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Bike geometry Everything you need to know about it

18 November, 2020
Bike geometry Everything you need to know about it 1

Welcome the beginning of your cycling journey, you will not understand anything about bike geometry and its effect on ride performance. However, over time, you will begin to suspect something.

If you haven’t heard something like that before, now you’ll start. The information is accurate and verified.

A friend’s bike won’t help here. Because he’s just different.
The difference between frames and their geometries is so big., if all (or almost all) of the equipment migrates from one frame to another. Then the opportunity opens up to find out the difference between them, and then compare your feelings with the numbers on paper. 

Be sure, intuitively you will feel everything right. Controllability, speed gain, cross-country ability, landing, convenience of aggressive pedaling will change. The list of sensations is incredible in size.

If you can evaluate the parameters of the frame of absolutely any bike and not on your equipment, then it is unclear why you are reading this article) there will be nothing new for you)

So we suggest you to read someting new: Jose Maria Jimenez “El Chava” a cycling star from Spain

Bike geometry for dummies

Many have heard that the frame is selected by height and ETT parameter.
The frame size (seat tube length) is defined by a long straight line extending from the center of the BB assembly to the top of the seat tube. 

This method has large errors, since some bicycles have a built-in seatpost or a different angle of inclination of the seat tube. The overwhelming majority of bike manufacturers, especially in the middle and budget segments, indicate the size of the frame based on the length of the seat tube.

The ETT parameter is more often used by advanced users, it characterizes the effective length of the upper pipe. To calculate the ETT parameter, the bike frame must be set in the position in which it will ride. Further, from the diametrical center of the upper part of the head tube we draw a horizontal line to the diametrical center of the seat. 

You will always run into the seatpost, not the seat tube. Of course, this will be the case if all measurements are carried out correctly.
It is important to understand that ETT from different manufacturers in one size can differ by an amount from 0.5 to 5-6 cm.

This is a lot. You will sit upright on one frame and stretch out on the other. Of course, you can play with the stem length to compensate for the fit, but these frames will still ride in completely different ways.

Changing the effective length of the top tube entails a change in the wheelbase, which makes the bike softer on bumps and faster, but then its handling is sacrificed, for example. 

In the world of even more advanced cyclists, frame size and ETT are practically not taken into account, completely different values ​​are taken as a basis.

Yes, from this very moment we start talking about geometry itself.

Stack and Reach

Stack and Reach Bicycle
These two parameters are now very popular among advanced cyclists.

Stack 

  • Is the length of a vertical straight line going from the diametrical center of the top of the head tube down to an imaginary horizontal line going from the center of the bottom bracket.

Reach

  • Is the length of the horizontal straight line extending from the diametrical center of the top of the head tube to an imaginary vertical line exiting the bottom bracket assembly.

Yes, in fact, it does not really matter how you measure, starting from the bottom bracket or head tube. You will get the same parameters anyway.

They allow you to quickly assess your fit on a bike without in-depth study of its parameters. The frame dimensions and ETT do not give such an idea, they simply characterize the height of the seatpost and the length of the upper tubes.

Of course, one can argue that the landing can be adjusted using the height of the seatpost with and without offset, using the angle and length of the stem, the width of the handlebars, the presence or absence of the handlebar spacers, however, Stack and Reach are defining the basis, since these values ​​cannot be adjusted and are selected only once.

  • With a fixed Reach, increasing the Stack puts you in a more upright position and straightens your back.
  • With a fixed Stack, increasing Reach brings you into a more extended position, with your arms extended forward. 

Bike geometry: Agressive or Relaxed position?

That is, if you are looking to buy a new frame and find your fit too elongated and / or aggressive, then it makes sense to think about a higher Stack and a shorter Reach.
If the fit is too straight, then the Reach gets longer and the Stack gets lower.

Specialized Tarmac Bike geometry

As an example, we propose to consider the Specialized Tarmac, where in the 56 size the Stack is 565 mm and the Reach is 395 mm. It’s not clear yet, I know. Now let’s pay attention to the Specialized Roubaix in the same 56 size, it has a 605 mm Stack and 384 mm Reach.

Specialized roubaix Bike geometry

Now it becomes clear that the size, to which so many users appeal, does not play such a big role, if we talk about landing. From the numbers we can see that the Tarmac with the lower Stack has a more aggressive and extended fit. 

The Roubaix, on the other hand, offers a high riding position, which is not surprising since it is marketed as an endurance bike. That is, such a bike is designed for long rides, where an aggressive landing will take away extra energy, here the convenience and duration of screwing is important, and not their aggressiveness. It is not speed that comes to the fore, but comfort. 

Of course, all of these statements are true when using the same stems, handlebars and seatpost. With their help, landing on any bike can be made, both more elongated and more straight.

Confused?

A completely natural question arises – why start from Stack and Reach, if the fit can still be changed?

The point here is that when conceiving a bicycle with certain characteristics, engineers put in it certain stiffness parameters at different nodes.

It turns out that you took a hard Tarmac, but ended up landing higher. The ride will certainly become more comfortable, but the frame will still remain very rigid, it will hurt to ride. 

The Roubaix is ​​similar, with a softer frame on some knots to provide more comfort. Pulling the fit on it will make you faster, but some of the aggressive screwing energy will now be wasted on deforming the “flexible” frame members that the Tarmac lacks. 

In addition, stem length will inevitably increase, which will affect both the appearance of the bike and its weight. Even if not significantly, but still.

Controllability will not remain aside either, it will become worse. Depending on the parameters set by the manufacturer, it will become either more lethargic or more nervous.

