A fork with 160mm of travel can be installed on a 140mm bike only when the head tube is strong enough to handle the additional 20mm.
If that condition isn’t met, the rider will lose the warranty of the frame and risk frame cracks or complete disintegration that may lead to serious injuries.
How Forks Stress The Head Tube
- Stress A – the constant stress that the fork exerts onto the headtube while riding
The rider’s weight presses the frame towards the ground. That pressure is fairly constant apart from weight shifts done to increase pedaling efficiency (e.g., riding out of the saddle).
Since the head tube is attached to the frame at a non-vertical angle, there’s extra stress on the connection points between the head tube, the downtube, and the top tube. The slacker the head tube angle is, the higher that stress is.
That said, the constant stress is fairly low and highly unlikely to result in a major crack of the frame even when the head tube is not designed for a fork with massive travel.
The true danger comes when there’s impact.
- Stress A – the stress that the fork transmits onto the frame upon impact
Ultimately, the fork acts as a lever against the head tube. The longer the fork is, the larger and stronger the lever becomes and so does the stress exerted against the head tube.
The head tube angle influences the stress applied to the head tube too. If hypothetically, the head tube angle was 90 degrees, the stress would be at its lowest.
The slacker the head tube angle is, the more stress the fork can apply onto the head tube upon impact.
For that reason, the head tubes of frames designed for extra-long forks are over-built.
The graph above illustrates how the fork’s leveraging force increases as the head tube angle gets slacker.
What Would Happen If I Install a 160mm Fork On a 140mm Bike?
- Extra Stress On the Head Tube
As already explained, the longer fork will exert more force onto the head tube and thus increase the chances of frame failure.
- Slacker Head Tube Angle
Installing a longer fork is one of the common methods used to slacken the head tube angle of a bike. 20mm of extra travel would slacken the head tube angle by about 1 degree.
Therefore, in this case, the head tube angle will get about 1-degree slacker.
- Slacker Seat Tube Angle
The longer fork will raise the front end and thus slacken the seat tube angle too. Technically, the slacker seat tube angle is a downside because it will position the rider closer to the rear wheel and thus hurt the pedaling and climbing efficiency of the bike.
That said, the change isn’t dramatic enough to negate the benefits that would come from the slacker head tube angle.
- Higher Bottom Bracket
The bottom bracket height will increase slightly too. A higher bottom bracket offers greater frame clearance at the expense of stability and sharp cornering. However, in this particular case, the elevation isn’t large enough to make the most dramatic difference.
In most cases, a 1-degree slackening of the head tube angle achieved by lifting the front end results in about 7mm of bottom bracket elevation.
- Worse Steering at Slow Speeds
Longer forks and slacker head tube angles hurt the bike’s steering at slow speeds because the front wheel is too far forward and there’s less weight on it. As a result, the steering feels a bit like a “boat” in the sense that the front end is too elongated for snappy maneuvers. Hence the absence of slack head tube angles on BMX and dirt jumpers.
- Worse Climbing Capabilities
A slacker head tube angle will make climbing more difficult due to the following effects:
- The rider’s back angle will be more vertical and there will be extra drag.
- Maneuvering at slow speeds will be more difficult.
- There will be extra weight on the rear wheel.
- The front wheel traction will be reduced.
- The front wheel may be lifted accidentally.
- The wheelbase of the bike will increase.
You May Not Have To Buy a New Fork
It’s important to note that this particular upgrade may not require a new fork because some models have adjustable travel. For instance, the Manitou Mattoc Comp fork has a 140-160mm travel adjustment via travel spacers inside the fork. To switch between the high and low settings, it’s necessary to remove the spacer.
Is The Switch Worth It?
The main incentive to increase the travel from 140mm to 160mm is to conquer more aggressive downhill terrain. If that isn’t your goal, then the change is probably not worth it given that the pedaling efficiency of the bike uphill will be hurt significantly.
Truth be told, even 140mm of travel could be too much for some technical climbs. Many climbers prefer 100-120mm of travel for maximum snappiness with some suspension effect.
Summary: What You Need To Know
- Longer forks with more travel act as longer levers and exert additional stress on the head tube.
- If the head tube of the frame can take the additional stress that the new 160mm fork will produce, the switch from 140mm to 160mm is a viable option.
- If the head tube is rated only for a fork with up to 140mm of travel, the extra 20mm will void the warranty of the frame and create an opportunity for cracks or maybe even complete disintegration.
- The switch from 140mm to 160mm makes sense only when the bike will be used for difficult downhill descents and no climbing outside of fire roads.
- If the bike will have to serve as a technical climber, 160mm will be too much. In some cases, even 140mm is on the slacker side.
- The switch from 140mm to 160mm may not require a new fork as some models have adjustable travel.