Author Topic: Cheap Dyno hub. A problem or just rubbish (Read 6103 times)

The Family Cyclist · « **on:** 23 October, 2018, 07:50:27 am »

I've purchased a cheap Dyno wheel from Decathlon to keep me going till I get something better and be a spare. It's a dh3n 31 according to the website and says nexus on the actual hub.

Now I wasn't expecting much, it is £30 quid at the end of the day but the drag appears huge. Haven't fitted tyre yet but if I give it a bug spin it only goes round about 5 times and there is an audible resistance (I know resistance isn't a noise but it's a kinda brake rubbing noise which they're not)

Has anyone had any experience of these. I don't know if it really is that bad a hub or maybe bearings over tightened. The actual output seems alright as lights on when just spinning wheel

jiberjaber · « **Reply #1 on:** 23 October, 2018, 08:08:06 am »

They all have a certain resistance when spinning it and not in contact with the ground due to the magnets in the actual dynamo - even a SON one has it and will slow to a stop over a few revolutions from a hand push. Maybe compare it to someone else's hub locally to ensure its not more than that?

Ripio · « **Reply #2 on:** 23 October, 2018, 08:08:47 am »

Wait until you have tried it on your bike, its hard to judge dynohub wheels out of the bike.

The Family Cyclist · « **Reply #3 on:** 23 October, 2018, 08:12:34 am »

Thanks. I'll stick a tyre on and have a go. Oh and movers can this go into the knowledge posted in wrong place

Edit moves it myself. Didn't realize could do that

T42 · « **Reply #4 on:** 23 October, 2018, 08:16:11 am »

Don't worry about it, they all do that.

AIUI the main difference between the cheaper Shims and the pricey ones lies in the bearings and the weather protection. I have an idea, too, that when you're riding the cheaper ones have less drag when your lights are on than when they're off.

rogerzilla · « **Reply #5 on:** 23 October, 2018, 08:18:31 am »

Adjust the bearings (LH cone)! They can be very tight out of the box these days - Shimano quality control has gone down the pan for anything below Dura-Ace - and it makes a huge difference to lights-off drag. You want slight play that disappears when the QR is tightened, as usual.

andrew_s · « **Reply #6 on:** 23 October, 2018, 09:43:31 am »

Quote from: rogerzilla on 23 October, 2018, 08:18:31 am

Adjust the bearings (LH cone)!

Add some extra grease too, whilst you are at it.
The right-hand side shouldn't (*) be undone, but whilst the LH cone is off/loose you can often push the innards across enough to get a sufficient gap under the seals to get some in using a grease gun and thin squashy tubing.

(*) If you start undoing RH locknut & cone, you can break the connector wires. It is possible, with sufficient care, but Shimano say not to do it so there will be no warranty come-back if you do.

Brucey · « **Reply #7 on:** 23 October, 2018, 11:39:29 am »

yep, IME the bearings will be too tight and there won't be enough grease either.

DH-3N31 exists in two forms, -NT and -QR for nutted and QR type

http://si.shimano.com/pdfs/ev/EV-DH-3N31-NT-3545.pdf
http://si.shimano.com/pdfs/ev/EV-DH-3N31-QR-3546A.pdf

they are really quite different from one another.

Needless to say the best adjustment for the QR version is like any other cup and cone QR hub; a tiny bit of free play in the bearings that just disappears when the QR is fully tightened. The correct adjustment for the NT version is no free play, but this can only be determined by finding a setting where there is a little free play (with the locknuts tight etc) and then adjusting the LH cone a fraction more.

I have had new shimano hub generators in which the bearings were set so badly that the correct setting was a full 1/4-turn looser. OK the hubshell is a bit squashier than most others (the ends deflect more easily) but that adjustment is terrible and will destroy the bearings in short order.

cheers

Karla · « **Reply #8 on:** 23 October, 2018, 12:00:39 pm »

Consider how hard you pushed the wheel when you span it. If it felt like you used about as much energy as if you'd lifted a kilogram weight by one metre, then that's ten Joules of energy you've used.

