What is moving?

Oct 15, 2010
Quick question. The magnet on my front wheel from my cycle computer is about 2 or 3mm from the sensor, but when I stand up to power up a hill I can hear it making contact. Anyone know why this would happen? Would it be my carbon fork flexing? or my not so fancy wheel? I can't figure it out, and it makes me a little uncertain how aggressive I can be. I don't want anything to brake. If it helps, I am on about a ¥100,000 bike and weigh 87kg.


Maximum Pace
Sep 3, 2009
Do you hear the noise approximately after a full revolution of the wheel?

If the magnet and sensor are touching you should see some light scratches on the sensor. If the sensor has scratches, there is a big possibility that your wheel might be flexing.

No scratches on the sensor, means something else is making the noise.


Sep 24, 2010
I have noticed that at slower speeds on my bicycle a sound comes everytime the magnet passes the sensor. I initially thought that something might be scratching against the magnet but found nothing... I believe that sound is from the magnet-sensor arrangement, though I couldn't find any logical reason behind it :(
Since the sound goes away (or is imperceptible) at higher speeds I don't bother about it.


Maximum Pace
Dec 3, 2010
I know that when I stand up and kick hard I can make a bike's frame flex a LOT.

I used to have a buddy back in Canada who had a very nice custom built steel framed racer bike, were were about the same size, but he weighed about 65Kg, I weighed 84Kg, on a steep hill climb I could flex that bike enough to make the wheels rub the frame or fork, he hated it when I did that :D One reason when I bought a road bike I went with a super stiff Cannondale aluminum frame bike, a LOT harder to flex.

You may very well be rubbing a tire on the frame when you climb.


Maximum Pace
Some flex in the fork and front wheel when standing to climb is to be expected, especially at your (our) weight. It doesn't necessary mean anything is about to break, although I've had wheels that were very flexy and broke a spoke on them after about 3000kms (rear wheel, not front). Try moving the sensor another mm or two further away. If should still pick up the magnet at that distance.
May 22, 2007
Would it be my carbon fork flexing? or my not so fancy wheel?
At that price point maybe you have Shimano WH-R550 wheels? Radial spokes on the front? I had them originally and found that they flexed a lot when dancing. (I tore apart two Y's house-brand frames within a few weeks!)

You can find out whether your magnet/sensor are contacting by putting a little masking tape on the sensor. Same for the tire/fork. Unless you are using a 1:1 gear ratio (unlikely on a road bike) the magnet shouldn't hit on every rotation because your pedals and wheels will not be rotating at the same speed.

You could also reverse the front wheel and see if the problem goes away. That might indicate a loose or weak spoke.

Also, it's my experience that bike noises can be very difficult to pin down. I once had a squeak that I was sure was my seatpost. No - my crank. No - my headset. Turned out to be dry derailleur jockeys.
Oct 15, 2010
I have Ambrosio WS 23 rims (http://www.ambrosiospa.com/catalog_eng/index.php?option=com_wrapper&Itemid=58) on my Bianchi Via Nirone 7 Aluminum frame/carbon fork. It must be some sort of flex, but with all the muscle (from the drive train) and weight (being on an incline when going up hill) I still can not really figure out what would make the front end flex. A good example is when I am powering up (from a near stop) the center ramp on a short overpass near my place - I always hear that ting... ting... ting.... I will move the sensor a little further away from the magnet (quite hard to do when the forks are like blades and not round) and if the noise goes away, I will relax about it. Thanks for your prompt informative responses.

m o b

Speeding Up
Jun 22, 2008
What would Dr. Hasselhoff do?

Coming back to the original question, whether the fork or the wheel or both flex: I assume that the sensor is attached to the fork, almost vertical below the front brake. When the magnet is close to the sensor it is on a spoke that is located in the upper half of the wheel, almost at 12:00 position.

I don't believe that such spoke will flex much in this position as it isn' subjected to lateral loading or bending moments (perhaps to tensile loads as the whole wheel is one system and each spoke interacts with each other, but there wouldn't be a bending effect necessarily). I could imagine that the spokes in the lower half of the wheel would flex, as there is a difference between the center of gravity of the weight of the rider and the bike (Transmitted to the wheel through the axle) and the point where the wheel touches the road surface. So, if anything bends on the wheel, it would be the rather lower half, not the upper.

