Description Of The Problem: During braking the front wheel of the bike is wobbling (moving back and forth). The harder and more intense the braking is, the more noticeable the unwanted movement becomes.
Possible Sources Of The Problem
1. Non-secure Headset Bearings
The most common source of front end wobbling during braking are improperly “pre-loaded” and therefore non-secure headset bearings.
The term “pre-load” refers to the initial tightening of the bearings. The goal of pre-loading is to ensure that the rolling elements of the bearings are in contact with the lower and upper headtube races.
The “pre” in “pre-load” indicates that force is applied to the system in advance before the effective loading.
If there’s no pretension, the bearings would still operate but there will be play between them and the headtube races.
There are two types of headsets – threaded and non-threaded. The threaded headsets are found on older bikes as well as newer models going for a vintage look.
Threaded headsets require forks with threads on the steerer tube. The bearing pre-load is done via the upper head tube race which threads onto the steerer.
Threading the race further increases the pretension whereas untightening it decreases the applied force. Then, the adjustment is finalized by a locknut that also threads on the steerer.
Ideally, the fork will rotate freely and there won’t be noticeable play. It takes a bit of time and skill to select the right setting because it’s difficult to find the spot that offers both freedom and no play.
If the bike is newer, it probably has a threadless headset. As the name suggests, those types of headsets are not threaded onto the fork’s steerer.
Threadless headsets are pre-loaded by a bolt that inserts into a star fangled nut or a compression plug (carbon bikes) wedged into the steerer of the fork.
By tightening the stem bolt, the user is pulling the fork and the stem towards each other. Then the stem’s clamping bolts are tightened to fully secure the fork to the frame.
If the front end is stuttering when braking and the bike has a treadless headset, the most common source of the issue is play created by insufficient preload.
The basic protocol for fixing it is as follows:
Step 1: Loosen the stem’s clamping bolts
Step 2: Tighten the bolt inserting into the steerer tube to pack the headset and pre-load the bearings.
Step 3: Re-tighten the pinch clamp bolts
Note: In some cases, the problem comes from a fork steerer that’s sticking out ever so slightly above the stem. When the top cap bolt is tightened, it cannot press the stem and the headset together because the top cap is not in contact with the stem.
There are two solutions to this problem:
- Cut the steerer slightly
- Add a smaller spacer under the top cap
Consult the video below for more information:
Damaged StarFangled Nut
If the star-fangled nut is damaged (e.g., stripped threads), the top cap bolt won’t have much to hold onto too and will fail to compress the headset. The solution is to punch out the old star-fangled nut and install another one.
Worn or Broken Headset Bearings
If the headset’s bearings are worn or even broken, the headset will have some play. The only solution is to replace the ball bearings with new ones. The replacement units are not expensive.
A suspension fork can be the source of the play too. For example, if the bushings of the fork (sleeves in the fork’s lower legs guiding the upper legs) are worn, the fork will have some play.
This is a fairly rare issue with modern forks, however.
Front Hub Play
If the front hub of the wheel has worn bearings or has been improperly assembled, it can create instability too.
For example, cup and cone bearings require a fine adjustment to eliminate play in the hub while still allowing the wheel to rotate freely. If the bearing system isn’t adequately pre-loaded, the wheel can move unnecessarily.
That said, the front hub will not produce the same type of stuttering as a worn or improperly packed headset.
Insecure Front Wheel
If the wheel isn’t secured to the fork, it can create a noticeable play. The wheel should be fully seated in the fork’s dropouts and the retaining mechanism should be tightened adequately.
The most common retaining mechanism is a quick-release skewer. Truth be told, many recreational cyclists are unfamiliar with the function of the quick-release skewer and do not know how to tighten it.
I’ve seen people who ride their bikes with the quick-release lever in the unlocked position.
If you do not know how to properly tighten the retaining mechanism, consult a professional. This issue shouldn’t be taken lightly because a failure of the front wheel results in an instant accident.
If the bike is an old-school MTB with cantilever brakes, the brakes could be the source of the issue.
Cantilever brakes are known to cause vibrations of the fork during braking. The problem is known as fork shuddering and manifests when the brake cable hanger is part of the headset.
Here’s what happens:
During braking, the wheel is partially or fully locked by the brake. However, the inertia keeps pushing the bike forward. Since the wheel is prevented from spinning freely, it “bites” the ground. Then, the ground pushes the fork backward, and the fork has no choice but to bend.
When the fork bends, it stretches the brake cable. As a result, the brakes grab the rim even harder and the fork flexes again. The cycle continues until the bike slows down or the rider stops pressing the brake.
The common way to resolve this issue is to mount a cable stopper on the fork (image below) and run the brake cable through it rather than the headset.
A cable hanger will reduce the bow effect by bringing the two anchor points closer together.
Having said that, brake juddering may steel appear but to a small degree.
Another solution would be to switch to V-brakes.