considering hill grades on first ebike build

Hi Folks,

I'm contemplating my first e-bike build and am wondering whether I should be factoring in a 17% grade hill that will be a part of nearly every ride as I choose motors and battery systems.

I'm electrifying an old Schwinn tandem, about 50 lbs, and I weigh in at 215lbs. The tandem may be carrying another rider around 140bls, or just my surfboard. A 25 mile range is more than adequate (5-10 ) much more common, and max speed at 20m full throttle is fine.

The hill in question has about a 1/2 mile run and 385 ft rise. It's easy to use Grin Tech's simulator to find the speed/range numbers, but I'm worried about buying parts that will burn out on that hill. Is this a valid concern, and how can I address it by making proper build choices?

Thanks for the help.

encinitas said:

I'm contemplating my first e-bike build and am wondering whether I should be factoring in a 17% grade hill that will be a part of nearly every ride as I choose motors and battery systems.

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Absolutely! Do you have a preference for geared or direct drive hub motor? Or are you thinking about a middrive solution?

https://www.electricbike.com/motor-tech-learn-the-terms-part-1/

Do more than just factor it in, because with the loads you are talking about the hill needs to be your single biggest consideration.

OK, I soft-pedalled my question--I am definitely factoring it in, except I don't know how to factor it in. That's the guidance I am seeking!

In principle, I like middrives, I like pedal assist and using the gears along with the motor. But I'm trying to minimize my costs on this first bike, so I was heading toward a rear hub motor.

Realistically, 90% of it's use will be to go over that hill to get to the beach, a 5 mile round trip, 5-7 times a week.

So, again, I'm looking for feedback in terms of size of motor, or whether a hub motor is even viable for this use, etc.

The rear Bafang BPM is pretty cheap, and it'll easily get you up that hill. If you can afford a 48v battery, 20A should be enough, otherwise a 36v one at 25A or more. I just built a bike with the 260 rpm (code 14) 48v BPM and the 23A S12S controller from BMSBattery. It maxes out at 24 mph, and got me (220lbs) up a half mile hill that averages 20% and peaks at 31%. The S12S sine-wave controller is IMHO one of the best when matched with their LCD3 display. It's specially good if you like to pedal. I just used the default settings, but there's loads of settings to play with if you get bored.

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d8veh, thanks for the recommendation. I will look at that set up, as I do like to pedal. What is the spec on the lcd3 display?

Sorry, I don’t have an answer but I do have a question. I only have experience with 1 motor which is a slow wind DD hub.

Are geared hubbies THAT much better at climbing hills that they can run up a hill at 30%+?





And I thought my mile long 8%-10% hill was tough. At my max (~1,500W) it doesn't generate much heat but I assume it would be struggling at 17%-20%. Maybe with more power.

How much it would struggle would depend on wheel size and gross bike weight. 1500W in a 26" wheel with a 1000W motor will take a smaller person up a 20% grade pretty good. Same motor running 100V and 40A controller allows me to accelerate up 20% grades with ease and I'm 270 lbs.

We just heard it will be a lot of weight. I think you need to look at larger motors, or two motors, or a very low geared mid drive.

Lots of appeal to me, for the idea of a direct drive rear motor, in about 1000w size. Then add a front gear motor, like a Mac or whatever to the front. Then you'd have up to 2000w to get up the hill.

A simple switch can kill the power to the front motor controller when running on just the rear one. One throttle can control both, but just have a switch you can flip to start the signal going to the front motor.

On the other hand, one large motor works great too. I run one large motor on my heavy cargo bike, and fear no hill. I run about 2000w to a vintage crystalyte 5000 series motor.

The downside, either way you slice it, you will need a nice strong battery to dish out 40 amps of 48v. So your battery cost will likely be pretty high.

The bafang motor does sound like a possible solution. At the risk of setting off somebody, worth noting that that particular one is a slower wind motor. But it will be running at a very inefficient rpm going up that hill. 220 sure, but the second person will make the load bigger, while adding only about 100w of pedal power. The surfboard will weigh less, but it won't pedal.

