Human-Powered Parade Float
July 8, 2011 6:26 AM   Subscribe

How much weight can a human-powered parade float (featuring two side-by-side 10-speed bicycle drive trains coupled to the rear axle of the float) reasonably pull along a flat stretch of trolley tracks?

We are building a human-powered parade float/kinetic sculpture for a festival this fall (not Burning Man). The float never needs to travel more than five m.p.h., but we would like to load the float with 8-10 people, and various art pieces to create a fine spectacle as the float travels through the festival along a set of streetcar trolley tracks. The float will be powered by two side-by-side 10-speed bicycle drive trains (welded to the float deck, and coupled by chains to a sprocket on the rear axle of the float). The stokers will be in a seated position, and will consist of two reasonably healthy adults. The bicycle drive trains will take advantage of the 10-speed bicycle gearing mechanism (we are leaving the derailleurs on, and the stokers will be able to freewheel when tired). The rear axle sprocket will likely be in the 10-15 inch range, and the wheels will be 18 inch aluminum car tire rims, chopped in half to fit in the trolley track groove.

My question is: even if we give the rig a nice shove to get it moving along the tracks, what is the maximum weight we can build the rig so that the stokers can actually keep the whole thing moving down the tracks?
This is simple high-school physics, I know, but I was a French be nice.
posted by piedrasyluz to Sports, Hobbies, & Recreation (20 answers total) 3 users marked this as a favorite
The physics isn't actually that straightforward. It sounds like you will have metal wheels on rails, so the rolling resistance should be very low. The initial acceleration will be more challenging, so having people push to get it going will help. If the course is not perfectly flat, any slight incline will be tough. My gut feeling is that 8-10 plus the structure itself and art pieces will be doable.

The counterpart is, stopping that much mass is not easy, so make sure you have very good brakes.
posted by exogenous at 6:39 AM on July 8, 2011

Sorry, by 8-10 I meant 8-10 people.
posted by exogenous at 6:39 AM on July 8, 2011

I don't think you're going to do it with just 2 people powering it. When pedaling just myself on a modern hybrid bike I probably average ~14mph, though I can get up into the 20s for brief periods if I really work at it. You're wanting a speed that's only a little over 1/3 as fast, but with far more than 3x as much weight. Each rider pulling 5-6 people (including himself) plus the structure to carry them, plus sculptural elements, plus the likely possibility that this is all cobbled together out of improvised materials that are far heavier than the frame and wheels of a nice bicycle -- no way can I see this working.
posted by jon1270 at 6:54 AM on July 8, 2011

We moved industrial kitchen equipment on a trailer powered by bicycles, and that was probably 1000-1500 pounds...but that was rubber on road pulled by a single bicycler. Literally, YMMV.
posted by schyler523 at 7:00 AM on July 8, 2011

Aerodynamic drag increases with the cube of velocity, so (all other things being the same) going 1/3 as fast would produce 1/27 the drag. Of course a parade float is huge and draggy but it is also going very slow (walking pace is around 2 mph I think) so drag isn't too bad.
posted by exogenous at 7:02 AM on July 8, 2011

Assuming a six square meter frontal area, level steel track, and a weight of 1 metric ton, looks very doable:

Frontal Area 6.00 m2
Coefficient Wind Drag 0.50 dimensionless
Air Density 1.226 kg/m3
Weight 1000.0 kg
Coefficient of Rolling 0.001 dimensionless
Grade 0.000 decimal
Wind Resistance 7.4 kg m/s2
Rolling Resistance 9.8 kg m/s2
Slope Force 0.0 kg m/s2
Cadence 100. rev/min
Crank Length 170. mm
Pedal Speed 1.78 m/s
Average Pedal Force 19.3 kg m/s2
Effective Pedaling Range 70. degree
Effective Pedal Force 49.6 kg m/s2
Speed 2.00 m/s
Power 34.3 watts

Plug in your numbers here:

Coefficient of Rolling for steel wheels on steel tracks is 0.001 - 0.0025.
posted by Nothing at 7:13 AM on July 8, 2011 [1 favorite]

For comparison, see the results for a normal bicycle at 8m/s:

Average Pedal Force 156.5 kg m/s2
Effective Pedaling Range 70. degree
Effective Pedal Force 402.3 kg m/s2
Speed 8.00 m/s
Power 278.5 watts
posted by Nothing at 7:15 AM on July 8, 2011

Have you broken it down and looked at what you're considering? Using some ballpark numbers:

-10 people is about 1,800-2,000 pounds
-Structure (steel? wood?), say another 1,000 pounds
-Wheels are about 20 lbs each
-Bikes will be about 20 lbs each

Conservative estimate, let's say about 1.5 tons. It sounds like a lot, but honestly two people can probably get that much weight moving. You might consider gearing down the bikes even more (i.e., don't do a simple 1:1 from the bikes to the drive wheels), especially since you're going so slowly.

From here, let's guess a rolling resistance of about 0.005. Frictional forces from rolling resistance for the assumed weight will be about 17.5 lbf, and we'll also assume wind resistance is negligible (hope you don't have a headwind!). This is just resistance from wheels meeting track; you'll have inefficiencies in the drivetrain and bearings that will add some extra forces that you need to overcome. Let's say 20 lbf as a nice even number. This might be higher if your construction tolerances are poor and, for example, your wheels toe in to the track.

To find the power required, let's assume you're going at your top speed of five miles per hour. 20 lbf * 5 mph = 100 lbf mi/h * 5,280 ft/mi * 1/60 h/min = 8800 lbf ft/min or about 1/4 horsepower, which is about 200 W.

