Angles and Sizes, and How They Affect Your Bike

Geometry
Bike geometry is a big term and comprises of many complex angles and lengths. Most of these remain standard for the most part and do not greatly affect the ride characteristics of a bike. However there are a few measurements that can help you select the perfect frame or bike.

1. Head Angle is the angle of the head tube and fork with the ground. Cross-country bikes have tight head angles of 70-71 degrees. All mountain and downhill bikes will have a slack head angle from 66-69 degrees. A slack head angle will allow the bike to fly through technical sections easier; the downside is slacker head angles mean slower handling and climbing. The fork length directly affects this angle. Putting a 160mm fork on a XC bike designed for 100 will give the bike a slack head angle.
2. Seat Angle refers to the angle of the seat to the angle of the chainstay. Steep angles will put you almost directly over the cranks for optimum pedal efficiency whereas slack angles will sit you back, to more easily take on the technical stuff. Bikes with slack seat angles are paired with slack head angles and vice versa.
3. Bottom Bracket Height is the clearance your bike has. Your bottom bracket is the lowest point on your bicycle and when you’re getting into technical terrain and hoppin’ rocks, clearance can become a concern. All-mountain and downhill style bikes will have higher bottom bracket heights than cross-country bikes. Nevertheless the higher the bottom bracket height the higher your center of gravity. High bottom brackets mean slow cornering but can save your chain rings from bashing into rocks.

Fit
Fit is very important when it comes to bikes. Improper fit can lead to poor control, loss of power, and even pain and joint/muscle problems. This is just a quick guide to show you what different angles and sizes do to change the performance of a bike. For info on the actual fitting process do some searching or visit your local bike shop.

1. Frame Fit is based on a couple of factors: Your height will of course be the main factor, but another thing to keep in mind is your riding style. For example a 6’2” person will be a perfect fit on a 21” cross-country hardtail, however they could also fit on a 20” or even a 19”. For long rides with less technical terrain, a larger bike will be better, in our example the 21”. Say our rider wants a hardtail but will be riding on more technical terrain with quick turns. In this case a 20” or 19.5” would be a better choice. A smaller frame will give you more control over the bike in tight and technical sections. Longer bikes will be more stable and comfortable for longer rides. I have a 20” single speed and a 19” geared hardtail. My single speed is great for cruising through national forests, while my geared hardtail is my first choice for anything really technical or fast.
2. Stem length and Angle will change your riding position on the bike. For more climbing run a longer stem (90mm –120mm), with 0-10 degrees or rise. This will keep more weight forward and put you in a more efficient pedaling position. For downhill and technical terrain run shorter stems (40 – 70mm) with 10-15 degrees of rise. For a combination run a stem in between these measurements.
3. Seat Height is a pretty easy adjustment. Sit on your seat and stabilize the bike. Put the heel of one foot on the pedal and bring the pedal down so the crank arm is parallel with the seat tube. At this point your leg should be able to straighten out completely. If you cannot straighten your leg out fully raise your seat, and if it feels like a stretch lower your seat. For more technical riding run your seat lower. This will sacrifice pedaling power but give you more room to move around as you conquer the rough stuff.
4. Seat Angle: For climbing point the nose of your saddle slightly up, for downhill and jumping put it slightly down. If you do both, keep it level.
5. Seat Position (forward and backward position) will change your riding posture. This will mainly affect your lower back. The easiest way to adjust this is just try to feel it out (concentrate on how your lower back feels, tight or stretched) or have an experienced rider watch your position.
6. Lever Position should be set up so when you brake, your arms and wrists will be in a straight line. For braking on flats your levers will be tilted downward more than for downhill. Choose a setup that will be comfortable for the majority of terrain you are riding.

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• August 29, 2008 -- Mountain Bike Frame Materials