Sprag clutch vs freewheel

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flat tire   1 MW

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Sprag clutch vs freewheel

Post by flat tire » Jul 30 2019 4:47pm

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
Last edited by flat tire on Jul 30 2019 8:26pm, edited 2 times in total.

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spinningmagnets   100 GW

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Re: Sprague clutch vs freewheel

Post by spinningmagnets » Jul 30 2019 5:06pm

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...

Image

Sprag clutch on the TSDZ2 mid drive...

Image

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ferret   1 kW

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Re: Sprague clutch vs freewheel

Post by ferret » Jul 30 2019 6:24pm

Don't believe everything you think.

Cove GNG-Spot build thread

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Re: Sprague clutch vs freewheel

Post by flat tire » Jul 30 2019 6:37pm

ferret wrote:
Jul 30 2019 6:24pm
https://onyxrp.com/
Good to know someone's doing it right!

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Re: Sprague clutch vs freewheel

Post by dustNbone » Jul 30 2019 6:52pm

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.

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Re: Sprague clutch vs freewheel

Post by PaulD » Jul 30 2019 7:09pm

dustNbone wrote:
Jul 30 2019 6:52pm
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.
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!

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Re: Sprague clutch vs freewheel

Post by flat tire » Jul 30 2019 8:25pm

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

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neptronix   100 GW

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Re: Sprag clutch vs freewheel

Post by neptronix » Jul 30 2019 9:15pm

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

Is low rolling friction typical for sprag clutches?
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dustNbone   10 kW

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Re: Sprag clutch vs freewheel

Post by dustNbone » Jul 30 2019 9:49pm

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.

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Re: Sprag clutch vs freewheel

Post by Grantmac » Jul 30 2019 10:10pm

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.

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Re: Sprag clutch vs freewheel

Post by wturber » Jul 30 2019 10:54pm

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.
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Toorbough ULL-Zeveigh   10 MW

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Re: Sprag clutch vs freewheel

Post by Toorbough ULL-Zeveigh » Jul 31 2019 12:57pm

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.
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Re: Sprag clutch vs freewheel

Post by PaulD » Oct 02 2019 11:25pm

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).
revel sprag.png
revel sprag.png (407.63 KiB) Viewed 202 times
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.
Shimano steps roller clutch2.png
Shimano steps roller clutch2.png (410.17 KiB) Viewed 202 times
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.
bosch roller clutch.png
bosch roller clutch.png (413.14 KiB) Viewed 202 times

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