how weight affects speed and distance

kisshu

kisshu

100 mW
Joined
Jun 6, 2007
Messages
47
Location
CT, USA
does anyone know how weight affects speed and distance for any given motor voltage set up

for instance if the total weight varies by 50lb how would that affect the speed and distance over various grades?

a flat grade might just affect acceleration but not top speed but with grades of 5, 10, 20% how much would the resulting effect be from increased weight?

i'm trying to determine the benefits of a much lighter frame battery setup

thanks
 
Experiment with the weight and slope variables:

http://www.kreuzotter.de/english/espeed.htm
 
thanks! i get speed differentials of roughly 1 - 2 mph and power consumption of up to 100 watts more for 20% grade at 50lbs more weight
and 5mph -not too much difference; but it increases to 300 watts difference at 15mph- so if you really want to rip up a hill that extra 50lbs makes over 300 watts difference in power consumption- but at lower power consumption 600 watts the difference in speed it only 1- 2 mph with a 50lb heavier bike.

however a heavier bike carries more momentum against the wind and is less affected by wind speed - and moves faster down hill.

:D

thanks xyster for your fast response
 
kisshu - a heavier bike is not affected less by the wind. the weight of your bike doesn't affect the aerodynamic drag.

i like to be able to lift my bike. i worry that a heavy bike will stop slower and be less responsive. these are the main reasons to stay away from lead acid. i think lead acid is "less cool" to

:D
 
kisshu said:
however a heavier bike carries more momentum against the wind and is less affected by wind speed
Click to expand...

If you mean speed is affected less by transient wind gusts I would agree. Otherwise no. Weight should have no necessary relationship to wind resistance at all
 
If you spend any time pedaling or kicking, the lighter the better.

tks

Lock
 
I've been thinking about the weight issue lately too, and I think its VERY important to the performance of an ebike. Consider two ebikes that are identical except one weighs 250 pounds (including rider) and the other weighs 350 lbs:

1) They will have identical wind resistance in ALL situations.
2) When going up a hill, the heavier ebike will require the additional energy necessary to move a 100 lb object up the hill.
3) When accelerating, the heavier bike will require the additional energy to accelerate the additional 100 lbs up to speed.

Note: The physics formulas for going uphill, and accelerating are PROPORTIONAL formulas. THerefore, a bike that's 20% heavier will require exactly 20% more energy to climb a hill and will also require exactly 20% more energy to accelerate. Also, if the motor is outputting the same amount of power, the heavier bike will travel 20% slower uphill, and accelerate 20% slower.

However, these are just the formulas for motion. In the real world there are additional factors that favor the lighter bike. For example, the motor will function more effeciently with the lighter bike because it won't be asked to run a heavy load at a slower speed. Also, you may not be able to accelerate to optimal speeds uphill, straining the motor in a bad way. So in reality, the lighter bike will be significantly more effecient than the heavier one, but the only way to know the exact difference is to test a real bike.

Here's what's important to know:

1) Wind resistance is MUCH less significant at slow speeds. But as you go faster, wind resistance goes up EXPONENTIALLY. So if it takes 100 watts to go 10mph, it may take 500 watts to go 20 mph.

2) Most ebikes will perform great on flat land. On flat land weight is not very important. It will still take more energy to accelerate, but one the bike is moving fast, weight has absolutely no effect. The only issue is wind resistance. And all the motor's power is used to fight the wind.

3) When climbing a hill, wind is much less significant because you slow down, and much of the motor's power is used to pull the bike uphill.

4) A crutial isse is when you must ACELERATE UPHILL. This is where wind resistance is basicly insignificant. Almost all the motor's power is used for acceleration and hill climbing.

I think too many poeple focus on top speed as a measurement of their ebike's performance. I think a better measure of performance should be how well it accelerates uphill. Getting a ebike to go fast on flat land is the easy part. But when using an ebike as a vehicle in the real world, you want a bike that will get you up a hill, and accelerate well in "stop and go" traffic. These are the real issues. Also, if you design a bike to perform well on hills, it should automatically have a good top speed.

Here's an example:

Energy Needed to accelerate 100 lbs to 15mph

100lbs = 45kg
15mph = 6.7 meters per second

Kenetic Energy = 1/2 * Mass * Velocity * Velocity

http://www.csgnetwork.com/kineticenergycalc.html

KE = 1/2 * 45 * 6.7 * 6.7 = 1010 Joules

KE for 350 lb ebike = 3535 Joules
KE for 250 lb ebike = 2525 Joules

So it will take an additional 1010 Joules to accelerate

This is the total energy needed to get each ebike up to 15mph. It is INDEPENDENT of how much time is used to accelerates. For example, you could deliver the energy slowly at 10 watts (1 watt = 1 Joule per second) , and it would take 100 seconds to reach 15 mph. Or you could deliver it in a 1010 watt burst, and it would take 1 secont to reach 15mph.

