What fault in the steering mechanism can risk causing the rider to fly over the bars when braking?

Question

By steering I collectively mean the area of handle bars, headtube, and fork.

So last year I bought a second-hand bike. Never really had a problem with it but today I took it to the shop to change worn out tires (+ inner tube) and cracked pedals. When I picked-up my bike, the mechanic informed me that he also found a problem with the steering. He proceeded to demonstrate by applying weight/pressure on the front wheel through the handle bars. Unfortunately I did not hear the specifics properly since we were talking with face masks on. (He didn't fix it because it would throw off the cost estimate I was quoted.)

He then just admonished me to be careful when braking as I risk flying over my handle bars. I rode the bike on the way home (4.47 km, 17 m elev gain, 28.8 kph max speed) and I did not feel anything amiss or concerning.

If I had to guess, his concern might be in my suspension-type fork. Honestly, this is the first (and thus far only) bike I've ridden with a suspension fork and I think it is not as responsive as suspensions are supposed to be. But again, I've ridden this for a year, over various terrain, and there wasn't really any issue.

So what defect could possibly give me this risk? Is it worth servicing?

Answer 1

As per comments, it sounds like your bike has front suspension forks.

When doing a hard/panic brake, the front wheel will slow faster, and the suspension will compress ("load up") and acquire stored energy. Once the rest of the bike/rider combo slows to the same speed, the suspension will unload and relax.

This can result in odd handling, potentially throwing the rider if they're unprepared. A good example is turning - with the front wheel pointing around a corner, the suspension relaxing will give a vector of force toward the outside of the turn, which can be a surprise.

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Separately your description of

(mechanic) applying weight/pressure on the front wheel through the handle bars

(fork is) not as responsive as suspensions are supposed to be

suggests they were compressing the front suspension and showing you how it wasn't working very well. This is common with suspension forks that haven't been serviced regularly. Ultimately a suspension fork will seize up and become a heavy rigid fork.

This may be an issue if you ride off road a lot, but for on-road usage the gradual loss of suspension is unnoticable, and isn't really needed for most roads.

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Lastly, C19 doesn't transmit over the `phone. Consider calling the bike shop and ask for a clarification. No masks required, and you'll be able to take your time.

Answer 2

On suspension forks, there could be a small play between the lower part to the fork (the bottles) and the upper tubes that slide in (the stanchions). That is due to the tolerances in the bushings that these tubes slide on. When these get worn the tolerance gets too large so the wheel (noticeably) moves back and forth relative to the bike when squeezing/releasing the front brake with the bike in movement.

This is usually demonstrated by applying the front brake and rocking the handlebars back and forth with the wheel on the ground. The bike will noticeably move back and forth, often with a clunking sound.

I'm guessing that's what your mechanic told you. That feeling in the fork can yield nervous reaction on the rider and overall makes for an unpleasant ride but I doubt this alone can cause an OTB accident. It may be expensive to fix if the bushings for the fork are expensive or hard to find. Some forks are not serviceable, especially cheap ones, thus the repair is to swap the fork.

I have seen riders of very badly maintained bikes perform panic braking where a combination of worn and misadjusted parts produce an weird effect where the brake seemingly releases and reapplies by itself. That could cause an OTB because at some point the brakes grab more than the initial reaction seems to be. I mean, the rider perceives an initial deceleration and thinks for a moment that is all the stopping power available. Suddenly, there is more and... well, you get the picture. However, I have seen this happen only on bikes made with cheap steel parts, where the huge flex on these parts allow for such extremes.

The other failure I can think of, related to suspension forks is some worn seal in the damping mechanism or similar that would make the fork "sink" too much in an unpredictable way, increasing weight transfer to the front, or said in other words, helping initiate the rotation of the rider+bike towards the front. This may be expensive since the internal part may not be available or difficult to find, or available only as a rebuild kit.

I can also imagine the fork preload being too low (suspension too soft) thus allowing the crown of the fork to make contact with the tire, which may result in an OTB. This however is usually easy to regulate, unless the respective mechanism is broken.

In general, severely worn or damaged suspension forks may be expensive to repair, to the point that it's often easier to swap it (In some cases the rebuild kit is available at an affordable price, but don't bet on it)

Also, pay attention to the headset bearing (The one that allows for rotation). It is normally easy to adjust and the ball bearings are usually cheap to replace, but if other part of the headset is damaged, it may render the repair more expensive than the initial budget. A loose headset may fell the same as described in the first paragraph, and may affect any fork type.