Building a dynamo wheel

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Parts used, 32 hole Mavic CXP rim, Shimano DH3 N80 hub, Sapim double butted spokes

Being self sufficient for gadget power and lighting has always been high on my list of priorities so a dynamo hub was always on the cards. The Schmidt SON hub is reckoned to be the best one to get, lightest and lowest drag,  but they are expensive so I decided early on to use the cheaper Shimano option to get started and then upgrade to a SON at a later date, closer to the race. Both output a 6 volt 3 watt AC current but in tests the Shimano is reckoned to have slightly more drag, especially when under load. £150 difference in price is quite a difference though so it was either wait for some time for a SON or get dynamo power now.

I’d always fancied learning what I’d seen as the black art of wheel building so this seemed a perfect reason to have a go, if I made a complete mess of it I could always bail out and take the parts to a wheel builder. There is masses of information online so I was confident of a half decent outcome.

Without going crazy I’ve chosen the CXP Pro rim from Mavic, strong and light and not too expensive. It is an old design and maybe just a little too narrow for the wider tyres I want to run for TAW2018 but I can always rebuild the wheel closer to the race once I have some experience in building and once I figure out what type of rim I want. I’m curious about tubeless but I need to find out more.

The lacing of the wheel is actually very quick once you see the pattern of where the spokes have to go. A 32 hole rim with three crossings per spoke should give a strong front wheel, the Shimano hub has quite a large diameter flange so these should be pretty sturdy wheels. I’ve laced them up with Sapim spokes which are 2mm diameter either end , waisting down to 1.8mm in the middle. This makes them tension up nicely, again it should ensure a tough wheel

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Beginning the lacing

Once the first sets of spokes go in the wheel starts to take shape quite quickly. When all the spokes are in their correct position you wind the spoke nipples on until no thread is visible on the spoke. This turns the floppy, rattly and rather saggy rim and hub assembly into something much more wheel-like. Once all 32 spokes are in and roughly tensioned the wheel goes into a truing stand for the actual process of truing, both laterally and radially. It’s important also to ensure the rim stays round so a degree of evenness is essential in the tightening  process. I don’t yet have a truing stand so the front forks of my bike will suffice with a couple of cable ties on the fork legs to act as indicators for side run out

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The front wheel is much easier than a rear since you don’t have to worry about dishing the wheel to compensate for the thickness of the cassette.

Once the rim is laced and rough tensioned it is time to true it. This is most definitely a job that requires supreme patience and a degree of skill as well as peace and quiet.

A job for another day

Impatient to finish the wheel the another day was the very next day. In the absence of a wheel truing jig I fitted the rough tensioned wheel in the front forks of a bike up on the work stand and using two cable ties on the fork legs as pointers set about firstly making the wheel run true radially then getting it to run with no sideways runout. It isn’t a job you can rush but after a while with a little practice the roughly tensioned wheel starts to behave. Gradually after seeming to chase your tail a little you get to know how much to tweak each group of spokes to get it to move where you want. I took it out of the forks a few times and pushed the wheel rim down hard with both hands on opposite sides of the rim, with the hub resting on the ground. Pushing down like this and trying several different positions on both sides releases any built up stresses in the spokes, hubs and rims and should ensure that the wheel stays running true. The stress relieving will push the wheel a little out of true but after a couple of goes it will stay running true and then it is time for the acid test of putting a tube and tyre on and riding it.

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First few rotations it gives a few tinkling sounds but then it just rolls obediently with no drama and that is another very useful bike building skill learnt. I’ve now done nearly 1000 miles on the front wheel and it’s hit a number of potholes yet still runs true.

I’ve coupled my dynamo hub up to some B&M LED lights front and rear and to charge devices I have the Sinewave Revolution USB charge unit. With the lights running the charging times seem very slow but with lights off the phone and Wahoo Bolt Elemnt both charge up reasonably well on longer rides. Once I understood the limitations of 3 watts of charging power I’ve got on quite well with it. Below 5mph there isn’t sufficient power being generated to even run the lights properly. I’ll probably choose to carry and charge an auxiliary battery pack for any late night phone or Wahoo top ups. Luckily in Ireland there aren’t any really long slow climbs just lots of short vicious ones!

 

 

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