This post investigates the relationship between different seat tube angles and knee pain.
First, let’s begin with the definition of a seat tube angle.
The seat tube angle (STA) is the angle between the seat tube and a horizontal line (parallel to the ground) running through the bottom bracket. (image below).
When we’re examining the effect of the seat tube angle on the knees, it’s more useful to look at the so-called effective seat tube angle as it reveals more information about the true geometry of the bike.
The effective seat tube angle is formed by a horizontal line passing through the bottom bracket and a line passing through the bottom bracket and the middle of the junction where the seat meets the seat post. (image below)
The Seat Tube Angle’s Effect On Knee Position
A steeper (larger) seat tube angle puts the rider closer to the front wheel; a slacker (smaller) seat tube angle positions the rider closer to the rear wheel.
An extra steep seat tube angle is more likely to cause knee pain/discomfort than an extra slack one.
Why?
A steeper seat tube angle opens up the hip angle (the angle between the torso and the femur) by pushing the hips forward.
The more open the hip angle is, the less the posterior chain muscles (glutes and hamstrings) can contribute to the pedaling motion as they’re already shortened.
Meanwhile, the knee angle gets smaller and the knees have to travel further past the toes.
The smaller the knee angle, the more the quadriceps have to work. This outcome isn’t bad in and of itself. The downside is that there is also extra stress on the knee joints. If the rider has a history of knee pain, the additional stress could cause irritation.
The effect is amplified when the rider has long femurs. The longer your femur is, the more your knees have to travel past your toes at the top of the pedaling stroke.
Once again, this isn’t inherently bad when the joints are healthy, but the increased strain could stress the patellar tendon and other connective tissues near it.
Reducing Knee Stress Via The Effective Seat Tube Angle
The options are:
- Get a frame with a slacker seat tube angle
If the seat tube angle of your current frame is too steep (and the rest of the recommendations here don’t make a difference), the only option is to replace the frame with one that has a steeper STA.
- Slacken your effective seat tube angle via the saddle
The saddle’s position directly affects the effective STA of the bike. Moving the saddle forward steepens the seat tube angle; moving it towards the rear wheel slackens the angle.
In this case, moving the saddle backward could be a solution. The “layback” placement will close the hip angle and open up the knee angle.
As a result, the glutes will have more work to do at the expense of the quadriceps. The knees won’t protrude as much over the toes and will experience less stress.
The downside?
The smaller hip angle will stretch the rider and put more stress on the back and potentially the perineum.
This happens because the slacker seat tube angle lengthens the effective top tube angle.
The effective top tube is the horizontal distance from the point where the top tube and headtube meet and an extended line from the seat tube. The term has become widely used due to the popularity of sloping top tubes.
The lengthened effective top tube can be “countered” with a shorter stem.
- Slacken The Effective Seat Tube Angle With a Set-back Seat Post
You can also get a seat post with a curve. Those models slacken the effective seat tube angle by positioning the saddle closer to the rear wheel.
Additional Factors That May Be Behind The Knee Pain
It’s entirely possible that the knee pain comes from other issues rather than the seat tube angle.
Those would be:
- The frame is too small
A frame that’s too small for you will prevent your joints from extending optimally and can cause excessive stress.
For that reason, people avoid commuting on bicycles such as dirt jumpers because those models are not designed for optimal and comfortable pedaling.
- Poor Bike Fit
Even if you have the perfect frame for you, a poor bike fit can make it feel painful. You don’t need a USD 200 bike fit done by experts (although it wouldn’t hurt), but it’s necessary to examine the seat height, the stem length, and the crank length to ensure that your bike fits you well.
The seat height is the most important value. If it’s too low, the knee angle closes again and there’s more stress on the knee joint.
- Pedals and Shoes
Clipless pedals are very efficient, but they are also known to cause knee pain because:
- The joints are restricted.
- You are pushing through the ball of the foot.
If the pedals are the problem, the solutions are:
- Experiment with another model
- Use another foot retention method (cages, straps, clips without straps…etc.)
- Flat pedals without retention (the most knee-friendly option).
Summary: What You Need To Know
- An overly steep seat tube angle increases the stress on the knees by reducing the input from the glutes and closing the knee angle. The steeper the seat tube angle, the more the knees travel past the toes. This isn’t inherently bad, but in some cases, the extra stress causes pain.
- By slackening the seat tube angle one can reduce the stress on the quads and knees. Aside from getting a new frame, this can be accomplished by sliding the saddle towards the rear wheel and/or getting a “layback seat post”.
- The slacker seat tube angle will increase the effective top tube and thus put more stress on the rider’s back. If this is a problem, a shorter stem will help.
- A small frame or an improper bike fit could also cause knee pain.
- Clipless pedals stress the knees too.