So suppose you wanted to accelerate to 15 mph in 8 seconds (realistic):

350 lb bike needs (3535 joules / 8 seconds) = 441 watts
250 lb bike needs (2525 joules / 8seconds) = 315 watts

So it takes an additional 126 watts to accelerate the heavier bike to 15mph.

Note: Eventhough both these bikes will accelerate to 15mph in 8 seconds, the heavier bike will still require 1010 more Joules of energy to accelerate. This energy will be missing from the battery, which will reduce the range of the heavyer bike.

Now consider both bikes going up a 100 meter hill that rises 10 meters:

Potential energy = weight * height * 9.8

http://hyperphysics.phy-astr.gsu.edu/hbase/gpot.html

Weigh difference = 100lbs = 45 kg

PE = 45 * 10 * 9.8 = 4410 Joules

PE for 350 lb bike = 15435 Joules
PE for 250 lb bike = 11025 Joules

So it takes 4410 more Joules of energy to get the heavier bike up a hill that climbs 10 meters. Again, this number is independant of the time it takes to climb the hill. But suppose you wanted to climb the hill going 15 mph.

15 mph = 6.7 meters per second
100 meter hill / 6.7 meters per second = 15 seconds to climb hill at 15mph

So it will take 15 seconds to rech the top of the 100 meter hill.

So the 350 lb bike will use 15435 Joules in 15 seconds =1029 j/s = 1029 watts. Likewise the 250 lb bike will require 735 watts to climb the hill.

So the heavier bike requires an additional 294 watts to climb the hill.

Now consider accelerating up the 100 meter hill. However, we must adjust the amount of time to accelerate to the top because the bike can't accelerate quickly going uphill, so assume that it will take 20 seconds to climb the hill, and it will reach 15 mph at the top of the hill:

Joules needed for 350 lb bike = energy needed to accelerate to 15 mph + energy needed to climb hill

Energy for 350 lb = 3535 Joules + 15435 Joules = 18970 Joules
Power needed for 350 lb bike = 18970 Joules / 20 seconds = 949 watts

Power for 250lb bike = 677 watts

The best way to understand these numbers is to get a pedal-bike and ride it. How much harder is it to accelerate than to maintain a speed? How much work does it take to get up hills?

I beleive evs must use similar engineering to pedal-bikes. Look at all the effort to make bicycles light. Weigjt is a huge issue. If you ride a bike that has 10 lbs of extra weight, your batteries must accelerate that 10 pounds every time you come to a stop sign. And it has to pull that 10lbs up every hill.

Also remember that if you can shave 20% of the weight from your bike, it will go 20% farther on a charge (provided you drive exactly the same) or it will go 20% faster up hills, or accelerate 20% faster. This is not exact, it will be aslurring of numbers, but its a good guideline.

I'm trying to make my next bike out of aluminum.
 
There's a simple formula:

Less Weight = More Money

Whether it's batteries or frames the more you spend the lighter weight you can go. At some point it really all comes down to economics, but it's always better to have a lighter bike. But, you also want a bike that gives the performance on the road that you want. If you use a lightweight fork that breaks on you (think of those people trying to use aluminum forks with front hub motors... yikes!) then all the money you spend is not exactly worth it.
 
Great examples there Beagle. I agree, uphill acceleration is a much more useful parameter for measuring real world performance.
I'd also like to build a lightweight frame that's designed to cope with the increased stresses produced at ebiking speeds, but isn't over-engineered for its purpose (like my motorbike). I'm with Safe on this one though, steel is a more forgiving material to use for a prototype, and the weight penalty for building a frame in steel rather than aluminium is minimal. It's also easier to fix when something breaks :D
 
Malcolm said:
I'm with Safe on this one though, steel is a more forgiving material to use for a prototype, and the weight penalty for building a frame in steel rather than aluminium is minimal. It's also easier to fix when something breaks :D
Click to expand...

Thanks Malcolm. Actually I agree with both of you. But actually I think we're both right.

My main point about weight is that there's a "snowball effect" from reducing weight. In other words, when you reduce weight from the frame and batteries, you can then use lighter wheels, and smaller motor.

