A fork with 170mm of travel can be installed on a bike that originally has a model with 150mm of travel only when the head tube is strong enough to handle the increase.
If that condition isn’t met, the warranty of the frame will be lost, and the head tube might crack or even “rupture”.
More Travel = Longer Fork = More Head Tube Stress
A longer fork increases the leverage transmitted to the head tube.
During normal pedaling, the fork doesn’t impose a lot of stress on the head tube. But even the slightest impact (e.g., landing a small jump) increases the torque exponentially because the fork acts as a lever pulling the head tube away from the frame.
For that reason, forks with more travel require stronger head tubes. If that condition isn’t met, the head tube can crack or even break off unexpectedly. When that happens, the rider will lose complete control over the bike and crash.
Thus, it’s of utmost importance to never ignore the limits of the frame.
Slacker Head Tube Angle = Even More Stress
The head tube angle (HTA) is the angle between the head tube and the ground. The smaller/slacker, the head tube angle is, the more stress the fork generates against the head tube.
How Will a 170mm Fork Impact The Geometry?
A switch from 150mm to 170mm of travel will produce the following changes to a bike’s geometry:
- Slacker Head Tube Angle
The longer fork will elevate the front end and consequently slacken the head tube angle. In fact, many people get a longer fork precisely to accomplish this effect.
In general, 20mm of the extra travel slackens the head tube angle by 1 degree.
The main advantage of a slacker head tube angle is that it makes the bike more capable when descending on off-road terrain. The front wheel is further in front of the bike and eats the hits while the rider is positioned closer to the rear end.
Another bonus is that you don’t feel like you are about to flip over the handlebars.
- 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.)
20mm of extra fork travel slackens the seat angle by about 0.5 degrees.
In general, a slacker seat angle is considered a negative in the world of modern MTBs where the tendency is to combine ultra-slack head tube angles with steep seat angles for pedaling efficiency and better mass distribution.
That said, the effect, in this case, can be negated largely by steeping the effective seat tube angle via the saddle.
- Stack Height Increase + Reach Decrease
The elevated front end will increase the stack height of the bike by about 16mm. The stack height is the vertical distance between the bottom bracket and the top of the head tube.
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.
If you want to learn how the stack height affects a bike’s reach, consider reading the dedicated article.
- Higher Bottom Bracket
The longer fork will elevate the bottom bracket too by about 5-7mm. A taller bottom bracket provides frame clearance but hurts the bike’s stability because the center of mass is higher. The cornering of the bike will suffer too.
- Longer Wheel Base
The slacker head tube angle will position the front wheel further away and will therefore increase the wheelbase of the bike. A longer wheelbase improves stability but hurts maneuverability.
Climbing Will Become More Difficult
Forks with 170mm of travel are not designed for technical climbing. The slack head tube angle makes the steering of the bike a lot less responsive at slow speeds and gives the feeling that the front wheel will go up any second.
Truth be told, even the original 150mm could be too much for many climbing sections. If technical climbing is important, it’s wiser to go for a fork with about 120mm of travel, albeit exceptions exist.
The steering isn’t the only issue, however. The slacker head and seat angles will position the rider’s weight closer to the rear wheel and make it more difficult to maintain a high cadence when going up a hill.
In addition, the rider’s back will be more vertical and thus create more drag that would hurt the overall efficiency.
When Is This Transition Worth It?
The transition from 150mm to 170mm could be a viable option when the following conditions are met:
- The frame can withstand the additional stress.
- The goal of the rider is to cover more difficult downhill sections rather than to climb.
- The aforementioned changes to the geometry are not deal-breakers.