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 02-05-15, 06:27 AM #1 erashish14 Newbie Thread Starter   Join Date: Jun 2010 Location: India Bikes: Posts: 3 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Air Drag & Rolling Friction Calculation Hi Experts, I like to know, how Air drag & Rolling friction via Bottle dynamo can be calculated i.e. Formula for the same. Actually my nephew is 15yrs old, he loves riding bicycle a lot, he carry his bicycle every where, whether its day or night, so i'm planning to install bottle dynamo on his bicycle wheel (front & rear), which help in providing light during night while riding, but i'm confused, did those bottle dynamos will make his bicycling harder, if yes, then how much, how to calculate that. hence i required formulas for air drag & rolling friction calculation & other parameters which effects his cycling. Thanks in advance Ashish
02-05-15, 05:58 PM   #2
christo930
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 Originally Posted by erashish14 Hi Experts, I like to know, how Air drag & Rolling friction via Bottle dynamo can be calculated i.e. Formula for the same. Actually my nephew is 15yrs old, he loves riding bicycle a lot, he carry his bicycle every where, whether its day or night, so i'm planning to install bottle dynamo on his bicycle wheel (front & rear), which help in providing light during night while riding, but i'm confused, did those bottle dynamos will make his bicycling harder, if yes, then how much, how to calculate that. hence i required formulas for air drag & rolling friction calculation & other parameters which effects his cycling. Thanks in advance Ashish
Bottle type generators suck and they significantly add to the resistance of your pedaling as well as ruining your tire. (the bottle kits come front and rear all running from 1 bottle) Not only is the bottle stealing power to generate electricity, but it is stealing power by being tightly compressed against the sidewall. The electrical problem is that they don't have a voltage regulator in them and when you are going downhill, you end up with too much voltage and blow the lamp and those lamp bulbs are impossible to find. When one light blows, the other blows.

You would be MUCH better off using a Shimano hub generator and lights kit. The resistance is far less and the voltage is much more stable. They use LED lights which don't blow out and everyone I have ever talked to said they couldn't feel the difference in the pedals. I'm pretty sure they are only 3 watts (6V and 500mA). The downside is that if you don't know how to lace a rim, you will have to buy a built rim with the hub. In the long run, this is by far the best way to go for on-board generation.

The other thing is that LED lights are dirt cheap and the batteries last a long time. Even a cheap dollar store led flashlight will be brighter than either option above and run for many, many hours on 3 alkaline or rechargeable batteries. You can go monster and get a 5 hour battery pack and a super bright light (well over 1000 lumen, with some of them going to 2000 or more, brighter than a motorcycle). That is the way I went. I paid <\$25 shipped for a light and a 5 hour battery (more like 3) that can also strobe. The strobe is amazing and people can see you for blocks. You can also put them on your head with the included band which means you can put that light anywhere it's needed.

As for calculation, it's impossible with the bottle lights because it depends on how hard it's leaning on the tire. But they are very inefficient and unreliable which is why nobody carries them anymore. The hub motor, even if it is only 50% efficient will rob you of about 6-10 watts of pedaling power.

Air resistance will be unnoticeable. Your nephew's clothing will probably have a larger effect than the lights or the bottle. His body is the overwhelming majority of the air resistance.

Last edited by christo930; 02-05-15 at 06:24 PM.

 02-06-15, 06:18 AM #3 erashish14 Newbie Thread Starter   Join Date: Jun 2010 Location: India Bikes: Posts: 3 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Thanks Dear christo930, Thanks for such a elaborate explanation, but if you don't mind, is there any formula for calculating air drag & rolling resistance i.e. how much more power is required to maintain the same pace while riding a bicycle, if i add 2 bottle dynamo or say 1 bottle dynamo for lighting the path. Thanks in Advance Ashish
02-06-15, 12:47 PM   #4
christo930
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 Originally Posted by erashish14 Dear christo930, Thanks for such a elaborate explanation, but if you don't mind, is there any formula for calculating air drag & rolling resistance i.e. how much more power is required to maintain the same pace while riding a bicycle, if i add 2 bottle dynamo or say 1 bottle dynamo for lighting the path. Thanks in Advance Ashish
Air drag is a rounding error for a bottle type generator. Much of the bottle sits right in front of the fork, at most it will add 2 square inches of additional surface, the light itself will add more air resistance than the bottle (but the light is in front of your body meaning the air was going to hit you anyway). There is no way to estimate upfront how much additional resistance a bottle would put on the ride because you cannot know in advance how hard the bottle will lean on the tire. If you want to rough it, take the electric output of the bottle, double it and then add some more and that will be a rough estimation but will not take the pressure on the tire into consideration. I used one of them when I was a kid on a 20" wheel kid's bike and I can recall that it added a LOT of resistance to the pedaling, keeping in mind I was a small child when I used it.

The bottle light generators that I've seen are 2 light generators producing 2 different voltages, one for the front lamp and the other for a rear lamp. I've never seen one with caps to keep the lights going when you are stopped or going to slow to generate enough to light the lamp. Point is, you won't be putting 2 bottles on the bike and there will be no light when going slowly or stopping. If you do decide to get one, take the light apart and put a voltage regular in the light and make sure the black part of the voltage regulator is resting on a highly thermally conductive metal. 12V regulators are easy to find and easy to install, but I think the back light uses 6V and you might have to special order it. If you have some electronics knowledge you can add a decent sized cap to the light as well so the light keeps going while slowed down. Just keep in mind that these are VERY inefficient, not only are the bottles themselves inefficient, the light bulbs are inefficient and they don't make them for LED lights. Almost all of the extra effort will go to inefficiencies and a very inefficient lamp (generating a lot of heat and little light).