Sprag clutch vs freewheel

flat tire · Jul 30, 2019

So I got a fancy custom mid drive kit and the freewheel is actually a sprag clutch.

This is freaking AMAZING.

It doesn't make any noise when spinning free and locks up immediately when the torque comes.

WHY are these not more popular? Why don't they use them in cassettes? Who the hell wants their bicycle to make that annoying clicking sound when coasting?

*edited because I learned how to spell the thing properly

spinningmagnets · Jul 30, 2019

Sprag clutches are more expensive. Works great, though...
In the pic below, sprag is on the left, ramped roller in the center, and common bicycle pawls on the right...

Sprag clutch on the TSDZ2 mid drive...

ferret · Jul 30, 2019

https://onyxrp.com/

flat tire · Jul 30, 2019

ferret said:
https://onyxrp.com/

Good to know someone's doing it right!

dustNbone · Jul 30, 2019

Sprags are also substantially heavier for the same holding force, not that it should really make a difference as both are pretty light at bicycle levels.

PaulD · Jul 30, 2019

dustNbone said:
Sprags are also substantially heavier for the same holding force, not that it should really make a difference as both are pretty light at bicycle levels.

I agree that Sprags are way heavier, but I wouldn't say that both are light. CSK30 sprags such as in the TSDZ2 don't seem to last very long (search the forum) under those that pedal. CSK35s also seem borderline (according to some forum members). CSK40s are like boat anchors, but have been used before.
https://endless-sphere.com/forums/viewtopic.php?t=47618&start=275

I've worked on a custom solution to bring the weight to a reasonable level with mixed results.
I wanted to try it out and yes, I'm with flat tire on this one. They feel amazing. Smooth, instant engagement, and no clicks!
I'll try to share more soon!

flat tire · Jul 30, 2019

The function is so good and the absolute weight low enough that I'd say sprag clutches should be on basically every bicycle.

neptronix · Jul 30, 2019

I had no idea there was a sprag clutch bike hub maker out there. Awesome.

Is low rolling friction typical for sprag clutches?

dustNbone · Jul 30, 2019

Friction should be lower than a pawl clutch for sure. You're not overcoming the spring with every "click", just the friction of the sprags smooth surface sliding over the drum, and that should be very minimal with proper lubrication.

They should also be very reliable, no spring, no teeth to break off.

Grantmac · Jul 30, 2019

I think with the right programming manufacturers could eliminate every freewheel/sprag from the system including the hub with exception of the one at the cranks which could be a sprag. All it would "cost" is 25w or so.

wturber · Jul 30, 2019

Shimano used to make a silent freehub. But not any longer. Gotta figure there's a reason they didn't take the market by storm. Cost to manufacture, weight, reliability ... surely something(s).

Also, FWIW, the freewheel clicking doesn't bother me at all and it surprised me to hear that it really bothers some people. It might be that I so wanted a 10-speed when I was a kid that I associated the ratchet sound with positive feelings and that makes it pleasant, not annoying.

Toorbough ULL-Zeveigh · Jul 31, 2019

autobike used a sprag or sprag-like zero backlash bb by necessity.
CT must have had 110% warranty return on them b/c when i went to get it exchanged the next day all of them were in worse shape with zero crank engagement whereas mine only had a pair of bad spots.
same thing with this Lee Valley manual insert bit screwdriver (multiple dead spots until complete failure).
having owned both that broke almost immediately with light use i would not want anything with a sprag in it for free.

Miles figures that these failures are the result of being undersized for the task.
which means it would be even heavier, bulkier & more expensive to do the job of an equally capable ratchet-pawl.

PaulD · Oct 2, 2019

Ok, I finally had time to get some pictures of my sprag experiment. I think the outer ring might not be stiff enough, but testing will prove that out. One might think it's an advantage to have a larger diameter, but it requires more material to achieve sufficient stiffness. This is about 200g heavier than my aluminum freewheel (also specific to the Sempu T4 BB).

For reference, here is a roller clutch from a Shimano Steps E6000. Heavy, thick wall hardened steel bits. Not shown is the outer ring/sprocket which is solid steel.

Bosch on the other hand can use a bit lighter setup since the human torque goes through a set of step-up gears to reduce the torque.

Bullfrog · Jan 4, 2020

Does anyone know what type of clutch the MAC ebike motor uses...is it a sprag, ramped rollers, or something else?

Pretty sure it is not a pawl type since it freewheels very smoothly without any clicking

.

I plan to drill out the rivets that hold mine together next time I pull the motor off of the bike. Mine will not release but it does not slip either so I am getting regen. Functioning like the GMAC (which doesn't have a clutch at all) at this point. I am currently cooling my MAC by putting 5 ounces of distilled water and 5% by volume Motul MoCool (corrosion inhibitor) in the motor. Sealed the motor/hub interface with Permatex Ultra Gray. My bet is corrosion is the cause of the clutch not releasing but I won't know for sure until I take it apart.

Thanks

Sprag clutch vs freewheel

flat tire

100 kW

spinningmagnets

Moderator

ferret

1 kW

flat tire

100 kW

dustNbone

10 kW

PaulD

1 kW

flat tire

100 kW

neptronix

Administrator

dustNbone

10 kW

Grantmac

10 kW

wturber

1 MW

Toorbough ULL-Zeveigh

1 MW

PaulD

1 kW

Bullfrog

10 kW

Similar threads