Author Topic: SON dynamo slipping  (Read 1695 times)

SON dynamo slipping
« on: June 30, 2020, 10:38:53 am »
A while ago I broke the spades on my thru axle SON dynamo due to it slipping the wires pulling on the spades.

(full details of that story here: https://yacf.co.uk/forum/index.php?topic=107737.msg2279629#msg2279629 )

Dynamo repaired (under warranty), but the problem still persists. Even if I clamp the wheel in really tight it slips and the position of the spades moves over the course of a ride ...at some point putting the connectors and wires under stress.
I can't really understand the physics of why this happens. I assume it moves when braking.

I can't be the only person who has this problem  ???  ...although searches don't turn up any other tales.

QR dynamos have a serrated surface where they meet the fork, but the thru axle ones are smooth.

Wondering what would be the best solution to stop the slip:
  • Cutting some groves into the dynamo? (strangely this idea doesn't appeal!)
  • Mounting a thin locking washer on the dynamo or the fork (how would I get this to stay put? epoxy? superglue?)
  • Any other ideas?


LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #1 on: June 30, 2020, 10:41:49 am »
HK has had this a couple of times on her Kinesis fitted with a carbon fork and similar disc SON Hunt wheel. The axle rotated forward but I have not thought through the likely cause. Tightening the through axle seems to stop it in her case.

My first guess is that disc brake forces flex one fork leg more than the other. If there isn’t sufficient friction around the full circumference of both ends of the through axle, there is an opportunity for the axle to progressively ‘pivot forwards’ on diagonally opposite points, ratcheting as the fork flexes and returns to its rest position.

Are the inner faces of the fork entirely smooth and parallel where they contact the hub?
Wheel meet again, don't know where, don't know when...

Re: SON dynamo slipping
« Reply #2 on: June 30, 2020, 12:55:15 pm »
My first guess is that disc brake forces flex one fork leg more than the other. If there isn’t sufficient friction around the full circumference of both ends of the through axle, there is an opportunity for the axle to progressively ‘pivot forwards’ on diagonally opposite points, ratcheting as the fork flexes and returns to its rest position.
That's a good explanation and it makes sense. It never moves a large amount in one go, but the sum of the small increments are noticeable over the course of a ride. On longer rides I've had to reset it to the original position.

Are the inner faces of the fork entirely smooth and parallel where they contact the hub?
Smooth, yes & have the appearance of being parallel. It's quite a loose fit, but I tighten the axle as tight as I can possibly make it (more than feels comfortable) & it still slips.

zigzag

  • unfuckwithable
Re: SON dynamo slipping
« Reply #3 on: June 30, 2020, 01:38:14 pm »
half serious suggestion: apply a ring of superglue on the axle ends and dip them into sand. use acetone to remove the sand if the hub/wheel has to be sold on.

Re: SON dynamo slipping
« Reply #4 on: June 30, 2020, 01:40:52 pm »
Identical problem here, which I attributed to an issue with the axle rather than the dynamo. I always managed to spot and correct it before any damage to the dynamo spades or cabling. Tightening the axle made no difference.

Moving the lever to a 3pm position rather than its usual inline with the fork position does appear to have fixed it – in that it hasn't happened on the few rides since this change. Not sure of the cause or why this solution worked yet...

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #5 on: June 30, 2020, 01:49:25 pm »
I wonder if this is a widespread problem but rotation of the axle in a standard hub isn't easily detectable (or indeed important). Perhaps somebody could mark normal through axle hubs and see if those axles also rotate over time.
Wheel meet again, don't know where, don't know when...

Re: SON dynamo slipping
« Reply #6 on: June 30, 2020, 02:33:26 pm »
half serious suggestion: apply a ring of superglue on the axle ends and dip them into sand. use acetone to remove the sand if the hub/wheel has to be sold on.

I like it! I think I'll give that a try.

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #7 on: June 30, 2020, 02:34:58 pm »
I don't know if that approach would tend to abrade the carbon fibre (epoxy at least) of the fork over time. Carbon paste might be a better option, albeit temporary.

https://www.reddit.com/r/cycling/comments/a74rrc/son28_thruaxle_terminals_rotating_when_in_use/ is the only directly relatable incident that I have found on the interwebs.
Wheel meet again, don't know where, don't know when...

