Quick experiment in motor Drag

[Ypedal]

2 nights ago, me and the neighbors took the dogs out with the bikes for some quick exercise.. after a dozen laps of full out running.. tired and quiet dogs...

Me:
Downtube Folder FS ( full suspension )
Crystalyte rear 405 in 20" wheel
Tires : road tires, Kenda front and generic slick on back at 50 psi
36v 8ah NImh battery, plugged in and controller powered up
Rider = 175 lbs

Neighbor:
Raleigh mtb, rigid frame
26" wheels
Tires : generic flat strip down the middle knobs on sides 40 psi
Rider = 180 lbs

We set out from a rolling start, at 1 or 2 km/h, side by side, and let the bikes roll down the hill on their own..

http://video.google.ca/videoplay?docid=5487756685545827972&q=72v+norco

This is the piece of road used.. ( not the bikes used.. but just for reference on the slope )

We started off equal.. stayed neck and neck all the way down, about 10 ft before the end of the street i started to gain on him by about 2 ft then had to hit the brakes....

Conclusion : Brushless, gearless hub motors do not provide significant drag when coasting. at 36v on a 405 in a 20" wheel regen does not show up on the DrainBrain until aprox 45 km/h and no noticable braking until 50 +... At 72v on my 409 ( again on a 20" wheel ) forget regen below 60 km/h.. ( and at these speeds, wind is a bigger factor than motor drag ! )


Has anyone else tried this ?

 

[safe]

My bike has a freewheel... no drag...

 

[Ypedal]

That's exactly my point.. so did my neighbors bike.. but we were coasting at the same speed.. :wink:

 

[knightmb]

Although not scientific evidence (just anecdotal ), in my bike club, against another 150 other bikers, mine always rolls downhill the fastest, hands down.

There are people in the club that have $5K road bikes that are suppose to be flawless in friction, weight, etc. But every time on the "downhill coast" I'm always riding the brakes to keep from taking off ahead of them with no motor power at all. Some think I'm using the motor, but I have to always tell them it's just the bike weight (not really sure).

I've always wondered this myself. The front hub motor has some friction if you spin it by hand. If you spin it by hand free-wheel in the air it doesn't spin long before it stops. So it looks like is has a lot of friction. But I think that's just all relative because even if my back wheel can spin for 10 minutes before it stops with one push as compared to my front wheel, the amount of friction must really drop it to neigh zero after about 5MPH or higher I'm guessing.

 

[knightmb]

My bike has a freewheel... no drag...

I think though there is a point in which friction of the hub becomes moot after only a short amount of speed. Otherwise, people in my bike club would be passing me at over 55MPH on some of the hills. One hill in our ride, I can hill over 60MPH just from gravity alone and no one in the bike club can keep up, even if I'm in the lead taking all the wind. That's while taking my draft, which has always surprised me.

 

[Drunkskunk]

I haven't done this test, but I have played around with how much the motor drag effects me. I have one of the dual Clyte motors, 4012/408. It also has an "off" position that disconects either motor from the FETs.

On the off position, you don't notice any drag, there is no noise from the motor when coasting.

On the 408 setting, there is a small amount of drag. flipping the switch to 408 from off you can't feel, but you can hear the motor humming faintly during coasting.

On the 4012, there is a little more drag. flipping the switch to 4012 from off you feel it slightly, about like having the tire suddenly lose 5 pounds of pressure. There is enough of a hum for me to notice, but it sounds like tire noise.

Coasting next to my girlfriend's bike in the park, there didn't seem to be any real diffrence between my bike with the motor on 4012, and her non electric bike. I ride with the power off when I ride with her, but I leave the batteries on the bike.

 

[Kyle]

Interesting experiment!
Just thinking out loud:
Two or more factors come into play.
1. Gravity pushes equally on two different weights. The ol' two objects of different weight dropped off a tall building will reach the ground at the same time (given the aero drag is the same).
On the other hand:
2. More weight in motion = more momentum, ie more force to overcome aero drag.

Hmmm?

So the drag of the Hub is less than the force of momentum from the extra weight. But at only few a feet of difference, it must be pretty much equal.