Consider a steel bike with SLA batteries: The bike might weigh 120lbs so you might have to put 3 inch steel wheels on it, and heavier forks etc. Then the whole thing would need a slightly bigger motor, making the bike weigh more. Then you'd need heavier batteries to power the motor, and then you'd need to make a heavier frame to support the weight etc etc etc.

However, a bike made from lithium-ion batteries and aluminum frame could weigh as little as 60 lbs, it could use aluminum wheels and forks. It could really be a bicycle weight vehicle. It would need a smaller, lighter motor. etc.

Just consider batteries alone: My SLA batteries weight 45lbs total--I have 3 18ah batteries. THe equivalent li-ion would weigh about 12lbs. But these batteries wouldn't be nearly equiavalent in the real world because the bike with li-ion would have 32 less pounds of weight to haul up hills. Or it could go about 13% farther on a charge.

Slower battery drain and lower torque situations would also significantly improve efficiency.
 
There is a ballance point to consider between weight and safety etc.. i totally agree.

My full suspension bike with 72v 8ah of NIMH weigh 76 lbs.

I'm about to swap out

72v 8ah at 25 lbs ( nimh )
for
72v 20ah at 30 lbs ( LiMn )


Not only am i adding a meere 5 lbs, i'm gaining double the range with higher operating voltage the entire time.

My folding bike was a lightweight in comparison, and yes, it required less power to operate.. but it was just not safe enough above 35 km/h for my taste.

I just had an idea.. check the poll section in a minute ! lol.
 
well, when i was playing with the simulator wind speed seemed to have an exagerated effect on the lighter bike. the mass of the bike would come into play say in this instance: two bikes same aerodynamics traveling on level surface at same speed coast against head wind exerting same force against each bike - the higher mass bike would carry more momentum and decelerate at a slower rate than the lighter bike -same principle as accelerating uphill - the higher mass bike requires more energy to move.

so, i was considering frame weight mainly as batteries can be upgraded over time. and was trying to determine what the difference of alluminum vs. steel is and frame design.
the mongoose is 90lb with lead acid- i want to add better batteries;the optibike is 71lb (600 watt) but aluminum compact design - i want to be able to carry extra batteries on the frame and a rack to carry some groceries or run an errand. i need an interim solution untill i can build a cheaper geared bike. also i'm tall and need alot of room its hard to find a L or XL used electric bike.

but i agree accelerating uphills is major importance for me as most of the speed limits on the roads i would travel are 15 to 25 ; 20-25 top speed is usefull for me but i would rather use a gearing design to tackle hills rather than use more power or slow to a snails pace.
 
kisshu said:
well, when i was playing with the simulator wind speed seemed to have an exagerated effect on the lighter bike. the mass of the bike would come into play say in this instance: two bikes same aerodynamics traveling on level surface at same speed coast against head wind exerting same force against each bike - the higher mass bike would carry more momentum and decelerate at a slower rate than the lighter bike -same principle as accelerating uphill - the higher mass bike requires more energy to move.
Click to expand...
http://tinyurl.com/yqlmb
 
F=M x A so the heavier the mass, the higher the force required.
 
You have to think in terms of the "big picture". If you weigh 185 lbs and your bike weighs 80 lbs then your bodies weight is more significant than the bike weight. A 10 lb difference in the weight of the bike is really in terms of hill climbing ability only about:

185 + 80 = 265 total lbs

10 / 265 * 100 = 3.77%

Three point seven seven percent is something that might win you a bike race, but with an electric bike you can just downshift and go slower up hills or if you want lighter weight you just pay more money for lighter stuff. (or simply increase your voltage or amperage for more power) We're not at the same STAGE of development in the electric bike world as bicycles are. Everyone is not running the same stuff yet and so until this type of vehicle matures to a level where you need to be competitive for reasons of winning a race on an uphill it's more a matter of how much money you want to spend.

You could spend thousands of dollars on this hobby for a 3.77% weight gain... :wink:
 
My first project cost about $600 initially (shown in picture) and with repairs it's getting up to near $1000. My second project will likely start a little higher, ($700?) but not by too much. I just can't see throwing too much money into something that is so early in the development cycle.

I have already gotten my batteries paid for twice compared to an automobile, so it's been good that way.

Today I was out riding and my master link for my #25 scooter chain fell off. (connects to the transaxle) I found the chain, but not the master link. When I replaced the chain awhile ago I forgot to order a new master link and decided to use the old one that didn't really fit right. So it's stuff like that (a $5 part) that I keep having to add to the bike.
 
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