Why are the seat tube angles of mountain bikes getting bigger and bigger?
A few days ago, I accidentally flipped through a video of a electric mountain bike race in the 1990s. I really sighed the rapid development of electric mountain bikes from the bottom of my heart. Taking this opportunity, we will slowly talk about the situation and meaning of the geometric angle change of the mountain bike.
In recent years, the geometric angle of the mountain bike frame has been developing toward the three characteristics of steeper seat tube angle, looser head tube angle, and longer front triangle. The seat tube angle is particularly important for the influence of the pedaling position.
Generally speaking, the ergonomic seat tube angle is 72-73 degrees, which is also the most common and recommended angle we use on road bikes. Of course, this is not absolute, and it will fluctuate up and down by one or two degrees from person to person. And according to the different front and rear positions of the seat installation, this angle will also have a range of plus or minus three degrees.
However, most of the full suspension bicycles we have seen are equipped with a steep seat tube angle, which means that the angle with the ground is larger. Recently, our common ones are around 75 degrees. Of course, this is not the result of a random design, but is determined by the environment and performance of the bike.
Generally speaking, in addition to steep downhills, full suspension bicycles also have to deal with steep uphill. When encountering an uphill slope, a slope of about 10% will reduce the seat tube angle by about 6 degrees. Because on a full-suspension vehicle, when going uphill, the weight distribution will be more rearward. At this time, the stroke of the rear suspension will also be pressed down more than the front suspension.
For a typical 150mm travel soft tail bike, this kind of rear shock compression will make the seat tube angle again about 3 degrees of backward tilt. Therefore, if the rider wants to adopt a seated pedaling position, a steeper, a larger angle of the seat tube, can compensate for the lost angle, so as to obtain a relatively more comfortable and reasonable pedaling position. This is the first reason, for angle compensation in climbing.
The second reason is that the steeper seat tube angle adjusts the center of gravity of the vehicle. As the slope increases, the driver’s center of gravity will become more and more rearward. When the vertical position of the driver’s center of gravity exceeds the contact point between the rear wheel and the ground, the head will be warped. This is why when climbing some very steep slopes, the body is pressed forward in order to prevent head lifting.
Turning to the seat tube angle now, the steeper seat tube angle means that the position of the rider who occupies the heaviest weight will be closer to the center of the bicycle, that is, the middle position of the front and rear wheels. This allows the rear center to be longer and the center of gravity farther from the critical point, which is more conducive to facing steep roads, increasing stability and grip.
The last reason is also related to handling, but also to stability and comfort. On a soft-tail bicycle, when the rear wheel passes a bumpy obstacle, the rear wheel will be bounced. Although the shock absorber will absorb part of the vibration, it will still cause the entire bike and people to move. Bounce is a directional movement roughly around the front wheel.
Therefore, at this time, if the rider is farther away from the rear axle, that is, the position of the rear hub, the vibration will be relatively smaller and the riding comfort will be better.
If you still don’t understand, think about our older generations who like to sit in the front when they ride in the car, because they always say that they are panicking behind. Although the comfort of our long-distance buses is getting better and better, we have to say that the rear is indeed upside down than the front of the car, which is the same as the above.
For the above three reasons, the seat tube angles on soft tail cars are generally designed to be steeper, and the steepness varies with different uses. The largest is 79 degrees, and the smaller is smaller. There are also 74 degrees and 75 degrees.
Well, the last question is that on many soft tail bike, unlike road bike, the middle tube is straight and the angle of the seat tube is the same as that of the middle tube. And because of different designs, there will be a variety of weird shapes.
At this time, you can determine the seat tube angle by the angle between the line from the center of the seat to the bottom bracket and the ground to determine the actual seat tube angle. And this angle will become slacker as the seat rises, that is, smaller. So, you need to pay attention when choosing a frame. This is why a smaller frame will reduce the efficiency of the rider in the uphill pedaling and make it easier to “roll over”.
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