# Serious math help needed

1. Aug 11, 2008

### puppetmaster

serious math help needed!!

hello,
first off i would like to say that im new here, and this being my first post, so if i did something wrong, don't flame me. im a designer by trait and im currently trying to build something but unfortunately its completely out of reach of my math skills, so thus why i seek help from all you professionals.
what i need to find answeirs to are the following questions. however if what i post does not give enough info please let me know so that i can provide everything you need to figure out my problem.
ok here goes:

im building a homemade dynameter for my pocket bikes. unfortunately they do make these so im designing one my self. so what i need to find out is this:

i have a 10" (outsidediameter) drum that wieghs 60lbs,
i need to know if you can find the rpms of the motor spining it
by how long it takes to spin the drum from 0 - 30mph.

also i need to find the horse power the motor has by using this.

so here it is.

the drum (10" outside diameter)
wieghs 60lbs
what is the rpms of the motor spinning by moving it in the (TIME) it takes to go from 0-30mph.
once the rpms are achieved can i find how much horse power the motor has?

anybody who can help me out i would be more than appreciative to.

puppet.

2. Aug 12, 2008

### lewdtenant

Re: serious math help needed!!

Ok, I'll give it a shot:

OD = 10" = 25.4 cm = .254m
Weight = 60 lbs = 27.3 kg
30mph = 13.4 m/s

Circumference = pi*OD = 0.80m

Measure the time it takes to go from zero-to-thirty. Call this t.

Power = Work/time = (Force * Distance) / time ---> where Force = mass * acceleration

We need to find acceleration.

You will need to construct a graph:

the y axis should be measured from 0-30mph in 1mph increments.
the x axis should be measured from 0-t in 1 second increments.

Plot (t, 30mph)
From this point draw a straight line through the origin.
The slope of this line is the acceleration. (Which should be the same regardless of whether you kept going though 30mph or not).

Also, forget the weight of the drum. What is the weight of the bike? Convert to kg.

Power = (mass * acceleration * distance) / time

Plug and Chug. Make sure your units are correct. I like SI units.

P.S. I have no idea if this is right.

3. Aug 12, 2008

### puppetmaster

Re: serious math help needed!!

hey thanks for the help. now i have to just figure out to incorporate all this information into a visual basic program that i can just plug the numbers in. forget the wieght of the drum, the wieght of the bike? how come? the rear tire has to spin the drum. to get the reading from the motor there needs to be some type of resistance that the bike needs work for. but thank you for the help.

4. Aug 12, 2008

### lewdtenant

Re: serious math help needed!!

i don't know man, i just thought i'd get the ball rolling ya know

good luck with everything, you should probably wait for someone else to come along to help.. that's probably the best advice i can give you

5. Aug 12, 2008

### Cvan

Re: serious math help needed!!

By 'from zero to thirty miles per hour', do you mean that the drum is moving on the ground, or that it's rotating about it's axis with the tangential speed at 30 miles per hour. Also--how thick is the drum?

Additionally, in regards to lewdtenant, the graph constructed in that fashion would have some other properties in practice: The graph, taken as the divisions of time become smaller and smaller (eventually going as the limit goes to zero, where you can take an integral), won't necessarily be a straight line because of how energy is distributed into the rotational energy of the drum. You would get an average from this though, which is what you're looking for probably. The power would be the area under this curve (barring of course, the efficiency of the motor itself). Average force turns out to be (1/(change in time))*(integral F*V dt)

You're probably better off seeing if you can get a model number off the motor and looking up the juice it draws, then doing a quick and simple gear ratio proportion to get the RPMs. Why do what's already done for you? Huge part of engineering.