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. Commented Dec 19, 2013 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 Commented Dec 19, 2013 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. Commented Dec 19, 2013 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
Commented Sep 13, 2014 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
Commented Dec 23, 2014 at 20:59

I happened to do just that.

I took an old hand pump and an inner tube to the bath tub. I 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 was 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, let's see if there are 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 at 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, I could pick up the front tire and it would slowly start to rotate on its 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 felt 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 its shape, but there was no pressure to force the leak when I wasn't on the bike (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
Commented Dec 19, 2013 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. Commented Dec 19, 2013 at 16:37
• 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. Commented Dec 19, 2013 at 17:14
• I ran a similar experiment with the rear tire during my teenage years. I remember that I was surprised how little effect either in weight or in riding I perceived. I used a 26 x 2.1 MTB "BSO" at the time and used to participate in mini races around the block with my friends, which I used to win. Opposed to them, I used to start in high gears and accelerate slowly, overtake near the middle of the loop and by the final quarter, it was just matter of keeping momentum. I think my water filled tire was "consistent" with the strategy, but not really decisive. (to be continued...) Commented Sep 16, 2022 at 23:57
• (... contd.) I felt no significant difference in acceleration or braking, that is, I did not felt I needed a lot of extra effort getting up to speed nor braking. I did felt the bike less agile in tight, fast turns. I guess that the water is not really forced to rotate at the same speed as the wheel, it is not solidly coupled (viscous coupling comes to mind). So I guess the effect is partially the same as attaching a large-ish water bottle near the axle. Commented Sep 17, 2022 at 0:04

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).