Nevertheless, the hill is only a half mile. Pedal hard enough, and it might not overheat the motor. But the rest of the ride after that hill better not be too much hill, because that half mile will really heat it up.

Modifying the bike to have a 20" wheel, but still have the same height, is a really good option to look at. in 20" wheel, you have a real, lower gear.

This video shows what a 500w geared hub-motor can do. He's got 29" wheels on that bike. With 26" wheels it could manage steeper.

Obviously with a 5:1 reduction ratio, you get a lot more torque than the same size DD motor, or the otherway round, you can get the same torque with a much smaller lighter motor.

[youtube]RIHJPOL3_HQ[/youtube]

Yep, but he doesn't weigh what is being discussed.

I did a maximum weight test to failure, and fried the hell out of a 1000w gear motor easily. 6% grade for several miles. Weight, rider, cargo, bike, battery, was 450 pounds.

The weight matters almost as much as the grade, if power is limited to 1000w or so. So does hill length, you can make all kinds of heat and get away with it if the ride is not miles long. Fortunately, I think this guy is talking about only one really steep, but half mile long hill. It just might fly fine with a Mac 10t or similar motor. For it to work best though, he'd need himself, bike, motor and battery, and board to weigh less than 300 pounds. He'll weigh a lot more with the second rider, but a rider can pedal hard. He'll be very close to the max a single 1000w motor can do with him and the board. But again, the short hill will not fry a motor right away. But if it's still uphill after that steep part, then he could be in trouble with a smaller motor.

Two motors, 2000w, works oh so much better. Or one large motor that easily handles 2000w.

385ft rise is 117m height gain.

If the bike weighs 450lb (204kg), the energy (E) required to gain that height = 204 x 9.81 x 117 = 234,000 Joules

If the hill is 0.5 miles long and you want to climb it at 20mph, time taken is 0.5/20 = 0.025 hours, or 1.5 mins, or 90 seconds

work rate is 234,000/90 = 2600 Watts

If the motor efficiency is 70% at this speed and load then required motor input power is 2600/0.7 = 3714W

Worse still, we've ignored air resistance and rolling resistance, which would total about 400-500W on level ground. So you could be looking at over 4000W.

Obviously, if you half the speed then the power required will also half (approximately due to air resistance being non-linear), but you can see that 1000W doesn't get a heavy bike up a hill very fast at all. From a practical standpoint you may also struggle to find a motor that is correctly geared to allow heavy loading at low speed (say 5mph).

The code 14 BPM is a nice compromise between torque and speed. It's quite happy going down to about 7 mph as long as you don't do it for too long. If that hill is fundamental to the kit choice, you could always go down to the 201 rpm version, which I think is code 16, and it does about 15mph top speed, which I think most people would find too slow.

d8veh said:

The code 14 BPM is a nice compromise between torque and speed. It's quite happy going down to about 7 mph as long as you don't do it for too long. If that hill is fundamental to the kit choice, you could always go down to the 201 rpm version, which I think is code 16, and it does about 15mph top speed, which I think most people would find too slow.

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Be careful that you aren't dipping into the land of mythical high torque windings. If the planetary gearing is the same gear reduction, and only the windings of the stators are different for the different codes of BPM, then they can all make only the same torque and all have only the same hill climbing capability. What something can do for a one block climb only demonstrates the torque capability without considering heat dissipation. At a half mile long the grade could be even less than the 30% in that video, and the MAC would certainly be pushed to failure.

I plugged your 220lbs plus I assumed a pretty light ebike at 60lbs into the Kreuzotter calculator, and to climb a 20% grade (you said 20-30% for half a mile) and at just 12mph (half of your top speed) 1272W is required at the wheel, but you aren't even inputting that much power. Now maybe you backed off the throttle a bit to lower the power required, and pedaled your ass off for that few minutes to significantly assist the motor, but something isn't right.

