Bicycle Mechanics - What is the purpose of derailleur pulley teeth?

I keep reading over and over, where people post about their derailleur pulleys being worn. The typical response on this forum, is..."if it is still shifting properly, just leave them alone." As someone who makes his living working on mechanical things, I am confused by this repeated statement.

It flies in the face of preventative maintenance and sounds like strange advice. Yet it is prescribed so often that I am inclined to believe there is some truth to it. So what is the purpose of having derailleur "teeth" cut into derailleur pulleys?

I'm not trying to start a huge debate, i'm simply asking if anyone KNOWS why they are there. Why aren't the pulleys round, with no teeth?

What would stop the chain from dragging along the derailleur cage if there were no teeth? The purpose of the derailleur is to guide the chain... kind of pointless if there's nothing to actually guide the chain with.

They are freewheeled, and probably could do just as well with fewer teeth, depending on chain wear. Only with a toothed wheel, the pulley wheels have to rotate with the chain, so they wear is more evenly. A smooth pulley wheel might bind at its spindle and wouldn't have the chain to turn it. That might result in faster and uneven wear, maybe even making a soft pulley wear out of round.

Years ago Huret, I believe, made a rear derailleur that had toothless jockey wheels. Since they had no teeth to force them to rotate with the chain, the chain tended to slide along them and they wore out quite quickly. A whole industry sprang up to provide aluminum replacements - with teeth.

More precise movement of the chain when you shift, and therefore more precise shifting.

If the pulleys didn't have teeth, as the deralieur moved over to shift, the chain would fall off the pulley, or go to the edge of the pulley, in either case, shifting would be compromised.

So far, all good reasons to replace worn pulleys. Where is the camp that normally says to leave them be????

http://www.parktool.com/images_inc/repair_help/der401.jpg


http://www.parktool.com/repair/readhowto.asp?id=76

Wow, quite the picture. I'd keep using the worn ones, though, unless the chain slips or falls off the pullies while trying to shift.

Wow, quite the picture. I'd keep using the worn ones, though, unless the chain slips or falls off the pullies while trying to shift.

I've got an old derailleur laying in the garage, where the teeth look like the one on the right. It started to shift like crap. I guess my question is......when you know you have a part that might be suspect and start causing trouble, why leave it on there?

Maybe if you didnt ride every day year round like me, but I like to keep stuff replaced. I'd rather ride than have failures.

I've got an old derailleur laying in the garage, where the teeth look like the one on the right. It started to shift like crap. I guess my question is......when you know you have a part that might be suspect and start causing trouble, why leave it on there?

Maybe if you didnt ride every day year round like me, but I like to keep stuff replaced. I'd rather ride than have failures.

A worn derailleur pulley isn't going to break anything else and isn't a threat to life and limb. It'll just continue to wear until it doesn't work very well any more. A worn chain, on the other hand, will wear everything it touches until you have to replace it all together. Worn rims and brake pads or corroded and frayed brake cables can all be very dangerous.

So, if derailleur pulleys are getting worn but they still shift OK, there isn't a pressing need to change them. You might want to reconsider that if you're leaving on a long tour, though.

Also be careful when changing them. I was sprinting downhill behind a guy once when I started getting pelted by some tiny bits. Then the guy goes down and I swerve around him just in the nick of time while 5 guys behind me piled into him. He had swapped in new shiny anodized alloy pulleys and didn't tighten down the cage-bolts enough. It came apart on the sprint and jammed his chain, RD and rear-wheel into a mess... not fun at 40mph+... :(

I'm not philosophically in either camp (replace or ignore) at the moment, but it seems to me that the SIDE WALLS of the derailleur cage are what become the effective shifting mechanism once the teeth are gone. Since there is more distance between the cage side plates, shifting should logically become sloppier once the teeth are gone. Additionally, there should be more friction as the chain rattles against the side plates than if the chain is being centered by the jockey wheels. Of course, all this assumes that the teeth are TOTALLY gone, and that the chain is free to move laterally within the cage plates of the derailleur.

So long as the jockey wheels are doing their job (keeping the chain centered and out-of-contact with the side plates), it would seem that the condition of the teeth would be academic. In other words, significant wear to the teeth themselves would make no difference in shifting so long as the side-to-side width of the jockey wheel is still sufficient to control the chain. Once the jockey wheels become narrowed by wear to the point where they no longer center the chain (even under the lateral load of shifting), then it would make sense to replace them.

