# What are the effects of completely filling tires with water instead of air?

As absurd as it sounds, I was curious as to how a bike would handle with the front tire filled with water. I wonder if steep hill climbs would be easier (particularly for a mountain bike):

1) to bring the center weight forward

2) to use the increase rotational momentum to carry the speed through to the next pedal stroke.

• 1) The bike would be a lot heavier. 2) The tire would be a lot stiffer. 3) If there was any air in with the water the bike would likely behave unpredictably at moderate speed because the water would slosh around. – Daniel R Hicks Dec 19 '13 at 12:17
• If my calculations are right, the water would weigh around 10 pounds. With a rim of radius R and a tire of radius r, the volume of the tire should be: (2 * pi * R) * (pi * r^2). For a 26" rim and 2" tire, R = 33cm and r = 2.54cm, so that's about 4200 cm^3 or 4200 ml, water weighs about 1 g/ml, so that's 4200g, or 9.25 lbs – Johnny Dec 19 '13 at 16:27
• Your friends probably wouldn't wait for you as you crawled up climbs and pinballed off of trees on flats and downhills. – joelmdev Dec 19 '13 at 17:15
• Thanks, Mike! I actually want to inflate tires with water for an entirely different (non-bicycle related) experiment, and I wasn't even sure it was possible with a submerged bike pump... you have facilitated my experiment with your creative and sharing nature :) – user13758 Sep 13 '14 at 22:23
• Great Experiment ! ! I bet just a little water would reduce the natural migration of air through the tube! Have you ever tried to put about a table spoon of water - just to seal the microscopic pores in the rubber? I will try it now Thanx for the question and answer. I think it is perfectly fine to ask AND answer your own question - this IS brainstorming regardless. – user15822 Dec 23 '14 at 20:59

I happened to do just that.

I took an old hand pump and an inner tube to the bath tub. Submerged the pump and pre-filled the tube with water, complete with burping out any air bubbles. With the tube moderately full and free from air, I put it on the rim with the tire. Then the final pressure up.

The first thing to notice is that the pump operates with lots of resistance (water flows a lot slower than air through those tiny parts).

As I pushed each stroke of the pump, the resistance difficult yet constant. I kind of expected the resistance to increase as I pressured up the tire, but it didn't. It was a solid stopping point, halfway through the last stroke the tire wouldn't take any more water. (I shouldn't have been surprised, water is one of the least compressible substances around!)

Now out for a ride to test this out!

Indeed to carry the bike the front was a bit heavier (heavier is exactly what most cyclists would not want; but hey, lets see if there's some benefits).

Oddly, the bike didn't feel all that different. Just slightly front heavy. But once I got some speed the steering became really quite stable. A ride through the forest and I could quickly pin point my leans for corners, it was soooo smooth! Like the steering of a motorbike; I assume the extra rotational inertia in the front wheel added to the smoothing gyroscopic effect.

Mucking about, I tried a wheelie. It took a bit of getting used to for lifting it up, but once up it was also very very smooth. I was never that good an wheelies, but now I seemed to be able to ride for as long as I wanted. Try balancing a baseball bat with the heavy side down, it's actually much easier with more weight at the top! And the extra gyroscopic effect made it easier to steer the wheelie.

When stopped at a light or something, pick up the front tire and it would slowly start to rotate on it's own.

It was fun, so I kept it.

The next phenomenon was a puncture. sure enough, one day riding on a nice sunny day... I feel tiny sprinkles of water; I had a tiny tiny leak and I was actually spraying a very fine stream. The tire hadn't lost much though, so I double timed it to my destination in hopes of avoiding changing in the field. On arrival, the tire seemed really quite full still. When off the bike the tire really wasn't getting flat. I left it. I continued to ride for a week or so, sometimes the leak would come back... sometimes it would go away.

I'm speculating, as I only did this once... but it occurred to me that with water being incompressible, the small amount of leaking water was very little volume but nearly all of the pressure. The tire was still full and held it's shape, but no pressure to force the leak when I wasn't on it (it was obviously a very small leak).

Eventually I had to add some air, now and again (too lazy to put water in, and too curious to see how bad the water/air mix would be). Soon the water/air mix was sloshing inside and causing a lot of resistance, and that became the end of my experiment.

In the end I did not see any improvement for hill climbs, or maintaining pedal momentum; only wheelies and cornering.

• On a mountain bike, this might be okay as you don't need the "suspension" coming from an air-filled tire. I believe that on the road this is rather different. But nice experiment! – arne Dec 19 '13 at 8:50
• I wonder how long the water keeps spinning around the inside of the tire when you come to a sudden stop, and if the resulting gyroscopic forces make it easier to balance at a stop. I think the water would only have a moderate effect on pedaling momentum since it's so loosely connected with the tire - it doesn't really "push" the tire around in a circular motion since the water is free to flow in a circle independent of the tire. – Johnny Dec 19 '13 at 16:37
• @CarsonReinke - It is OK to answer your own question: Stack Exchange has always explicitly encouraged users to answer their own questions. If you have a question that you already know the answer to, and you would like to document that knowledge in public so that others (including yourself) can find it later, it's perfectly okay to ask and answer your own question on a Stack Exchange site. – Johnny Dec 19 '13 at 16:40
• There's nothing weird about asking a question with the immediate intention of answering it. There is something weird about actually filling your bike tires with water. – joelmdev Dec 19 '13 at 17:14
• Salute to you! Quite the experiment here. Upvoted. – D.Salo May 29 '14 at 21:31

Putting more weight on a bike is not going to make it easier to climb a hill.
It is just weight you have to carry up a the hill.

As for rotational momentum to carry you through the stroke that is just rotational momentum you need to put back in the wheel on the down stroke.
If you are stalling out on a hill climb a better solution is a lower gear (and less not more weight).