Help Me With A Physics Concept

[Bill Verburg]

Originally Posted by jyl

Hey, I noticed something else - tell me if this sounds correct.

If this drive produces constant force (20 N) all the time that it is turned "ON", then it produces constant acceleration all the time. Whether you want it or not. When you are coasting fast down a steep incline, when you are holding position amidst other riders in the peloton, when you are trying to slow down for a turn - you don't want acceleration, but you will get it anyway. It is like a car with a stuck throttle. The only way to stop the constant 20 N is to turn the drive off. But when the drive is turned off, the wheel is very imbalanced as discussed above.

Not so, there is only acceleration when the input is greater than the opposing forces of friction and aero. 20N is only ~4.5lbs, but again power depends on speed so if you have 20N @ 10000rpm that's a lot of power, but 20n at 10rpm not so much

[aschen]

I think another thing that may be giving OP a bit of pause is the simple fact that 250w aint all that much power. Its around 1/3 HP which would sound positively puny except for the fact that reference is continuous human cycling power which is a pretty low bar.

Also consider that state of the art batteries are something like 10-12lbs for a usable KW-Hr. The difficulty of implementing/packaging this guy practically guarantees a low effective efficiency. Overall I think you are right that it is feasible but not practical.

I saw the other cheater device in the news and I think it is very well packaged. If you really want to hide such a device I think some sort of geared hub motor might work if they still use those spokeless wheels where you might hide the assembly. It is much more straightforward to react the torque through the bice frame of course

Only 0.01% related but the application of inertial torque to a cube with no external force. This is cool as hell and might give you a headache if you try to do a free body diagram

https://www.youtube.com/watch?v=n_6p-1J551Y

also those star wars robots use the same drive mech with offcenter mass as proposed here. This guys is particularly eloborate

https://www.youtube.com/watch?v=61P_2zvbxd8

[flatbutt]

Originally Posted by jyl

You guys are helping me with the intuition.

Here's where my hangup has been. Suppose I have a spaceship of mass M with a rocket drive that produces a constant force F. It is far from any planet or sun, so there is no friction opposing its movement. Force F accelerates the spaceship to velocity V at time T1, then a while later it has accelerated to velocity 10*V at time T2. At time T1 the power of the drive is F x V. At time T2 the power is F*10*V. I was/am having trouble understanding intuitively why the constant force is producing more and more power.

Are you taking into account the decrease in fuel mass that must be moved as you burn fuel?

[jyl]

Originally Posted by Bill Verburg

Not so, there is only acceleration when the input is greater than the opposing forces of friction and aero. 20N is only ~4.5lbs, but again power depends on speed so if you have 20N @ 10000rpm that's a lot of power, but 20n at 10rpm not so much

You're right, I was confused.

Aaschen's comment that the drive could hold the weight at any point on the rim is interesting and, I think, disproves my assumption that the drive must produce a constant 20 N of force. If the weight is parked at 5 oclock it will produce less force (tangent to the rim). If it is parked at 6 oclock it will produce no force (tangent to the rim).

It also seems that, if the weight has very low friction to the rim, then it might not require much energy to hold it at 6 oclock. So maybe it could remain in this "idle" low energy state for most of the race, then switched to 3 oclock for the key attack or sprint or climb.

Yes, this whole discussion has emphasized how pathetically weak the human engine is.

[livi]

This thread makes me feel stupid.

[aschen]

Originally Posted by jyl

You're right, I was confused.

Aaschen's comment that the drive could hold the weight at any point on the rim is interesting and, I think, disproves my assumption that the drive must produce a constant 20 N of force. If the weight is parked at 5 oclock it will produce less force (tangent to the rim). If it is parked at 6 oclock it will produce no force (tangent to the rim).

It also seems that, if the weight has very low friction to the rim, then it might not require much energy to hold it at 6 oclock. So maybe it could remain in this "idle" low energy state for most of the race, then switched to 3 oclock for the key attack or sprint or climb.

Yes, this whole discussion has emphasized how pathetically weak the human engine is.

A minor interjection then I can let it die, I promise.

If the weight is at 5 o'clock it still exerts a 20N force. It just does not create as much torque on the wheel, which is the important aspect here. Only a component of the force is orthogonal to the wheel so less torque is created on the wheel and less power will be required of the device to keep it steady.

You can imagine in our example of coasting along at 6 oclock, the device creates no torque and would require no force to keep in place (with the hypothetical frictionless conditions).

This follows intuition as if you made a completely dumb and passive device to ride in the wheel on rollers it would "want" to stay at the bottom. IT would require more and more torque to have it advance along the wheel, until it maxed out at 3 oclock


sorry, I like to think about this sort of stuff

[flatbutt]

Originally Posted by livi

This thread makes me feel stupid.

Except for the fact that YOU can save a life!
How are ya Markus?

[herr_oberst]

Originally Posted by livi

This thread makes me feel stupid.

Jeez, imagine how I feel....

[jyl]

Originally Posted by aschen

A minor interjection then I can let it die, I promise.

