Inexpensive Rigid Geared MTB?

On Apr 2, 4:27*pm, DanKMTB wrote:
On Apr 2, 3:29*pm, Chalo wrote:





DanKMTB wrote:

Assuming the same motor, how much of a difference will 700C vs. 26"
wheel make? *

It depends on how close your system is running to its limitations. *To
keep an electric motor cruising in its efficient speed range, it needs
to be more powerful than absolutely necessary to maintain your
cruising speed. *That way, it can quickly get up on top of its speed
range where power tapers off and efficiency reaches towards a
maximum. *If your motor is struggling along in its high-power, low-
efficiency range with the smaller wheel, then it can become much worse
when the wheel diameter is enlarged.

It's a good idea to establish how fast you want to go, then use a
power and speed calculator to figure out how much power that will
require. *Then try to find a motor that has a peak of about twice that
much power, but matches your cruising power requirement with the right
wheel RPM. *The ebikes.ca simulator is great for this latter task.

http://ebikes.ca/simulator/

Say for instance you use the calculator at Analytic Cycling to figure
that you can go 23-24mph with 400W at the wheel. *The ebikes.ca
calculator shows that the Nine Continent 2807 motor in a 700c wheel
with a 36V battery and 35A controller matches that speed and load
together perfectly, and it does so at close to its efficiency peak.
You'd be able to cruise at about 84% efficiency, with strong
acceleration up to that point.

The Crystalyte 404 in a 700c wheel with a 36V battery and 20A
controller also has a fairly good match of power and rpm for the
example application, but it meets that power and speed a much lower
efficiency, and will spend more time accelerating to that speed
because of its low output at speeds below your desired cruise. *At the
same 23-24mph, your efficiency would be about 70%, with weak
acceleration and poorer efficiency at lower speeds.

Clearly there would be more torque with the smaller
wheel, but my route is not too hilly and doesn't involve a ton of stop
and go. *I expect the 700c will work fine. *

Sure. *But choose a motor that tops out not too much above your
desired cruising speed. *It won't allow you to have bursts of higher
speed, but it will cruise efficiently at full throttle.

Chalo

I just plugged my #'s into the calculator. *Weight makes surprisingly
litle difference, bike weight being 50lbs or 80lbs is only the
difference from 360-380W to maintain my 23MPH. *I think it's safe to
assume added drag from the panniers, but I'm not sure how much. *So
400W is probably an excellent estimate, all things considered.- Hide quoted text -

- Show quoted text -

Something I should have mentioned in my last post, is that the motor
i'm looking at is considered to be very, very similar to the Clyte
407. With 48V it seems it'll be in my cruising range well below the
peak of the efficiency curve, even with a 700C wheel. A 36V pack
would hit my desired speeds right around the peak of the efficiency
curve. There may be something to this 36V thing after all,
considering I don't want to go any faster than 25MPH, and consider
22-23 a good cruise speed. Of course, I don't know how bad it would
struggle up hills with only 36V pushing it.

DanKMTB wrote:

With 48V it seems it'll be in my cruising range well below the
peak of the efficiency curve, even with a 700C wheel. *A 36V pack
would hit my desired speeds right around the peak of the efficiency
curve. *

I was gonna say that, but it appears you figured it out yourself.
Another benefit may be to cruise on full throttle, rather than having
to throttle back to keep the amp draw reasonable.

There may be something to this 36V thing after all,
considering I don't want to go any faster than 25MPH, and consider
22-23 a good cruise speed. *Of course, I don't know how bad it would
struggle up hills with only 36V pushing it.

The speed in either case will sag until the power output matches power
demand, which means you'll fall to a lower speed before reaching
equilibrium on 36V. I found during my experiment with e-bikes that my
inclination was to give 'er hell on the pedals to make up the
difference when climbing. Your conditions or tastes may indicate a
different course.

It does seem like a relatively insignificant hassle to try 36V or the
equivalent before increasing your system voltage if your find it to be
necessary. If 36V does it, then you have a lighter and less expensive
bike for your efforts.

Chalo

DanKMTB wrote:

I also
think that on any decent decline, sustaining high teens to low 20 off
the motor would be reasonable, to save battery life. *I don’t know how
much of a difference it would make in saving battery, but it’s doable.

