LEGO Technic Free Wheel (Sprag Clutch)

This Free Wheel mechanism allows the output to spin faster than the input, and when the direction of the input is reversed, the output becomes disengaged and can rotate in either direction.
Free Wheel mechanisms are typically found in bicycles, engine starters, agricultural equipment, helicopters, and lathes.
More information about Free Wheel mechanisms: en.wikipedia.org/wiki/Freewheel
More information about the Sprag Clutch:
en.wikipedia.org/wiki/Sprag_c...
Based on a design developed by piterx and DrJB on the Eurobricks Forum (www.eurobricks.com/forum/index..., this version has been modified to improve the strength and reliability of the mechanism.

[@locke_ytb]

Ohoho, my brain is just so excited to play with Lego Technic for some reason. I just want to spew all my amazing ideas out.

[@Unbrickme]

Wow!!! Absolutely genius design!

[@adrian_zombturtle148]

1:21 i think this clutch needs a clutch just look at them poor gears

[@slfroden]

Indeed. This is just a prototype, and improvements can definitely be made. Stalling the output does put a lot of stress on some of the pieces, but at least the gears aren't 'clicking' (clicking is generally a bad thing and means the gears/axles are deforming).

[@gerrettmcqueen7001]

This is the kind of clutch found on weedeaters

[@riythemusicguy7696]

"comedy in the future will be randomly generated!" Is what this comment reminded me of

[@user-hv1iy9ge9i]

imriy FPS* W E E D E A T E R

[@psychedelicmonkey5848]

*centrifugal

[@DashzRight]

Nope

[@slfroden]

+Dan Bossons - That is the Lego L Motor. It was released in 2012 and has been used in about half a dozen sets so far, including this year's Crawler Crane (Set 42042) and Mercedes-Benz Arocs 3245 (Set 42043).

[@LochyP]

I like how you show all the pieces as you slowly attach them. Good job!

[@HondaCivic-lj4ri]

This is the basic idea of a mountain bike (or any bike with a freewheel system)

[@slfroden]

It is the same basic idea, however most bikes with a freewheel system use a ratchet and pawl mechanism, which gives you that distinctive clicking noise when you aren't pedalling.

[@petrsokol588]

That's the most clever thing I've ever seen! You're a true genius.

[@isaactaylor2282]

Hate when people say: "Needs more work or something to that extent." It's great just how it is and if he needs your expertise, I'm sure he'll ask.

[@CarlosRamirez-ns7qw]

According to my calculations...

[@arifanwari3168]

Excellent, why the 311 dislikes? Ignore them bud, good work.

[@slfroden]

Thanks. People like and dislike different things. 311 dislikes with 750,000+ views isn't too bad though.

[@noscopesallowed8128]

Now this is cool and useful. Awesome dude!

[@5murry570]

Very nice, I had a much larger version, but it did allow large torque transfer. I like this because then brakes on a model are actually useful.

[@seagangstab2637]

Coolest Lego build I've seen to date! Thanks for posting:)

[@slfroden]

+seagangsta b - Thanks :-)

[@zangryomani1257]

this should be put on lego trains. so we get a more realistic deceleration. and also a better down hill effect

[@Tenecifer]

Make your own ! 😁

[@LordStexx]

Cool, lego technic is a great one

[@rc4life860]

Now that’s really cool! Good job man! Would be awesome if you could make something like this in a lego car

[@limer.6355]

Nice one . Would like to see more

[@retrox684]

This is how go karts work

[@zandkoek]

RetroX no go karts work with a centrifugal clutch, those use centripetal force to engage. This one uses gears to engage

[@retrox684]

zandkoek notice how it has two arms that swing out? They also stay in when the motor is off so the tire can free roate. They only engage when the motor spins and if it spins slow enough it wouldn’t engage, just like a go kart clutch

[@diegootero1686]

@@retrox684 no, it's based on rotation but not rpm. You can see how he spins it manually very slowly and It engages anyway

[@josephhalton6257]

@@retrox684 too much complicated talk here I will leave

[@itachiuchiha7411]

gerbullo crolello everything else is true except for the braking. when he stops the wheel the motor has to stop because there is still torque being applied to the clutch

[@berellevy1]

This is genius

[@valtterikaisla3038]

absolutely impressive!

[@Hypn1c]

Good job!