If the weight is at 5 o'clock it still exerts a 20N force. It just does not create as much torque on the wheel, which is the important aspect here. Only a component of the force is orthogonal to the wheel so less torque is created on the wheel and less power will be required of the device to keep it steady.

You can imagine in our example of coasting along at 6 oclock, the device creates no torque and would require no force to keep in place (with the hypothetical frictionless conditions).

This follows intuition as if you made a completely dumb and passive device to ride in the wheel on rollers it would "want" to stay at the bottom. IT would require more and more torque to have it advance along the wheel, until it maxed out at 3 oclock


sorry, I like to think about this sort of stuff

I think it is interesting too. Bike races are won by small differences in the power of the engines (riders) and those engines are already very weak, so hidden motors don't have to produce much power to make a difference.

For example, the rider with the fastest time up Alpe d'Huez in the 2015 Tour de France pedaled at 340 watts for 40 minutes, ascending a 8% grade at 21 km/h. He lost the Tour overall victory by 1m30s. If he'd had sometime like 30 watts more, he'd have won. Or something like 15 watts more on that stage and a couple of the earlier stages. Or about 30 watts more on an early stage where he lost 1m30s in crosswinds.

At 11 m/s (25 mph), to get 30 watts from this concept would only require a 0.27 kg weight. Right? 0.27 kg x 10 m/s^2 = 2.7 N. 2.7 N x 11 m/s = 29.7 watts. So, about a tenth of the 2 kg weight we've been talking about. And a tenth of the battery.

Modern carbon bike wheels are pretty deep, there is some volume to work with in there.

Racers are followed by team cars carrying spare wheels and bikes. Changing wheels (for flat tires) is common, changing bikes is increasingly so. A racer doesn't necessarily start the day's race on the same bike or wheels that he finishes on.

The UCI (sports governing body) is using magnetic field sensors to screen bikes for motors, because (I think) the existing seattube motors contain permanent magnets, and then confirming by removing seatposts and bottom brackets to look inside the frame.

They have bought xray machines to some races, but those are not mobile and I don't think they are used that much. At each race there are say 200 riders and 400 bikes and 600 wheels strewn over 160 km of public roads, busy start and finish areas, team cars and trucks and buses. Impractical to physically impound all that equipment and run it through an xray and be assured the wheel used in the race is the same as the wheel that was xrayed.

A hidden motor that does not use permanent magnets, and is concealed in a component or a part of the frame that cannot be non-destructively opened, and that looks identical to current race bikes, would be hard to detect using the sport's current screening methods.

[LEAKYSEALS951]

[QUOTE=jyl;9003348]For example, the rider with the fastest time up Alpe d'Huez in the 2015 Tour de France pedaled at 340 watts for 40 minutes,[QUOTE]

Good lord- did all the antidoping stuff work and power levels go down? 340 is a lot- but when I was in the best of best (for me) shape, I could put out 300-320 for 45 minutes using a calibrated SRM system. I weighed 150 dead even- which is about 4.5 watts/kilo. I find it hard to believe I was within 90% of a TDF rider. The guy must have been light as heck and have a killer power to weight ratio. I always heard Lance putting out 400watts for extended periods of time. I thought professional riders were around 6-7 watts/kilo.

P.S-I probably couldn't even do 140watts now! ha!

[jyl]

The guy weighs about 125 lb, and this was after 20 days of intense racing and was the third big mountain climb of that day.

But, yeah, the anti doping is working. His time of 39-40 minutes (can't recall exactly) was 3 minutes slower than the Pantani (max dope) record. I don't know if anyone will ever beat Pantani's time. Maybe when bikes weigh 3 kg.

[LEAKYSEALS951]

Originally Posted by jyl

The guy weighs about 120 lb, and this was after 20 days of intense racing and was the third big mountain climb of that day.

Okay makes sense- so 120 lbs= 54 kg 340w/54kg= about 6.3 watts/per kilo. That sounds better (especially late in the race)!

[Bill Verburg]

I loved the one about the Tor rider that, swapped in lead filled water bottles at the top of the climbs so that he'd go faster downhill.

[jyl]

Originally Posted by LEAKYSEALS951

Okay makes sense- so 120 lbs= 54 kg 340w/54kg= about 6.3 watts/per kilo. That sounds better (especially late in the race)!

Pantani was > 7 w/kg!

Today, if someone climbed like that, he'd be covered with urine and punches from the crowd.

[aschen]

I like for my RC helis to have at least 250 w / kg so they don't feel sluggish!

[livi]

Originally Posted by flatbutt

Except for the fact that YOU can save a life!
How are ya Markus?

My Life is bordering on sublime, thanks! You too I hope.

[livi]

Originally Posted by herr_oberst

Jeez, imagine how I feel....

I have found that a healthy amount of Johnnie Walker Black Label makes that feeling go away. Along with cramp in my lower legs and feet during night. A twofer I Believe you call it.

[Charles Freeborn]

Or, better living through magnets...
https://www.facebook.com/1507529206213982/videos/1519998604967042/