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# Bicycle Water Ballast

 (+4, -5) [vote for, against]

Today I used tried out my new back wheel pannier for the first time to carry home some groceries on my bike. I felt a weir sensation when going on level ground. It took much less pedaling power to get going quickly, and stay at that speed than usual.
This got me thinking about gliders and how adding extra weight actually improves their speed. It's a trade off of course. By carrying that extra weight, the glider can fly faster, while maintaining the same glide ratio as before. The down side is that if you find a source of lift, you won't be able to utilize it as well and it will take longer to gain a given altitude.
On a bike it is very similar. Finding lift means going up hill. Gliding means going down hill. If a bike carried water ballast, it would be tougher to go up hill, but on flat or even down sloping terrain, I can't help to think that there would an advantage. I'll need help to crunch the actual numbers, but my thinking is along these lines. While carrying 0 kg ballast, you can comfortably sustain 30 km/h. While carrying 20kg ballast, you can sustain perhaps 33 km/h.
Before you fish this by saying, "ya, but it takes you much more effort to get to the 33km/h in the first place". My answer is that for a sufficiently long journey without many starts and stops, this may actually be more energy efficient.
I'm sorry that I can't provide the hard math to prove it. This idea is partially a question to the engineers out there - how would you calculate the trade off between, power applied to the pedals, inertia, wind resistance, and tire friction?
On a glider, the result ends up looking like a upside down U when plotted on a sink rate/speed graph. This helps the pilots pick the optimal ballast for the conditions. Weak lift + short journey = no ballast. Strong lift + long journey = lots of ballast. I'm wondering if something similar is possible for a bike. Lots of hills + starts and stops = no ballast. Long journey + against the wind + no hills = ballast.
 — ixnaum, Sep 15 2009

bicycle with speed governor bicycle_20with_20speed_20governor
my first idea on HB [xaviergisz, Sep 15 2009]

rolling resistance http://en.wikipedia.../Rolling_resistance
Normal force is a multiplier of the coefficient [Sparkyplugclean, Sep 16 2009]

Eppure, si muove più velocemente http://techtv.mit.e...nd-coin-in-a-vacuum
As a kid, I loved the 5 story tall version at the local science museum. [mouseposture, Jan 09 2011]

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 Wind resistance goes up with the cube of speed, no matter what. So even if the extra inertia helps you maintain your speed better, you're putting more energy in to do it. Plus rolling resistance increases linearly with weight, so you're losing more energy there.

 That cube of the speed thing also means that the slow climbing is more efficient (in terms of total energy of the bicycle rider system) than the fast downhill. Thus the energy expended to climb a hill is always more than the energy recovered going down the far side. Since the more weight you're carrying, the more energy you spend going up, more weight equals a much higher total expenditure.

 The net result is that more weight is not a good thing under any circumstances except going down hill, and there it's offset by the climbing in the first place. There's a reason why good climbers are either skinny guys or guys with legs that resemble battleship guns.

What the extra inertia does is smooth out your power input. Thus if you have a stomping pedal motion, or lots of very small ups and downs it's a lot easier to glide over them without noticing, but you're still putting more energy in.
 — MechE, Sep 15 2009

.... downhill maybe, on the flats no way. A bicycle isn't a glider and so there really is no correlation. All you noticed was that the heavier load rolled further for a given speed and that you could exert yourself then relax which feels easier to the less hardcore rider.
 — WcW, Sep 15 2009

...and, as [MechE] says, the "King of the Mountains" cyclists in major events are always the thin, wirey guys, not the guys who have all the power and speed.
 — hippo, Sep 15 2009

// I felt a weir sensation //
Stay away from weirs on your bicycle.

Perhaps you should propose a system of ballast stations at the top of all downhill sections.
 — coprocephalous, Sep 15 2009

//a system of ballast stations at the top of all downhill sections// Presumably, to avoid stopping, they use well-aimed hosepipes to fill your ballast tanks?
 — pocmloc, Sep 15 2009

I like thinking of a road course that is generally downhill, but with smaller hills on it. It would be a good feeling to not have to pedal at all on these sections. Also for those (like myself) that bike to and from work/school each day on a path that is defined as mostyl uphill or mostly downhill, one could, for example, fill up ballast tanks in the morning, coast faster and easier to the destination, empty and return uphill as normal. [+].
 — jellydoughnut, Sep 15 2009

//ballast..downhill// well-aimed hosepipes and those jokers that always spray me with them. My ballast bags were full on the downhill, and that poor pedestrian... well, I just couldn't stop.
 — Sparkyplugclean, Sep 15 2009

Ok I admit, I was way too optimistic by including flats as one of the possible scenario. In my defense, I did explain that I don't know what the trade off is and what the actual "benefit" scenarios are.