Blodwyn Pig

  • what a nice chap
Re: SON dynamo slipping
« Reply #8 on: June 30, 2020, 02:45:39 pm »
had a son hub built in to a wheel, by none other than Harry Rowland, and he said, 'sommit wrong with that hub, kept slipping in the jig, but it has never slipped on the bike, before or after.   ???

Re: SON dynamo slipping
« Reply #9 on: June 30, 2020, 03:50:55 pm »
to my mind what may be happening is this;

- when riding normally road shocks force the axle and the hub to sit as high as possible in their range of allowable positions (dictated by the amount of clearance between the axle and the hub, as well as the axle and the dropouts).

- when the brake is applied the axle and the hub are forced backwards in the LH dropout.

One or other of these movements may result in more of a rolling contact (between the hub and the axle) than a straight shove, and the net result may be that the hub will 'precess' in one direction with repeated brake applications.

Fundamentally improving the diametral  fit between the axle and the dropout, as well as between the axle and the hub, ought to improve matters. Improving the grip between the hub and the dropouts will only work if it inhibits movement entirely, which (IMHO) is a big ask.

cheers

Tim Hall

  • I want to eat the fleeting shade of your lashes
Re: SON dynamo slipping
« Reply #10 on: June 30, 2020, 03:59:37 pm »
to my mind what may be happening is this;

- when riding normally road shocks force the axle and the hub to sit as high as possible in their range of allowable positions (dictated by the amount of clearance between the axle and the hub, as well as the axle and the dropouts).

- when the brake is applied the axle and the hub are forced backwards in the LH dropout.

One or other of these movements may result in more of a rolling contact (between the hub and the axle) than a straight shove, and the net result may be that the hub will 'precess' in one direction with repeated brake applications.

Fundamentally improving the diametral  fit between the axle and the dropout, as well as between the axle and the hub, ought to improve matters. Improving the grip between the hub and the dropouts will only work if it inhibits movement entirely, which (IMHO) is a big ask.

cheers
Surely a thru axle (see the OP) doesn't have the ability to move in the drop out?
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #11 on: June 30, 2020, 04:33:20 pm »
Many through axles are anodised. If there was this rocking motion between axle and through axle, wouldn’t there be some abrasion of the anodising? If this rocking is the problem, greasing the surface of the through axle should reduce friction, possibly preventing rotation.

I have read a few instances of through axles (particularly DT, which is a wingbolt) loosening in use. I don’t know whether this might be related to this rotating hub axle behaviour. I wouldn’t have thought so, because of precession.
Wheel meet again, don't know where, don't know when...

Re: SON dynamo slipping
« Reply #12 on: June 30, 2020, 04:52:35 pm »
Thanks all, the input is appreciated.

I had a proper look at it and the face of the (carbon) fork where it has contact with the the hub is alloy.

I've put a generous dollop of gritty carbon assembly paste on both sides and done it up tightly.
I'll take it for a spin tomorrow (no pun intended) and see if it holds. May try a permanent fix with some silicon carbide grit and superglue.

Blodwyn Pig

  • what a nice chap
Re: SON dynamo slipping
« Reply #13 on: June 30, 2020, 05:15:15 pm »
Was thinking, without going to the shed to look, but could you not remove the 2 outer serated nuts and slip on a tab washer, (home made probably) and then refit the locknuts and do up tight, making sure both tabs (that would be longer than the depth of the nut) would locate the bayonets in the desired position, and said tabs would sit in the dropout, below the nut, and prevent rotation, like.

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #14 on: June 30, 2020, 05:35:32 pm »
Through axle hub with no locknuts, so no dice.
Wheel meet again, don't know where, don't know when...

Blodwyn Pig

  • what a nice chap
Re: SON dynamo slipping
« Reply #15 on: June 30, 2020, 05:37:10 pm »
Through axle hub with no locknuts, so no dice.

Ah! didn't read that bit....doh.

Re: SON dynamo slipping
« Reply #16 on: June 30, 2020, 06:56:30 pm »
I don't know if that approach would tend to abrade the carbon fibre (epoxy at least) of the fork over time. Carbon paste might be a better option, albeit temporary.

"Carbon paste" *is* sand suspended in grease. Despite the name, there is no carbon fibre in it.