Note that the Crystalyte controller allows current to flow both ways.
That is, if you encounter a hill steep enough to propel you above the
motor's free speed, the motor will generate back EMF that flows to the
battery, causing electromagnetic braking that clamps your top speed to
pretty near the motor's free speed.

I don't know whether this will harm LiFePO4 batteries, but I know it
can screw up Li-poly batteries. SLA batteries, such as I used, accept
the reverse current as charging.

Chalo

Chalo wrote:

DanKMTB wrote:

I also
think that on any decent decline, sustaining high teens to low 20 off
the motor would be reasonable, to save battery life. *I don’t know how
much of a difference it would make in saving battery, but it’s doable..

Note that the Crystalyte controller allows current to flow both ways.
That is, if you encounter a hill steep enough to propel you above the
motor's free speed, the motor will generate back EMF that flows to the
battery, causing electromagnetic braking that clamps your top speed to
pretty near the motor's free speed.

I don't know whether this will harm LiFePO4 batteries, but I know it
can screw up Li-poly batteries. *SLA batteries, such as I used, accept
the reverse current as charging.

Note also that if your battery can't safely accept the regenerative
charge the motor supplies on fast downhills, you can shunt the reverse
current to a power resistor or similar load.

Chalo

On Apr 3, 12:24*am, Chalo wrote:
DanKMTB wrote:

With 48V it seems it'll be in my cruising range well below the
peak of the efficiency curve, even with a 700C wheel. *A 36V pack
would hit my desired speeds right around the peak of the efficiency
curve. *

I was gonna say that, but it appears you figured it out yourself.
Another benefit may be to cruise on full throttle, rather than having
to throttle back to keep the amp draw reasonable.

I did figure it out for myself, but promptly confused myself again.
There is concern that even a 408 motor will not provide sufficient
torque to get up hills. The guys doing this for real seem to be
swayed toward the 48V systems for that reason. A 409 motor, if the
clyte 409 is still available, may give me the sweet spot I want @ 48V
while providing plenty of torque. There is the concern of going
overboard in search of torque and losing overall effeciency in the
process.


There may be something to this 36V thing after all,
considering I don't want to go any faster than 25MPH, and consider
22-23 a good cruise speed. *Of course, I don't know how bad it would
struggle up hills with only 36V pushing it.

The speed in either case will sag until the power output matches power
demand, which means you'll fall to a lower speed before reaching
equilibrium on 36V. *I found during my experiment with e-bikes that my
inclination was to give 'er hell on the pedals to make up the
difference when climbing. *Your conditions or tastes may indicate a
different course.

I don't want to have to work too hard on the way to work. On the way
home I don't care, but I don't want to arrive at work tired and
sweaty. The concern is what speed I would fall to on a 36V system
before getting to that sweet spot.


It does seem like a relatively insignificant hassle to try 36V or the
equivalent before increasing your system voltage if your find it to be
necessary. *If 36V does it, then you have a lighter and less expensive
bike for your efforts.

Not too much less expensive and lighter, because if I go 36V I'll go
with twin 20Ah packs. If I go 48V I'll go twin 15Ah packs. Bumping
the 36V system to 48 would require another two 12V packs, meaning I
now have 4 packs to carry and charge. That's a significant hassle in
my book. I will be using a controller and motor that will be fine
with either option, but I don't relish the thought of having 4 packs
to deal with.

On Apr 3, 12:31*am, Chalo wrote:
DanKMTB wrote:

I also
think that on any decent decline, sustaining high teens to low 20 off
the motor would be reasonable, to save battery life. *I don’t know how
much of a difference it would make in saving battery, but it’s doable..

Note that the Crystalyte controller allows current to flow both ways.
That is, if you encounter a hill steep enough to propel you above the
motor's free speed, the motor will generate back EMF that flows to the
battery, causing electromagnetic braking that clamps your top speed to
pretty near the motor's free speed.

I don't know whether this will harm LiFePO4 batteries, but I know it
can screw up Li-poly batteries. *SLA batteries, such as I used, accept
the reverse current as charging.

