# What materials could be used to make a strip that bends when wet?

A bimetallic strip is a band of two metals attached in parallel that expand at different rates in response to temperature changes, thus bending the band, often used for measuring temperature. A bike project I'm attempting would be greatly improved if I had a band that bent in response to moisture changes. Specifically, brake/rim dust being fed back into the braking system is considered the top factor for rim wear in wet weather riding, and I've attached a prototype sponge to my brake pads to clean the rim when I brake.

It works well but I don't have the sponge on all the time since it would just be rubbing a dry rim, and it's a pain to engage, so if I want to make it useful to more than just tinkerers it needs to auto-engage. There are a lot of other issues to consider even if I had a material that bent when wet, but this is the thing I know the least about how to design.

I would like to tinker with an arm that closes a 2-5mm gap when wet from rain. A separate sponge or squeegee will deal with brake dust primarily, but some possible contamination from brake dust and road salts should be expected.

The closest thing I've been able to find is a polymer sheet that dances around on a wet surface, which tells me it could be done, but there's got to be a simpler way.

Even using something like a strip of swellstop (a rubber+sand compound for sealing concrete) glued to a non-expanding rubber might work, but I suspect swellstop doesn't contract when dry, and probably doesn't dry quickly either.

It's relatively easy to find materials that expand when wet, but seems hard to find ones that return to their original shape when dry.

Do I really have to resort to "smart" polymers? Is anyone familiar with a material that is less exotic that could fill my need?

Finally, if I can't find one, what's my next step? I'm a dog flying an airplane when it comes to designing something like this from scratch, do I need a team of PhDs? Where would I even start?

• What exactly are you trying to accomplish here? State all of your constraints. Are you talking about pure water? If not, how do you plan to deal with the inevitable contamination issues? There are many other ways to detect water. For example, it has a very high dielectric constant relative to most other materials, so capacitive sensing is very effective. Apr 16 '15 at 18:05

I do not know of any material which will meet your needs.

However, I can offer a potential solution. You stated that your sponge is "a pain to engage" and I think that is your real problem. If you can make your sponge easier to engage and disengage, then you don't need the exotic material that you are looking for. I don't know what type of brakes or bicycle you have, so I won't try to offer specific solutions on how to engage or disengage your sponge.

You may also want to consider measuring how much the sponge interferes with your normal riding. Measuring the power required to turn the wheel against the sponge is quite simple.

1. Get a small spring scale and hook it on the end of one of your spokes with the sponge engaged and the wheel stationary
2. Measure the length from the axle to the scale. Call it $l$.
3. Slowly pull on the scale until the wheel starts to turn and note the force it took to engage. Call that force $F$. Make sure your are holding the scale perpendicular to the spoke.
4. Measure the diameter of the wheel. Call it $d$.

We can now find the torque exerted on the wheel by the sponge.

$$T = Fl$$

Given the torque, the power required to pedal against your sponge, as a function of bicycle speed is given by the following equation. If your force ($F$) is in Newtons, your lengths ($l$ and $d$) are in meters, and your speed ($S$) is in meters per second, the answer will be in Watts. If your units are different, just use Wolfram Alpha and it will convert your units for you.

$$P = 2TS/d$$

I suspect that $P$ will be small compared to the overall effort of riding a bike (50W to 100W) and if it is, you don't need to worry at all!

• That's a cool way of measuring it! I think it will be extremely small. I'm also concerned about prematurely wearing away the sponge. Worth just trying to see though. Apr 16 '15 at 21:37