Watts that you say?

[tom2slow]

Hi All,

Someone said to ask questions..

I see a lot of posts that say things like 'it will go X mph because its 36 v (or insert your own voltage)
But isn't the real information in the watts? I mean horsepower to weight ratios have always been the telling fact in go fast cars....and watts = horsepower x fancy math.
And I am sure I weigh a bunch.

I guess w/o the smart aleck my question would be: To go a certain speed I have to develop (and deliver to the road) a certain wattage, correct?

To me then, this means my trike is always gonna require more amps/have less range than a 2 wheeler, or when I add a laptop to the pans I am gonna lose a mile in range.
Yes there is rolling resistance, trebly so on a trike, and aero is a factor, and that's all a drag too. But aren't you really interested in the same thing as John Force?

--T

 

[spinningmagnets]

Statements like "I went X-mph on Y-motor" is important info. Watts is Volts X Amps, so in theory you can get the same power from high volts and low amps, or...low volts and high amps. Using high amps can result in needing fatter wire, bigger connectors, and having more of your batteries capacity used up in making heat, rather than power.

Once you find out that higher volts are more desirable, what can you afford? So far nobody I know makes a 44V LiPo pack, so if you want 44V you have to series two 22V packs. That cuts your frame space in half for battery range. Lots of choices.

Motors come from the factory with certain windings. A simple explanation is that in the motor-space allowed for copper wire windings, you can use many wraps of thin wire, or a few wraps of thicker wire. The number of RPMs you get per volt applied is called its kV. Unless you want to custom re-wind a motor to get your desired top-RPM at your most-desired battery pack voltage, you are stuck with the available motor-windings.

 

[Ypedal]

roughly speaking..

when talking hub motor, the voltage is directly related to wheel rpm, more volts = more speed

but.. when using a motor with a chain and sprokets, more volts still equals more rpm but you have the option of changing the sproket sizes to provide the wheel speed you want giving the motor a mechanical advantage via gear reduction..

Add to that the info posted above that X motor can be built to spin faster or slower by how many turns of copper on each pole has... things are variable.

In general, 36v for 20mph, 48v for 25 to 30mph..

How quickly you can accelerate is determined mostly by " Amps " .. so volts x amps = watts..

You could cut the amp limit in half and still get the same top speed, but it would take longer to get there as you would accelearte slower.

alot of exceptions to those rules, but in general.. that's that.

 

[tom2slow]

ok, got all that, I was thinking of a on the chain, Cyclone type equation. Had not thought a hub motor through all the way

So here is a bottom line question. I recently changed job and my commute went from just over 8 miles that I have done for years in a half hour with no shower required to 20 miles with no shower available and I report at 0600. Am I crazy to try to get this ride down to an hour'ish and keep the human factor to a bearable level? Coming home I could care less, but getting there, in the dark (which I have done for years at the lesser value)

--T

 

[dogman dan]

I do 15 miles on mornings I ride the e-bike. I don't do it in the dark at all anymore. Just too unpredictable to go that fast on a bike in the dark. Light yourself up all you want, but you come along at 30 mph and they will pull out into your path thinking you are a bike going 15 mph.

What made me quit riding in full darkness was the day I hit a pedestrian while riding 27 mph. Knothead was wearing a black hat, black coat, black pants and black shoes. Fortunately I just caught his shoulder and didn't hurt him, and I survived the over the bars ok too. But now I wait till the predawn, when I can see knuckleheads in the bike lane better. I slowed the motor down to 20 mph max, and now I get crossed about 90% less than when going 30.

If you keep your top speeds more reasonable, allowing you time to dodge the unexpected, you can expect to do 20 miles in about 75 min. A 36v hubmotor's typical speed of 20-25 mph is what I'm saying here.

A LOT, of course will depend on your actual route. Traffic on it, do you have a wide shoulder or bike lane, how many chicks driving and putting on makup do you have to get past, etc etc.? For me, the danger is ninjas walking to the bus stop in the dark.

