Condensed Answer: A downhill fork can be mounted on an enduro frame. The extra travel of the fork, however, will increase the stress on the head tube and change the geometry of the bike.
If the new travel exceeds the conditions of the frame manufacturer, the warranty will no longer be valid due to the possibility of cracking the head tube.
Head Tube Stress
The steerer of the fork passes through the frontal tube known as head tube.
The greater the travel of the fork (compression and subsequent decompression), the higher the stress on the head tube and the welds connecting it to the down and top tube.
Frames designed for long-travel forks come with reinforced head tubes. The reinforcement includes larger and stronger welds. Sometimes manufacturers add gussets or an additional diagonal tube connecting the head, top, and down tube.
If a long-travel fork is installed on a frame designed for less travel, the head tube can crack or break off the frame and cause an accident. For that reason, manufacturers would not respect the warranty when the user relies on a fork exceeding the permitted travel.
Downhill forks have about 200mm of travel whereas enduro forks max out at about 180mm. Therefore, an enduro frame is technically not suitable for a downhill fork by default.
Additional Downsides To Installing a Downhill Fork On An Enduro Bike
- Changes To The Geometry
Double-crown forks are longer and consequently raise the front end of the bicycle. This change results in three modifications to the bike’s geometry:
a. Slacker Head Tube Angle
The longer fork will slacken the head tube angle. A slacker head tube angle makes it easier to overcome obstacles when going downhill but has a negative effect on technical maneuverability and climbing.
b. Steeper Seat Tube Angle
The raised front end will push the seat back and steepen the seat tube angle. As a result, more of the rider’s weight will be over the rear wheel. The new position will make pedaling uphill harder.
c. Higher Bottom Bracket
The bottom bracket of the bike will get higher too. The extra clearance between the frame and the ground will reduce the chance of hitting an obstacle on the trail, but the higher center of mass will result in instability. (Another bonus is the lower likelihood of hitting the ground with the pedals a.k.a. pedal strike).
In extreme cases, it may also become difficult for the rider to reach the ground with their feet while sitting on the saddle.
- Extra Weight
The second crown as well as the extra length of the fork result in more weight. In some scenarios, the extra weight could be over 1kg/2.2lbs.
- Limited Steering Angle
A double-crown fork reduces the maximum steering angle because the fork’s upper portion is wide and can come in contact with the frame.
Additionally, during a crash, the fork can damage the frame due to contact between the upper legs and the front end of the bike.
FAQ: Is it possible to reduce the chance of damaging the head tube?
The only way to reduce the stress on the head tube when performing this conversion is to get a double-crown fork with travel very close to the maximum number allowed by the frame.
For example, if the frame is rated for 160mm of travel, a double-crown fork with 180mm of travel would technically put less stress on the head tube than a model with 200mm travel. Hence why dedicated double-crown enduro forks have about 180mm of travel (read more).
There are also older double-crown forks with 160mm of travel, but those models would defeat the purpose of this modification. Why use a double crown fork when a single-crown can offer the same or more travel for less weight?
Another option would be to get a custom frame with a head tube reinforced to the maximum. That route, however, will not be cheap.
What Are The Advantages of Installing a Downhill Fork On an Enduro Frame?
The main technical motivation to install a downhill fork is to benefit from the fork’s extra travel and stiffness. Those two qualities allow the rider to cover harder off-road terrain with greater precision.
Another reason would of course be the appearance of the bike. Downhill forks look “serious” and some people install them even on hardtails for the looks.
Summary: What You Need To Know
- Only enduro frames that are “downhill certified” can accept a dual-crown fork safely and without voiding the warranty. This greatly limits the available options because most enduro bikes aren’t made for pure downhill. Consequently, they aren’t overbuilt enough to sustain the stress that a double-crown fork would put on the frame.
- A dual-crown fork will slacken the head tube angle, raise the bottom bracket and position the saddle too far back. As a result, the bike loses technical maneuverability and climbing ability.
- A dual-crown fork adds additional weight and can easily come in contact with the frame during a fall and damage it.
- A dual-crown fork is stiffer than a single-crown fork and doesn’t creek.
- A single-crown fork with lots of travel allows the rider to cover downhill terrain and further diminishes the incentive to install a dual-crown fork on a frame that isn’t designed for one.
- Many people want a dual-crown fork for the looks rather than the performance.