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Wednesday, March 10, 2010

Passing Geometry

Understanding Bike Geometry When Choosing A Bike:

I know the word, but what does it mean?


ST (ct) Seat Tube.

The seat tube determines the height of the bike or your frame size. This is generally measured from the center of the bottom bracket to the top of the top tube.

TT (cc) Top Tube Center to Center.

This is also known as the “effective top tube”. This measurement ranges from the center of the seat post to the center of the steering tube. This measurement determines the rider’s cockpit size and reach. The TT (cc) measurement is important to understand so that the rider will not be too stretched out or scrunched up. Most riders can feel when they are too stretched, but often make the mistake of getting too scrunched and not allowing a good position for proper breathing. Good breathing technique is vital for climbing and hard rides.

TT (hz) Horizontal Top Tube.

This is the actual length of the top tube. On compact frames this measurement may be longer than the effective top tube.

ST angle: Seat Tube Angle.

The angle of the seat tube is measured in relation to the ground, and tends to fall somewhere between 68 and 75 degrees.

A frame's seat tube angle determines the position of a bike's saddle related to its bottom bracket. Since the bottom bracket is the axis around which the bike's cranks and pedal rotate, this affects the orientation of the rider's legs and body in relation to the bike. Seat tube angle also influences how the rider's weight is distributed between saddle and handlebar.

A steeper seat tube brings the bike's saddle more into a vertical line with the bottom bracket, thrusts the rider forward into a more aerodynamic position, situates more of the rider's weight on the handlebars, and so the rider employs more of the hamstring muscles. Triathlon bikes often have steep seat tubes, because they're built exclusively for speed and tend to be ridden by people who are strong runners (and therefore have well-developed hamstring muscles).

The shallower the seat tube, the further the bike's seat is pushed behind its bottom bracket. This is a less aerodynamic position, but it also puts more demand on the thighs and glutes (which tend to be well developed in cyclists), and places more of the rider's weight on the saddle, to increase rider comfort.

A steeper seat tube makes a shorter seat tube for a frame of a given size, and making for a stiffer, more responsive feel. A shallower seat tube requires longer chain stays which can allow for grater comfort over rough terrain.

The steeper seat tube enables a more direct transfer of power through the rider's legs, although a shallow seat tube angle puts the rider in a better position to exert power during low cadence riding, such as seated climbing.

HT Angle: Head Tube Angle

Most head tube angles vary from about 69 degrees (when 90 degrees is perpendicular to the ground) to 75 degrees. In general, the steeper your head angle (closer to 75 degrees) yields quicker and more responsive the steering. Responsiveness can be explained by the following:

a) - A steeper head angle allows for a more weight-forward riding position, placing the body and head closer to the front of the bike, allowing the rider to see the road ahead sooner.

b) - A more upright head angle allows the front wheel to be tucked down under the rider, thus easier to steer.

c) - With the rider's weight more aligned with the turning angle, the bike will turn with more subtle weight and shoulder shifts rather than large arm movements.

You Gotta Have An Angle:


Bikes with the steepest angles are the dedicated road racing bikes, which have angles averaging 74 degrees for both the head and seat tube. Racers want quick, agile and efficient machines, geared towards speed with comfort being a bonus if you get it. A steep head tube angle means that most of the rider's weight will be evenly distributed between legs and shoulders. A racer has to be in good shape to ride with more weight on the shoulders with eyes focused directly on the road in front rather than the trees and mountains.

Touring / Sportive/Recreational Road Bikes:

Steep angles allow for a very efficient ride. You can get racing angles to fit this category as well. In this category however, you get the luxury of having a slightly slacker HT angle for a more upright position, better for taking in the scenery, watching for cars ahead, and a little less interest in controlling the bike. In a more upright position, more weight gets transferred to the sit bones, which is usually more comfortable.

BB drop: Bottom Bracket Drop

If you were to draw a line horizontally between the front and rear dropouts and a perpendicular line through the center of the bottom bracket. The distance between the center of the bottom bracket and the first line, measured along the second is what is known as he "BB drop, which measures how far below the axles the bottom bracket sits.

Mountain, cyclocross, and track frames bottom brackets tend to be quite high (mine is 65mm). The reason is to keep stones, dips, roots or a severely sloping track from causing a pedal strike, hitting the chain rings or frame. The bottom bracket in these bikes is built up and way from these dangers which could end a ride or a season in a hurry. A higher bottom bracket also shortens the chain stays and down tube of a frame, which are put under the greatest stress under riding conditions, which translates into a stiffer, more responsive feel.

The advantage of stiffness and safety come at the cost of stability; the lower the bottom bracket, the lower the bike's center of gravity, for a more stable ride. Touring and commuter frames often have a lot of drop, to be more practical than concerned about winning a sprint. Many good road bikes tend to have medium BB drop for the best of both worlds.

CS: Chain Stays. The chain stays run from the bottom bracket to the rear dropouts. This can be measured as "actual" or "effective" length due to the fact that the bottom bracket drop will increase or decrease the length of a frame's chain stays without changing the position of the wheel.

Most models tend towards shorter chain stays. Short stays makes them stiffer and minimize the flex and therefore adds greater power transfer from the pedal stroke. This shortens the wheelbase for a snappier feel and low speed turning. It also puts more of the rider's weight over the rear wheel for optimized traction. Mountain bikes use short chain stays to better dig in to fight up loose climbs.

A longer wheelbase with longer chain stays makes a more stable bike at higher speed. The long chain stays add more flex. They push the rider forward off the rear wheel to add to the comfort of the ride. These bikes will often have a more shallow seat tube angle; great for seated climbs and women's bodies. The longer stays also allow greater tire clearance for larger tires to be used and making it easier to remove the wheel. Another added benefit of longer chain stays is that they will keep the rider's heels from hitting a rear pannier rack. This is generally why you see commuter and touring bikes with longer chain stays.

SO: Standover.

This is how high the top tube will be from the ground. This measurement may vary based on tire size. Some companies measure this differently, so it is good to double check before ordering.

HT Length: Head Tube Length

This may vary to allow for greater aero position.

WB: One fundamental measurement is wheelbase. The WB is calculated either by measuring the center of the hubs or the center of the spot where the wheels meet the ground; these distances must be equal (assuming the wheels are round!). A longer wheelbase should make a bike more stable in a straight line but less agile in turns. It may make a bike feel more comfortable or smoother due, in part, to the flexy stays.

Stem: Stem length is a completely adjustable parameter to adjust the feel of a bike. Most stem lengths are selected based on fit, but a rider can opt for a longer stem with a forward seat position to create slower, more stable steering, or the opposite effect with a shorter stem. Stems may vary from 130mm to under 90mm.

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