[@searntlcu8263]

thanks i might use this in my rc Chiron project

[@wesleysavage444]

Very cool. This would be great for a complex motorized bike model.

[@AtomicChristmas]

this is like a ratchet but using friction instead of gearing so it spins smoothly without any click-clacking

[@ironbullet9956]

Nice job!

[@blake790]

That's smart bro

[@yuvallegomotorized]

Cool idea 👍

[@KacpiJK]

nice work! Thanks bro ;)

[@brickphysics1506]

Awesome idea. Thank you. : )

[@izumfog]

It's ingeniously!

[@ScaledChannel]

Very nice! good to know!

[@MaxImagination]

Thats really nice! Good build! I want to see it built into a vehicle 😊

[@slfroden]

Thanks. I haven't built this into a vehicle... yet. If you use this mechanism in a vehicle, please upload a video and send me a link so that I can check it out.

[@socialus5689]

Cool idea!

[@WilliametcCook]

All I see is magic.

[@randombird390]

Kind of like my slider (It’s a bike you can sit down in and very low and it’s easier to drift in it)

[@latassedethe4931]

What kind of sorcery is this?

[@emeraldleetiger]

So basically the same concept as a centrifugal clutch.

[@slfroden]

This is similar to a centrifugal clutch, but not quite the same. With a centrifugal clutch, the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). This is more like a sprag clutch.

[@JaceN3D]

@@slfroden if you put a rubber band on it, will it be a centrifugal clutch? so it has to spin fast enought to le the catches reach out

[@slfroden]

@@JaceN3D Yes, I think that would work. LEGO pieces, bring made from plastic, are fairly light, so you would have to spend a fair bit of time fine tuning the system.

[@gaminghub8108]

Perfect for sequential gearboxes.

[@DesignJunkie58]

This is an ingenious idea! Soory for my bad spelling :0

[@HexhamFoodCo]

nice work i am going to try and incorporate into a dragster racer as part of the pull back force accumulation to deliver a faster start, like holding a car on a hill. thnx

[@slfroden]

That sounds like a great idea! Once you have finished your dragster, please take a video and post a link here so that I can check it out.

[@user-bs1su4xm6s]

これはスゴイ。ってか発想が神がかってる

[@Onewaygoon704]

Isnt this just a centrifugal clutch?

[@slfroden]

This is similar to a centrifugal clutch, but not quite the same. With a centrifugal clutch, the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). This is more like a sprag clutch.

[@MarinusMakesStuff]

Is it safe to call this a centrifugal clutch? Like the ones you see in semi-automatic clutch mopeds?

[@slfroden]

This is similar to a centrifugal clutch, but not quite the same. With a centrifugal clutch, the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). This is more like a sprag clutch.

[@MarinusMakesStuff]

Ah yes exactly. If you were to add rubber bands, springs or cones with balls and a v-shaped belt it would come closer to a centrifugal clutch of course. I hadn't heard of a sprag clutch but I think that's more down to a language barrier. I wonder what it's called in Dutch (looked it up and they call it a sprag clutch or a 'vrijloop' in Dutch).

[@slfroden]

To be honest, I didn't know that it was called a Sprag Clutch either until Connor Cruz pointed it out to me in the comments.

[@UncleRanger]

Good idea!!

[@lipcsey]

Nice one, think it can withstand the torque of multiple RC buggy motors as well?

[@dellie4475]

Yes.

[@astodon]

nice

[@v.t.o.l.friedchicken6973]

I don't quite get it. If you reverse the current and have the motor rotate the other way, wouldn't the "clutch pieces" rotate in other direction and still engage the wheel? It seems like the plastic light grey piece that encases the clutch pieces also rotate with the wheel so if the clutch pieces were to spin outwards, wouldn't they engage the grey piece as well? Can you please explain why the wheel only spins in one specific direction and spins freely in the other? I read the article you linked to in the description but I still can't quite understand it. Thanks

[@slfroden]

+V.T.O.L. Fried Chicken - When the motor spins in one direction, it rotates the two rubber pieces outwards, but they can only rotate so far until they engage with the outer casing, at which point they lock together and the motor's rotation is now transferred to the wheel. When the motor spins in the opposite direction, it rotates the two rubber pieces inwards, which disengages them from the outer casing, and therefore stops transferring the motor's rotation to the wheel, so the wheel is able to spin freely. When the two rubber pieces rotate inwards, they can only rotate so far before they hit the piece in the center, so they can't rotate all of the way around and engage with the outer casing at all, so they remain disengaged until you reverse the direction of the motor again. If they weren't limited by the piece in the center (and they were spaced further apart so they wouldn't hit each other), then they would rotate all the way around and engage with the outer casing again. I hope that explanation helps.

