This post presents all the methods used to slacken the head tube angle of a bicycle.
First, it’s necessary to explain what a slack head tube angle is.
The head tube angle (HTA) is formed by intersecting a line passing through the bike’s head tube and another horizontal to the ground.
In simpler words, the HTA is the angle between the head tube and the ground.
A slack head tube angle means a smaller head tube angle and positions the front wheel further in front of the rider. The benefits of slacker head tube angles are explained in detail in this post.
Methods For Sclakening The Head Tube Angle
Ultimately, slackening of the head tube angle is achieved by elevating the front end of the bicycle or dropping the rear end.
1. Increase The Travel Of The Fork
Many forks, even some that are considered entry-level, allow users to adjust their travel. (The travel of a fork indicates how much the fork compresses.)
For example, I have an old Manitou Markhor Air that can operate at 80/100 and 120mm of travel.
By default, the fork is set for 100mm travel. However, it comes with one extra spacer that you can add to the fork’s inner leg to reduce the travel to 80mm.
If you want to increase the travel from 100 to 120mm, you can remove the spacer that’s already inside the fork.
In general, 20mm of extra travel would slacken the head tube angle by about 1 degree.
A more radical solution would be to replace the fork entirely with one that already has more travel. (e.g., 120mm vs 160mm).
This approach will work fairly well as long as the head tube is strong enough to handle the extra travel of the fork.
Before purchasing a new fork make sure that the frame is compatible with it. Usually, manufacturers list the travel that their frames can support. If you don’t respect those recommendations, you will lose the warranty and risk the integrity of the frame.
In some cases, the head tube will simply crack, but it’s also possible for the frame to disintegrate completely. Sudden failure of that sort is more common for carbon and aluminum frames. Steel is a bit more resistant, but it can break off too.
2. Use a Headset that angles The Fork
There are special headsets that angle the fork’s steerer inside the head tube to change the head tube angle. Most models can steepen or slacken the head tube angle by 2 degrees.
In the graph above, HTA 1 represents the original head tube angle that the bike will have when relying on a standard headset that doesn’t angle the fork’s steerer.
HTA 2 shows the new slacker head tube angle that forms once the geo-altering headset is installed.
Note: Similar headsets are available only for tapered headtubes and forks that start narrow at the top and get wide at the bottom.
FAQ: What are the downsides of using an angled headset to slacken a frame?
There are two major downsides:
- You need a longer fork steerer
If you use the original fork that already has a cut steerer, it will still slacken the head tube angle but will simultaneously drop the front end and the bottom bracket to a lower level. The new position could feel “weird” and will also increase the chance of pedal strikes during turns.
- Extreme Stress On the Head Tube
An angled steerer will place even more stress on the head tube than the default alignment of the fork. Also, the new headset will more than likely void the warranty of the frame. Thus, it makes sense to use this method only if the frame is extremely strong and capable of operating with a super slack HTA.
- Creaking
Many users of angled headsets report annoying creaking coming out of the headset. Whether this effect will manifest depends on the headset, frame, and fork model.
3. Reverse Angle Spacer
Another option to slacken the head tube angle is to replace the original crown race with a reverse angle spacer. This is a special type of spacer that will slacken the original head tube angle by 0.5°.
This option works only for tapered forks and head tubes.
4. Use a Shock with a Shorter Travel
Another way to slacken the head tube angle of a full-suspension bike is to use a shorter rear shock. This method lowers the rear end of the bike and slackens the head tube and seat tube angles.
In this case, 10mm of travel reduction equals about 0.8 degrees of change. So, if a 165mm shock is replaced with a 125mm one, for example, the 40mm reduction will slacken the head tube angle by about 3.5 degrees.
The main issue with this method is the lower bottom bracket (hello, pedal strikes) as well as the firmer suspension with reduced off-road capabilities.
5. Install Offset Bushings
Offset bushings decrease the effective distance between the rear shock eyelets (mounting points).
The main advantage of offset bushings is that they reduce the effective length of the shock but do not affect the travel. (They can also be used in reverse to increase the length of the shock that you can fit on your bike.)
Offset bushings lower the bottom bracket by about 10mm and slacken the head tube angle by 1 degree.
The main advantage of offset bushings (in comparison to simply getting a shorter shock) is that the travel of the shock is not reduced.
Also, the changes to the geometry of the bike are nowhere near as aggressive as those caused by the usage of a much shorter shock or a smaller rear wheel.
You can find more information about offset bushings on the official page.
6. Different Tires
Back in the day, I accidentally slackened the head tube angle of my commuter hardtail. Originally, the bike came with knobby tires designed for XC terrain. But since I was using the bike primarily for paved roads, I replaced the original tires with slicks.
Eventually, the rear tire got a massive cut that kept reopening and causing flats. To save money and time, I placed the front slick tire at the back and one of the old knobby tires at the front.
