Downhill bikes have slacker seat tube angles to preserve their slack head tube angles and short reach all while protecting the rider from flipping over the handlebars by putting most of the weight on the rear wheel.
Slack seat tube angles have downsides, but those negative properties are close to unessential when descending (the primary purpose of a downhill bike).
The table below contains the actual seat tube angles of popular downhill models.
If you are not familiar with the terms seat tube angle and effective seat tube angle, I recommend this post.
|Model||Seat Tube Angle|
|Commencal Supreme DH V5||67.1°|
|Santa Cruz V10||58°|
|Nukeproof Dissent Carbon 290||66°|
|Nukeproof Dissent Carbon 297||66°|
|Canyon Sender CFR 29||65.9°|
|Scott Gambler 900||66.8°|
|Scott Gambler 920||66.8°|
|Santa Cruz Megatower 2.0||77.2°|
|Trek Sessions 29 9.9||56°|
Conclusion: The average seat tube angle of a modern downhill bike is about 66°.
Meanwhile, the average seat tube angle of XC bikes is 74.9° and that of trail bikes is slightly steeper at 75.9°. (As a side note – the seat tube angle of modern road bikes is about 73°.)
This leads us to the question:
Why is the seat tube angle of downhill bikes so slack?
- The Front End Of The Bike Dictates The Entire Geometry
The front end of the bike is the primary focus and factor. The head tube angle of downhill bikes is very slack (small) E.g., 69° (The head tube angle is the angle formed by the ground and the head tube).
A slacker head tube angle has the following benefits:
- It’s easier to overcome obstacles because the wheel is further in front of the right and there’s less weight on it.
- A slacker seat tube angle is safer when descending due to the lower chance of flipping over the handlebars.
Thanks to those two properties a slacker head tube angle allows riders to descent extremely fast on off-road terrain in a fairly safe manner.
In different, XC bikes have steeper head tube angles which are better for climbing but not suitable for pure downhill terrain.
Normally, the slack head tube angles of MTBs are combined with steeper (<73°) seat tube angles to negate some of the downsides that a super slack head tube angle produces (e.g., difficult climbing, reduced maneuverability at slow speeds, extra weight on the rear wheel…etc.)
However, downhill bikes have a shorter reach (learn why).
The term reach refers to the horizontal distance from the center of the bottom bracket to the middle of the head tube.
The short reach of downhill bikes makes it non-practical to use the steep seat tube angles found on other MTBs because the rider will be too compressed.
In some cases, the rider’s knees may even touch the handlebars.
So, how do you negate that?
You move the saddle backward via a slacker seat tube angle.
The Downsides of Slacker Seat Tube Angles Are Negated
A slacker seat tube angle has a number of downsides (e.g., difficult climbing, more weight on the rear wheel, higher chances of looping out…etc.), but all of those are not essential in the world of downhill.
A downhill bike is a highly specialized unit made solely for descending. Climbing and even pedaling on flat roads is not optimal on a downhill bike. For that reason, downhill riders use lifts after a descent to get back up.
Also, downhill riders are not particularly concerned that they may “loop out”. (Looping out occurs when the rear wheel slides under and in front of the rider as it often happens during a failed wheelie attempt.)
It’s close to impossible to loop out during a descent due to the center of gravity. However, going over the handlebars is a real concern. For that reason, the head tube angle must remain slack at all costs. A slacker head tube angle is a safer head tube angle in this case.
The short reach that downhill bikes come with also provides benefits (e.g., keeping the bike nimble despite its longer wheelbase) that are more important than the downsides produced by an overly slack seat tube angle.
Note: The term wheelbase indicates the distance between the front and rear axle. Long wheelbase = stability at higher speed; shorter wheelbase = agility and aggressive cornering
Downhill Bikes Have Suspension With a Lot Of Travel
Finally, it’s necessary to mention that downhill and freeride bikes come with massive suspension that has the most travel out of all MTBs.
The rear suspension has about 220mm travel whereas the fork has 200mm. (Of course, the numbers depend on the model.)
To accommodate the rear shock, the frame is often made with a slacker or even a curved seat tube that points in front of the bottom bracket. The result is a slacker actual seat tube angle.
Summary: What You Need To Know
- Downhill and freeride bikes have the slackest seat tube angles of all MTBs at about 66°.
- The main purpose of the slacker seat tube angle is to accommodate the slack head tube angle which makes descending on off-road terrain faster and safer. By keeping the angle slack, the rider remains in a safer position that lowers the chances of flipping over the handlebars.
- The slacker seat tube angle of downhill bikes makes it possible to keep the short reach of the frame and the slacker head tube angle.
- Since downhill bikes are used for descending, the downsides of slacker seat tube angles such as difficult climbing, looping out, pedaling comfort…etc. are irrelevant.