[@verunaokumura4836]

good one, fantastic!

[@slfroden]

+Atomic Arctic - Thanks :-)

[@verunaokumura4836]

slfroden oh btw, can you make 2 looping motor lego generator? i requested this because i haven't seen one before making this. i hope you are reading this and know how its work. and dont forget to use 2 gear, one with the smallest gear (8 tooth) , and one with the biggest gear (i think it was 64 tooth).

[@slfroden]

+Atomic Arctic - I'm not sure what a '2 looping motor Lego generator' is. Can you provide some more information or a link?

[@jackradzelovage6961]

make actual use of the reverse gear in so many lego transmissions.... and make a reason to include real brakes..... i like it!

[@alessandrocorucci799]

Genius👍👍

[@sdsinghji7656]

Nice

[@ruebenwalls3730]

Nice

[@marigo856]

Nice

[@linhtuong7908]

Zesty_ Sketchz nice

[@nootaboot7042]

@@linhtuong7908 nice

[@kaanosphere]

@@nootaboot7042 noice

[@CornyFlakez]

You just earned a like and a sub ;P

[@slfroden]

+KKGamer 1610 - Thanks :o)

[@CornyFlakez]

Really, great mech!

[@hsworksss]

It can be used for a technic moc and have the engine running but the wheels are not like a handbrake

[@joo-fy4xo]

You dont even know what handbrake does

[@snucc1000]

BMX Freecoaster 🤔

[@HannyDart]

it kinda isnt a sprag clutch (cause of the gearing) but then again it is, cause the force that presses the sprags outside is still provided by the torque of the input shaft... props :D

[@slfroden]

Yeah, it's hard to define exactly what this clutch is, but the definition for a sprag clutch is the closest that I have found so far.

[@justinw.2376]

I read the description. How does this allow the output to spin faster than the input? It’s one to one ratio.

[@tiemenjansma4685]

I think because the wheel can spin freely and it keeps accelerating a little bit when the motor starts spinning.

[@justinw.2376]

Adolf Hitler that’s not gonna allow the output to spin faster than the input.

[@ONLYUSEmyTOILET]

The motor won't spin the output faster than the input, it simply allows it to spin faster if external forces are applied. Think of it like this: If you pedaling your bike as you encounter a downward hill. You keep pedaling at a steady speed but you are now rolling a lot faster. You realize you don't need to pedal down a hill, so you stop. The pedals (input) is now completly still, but the wheels (output) are allowed to keep turning thanks to the sprag clutch.

[@justinw.2376]

OUMT ya but with this design, it won’t free wheel at all like a bike would while you are still peddling. This is really more like a regular clutch but a cone shape

[@slfroden]

+OUMT is correct. This clutch works based on the difference in speed between the input and the output. If the output is slower than the input, then the rubber shoes engage and the output becomes the same speed as the input. If the output is faster than the input, such as when you stop the input (stop pedalling) or an external force is applied (rolling down a hill), then the rubber shoes disengage, allowing the output to 'free wheel'. It can still 'free wheel' while you are pedalling, as long as the output is going faster than the input.

[@manmohanverma8831]

Good

[@feoranis26]

Interesting. Is there instructions? I’d like to use that.

[@slfroden]

I don't have instructions for this model. If you study the video closely and are familiar with Lego Technic pieces you should be able to see enough information to recreate this model yourself. Remember that this is just a prototype, so you could also use different pieces to get similar results.

[@pifpaf3329]

I'm thinking about lock version, so a reverse moter direction would be available.

[@slfroden]

+pif paf Do you mean a lock feature that you can enable and disable as needed? That would be a good idea.

[@qubi]

his can be to the lego krimes ! cool swing!!

[@legosadsad237]

Nice 👍😊👍☺️

[@saynotocappiescommie7131]

how to enforce 1 way lanes

[@dominickstellato4076]

@slfroden This is basically a centrifugal clutch right?