A dynohub is rated at three Watts, right? That means it uses three Joules every second. You've given the wheel 10 Joules and the hub is taking 3 away per second ... I'll let you do the math.

Kim · « **Reply #9 on:** 23 October, 2018, 12:11:49 pm »

Quite. I find it quite uninituitive how little speed you can put into a wheel by spinning it by hand (or even using sneaky dynamo-testing tactics like holding the chuck of a power drill against the tyre), compared to riding at even a gentle pace.

Also note that without the tyre, there's not a lot of flywheel mass there to store energy.

OTOH, while it's normal for dynamo hubs to feel implausibly stiff by hand, and they can make interesting buzzing noises due to the 'cogging' effect (particularly if they vibrate a truing stand or something), they shouldn't actually be making rubbing sounds.

mattc · « **Reply #10 on:** 23 October, 2018, 12:40:06 pm »

Quote from: Karla on 23 October, 2018, 12:00:39 pm

Consider how hard you pushed the wheel when you span it. If it felt like you used about as much energy as if you'd lifted a kilogram weight by one metre, then that's ten Joules of energy you've used.

A dynohub is rated at three Watts, right? That means it uses three Joules every second. You've given the wheel 10 Joules and the hub is taking 3 away per second ... I'll let you do the math.

[my bold]; and if you don't know how much this is, you should probably avoid fettling anything!

Bantz aside, this is quite a useful backoftheenvelope guide - nice one K!

Brucey · « **Reply #11 on:** 23 October, 2018, 01:12:37 pm »

the other way of looking at it is the amount of KE the wheel has when you give it a spin.

Very roughly a rim weighs 500g and a rim + tyre weighs more like 1kg.

If you spin the wheel at about 10mph then that is ~5m/s

So without a tyre the KE is (mv^2)/2 = (0.5 x 5^2)/2 which is 7.5 joules

With a tyre the KE about double that.

So with the light on a tyreless wheel is likely to stop in about two seconds. Without the lights on it will be about three or four seconds if the drag is about 2W. IME most shimano hubs (when fettled and the seals are run in) drag between 1 and 1.5W (in round numbers, lights off).

The Nexus ones which have a ~90mm diameter generator on the RH side of the hub are rather more draggy than that, lights off, but they are (amongst shimano ones) the exception. [edit; see McWheel's link for more details of the lights off drag of HB-NX** models.]

cheers

grams · « **Reply #12 on:** 23 October, 2018, 01:51:22 pm »

The notchiness means the hub is constantly absorbing and releasing energy, for little net loss. But when you spin it the wheel will stop spinning the moment the kinetic energy is less than needed to overcome one notch.

alexb · « **Reply #13 on:** 23 October, 2018, 01:54:59 pm »

I had one of these for ages. They're great hubs. Cheap and just work. for commuting they're almost the perfect hub. Mine gave me over 5 years hassle free service and I sold it with the bike, so it's probably still working fine for someone else.

vorsprung · « **Reply #14 on:** 23 October, 2018, 02:50:56 pm »

please be very careful adjusting bearings in Shimano generator hubs. It is very easy to break wires and nearly impossible to reattach them

rogerzilla · « **Reply #15 on:** 23 October, 2018, 03:39:51 pm »

It's safe if you only touch the LH side. My new DH-3N80 (built up last week) was as tight as a crab's arse but spun much more freely - not much worse than a normal front hub - when properly adjusted.

I used to have a SON 28 (original type, iddu has it now) and a properly-adjusted 3N80 spun noticeably more freely with the lights off than the SON 28. It's hard to make a comparison with the lights on, as neither spins for very long.