Here is a thought experiment: There are two cloned racers (same physical ability) weighing 60 KG. One with state of the art light weight bike at 7 KG (total 67 KG). The other with a bicycle equipped with a water ballast system that adds adds extra 40 KG inside the aerodynamic frame of the bike (107 KG total). Who will win? I guess more accurately, who will win with what slope?
On one extreme, with the slope being nearly level, the light bike would win. What about the other extreme? 90 degree drop straight down? In that case terminal velocity comes into play. Please correct my calculations .. this is the first time I've calculated anything like this in a while:

C=0.8 (drag coefficient estimate)
m=67 and 107 kg respectively
g=9.81 m/s^2 (they are on earth)
p=1.2 kg/m^3 (approx density of air)
A=0.3 m^2 (approx projected area)

Vt=SQRT(2mg/pAC)
Vt-heavy=307 Km/h
Vt-light=243 Km/h

Clearly, the heavy bike kicked some serious ass. Now unless these people had a parachute they are dead. I can't wrap my head around doing a calculation for a more reasonable slope taking into account rolling friction etc. One of the biggest obstacles to that calculation is that I don't know how to take slope into account for rolling friction. For example, the wheel friction on a level slope will be higher than on a 89.9 degree slope (i'm only guessing I need some trig function to take that into account) ... help! ...
... wild guess... is it F=CN * cos( down_hill_angle ) ? ??
 — ixnaum, Sep 16 2009

 Weight helps on a smooth downhill, but other then the aforementioned good and bad commute directions (or free-ride mountain biking) you never have a pure downhill. Free Riders slow down because of the difficulty of rough terrain downhilling.

 Commuters slow down because of pavement. I have hit 64 kph on a downhill, but only with the advantage of seeing several riders in front of me to pick out a clean line. At those speeds, a small piece of gravel can be dangerous, a large piece, or worse a blown front tire can be deadly. And those speeds can be attained by fairly light riders on a relatively moderate downhill. This might gain something on very moderate (1-2% grades) downhills, but only there, by 4-5% most people will be reaching for the brakes not extra speed. (5% is 5 meters in 100, or about a 3 degree downslope. Major roads rarely go above 6%, minor roads rarely go above 8% or 5 degrees).

 The only people who are going to go that fast routinely (or faster) are racers who had to climb to get there first. Also, at high speeds, a nice tight aerodynamic tuck is going to do you more good than extra weight. The equivalent to shaving off 1-2 pounds in a climb, or adding it in a downhill is a projection into the air stream the size of a pencil, end on.

A state of the art bike these days is closer to 3-5 kilos, and there's a reason for that.
 — MechE, Sep 16 2009

The pros get ultralight bikes because they know their course isn't all downhill. The heavy bike kicks ass down hill. I have a mountain bike and have coasted past fellow riders on lighter road bikes downhill without any extra effort. It's a disadvantage uphill, and breaks even on flat. Anything improved by retention of momentum is equally lost by difficulty of acceleration. There's no such thing as a free lunch.
 — jellydoughnut, Sep 16 2009

Also, acceleration due to gravity on a slope is the component along the slope, friction involves the component of weight into the slope, so the acceleration will be the sine of the slope angle, the rolling friction will be the cosine. Rolling friction is pretty much negligible compared to air resistance above about 20 kph though, so the decreased acceleration due to gravity in the terminal velocity equation is your major player, and remember, you're probably dealing with a less than 10% down angle.
 — MechE, Sep 16 2009

[Jelly] if the road bike riders go into a full aero tuck, they'll blow past you. Wind is more important than weight.
 — MechE, Sep 16 2009

 Ah, memories.

 In my earlier youth, I rode/hitchhiked an old "waterpipe" ten-speed from San Diego to San Francisco and back.

