# Do pistons stop in an engine when reversing direction? **URL:** \ **Category:** Factual Questions\ **Created:** [November 6, 2000, 6:13am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913 "2000-11-06T06:13:36Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 4, 2018, 1:12am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/21 "2018-01-04T01:12:48Z")
> [@Chronos](#): > > I seem to recall reading that some very high-end engines are designed with a slight twisting motion like **galt** described (lo, these many years ago), precisely to avoid the static friction issue. Umm… you have the entire rotary momentum of the engine to help break free any pathetic static friction. I can’t see any advantage to an over-engineered “twisting” design. ---
**Author:** ![jtur88](https://avatars.discourse-cdn.com/v4/letter/j/e9c0ed/32.png) [@jtur88](https://boards.straightdope.com/u/jtur88)\ **Post date:** [January 4, 2018, 4:00am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/22 "2018-01-04T04:00:03Z")
Before you say it stops for an infinitely short period of time, remember that the material of the piston is not infinitely rigid, and has a certain very small fluidity. So when the piston comes to the end of its stroke and reverses, it distorts ever so slightly, so different molecules in the piston’s composition come to a stop at different times, whose interval is not infinitesimal, but theoretically measurable. Think of the bounding rubber ball analogy, where the molecules of the ball do not all stop simultaneously – so there is never an instant when every molecule of the ball is motioness simultaneously. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 4, 2018, 4:03am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/23 "2018-01-04T04:03:51Z")
> [@jtur88](#): > > Before you say it stops for an infinitely short period of time, remember that the material of the piston is not infinitely rigid, and has a certain very small fluidity. So when the piston comes to the end of its stroke and reverses, it distorts ever so slightly, so different molecules in the piston’s composition come to a stop at different times, whose interval is not infinitesimal, but theoretically measurable. Think of the bounding rubber ball analogy, where the molecules of the ball do not all stop simultaneously – so there is never an instant when every molecule of the ball is motioness simultaneously. That’s pointless pedantry. You could say the same thing about the paperweight sitting on my desk - all it’s atoms are vibrating by some small amount. ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [January 4, 2018, 4:21am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/24 "2018-01-04T04:21:53Z")
> [@beowulff](#): > > That’s pointless pedantry. You could say the same thing about the paperweight sitting on my desk - all it’s atoms are vibrating by some small amount. Pointless pedantry…the best kind of pedantry. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 4, 2018, 4:34am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/25 "2018-01-04T04:34:37Z")
Put it this way: Regardless of the details of atomic vibration and so on, engine pistons _do_ stop for long enough for static (rather than dynamic) friction to become relevant. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 4, 2018, 11:52am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/26 "2018-01-04T11:52:03Z")
> [@Chronos](#): > > Put it this way: Regardless of the details of atomic vibration and so on, engine pistons _do_ stop for long enough for static (rather than dynamic) friction to become relevant. Given there is a film of oil floating between the rings & cylinder wall, I am wondering if there is much of a difference between the dynamic and static friction. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 4, 2018, 12:04pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/27 "2018-01-04T12:04:02Z")
> [@Chronos](#): > > Put it this way: Regardless of the details of atomic vibration and so on, engine pistons _do_ stop for long enough for static (rather than dynamic) friction to become relevant. not as relevant as the flexing of the rings as they transition from purely sealing to sealing and oil-scraping. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [January 4, 2018, 12:17pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/28 "2018-01-04T12:17:25Z")
If you ever wondered about the design of the pistons themselves, this is why they are made to have as little mass as can be consistent with the strength required. A huge, but slow-revving marine diesel engine can have cast steel pistons. The pistons in an F1 or Nascar engine are probably made from a Titanium alloy. Most cars have aluminium pistons. There will also be a small amount of elasticity in the connecting rods, which will absorb some of the “jerk”. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 4, 2018, 12:39pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/29 "2018-01-04T12:39:25Z")
There’s a famous thought experiment about this topic: A locomotive is travelling at 100mph west on a straight track - it has a head-on collision with a bluebottle fly which is travelling at 10mph east. The train obviously wins in this scenario and the fly’s direction is travel is quickly reversed… BUT… There has to be a point at which the fly’s motion goes from eastward to westward - and this transition involves traversing zero - so if the fly is momentarily motionless, and is still in contact with the train, the train must also be considered to be motionless in that moment. I mean, I don’t think it works like that in reality - this is supposed to be an argument that gets you thinking about what really happens (which I think is): parts of the fly stop moving and very small regions of the material of the train also stop moving as individual small parts of the fly are reversed in direction - compression of materials and, I suppose, fields, also takes up the slack ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [January 4, 2018, 1:05pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/30 "2018-01-04T13:05:19Z")
