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I recently changed the speedometer on my bike, and had some problems to adjust the magnet on the spokes as well as the sensor. (The spokes are flat, and the magnet that came with the sensor was intended for round spokes. And there's not much clearance between the fork and the spokes.) But in the end I managed to get it up and running with a couple of mm gap between the two.

However. When I accelerate hard or go up steep hills I notice that the magnet hits the sensor. Apparently the front wheel flexes significantly, even when I'm not standing up or pulling seriously at the handlebars. Is this normal? How much flex would one expect in the front wheel?

It's a new CF road bike and the wheels are 'FSA Team 30', decent entry-level aluminium wheels, according to most reviews.

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  • A lot of wheels will be specc'd for the spokes to have a certain tension, so it may be worth getting this information, and getting the wheel checked (your LBS will likely have a tool for this). The trouble is, I know some (good) wheelbuilders who judge tension simply by touch, so it's not an exact science. Obviously there will be a certain amount of flex, but you're asking "how much is too much" and that's a difficult one to answer.
    – PeteH
    Sep 9, 2014 at 11:02
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    I think FSA Team 30s equate to Vision T30s, and the user manual is at http://www.visiontechusa.com/wp-content/uploads/2014/08/ZS103-Vision_WH-VT-602_Chinese_20130628_v2.pdf. Recommended spoke tension for the front wheel is on page 10, 85-100 Kgf. I'm sure that would make sense to a shop with the necessary tool.
    – PeteH
    Sep 9, 2014 at 11:24
  • @PeteH Thanks for the link! I'll have a look at the weekend. I don't think the spoke tension is of though. All spokes 'ping' satisfyingly and seem to be 'in tune'.
    – Popup
    Sep 9, 2014 at 11:37
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    Mount the magnet nearer the hub and you should keep the movement within range.
    – Emyr
    Sep 9, 2014 at 13:56
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    That would probably solve the immediate problem, but not my curiosity.
    – Popup
    Sep 10, 2014 at 9:30

2 Answers 2

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The amount a wheel will flex will vary greatly depending on front/rear, spoke count, rim material and rider weight.

The pedalling force that drives the bike passes through the rear wheel, so it is not uncommon for carbon rear wheels to flex significantly when a stronger/heavier rider is climbing.

For an aluminium rimmed front wheel with spokes that are correctly tensioned and a rider not exceeding the weight limit for the wheel then several mm of flex are not normal.

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Are the spokes correctly tensioned? You shouldn't get several mm of flex from a new wheel just from climbing or accelerating.

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  • I haven't measured the torque, but they all 'ping' satisfyingly, and roughly in tune.
    – Popup
    Sep 9, 2014 at 12:17

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