How Geometry Impacts the Ride of a Bike

Dec
5

Posted by Coach Adam Sczech

Ride a variety of bikes, and you will discover there are those you like and feel comfortable on, and those you don't. You can feel that a bike rides better for you, but have you ever wondered why? Even if we eliminate as many variables as we can between bikes (3 points of contact on the bike (fit), wheels/tires, shoe/pedal, saddle, and clothing), we will likely still have a preference for the ride of one bike over another.

 

Why is that?

 

The answer is geometry. Any fitter will tell you the geometry of a bike is important because it determines if a person will be able to ride a bike in a comfortable and efficient position. That is accurate, but geometry also explains how the bike will ride for an individual. A rider can get their 3 points in space (fit) in the exact same place on many bikes but have vastly different experiences on each. This is because of the more in-depth measurements of a bike, mainly the Stack, Reach, Head Tube Angle, Trail, Front-Center, Rear-Center/Chainstay, Wheelbase and Bottom Bracket Drop.

 

Let’s start with some definitions:

  • Wheelbase - The distance between the center of the Fork Dropout and the Center of the Frame Dropout
  • Stack - The distance between the center of the Bottom Bracket and the center of the top of the Head Tube perpendicular to the Wheelbase
  • Reach - The distance between the center of the Bottom Bracket and the center of the top of the Head Tube parallel to the Wheelbase
  • Bottom Bracket Drop - The distance between the center of the Bottom Bracket and the Wheelbase perpendicular to the Wheelbase
  • Head Tube angle - The angle measured behind the Fork by extrapolating the Head Tube to the Wheelbase
  • Trail - The distance between the center of the Steerer Tube and the center of the Fork Dropout parallel to the Wheelbase

http://community.terrybicycles.com/wordpress/wp-content/uploads/2011/02/frame_geometry.jpg

 

These measurements may seem complicated but are really quite understandable. The Wheelbase determines two points where you and your bike touch the ground. The Stack and Reach, when coupled with your fit, determine the center of mass on the wheelbase. Most road, tri and xc mountain bikes are designed to be most stable between a 60/40 - 70/30 weight distribution between the rear and front wheel (wheelbase). Too much weight back makes handling on the bike feel slow, while too much weight forward makes the bike “twitchy” and unstable. The other center of mass measurement is the Bottom Bracket Drop. The greater the drop the more stability because of the lower center of mass, but the lower the drop the less clearance between the pedal and the ground. It is important to remember a bike with a low bottom bracket will be more stable, but less fun in corners. 

 

The second set of measurements (Head Tube angle and Trail) describe the angle of caster, which are less obvious but more important to the handling of the bike. One way to understand angle of caster is to think of a typical shopping cart. The two front wheels are caster wheels, the axle of the wheels is perpendicular to, and offset from the steering axis. This is the same as how the front wheel of a bike is attached. The Trail of the bike is the big determiner of how a bike is going to handle, and is basically the distance from the axle of the wheel to the steering axis (Head Tube). Coming back to the shopping cart analogy, the wheel follows the steering axis (negative Trail) but it is possible to flip one of the front wheels around (positive Trail) and have it continue to go straight, however as soon as the cart starts to change direction the wheel will flip right back.  A bike with a big trail is very stable but difficult to turn, it may even feel “floppy” because when the wheel turns it want to turn all the way. While a bike with a small trail can be “twitchy” because small riding inputs will cause the wheel to turn, but it also takes little effort to correct the wheel. Fun fact: The Trail should be between 4 cm and 5 cm for a a nice neutral handling bike.

 

So, when looking for a new bike pay attention to the geometry to ensure it will ride like bikes you have had and enjoyed in the past. You can enlist the help of a certified bike fit professional to aid in the selection and fit.

 


Coach Adam Sczech is an IRONMAN Certified Coach, USAT Level I Certified Coach and a certified Bike Fitter based out of the Western Slope of Colorado. Adam has years of experience coaching beginners, juniors, elites, and clubs as well as a year focusing specifically on special needs athletes. Adam's expertise with bike fitting is extensive with over 15 years and 8,000 fits for athletes that include two world record holders, a national champion, several IRONMAN Pro/Age Groups winners, and an ITU winner. He has completed several full and half Ironman races, as well as numerous Olympic and Sprint races. Adam grew up competing in speed skating and hockey in the midwest before playing football in high school and college.  After an injury in college football he took up road cycling, and competed on the road and track until making the switch to triathlon in 2009. Adam can be reached via email at adam@teamMPI.com.

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