The little knob fell off my tire - the one that is for air. Do I need that? Will I lose air from the tire?

its called a valve stem cap, and the sole purpose is to keep dirt out of the valve stem. you will be better off getting a new one when you get a chance to, but its not a do or die priority

you will not loose any air even slowly without the cap. If you do it is the schrader valve internal to the stem. Even with a cap on the valve steam if it still leaks down slow the schrader valve is defective not a cap issue

hey susan! I think you may need the cap, the only reason I say this is because even though air won't just "fly" out of the tire I believe you will be losing air. It may take time for you to notice any significant amount of loss but I would just check the air pressure more often to see it I need to get a replacement cap.

The type of vavle used in auto wheels is called a Schrader valve, They have a spring loaded center piece that is pushed in when you add air. When you pull the air chuck way, the spring pushes the valve shut.

At higher speeds, the centripical force generated by the spinning wheel can cause the spring to compress, causing you to lose air. Yes, I've seen this happen and we require it on all race bikes.

You should be ok, without it, but I would suggest getting some new ones to replace it. They are usually sold in packs of four or five and are very cheap.

No, you don't need it to keep the air in. There is a pressure valve on the tire stem. The little cap keeps debris out of this valve. Any auto shop will gladly give you one for free to replace the missing one.

Good luck!

If your tire is still full of air you have nothing to worry about. It was just the little plastic cap that serves no purpose other than to keep dirt out of the valve.

You don't need it, precisely. It keeps dirt out of the valve stem, and it is possible for a rock or piece of debris to let air out of your tires. But, it is extremely unlikely.

I wouldn't worry about it.

You wont loose air but it'll get dirt and road debis in it,if you have a bicycle take it off and put it on the car tire,thats all.

No, the cap is to keep dirt from getting in the valve, but it would be advisable to get another one, you don't want it to clog, you can get them at any parts store, very affordable.

I am going to assume that is the cap you are talking about. It purpose is the keep the valve stem from getting dirty. You will be O.K.