> [@Mangetout](#): > > There’s a famous thought experiment about this topic: > > A locomotive is travelling at 100mph west on a straight track - it has a head-on collision with a bluebottle fly which is travelling at 10mph east. The train obviously wins in this scenario and the fly’s direction is travel is quickly reversed… BUT… > > There has to be a point at which the fly’s motion goes from eastward to westward - and this transition involves traversing zero - so if the fly is momentarily motionless, and is still in contact with the train, the train must also be considered to be motionless in that moment. > > I mean, I don’t think it works like that in reality - this is supposed to be an argument that gets you thinking about what really happens > (which I think is): > > parts of the fly stop moving and very small regions of the material of the train also stop moving as individual small parts of the fly are reversed in direction - compression of materials and, I suppose, fields, also takes up the slack sounds like a suggestion for theslowmoguys. ---
**Author:** ![jtur88](https://avatars.discourse-cdn.com/v4/letter/j/e9c0ed/32.png) [@jtur88](https://boards.straightdope.com/u/jtur88)\ **Post date:** [January 5, 2018, 2:25am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/31 "2018-01-05T02:25:34Z")
> [@beowulff](#): > > That’s pointless pedantry. You could say the same thing about the paperweight sitting on my desk - all it’s atoms are vibrating by some small amount. Which is exactly my point. Introducing the word “infinitesimal” into the discussion, opens the door for exactly the kind of pedantry that we both wish to avoid. To say the stop is infinitesimal, you need to define both “stop” and “infinitesimal”, not just one or the other. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 5, 2018, 3:35am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/32 "2018-01-05T03:35:45Z")
Do you really want us to go all epsilon-delta here? Suffice to say that, under the definition of “stop” used by all physicists, that it does stop. If you’d rather use some nonstandard definition, then you go ahead and tell us what definition you’re using, but standard definitions are standard for a reason. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [January 5, 2018, 4:20am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/33 "2018-01-05T04:20:56Z")
It definitely stops. Just like the surface of the wheel in contact with the ground is stopped no matter how fast the car is going. (Unless the car is skidding) ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [January 5, 2018, 4:32am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/34 "2018-01-05T04:32:23Z")
Cool thread, folks! I just learned from this thread that **Una** ’s name used to be **Anthracite**! When I was in 8th grade general science class, way back circa 1965, the teacher told us that thought experiment about the fly and the locomotive. I think it would be fun to go all epsilon-delta. I’ve haven’t had to think about epsilon-delta for 20-some years! ETA: And higher derivatives for sine and cosine functions sure get interesting! Oh wait a minute, no they don’t. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [January 5, 2018, 5:01am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/35 "2018-01-05T05:01:22Z")
Q: What’s the last thing that goes through the fly’s mind when he hits the train’s windshield? A: His asshole. > [@jz78817](#): > > no. the wrist pins prevent the pistons from doing what he describes. they simply cannot “twist” in the cylinder bore. This is the key. The piston is attached to the piston rod by the wrist pin; the piston rod is attached to the crankshaft with bearings at the other end. The piston goes back and forth. During intake cycle, the rod draws the piston down, so it is pulling the piston. When it reaches bottom, it reverses and is pushing not pulling. During compression cycle, the piston is pushing against the pressure of the fuel-air mixture in the cylinder. It reaches top and stops, but is still pushed against the compressed air - then pushed even more on the downward stroke as the mixture burns. The cylinder reaches bottom and stops, reverses direction, Now it is pushing the exhaust gas out as the cylinder goes up. At the top, it stops and begins drawing air in in a new intake cycle. The piston is now being pulled down. The biggest strain on the wrist pin, that risks shaking the piston/wrist-pin/rod apart, is the change between pushing the piston and pulling it (and vice versa). If the bearing for the wrist pin has any slack, then you do get a slapping problem on the cycle and deterioration of joint and eventual failure. Slack in a mechanical connection is not good. But also keep in mind, the cylinder’s motion, as the post suggested before this thread was an adult, describes approximately a sine wave - so there’s no abrupt stop and change direction; it’s not like hitting a brick wall. The motion and direction change is as smooth and even as something doing several thousand RPM can be. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 5, 2018, 6:01am UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/36 "2018-01-05T06:01:04Z")
Have we discussed a photon in a vacuum? If you hold up a mirror, is the photon stationary as it bounces off the mirror? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 5, 2018, 1:52pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/37 "2018-01-05T13:52:19Z")
Although, the fact that the derivatives of sine and cosine are so uninteresting is, itself, quite interesting. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [January 5, 2018, 4:59pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/38 "2018-01-05T16:59:47Z")
The derivatives of _every_ function are interesting. Why, if there was one that wasn’t interesting, then that would be the most interesting of all! ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [January 5, 2018, 6:01pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/39 "2018-01-05T18:01:38Z")
> [@CurtC](#): > > The derivatives of _every_ function are interesting. Why, if there was one that wasn’t interesting, then that would be the most interesting of all! But there isn’t just one uninteresting derivative. There’s an uncountable number of them. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [January 5, 2018, 6:10pm UTC](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913/40 "2018-01-05T18:10:57Z")
> [@ftg](#): > > But there isn’t just one uninteresting derivative. There’s an uncountable number of them. I find that interesting, but only finitely so - my level of interest in each uninteresting derivative halves with each successive uninteresting derivate. What are the implications? [Previous page](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913.md?page=1) [Next page](https://boards.straightdope.com/t/do-pistons-stop-in-an-engine-when-reversing-direction/39913.md?page=3)