A 180mm fork can be installed on a 160mm frame only when the head tube has the necessary strength to endure the additional stress.
If that’s not the case, the frame’s warranty will be lost and the rider will be at greater risk of cracking the head tube or experiencing complete disintegration of the head tube resulting in an uncontrollable crash.
More Travel = More Head Tube Stress
The graph above illustrates the head tube angle (HTA) of a bicycle. The HTA is essentially the angle formed by the head tube and the ground.
Since the fork’s steerer passes through the head tube, the fork assumes the HTA of the bike in relation to the road.
The fork exerts pressure against the head tube all the time by acting as a lever with the front wheel being the “input point”.
The graph above illustrates a high mechanical advantage (leverage). Side B has the mechanical advantage thanks to the longer lever arm. As a result, it can generate more torque.
The same happens when a bike has a longer fork. The fork is the input force whereas the head tube receives the stress.
Naturally, the stress multiplies when the front wheel faces an impact (e.g., an obstacle, landing a jump…etc.)
If the head tube isn’t strong enough to endure the generated torque, it can break partially or fully.
For that reason, frames designed for forks with lots of travel have reinforced head tubes and sometimes rely on gussets.
The Geometry Will Change
A switch from 160 to 180mm of travel will trigger the following changes:
- Slacker Head Tube Angle
The longer fork will elevate the front end by about 20mm and will therefore slacken the head tube angle by 1 degree.
A slacker head tube angle makes the bike more capable when descending on off-road terrain because the front wheel sits further away from the rider and eats the irregularities.
And since the center of mass is closer to the rear wheel, there is no feeling that you’re about to flip over the handlebars any second.
- Slacker Seat Tube Angle
The higher front end will slacken the seat angle too. The seat tube angle is the angle between the seat tube and a horizontal line passing through the bottom bracket.
Modern MTBs combine slack head tube angles with steep seat tube angles. The purpose of the steeper seat tube angle is to counter some of the negatives that come with a slack head tube angle (e.g., too much weight on the rear wheel.)
Thus, this particular change caused by the additional travel is a negative. However, it can be negated by sliding the saddle forward to steepen the effective seat tube angle.
Most of the time, 20mm of front-end elevation slackens the seat tube angle by less than 1 degree.
- Taller Bottom Bracket
The taller front end will elevate the bottom bracket 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.
- Longer Wheelbase
The slacker head tube will push the front wheel away from the frame (or the frame away from the front wheel depending on how you want to look at it) and thus increase the distance between the front and rear axle known as wheelbase.
A long wheelbase increases stability but makes the bike less agile.
- 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.
All Climbing Efficiency Will Be Lost
Forks with 160mm of travel already make climbing difficult due to the “boat-like” steering at slow speeds. 20mm extra will make the issue even worse.
Ultimately, climbing favors steep head tube angles and forks with less travel. That combination provides precision and additional front-wheel traction.
Forks with lots of travel and slack head tube angles have the opposite effect. They push the rider towards the rear wheel and thus hurt the traction of the front wheel and the pedaling efficiency when going uphill. The slack angle introduces dead points to the steering and gives “floppy” feedback.
That being said, most people looking to perform this transition probably don’t care about climbing all that much, to begin with.
When Does It Make Sense To Go From 160mm to 180mm?
A switch from 160mm to 180mm makes sense only when the following conditions are met:
- The frame can handle the stress.
- The rider wants to make the bike even more aggressive and conquer downhill terrain.
- The rest of the geometry agrees with the switch.
If the overall frame geometry and the rest of the components aren’t already good enough for aggressive off-road riding, 20mm of extra travel will not make the bike a downhill machine.
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)