Condensed Answer: A downhill fork can be installed on a hardtail only when the frame is strong enough to handle the travel (compression) of the fork.
If the frame isn’t specifically built for long-travel forks, then the geometry of the bike will be dysfunctional and the head tube can literally rip off the frame.
Head Tube Stress
The head tube is the frontal tube through which the fork’s steerer passes. Consequently, the head tube receives a lot of stress from the front wheel.
The strength of the head tube depends on three main factors:
- Frame geometry
Long-travel forks stress the head tube more. For that reason, one shouldn’t neglect the travel limitations provided by the frame manufacturer.
For example, if the frame is designed for 100mm travel, and you put a fork with 160mm travel on it, the new unit will alter the geometry of the frame and subject the head tube to a non-tested level of stress.
The outcome could include a cracked head tube. In some cases, the entire head tube may rip off the bike and cause a horrific accident.
Dual-crown forks offer around 200mm of travel which is significantly more than basic forks (100-120mm) as well as long-travel single-crown forks which usually go up to 180mm.
If the frame in question isn’t explicitly listed as a unit supporting long-travel forks as well as dual-crown forks, then the frame isn’t safe to use with a dual-crown fork.
- Extra-slack Head Tube Angle
The head tube angle is the angle between the head tube and the ground. A slacker head tube angle (raised front end) increases the bike’s stability when going downhill but hurts the handling, especially at slow speeds.
A double-crown fork will greatly slacken the head tube angle and hurt the bike’s climbing ability too.
- Higher Bottom Bracket
The bottom bracket of the bicycle will also be raised by a notable amount. A higher bottom bracket increases the frame’s clearance and reduces the chance of hitting an obstacle on the trail but makes the bike less stable due to the higher center of gravity.
Or in simpler words, the maneuverability of the bike could feel “weird”.
A dual-crown fork isn’t cheap and can easily reach the price of an entire hardtail. The fork won’t be the only expense, however.
Since many dual-crown forks require a 20mm axle, it may also be necessary to replace the hub on the existing wheel.
- Limited Steering
In some cases, the maximum steering angle of the bike will be limited because the fork will hit the frame when turning.
- Limited Frame Choice
The number of rigid frames that can handle a dual-crown fork is very slim. In fact, one would be hard-pressed to find a manufacturer that officially lists a hardtail frame as compatible with a dual-crown fork.
Truth be told, most people who install a downhill fork on a hardtail, use rigid frames that can support single-crown forks with at least 180mm of travel and do so at their own risk.
If a dual-crown fork is used on such a frame, the frame warranty would be voided.
- Extra Weight
A double-crown fork has more parts and is therefore heavier. Of course, there are exceptions to this rule.
- Low Benefit When Combined With a Hardtail
The main purpose of a suspension fork is not to provide extreme comfort but to increase the front tire’s traction with the ground.
When the wheel meets an obstacle, the fork compresses and keeps the tire on the ground. Without suspension, the wheel would bounce off the irregularities and lose traction.
Forks with larger travel increase the size of the bumps that the wheel can overcome and provide additional cushioning upon landing. Consequently, the rider can cover extreme off-road terrain while maintaining high speed.
However, when the rear end isn’t suspended, the effectiveness of the fork is reduced because the rear part of the bicycle bounces around and requires the rider to either exert more effort to control it or to slow down.
Or in other words, the rear of the bike acts as a “bottleneck” preventing the fork from offering its full capabilities.
The Benefits Of Installing A Double-Crown Fork On a Hardtail
Truth be told, there are no tangible benefits to installing a double-crown fork on a hardtail other than unique appearance and street cred.
The travel of the fork is past the point of diminishing returns when installed on a hardtail frame.
Maximum effectiveness can be achieved by using a frame designed for a long-travel fork and combining it with a single-crown fork with 160-180mm of travel.
The extra 20mm or so of travel that double-crown forks provide will make no practical difference when the rear of the bike is rigid.
FAQ: If double-crown forks on hardtails are a no-go, why does this combination exist?
A couple of reasons come to mind, namely:
The vast majority of people who run this combo are children or teenagers who want a “real” MTB bike, but can’t afford one. This combination is also popular in the developing parts of the world.
- Coolness Factor
Hardcore hardtails are a statement. Some people build them up not because they can’t afford a full-suspension bike but to look different.
Summary: What You Need To Know
Technically, it’s possible to install a double-crown fork on a hardtail, but the practice is not a good idea for the following reasons:
- The vast majority of hardtail frames are not strong enough to survive the 200mm travel of a DH fork. Consequently, the head tube of the bike may crack or break off the frame.
- Most of the frames that can support a double-crown fork are old and hard to find. Two examples would be the legendary .243 and Banshee Morphine models. Those frames are extremely strong, but they are also expensive and scarce.
- The rigid rear end of a hardtail acts as a bottleneck preventing the bike from covering extreme off-road as fast as double-crown forks would otherwise allow.
- A double-crown fork makes extreme changes to the geometry. It slackens the head tube angle, raises the bottom bracket and throws the seat too far back. As a result, the bike becomes difficult to maneuver in tight spots. For example, making a simple U-turn will require a lot of room. Climbing will also become close to impossible.
- A single-crown fork with lots of travel (160-180mm) can be installed on a great number of hardtails and suits the purpose of a downhill hardtail better.