High-engage BMX freewheel adapted to 80T sprocket

[Barncat]

for those of us who are building chain drive bikes, the performance of cheapo flanged freewheels that adapt rear sprockets is very disappointing. just two weak pawls, lousy tolerances, and a lot of lash to take up when you hit the throttle.

i've devised a way to adapt a good quality bmx freewheel (in this case an Origin8 Hornet) to an 80T or other 4 hole #25 sprocket- without a lathe, or welding which would damage heat treatment. these engage in just 3.33 degrees of rotation with 9 pawls- only 3 of which are engaged at any one time- which is a 50% higher engagement than 2 pawls, plus the leaf springs are way stronger. too strong in fact, which adds drag and noise when coasting. it's fairly easy to weaken the springs by straightening them a bit with needlenose pliers to tailor the drag and noise. the tolerances are much better on a performance freewheel, and indeed they're a marvel of engineering given all they must do in very limited space.

the key is to buy a 16T freewheel because the teeth spacing aligns perfectly with 4 hole sprockets- every 90 degrees. the Hornet has a 55mm diameter sprocket body. go buy a bimetal 2-1/8" hole saw, and drill through a bit of 1/4" aluminum plate. hole saws always have a lot of runout, and mercifully in this case that produces a nominal 55mm hole plus about .003-.005", so this creates a backing plate to bolt to. the Razor brand et al 80T sprockets are 54mm ID, so some very careful filing with a big half round will bump that out to press on to the freewheel sprocket body. go slow and check your fit up when filing... my lathe is up north so i went all cave man.

i'm still deciding on final fastener selection. will report back...

 

[Chalo]

Noisy freewheels ratchets are hugely overrated, given that the entire torque load is carried by one or two pawls at any given time. Unless the freewheel bearings are egregiously sloppy, there isn't enough wiggle room in the mechanism to engage even three pawls at a time.

Given the recent fad of excessively fussy and draggy freewheel ratchets, you'd think that all the simple, free-turning ones that came before had some kind of problem. But they didn't. Like too-thick spokes, umpteen-speed cassettes, or hydraulic whatevers, rackety freewheels and freehubs are just a way to dislodge money from gullible buyers.

Back in 2003, I machined a 144T sprocket and carrier to fit an ordinary 16T ACS freewheel. It worked fine then, and it still does (though I haven't run it in years). It's very noisy because the huge aluminum sprocket surface resonates the sound of the pawls and bearings.

 

[Barncat]

Chalo- i'm no fan of noise either in the case of the freewheel, or $$$$ components that are just hype. noise = drag, and it's aesthetically unappealing. this is why i stated that i will literally tune the mini leaf springs to minimize it. but it's a rob Peter to pay "Pawl" situation- those extra pawls are necessary to get the quick engagement that is the whole point of these newer units- a performance advantage that is not debatable. the tolerances on new stuff are such that i'd be surprised if all 3 aren't carrying load.

i can tell you in no uncertain terms that the cheapo freewheel adapters- the only ones currently available to my knowledge- are not suited to transmitting 6-7hp or more to the back wheel. i've had ratchet teeth skip with disconcerting clunks. surprised they didn't break, yet.

will likely get some test miles on this setup tomorrow on the OCC stingray. i expect significant power transmission improvement.

 

[Chalo]

But it's a rob Peter to pay "Pawl" situation-

LOL... well played. I commend you, sir.

You can quiet the racket somewhat with heavy oil, e.g. Phil Tenacious Oil, chainsaw bar oil, etc. Don't use grease because it will stick the pawls down sooner or later.

 

[Barncat]

what cyclist wouldn't get a kick out of that pun.

yes, lube viscosity will affect db level... noted re the grease, which i might have been tempted to use. thanks.

to the site administrator- my bad for not putting this thread in bike mechanical and structural. please move it if and when you see this note. thanks!

 

[calab]

But it's a rob Peter to pay "Pawl" situation-

That is a good pun

 

[Barncat]

i reassembled the freewheel. not a task i want to do every day, with some 90-100 loose 1/8" ball bearings, but it went ok. i used chainsaw bar oil i had on hand which must be very similar to Phil's Tenacious and way cheaper, so good tip Chalo. went with 4 button head socket screws since they're a high quality fastener and i needed clearance with the frame dropout. i put the 80T sprocket on the inside of the freewheel cog, and miraculously did not need to realign the motor to compensate with the front sprocket.

on the road it is of course a bit buzzy when coasting but from my perspective with the wind noise it's sort of a wash. engagement is solid as expected and is an upgrade.

eliminating freewheel slop did not completely quiet the thing down from standing starts and i think i can attribute some related noise to battery mounting and a vibrating plastic cover, so that's another issue i'll track down and solve...

(EDIT update): i've determined that the brief minor low frequency sound from standing starts is a cumulative controller/motor, sprockets, chain noise amplified a bit by the large rear wheel and tire. that Stingray wheel of course looks cool and is a design feature so it is what it is... it handles 6-7hp fine otherwise. i'd be curious whether a better FOC controller would eliminate a bit of the undesirable sound but no plans to swap that out...

i did pick up a CSK one-way bearing to experiment with as the Holy Grail alternative to freewheels. they do indeed engage instantly and are silent. i'm not about to spend 500 bucks on building a wheel with a new mtb roller clutch hub, which aren't designed to transmit multiple HP anyway. it's a topic for another thread. will report back if i get anywhere with it.