saw an interesting thing on a bike at the local library today. bike had this plastic cover over the valve stem and was wondering if anyone knows the purpose of this device and where i can get them if they are useful
also the bike had one of those generators that rub against the rim of the rear wheel to generate electricty for the lights. do these type of generators provide alot of rolling resistance while riding the bycycle ? where can i get one of these also.

The generators set you are looking for are not popular as they once was. But they are still available at QBP which you LBS can order from. They range anywhere from $ 20 and up. Yes they do add resistance to the tire and also wear the tire sooner. The plastic cover you saw maybe lights too and they can be order at the shop too.

Those generators run against the tire sidewall, not the rim. They are much less common than in the past. Here is a higher-quality one - the light would be bought separately.
https://harriscyclery.net/itemdetails.cfm?ID=1580

There are also generators that run centered on the tread of the tire.

Much more rolling resistance than a battery-powered LED headlight (which would have none).

You saw a plastic cover on a valve stem and you don't know what it is but you want one? hmmmmmm
Could it have been a battery-powered light of some sort? Wasn't a valve cap, was it? (Plastic cover that screws on)

i thought maybe the see thru valve cup cover was to keep the air from escaping from the tire tube, so one of those would be nice to have

wut. See through valve cap?

And a hub dynamo, built inside the front hub, will add far less resistance than a bottle (sidewall) dynamo. The good thing about bottle dynamo is, when it's not in use, there's zero added resistance. With hub dynamo there's always some, even when it's switched off. If you're interested in dynamo powered lights, both are worth looking into.

--J

Yeah, there is some resistance from a bottle generator. I have one on an older bike and it puts out a pretty bright, wide beam of light. I can feel the difference with the generator, but it's not too bad.

There are also hub generators (aka "dynohub") that have less resistance than either tire-driven type, but it does mean you need to build a wheel around them:

Here's another bottle generator: https://www.velo-orange.com/spdy.html It's a little cheaper than the Busch & Müller linked to from Harris, but probably not as nice.

There are 3 basic types of generators:

1) "bottle" generators that have a roller that goes against the side of the tire. They can be disengaged so they don't touch the tire when not in use, so they have no drag. When in use they tend to have the highest drag when in use. They also tend to wear out the tire sidewall. Maybe it wear out before the tread wears out from riding, maybe it outlasts the tread. YMMV. They also can be difficult to get properly aligned so they get good grip on the tire. They also have a reputation for slipping when wet, which is when you really don't want your lights to get flakey on you. They're the cheapest, and you get what you pay for.

2) roller or "bottom bracket" generators. They mount behind the bottom bracket and have a roller that contacts the center of the tire tread. Completely disengages so there is no drag when you're not using it. I'm not sure how the drag compares to a bottler generator when in use, but I suspect it would be lower. I'm also pretty certain it will be higher than most hub generators. They're pretty rare now days.

3) Hub generator. This is built into a hub for the front wheel, so it requires buying/building a new front wheel. They have the lowest drag when in use, but also have a slight drag when not in use. But the drag is very low in both cases. The best ones have drag about equal to rising about 5 feet per mile, so it's unlikely that you'd even notice it. They're the best, but the most expensive. They're what most people use now days.

If you want to get into lighting and don't want to spend much to start, you could start with a decent LED light and a bottle generator. Go ahead and spend the money for a decent light, since you will keep it, but go with a bottle generator to get started. Then plan on upgrading to a hub generator down the road. Both bottle and hub generators produce 3 watts at 6 volts AC, so you should be able to use the same light when you upgrade.

Just be careful to get a light that has a on/off switch. Lights are sometimes sold as "for bottle generators" that don't have a switch, because you can turn it off my disengaging the generator. But when you upgrade to a hub generator, your light will always be on. Although, the drag from a hub generator is low enough that some people just keep their lights on all the time to be more visible during the day. Only do this with LED lights, since you'll go through too many light bulbs with halogen lights. For that matter, only get LED lights, they are far superior to the halogens now days.

A good page on generators is https://www.nordicgroup.us/s78/dynamo.html although it is a couple of years old, and at least one link is out of date (the link to the bottom bracket generator at www.bicycletires.com - they no longer seem to carry it). Also, www.peterwhitecycles.com also has lots of good info.

Keith