 

[BLUESTREAK]

Hey DOGMAN how many NINJAS TURTALS have you wiped out? :lol: :lol: :lol:

 

[tom2slow]

That's a great point, I have to admit I was thinking get there, not the nimrods...I have tons of light, but you are right, they would think I was doing normal speeds. You will have to trust me, riding a trike in Denver is the closest I am ever gonna get to being a hot blonde or at least stared at like I was one.


Did have a Mule Deer jump out at me once in the Cherry Creek State park. That was truly exciting.

 

[blueb0ttle2]

Hi All,

Someone said to ask questions..

I see a lot of posts that say things like 'it will go X mph because its 36 v (or insert your own voltage)
But isn't the real information in the watts? I mean horsepower to weight ratios have always been the telling fact in go fast cars....and watts = horsepower x fancy math.
And I am sure I weigh a bunch.

I guess w/o the smart aleck my question would be: To go a certain speed I have to develop (and deliver to the road) a certain wattage, correct?

To me then, this means my trike is always gonna require more amps/have less range than a 2 wheeler, or when I add a laptop to the pans I am gonna lose a mile in range.
Yes there is rolling resistance, trebly so on a trike, and aero is a factor, and that's all a drag too. But aren't you really interested in the same thing as John Force?

--T

I am by no means an expert and I'm quite new to this game, but there are some very basic concepts of electronics that can answer many of the questions you have here.

-Relating Watts to horsepower is not "Fancy Math" , 1HP is roughly 750 watts (There's a difference between metric HP and Imperial, and there are many ways to measure HP, but 750 watts/HP is as good a number as any. They all float around there)

-1 Watt= 1Volt X 1AMP (P=VI, or power=VoltageXCurrent)
This is where voltage comes in. If you have 36 volts worth of batteries, and your motor and controller can take up to 30 amps you have 1080 watts maximum power or ~1.4 HP. However, once you get up to cruising speed you will probably only pull 15-20 Amps, which means you are running at about 540-720 watts. THAT wattage is the "continuous power" rating that most electric motors are advertised by.

-We tend to speak in terms of voltage as a rough guide for speed, and amps as an indicator of range and torque. I imagine that this is because, with the controllers and batteries we have available, we need to balance voltage and amperage to maximize range and minimize weight. So, our solution is to have between 24-48 volts worth of juice, and between 10-20 AH worth of capacity. This is on typical bikes though, there are some more daring souls that push the limits, and that is great, because such experiments will lead to better bikes

-Also there's what we call "I squared R loss", which essentially means the higher the current, the more energy gets lost through heat. So, in order to improve efficiency, we use higher voltages, rather than high currents on our bikes.

-Since you are using a trike, you will need more torque and therefore bigger batteries. That's not a bad thing though, the tricycle can more easily support the big batteries. My homemade electric bike keeps falling over, and all I have on it are two 12 AH batts. Think of what you can pile up on a trike

As Ebike hobbyists we rarely seem to go beyond basic algebra, it's in designing controllers and motors where the "Fancy Math" comes in. I am just beginning my training to be an electrical engineer, so naturally this "Fancy math" will be my stock and trade, but it need not frighten everyone

Guys, feel free to jump in and correct me if I made any mistakes. I'm sure I've made some :lol:

 

[blueb0ttle2]

Ah! I see Ypedal had already jumped in and pretty much said the same stuff I said. oops.

 

[tom2slow]

OK, so http://en.wikipedia.org/wiki/Bicycle_performance tells me that I MIGHT put out 200 to 250 watts. That's an interesting number when you look at adding 250 - 500 watt motors.

And yeah, one of the greatest parts of triking is no falling off. I have a 22 tooth climber in the ft and 32-11 in the back, on 20 inch wheels, if I have to I can pedal all day and never get anywhere

--T

 

[gestalt]