A spec refresher:
naitsirk said:
...I'd like to bring my 5yo daughter to school on my my bike. Her school is up one of the steepest streets in the western world
(31% ). I can get there by going up a series of 15-19% hills instead.
... We are 300 lbs together with the bike.
...I can get up there, but it is a sweat at about 3mph, and its no fun. I'd like to be able to wear work clothes and assist but not, ideally, break a sweat.
naitsirk said:
I neglected to say, this whole trip is 1.5 miles up hill, with a few 17% hills that are a block or so. Then it is 1.5 miles downhill to work and 1.5 miles mild uphill (5-7%) back home. So a mid-drive might allow me to have a smaller battery for the whole trip because of efficiency.
The business of possibly buying a new bike is sort of a major change - as are the short distances....
Since you are talking about toting a kid around, I'm wondering about what you plans will be in a year or two... It frankly looks like a cargo bike or mid-tail is you best bet if you are going to be trucking around a kid-sized passenger - which could be an important factor up front.
This plot may help you evaluate the suggested solutions - it shows the power needed to propel the bike up differing grades. You claim a present bike weight of 300lbs so I jacked it up to 330 to account for the electrification - this is probably a little shy but it's in the neighborhood.
It's important to realize that this is the power needed to move the bike - not the actual battery power you much expend to do so - which relies largely on motor efficiency. It has nothing to do specifically with electric power. For instance, you claim to be able to climb the 30% hill at ~3mph - the red plot shows you were putting out a bit less than 600W (300 vs 330lb vehicle) - which would definitely leave you dripping wet.
You mention a lower acceptable limit of 5mph and not working up a sweat as the target. From the plot we see that needs about 1000W - we might estimate you are only contributing 100-150W to stay dry, so we have a power shortfall of about 900W. This then is the magic number, so the Big Question becomes:
- "How to best get 900W-1000W to the road (after we throw away power on motor efficiency)?".
This is where the high load and resulting low speed comes in...
- At these speeds a DD hub motor will have efficiencies in the 25% range. Suggestions to use a big DD that is basically built to do 40mph and throw away 2700W for the primary Use Case doesn't sound very good - that's a heavy motor and high Amps, highish voltage battery that is essentially being wasted - the available high speed is actually unwanted. (If it were the only solution, it might be still be attractive, but...)
- A gear motor runs at 5 times the rpm which in part accounts for the improved efficiency at low rpm, but the gearing is fixed and there are no hulking big gear motors to do that job - a single MAC12T can't muster the power to carry the load unassisted at that speed and the resulting even lower speed will send the efficiency to very low levels. Estimates using the ebikes.ca simulator indicate 6% efficiency and overheating in 48 seconds - achieving less than 1mph at 50A.
- The dual MAC 12T approach comes in at about 80% efficiency but the fixed gearing and oversized motors (really designed to be going much much faster than required here) still leave it drawing heavy amps (~55A)although the voltage will be low (i.e. it's 80% efficient but the winding and gearing make for a natively low voltage/high amp solution).
- the StokeMonkey is going to be about 80% efficent as well but the gearing and winding make it a lower current solution. Unlike the MAC with fixed gearing, the StokeMonkey will always get you up the hill - just more slowly - and the motor is sized more appropriately for your desired speed on the flat. Other middrive solutions carry the same characteristics.
It should be clear that any suggestion that a 250W motor or 'any hubmotor kit will help' is nonsense - the requirement that you do not make a material contribution means that the motor must do the bulk of the work. The motor output power requirement of ~1000W is such that low powered kits will melt - the speed is so slow and the efficiency so low that virtually all the motor power will go to heat. Similarly, a single 12T also appears to be inadequate (although it seems workable for 20% grades).
The dual 12T will do the job and is a relatively easy build for any frame. It does have a top speed you don't need and is still using a lot of amps at your target speed (<60A). You could remedy this with an EM3EV INR18650-20R battery which Paul rates at 5C (de-rated from cell rating of 10C). The 17.5Ah triangle battery would give you 80A+ but a slightly smaller custom built battery might be a better fit since you don't really need the range - just short-term high C-rate. A Cycle Anaylst V3 would be a good addition to limit current and monitor temperature.
The StokeMonkey (or maybe other middrive) is probably going to get you what you want with a motor sized for your needs in all cases - good hill climbing (assuming appropriate gearing), limited top speed, and good enough efficiency while climbing that you can use a small battery solution.
The StokeMonkey was designed specifically for SF hills and is a proven solution. Frame design may be a concern. If you want to go with a new bigger frame, a Mundo or Edgerunner would give you a nice 'station wagon' vehicle that is durable and should haul kids for many years to come.
This 31% grade is the killer - go the long 20%-grade route and a single 12T should work as well.
This is basically a rehash of what dogman and others have already said, but with a couple of numbers thrown in to show 'why'...