Obviously, all the above paragraphs assume that the jockey wheel BEARINGS are still smooth and frictionless. If not, then energy is lost as the wheels are being turned (resulting in accelerated wear on the teeth) and the bearings should be maintenanced or replaced.

I may be off base with these musings - I'm typing as I think...

Always use red loctite on thease bolts

Years ago Huret, I believe, made a rear derailleur that had toothless jockey wheels. Since they had no teeth to force them to rotate with the chain, the chain tended to slide along them and they wore out quite quickly. A whole industry sprang up to provide aluminum replacements - with teeth.

Not on my Huret Jubilee, still working fine.

Think about it, worn teeth will usually mean that the bush is also worn, allowing the jockey pulley to yaw. That is what causes poor shifting, not the lack of teeth or otherwise.

Always use red loctite on thease bolts
Yeah, right. :rolleyes: Why not just weld them in place?

If you have a need to use loctite... stick with the blue. Then you can take them apart w/o melting everything.

Years ago Huret, I believe, made a rear derailleur that had toothless jockey wheels. Since they had no teeth to force them to rotate with the chain, the chain tended to slide along them and they wore out quite quickly. A whole industry sprang up to provide aluminum replacements - with teeth.

If memory serves me correctly, smooth pulleys were also used by Campagnolo at one time.

I have a couple of Huret derailleurs with smooth pulley wheels - the lower ones at least. The upper pulleys are still toothed (teethed?), presumably because this improves shifting somehow. But then again, they're French. So there may be no reason at all. Not that I'm protesting of course!

- Joel

A long while back, there was a third party that sold replacement gears for rear derailleurs. The product was called a "shift biscuit", and was a CNC machined gear, probably machined, anodized aluminum so it wasn't known for its wearing abilities.

I've always wondered what happened to this, or if there is really any point to this because most derailleurs already have rebuild kits that have a pulley gear in them for replacement.

If the third party gear was extra solid or had some new way to help with shifting, it would be great, but I think the shift biscuit product died because nobody really cared about a third-party product in their rear derailleus as compared to Titanium skewers, or other items.

A long while back, there was a third party that sold replacement gears for rear derailleurs. The product was called a "shift biscuit", and was a CNC machined gear, probably machined, anodized aluminum so it wasn't known for its wearing abilities.

I've always wondered what happened to this, or if there is really any point to this because most derailleurs already have rebuild kits that have a pulley gear in them for replacement.

I expect they came anodised in many different colours. In the 90's there was a craze of covering your bike in as many colour coridinated anodised parts as possible. The point was your pulleys would match all your other anodised bits. Once the coloured ano craze died out there wasn't any point to them.

Not on my Huret Jubilee, still working fine.

Think about it, worn teeth will usually mean that the bush is also worn, allowing the jockey pulley to yaw. That is what causes poor shifting, not the lack of teeth or otherwise.

IIRC, don't some of the old Huret pulleys have ball bearings?

I expect they came anodised in many different colours. In the 90's there was a craze of covering your bike in as many colour coridinated anodised parts as possible. The point was your pulleys would match all your other anodised bits. Once the coloured ano craze died out there wasn't any point to them.

IIRC, you are right about that. Almost everything was being spat out from aluminum billeted CNC machines with anodized colors, down to cute little peace signs to hold the wire for the canti levers.

A long while back, there was a third party that sold replacement gears for rear derailleurs. The product was called a "shift biscuit", and was a CNC machined gear, probably machined, anodized aluminum so it wasn't known for its wearing abilities.

I've always wondered what happened to this, or if there is really any point to this because most derailleurs already have rebuild kits that have a pulley gear in them for replacement.

If the third party gear was extra solid or had some new way to help with shifting, it would be great, but I think the shift biscuit product died because nobody really cared about a third-party product in their rear derailleus as compared to Titanium skewers, or other items.


I don't know if it was the same product, but the 3rd party pulley wheels I bought also had bearings vice simple bushings. I used them to reduce friction.

IIRC, don't some of the old Huret pulleys have ball bearings?

Correct, including the Jubilee. The only reason the chain would 'drag' on them is if they were adjusted too tight.