 At one point in the hills above Santa Barbara, I was downhilling it fast and furious carrying fifty pounds of luggage, in a tight tuck seated on the rear rack with my chin on the seat,when a Ford Pinto pulled up next to me, rolled down the window, and shouted "Sixty-five! I thought you might like to know!" and moved on.

I do remember thinking that if anything went wrong it would hurt. In your early twenties that's negotiable.
 — normzone, Sep 16 2009

Can I ask why my earlier trebuchet anno was deleted?
Whilst the trebuchet may have been flippant, I think the comment about peak density of water at 277K utterly relevant.
 — coprocephalous, Sep 16 2009

[coprocephalous] ... I definitely didn't delete it. Not sure who else has the rights to do that.
 — ixnaum, Sep 16 2009

//There's no such thing as a free lunch.//
Just wanted to point out that I'm keenly aware of that. I should have written more background into the idea, but the glider ballast I'm mentioning is jettisonable. You can start with 200 KG water ballast in your wings and if conditions change, or when ready to land jettison all or part of your ballast. This is the assumption for the bike water ballast system too. The aerodynamic profile of a bike with 50 KG water ballast would be equivalent to that carrying 0 KG. All that would change is the mass. That's why I find any references to "blowing past your competitor if you tuck in more" irrelevant since that's a constant. Both bikes and both riders have the same wind resistance, only one can start with a full tank of ballast on top of a hill and then jettison it at the bottom ready for the next climb.
Also in my opinion these anecdotal stories are fun and interesting. But, what's the math behind this? Can someone actually calculate who will win, on what surface, with which watter ballast? Obviously I showed that during free fall, ballast wins - at what downhill grade does this cease being the case?
 — ixnaum, Sep 16 2009

The aero tuck comment was in response to mountain bike vs road, not directly to the idea. The major point of my comment above is that safety concerns with regard to speed will kick in well before the heavier cyclist starts winning. Actually doing the math is difficult, because there are so many factors involved. Even if you cancel everything out though, and ignore rolling friction, the equation becomes simply Vt=Sqrt[2m sin(slope)g/pAV]. What becomes critical is determining how long of a slope it takes to reach terminal velocity on a , being generous, 10 degree down slope.

 — MechE, Sep 16 2009

[MechE] thank you very much for that formula. This is what I got:
Vt=Sqrt[2m sin(slope)g/pAV]

0.5% grade: Vt_heavy = 21 Km/h, Vt_light = 17 Km/h
1.0% grade: Vt_heavy = 30 Km/h, Vt_light = 24 Km/h
2.0% grade: Vt_heavy = 53 Km/h, Vt_light = 42 Km/h
5.0% grade: Vt_heavy = 68 Km/h, Vt_light = 54 Km/h

Notice that that's a constant improvement by a factor of 1.26 (not bad at all). Now, as you mentioned, at lower speed rolling friction DOES become increasing component. So this constant improvement will not hold for sure, it will be slightly lower for lower road gradients. In a race where seconds count, I can't help to think that the jettisonable ballast would give a rider an edge ...
 — ixnaum, Sep 16 2009

I should add, that of course on say a 0.5% grade the cyclists would still be pedaling, because they are nowhere near a "dangerous" speed for the conditions. However, that 1.26 improvement factor means that one rider pedals less hard, and/or can get into perfect aero-tuck position since they are not pedaling as much and not disturbing their form.
 — ixnaum, Sep 16 2009

 If the ride starts at the top of a hill, and you can truly find a way to add a significant amount of ballast without a) affecting the aerodynamics of the bike and b) increasing the base weight of the bike once the ballast is drained or dropped (this includes undrained liquid, then there is some benefit to this. I don't think you're going to find any cyclists who want to try it though.