With hauling a friend and surfboards on the table, then high speed is out. With parking at the beach and sand blowing into everything, ventilated cooling is out. With his loads I would question running any economical hubbie even in a 20" wheel, though it may be a great application for using an oil bath to keep the stator cool due to the limited length of the hill. It's a tandem, so the one thing he's got plenty of is space, which due to budget concerns tells me the best option is a cheap direct drive hubbie made for 20mph on 36V mounted in the rear area of the frame running at 74V-81V nominal and geared down to 20mph. The result is a stress free system with great efficiency for the other 24.5miles and will maintain 12-15mph on the hill without stress. Use just straight chain on the left side with fixed sprockets mounted to disk brake mounts and an idler to maintain chain tension for quiet running. The reason I say a DD hubbie is to have regen braking for going back down the hill and virtually eliminate brake maintenance, as well as add a some of that energy back to the battery.

The other options would be 2wd or find an electric scooter to cannibalize for it's motor and maybe controller, since they're designed for that kind of load, and come up with some kind of weird but cool beach hauler. That opens up higher speed though, which means more batteries, and I don't know about hauling surfboards around at speed.

A few thoughts about your situation.

Since you want to make round trips, going down that hill will also be required. A DD hubmotor will allow regen to save on brakes, a geared hubmotor will not. Replacing brakes frequently is expensive and not terribly fun, regen will prolong the life of your brakes almost indefinitely.

There are some new 2 speed geared hubmotors that have two different gear ratios built into them. They are particularly good at climbing steep hills and still making good speed. They may not be big enough for your needs however.

Two motors, front and rear, is an interesting possibility but the complexity and amount of equipment on the bike with dual controllers and extra wiring is probably not a good thing for a first ebike.

On a grade you MUST keep your speed up, or the motor will quickly overheat and burn. If you have enough torque it will not be a problem. If you don't, pedaling will help but heat will build up quickly if you don't keep moving.

The simulator at ebikes.ca makes an estimate of time before the motor melts. It may not be accurate but will allow comparison when changing things.

Cromotor?

John has it exactly right. A lower rpm motor will not make 1000w be enough power. Not for that weight.

A true lower gear, like a mid drive might make 1000w enough. But I can't say it will from experience. I cant wait to try a brushless motor mid drive like the bafang, but have not the money to try it now.

Nobody mentioned a stokemonkey yet. You can buy them at Grin cyclery again now. For sure one of those will do er.

I think 1500w through a typical cheap 1000w motor has a chance in 20" wheel, but it has no chance in 26".

Definitely look at big powerful motors. Cromotor, vintage 5304 crystalyte, etc. Something that can run at 2000w with no sweat.

Or two motors, 1000w each.

encinitas said:

...a 17% grade hill that will be a part of nearly every ride...

I'm electrifying an old Schwinn tandem, about 50 lbs, and I weigh in at 215lbs. The tandem may be carrying another rider around 140bls, or just my surfboard. A 25 mile range is more than adequate (5-10 ) much more common, and max speed at 20m full throttle is fine.

The hill in question has about a 1/2 mile run and 385 ft rise. It's easy to use Grin Tech's simulator to find the speed/range numbers, but I'm worried about buying parts that will burn out on that hill. Is this a valid concern, and how can I address it by making proper build choices?

Click to expand...

encinitas said:

OK, I soft-pedalled my question--I am definitely factoring it in, except I don't know how to factor it in. That's the guidance I am seeking!
...
But I'm trying to minimize my costs on this first bike, so I was heading toward a rear hub motor.
Realistically, 90% of it's use will be to go over that hill to get to the beach, a 5 mile round trip, 5-7 times a week.
So, again, I'm looking for feedback in terms of size of motor, or whether a hub motor is even viable for this use, etc.

Click to expand...

Lots of opinions and options here - let's flog the simulator a bit and see what it has to say and what rings true...

Your projected load in 215+140+50 lbs plus the weight of the drive system - let's say another 40lbs plus 5lbs of locks and chains (so your bike won't get stolen at the beach) so a minimum of 450lbs.

