[66]

I have a frame with a chain pulley (PDC). The teeth are wearing greatly. I will probably not be able to get a new one for a couple weeks but need to head to Utah in 7 days.

All this gets me thinking, how important are the teeth on a pulley on a frame design like the PDC, Balfa, Canfied etc..?

 

[thaflyinfatman]

Not very at all - all they do is locate the chain, they don't actually hold any load along the chain's axis. In theory you could just use a roller like an MRP one (no teeth, just a channel) but in reality they'd wear out far too fast.

 

[FlipFantasia]

that's wierd your's is wearing out so fast....my bb7 is 3 years old and the pulley teeth still look good on it....

 

[DHDror]

The pully on my 3 Y.O. BB7 also has nice teeth.

I think the the teeth are for better alignment of the chain in the midle of the groove and less noise as a consequence.

 

[ÆX]

check to see if the chain is going straight into the pulley?

 

[66]

The chain exits straight. I think it wore fast becase it's a soft aluminum. One of the changes for the next generation of the bike will be a steel pulley.

Here is a pic if anyone is interested

 

[aidanwray]

it makes no difference.

all that the pulley does is locate the chain in the right place, the teeth keep it in the right place and presumably help stop it falling off.

i.e. an mrp roller and toothy e13 cog both are different but do the same kinda job

 

[thaflyinfatman]

The chain exits straight. I think it wore fast becase it's a soft aluminum. One of the changes for the next generation of the bike will be a steel pulley.

Here is a pic if anyone is interested

Ok any wearing part being made out of aluminium was pretty dumb to start with. A steel pulley would last much much longer.

 

[beaverbiker]

what about aluminum chainrings? you want those made of steel too? it wore for different reasons than it being aluminum genius.

 

[Toshi]

what about aluminum chainrings? you want those made of steel too? it wore for different reasons than it being aluminum genius.

uh, please explain. steel is harder than aluminum, ergo it would wear slower, no?

 

[Ciaran]

what about aluminum chainrings? you want those made of steel too? it wore for different reasons than it being aluminum genius.

Chainrings are much larger thus spreading the wear out over a larger surface area. There is a reason why granny gears and rear cassette cogs are not made of aluminum.

 

[buildyourown]

Chainrings are much larger thus spreading the wear out over a larger surface area. There is a reason why granny gears and rear cassette cogs are not made of aluminum.

They make aluminum granny gears and cogs. FYI

The jockey pulley shouldn't wear as there is no torque on it. Derailleur jockey pulleys are plastic and they last forever. Did you check you chain? A stretched chain can destroy a new chainring in a single ride.

The pulley on my BB7 which is under identical loads lasted for a long time and it was aluminum.

 

[beaverbiker]

Chainrings are much larger thus spreading the wear out over a larger surface area. There is a reason why granny gears and rear cassette cogs are not made of aluminum.

that is true, when there is a torque being applied to the gear. as an idler pulley with little/no torque applied, the tangential forces acting on the tips of the teeth are near zero. these tangential forces are what wear the teeth down. the larger the tooth (a function of the number of teeth and the diametral pitch), the more area that these tangential forces are distributed over. so in a large chainring where the number of teeth is large for about the same diametral pitch as a smaller chainring, the area is larger. so you are right. but like some other dude said, they make small chainrings and small cogs out of aluminum as well. it all depends on how good you are at manufacturing them. to increase the fatigue life of a part there are many surface treatments that can be done (i.e., surface hardening causing residual compressive stresses on the surface of the part).

my point was that switching to a steel pulley may help, but it's not a solution. its obvious that the assumption of zero torque(zero tangential forces acting on the tooth) is wrong. efforts should be put towards figuring that out, which could lead to larger problems (i.e. wearing out a chain after 2 rides), instead of switching to a steel pulley.

 

[joelsman]

does the pully have friction when mounted or does it spin freely, you might need to put in some washers so you only clamp the stationary part of the bearing.

 

[zedro]

Not very at all - all they do is locate the chain, they don't actually hold any load along the chain's axis. In theory you could just use a roller like an MRP one (no teeth, just a channel) but in reality they'd wear out far too fast.

it would do more than wear out fast, it would probably break (especially a derailler pulley), or dig into the roller so much it'd bog things down. I use that type of pulley setup and its the one thing thats given me the most problems...i've had one pulley design self destruct, another fall apart or loosen up etc. That should have a steel cog if its to survive; the torque is super low but the load on the gear is quite high; deforming the flats between teeth really screws with th drivetrain. Also it seems like they use a 11t cog, ideally you'd want a higher tooth count to better distribute the load across the gear, and it makes the drivetrain a bit smoother (i started with a 11t, now use a 15t).

 

[Curb Hucker]

it would do more than wear out fast, it would probably break (especially a derailler pulley), or dig into the roller so much it'd bog things down. I use that type of pulley setup and its the one thing thats given me the most problems...i've had one pulley design self destruct, another fall apart or loosen up etc. That should have a steel cog if its to survive; the torque is super low but the load on the gear is quite high; deforming the flats between teeth really screws with th drivetrain. Also it seems like they use a 11t cog, ideally you'd want a higher tooth count to better distribute the load across the gear, and it makes the drivetrain a bit smoother (i started with a 11t, now use a 15t).

The pulley on my canfield is 15t and made of Ti, i have yet to have any issues with it other than a bit of wear.

 

[thaflyinfatman]

Hmm, I got an idea - make a carrier that has the same dimensions as a freehub body (well the splines or whatever they are anyway), and just use a rear cassette cog as an easily-replaceable, already-designed idler interface?

 

[66]

so it's sounding like even if i replace the pulley this may happen again. right? The chain is fine, it shouldn't be the source of the problem.

What should I change? The pulley points the chain into the chain ring fairly straight. the chain ring is bent so there is a little wobble. Hopefully that isn't the problem because if have never had a chainring/crank/chainguide stay straight for very long. especially with the bb height on this bike. larger pulley?