[@slfroden]

Dominick Stellato Yes and no. The set-up is similar, where the inner rubber 'shoes' are pressed against an outer housing, however in a centrifugal clutch the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). It uses the small 8T gears to force the rubber 'shoes' out. Reversing the direction of the input, or allowing the output to spin faster than the input (free wheel) forces the rubber 'shoes' to disengage completely.

[@adrix6180]

Tipically drum brakes

[@DesignJunkie58]

HOW sturdy is it to put into a MOC weighing 2KG's

[@slfroden]

I haven't built this into a 2KG model, so I'm not sure how sturdy it would be. It works quite well, and the only problem is that it is quite large. If you end up using this design, please post a link to your video so that we can see how it works.

[@lil_casperr3922]

Nutral and drive

[@doyouhavegainsonyourphone4507]

So a centrifugal clutch?

[@slfroden]

+foster Rich - Yes and no. The set-up is similar, where the inner rubber 'shoes' are pressed against an outer housing, however in a centrifugal clutch the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). It uses the small 8T gears to force the rubber 'shoes' out. Reversing the direction of the input, or allowing the output to spin faster than the input (free wheel) forces the rubber 'shoes' to disengage completely.

[@madcuzbad8205]

So, like a centrifugal clutch that only works in one direction?

[@slfroden]

+madcuzbad - Yes and no. The set-up is similar, where the inner rubber 'shoes' are pressed against an outer housing, however in a centrifugal clutch the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). It uses the small 8T gears to force the rubber 'shoes' out. Reversing the direction of the input, or allowing the output to spin faster than the input (free wheel) forces the rubber 'shoes' to disengage completely.

[@madcuzbad8205]

Alright, I was more or less trying to keep it simple, but now I know lol

[@creepjax]

neutral mode?

[@slfroden]

+creepjax - Yes, I guess that you could think of it that way. When the clutch is disengaged, it is in neutral mode, and when the clutch is engaged it is in drive mode.

[@aidanburns7797]

Thats cool plz do a how to vid

[@slfroden]

+Aidan Burns - I thought that this was a 'how to' video...?

[@fartonaut2291]

Neat.

[@gfuel7574]

Cool

[@HexhamFoodCo]

Will do

[@vovcickandarkashka]

Блин, всё видел, но вот центробежное сцепление в Лего вижу в первый раз

[@1_ESHO_1]

Я офигеб если сделают бензатилу

[@Zephyre09]

R

[@yarmikh2576]

Только это не центробежное сцепление, лепестки разворачиваются благодаря шестерням

[@d0gd0g19]

Я думал я один русский который его смотрит

[@chelovechik]

не один

[@adanlopez5251]

Do it’s drum breaks?

[@slfroden]

The mechanism in this video is similar to a drum brake in that it has 'shoes' that press against the inner surface of a 'drum', however there is one important difference. Drum brakes are typically engaged by an external input, such as pressing a brake pedal or engaging the parking/emergency brake in a car, whereas the mechanism in this video is engaged when the rotation direction of the input axle changes.

[@glowiever]

Like one way bearing ain't it?

[@slfroden]

A Sprag Clutch is a subset of one way bearings, but there are several different types of one way bearings.

[@ryanthedev288]

What if you made a life size Lego car???

[@slfroden]

A number of life size Lego cars have been made already by other people, including a driveable version of the Bugatti Chiron (www.lego.com/en-us/themes/technic/bugatti-chiron/build-for-real) which used over 1,000,000 Technic pieces and took over 13,000 man-hours to design and build. Unfortunately I don't have 1,000,000 spare Lego pieces of the right type, or the spare 13,000 man-hours that would be required to build a life size Lego car myself.

[@connorlever229]

I’m not an expert so would you explain where this would be useful please not trying to sound sarcastic but I don’t know anything about this sort of thing Cheers mate

[@slfroden]

Hi Connor - Check the description of the video, it should answer your questions. If you have any further questions that aren't answered, please feel free to ask.

[@connorlever229]

Thank you

[@domenicwilliamson8930]

@@connorlever229 if you have a gearbox with a clutch to follow up behind this the motor could still run during a gear change, whilst the wheels are freely spinning, without the motor stopping them, making a manual and auto gearbox more realistic

[@harrodharrod5239]

@@domenicwilliamson8930 That's what the clutch is for though.

