What is seat tube angle?
Traditionally, seat tube angle (STA) is defined as the angle between the seat tube and a horizontal line running through the bottom bracket.
In simpler terms, this translates to the angle between the seat tube and the ground (See Figure 1).
This affects the center of gravity and the distribution of weight on the wheels.
Steeper STA improves pedaling efficiency but makes steering more difficult and worsens stability as weight is shifted to the front of the bike (example: Triathlon Bike).
A slacker STA is more stable but creates inefficient pedaling (example: downhill mtb).
(Figure 1) Seat tube angle = the position of the saddle relative to a horizontal line through the crank axis of the bicycle
However, nowadays there are 3 additional ‘types’ of Seat Tube Angles:
Effective Seat Tube Angle (ESTA)
“Actual” Seat Tube Angle
“Virtual” Seat Tube Angle
Slack vs Steep
“Steep” angle moves the saddle forward = larger or ‘steeper’ angle
“Slack” angle moves the seat backward = smaller or ‘slacker’ angle
ESTA vs sta
The Effective Seat Tube Angle (ESTA) is similar to the traditional seat tube angle (STA) but with a few important differences:
Seat Tube Angle (STA) = Fixed Geometric Measurement
The Seat Tube Angle (STA) is a fixed geometric measurement of the frame, whereas the Effective Seat Tube Angle (ESTA) changes when the saddle position is moved
Instead of measuring the angle through the seat tube, the angle is measured from the bottom bracket/crank axis to where your butt makes contact with the saddle (ischial tuberosities or “sit bones”)
The Effective Seat Tube Angle Is Adjustable
This angle is adjustable. Moving your seat forward/backward will alter the effective seat tube angle. Also, if you are riding with a dropper post, the ESTA will be different when fully extended vs middle or dropped position.
The ESTA and STA, especially for mountain bikes, will have a significantly different angle. Keep in mind that the ESTA of an MTB is calculated to a “certain” saddle position, which means this is probably not the saddle position that you ride in, resulting in a different ESTA than the one listed.
Virtual Seat Tube Angle & "Undisclosed Saddle Height"
Since bicycle manufacturers measure Effective Seat Tube Angle from the center of the bottom bracket up to an undisclosed saddle height (reference point)
This angle is sometimes called Virtual Seat Tube Angle, since the reference point measures a “virtual saddle height”
Is virtual Seat Tube Angle the same as Effective Seat Tube Angle?
No. Virtual angle is based upon an unknown reference point whereas effective angle is based upon a known saddle height in relation to the bottom bracket
Actual Seat Tube Angle
(Figure 3) The seat tube does NOT intersect the bottom bracket
Actual STA: No BB & Seat Tube Interaction
‘Actual’ Seat Tube Angle pertains to mountain bikes or any bike where the seat tube and bottom bracket don’t intersect:
‘Actual’ Seat Tube Angle = the angle of the seat tube on the frame
The seat tube doesn’t actually connect to the bottom bracket as the seat tube isn’t straight (see figure 3)
Unless you are riding a mountain bike with the saddle fully extended or dropped, the ESTA is a more accurate measurement
Seat tube angle affects
5 seat tube angles: between 59 (E) and 99 (A) degrees
Steeper STA & Improved Aerodynamics
Increasing the Seat Tube Angle can decrease your torso angle, reduce wind resistance, and improve aerodynamics. These are all reasons why triathlon bikes are engineered with steeper Seat Tube Angles.
Road Bike STA = 72° to 76°
Triathlon bike STA = 78° to 82°
XC Mountain Bikes STA = the low to mid-70s°
Downhill Mountain Bike ESTA= low 60s°
STA & Muscle Activation
An increased Seat Tube Angle alters your knee angle, which results in muscle strength and contraction changes:
Increased power-output = improved climbing and sprinting
Less muscle fatigue = Improved pedal efficiency
Improved facilitation of the bike-to-run transition for triathletes
Able to maintain the same power-output while significantly reducing the muscular activation of the hamstrings. This allows the triathlete to maintain their normal running gait after cycling.
“A steeper STA can reduce the load on lower extremity muscles which might enhance pedaling efficiency at STAs between 59° to 89°”
Mountain Biking & Seat Tube Angle
Modern Cross-Country style mountain bikes with dropper posts are designed with steeper angles, which places you further forward while in the saddle.- This makes climbing easier since more of your body weight is transferred to the front wheel. However, descending becomes more challenging, unless you have a dropper post to get your seat out of your way.
Slack vs Steep: Seat Tube Angle Terminology
Steep STA & Pedal Power
A steep Seat Tube Angle will place you on top of the pedals for optimal pedaling. This is why an XC-MTB will have a steeper STA compared to the slacker (smaller angle) STA of a downhill mountain bike.
Descending with a steep STA is challenging…..so if you ride technical terrain, you better have a dropper post.
“Slack” STA of 72 degrees vs “Steep” STA of 82 degrees - (Figure 4)
Steep vs Slack
Steep = Larger angle = put pelvis further forward relative to the crank = optimal pedaling (‘steep’ angle moves the seat forward)
Slack = smaller angle = more stretched out = better for descending technical trails (‘slack’ angle moves the saddle backward, which is why downhill MTB have ESTA in the low 60s°)
“Because your seat tube angle determines where your saddle is in relation to your top pedals, it has an enormous effect on the way you pedal your bike”
Final Thought
Bike geometry can be overwhelming. Even if you fully grasp STA, simply adjusting your saddle forward increases your ESTA, thus modifying the distance between your handlebars and saddle, which alters your existing bicycle geometries.
Understanding the basic premise of the seat tube angle is important. Knowing this concept allows you to purchase a bike or adjust your current bike to best suit your style of riding. However, bottom bracket height, chainstay length, and especially head tube angle all affect the way your bike feels.
The only true way to understand bicycle geometry is to live it…..go out and ride.
Jesse is Director of Pedal Chile and lives in Valdivia, Chile (most of the year). Jesse has a Master of Science in Health & Human Performance and is an avid MTBer, snowboarder, reader of narrative non-fiction & taster of craft beers.
Sources:
Chen, Chia-Hsiang et al. “The effect of bicycle seat-tube angle on muscle activation of lower extremity.” (2015).
Cheung, Stephen S, and Mikel Zabala. Cycling Science. Champaign, Il, Human Kinetics, 2017.
Duggan, Will et al. “Effect of Seat Tube Angle and Exercise Intensity on Muscle Activity Patterns in Cyclists.” International journal of exercise science vol. 10,8 1145-1156. 1 Dec. 2017
Lopes, Brian, et al. Mastering Mountain Bike Skills. Champaign (Il) ; Windsor (On) ; Leeds (Ls), Human Kinetics, 2017.
Ricard, Mark D et al. “The effects of bicycle frame geometry on muscle activation and power during a wingate anaerobic test.” Journal of sports science & medicine vol. 5,1 25-32. 1 Mar. 2006
(Figure 4)
Silder, Amy, et al. “Influence of Bicycle Seat Tube Angle and Hand Position on Lower Extremity Kinematics and Neuromuscular Control: Implications for Triathlon Running Performance.” Journal of Applied Biomechanics, vol. 27, no. 4, Nov. 2011, pp. 297–305.
Umberger, B. R., et al. “DIFFERENCES IN POWER OUTPUT DURING CYCLING AT DIFFERENT SEAT TUBE ANGLES.” Medicine & Science in Sports & Exercise, vol. 30, no. Supplement, May 1998, p. 81.