Other than downhill mountain biking I'm not aware of any cycling which is all down hill, and in free ride, terminal velocity is defined as the speed at which you hit that tree, nothing to do with wind resistance.
 — MechE, Sep 16 2009

I'm pretty sure that the benefits gliders get from added ballast is due in some way either to their lack of rolling friction, or the fact that some other vehicle takes them up before they begin their descent.
 — ye_river_xiv, Sep 16 2009

Does anyone know what terminal velocity on a bike is? I'm not sure but I'm pretty sure it's above what a sensible rider would chance anyway. I went about 55 down the side of a paved mountain once and had only noticed that the wind was becoming a danger to my staying on the bike.
 — jellydoughnut, Sep 16 2009

ixnaum's calculations are pretty close, it is weight dependent.
 — MechE, Sep 16 2009

Who knows why, but I decided to revisit this idea more than a year later. I think I'm still trying to wrap my head around this. I re-read the comments and re-thought the things regarding rolling friction and I have to say that now I agree with the following comment:

//safety concerns with regard to speed will kick in well before the heavier cyclist starts winning. MechE//

The only way this idea could ever work is if someone invented bicycle tires with a rolling coefficient much lower than the current 0.0025 ... for example, 0.001 would start to be feasible...
hmm ... railroad steel wheel on steel rail has that coefficient according to Wikipedia... maybe in a half-baked bicycle race on down-hill rail road- tracks, the ballast laden bike would win :-) .... also from what I read larger radius of the wheel will decrease the rolling coefficient. If riders were allowed to change bicycles in the middle of the race, this might start to be interesting. Giant wheeled + heavier bicycle for downhill section ... smaller wheels and lighter bike for uphill. I know none of this is going to happen in a real race ... but I'm just trying to stimulate conversation to see if I finally understand how this works.
 — ixnaum, Jan 09 2011

 I missed this idea when it was posted, as I was on a bicycle trip. I was riding an overloaded recumbent, so I am going to speak as an expert when I say that weight is evil, and streamlining isn't much help.

 The comparison with gliders is interesting, but the main differences between gliders and bikes is that the gliders get to pick their hill, so to speak. A glider pilot can descend at any angle he chooses, and he has a little graph to show him the best speed-to-distance airspeed/angle. (And, as was said, he isn't doing the pedalling on the way up.) A bicyclist has to deal with the downhill that he gets, and all the dogs, corners and cars that it has.

 This idea would work, on some hills, in some circumstances, if you could pick up some weight at the top, and drop it at the bottom. Extra weight would take you downhill faster, and give you more speed to coast up the next hill, provided the streamlining was not affected. And provided the next hill was in the right place, was low enough to coast over and didn't have a dog on it. But those circumstances are ideal.

 Speaking of ideals, consider a idealistic bike with double the weight. It would take twice as long to get up a hill, and come down twice as fast, right? So it all evens out, right? But the time spend going uphill just doubled from one agonizing hour to two gut-wrenching hours, while the descent halved from ten exhilarating minutes to five white-knuckled minutes. The trip over the hill is now fifty-five minute longer, and a lot less fun. And actually, the climb uphill would more than double, due to friction, while the downhill would not be cut anywhere near in half, again due to friction and air drag.

 The first paragraph of the idea seems wrong. I don't know what was happening, but it isn't anything I've noticed. If it is referring to a streamlining effect, it is still a bit odd.

[NotationToby], your bit about CO2 balloons confuses me as to what you even thought would happen, and the ground effect part is even wronger.
 — baconbrain, Jan 09 2011

//Extra weight would bring you downhill faster// Ahh Galileo! Wherefore art though, Galileo!
 — FlyingToaster, Jan 09 2011

 In air with the same frontal area, yes, increased weight will bring you downhill faster. Terminal velocity, it is called. A heavier object has more oomph to get through the same amount of air.

 Try this. Go get two identical cardboard boxes, fill one with bricks, books or beans, and chuck them both off the top of a very tall building. The heavy one will hit the ground first because it has more weight to push it through the air's resistance.

 What Galileo showed was that in circumstances where air resistance is not a factor, such as with cannonballs from a medium-tall building, different weights do indeed fall at the same speed. People had thought that lighter objects, such as feathers, falling slower had to do with weight, Galileo showed that it had to do with density in air, which is precisely my point.

 A bike with filled panniers will go downhill faster than a the same bike with the same panniers empty, because of air resistance. Galileo wouldn't disagree.

 // Wherefore art though, Galileo! //

 If you were trying to write "Wherefore art thou Galileo?", the answer is "because he was the first of Vincenzo Galilei's children".

If you thought you were asking where Galileo is, you need to brush up on your Elizabethan.
 — baconbrain, Jan 09 2011

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