(the things you learn when your puppy chews on things it shouldn't and you have to phone the number on the tube)

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #17 on: June 30, 2020, 07:39:54 pm »
My understanding was that, to increase the friction coefficient with carbon surfaces, (at least some) carbon paste incorporated deformable plastic grains, rather than abrasive sand. I know the paste doesn't include carbon particles (which would be a lubricant).
Wheel meet again, don't know where, don't know when...

Re: SON dynamo slipping
« Reply #18 on: July 01, 2020, 08:25:11 am »
Just a thought.  How does the clamping power of thru axle mechanisms vary?  I don't have any, but some appear to be an 'external cam' type...
Destroying rainforest for economic gain is like burning a Renaissance painting to cook a meal.  EOW.

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #19 on: July 01, 2020, 09:14:14 am »
to my mind what may be happening is this;

- when riding normally road shocks force the axle and the hub to sit as high as possible in their range of allowable positions (dictated by the amount of clearance between the axle and the hub, as well as the axle and the dropouts).

- when the brake is applied the axle and the hub are forced backwards in the LH dropout.

One or other of these movements may result in more of a rolling contact (between the hub and the axle) than a straight shove, and the net result may be that the hub will 'precess' in one direction with repeated brake applications.

Fundamentally improving the diametral  fit between the axle and the dropout, as well as between the axle and the hub, ought to improve matters. Improving the grip between the hub and the dropouts will only work if it inhibits movement entirely, which (IMHO) is a big ask.

cheers

If this motion is occurring, there should be contact marks mostly along the middle of the through axle, due to flex of the axle between the supports?

If the movement is via diagonally opposite points on the hub axle, the contact marks should mostly be close to the ends? I will have a look at HK’s Kinesis later but she has only noticed a couple of instances of axle rotation, so it may not be obvious.

AO, some through axles are Allen key bolts, some use a variation of an external cam quick release and some are basically wing bolts.
Wheel meet again, don't know where, don't know when...

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #20 on: July 01, 2020, 09:28:25 am »
Stuff it, why not now?

Wiped off the grease first. The front through axle is labelled Formula FQR-12F and anodised black. The visible anodising degradation is about 1mm wide, nearly halfway round the circumference and about 10mm from the outside of the fork on the brake side (at the end of the hub axle) and a couple of little patches within the adjacent fork blade. The main wear mark can be detected by feel alone, just.

Nothing much visible on the driveside end.

https://www.flickr.com/photos/46271676@N08/shares/Bn3xiq should show the marks.

The through axle is typically orientated so that the main wear mark would align with Brucey’s hypothesis, rather than my guess.

Speculation:
It looks like the main issue is on the brake side of the fork. A closer clearance between the through axle and hub axle diameters would be good, though more difficult to insert. Increased friction between hub axle and fork, particularly on the brake side, should help.
Wheel meet again, don't know where, don't know when...

Re: SON dynamo slipping
« Reply #21 on: July 01, 2020, 12:00:26 pm »
Very interesting!
I had a look at mine and it's exactly the same (bar the slightly lighter colored table top):
https://flic.kr/s/aHsmPaRx4h

LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #22 on: July 01, 2020, 12:06:17 pm »
My photos are taken on a floor, rather than a table.

Those marks inboard of your hub’s semi-circumferential wear mark (which is caused by the end of the hub axle) are interesting. They don’t seem to align with anything in particular and don’t look to be a result of precession. What are their textures/ characteristics? What do you think they are caused by?
Wheel meet again, don't know where, don't know when...

Re: SON dynamo slipping
« Reply #23 on: July 01, 2020, 01:18:04 pm »
I can't work out what those are from. It's done about 25 000km, gets used with different wheelsets (without dynamos) and has done a fair amount of "heavy" off road.

The fact that they are both on the brake side and close to the "wear groove" looks like they are related to the same issue.
Maybe it's time for a new one ...although that wear feels superficial at most.


LittleWheelsandBig

  • Whimsy Rider
Re: SON dynamo slipping
« Reply #24 on: July 01, 2020, 01:23:22 pm »
Seems to be corrosion-related, rather than abrasion. Do you grease the surface?
Wheel meet again, don't know where, don't know when...