Wednesday, November 16, 2011

Bike Shop Horrors: Gutted

It isn't often that you get to see what is on the inside of bicycle parts, unless you are a mechanic like I am at a bike shop, or unless you are a BMX-ican.  (Because they can not live until they've torn apart their bicycle to the nth degree!)

You are one "twisted sister"!
Today's subject matter is provided to us by one White Industries Tandem Hub that suffered a defect in manufacture where-by the stamp put in the hub shell was a source for a hole/crack that developed. You can see it to the right side of the logo embossed into the aluminum shell there.

This hub was laced to a Sun rim by 40 spokes and resided in a Cannondale tandem at first, but was ultimately put to a cruel end as a loaner wheel last summer on a Co-Motion tandem.

That tandem team pulled a very loaded down trailer and the torque loading must have been pretty impressive as it twisted the entire aluminum structure, as you can see here.

Well, be that as it may, the hub was warranted, and a new White Industries Tandem Hub was sent out. So, I unlaced this destroyed hub, and figured, why not have a look-see inside o that thing!

Things that go "bzzzzzzzt"!

So, here we have the free hub body off, (you can see the cassette splines facing the table), and what you are looking at is the free wheel "pawls" and the springs that force the hardened steel pawls outward. These springs appear as flat peices of steel with curled ends that fit nicely into machined pockets in the star shaped end of the free hub body. They sit under the pawls, which themselves fit beautifully into their own machined pockets in the same star shaped end of the free hub body. Note: Neither the pawls or the steel springs are mechanically held in place. They are confined on one side by the free hub, and on the other side by the hub body, which houses the ring gear. (Coming up next!)

The blue-ish circular item in the middle is a cartridge bearing that rides on the axle, which is not shown here. One thing I find amusing, and that is all the talk you here about "points of engagement" when referencing the free turning of a hub before a pawl engages. Well, this is such a misunderstood term. Actually, there are only ever three points of engagement with this hub, because there are only three pawls. Hard to imagine all that tandem power being directed through those three hardened steel pawls, but that's how it works.

The other half of "the buzz".
 Here we see what those pawls engage into. This is the ring gear, which is another chunk of machined and hardened steel pressed into the aluminum hub body.

When you coast, those three pawls slide up the ramps, being held against the ring gear by those metal springs, and slide down off the ramps, down the vertical faces of the ring gear, and when this happens at speed, you hear the "bzzzzzt" as you coast, or the "clack-clack-clack" as you walk your bike. It's the sound of those three pawls slipping over those saw-toothed ring gear teeth in unison.

The bike is propelled forward as the gear on the cassette gets driven by the chain, and forces the pawls to rotate until the leading edge of a pawl runs against the vertical face of one of the ring gear's teeth. Since the spring is forcing the pawl outward, the pawl basically jams against the ring gear, and the force of your pedaling efforts is translated to the hub shell, through the spokes, to the rim, tire, and ultimately the ground.

How far the pawls rotate until they engage a ring gear tooth is the "free-play" in the hub that mountain bikers sometimes obsess over. It would be more accurately described in degrees of free rotation, instead of the misleading "points of engagement" that many cyclists refer to.

But at any rate, there is your inside look at the inside of a hub, and how it works!


Travel Gravel said...

It would be cool if you could use different densities/sizes/materials for the pawls, and maybe drill/machine them out in such a way as to make them produce a harmonic. Then you could "tune" your freewheel to a certain sound. Probably spend too much time coasting though!

james said...

In regards to your Phil Wood fat bike hub, do you think the problems you had (the noise, crunching and grinding) originated from the pawl and ring gear interface that you show in the picture?

Rob said...

GT- So "points of engagement" refers to the number of notches on the outside part of the hub any hubs have a higher number of pawls?

Matt said...

Cool, thanks for posting this! It answers several questions I'd had about hubs.

What it doesn't answer is why I didn't just become a shop rat as a teenager... too late now!

Matt said...

@ Travel Gravel -

Gives a whole new meaning to "recently tuned!"

Guitar Ted said...

@james: Yes, that is precisely where my issues with the Phil hun arose from. Their design actually had 4 pawls, and what appeared to be going on was that the pawls weren't seating up into the vertical faces on the ring gear properly. This meant that the pawls were only partially slid onto the faces of the ring gear and could slide up all the way, causing the pop, or at times go the other way and slide over the ring gear, which of course was felt as slippage. The second hub could be made to fail in this manner by hand.

@Rob: The ring gear only represents "possible places for the three pawls to engage". You only can have as many "engagement points" as you have pawls. (Just to be perfectly clear).

So, yes- more pawls = more points of engagement, but that comes with its own compromise in more drag in the hub while coasting and possibly a louder/weirder sounding hub. (See Chris King, Industry 9 for examples)

Also, pawls can have teeth machined into them that match the ring gear, so you could have a pawl machined with three matching "steps" for three ring gear teeth.

Rob said...

great info, thanks!

Steve Fuller said...

Seems that there would be some optimal number of both pawls and notches in the ring gear to maximize wear/life and reduce the degrees of rotation.