Why?
The rear wheel provides most of the rolling resistance. Thus, it made sense to keep using a slick tire at the back.
Knobby tires have a larger circumference than slicker models of the same size. Consequently, my new setup slackened my head tube angle a bit. The difference was small but not entirely non-perceivable.
Obviously, this method wouldn’t cause as much change if both tires are of the same type. However, there are circumference differences between knobby tires too. For example, a downhill tire has a larger circumference than one designed for light XC.
Different Wheel Sizes
Another way to slacken the head tube angle would be to rely on wheels of different sizes.
There are three options:
- А larger wheel at the front
- A smaller wheel at the back
- A larger wheel at the front and a smaller wheel at the back
The first is the most common. For example, some people try to put a 29″ wheel on a 27.5″ fork.
This transformation could work only if the fork has enough room for the new wheel plus some extra clearance to prevent debris from getting stuck between the tire and the fork arch.
The difference in radius between a 27.5″ and a 29″ wheel using the same tires is about 9.5mm. If the swap is made, the head tube angle will get slacker by about 0.5°.
If the fork can fit a wider tire, the slackening will be even more extreme because wider tires have a larger diameter.
That said, the clearance of the fork will greatly limit the “switch” and thus prevent over-slackening of the head tube angle.
An Important Note On Brakes
Wheel switching works best if the bike has disc brakes because the rotor is always at the center of the wheel regardless of wheel size. However, if the bike uses rim brakes (e.g., V-brakes), the new wheel will position the brake track far away from the brake pads.
A smaller wheel will require longer brake levers whereas a larger one will demand shorter brake arms. It’s not impossible to make the change work, but the process could be very frustrating.
If you want to learn more about this issue, I recommend reading this post even though you might not be interested in the exact modification on which the article focuses.
Built-in Frame Adjustments
Some frames (e.g., Specialized Stumpjumper) have a built-in technology into the frame called Flip Chip allowing the user to change the geometry of the bicycle. The flip chip can lower and elevate the bottom bracket.
Those changes affect the head tube angle too. The lower bottom bracket position slackens the HTA whereas the higher steepens it.
In addition, the Stumpjumper has an adjustable headset that affects the head tube angle by one degree. Consequently, the head tube angle of the bike ranges between 63.5º-65.5º.
The video below talks about those features of the model in greater detail.
The Stumpjumper is not the only model with the Flip Chip adjustment.
Below is a list of models with similar technologies:
- Giant Trance Advanced PRO 29
- Giant Trance 29
- Giant Trance X Advanced PRO 29
- Giant Trance X
- Giant Trance X Advanced E+
- Reign E+
- Salsa Spearfish
…etc.
The Effect On The Seat Tube Angle
Bike geometry is interconnected. Changing one angle often results in modifications elsewhere. In this case, the seat tube angle is the most affected property.
How slackening of the head tube angle will affect the seat tube angle depends on the used method. The list below contains some of the possibilities.
Option 1: Longer Fork
A fork with a longer travel will result in the following changes:
- Slacker head tube angle
- Taller front end
- Taller bottom bracket
- Slacker seat tube angle
The slacker seat tube angle in this case is a negative because it hurts some of the benefits that come with a slacker head tube angle.
For example, the rider’s weight shifts excessively towards the rear wheel and it becomes even more difficult to climb. Also, the chances of experiencing unwanted lifts of the front wheel are higher.
That said, the change isn’t extremely detrimental to the point where it’s necessary to abort the slackening of the head tube angle.
Also, you can steepen the effective seat tube angle by moving the saddle forward.
Option 2: Angled Headset
If the steerer of the fork is long enough, there won’t be noticeable changes to the geometry other than the slacker head tube angle.
If the original fork is used, the following changes will occur to a small degree:
Option 3: Larger Wheel/Tire At The Front
A larger tire at the front and/or a smaller one at back has the following effects on the bike’s geometry:
- Slacker head tube angle
- Slacker seat tube angle
- Higher bottom bracket
Option 4: Shorter Rear Shock
If you replace the rear shock with a shorter one, the following changes will take place
- Slacker HTA
- Slacker seat tube angle
- Lower Bottom Bracket
- Decreased shock travel
Option 5: Offset Bushings
Offset bushings change the geometry as follows:
- Slacker HTA
- Slacker STA
- Lower Bottom Bracket
Is it worth it?
The tweaks above can add some “playfulness” and “aggression” to a bike, but they are minor and cannot transform a sheep into a wolf.
Or in other words, it’s not possible to get a conservative XC bike and turn it into a trail bike by slackening the head tube angle by a few degrees. If the base isn’t there, no amount of adjustment will compensate.
That said if the head tube is strong enough to handle a much slacker angle, it will be possible to greatly increase the off-road descending capabilities of a bike by using a longer fork.
