A 170mm fork can be installed on a 160mm bike when the head tube is strong enough to handle the additional stress.
If that condition isn’t met, the frame warranty will be lost and the rider will risk frame damage that may result in a massive crash.
Extra Fork Travel = Extra Head Tube Stress
The extra length of forks with longer travel stresses the frame because the fork gains leverage against the head tube.
When the rider’s weight presses the bike towards the ground, the fork acts as a lever trying to pull away the head tube from the rest of the frame.
The effect is quite strong because the front wheel (the point connecting the fork with the ground) is located at the end of the fork where it can exert the most torque.
Consequently, forks with more travel require stronger head tubes to avoid cracks or a complete frame “rupture”.
This is especially true for MTBs that face a lot of impacts (e.g., enduro, downhill) because in those situations the stress created by the fork augments exponentially.
Slacker Head Tube Angle = Even More Stress
The head angle (the angle between the head tube and the ground) influences the stress on the head tube too.
A slacker head tube angle increases the stress because the fork has an even better position to exert torque at the connection point between the head tube and the down tube.
For that reason, MTB frames designed for forks with massive travel are made quite strong.
Frames have a travel limit specified by the manufacturer. If the limit isn’t respected, the frame’s warranty will be lost.
Changes To The Geometry Of The Bicycle
The 10mm longer fork will trigger the following changes to a bike’s geometry:
- Slacker Head Tube Angle
Getting a fork with more travel is one of the classic ways to slacken a bike’s head tube angle.
20mm of extra travel slackens the angle by about 1 degree. In this case, the slackening will therefore be only 0.5 degrees.
The main advantage of a slacker head tube angle is the ability to ride faster on difficult downhill terrain.
A slack HTA positions the front wheel further in front of the rider and makes it easier to overcome irregularities while also reducing the chances of flipping over the handlebars.
Hence the extremely slack head tube angles on downhill bikes.
- Slacker Seat Tube Angle
The elevated front end will slacken the seat tube angle too. The seat angle is formed by the seat tube and a horizontal line passing through the bottom bracket.
A slacker seat tube angle is considered a downside because it positions too much of the rider’s weight over the rear wheel and makes the distribution of mass uneven. Consequently, climbing becomes a nightmare, especially when the head tube angle is ultra-slack.
That said, it’s possible to counter this geo change and acquire an acceptable effective seat tube angle by adjusting the saddle setback.
- Taller Bottom Bracket
The higher front end will lift the bottom bracket (BB) by about 3.5mm. (1 degree = 7mm of BB elevation).
A taller bottom bracket increases the frame’s clearance but hurts the bike’s maneuverability. In this instance, the change isn’t drastic, however.
- Longer Wheelbase
The slacker head tube angle pushes the front wheel away from the frame and thus increases the bike’s wheelbase every so slightly. A longer wheelbase improves the bike’s stability but hurts its responsiveness and cornering.
- Taller Stack Height
The elevated front end will increase the stack height of the bike by about 8mm. 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 Suffer
Forks with longer travel make climbing difficult for the following reasons:
- The extra slack head tube angle positions a lot of the rider’s weight on the rear wheel and makes it difficult to maintain a high cadence.
- The extra slack head tube angle reduces front-wheel traction and increases the chances of accidental lifting of the front end.
- The back angle of the rider gets more vertical and creates additional drag hurting the overall efficiency.
That said, a fork with 160mm of travel is already “anti-climbing” territory. Thus, one can argue that the additional travel isn’t all that harmful because with or without it, the bike won’t climb well anyway.
When Is The Transition Worth It?
Truth be told, the difference between 170mm and 160mm of travel isn’t massive enough to significantly improve performance. If the overall frame geometry and the rest of the components aren’t already good enough for aggressive off-road riding, 10mm of extra travel will not make the bike a “killer”.
It’s also important to note that there are other factors besides fork travel that have a large impact. Those would be:
- Overall frame geometry and quality
- Suspension model and class
- Components class
- Condition (if purchasing second-hand)