Dura ace 7850 rear hub problems

I had a custom set of wheels built around a DA 7850 rear hub, and sometimes when you put the power back on after freewheeling, it makes a terrible noise. Like as though the teeth were not engaged correctly, and it has skipped to the next tooth. It only does this about once per ride, but it is very annoying.

Can the prawl set up be replaced or serviced?

I purchased the hub from Ebay, so i cannot take it back to the bike shop for warranty etc.

How old is it? Might just need stripping down, cleaning, and regreasing.

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Precision Sports

3 pawl system very similar to Campagnolo. I'll bet one of the pawls is greased/gunked up and sticking down. Take apart, clean all, OIL the pawls, grease the rest and put back together.

Peter, you may be right, but I can't visualize how a stuck-or-sticking pawl would cause an excessive amount of noise ...

Wouldn't the engagement of the other two pawls keep the recalcitrant pawl from moving past more than "saw" tooth?

I would be inclined to check the rear derailleur's indexing ... I reckon that the derailleur cable & housing may need to be cleaned-and-lubed and/or replaced.

If it were the indexing it would do it as you ride, not after idling then engaging. Could be enough crap in there that none of the three get it right at least once a ride. Might be riding half the time with only one pawl doing anything.

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Precision Sports

HMMmm. Well, I guess that's a reason to choose a DT (formerly, "Hugi") hub with its star ratchet mechanism!

May be the sticky pawl is allowing the freehub to partially engage and then make the noise to the next. Sure sounds like others I have seen, Campagnolo type.

Okay, thanks ...

I guess it was easier to imagine sticking pawls making an bad noise when freewheeling than when engaging UNLESS there was really a lot of gunk in the 'chamber' (as Phill P also suggested).

Of course, for those who don't already know, the Hugi-designed star ratchet can apparently become non-functional, too, if the wrong weight lube is used or if grit somehow impedes the 'piston'.

BTW. Are the Dura Ace pawls held in place with a wire band or are they loose ... like Campagnolo's?

A buddy here had a similar problem with his Bontrager Racelite hub in the colder temps last winter. At ~40F, when transitioning from coasting to pedalling, the pawls would sometimes fail to engage. He solved the problem by cleaning out the OEM grease and replace it with a lighter synthetic grease.

Thanks for the replies.

I live in a normal climate and the hub only has about 2000km on it from new-rarely gets ridden in the wet (maybe 5-10 times max), and always on normal roads (no massive amounts of dirt etc).

When i wash my bike, i only ever use normal hose pressure, no high pressure cleaners etc.

What are the odds of it gumming up so quickly from new?

Are there any online instructions on how to pull it down and clean it?

I have changed cassettes/chains etc, but i have never pulled a freehub apart.

Thanks Glen

Teeny spring like Campagnolo. The older 8s and early 9s Campagnolo freehubs had individual springs and no 'keeper'. The newer Campagnolo and shimano 7800 series freehubs, for alminum axles have a single spring keeper that holds them in place and also provides the 'spring'.

Thanks for the info.

The pawls in the 7850 are contained in the cassette body and you aren't really able to get to them. They are not external to the body and in the driveside face of the hub shell as Campy hubs are and the 7800 rub was.

I have a few 7850 rear hubs and have had the same issues with them making crackling noises.

I believe the 7850 hub has only three pawls and that they are offset such that not all of them are really engaged at any time. This was done to get quicker cassette body engagment without having to have too many indents in the drive ring.

I think Shimano F'ed up with the 7850 hub. It got pretty quickly replaced by the 7900 hub which has a different cassette body. I haven't heard any feedback on the 7900 hub to know if they have addressed any of the problems of the 7850 hub.

7850/7900 are essentially the same hub. Same cassette body and same tripawl system. 7800/7801 also use a very similar (if not same) tripawl system.

Well, shuck my corn..you are right. 7850 and 7900 have gone back to freehub bolted onto the hub body(better for axle strength, poor for servicability) and away from the exposed 3 pawl system.

I have not had the need to take apart a 7850 hub so I looked at the shimano tech docs and viola...

At least they are 7/8/9/10s compatible but as I mentioned, no service please, just replacement ala a bic lighter.