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DIY–How to choose the front fork behind the frame

Classic frames generally have both road and mountain performance. How to choose a front fork is almost equivalent to choosing the attributes of a road bike, mountain bike, and station wagon.

Choosing front forks with different parameters: length, slope, etc., will affect the frame’s seat tube angle (E), front/head tube angle (D), BB height (or H), wheelbase (G) and other parameters. Changes in frame parameters not only bring about changes in riding comfort, but the key is that the type of bike has also changed.

front fork

How to choose a fork

Analysis of the Mystery of Mountain Bike Frame Geometry

Most cyclists or cyclists complicate the geometry of the frame. Almost all modern off-road mountain bikes only change within a few angles, and a two-degree difference can immediately judge the height of the frame.

Twenty years of testing has produced a standard mountain bike frame geometry: 71° head tube angle, 73° seat tube angle, 23 inches of top tube length, 16.9 inches of chainstay length, and 12 inches of Bottom Brackets height . Although it is really difficult to tell the difference between the two different brands of electric bicycles, the riding feels of the two electric bicycles are significantly different, but the difference is really different!

A small change in the frame size formula can greatly change the handling of the bicycle. MBA tries to ride more than 40 mountain bikes every year, and there are no exceptions. We found that each bike has its own personality. Common sense tells us that a small change in the geometry of the frame does not seem to have a major change in the handling of the mountain bike. But things are not what you imagined. The different lengths and angles of the various parts of the frame really make you see electric bicycles with similar appearances, and they ride so differently.


Descriptive words like “legendary feeling” or “perfect balance” do not often appear in MBA magazine competitions and test rides. Frame designers fully understand that what we call frame geometry is the key point of the “angle” that can be counted with one hand. Any experienced mountain bike rider can tell whether an electric bicycle is good or not within three turns of the crank.

I tried to explain the geometry of the frame, the function of each data on the frame, and how the size of individual parts affects the overall frame of the frame, so as to help readers understand that the electric bicycle and your body fit seamlessly. And why the feelings of acceleration, cornering, downhill, and climbing are different for different cars.

seat tube angle

The seat tube angle refers to the angle at which the seat tube tilts back to compensate for the length of the rider’s legs. When the seat cushion is at the right height, that is, when the feet can be stretched perfectly, with the crank of the chainring pointing at three o’clock, your ankle must be below your knee.

Designers have found that a seat tube angle of 73° can satisfy most Knight. This angle can make up for a knight with short legs. When a tall person raises the seat cushion, the seat cushion position can be moved back, while a smaller person lowers the seat cushion.

The seat cushion position is slightly forward. Of course, there are exceptions to the figure, but when the 73° seat tube angle and the front and rear adjustment of the seat cushion are matched, almost all the riders can be placed in the correct and appropriate positions that match the crank.

Length of upper tube

The upper tube length provides arm extension, and the steeper the head tube angle makes it easier to maneuver down steep slopes

The length of the upper tube is measured by the horizontal tangent of the upper edge of the head tube extending to the length where it meets the seat tube. From a practical point of view, it is the distance between the control part of the bicycle and the upper part (Cockpit).

The length of the upper tube provides space for your upper body and allows your arms to be perfectly extended and held on the handlebars. On the other hand, it also allows the handlebars to “keep a safe distance” from your knees. Even when you get up and step on it, The knee will not hit the handlebar.

Top tube length notice
In fact, the faucet and the upper tube are independent of each other, and the riding feeling they affect is also different. For fans of off-road and forest riding, you should choose the top tube that allows you to match a faucet with a length of 100mm to 125mm. This combination is the best compromise between climbing efficiency and downhill handling and high skill requirements.

Chain stays

The correct chainstay length refers to the horizontal distance between the BB and the axis of the rear wheel. The rear triangle arm of the full suspension car replaces the function of the chain stay.

Bottom Brackets height

Refers to the vertical height from the ground to the center of the BB axis/crank axis. It determines the distance between the pedals and the ground when the crank turns to the lowest end and points to the “six-thirty” direction.

The main function of BB height is to keep a proper distance from the ground. In other words, the crankshaft must be high enough so that when you forcefully pass through a lane full of rocks and tree roots, the “chainring” and the crank/pedal will not collide. A car with a low BB lacks the space for effective cornering. When you want to accelerate pedaling and run from a curve to a straight road, the pedals may even hit the ground.

BB height notice
The best BB height for the off-road full suspension car family is between 12.5 and 13.5 inches; the front suspension car likes the 12-inch BB height. The higher the BB height, the longer the wheelbase must be, so that the car will ride smoothly and stably.

Total track

Is the distance between the front and rear axles.

A long-wheelbase car rides more smoothly, but turns slower and slower (longer radius of gyration). The movement of the body’s center of gravity during cycling will not affect the handling of the car; a car with a short wheelbase is more sensitive, but when riding on a bumpy road, you can feel the bumps. The magic number of the wheelbase is about 42 inches

Front/head tube angle

Refers to the angle of the frame head tube and the angle of the “fork axle”. The front angle determines the flexibility of the car’s handling and its inherent stability

71° is the most popular front angle, because this angle makes the front of the car rotate more lightly when climbing a hill. Shrinking it by one or two degrees can make the full-suspension car as stable as it is when it is slammed and braked. The height of the angle affects the steering geometry of the ebike.

If you are interested in motorized bike and want to know more, you could browse previous articles or visit our Official Website, also choose to leave your message below. If you are interested in electric bike, you can contact us.

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