Author Topic: Dynamo / wheel menace issue  (Read 883 times)

Dynamo / wheel menace issue
« on: 30 April, 2021, 09:40:27 am »
Hi Folks,

I have a SP v8 dynamo and it's been working fine until now. Suddenly the wheel feels like the front brake is on (it's not)? Any ideas? What to do?

Cheers, Doo ???
I dunno why anybody's doing this!

Re: Dynamo / wheel menace issue
« Reply #1 on: 30 April, 2021, 10:18:51 am »
Doo

Years ago, on one of Blacksheeps rides out of Chepstow, I found myself having to cycle down the first descent, and then grounding to a total stop on the subsequent climb.

Like you it appeared that my brake was on, this was not the case, and found that I was unable to spin the front wheel.

I had a SON dynamo and its transpired, after being sent to be serviced, that one of the magnets had moved causing excess resistance. Maybe the hitting of a pothole badly the week before was the cause.

I got the SON and wheel via St Johns Cycles who sent it to Germany for a service and it gave me no problems once back.

Anton 

Re: Dynamo / wheel menace issue
« Reply #2 on: 30 April, 2021, 05:23:46 pm »
Short circuit, usually.
And Darkness and Decay and the Coronavirus held illimitable dominion over all.

Re: Dynamo / wheel menace issue
« Reply #3 on: 30 April, 2021, 06:21:32 pm »
Email the distributor (Ison) and they'll rebuild a new one into your wheel for £45 or thereabouts.

Re: Dynamo / wheel menace issue
« Reply #4 on: 30 April, 2021, 06:33:55 pm »
Email the distributor (Ison) and they'll rebuild a new one into your wheel for £45 or thereabouts.
Or, if you can do the wheel building yourself, they'll do you the hub on it's own for £25. In both cases they'll be sold as reconditioned, though people do sometimes get a new one, I'm assuming because no recons were available.
Of course if it's under warranty, 2 years for SP, the retailer you bought from ought to sort it out for you. 

Re: Dynamo / wheel menace issue
« Reply #5 on: 30 April, 2021, 09:38:12 pm »
Hi Doo,

Experienced similar symptoms. Sent my SP dynohub to Taiwan in December for replacement or service. About 3 weeks turnaround cost about £40 inc postage. Dealt with eliza@sp-dynamo.com...excellent communication via email

Had to unbuild and re-lace wheel which will cost you extra.

All good since
#makewattsnotwar

Kim

  • Timelord
Re: Dynamo / wheel menace issue
« Reply #6 on: 30 April, 2021, 11:06:37 pm »
Short circuit, usually.

Not on its own.  Short-circuit drag isn't far off open-circuit drag.  AIUI on some hubs it's actually lower, because *handwaves* eddy currents.

(Hence the cunning strategy of limiting voltage by crowbarring the hub with a thyristor, rather than attempting to shunt the power in a zener diode.)

A sufficiently manky short might happen to hit the maximum power point for some speed, I suppose.
Careful, Kim. Your sarcasm's showing...

BFC

Re: Dynamo / wheel menace issue
« Reply #7 on: 06 May, 2021, 11:07:02 am »
Short circuit, usually.

Not on its own.  Short-circuit drag isn't far off open-circuit drag.  AIUI on some hubs it's actually lower, because *handwaves* eddy currents.

(Hence the cunning strategy of limiting voltage by crowbarring the hub with a thyristor, rather than attempting to shunt the power in a zener diode.)

A sufficiently manky short might happen to hit the maximum power point for some speed, I suppose.
Try riding with the hub wiring disconnected, this will indicate if its an intrinsic problem within the hub or a system issue.
The torque loading of a shorted out dynamo hub is massive - they normally turn the axle in the frame. If ridden with a very low resistance short the windings can overheat and kill the insulation (hub will be scrap and the rider will also be knackered). The turning of an axle in the frame can also cause the short due to wiring damage (chicken and egg debate).
The windings of SP PV8 hubs measure at 2.5ohms - you need a decent multimeter and leads to measure low resistances. The test load used for standard drag plots on the manufacturers sites is 12ohms (giving 3W load at 6V).
The other common cause of high drag on these hubs is bearing failure, there is usually a visible sign like brown water or grease around the end caps. I have accessed and rescued the bearings on PV8 hubs by timely flushing and re greasing, its a real pain to do. All of my bearing issues have been traceable back to riding through deep flood water, the rapid cooling of the air in the hub causes the unit to draw water in past the seals. I expect using a jet wash would not do these bearings (or others on bikes) any favours.
A usefull measure of drag is a run down test method - spin the wheel by hand and see how long it takes to stop. New PV8 builds (700c with tyre and tube) take about 12 seconds (wiring disconnected).