CityCyclingEdinburgh was launched on the 27th of October 2009 as "an experiment".
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I did some serious graphing over lunch:
Horizontal bars represent cadence, green being "knees don't explode", two pairs of vertical lines denote common speed zones. Each line represents an alternative sprocket size.
As you can see, the real hub is not very good at getting a large proportion of your journey into a sensible cadence:
Now, imagine if the Automatix shift point was increased to 16mph:
What an improvement! (I should be a bike hub designer, honestly.)
But you see the problem?
You're spinning out at 23mph and you're getting continual up and down-shifts at normal speeds.
That is some serious graphing! Maybe email it to SRAM, might be a good sideline for you.
From reading your experiences I get the feeling the hub isn't really designed for "real" work, more the sort of thing you'd stick on a beach cruiser.
Aye, but it's a shame because the build quality of it (so far - touch wood!!) is far superior to either of the Sturmey hub gears I tried.
The shift is so solid and secure-feeling (considering it shifts under load because it does so automatically) that it would make a really nice "workhub" if it only had a user-serviceable shift point.
For different values of 'user servicable', I guess. I've read that some have taken to modifying the springs to effect a later shift, essentially making the springs stronger so that the weights engage later. It might be worth experimenting with adding a second, weaker, spring each side so that you don't have to modify the originals.
Nice graphs Dave. I see the issue. Unlikely to be able to obtain a 25T sprocket for that hun, perhaps. What I'm missing is the third variable, chainring size. More expense, I grant you, maybe something you don't want to look at. However it might help extend that green zone a little?
Ultimately though, the problem is you only have two gears! If only there was an Automatix 3 speed, eh? Ratios of say 0.71, 1.0, and 1.37 ?
Also, I must say that your "normal use" range of speeds from 15 to 25mph is rather on the sporty sprinter side of cycling. I thought this bike was a trailer hauling workhorse, not a racer?
Well, it's used for all cycling inside the bypass. It takes 20-25 minutes for 5 miles on the commute (including red lights) which is a true average of 12-15mph (but those red lights mean riding speeds are presumably a good bit higher).
Adding the trailer doesn't make it much slower unless I'm hauling a kitchen appliance! I suppose it takes a couple of mph off on the hills?
Won't be "maintenance free" but fairly 'stripped down' - and a bit cheap looking -
Google Bikes Arrive on Campus: a Fleet by Hugger Industries, on Flickr
I recently built a 700c wheel, using the freewheel version of the SRAM Automatix hub. I installed it on a drop bar chromoly bike that I use for commuting. Using a 700c wheel with 700x32 tires, my shifting is consistent at 11.5mph. I used a 38t chainring and a 22t rear cog (a Shimano Nexus cog), to get 46 gear inches in low gear and 65 gear inches in high. With this gear setup, I can sprint through intersections and it shifts at just the right time. If I pedal casually, I have to get my pedal cadence up pretty high for it to shift. I like it this way, as it allows me to pedal at a comfortable 75rpm in low when climbing hills. I spin out at a fairly low 20-23mph, but I'm a spinner and the hub allows me to spin comfortably at a reasonable cadence. The hub works splendidly with this setup, but I think the 700c wheels are part of the reason for this.
Apparently there are three shift points depending on which hub you get: 12, 14 or 18 km/h (7.5, 8.7 or 11.2 mph), yet mine shifts consistently at just over 10mph. Weird!
I am thinking of opening it up to do something to the spring / add another one. The operation is so sweet, but the shift point so inappropriate, that it seems worth the risk.
In my inspection of the hub, it appears that the spring could be replaced without too much difficulty. I don't know if the existing spring could be wound tighter. If you made a new spring, I think it would be a trial and error experiment.
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