[@domenicwilliamson8930]

@@harrodharrod5239 yeah I know, I explained in that paragraph up there

[@potatolol4579]

So it allows it to spin one way and freely with no power, but reverse does nothing. Interesting.

[@vishouscat]

connect it to a transmission and you'll have an automatic clutch

[@LucasTheF]

So when a engine idles it spins backwards??!?

[@LucasTheF]

VishousCat ^^^

[@vishouscat]

most likely you'd use an electric motor as you'd need a legit centrifugal clutch for an engine

[@slfroden]

When the motor is stopped, the wheel is free to spin either forwards or backwards. When the motor is on and spinning forwards, the wheel also spins forwards. When the motor is on and spinning backwards, the wheel is free to spin forwards or backwards again.

[@CBK-rf6tl]

How to build it?

[@slfroden]

+CBK 1000 - I don't have Building Instructions or a Tutorial for this model that I made over 3 years ago, but If you study the video closely, you should be able to see all of the pieces required. This is just a prototype, so you could use different pieces.

[@MrSnake322]

old automatic transmissions use it as well but not agricultural equpment or lathes

[@slfroden]

+Snake322 - A quick Google search provides results that contradict your statement.

[@MrSnake322]

please show me a farming equipment that uses it(genuinely curious) and youre right. just thought about what happens when you stop a lathe, stops slow not instant.

[@slfroden]

+Snake322 - There are many examples of sprag clutches for tractors. Here is one: www.agriandgarden.com/sprag-clutch.html That page also lists the tractors that the sprag clutch is compatible with.

[@danbossons2192]

Which motor is that? Sorry about the necro.

[@dovydasnorkunas8113]

+Dan Bossons L motor

[@danbossons2192]

Cheers

[@janekgarwol7869]

bardzo mi się podoba

[@slfroden]

+Jan Garwol - Translation: 'I like it very much' Thanks. :-)

[@imdedinside8053]

Neat

[@Turambar3791]

At last we won't use back movements never more :D

[@Legominder]

Awesome: Freefloating mode for RC cars! Just the reverse gear to be sorted out😂

[@slfroden]

Thanks :-) Adding a reverse gear would be reasonably easy. Instead of having this mechanism go straight to the wheel, add a gearbox between them that lets you select either forwards or reverse.

[@overwatch_geek7651]

this has technic in it

[@anthonynalbandyan1539]

Im sure he meant that the "Free Wheel" is lego technic.

[@hellman431]

i didn't notice i watched this video it in double speed weird..

[@dootdoot586]

Kinda like a centerfugal clutch

[@slfroden]

This is similar to a centrifugal clutch, but not quite the same. With a centrifugal clutch, the clutch engagement is based on the RPM of the input, and it engages more as the RPM increases. With the set-up shown in the video, the clutch engages based on the rotation direction of the input rather than the RPM (the RPM are still a factor, but not as much as the rotation direction). This is more like a sprag clutch.

[@szymonkierzenkowski7622]

woow

[@blackgriffinxx]

they so need to make a metal Lego kits

[@davidreichert9392]

It wouldn't work. The mating of Lego parts relies on the softness of the ABS. Metals are too hard.

[@blackgriffinxx]

look in to that. Found the closet thing to metal Lego was Erector sets stamped out metal with per-drilled holes and screws . Not as kid friendly as Lego. Still handy in life to fix and made many thing

[@slfroden]

There are stores on Bricklink that do sell metal (aluminum/aluminium) parts that are compatible with Lego. eg: store.bricklink.com/Eezo There are also users on Shapeways selling 3D printed parts that are compatible with Lego, and some of those parts can be printed in metal. eg: www.shapeways.com/designer/efferman/creations Both of these options are a lot more expensive than genuine Lego parts made from ABS plastic. I don't own any of these metal parts, so I cannot comment on their quality at all.

[@NathanB2002]

blackgriffinxx try mechano

[@tarzaan2603]

Great!!! Could we see it in a car?

[@slfroden]

I have not used this mechanism in a car yet. After a discussion on the Eurobricks Forum (www.eurobricks.com/forum/index.php?showtopic=92955), I built this mechanism to test it out in 'real' life, and to show a few changes that I felt improved the design. I had no plans to use this mechanism within another (more complete) model, but I will use this mechanism if I work on a model that calls for it. If you happen to use this mechanism in a car or other Lego model, please post a link so that I can check it out.

