Sturmey Archer X-RF8 with 250W Motor
I have an Electra Amsterdam Dutch-style city bike (700x38c wheels) with a Nexus Inter-3 hub on the rear and a 250W motor on the front wheel. The 250W motor is able to get my bike up to about 15 or 16mph on its own. To ease the load, I pedal along with it, but the highest gear on the Nexus 3 feels too low. The bicycle accelerates quickly but the top speed feels limited.
I've been thinking about getting the Sturmey Archer X-RF8 hub (talked about here http://www.bikeforums.net/bicycle-me...r-8-speed.html) but I'm wondering if the direct drive gear being the lowest one would be too high, even with the motor. What's the smallest sprocket/largest chainring combo available for this hub? I don't mind compromising a bit of acceleration as long as I can get a higher top speed (since the 250W motor cruises at its highest speed) but I don't want the bicycle to feel like an immobile tank either.
8th is 3.25x 1st, so make the chainring smaller , the hub is a 20,23 or 25t.. 23 and 25 t are in 3 or 4 32nd thickness .
so potentially a double Crank/chainset is possible as 8 speed chains will work on a 3/32 cog.
The biggest issue with IGHs is the low end input torque. A smaller chainring running to a 25t rear cog can exceed the input toque for the hub and tear loose some teeth or pawls.
IDK about S/A hubs, but for example, the Rohloff and Alfine recommend keeping input torque ratio Above 2:1… that is, for a 25t rear cog, a chainring no smaller than 50t. and that is their recommendation for just RIDER… not factoring in the additional torque of rider plus motor.
The nice thing is that you can generally go as small as you like on the rear cog, which is where you will get your high gear extended, because that actually lowers input torque.
The difference between the low gear with a 20 versus 25 will be minor in terms of feel…( especially considering that its in the low gears where the motor is the most help ) but it becomes very noticeable at the higher gear ratios.
The S/A 8 has a 300% Plus gear spread…. so I think you will be much happier than with a 3 speed.
I swapped out a 23t cog on my Alfine for an 18t… the bike still climbs a 20% grade just as well…. but it added nearly 10 mph to the top speed in highest gear.
OK, it's good to get an idea from someone else with a similar experience. I've been riding around using only gears 2 (direct drive) and 3 (1.33 ratio) on my nexus to try to get an idea of what starting in a 1:1 gear is. Definitely more sluggish than starting with 0.67 on 1st gear, but negligible as soon as the motor kicks in. What's the highest speed you've been able to sustain (I.e. not pedaling at 120 RPM) on your 18t Alfine?
Here's the thing... The gear ratios listed for any IGH are based upon the input... That is...a 1:1 first is one to one based upon the sprocket turning... Not your crank.
ergo... You change the cog or chainring, you change the real level of that first gear... But not by much, because 1:1 doesn't multiply the effect of a lower ratio.
however... If you have an 18t cog, and a 48t chainring, that's a 2.6:1 ratio at the rear cog. If the IGH has a high gear of 3:1, you are multiplying 2.6 times 3.... So the effect of changing the rear cog is far more dramatic the higher the chainring/cog ratio is to begin with.
on my bike I installed the Bafang 750 watt Bottom Bracket drive. I tried it with a 23t cog and 49t chainring... And got a very low gear... But in 8th I couldn't spin my legs fast enough to get any more speed.
i swapped out the 23 for an 18, and I ended up with a very slightly harder 1st... But a much faster 8th.
The motor, however, makes the low gear feel just as low in terms of rider Input.
as to top speed, the Bafang can be user programmed to a 50kph limit... About 31 mph. But I find that the throttle control does NOT really give you full motor power. Throttle seems like it offers up about 75% of full power. It may be that the controller doesn,t allow the throttle to make full use of the 48v my battery can output.
I can cruise without pedaling on level ground at around 28 mph, throttle only... But in 8th gear, with just very easy pedal input, I can get the thing up to 35, no sweat.
however... Its a city bike geometry. Carbon fork, aluminum frame. I,ve larded it up with a heavy kickstand, racks, and a brooks flyer ( I could easily strip 5 lbs from this machine with lighter components)
i have had to up to 40 plus on downhills and that's just plain scary.
my experience is that bikes, inherently, are unstable at speeds over 30-35. You need something a lot heavier, and well sprung to go faster with any sense of security... That's more like motorcycle territory.
i wanted a machine I could easily carry upstairs... And, of the power ran out, still muscle uphill back home.
i find the Bafang 750 to provide the perfect balance of power and speed for anyone who lives in hilly terrain.
being able to easily pedal at 25-28 mph is perfect.
The 8fun/Bafang motor I have on the front wheel does feel gutless at times. If I try to roll off without pedalling, I can hear it straining because it makes a noticeable "clunk!" Of course, I avoid this like the plague and always help it along by a pedal start.
Will post back a review of my experience as soon as the hub comes in. Thanks!
I am unfamiliar with any Bafang front wheel hub motor... But the front wheel is basically the worst place to power a bike. You need the motor most on uphill stretches, when the C of G is thrown way back... And the front wheel has the least traction, and the least weight. Think about how easy it is to get a front wheel drive car to spin its front wheels...
mid motor places the motor weight lower... And well centered in the frame... Especially with a down tube mounted battery... But beyond that...it puts the power into the rear wheel thru whatever gearing system you put on the rear wheel. Keep in mind that bicycle frames are designed from the start around the idea of power originating at the Bottom Bracket and being transferred into the rear wheel... So every bike is already optimized for torque forces between BB and rear wheel.
a rear hub motor is a second best... It places weight higher... But not by much...and at least puts power where you need it. Although it may tend to shift too much weight too far back...making wheelies on an uphill more likely.
but again.... A 750 watt motor essentially DOUBLES the torque of a well conditioned rider. That's a lot of strain on internally geared hubs designed around 1man-power loads.
i advise you look up the minimum input ratio for the hub you plan to use and choose a chainring and cog combo that is somewhat higher. ( e.g. If they say don't go lower than 2.12:1... Maybe go with 2.3... Just to allow for the maximum torque you and the motor can put out )
but of all the ebikes I have test ridden... The Bafang BB conversion I made has the best balance, fore and aft, and the most bike-like handling.
Okay... I just looked up the speed numbers with the X-RK8... And according to Sheldon Brown...the top gear on this hub would put out 50 mph... On a 700c wheel at 80 rpm.
now I want to get me one!
his calculator was accurate for the performance of the Alfine... Would be interesting to see how close the X-RK8 numbers come out.
I agree that mid-drive is probably better ... but I can't really modify my Amsterdam to fit that. That'll go for the next build. I plan to add a cargo rack to the front to attempt to stabilize the weight by countering the rack/Wald baskets on the back.
Hopefully the bike will hold! :D
Unfortunately, I also can find not one good review of the Sturmey Archer 8s.
nobody who has them seems to be happy with them... Most reporting problems with them drifting out of gear, and consequently tearing up internally.
i am finding reports of them breaking after 120 miles and less, and these are all reports of folks just pedaling... No electric motor users.
if you get the Sturmey, please be sure to post your impressions of it.
another nice thing about a BB motor, is you can always just put on a different rear wheel...
I plan to lace up a couple of others to see which IGH gets me the best performance and durability.
perhaps a NuVinci, and almost certainly an Alfine 11... Which has two gears higher than their 8.
I was thinking the SRAM 9... But it, too, has almost universally bad reviews for durability...
What is it about the Amsterdam that won't allow a BB motor? Is it an oversize or eccentric BB?
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