SG8R31 Shimano hub

[XXL1960]

Hi, I have a Kalkhoff Agattu with a Shimano 8speed hub. Yesterday my son took the bike out and the bike lost drive.
The pedals go round but the rear sprocket just spins. If you lift the back wheel off the ground it spins but as soon as any resistance is met it just spins.
I fitted a random back wheel from another bike and its OK (obviously not use on the road) so everything points at the hub.
Any ideas on a simple fix please?
The bike is a few years old but low mileage.
TIA

 

[sjpt]

Assuming it just spins in all gears it sounds as if the freewheeling mechanism isn't working right. On most systems that would most likely the pawls are gummed up and can't engage properly. I'm not sure it that applies to that gear; I just read that some of their 8 speed hubs have a silent clutch (roller clutch?) instead, but I'm not sure how that works.

 

[Nealh]

Sounds like the hub whether a nexus8 or Alfine8 needs a service,
They are servicable by the competant diyer with the right tools but is probably better having a LBS familiar with them to have it serviced.

It depends on the model as to whether which type of pawls or clutch type is sued.

Cycling uk forum has a few members more at ease with their maintenance and a large knowledge base.

All I have ever done with my Alfinee 8 is to remove the internals and give it a dunk in ATF then allow it to drip dry before refitting.

 

[XXL1960]

Assuming it just spins in all gears it sounds as if the freewheeling mechanism isn't working right. On most systems that would most likely the pawls are gummed up and can't engage properly. I'm not sure it that applies to that gear; I just read that some of their 8 speed hubs have a silent clutch (roller clutch?) instead, but I'm not sure how that works.

Thanks for your advice. I am sort of OK at mechanical DIY I will strip it down and take a look as I guess I have nothing to loose.
Regards
Stan XXL1960

 

[XXL1960]

Sounds like the hub whether a nexus8 or Alfine8 needs a service,
They are servicable by the competant diyer with the right tools but is probably better having a LBS familiar with them to have it serviced.

It depends on the model as to whether which type of pawls or clutch type is sued.

Cycling uk forum has a few members more at ease with their maintenance and a large knowledge base.

All I have ever done with my Alfinee 8 is to remove the internals and give it a dunk in ATF then allow it to drip dry before refitting.

Thanks for your advice. I am sort of OK at mechanical DIY I will strip it down and take a look as
Regards
Stan XXL1960

 

[Nealh]

It may just be sticky pawls on the freewheel end so one can try dripping some ATF or light gear oil to see if they loosen up.
Some times cable adjustment can be an issue so check the yellow marks meet when in 1:1 gear so 4 th or 5th gear.

It really depends on the exact model for more advice, but I would for sure ask on cycling uk forum and some one will give a comprehensive answer usually.

 

[AndyBike]

SG-8R31 is the nexus 8 speed.
This might be helpful to you Stan. Lets you know whats in there before you make a start.

Forewarned is forearmed

Like button, bottom right

In the vid, when he removes the mechanism from the body initially, we see the hub pawls that are responsible for driving the hub. If you are saying yours just spins, I'd maybe look closely at the pawls and ratchet ring first to see if those are engaging. Might find that the grease or muck has stuck to pawls inward so they arent engaging, or maybe even the opposite and the pawl retainer has come lose and the pawls have fallen out of place.

He also has a reassembly vid.

 

[XXL1960]

Massive thank you to all who replied and after a few days of fearing the worse and looking for a replacement hub I stripped the back wheel down and it was just the sprocket that was spinning on the hub! Ordered one from Bankrupt Bike parts and its all good again now.