[@tarzaan2603]

S. L. Froden thanks for the extra info, how could I get legos on the cheap? I'm broke but I want to learn!

[@slfroden]

Unfortunately Lego is not a cheap hobby. - The cheapest way to get Lego would probably be to find second-hand Lego on eBay or Craigslist (or slmilar) or at local garage/yard sales or op shops/thrift stores (eg. Good Will stores). - The next cheapest option is usually to buy Lego sets with a good 'price per piece' ratio. If you can find a store that has a sale, you can get some good deals. If you are just after a few specific pieces, you will have to balance the number of parts that you want/need in the set versus the other parts in the set/s and determine if this is worth it to you. - If you only want specific pieces, you can order from sites such as BrickLink or BrickOwl. - Lastly, you can order individual pieces directly from the Lego website, but this usually isn't the 'cheap' option. Depending on your location you may be able to rent Lego. Search the web for 'rent Lego' to see various companies that offer this service, Some websites that I use to research Lego sets and pieces are brickset.com, bricklink.com, and rebrickable.com.

[@tarzaan2603]

S. L. Froden rent legos!! That's insane hahahahhahahahahahhahaah I'm not that broke, honestly I can afford legos, but I'm going to complain about the price the whole time.

[@volbla]

Swanky.

[@kb.bit1]

So basically how bikes work?

[@slfroden]

Yeah, a similar principle, just with a slightly different implementation.

[@clemensbmx1235]

How to do ?

[@slfroden]

+Clemens Brombeer - If you study the video closely, you should be able to see all of the pieces required. This is just a prototype, so you could use different pieces. Please refer to the Eurobricks forum for more information (link in the video description). If you have a more specific question, please let me know.

[@JoranPrins]

what has a free wheel for function?

[@slfroden]

+Joran Prins - Please refer to the video description.

[@JoranPrins]

oops, sorry no wi know

[@bolognatheroach8576]

What kind of wheel

[@slfroden]

This model is a prototype. You can use any Lego wheel and tyre that you like.

[@bolognatheroach8576]

S. L. Froden ok so what is the biggest wheel you have tell me what set it was from

[@slfroden]

The largest Lego 'wheel' that I have is the 'Hailfire Droid Wheel' (part x784) from set 4481. The largest tyre that I have is the 'Power Puller Tyre' (part 32298) from set 8466.

[@ASUSfreak]

Cool but what's the use of it? I understand why a clutch in forward direction engages and turns the wheel. I understand when motor is off the wheel freespins. But I don't understand why the motor has to be able to turn backwards when it doesn't have to spin the wheel??? It's a serious question, I just don't know if I ask it correct since I'm Dutch speaking... 😊

[@slfroden]

The motor doesn't have to be able to turn backwards and have the wheel spin freely, but in this implementation it is just how it works. It is the same on a bicycle that has a free wheel mechanism. When you (the motor) turn the pedals forward the chain moves which spins the rear wheel. When you stop pedalling the rear wheel can still spin freely (coasting). When you pedal backwards the rear wheel can also spin freely.

[@Godshole]

Twist and go mopeds (automatic motorcycle scooters) use a similar clutch mechanism. It improves fuel efficiency that they can freewheel a bit on down slopes or coasting to a stop. So the engine has to be able to disengage, or run slower than the road wheels. Not that you would run the engine backwards but it has that effect. I remember it being called a flywheel clutch but now I know it's proper name. Good demonstration :)

[@user-zv3oe4wj9d]

Is its torque be too small?

[@slfroden]

+王立煒 - Too small for what?

[@user-zv3oe4wj9d]

slfroden Propulsion（Sorry,my english is not very good）

[@slfroden]

+王立煒 - It depends on what you are trying to propel. It you are trying to propel something 'small', it should be able to transfer enough torque. If you are trying to propel something 'huge', then it probably won't be able to transfer the torque, and either this sprag clutch or another part of the drivetrain will fail. I'm not sure exactly how much torque this sprag clutch can transfer before it fails, but in the video it was able to make the tire slip on the rim of the wheel, and it was also able to stall the motor when I halt the output, so it is transferring quite a bit of torque there without failing.

[@user-zv3oe4wj9d]

Thank you for the explanation.