Thus, this leaves only the fork to bend. But then again, if the fork bends in such way that the lateral distance between the fork and the wheel on one side is reduced, why does the magnet touches the sensor and the brake pads do not touch the rim on the same side? If there is substantial bend in the fork, it should be more prounounced as further it is away from the wheel axle.

So far for the enegineering point of view.

Based on the last idea I would recommend, to move the sensor downwards on the fork if possible. The closer the sensor is to the wheel axle, the less bend is possible. Of course you can also just move the sensor a few millmeters away from the magnet, but that might have an impact on the sensor reception, in particular if you use a wireless sensor on low battery.


Maximum Pace
I still can not really figure out what would make the front end flex.
When you're out of the saddle and pushing hard, the bike is being tilted from side to side. This results in some sideways force on the front end, especially if you're also leaning forward and/or pulling up on the bars. Most of my bikes I can flex this way to some degree, although it was most noticeable on a Pedal Force I had a few years ago. Not a big deal, although stiffer front ends can feel a bit better on descents, in my experience.
Oct 15, 2010
When you're out of the saddle and pushing hard, the bike is being tilted from side to side. This results in some sideways force on the front end, especially if you're also leaning forward and/or pulling up on the bars.
This sounds like the issue. I didn't think there was that much force, but that little tilt from side to side seems like the only logical explanation. I was going though a tunnel at about 50km/h in the dark on my ride back from Okutama the other day, and encountered some sort of speed bumps that gradually got worse near the end of the tunnel. I would love to see a slow motion video capture of all the moment/flex in that situation. I almost flew off my bike, and the force of most of my weight hitting the front wheel at 50K and all those bumps seems like it would be about 100 times greater than a little tilt while powering up a small slope from a stationary position. Maybe my bike is not really designed for any tilting, or lateral flex. Anyways... Thanks. I feel reassured about what is going on.


Maximum Pace
Ever sit in a window seat of an airplane near the wings and looked out at the wingtips? Flexing all over the place. A certain amount of flex in certain directions is good, even necessary. Where those places/directions should be are up to the engineers, but (apart from weight) one of the reasons just about all sports bikes above a certain price point these days come with a carbon fork is that they can soften up the kind of bumps that you experienced in the tunnel, and therefore improve handling. That's why you almost never see aluminum forks (except suspension forks...)


Maximum Pace
May 25, 2009
Ok factory built wheels need a service and tightening normally after the first 500-800km. Im guessing since owning the wheels you have never had them serviced?

Take them in get them tuned, dished and the flex will go and thus the clipping will stop.

Also regarding flex..... my sponsor for wheels hate me as in sprints Im pushing out over 1100w of energy and I weigh close to 80kg.... my wheels need a service after each race as I normally put them out of true by about 2mm. These are super light sub 1kg wheel sets though :D
Oct 15, 2010
Wow. Yah, good guess. I nearly have 1,000km on my bike now, and have never had it serviced. Never had a bike serviced in Japan. The bike was poorly assembled by Evans and I noticed the spokes were not that tight, but it was my first road bike, so I didn't/don't know what is normal. I tried tightening the spokes a little by myself, but did not mess with them too much because I didn't want to make matters worse. It is still perfectly true, I can't get a little wobble out of the rear one though, but it doesn't bother me. What do you think it will cost for the service here? It if is too much, I would be tempted to just get a better set of wheels. It seems Easton EA50s Fulcrum Racing 5s or Mavic Aksiums get rave reviews and are not too pricey. I am not too concerned about weight, more focused on value for money and doing as much DIY as possible. I realize though that sometimes it is best to let the experts so certain things. Thanks.


Maximum Pace
Apr 8, 2008
I've had a similar thing happen to me in similar circumstances.

In my case it was the magnet rubbing against the sensor. Decent Look forks and decent Mavic wheels. Try moving the sensor and magnet further apart and / or a smaller magnet.