Dynamo output figures

[ScotRoutes]

Has anyone recently published output figures from dynamo hubs?
I ask as I've recently built a Road Plus wheelset round a Shimano XT8000 dynamo hub and I've noticed that during routine bike maintenance (e.g. pad adjustment) a few spins of the wheel will light up my Exposure Revo. The Exposure (SP) hub on my other wheelset seems to need a lot more action to produce the same level of brightness.

[SteveM]

IIRC DrJon did a blog post about this some years back

[mat_swan]

Smaller wheels have to spin faster

[ScotRoutes]

Smaller wheels have to spin faster

Ha ha - yes, I was thinking that too, and then I realised.

[Alpinum]

Some info on here with helpful graphs

http://www.forumslader.de/automatiklader/

[Wilkyboy]

CTC/Cycling UK wrote an article on it a couple of years ago. It disappeared from their site a while back, but I seem to have retained a copy: http://www.16inchwheels.uk/content/imag ... ynamos.pdf

[jameso]

Not quite the same Q/topic but I thought this was interesting,

http://mccraw.co.uk/hub-dynamo-friction/
found by a chap in the office when we were discussing what 3W dyno drag was worth in real terms last week (scientifically sod all, imo/ime). 1.5% when running if you put out 150W consistently - I have no idea what my W output is, nor care, but 150W doesn't seem unrealistic for a fairly fit rider over the duration most of us would have a light running for. It makes the very good point that

So far you are perhaps thinking “OK, so 5-10 minutes over a ten hour event isn’t such a big deal, but it’s still something I’d rather avoid”. Fair enough.

This makes it a good point to contrast hub dynamo drag with some of the other things that can hold you back. For instance, it was recently reported that some types of chain lube can add as much as 10W over the factory lube [3] while, at only 12.5mph, the difference between a Continental GP3000 racing tyre and Vittoria Open Corsa racing tyre is already 20W [4].

I'd agree - good tyres or TLS make way more difference.

[sean_iow]

I've wondered for a while if I would notice the difference of having the lights on or off. Over the winter I've just left them on all the time even in the day for safety. A couple of weeks back I had my normal front wheel back on the bike, with the tyre swapped from the dyno wheel so only the hub was different. I was over a minute quicker on my commute and it was the first time I'd got under 30 mins for the ride on that bike. I did the same for the next 2 days as well. When I refitted the dyno wheel with the lights off I get the same saving.

Obviously saving just over a minute in 30 isn't a big deal on my commute, but for say an event in Scotland where I might have 80 to 100 hours of riding time, 2 mins an hour adds up to finishing up to over 3 hours quicker I had thought it might just be in my head and there was no noticeable difference but I think from now at events I'll leave the lights unplugged unless these is a benefit to having them on.

Back to the OP's question, my revo requires only a few good spins to light up when the bike is in the stand but I have to give the wheel a hard spin to get the required speed for it to come on.

[jameso]

Interesting Sean. If you're saving a minute in 30, call it 3%, and your dyno is pulling 3W since I assume you're pushing on for a PB time, that suggests that your power at that pace is ~100W - that can't be right. So another conclusion - you're not pushing your pace at max for that 30 mins so there's more variation in the wind and how you feel that day/week etc than the hub being on or off? (sorry - continuing OT I know)

[whitestone]

100W is very low - checked Strava for yesterday's commute along the canal tow path - the flat segments were around 200W, any climbs were in the 260W region. I was on my hardtail with 650b plus tyres so no idea how accurate those values are. I wasn't pushing for any PBs but equally I wasn't hanging around

[sean_iow]

I ride at the same effort most days, but it's a bit more difficult to analysis as I'm riding a ss mtb on the road. Geared at 34/17 it's a bit low for the flat bits and i'm limited by my max cadence. The gains would be on the up-hills and a faster coasting speed on the down-hills? I did wonder if I was just getting fitter? Or maybe just coincidence that the first time I went under 30 mins was the same day I didn't have the dyno wheel in? I've got a lower gearing on now so don't have any more data to add to check but post HT550 I'll be back on the 34/17 to compare times - assuming I still want to ride a bike after the HT

Of course it's only a small power loss but knowing how fastidious TT riders are about these things I guess the gains are real. I've not seen a TT rider with a dyno light on

[jameso]

Not sure Sean, just speculating : ) and not intending to pick any holes in your findings here, more of my take on the general strava science we see more of now.

I've not seen a TT rider with a dyno light on

No... 3W counts if you're prepared to wear a lid like they do in pursuit of an A-road PB and I read that some randonneur challenges have a different rank or cut-off for non-dynamo riders, pretty sure it was something on the Compass blog, over a shorter-distance I think.

Back to OP - this says there's more than 3W going in at the speed many of us would average on road, as the 3W rating is at 12(?) kph for German stds. My front light is still getting brighter up to ~25mph so perhaps 1 min in 30 is realistic.