livedarklions

Just a fun thought experiment:

Imagine that someone built a smooth, perfectly level 100 mile road on the moon, and designed a space suit/bike combination that allowed you to ride on it. What would be the maximum speed you could achieve strictly under your own power in 1/6 g and a vacuum, and what would be the ideal gear ratio?

Nerd out all over the place. I'm more interested in seeing people's reasoning than in getting to the "right" answer as there's really no way to test it. But if someone wants to create a computer model with nifty looking animations, wheeee!

Qualifications: Rider has just arrived from earth and has not lost significant bone/muscle mass and assume air supply sufficient to do a 200 mile round trip.

phughes

Give me a bit, I'm suited up and will give it a try.

hillyman

Rode in Arizona. Close enough and no space suit saddle rash. Even have a crater.

phughes

As a jumpstart, the Lunar Rover had four 1/4 HP motors for a total of 1 HP. The average recreational rider puts out .35 HP over a two hour ride. It weighed 460 lbs on Earth, and could carry a payload of 1,080 Earth lbs. Tires will be an issue since the temperature can reach 260 F on the surface, and much colder if you happened to go through a shady section. Assuming a flat area, shade won't be an issue. Of course that is just the surface, the vacuum above will be much colder.

Oh, and the dust is very abrasive. I suggest waxing the chain...

General Geoff

Without an atmosphere, you could probably get up to 100 mph+ and stay there for awhile. Frictional losses of the drivetrain and tires deforming/gripping the road are minimal.

Regular pneumatic rubber tires would work fine on the moon IF there is a smooth, paved road there. The reason they didn't use em for the lunar rovers had more to do with avoiding potential flats and negotiating extremely rugged regolith terrain.

fietsbob

The bike will be feeling Lighter.. lower gravity..

livedarklions

We know that the max speed is well above 100 mph because people can do much better than that on earth when they ride behind a dragster: https://www.npr.org/2018/09/18/64922...w-world-record

So, in a vacuum, this should be even faster. Also, while everything is 1/6 earth weight, your legs will be able to put out the same force as they did on earth, so let's start the bidding on top speed.

I'll start the bidding at approximately 3x earth max, or about 567 MPH. Let's make that the over/under. If you think it's more or less, why?

Barry2 

If you hit the deck at 567MPH will you only receive 1/6 the road rash?

Barry

Pratt

Great question, but the course should have killer hills, at 1/6 gravity you'll fly up them

Ogsarg

That lack of gravity would also make it difficult to get traction between tire and surface making it difficult to accelerate and also might even make it difficult to balance on two wheels. Interesting question for the physics experts out there.

General Geoff

I dont think any normal bicycle will survive much past 120 mph without wheel balancing and beefier bearings.

Unca_Sam

What's escape velocity again?

Happy Feet

If we take the current speed record of 189.3mph as a max speed on Earth to model from:

Aerodynamic resistance should be the same as drafting behind a canopy (as in the earth speed record attempt) considering there is no atmosphere on the moon.

Gravitational pull/downward frictional resistance is 1/6th. If there is a linear relation (?) and all things being equal, one should travel six times faster in 1/6 gravity.

so I'll say a base figure of 6x183.9 or 1103.4 mph.

That assumes sufficient staggered gearing and run on distance to ramp up to that speed and a human with enough physical capacity to maintain a constant RPM until you get there.

I think however, there will be a reduction in that speed estimate due to frictional losses from the bikes components themselves.

woodcraft

to test, instead of a wind tunnel,

need a vacuum tunnel.

Barry2 

Hyperloop here we come

Barry

Pop N Wood

Not applicable. The dragster is creating a vacuum so the cyclist is getting a boost from the air pressure behind her.

Remove the person from the equation so you don't need to worry about losses of flexing the space suit. Instead assume some max wattage motor. The real work left is computing the rolling resistance of the tires, bearings and drive train. Would need a lot of gearing so losses could be substantial. Subtract some losses for frame flexing due to the irregular pedal motion and vibration due to imperfectly balanced wheels. Add a factor for the bearing grease becoming more viscous and clearances shrinking with temperature.

Max speed will be where these frictional losses equal assumed power output.

Would have to think about the effect of gravity on the process. Level tarmac so assumed no change in potential energy. Force equal mass, not weight, thus times acceleration so the reduced gravity has no real affect on top speed. With less normal force there would be less traction, but with only a half horsepower or so that may not matter either.

In all honesty I think energy losses flexing the suit would be the limiting factor.

Won't break 100 mph. Hell, in suit probably won't hit half of that.

livedarklions

Well, it's a moon bike, so definitely not normal. Not sure how the stresses compare when the gravity is reduced and air friction is eliminated.

ofajen

Fun question!

My back of envelope calc estimate for top possible speed is about 1500 km/hr.

I assume the rider can put out about 200 watts. I also assume 0.9 coefficient of static friction of tires and road. I also estimated rolling resistance based on 40 watts at 50 km/hour on Earth scaling linearly to high speed (200 watts at 250 km/hr) but reduced by the lower gravitational force (so top speed is 250 x 6). And that would be the dominant force to work against since there is no gravity term and no air resistance term.

The rider would be limited by available traction to accelerate at about 6 km/hr per second so it would take about five minutes to reach that speed (and at least that long to slow down before turning around!).

However, thermal failure of drivetrain components might be an issue at anything close to that speed. Also the road has to be super smooth and the wheels need to be true and round.

Otto

DrIsotope

The single biggest force a cyclist has to overcome is wind resistance. The moon has no atmosphere, so that is off the table. The rider then only needs to overcome minimal rolling resistance, and account for equally minor mechanical losses. According to the calculator here, a combined rider + bike of 150kg, unrestrained by drag, requires just 176W to maintain 60m/s (117mph.)

rydabent

I would suggest a fat tire trike. Three wheels would not sink as far into the moon dust. Also the open position of a trike rider would better accommodate a rider in a space suit.

ofajen

That site is using weight on Earth (of course) and the rolling resistance term includes the weight of bike and rider, so power requirements would be reduced by a factor of six on the moon. That would suggest 360 m/s at 176 watts on the Moon. That’s about 1300 km/hr.

Otto

DrIsotope

I don't think it would factor quite that heavily. On flat ground on Earth, gravity manifests itself as rolling resistance, so Lunar rolling resistance would effectively be 1/6th of that on Earth. But rolling resistance is a couple of percent at most here-- probably something along the lines of 2-3m/s @ 176W on the moon.

70sSanO

I hear that even the moon doesn't have bikes under $1000.

John

Barry2 

It's a NASA bike.... Getting it to cost over $1,000 no problem.

BTW - Did you know NASA left the keys in the Lunar Rover.
After all, what country is going to steal it?

Barry

datlas 

Pretty sure regular pneumatic tires would pop instantly in the vacuum of the moon. Solid rubber makes more sense. Hypothetically, of course.