To get some notion of your power requirements - before looking at specific motors and batteries, the simulator Load Line shows the score:
(I have used a Mtn bike profile, although the upright posture of a tandem should have a higher CdA - but at these low speeds, we take the simple preset instead of fussing too much)

View attachment 4
This is the power required to propel the bike at various speeds. Divide these figures by motor efficiency to get the battery power you need. So - we see that you would need 2450W of motor power to climb the hill at 15mph but only 400W to cruise at 20mph. This portion of the simulator relies on basics physics - not motor models - and is very accurate. Obviously the hill is the entire problem by a huge margin.

Before looking at specific motors, we can see that even at 10mph, we are looking at serious power - and small DD or even large gear motors are going to be in trouble because of the low loaded speed which will push them into very low efficiencies. Mid drives that yield only sub-10mph speeds (far too slow for hub motors to be efficient) look like a real possibility with only modest power requirements.

But - we want to see the true picture - so here's a run with a MAC 12T (BMC V2 Trq is equivalent) and a common 9C2807 as found in 1000W DD kits. The battery and controller for the 9C have been jacked way up to get the same rough performance as the MAC. This was done to simplify the plot - also the efficiency curves were omitted for clarity but you can see things look bleak even without the complete plot.


Here we see neither one has a chance - drawing about 2kW and 4kW battery power respectively and loosing the majority of that to heat at 33% and 16% efficiencies respectively. The Simulator 'Overheats In' estimate is conservative and assumes still air - but at 4mph it's a good indication that these will be smoked long before the hilltop at this low speed.

Bad choices.

This really should be sending you to the previous suggestions - really big DD, 2WD, or mid-drives.

Unfortunately the Cromotor was recently removed from the simulator since a sample motor was never submitted to verify the estimated simulation parameters - so that one is off the table for simulation. You can fiddle about with other big motors - but they will require fairly massive amps to be able to do the job. This is fine if you are going to jump into LiPo on your first bike - but if you are trying to be more conservative with an NCA battery from EM3EV or similar, then these brute force motors are not going to work out too well - you can't get the amps out of the battery. Here's a sample run using a big 5305. You can select different voltages and controller currents, but it's clear that this motor wants to go fast and is eating a fair bit of power. Since we are really just trying to get up this one hill, it would not be my choice - particularly because of the battery chemistry - others may feel differently.

View attachment 2
2WD gearmotors are a possibility. Here is a simulation with two MAC 12s:


To do 2WD analysis with the same motor and wheels, we just scrub the speed cursor to the right until the Load is twice the Motor Power.

This solution uses off-the-shelf stuff from EM3EV. Nothing Custom. I have reduced the controller current to only 35A (although 12FET controllers can deliver a great deal more with resulting higher speed) just to get the battery current draw down to 70A. Assuming we are using a 17.5Ah 14s9p triangle pack with INR18650-20R cells (10C) we might pull 10 x 17.5A = 175A derated by 50% or about 75-80A safely. Since speed is to the left of the discontinuous peak in the red Motor Power curve, we know that the motor power is controller-limited and that is the gating factor. The battery is a little saggy, so we might not get the whole 14.5mph, but it will definitely get up the hill. The added complexity involves bolting in a prelaced front wheel, bolting up a second controller, and running some wires. More stuff for sure, but...

Looking at 'Overheat In' we see that 6.4min / (60min/hr) x 14mph = 1.5mile range before the motors begin to overheat. This is a nice workable margin for a 1/2 mile hill.

This looks fairly attractive and delivers about 180lbs of thrust off the line which makes for a perky getaway.

The last option is a mid drive. An option is the the StokeMonkey since you have the room in the tandem. Here the simulator gets a little more interesting because there is gearing involved. The SM has a 16 tooth gear. The biggest granny gear on a stock DNP freewheel is going to be 32 tooth, but obviously there are other options there since a regular cassette can be used. This leaves the Left/Right BB gears as the last ratio. If you use a 50T on the SM and a 22T chainring on the right, we have an overall ratio of 16/50 x 22/32 (or 16/32 x 22/50 if you look at it another way) = 0.22. Applying this to our 26" wheel we get an effective wheel diameter of 26 x 0.44 = 5.7 inches which we plug into the simulator.