This Wiki link suggests an average "in-shape" human can provide about 3 W/kg on a bicycle, so two average-sized riders should be able to do about 400 W (in the lab under ideal conditions).

If you want to include the effects of wind (and, really, who doesn't?), let's assume your float is big and nasty looking, and guess a drag coefficient of about 1.5 and a frontal area of, oh, 10 feet wide and 15 feet high? Drag force at sea level will be about 15 lbf at five mph, so let's increase our force required to 35 lbf. This brings the power required to about 360 W.

After all this: yes, probably doable. I don't think two people on bikes could get the whole thing going (or if they can it will be very slowly!), and any headwind is going to give you some problems. If you allow one person to coast you will slow down. I suggest swapping out riders regularly.
posted by backseatpilot at 7:23 AM on July 8, 2011

I should add that, of course, this is all ballparking and you may want to rerun the estimate with more refined numbers. Things will get challenging for the stokers if you're going uphill or if it's windy. You might consider adding one or two more bicycles if you can fit them in.
posted by backseatpilot at 7:31 AM on July 8, 2011

Remember that you're going to need a good braking mechanism. Once you've got that thing moving, it's not going to be easy to stop.
posted by adamrice at 7:36 AM on July 8, 2011 [1 favorite]

Given the numbers above I'd be a little skeptical of this working well. Among the assumptions that worry me:

200 Watts per rider: This is closer to a Tour-de-France-level rider than someone who rides occasionally for fun. Holding 200W for sustained periods is very difficult for anyone but endurance cyclists. Also, it's been well established [1] that the output power for a cyclist is aided enormously by wind cooling, which you're basically going to get none of (eg, you need to output the power associated with a 15mph oncoming wind, but you will actually be feeling a 3mph less-cooling wind).

0.00000 grade: Even trolly tracks will have occasional grade. Even very short-scale grades may be tricky (eg, welds joining two pieces of track, where you have to "climb" a 3/16" rise, or a pebble, or a switch gap).

If there's any way to test this ahead of time, even at scale (1 rider, frame, steel wheels on track, sandbags to weigh down the frame to about half expected weight), it would probably be a very good idea.

[1] Dave Gordon Wilson, Bicycle Science
posted by range at 7:56 AM on July 8, 2011

With the speed so low, the aerodynamics of the float won't matter at all. The rolling resistance will matter, but that depends on the quality of your build. If the wheels and rails are both good quality, then getting going would be slow but there would be no problem maintaining speed. Having a tiny amount of driving force in relation to the mass of the object just means slow acceleration and won't impact your speed until air or ground friction start to matter. Everyone has seen videos of one "strongman" pulling a tractor trailer or a jumbojet on their own. It's not a question of whether you'll be able to move the thing or reach 5mph.

What will matter is making it up inclines. Even gentle inclines that you wouldn't notice on foot or making it over bumps in the tracks at rail junctions may prove impossible. That will be hard to predict without more information.

Being able to stop safely on a gentle downhill or even on level ground if you've built up any speed will also be trouble. Brakes made from bike parts are not going to cut it.
posted by dodecapus at 8:38 AM on July 8, 2011

I've done the Pedal Pub, which weighs several thousand pounds and is powered by 7 people (with up to 15 total riders). From experience, I know it can be powered by two people if they're in very good shape and pedaling hard. So I don't think the riders are going to be the problem in this scenario.

I suspect your biggest problem will be getting traction to start the thing moving, given the narrow range of gears that a standard bike drive train has. You could eliminate this problem by just having several strong people push it to start.

I imagine you could do some pre-design tests using a couple of bikes towing a small car in neutral.
posted by miyabo at 8:59 AM on July 8, 2011

Did this in the street with a one-bicycle float for last Halloween and Mardi Gras. The float probably weighed about 2-300 lbs and at max capacity had five people plus gear and coolers etc onboard. The only limit we found was the structural integrity of the float itself.
posted by Scientist at 11:25 AM on July 8, 2011

Nothing beats a test to check your math. Don't wait until race day.

...and "nothing" above... good link. but those zeros in important places are really significant if they are even a LITTLE non-zero.

Make a quick mock-up and test it out, per miyabo and others. Testing is good.
posted by FauxScot at 6:08 PM on July 8, 2011

Here's a tandem pulling a 1000 lb and here's a tandem pulling 1500 lb. Even the second group is less ambitious than what you're proposing, and judging from the sunglasses, jerseys, and clipless shoes, they have some pretty serious cyclists.

Would it be possible to power the float with two tandems instead of two bicycles? That would be much more reasonable.
posted by d. z. wang at 7:58 PM on July 8, 2011

Update: turns out, piedrasyluz and I know each other completely separately from MetaFilter, but it took this thread (and exogenous' intervention) to hook us up. We all live in the same neighborhood, which is the same neighborhood that the float will be operating in. I'm doing the welding, exogenous is involved, and piedrasyluz is doing lots of detailed planning between his junkyard runs. He and I tore up some bikes in my back yard Friday night, and met again last night for more planning (and beer). It looks like this is actually going to happen.
posted by MrMoonPie at 2:00 PM on August 1, 2011

The float will debut Saturday!
posted by exogenous at 7:35 AM on September 12, 2011

And it'll have this on it. Seriously, it'll be epic.
posted by MrMoonPie at 11:18 PM on September 16, 2011

Here is the end result
posted by exogenous at 6:09 PM on September 19, 2011 [1 favorite]

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