Chalo

I did not know that. I'll have to see if the cycle analyst can stop
power from coming back into the batteries. I doubt it. I would have
to shunt the reverse power somehow, I think. Then again, the system I
rode yesterday did not have any issues at all with being pushed pedal
only, or coasting down hills. The owner encouraged me to ride it sans
power to get a feel for the resistance and true effect of the weight,
which I did. He also said that he never uses power on any significant
decline. His LiFePo4 pack is a couple years old, as is his Clyte
controller. Perhaps his controller did not allow two way power? This
is something else I'll have to look into. Thanks for the heads up on
that!

DanKMTB wrote:

Chalo wrote:

It does seem like a relatively insignificant hassle to try 36V or the
equivalent before increasing your system voltage if your find it to be
necessary. *If 36V does it, then you have a lighter and less expensive
bike for your efforts.

Not too much less expensive and lighter, because if I go 36V I'll go
with twin 20Ah packs. *If I go 48V I'll go twin 15Ah packs. *Bumping
the 36V system to 48 would require another two 12V packs, meaning I
now have 4 packs to carry and charge. *That's a significant hassle in
my book. *I will be using a controller and motor that will be fine
with either option, but I don't relish the thought of having 4 packs
to deal with.

FWIW, the rule of thumb that seems to work for most folks on most e-
bikes is that increasing pack voltage at the same Ah capacity doesn't
increase range, only performance. At a higher system voltage you can
and will use more juice, and the difference tends to negate the extra
watt-hours capacity of the larger battery.

My point is that if you think the range offered by a 48V/15Ah pack is
more than adequate, that's probably about the same as the range you'll
get from a 36V/15Ah pack.

Chalo

On Apr 3, 2:37 pm, Chalo wrote:
DanKMTB wrote:

Chalo wrote:

It does seem like a relatively insignificant hassle to try 36V or the
equivalent before increasing your system voltage if your find it to be
necessary. If 36V does it, then you have a lighter and less expensive
bike for your efforts.

Not too much less expensive and lighter, because if I go 36V I'll go
with twin 20Ah packs. If I go 48V I'll go twin 15Ah packs. Bumping
the 36V system to 48 would require another two 12V packs, meaning I
now have 4 packs to carry and charge. That's a significant hassle in
my book. I will be using a controller and motor that will be fine
with either option, but I don't relish the thought of having 4 packs
to deal with.

FWIW, the rule of thumb that seems to work for most folks on most e-
bikes is that increasing pack voltage at the same Ah capacity doesn't
increase range, only performance. At a higher system voltage you can
and will use more juice, and the difference tends to negate the extra
watt-hours capacity of the larger battery.

My point is that if you think the range offered by a 48V/15Ah pack is
more than adequate, that's probably about the same as the range you'll
get from a 36V/15Ah pack.

Chalo- Hide quoted text -

- Show quoted text -

Cool, thanks for the tidbit. Generally I'd agree, but I plan to be
using a cycle analyst to limit amp draw. While I will have a system
capable of pulling 30A, I'll have it regulated to 20A. I think,
anyway. Of course, it's subject to change once I get the stuff and
start playing with it.

I'm unable to find a combo I'm happy with that will give me peak
efficiency at the speed I want (20-23 MPH) @ 48V. I'm leaning very
heavily toward 36V at this point. If I can use even less power and
get even more range, awesome, but I still think i'll go with 40Ah. It
works in the budget I have planned, and extra range can't hurt. The
difference in price between two 15Ah packs and two 20Ah packs @ 36V
isn't enough to justify giving up the extra 10Ah worth of range.

I'm also a pedal masher. I have a plethora of bikes, but the 3 I ride
most are all singlespeed, and one of them is a MTB that I ride off-
road most of the time. Another is a road bike that I've done this
planned commute on many, many times. One of the two I just mentioned
will probably become the donor for this project. That said, I think
if there's a hill or two on my route that requires me to mash some
pedal to get it done at a decent clip, I will. A few minutes of hard
work a couple times over the course of a 35 mile ride isn't going to
tucker me out for the day, nor will it leave me arriving at work a
sweaty mess. I determined in my test ride that this will be assisted
pedaling, not an electric moped. With that in mind, 36V seems to fit
what I want. I just hope it's enough torque when I get it, and I
don't end up having to bust hump up every incline to get my overweight
bike up the hill. From reviews of other guys running 36V systems I
don't think I will. There is that nagging doubt, but I think I’ll
suck it up.