# what is this part on my bike tire? **URL:** \ **Category:** Factual Questions\ **Created:** [June 6, 2015, 2:51pm UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788 "2015-06-06T14:51:30Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![cornopean](https://avatars.discourse-cdn.com/v4/letter/c/8c91f0/32.png) [@cornopean](https://boards.straightdope.com/u/cornopean)\ **Post date:** [June 6, 2015, 2:51pm UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/1 "2015-06-06T14:51:30Z")
Notice this part I circled. I don’t get what the function of this part is. can anyone tell me what good this part is? [http://www.mediafire.com/view/lcuj1bqb1ki1crd/Capture.PNG](http://www.mediafire.com/view/lcuj1bqb1ki1crd/Capture.PNG) ---
**Author:** ![Doug\_K](https://avatars.discourse-cdn.com/v4/letter/d/b5ac83/32.png) [@Doug\_K](https://boards.straightdope.com/u/Doug_K)\ **Post date:** [June 6, 2015, 3:34pm UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/2 "2015-06-06T15:34:07Z")
> [@cornopean](#): > > Notice this part I circled. I don’t get what the function of this part is. can anyone tell me what good this part is? [http://www.mediafire.com/view/lcuj1bqb1ki1crd/Capture.PNG](http://www.mediafire.com/view/lcuj1bqb1ki1crd/Capture.PNG) That’s the coaster brake. It’s what causes the bike to stop when you backpedal. But it’s not part of the tire, it’s part of the wheel. More specifically, part of the hub. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 6, 2015, 3:34pm UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/3 "2015-06-06T15:34:08Z")
Does your bike have a hub brake (‘coaster brake’)? If so, I’d say it’s the [brake arm](http://sheldonbrown.com/coaster-brakes.html). EDIT: Dammit, **Doug K.**! Well, it serves me right for looking for a cite. 😛 . ---
**Author:** ![chappachula](https://avatars.discourse-cdn.com/v4/letter/c/d2c977/32.png) [@chappachula](https://boards.straightdope.com/u/chappachula)\ **Post date:** [June 6, 2015, 3:35pm UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/4 "2015-06-06T15:35:28Z")
It looks like the arm of a coaster brake. But in real life, (unlike your pic) that piece has to be [connected to the frame of the bike.](https://www.google.co.il/search?q=coaster+brake&biw=1113&bih=723&tbm=isch&imgil=yNxKhmOqOb3ZTM%253A%253BstNgAGOQ5YVVxM%253Bhttp%25253A%25252F%25252Fbikeville.blogspot.com%25252F2013%25252F01%25252Fcampagnolo-coaster-brake-hub.html&source=iu&pf=m&fir=yNxKhmOqOb3ZTM%253A%252CstNgAGOQ5YVVxM%252C_&usg=__5VpdZ6Ne-uqEbWJ2apF85uxSg70%3D&ved=0CDoQyjc&ei=mBFzVZraAqq17gby4YHICA#imgrc=yNxKhmOqOb3ZTM%253A%3BstNgAGOQ5YVVxM%3Bhttp%253A%252F%252F2.bp.blogspot.com%252F-78WBoicq_h4%252FUOS76vsX4UI%252FAAAAAAAAHzw%252Fmh9K1lzhm6w%252Fs1600%252FCampCoasterBrake.png%3Bhttp%253A%252F%252Fbikeville.blogspot.com%252F2013%252F01%252Fcampagnolo-coaster-brake-hub.html%3B805%3B540) . ---
**Author:** ![Doug\_K](https://avatars.discourse-cdn.com/v4/letter/d/b5ac83/32.png) [@Doug\_K](https://boards.straightdope.com/u/Doug_K)\ **Post date:** [June 6, 2015, 3:49pm UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/5 "2015-06-06T15:49:17Z")
> [@Johnny\_L.A](#): > > Does your bike have a hub brake (‘coaster brake’)? If so, I’d say it’s the [brake arm](http://sheldonbrown.com/coaster-brakes.html). > > EDIT: Dammit, **Doug K.**! Well, it serves me right for looking for a cite. 😛 > > . You must have hit “submit” at the same time I did. Your reply showed up immediately after mine when I hit “submit”. ---
**Author:** ![Orson\_Carte](https://avatars.discourse-cdn.com/v4/letter/o/ed8c4c/32.png) [@Orson\_Carte](https://boards.straightdope.com/u/Orson_Carte)\ **Post date:** [June 7, 2015, 1:01am UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/6 "2015-06-07T01:01:54Z")
Of greater curiosity, where’s the sprocket gone? ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 7, 2015, 11:26am UTC](https://boards.straightdope.com/t/what-is-this-part-on-my-bike-tire/721788/7 "2015-06-07T11:26:31Z")
When you apply a coaster brake the wheel (driven by the inertia of you and your bike) is being connected through the friction part of the brake to the inner part of the hub. This applies tremendous amounts of torque to the inner hub. That arm is strongly connected to the inner hub and reaches out to attach to the frame. This takes the strain from the torque on the inner hub and applies it safely to the frame. Without the arm, application of the coaster brake would twist the axle in the dropouts.