A 150mm fork can be installed on a 130mm bike only when the head tube is strong enough to handle the added stress.
The longer travel will hurt the climbing capabilities of the bike and can therefore be a major downside when commuting and gravel riding.
Why Do Longer Forks Stress The Head Tube More?
The fork acts as a lever exerting force against the head tube. As the length of the fork increases so does the stress generated at the head tube’s connection points.
The rider’s weight presses the frame down and with it the fork and respectively the rear wheel. As a result, the fork is constantly exerting pressure at the headtube with the front wheel as an input point.
The stress generated by the fork depends on the fork’s length and the head tube angle (HTA) (the angle between the head tube and the ground.
The slacker the head tube angle is, the more stress the fork can produce. Respectively, a steeper head tube angle diminishes the stress that the fork generates.
It’s also necessary to mention that MTBs face a lot of impacts (e.g., landing a jump) that augment the stress generated by the fork exponentially.
For that reason, the frames designed for a lot of travel have stronger head tubes.
Before installing a fork with 150mm of travel on your bike, consult the specifications of the frame to see if it can handle the stress. If you can’t find any, contact the manufacturer.
Changes To The Geometry
Switching from 130mm of travel to 150mm will result in the following geometrical changes:
Slacker Head Tube Angle
The additional 20mm will slacken the head tube angle by about one degree.
The advantages of a slack head tube angle are:
- The bike is more stable when descending on downhill terrain because the front wheel sits further away from the rider and can eat the hits.
- The rider is positioned closer to the rear wheel and doesn’t feel like he is about to flip over the handlebars.
Slacker Seat Tube Angle
20mm of front-end elevation slackens the seat tube angle by approximately 0.5 degrees. (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 negative for the following reasons:
- The rider is positioned even closer to the rear wheel. As a result, the center of mass becomes too imbalanced. The front-wheel traction is reduced, and the pedaling efficiency is hurt.
- The rear wheel becomes harder to spin due to the additional mass sitting on it.
To avoid the issues above, modern MTBs have moved towards steeper seat tube angles.
That said, the issue can be remedied by steeping the effective seat tube angle via the saddle.
Taller Bottom Bracket
The bottom bracket will be elevated by approximately 7mm.
A taller bracket gets the frame further away from the ground but hurts the bike’s stability and cornering capabilities due to the higher center of gravity.
Slightly Longer WheelBase
The term wheelbase refers to the distance between the rear and the front axle.
The slacker head tube angle of the bike will increase the wheelbase every so slightly.
A longer wheelbase provides stability but hurts the sharpness of the bike’s handling. In this case, the change is minimal.
Taller Stack Height
The elevated front end will increase the stack height of the bike by about 4mm. The stack height is the vertical distance between the bottom bracket and the top of the head tube.
Decreased Reach
The stack height increase will decrease the reach. The reach is the horizontal distance between the middle of the head tube and the center of the bottom bracket.
Climbing Will Be The Main Issue
A fork with 150mm of travel isn’t the most aggressive out there, but it will certainly have a negative impact on the bike’s ability to conquer technical hills.
This happens for the following reasons:
- A longer fork slackens the head tube angle and makes the bike less responsive. In different, a steeper HTA increases the bike’s responsiveness. Hence the very steep HTAs on freestyle BMXs.
- A longer fork positions the rider closer to the rear wheel and makes it more difficult to maintain a high cadence while conquering a hill.
- The distribution of weight closer to the rear wheel reduces front-wheel traction and greatly increases the chances of experiencing unwanted front-wheel lifts on technical climbs. When that happens, the rider loses control over the bike.
- The riding position requires a more vertical back angle which increases the drag created by the torso.
When Is This Transition Worth It?
Going from 130mm to 150mm of travel is logical only when the following conditions are met:
- The head tube is strong enough to handle the increased pressure.
- The rider wants to cover more aggressive off-road sections and doesn’t mind the decreased climbing capacity of the bike.