This looks pretty darn good. It uses little power exactly as predicted in our initial LoadLine analysis and will climb forever without overheating. This means a much thriftier battery. You might play with the gearing to further improve the situation, but even this simple setup works well. I used a 54V EM3EV triangle pack for this run, looking to an easy mounting situation, but with the low currents, it could be a cheaper version with more capacity and a lower C rating for more range than the one used in the 2WD simulation. As with the 2WD approach, this approach has massive torque off the line. Temperature monitoring is not required and you could use a simple CA V2 or even cheaper Wattmeter to track your battery - more savings.

There are other mid drive options, but this shows at least one very robust off-the-shelf solution. From here you should be able to go back to the simulator and work out battery requirements and available max speeds for your choice.

I hope that I haven't fat-fingered a value in the simulator, but you get the drift of the procedures and can verify the stuff yourself.
Hope this helps...

It gets to the point of a good battery now.

Thanks to everyone who has put time and thought into this problem. The discussion has helped me, but also challenged my memory of high school physics, and I have some questions.

To sum up what I've read so far, with my load of 450lbs, I need 2400W to get up that 17% hill at 20mph. Now, I'm fine with taking that hill at 7-10mph and doing so with light pedaling effort (i.e., the motor is doing most of the work). At that speed, 1300-1500W was suggested as about what was needed.

To discard some other variables: I am not entertaining putting on two hub motors; I'm fine with reduced speed up the hill and 20mph on the flats; a stokemonkey kit at $800 with no batteries seems expensive; .

The issues I'm having problems with is motor efficiency, heat generation and rpm. Am I correct in understanding that an underpowered motor would struggle with the load on the hill, turn at lower RPMs and generate a lot of heat, potentially burning out?

And that some of the solutions were 1) a geared motor like the Bafang BPM, 2) hacking a hubdrive into a homemade stoke monkey, 3) going with a huge DD motor.

I saw a bit of mention of windings and rpms, and saw a lot of back and forth on other forums. It's unclear to me whether a smaller geared motor is better than a big DD for this hill.

I would much appreciate enlightenment on what causes efficiency to drop and cause overheating and if there is a setup that minimizes cost, allows for steady, say 10mph hill climb and 20mph flat running that does not overly stress the motor. I like the stoke monkey concept, but obviously, a simple hub is the easier set up.

Alan B said:

A DD hubmotor will allow regen to save on brakes, a geared hubmotor will not. Replacing brakes frequently is expensive and not terribly fun, regen will prolong the life of your brakes almost indefinitely.

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Replacing brake pads is very easy and cheap compared to replacing the bike frame, hub motor axle, or even just the phase wire leads. Reversing torque (from regenerative braking) is many times more problematic and destructive than torque in only one direction. Until hub motors come with better torque anchoring, this is a major practical consideration at the power and torque levels being discussed.

It is true that proper torque arms are important for all setups, but it is a one time investment that costs much less than replacing brake pads every few months over the life of the bike for a serious commuter in hilly situations.

Regen may hasten a failure with inadequate torque arm and lock washer setups, but failure without regen would likely occur at some point anyway.

I've never used torque arms or plates and I do use regen braking. I don't have to use the actual brakes until I get to under 10mph for a normal stop. I'm still using the original brake pads after 10K+ miles. The wheel is solidly mounted in standard steel dropouts with a jam nut on the inside on one side. It has never come loose on any bike I've built up. That's 4 so far with a 1000W motor and 18-24s lipo. Currently building another with a 3000W motor and I won't be using torque arms on it either.

If the dropouts are steel, and they have enough material and are wide enough they may be able to absorb the torque (but insure that the axle is fully to the bottom of the slot). If the dropouts are not sufficiently thick, the steel of the axle or of the frame will deform and the axle will rotate. One example of this is on my Greyborg the Chrome-moly steel dropouts are quite thick and they absorb the torque of the Cromotor and the regen.

If the dropouts are aluminum then steel torque arms are recommended. Aluminum dropouts can work if they are very very thick of a hard alloy, but the usual bicycle aluminum dropouts will fail and the axle will cut right through them.

Using high performance lockwashers on the axle nuts also helps, NordLock is one example. They must be used properly but when they are the nuts won't work loose.