Below we define the important measurements that determine the shape, fit and handling of a mountain bike and explain how they affect riding.
We’ll start with the basics, including their less obvious aspects, before discussing some of the lesser mentioned but equally important geometric topics. Finally, we will delve into how the often misunderstood concept of trajectory affects handling.
The length of the seat tube determines the size of the bike more than the “small, medium or large” design. This is because it defines the minimum and maximum height a saddle can be set to, and therefore the range of heights a rider can ride the bike comfortably, or how low they can drop the saddle to descend.
For example, two mid-sized frames often have different seat tube lengths for different riders. While seat tube length does not directly affect bike handling, important handling and fit measurements such as reach must be compared to seat tube length to determine bike length relative to rider height.
The ratio of reach to seat tube length is particularly useful – some modern bikes have longer reach than seat tube dimensions.
Definition: The length from the top of the steerer tube to a horizontal line crossing the center of the seatpost.
The Efficient Top Tube (ETT) gives a better idea of how spacious the bike feels when you’re in the saddle than using the base tube measurement (from the top of the head tube to the top of the seat tube).
Combined with the stem length and saddle offset, this gives a good indication of how the bike will feel when riding in the saddle.
Definition: The vertical distance from the bottom bracket center to the top of the head tube center.
This determines how low the bar can be in relation to the carriage. In other words, it defines the minimum bar height without spacers under the bar. The stack also has an important but rather unintuitive relationship to rates…
Definition: The horizontal distance from the bottom bracket to the top center of the head tube.
Of all the usual numbers in bike geometry charts, offset gives the best idea of how a bike fits. In addition to stem length, it also determines how roomy the bike is out of the saddle, and the effective seat angle, which also determines how roomy the bike is in the saddle. However, there is a small caveat, it has to do with stack height.
Take two identical bikes and raise the head tube of one bike so it has more stack height. Now if you measure the range of these two bikes, the one with the longer head tube will be shorter. This is because the head tube angle is not vertical – so the longer the head tube, the more back the top of it, and therefore the shorter the reach measurement. However, if you use the headphone pads on the original bike so that the handlebar height is the same, the riding experience on both bikes will be the same.
This demonstrates how heap height affects range measurements. When comparing stretch distance between bikes, keep in mind that bikes with higher rack heights will feel longer than their stretch readings would suggest.
The easiest way to measure range is to put your front wheel against a wall, then measure the distance from the wall to the top of the bottom bracket and head tube and subtract.
Definition: The distance from the center of the bottom bracket to the center of the bottom of the head tube.
Like reach, downtube length can indicate how roomy a bike is, but this is also complicated by other factors.
Just as reach depends on stack height (the difference in height between the bottom of the bottom bracket and bottom bracket), so does the length of the downtube. head tube.
This means down tube length is only useful when comparing bikes with the same wheel size and fork length, so the bottom of the head tube is about the same height. In this case, downpipe length can be a more useful (and measurable) number than length.
The longer the front center, the less likely the bike is to lean forward over big bumps or hard braking. This is because the rider’s weight will naturally be behind the front contact surface. This is why cross-country enduro and downhill bikes have long front centers.
For a given rear center length, a longer front center reduces the proportion of rider weight supported by the front wheel. This reduces front wheel traction unless the rider shifts their seat forward or the center of the rear wheel also becomes longer.
Definition: The horizontal distance from the center of the bottom bracket to the rear axle (staystay length).
Since the center of the front wheel is usually much longer than the center of the rear wheel, mountain bikes tend to have a natural rearward weight distribution. This can be counteracted if the rider consciously puts pressure on the bar, but it can be tiring and takes practice.
With all of the rider’s weight on the pedals, the ratio of the center of the rear to the total wheelbase determines the weight distribution front and rear.
The rear center of a typical mountain bike is about 35% of its wheelbase, so before the rider puts weight on the handlebars, the “natural” weight distribution is 35% front and 65% rear.
A front wheel with a weight of 50% or more is usually ideal for cornering, so bikes with a shorter center wheelbase in the rear must apply more traction pressure to achieve this.
On steeper descents, the weight distribution becomes more forward anyway, especially under braking, so this is most relevant for flat corners.
The resulting longer rear center makes it easier (with less fatigue) to achieve a more balanced weight distribution, which is good for front wheel traction at straight corners.
