SRAM DD3- 24 Issues

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NUKe
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SRAM DD3- 24 Issues

Postby NUKe » 10 Sep 2019, 10:22am

I have an issue with DD3 hub on the grasshopper. It only occurs in first, on the Hub and low gear on the derailleur, climbing steep inclines. Feels a bit rough/rumbles is the only way I can describe, don’t think its slipping. In 2nd and 3rd on the hub its fine in 1st hub and in higher gears on the derailleur its OK? Any suggestions? I have ruled out the cable pull, its not the BB, Chain is getting to the point its probably needs changing , but I don't think its that. Iwould expect bearing to manifest in other gears
NUKe
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Brucey
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Re: SRAM DD3- 24 Issues

Postby Brucey » 10 Sep 2019, 11:10am

the drive to the hubshell in first and second gears is taken via the pawls on the left side of the mechanism. You can get problems here which manifest themselves worse in first gear simply because the torque is higher than in second. There is also a different centring force on the planet cage in second vs first which may affect things too.

The fundamental problem is that the hub designers went for 'low backlash' at the expense of 'reliable two-pawl engagement'. Two pawls (from four) in an opposing pair are meant to transmit the drive at any one time. However the parts are not made perfectly concentric and the chances of getting only one pawl engaged instead are at least doubled because of the low backlash design. Effectively this means that instead of (say) twenty degrees between engagements, so about nineteen degrees where both pawls engage, one degree where only one engages, you might have ten degrees between engagements, with only nine degrees where both pawls will engage.

You can tell the hub is bad in this respect if you backpedal (or turn the wheel backwards) very slowly in gear 2. You should hear 'click' or at worst 'click-k' as both pawls go at the same time. If it is really bad you will hear 'click-click'. If this sound is different when you have turned the part that wasn't moved through 180 degrees, this means that there is more than one non-concentric part in the hub (not uncommon) and it never will sort itself out.

When one pawl only engages, there is large reaction load between the planet cage and the axle. This is pretty draggy and you can tell if is has been happening because there will be a groove worn in the axle. Once this groove is present, the concentricity of the parts further reduces and this increases the chances of 'one pawl' engagement. Its a vicious circle.

The other thing that happens (probably not coincidentally) is that the spring which works the low gear pawls (which is like a fork seal spring) and the pawls themselves tend to suffer and this can lead to weird noises and eventual pawl failure.

So my suggestion is that you remove the LH cone and locknut, pull the internal and take a look at it. Damage to the pawls or the spring will be fairly obvious. Then remove the planet cage (circlip on the left IIRC) and check for wear on the axle, and free play vs the planet cage.

Dunno if you can still buy DD3 spares or not, but be prepared to have to replace the hub entirely; IME this fault is very common and by the time the hub is making funny noises the chances of it not needing a major repair are pretty slim.

cheers
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Brucey
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Re: SRAM DD3- 24 Issues

Postby Brucey » 10 Sep 2019, 11:13am

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NUKe
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Location: Suffolk

Re: SRAM DD3- 24 Issues

Postby NUKe » 10 Sep 2019, 11:51am

Brucey thank you a very comprehensive, explanation, I will take a look at the first chance, Also thanks for the link stuff is becoming rare for these hubs I have no idea of the history of my hub as the bike was secondhand and I have done 17K on it. Iti looked good when I stripped it 10k miles ago. I will take a look at the weekend . thanks for the link as well
NUKe

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NUKe
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Joined: 23 Apr 2007, 11:07pm
Location: Suffolk

Re: SRAM DD3- 2r resolved.

Postby NUKe » 10 Sep 2019, 9:50pm

Now fixed. I was about to strip down as per your recommendations and noticed the right hand wheel nut was only hand tight. I must have got distracted when doing a job at the weekend.
NUKe

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Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: SRAM DD3- 24 Issues

Postby Brucey » 10 Sep 2019, 10:34pm

good result!

cheers
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