YPedal: What was the total weight of each rider/bike

Later
Kyle

 

[Ypedal]

I can put both bikes on a scale and weigh them.. but my folder is aluminum, with a 15 lbs hub motor and 12 lbs battery.. his bike is a steel frame no suspension with regular wheels and no motor.. so both bikes are very similar in weight as tested..

Both riders are within 5 lbs of each other ( as of last month.. weighing the dogs we both got on and then got on again with dogs in our arms.. see numbers in my post above.)

 

[D-Man]

I thought I had some drag with my 408 brushless until I switched to slicks. Now it coasts really far with 30 lbs of lead on board.

 

[maxwell]

Weight really comes into it when going uphill!

 

[safe]

In "Gravity Bike" racing someone realized that more weight means a faster bike, so now they are starting to weigh the bike and rider beforehand and using weight to compensate. The bike/rider unit must fit into a specific weigh limit. Light riders are allowed to add ballast.

All that extra weight works against you uphill obviously... (as just mentioned)

 

[Lowell]

My 100lb ebike coasts down hills much faster than my unassisted lightweight road bike. Heavy bike + aero = downhill fun! Have good brakes ready at the bottom though...

 

[safe]

My 100lb ebike coasts down hills much faster than my unassisted lightweight road bike. Heavy bike + aero = downhill fun! Have good brakes ready at the bottom though...

Imagine my 140 lb bike going downhill... that's why I can get up to 50 mph so easily on the downhills... but up the same hill I'm lucky to be able to pull first gear at 16 mph... :lol:

 

[Kyle]

Non-hub bike
180+30= 210

Hub bike
175+12+15+30= 232

Do these weights sound about right?

I guess the another question would be "lenght of hill?"

next question:
Physic's Formula to make sense of the info?

any suggestion?

Kyle

 

[chuck]

Never finished, but was a physic major at the Univerity of Northern Iowa for 2 1/2 years. Potential Energy, PE, is Mass x Acceleration (gravity). The PE of the heavier bike is greater. On earth, the resistance of the wind is the same for both bikes, but, one bike has more PE. Without an opposing force, the friction of the air, both bikes would accelerate the same. The stored energy of the bikes is used to accelerate down hill. The same amount of PE is used to overcome air resistance on both bikes. Both bikes will reach a terminal velocity on an infinitely long hill when there PE=wind resistance. The heavier bike's terminal velocity will be larger than the lighter bike's. With out wind resistance, both bikes would keep accelerating.
Hope you understand, my ability to get ideas across has always been my weak point, but, rest assured, I know perfectly well what I meant to say.

Chuck

 

[knightmb]

Makes sense to me. Each object has it's own terminal velocity, so if you dropped a paper ball and a bowling ball at the same time, they fall at the same rate until the paper ball hits it's terminal velocity in which the bowling ball would fall faster. I guess it would take a really long fall for that to happen, but when you consider how steep hills are, especially long ones. A bike is certainly not aerodynamic with a rider and it's shape. I think that's why a lot of riders struggle to get above 25 MPH on a pedal only bike, the wind resistance really starts to kick in. Apply that to (2) separate bikers, among the many variables such as tire friction, frame, rider position, etc and I can see how some extra weight would start to give an advantage in the downhill coast. It's just odd that it's such a "noticeable" advantage. But when you think about it, if the bike next to you were doing 0 MPH and you were doing 0.25 MPH, you would still pull ahead anyway, so given one biker rolling 20 MPH and another rolling 20.25 MPH, it might not really seem so odd. It's not like you both start to roll and you zoom out so fast, the other guy misses you on a blink.

This also brings to mind a Mythbusters episode where they were racing downhill toy cars and a real size car. The real size car always won in a rolling race. They suspected the toy cars would reach their max velocity before wind resistance wouldn't let them go any faster.

 

[Kyle]

So:
no hub
210*32= 6720 FT/LBS/SEC SQ.
sq root=81 FT/LB/SEC

with hub
232*32= 7424 FT/LBS/SEC SQ.
sq root=85 Ft/LB/SEC

How is the math so far?
Physic class was in 1975, so please any corrections would be appreciated!
I'm not sure how to get "rid" of the "seconds squared' from the acceleration of gravity.


Since the two biker's were so close together at the end, let's call it a draw.(to keep it simple)

????for a 4 ft/lb thrust difference ~= 20 watts drag at that speed???

Please check this.

Later
kyle