However, the longer the rear center, the more weight the rider must carry (using a bottom bracket) to lift the front wheel. So a shorter rear center reduces the amount of manual work, but increases the amount of work needed to properly load the front wheel through the handlebars.
Definition: horizontal distance between front and rear axles or contact surfaces; the sum of the rear center plus the front center.
It is difficult to determine how the wheelbase affects handling. Since the wheelbase consists of a rear center section and a front center section (the latter in turn is determined by reach, head angle and fork offset), different combinations of these variables can produce the same wheelbase but different handling characteristics. .
In general, however, the longer the wheelbase, the less the rider’s weight distribution will be affected by braking, incline changes, or rough terrain. In that sense, a longer wheelbase improves stability; there is a larger window between when the rider’s weight is too far (above the handlebars) or too back (the loop). This can be bad, as manual or bow twist requires more effort.
There is also a downside to tight corners. The longer the wheelbase, the more you need to turn the handlebars (this is called the handlebar angle) to get the bike through a given radius of turn.
In addition, the difference between the arcs that the front and rear wheels go through will be greater. This is why long wheelbase vans tend to pinch their rear wheels on the inside of corners. Of course, mountain bikes don’t turn the same way as vans or even motorcycles – the rear wheel can bounce or skid in tight turns if needed.
The higher the bottom bracket height, the higher the rider’s center of gravity, so the bike leans more easily when hitting bumps, hard braking, or steep climbs. In that sense, a bottom bottom bracket improves stability in the same way that a longer wheelbase does.
Ironically, the bottom bracket also makes the bike more agile in corners. When the bike rests on a corner, it pivots around the roll axis (the line along the ground connecting the two contact surfaces). By lowering the rider’s center of mass closer to the roll axis, the rider’s weight drop decreases when the bike leans into a turn, and the rider’s momentum when changing lean angles (when turning from left to left), for example, is reduced. .
The height of the center of gravity of the rider and the bike above the roll axis is called the roll moment: the greater this distance, the slower the bike will change lean direction.
As a result, bikes with lower bottom bracket heights tend to get in and out of turns more easily.
Bottom bracket height is affected by suspension sag and dynamic ride height, so longer trips require a higher static bottom bracket height to compensate for the increased suspension travel. See the sections below on sag and dynamic geometry.
The disadvantage of a low bottom bracket is obvious: it increases the chance of catching on the pedals or sprockets on the ground.
It’s also worth remembering that the center of gravity of the bike and the rider is usually more than a meter above the ground, so lowering the bottom bracket by a centimeter (a quantity that greatly increases pedaling) makes a small percentage difference.
Definition: The vertical distance from the axle junction to the center of the carriage.
The drop of the bottom bracket itself is not as important as some might think. Some people see how the distance the bottom bracket hangs below the axle directly determines the bike’s stability in turns, as if the bike’s roll axis (the line that turns when leaning into a turn) is at axle height.
This argument is used in the marketing of 29″ wheels, claiming that the bike is more stable due to the bottom bracket being slightly lower (rather than higher) than the axle.
In essence, the rolling axis is – roughly speaking – a line connecting the contact surfaces of the tires. The important measurement for turns is the height of the center of mass above this line, not the height of the bottom bracket relative to the axis.
Installing smaller wheels will lower the height of the carriage, but will not affect the drop of the carriage. This allows the bike to change lean direction much faster because the bike and rider have a lower center of mass.
Interestingly, some bikes (like Pivot’s Switchblade) have height-adjustable “chips” to compensate for different wheel sizes. The bottom bracket height remains the same as the smaller wheel, but the bottom bracket height changes.
This resulted in a much smaller change in the bike’s handling, suggesting that bottom bracket height was important rather than bottom bracket drop.
However, dropping the bottom bracket is still a useful measure. BB height depends not only on wheel size, but also on tire choice – comparing bottom bracket drop between bikes for a given wheel size eliminates this variable.
First, the head tube angle affects how far the front axle is in front of the rider. All other things being equal, a looser head tube angle increases the front center, making the bike less prone to lean forward on steeper descents, but reducing the rider’s weight to front contact surface ratio. As a result, riders may have to push harder on the handlebars to avoid understeer in flatter corners with a lower head angle.
Post time: Nov-15-2022