Bike Geometry 1
A traditional steel bike is made of 12 tubes - 9 tubes for the frame and 3 more for the fork. (3 more if you consider the stem, which is part of a VELOPILGRIM MOKSH frameset.) The shortest of the 9 tubes in a frame is the Bottom Bracket. The remaining tubes are arranged at specific angles and distances in relation to the Bottom Bracket. This arrangement defines the bike's geometry.
The bike geometry influences everything on the bike from the bike's handling, how comfortable the bike is, how much power you can produce, how aero the rider is, to the bikes aesthetics and proportions. But while designing a custom bike, the 2 primary criteria considered are: (1) Establishing the 3 contact-points of the rider that facilitate the most efficient use of the riders muscles. a.k.a. Bike-fit (2) Design a bike that best handles the intended use case scenario of the rider. If these 2 design criteria are met, it becomes the most important consideration for the rider. Even more than the material used to construct the frame. In this post, we'll only cover the 1st objective:
How geometry decides a rider's fit.
Every rider has a different height, weight and or other physical attributes. So a few measurements needs to be made of the rider to make a road bike that fits the rider like a glove.
At VELOPILGRIM, we take the following measurements of a rider for the MOKSH road bike:
2) The Torso length of the rider (top of Pubic Bone to top edge of Sternum)
3) The riders Upper Limb (Acromion to Wrist fold-center)
4) Weight of the rider
(Additionally, we need the the shoe size & shoulder width. But let's keep those for another post.) The length of the riders torso and upper limbs help decide the reach while the riders inseam helps decide the stand-over height (especially when a bike like the MOKSH has a flat top tube.) A combination of the above 2 helps decide the stack height.
Now, to match the above measurements on to a bike that fits well, these are the key measurements from the bike geometry that needs to be considered.
1) the Seat Tube (ST) length,2) the Head Tube (HT) length
3) the ST angle
4) the Saddle set-back
5) the Top Tube (TT) length and
In a very basic manner, the riders Stack measurement can be translated on to the bike frame, by adjusting the length of the ST and HT. And the Reach, by adjusting the TT length, Saddle Set-Back and Stem length.
Here we are not only trying to make sure the bike's contact points are fit for the rider in an anatomically correct way, but also other considerations like positioning the riders' weight optimally between the 2 wheels or making sure there is no toe-overlap with the front wheel. Another consideration could be the weight distribution of 2 riders of the same height may vary based on their inseam v/s torso split as well as their body type. So depending upon the situation, the Seat tube angle and the saddle setback can also be adjusted to fit the rider correctly. Once all these measurements are decided, the frame builder then works out the rest of the dimensions of the bike.
What happens when the fit is not right?
When even minor imperfections exist in the fit of a bike, the rider's body compensates by using other muscles for support. Muscles which are not meant to be used for the intended action. Riders do not realize this overuse during rides. But it may be felt as recurring pain. Over extended periods of times, ignoring this pain or not correcting the posture may even result in long - term injury. This is because a long distance cyclist spends a huge proportion of time doing repetitive motions - the upper body acts as an extension of the bike to maintain balance, while the lower limbs complete close to 500 rotations per km. That's more than 5 million rotations a year for many randonneurs or endurance racers. In between these repetitive actions, a correct bike fit can help the body work seamlessly, at full potential, while a wrong fit can break the body down.
So what about off-the-shelf bikes?
In an off the shelf frame, all geometry parameters are set in stone. Most of todays' Big Name bands were startups, long time ago. They had small production lot sizes and lot of customization options back then. As sales increased, manual labor (and customization) gave way to mechanization. Nothing wrong with that. But, in a factory environment, mass production can be done only when a lot of parameters are defined and controlled. Which means less customization and only limited ways to change the fit for different riders of similar height. This is the equivalent of being handed over a compromise in exchange for the money you pay.
But in the case of a custom frame - the rider's dimensions are taken 1st - So the rest of the frame is built around these parameters. This also involves compromises in the overall design as the riders measurements cannot be changed. The frame builder decides on the rest of the parameters based on the use case scenario of the bike, riders preferences and the builders experience. This is the equivalent of being able to chose the compromises that we like rather than having no choices at all. For example a rider with a short torso (and thereby a frame with a short top tube) will have to decide between stable handling, toe clearance or max tire size. But the said rider will not have to battle with reach issues in the 1st place. So in my humble opinion, a custom bike is the best way to practice and enjoy our beloved sport for many, many years.
Bike Geometry - https://www.daccordicycles.com/
Vitruvian Cyclist - https://bikeon.tumblr.com/post/153898085597/blog-pedalnorth-com-legendary-regrannapp
Never Compromise - https://in.pinterest.com/pin/292452569546286598/
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