Brucey · « **Reply #16 on:** 23 October, 2018, 04:48:14 pm »

Quote from: vorsprung on 23 October, 2018, 02:50:56 pm

please be very careful adjusting bearings in Shimano generator hubs. It is very easy to break wires and nearly impossible to reattach them

Quote from: rogerzilla on 23 October, 2018, 03:39:51 pm

It's safe if you only touch the LH side. My new DH-3N80 (built up last week) was as tight as a crab's arse but spun much more freely - not much worse than a normal front hub - when properly adjusted.

I agree with Roger; it is perfectly safe if you adjust the LH side only.

BTW with any cup and cone bearing hub you should lock the RH cone and locknut (intending not to ever loosen it again) and adjust on the LH side. The reason for this is that if the RH cone is not perfectly tight and starts to precess, the hub is destroyed. If the same thing happens on the left, the bearings merely loosen slightly.

If there is no tab washer on the hub, and especially if you are running vertical dropouts and rim brakes (which is tolerant of QRs not being very tight), there is an increased chance of the RH cone moving in service. In this case it isn't a bad idea to use red threadlock on the RH parts when they are assembled. This won't stop them coming apart when they need to, but might possibly spare you a disaster.

cheers

mattc · « **Reply #17 on:** 23 October, 2018, 07:15:13 pm »

Quote from: alexb on 23 October, 2018, 01:54:59 pm

I had one of these for ages. They're great hubs. Cheap and just work. for commuting they're almost the perfect hub. Mine gave me over 5 years hassle free service and I sold it with the bike, so it's probably still working fine for someone else.

Top tip

My commuter bike's dyno-hub is creaking alarmingly, so I may as well buy a new wheel for £30! Then fettle the (slightly nicer) old wheel at leisure.

The Family Cyclist · « **Reply #18 on:** 23 October, 2018, 08:24:59 pm »

Had a quick fettle this afternoon. Bearings were far too tight. Adjusted and much better spinning from the wheel. Just need to try it in anger

McWheels · « **Reply #19 on:** 23 October, 2018, 10:17:07 pm »

I've been using a second hand DH3n30 for over 3000 miles. No worries at all. And before that a new one in the old commuter bike for 7500 miles. It's now been transferred SWMBO. Still the same, kept outside (under shelter) but no rattle or looseness in either. Hopefully the 3n31 is at least as good as its predecessor.

Also a long time ago I dug this out of the internet and it's been quite helpful to explain why I don't see the large price increase for high end dynamos as worth it. http://veloweb.ca/resources/VBQgenerator.pdf

mattc · « **Reply #20 on:** 29 October, 2018, 11:32:20 pm »

Can anyone confirm if this wheel is a wide 700c?

https://www.decathlon.co.uk/28-front-hybrid-dynamo-wheel-id_8133890.html ( a DH-3N31-QR hub)
(I believe the "28" designation is a bit of a minefield, historically $:-\$ )

whilst this is clearly a 26" :
https://www.decathlon.co.uk/26-hybrid-front-dynamo-wheel-id_8131687.html ( a DH-3N30 hub)

What did Mr faro buy??

Brucey · « **Reply #21 on:** 29 October, 2018, 11:50:56 pm »

'Hybrid' and 'trekking' are clues that it is a 700C wheel. Other wheels they sell as 28" wheels also mention that they are '700' size.

cheers

The Family Cyclist · « **Reply #22 on:** 30 October, 2018, 06:43:41 am »

I went for the 28/700

Torslanda · « **Reply #23 on:** 30 October, 2018, 08:27:31 am »

On the basis of the pictures and a touch of experience my guess would be the rim is around 17 -19mm wide, tyre size 28-35mm? Ish?

The only downside appears to be a single wall rim which will have a tough time on a commuter but what do you want for 30 quid...?

mattc · « **Reply #24 on:** 30 October, 2018, 02:12:09 pm »

Quote from: Torslanda on 30 October, 2018, 08:27:31 am

On the basis of the pictures and a touch of experience my guess would be the rim is around 17 -19mm wide, tyre size 28-35mm? Ish?

The only downside appears to be a single wall rim which will have a tough time on a commuter but what do you want for 30 quid...?

... I just want something that fits on my bike.