A 160mm fork can be installed on a 120mm hardtail only when the head tube is strong enough to endure the additional stress coming from the extra length.
If the head tube doesn’t have the needed integrity, it can crack or completely fall off.
A 160mm fork will also change the geometry of a 120mm bike to the point where the switch will be of questionable usefulness.
Longer Fork = Extra Leverage = Extra Stress
The longer the fork is, the greater its leverage against the head tube becomes. During regular pedaling, this isn’t a major issue because the stress on the head tube isn’t enormous.
However, if there’s an impact (e.g., landing a jump) the stress that the fork can exert at the head tube junction elevates massively. If the frame isn’t strong enough, it can crack. Sometimes, the head tube may completely “rupture”.
When that happens a fall is guaranteed as the rider completely loses control over the bike and may collapse face-first on the ground.
Slacker Head Tube Angle = Even More Stress
The head tube angle (HTA) of a bicycle is essentially the angle formed by the head tube and the ground. The slacker (smaller), the head tube angle is, the more stress the fork can exert at the head tube junction.
A slacker head tube angle increases the exerted force because the fork has better leverage to pull the head tube away from the frame.
In different, the steeper the head tube angle, the less stress the fork can produce against the head tube.
Changes To The Bike’s Geometry
Switching from 120mm of travel to 160mm will result in the following changes to the bike’s geometry:
- Slacker Head Tube Angle
The longer fork will elevate the front and thus slacken the head tube angle.
In general, 20mm of extra travel would slacken the head tube angle by about 1 degree.
Consequently, a switch from 120mm to 160mm will make the head tube angle 2 degrees slacker.
A slacker head tube angle is beneficial for aggressive off-road descents because it positions the front wheel further in front of the rider and makes it easier to overcome obstacles. Also, it’s much harder to go over the handlebars because the center of mass is closer to the rear end.
The downside is that climbing becomes more difficult for the following reasons:
- More weight on the rear wheel makes it harder to maintain a high cadence while climbing.
- Reduced front wheel traction
- The more vertical back angle adds additional drag hurting the rider’s pedaling efficiency.
- Worse maneuverability at slow speeds
- Slacker Seat Tube Angle
The longer fork will slacken the seat tube angle too. (The seat tube angle is the angle between the seat tube and a horizontal line passing through the bottom bracket.)
A slacker seat tube angle is considered a downside because it places the rider even closer to the rear wheel and thus hurts climbing and pedaling efficiency.
- Taller Bottom Bracket
The higher front end will elevate the bottom bracket too.
In general, 1-degree of head tube angle slackening achieved by lifting the front end results in 7mm of bottom bracket elevation. In this case, the bottom bracket is expected to raise about 14mm.
This is a significant increase that many people will notice. A taller bottom bracket increases the clearance between the frame and the ground but creates instability because the center of mass is higher.
The cornering responsiveness of the bike will be hurt too.
- Longer Wheel Base
A longer fork will slacken the head tube angle and position the front wheel at a greater distance from the rider. As a result, the wheelbase of the bike will increase. (The wheelbase is the distance between the front and the rear axle.)
A longer wheelbase improves the bike’s stability at the expense of maneuverability and cornering.
Is The Transition From a 120mm Fork To a 160mm Fork Worth It?
This switch makes sense only when the following conditions are met:
- The frame is designed for aggressive riding and is strong enough to handle the longer fork.
- The rider wants to improve the bicycle’s performance on off-road downhill.
In all other cases, the switch isn’t very logical and can be dangerous.
If you plan on using your bike for technical climbs, a good fork with 120mm of travel is a better choice because it will keep the head and seat angle steeper.
A steeper head tube angle is better for climbing for the following reasons:
- Snappiness (a steeper HTA makes the bike a lot more responsive. Hence the very steep HTAs on BMX bikes.)
- More front-wheel traction
- Less weight on the rear wheel
- More aerodynamic position
Summary: What You Need To Know
- More travel makes the fork longer. The longer the fork is, the more it stresses the head tube upon impact because it acts as a longer lever with a greater mechanical advantage.
- The slacker the head tube angle is, the more stress the head tube has to endure.
- A fork with 160mm travel can be installed on a 120mm hardtail only when the head tube is strong enough to support the additional stress that the longer fork will generate. If that condition isn’t met, the switch should be aborted to avoid accidents and preserve the frame’s warranty.
- The additional 40mm of travel will slacken the head and seat angle by about 2 degrees while elevating the bottom bracket by approximately 14mm.
- The slacker head tube angle will improve the bike’s performance on off-road downhill but will harm the bike’s climbing capabilities.
- A fork with 120mm of travel works better when the goal is to conquer technical climbs as it keeps the head angle steep which in return results in a more responsive steering.