Stack + and Reach +

The + prefix is ​​used by Canyon and allows you to get, in general, the same parameters, only now not from the top edge of the head tube, but from the top points of the steering wheel itself. While Reach and Stack are unchanged on a bare frame, Stack + and Reach + can vary depending on the handlebar installed, its height due to the spacer rings under the stem, and the stem angle.

Seat tube angle

Bike geometry 101: Learn why frame angles & trail matter | road.cc

Seat tube slope is measured using a horizontal line from the center of the bottom bracket and a line along the seat tube. The seat post that is inserted into the seat tube is not included in the measurement as it differs from model to model. In addition, there are pins with and without offset.

Seatpost offset is the amount of deviation of the saddle position from the horizontal of the seat tube. If you look at your bike, chances are the seat has some bend near the saddle itself, which is its offset. 

There is also zero offset so the saddle attachment point does not deviate from the horizontal of the saddle tube. The seat tube is usually angled about 73-74 degrees. 

Decreasing this parameter causes the saddle to move closer to the rear wheel, placing you “behind the carriage”.  Increasing the degree moves the saddle closer to the handlebars, allowing for a more aerodynamic landing as you will already be “in front of the BB”.

Large seat tube angles, around 80 degrees, are found on racetrack and triathlon bikes. They allow the back to be straightened as much as possible to provide minimal drag, the fit is very aggressive and promotes efficient twisting. True, it will be very difficult for an average cyclist to ride such a bike for a long time without some preparation.  

Head tube angle

Bike geometry 101: Learn why frame angles & trail matter | road.cc

The angle of the head tube is measured using a horizontal line through the wheel axis and a straight line through the center of the head tube.

On mountain bikes, the head tube tilt angle ranges from 62 to 73 degrees, road bikes are more often located at more than 70 degrees.

Decreasing this angle results in a longer wheelbase and softer handling, it becomes more lazy and relaxed. For a mountain bike, that’s it.
However, in order to keep the wheelbase at a certain level, some manufacturers shorten the upper tube of the bicycle.

When adjusting the fit, it is worth considering the tilt factor of the head tube. Let’s say it is 73 degrees and you add 25mm rings below the stem. The final handlebar height will change by about 24 mm, but it will move about 7 mm closer to the saddle.

Trail

Trail is measured between the center line of the head tube all the way to the ground and a straight line that extends vertically down from the wheel axle.  Trail depends on head tube angle, fork offset and wheel size. 

Fork offset is the amount by which the wheel axis is offset from the head tube axis.

Increasing the angle of the head tube, decreasing the fork offset and the wheel size leads to a decrease in the Trail parameter, which makes the bike much sharper in handling. 

Smaller Trail is more common on road bikes. The Extended Trail is used on mountain bikes and increases as the discipline becomes more aggressive. It makes the bike more stable in handling and forgives a lot of mistakes on the road.

Take the 56 frame size Trek Madone road bike as an example . It has a 73.5-degree headtube angle and a 40mm fork offset, which brings the Trail to 58mm.

Bike geometry

The Cyclocross Trek Boone has just 72 degrees in the head tube and 45mm fork offset, giving it a 67mm offset.  The difference of almost 11 mm dramatically affects the handling. The road bike needs more maneuverability, and the cyclocross needs to be stable and predictable in the absence of roads, so it has more Trail.

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The Trek Domane, for example, is an endurance road bike and doesn’t need the control of Madone, so the fork offset is 48mm and the Trail is 61mm.

Front center

Front Center is measured from the center of the carriage assembly to the center of the front axle. It should be borne in mind that on bicycles with different head tube angles and with different fork offsets, this parameter may not differ at all.  

With its help, it becomes clear whether your foot will cling to the front wheel or not. Of course, this is true for toothy tires and owners of large shoe sizes.

Bike geometry Chainstay Length

How to read a bike geometry table: the numbers made easy | road.cc
Source: Road cc

The length of the chainstays is measured from the center of the carriage to the center of the rear wheel. The shorter they are, the more efficient the bike is in terms of pedaling efficiency. This is due to the greater rigidity of the structure.

Chainstays are always shorter on road bikes than on mountain bikes. Although in the road bike environment, short chainstays are not always needed and these values ​​fluctuate depending on the discipline. 

The most aggressive ones have stays so short that engineers have to make a seat tube with a recess under the wheel. Of course, in such conditions, the tire size is very limited.

With MTB bicycles, everything is somewhat simpler, the feathers will fit huge tires along the length, but now there will be no more distance between the feathers.

Wheelbase

How to read a bike geometry table: the numbers made easy | road.cc
Bike geometry

The base is measured between the axles of the bike. Controllability and softness of the course depend on it. 

The frame can still be rigid. It just increases the smoothness of overcoming obstacles.  The shorter the wheelbase, the more nimble the bike becomes. The longer the base, the more stability in the run.

Now do you understand the importance of Bike geometry?

If we take a road bike and a gravel road for comparison, then the latter will have a larger wheelbase, since it needs control stability. 

Carriage height relative to ground

It is measured from the ground to the center of the bottom bracket and represents the bike geometry flotation.

That is, the height of the obstacle depends on its height, which you can overcome without taking off the wheel, without clinging to the stars of the connecting rods.
In addition, the height of the carriage angle characterizes the displacement of the height of the center of gravity. 

The lower it is, the more stable the bike is.
Well, of course, the taller the carriage, the more cornering tilt can be achieved without stopping pedaling, which is very important for road bikes.
It should be borne in mind that a change in the tire diameter leads to a change in the carriage height.

By: Berthy Perez