In theory how fast can this scooter go?

  • Thread starter david90
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  • #1
david90
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I just bought a 1500x motorboard scooter and it is powered by two 350 watts electric motor. The wheel is friction driven and the drive train is very direct.
If I weigh 150 lbs and the scooter weigh 22 lbs, in theory how fast can I go if the to motor is at peak performance? The scooter's specs says max speed is 12mph for my weight but I think it electronically limited. if so I want to remove the limit and

i think the gearing is 1:1
 
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Answers and Replies

  • #2
Pengwuino
Gold Member
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Aww this looks like a fun problem but I am too lazy to have a go at it :(
 
  • #3
FredGarvin
Science Advisor
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What's the wheel size?
 
  • #4
david90
309
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6 inches wheel
 
  • #5
krab
Science Advisor
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david90 said:
I just bought a 1500x motorboard scooter and it is powered by two 350 watts electric motor. The wheel is friction driven and the drive train is very direct.
If I weigh 150 lbs and the scooter weigh 22 lbs, in theory how fast can I go if the to motor is at peak performance? The scooter's specs says max speed is 12mph for my weight but I think it electronically limited. if so I want to remove the limit and i think the gearing is 1:1
Actually, your weight doesn't come into speed directly, since in absence of friction, a constant speed requires no power (Newton's Law). I will assume you are standing. Then the air resistance is similar to that of a bicycle or motorcycle. If we assume air resistance is the only loss, your top speed, assuming optimum gearing, is 30mph times cube root of horsepower. 700 watts is about 1 hp, so this predicts 30mph. But there is probably a significant rolling resistance as well, maybe using half your power. This still predicts 24mph. I would guess your scooter is a 1-speed, and that's the reason it will only go 12mph. If it were geared for max top speed, your acceleration would be abysmal; same as when you start your car in top gear. So it is geared for a reasonable acceleration.
Your current gearing gets 12mph or 18 ft/sec from 1hp. 1hp=550 ft-lbs/sec, so will get a forward force of 30 pounds. Since you are 172 pounds, this gets you an acceleration of 0.17 g, or 5.6 ft/s/s. IOW it will take 3 or 4 seconds to reach 12 mph. At this speed, you have some available force left, so without a taller gear to shift to, there is probably, as you are guessing, an electronic limiter to keep the motor from flying apart.
What you need to reach 24 mph is a transmission!
 
  • #6
GOD__AM
210
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It could probablly do 100mph+ if you throw it off a high enough cliff...sorry o:)
 
  • #7
FredGarvin
Science Advisor
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I think I am off in my estimates of losses due to friction and the belt drive and thenbacking it out through the tires. My numbers are coming out quite low. I wasn't taking into account acclereation either but at a constant speed.

I don't recognize the cube root of the HP relation to wind resistance though.
 
  • #8
Stingray
Science Advisor
671
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Aerodynamic drag will not be the dominant factor here. It also does not scale as the square of the speed (power as the cube) when going this slow.

Anyway, the motors put out 350 watts maximum. This is at one particular (shaft) speed. To give you an idea of what's going on, electric motors almost always have torque outputs that decrease monotonically with increasing rpm. The current drawn also decreases monotonically.

Despite this, power output initially increases (since power is proportional to both torque and rpm), peaks, and then decreases again. Efficiency (input versus output power) also has a single peak, and this usually occurs beyond the point of maximum power. It is very likely that 12 mph is already well beyond where the power peak is, so increasing max speed should be possible by changing gear ratios (with a transmission of some sort, or different wheels). It is impossible to say how fast you could go without much more information. Your acceleration also might become unacceptably poor.
 
  • #9
Antiphon
1,683
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Wear a helmet.
 
  • #10
david90
309
2
nah I'm too cool to wear one.
 
  • #11
david90
309
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What are some ways to improve speed or effciency? The engineering of this scooter is pretty good so it's going to be tough to think of an idea that could improve it.

The scooter uses this battery pack
http://www.portablepower.com/items/search/all/1/all/37L162S1 [Broken]
and it needs 4 of them to make 16v @ 8 amp hour. Is there another battery pack that has the same rating but weigh less? Wouldn't it be better if I put together 1.5 volt cells to make an equiv. battery pack?

here is the scooter
http://www.gosportinggoods.com/store/index.php?main_page=product_info&cPath=1&products_id=14
 
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  • #12
abercrombiems02
114
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using some very basic mechanics and lots of assumptions about what the effective friction would be I got an answer of about 10.5 mph. So I'd say that that 12 mph is really the top speed you would get on it. If you think about 700 watts, that almost 1 horse power. So, in theory, think how fast a horse could pull your wieght for an hour at a constant speed. 10-12 mph would seem reasonable.
 
  • #13
david90
309
2
i'm going to use a hand held gps to measure my speed. I'll let u guys know.
 
  • #14
david90
309
2
on a fairly smooth surface, I topped out at 13.5 mph. On a really smooth surface I can probably get to 15mph. Thanks all.
 

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