. Another way to accommodate incorrect frame length is by changing the stem to shorter or longer one, again to a certain degree, since you should stay within 90-130 mm lengths in order to avoid steering abnormalities. If you need a stem outside these limits, then you'd rather change the frame size.




Horizontal length of the frame


1. Intro

2. What kind of a bike?

3. For how much?

4. Where to buy?

5. How to buy?

6. The right size

7. Initial adjustments

8. The weight

9. Hardtail or a fully?

10. Obligatory accessories






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17. junij 2004 

Copyright Marjan Tkavc