ok guys… i need to build a rover that will weigh about 5lb. i’ll probably be using 4 3.5" offroad tires and i need to know what torque would i need to drive this thing on a flat cement surface.
thanx
That requirement is really pretty easy to take care of. Pretty much any motor from GHM-01 to GHM-04, or PGHM-01 would do nicely. The PGHM-02 and 03 would be too slow, and the PGHM-19 or 20 would be overkill.
thanx for your answer but i was looking forward for some numbers… to be more precise i would need to know what the rolling friction coefficient is (i hope i got this right: opposing_force=rfcweight9,8). i’ve been searching for this for a few days now but i didn’t find anything relevant. i also have some acceleration requirements for this and that’s why i need to know the actual value of the rfc on ate surface i mentioned to have a starting point.
p.s: it’s a bigger project from my university and i’m in charge of the drivetrain. i hope i made some sense because it’s been a while since i had my mechanics exam…
I dunno, I don’t design rovers that way. I build a chassis and stick different motors in it until it works the way I want. 
No math… LOL
This is where you break out your wallet, your inclined planes, and your pull scale to determine the values you seek experimentally. If someone just gave you the numbers the stuff would cost hundreds instead of tens of dollars and you would be spared the full value of your learning experience. Sounds rough but here’s the reason I bothered to write this up. It’s great to be able to pull the numbers from a spec. sheet and crunch the numbers, without a doubt. What if you don’t have a spec. sheet, or the equipment you are solving the problem from has changed and worn enough that it no longer complies to the pristine, right off the line, manufacturers specs? You make some attempt at de-rating it’s performance based on a WAG? You examine it’s current performance and efficiency data to get a better WAG at where it is now operating? Solving design issues occasionally, sometimes more than occasionally, requires the ability to know how to get the data for a device or system with previously unknown parameters. This is kind of one of those places where hands-on deviates from text books and tests. This is a simple problem in your case, I think it was somewhere around a second yr. physics lab level type of problem, that you can solve with some simple tools and observation. Besides, if memory serves me correctly, professors eat this kind of stuff up because it shows kids are actually able to think problems out rather than just follow the bouncing ball and sing along.
I do the same thing sometimes when I have a bunch of yet unused brackets and I get bored. I just think of something I might build and start building. That’s how I started the design for WALTER’s new top deck arm… 
I think the Servo Erector Set is better than a jigsaw puzzle sometimes, and I can make so much more with it. I haven’t even used all the different bracket and such yet.
8-Dale
i completely understand what you mean, Eddie. i know that the best thing would be to actually measure the stuff i need but for fore some reasons i can’t do that (i’m not from the us, i must order all the parts at once, without the actual wheels any experiment won’t be completely reliable). i study computer science (focusing on embedded systems) and mechanics wasn’t really my favorite so i did the most obvious thing: i asked somebody who should have some experience in this field, you guys, and go on from there. yes, i am aware that theory!=practice at all times but in this case it would be close enough.
the difficulty with this is this is a hobby group and most stuff is done, as jim and dale pointed out, either by trial and error or by looking at what other folks have created (frequently through trial and error.) most of the wheels and used for these projects are loosely spec’d and actually trying to predict the rolling coefficient of friction for model truck tires… just pick a number and round up. You may be able to find a platform using the same wheels you are and see if you can figure out what torque range the motors are operating at given their top speed. most of the motors do have the basic spec sheet so you get the graph plotting rpm vs. torque, current, and efficiency. sorry I don’t have much else to offer. 
pcradu - with all do respect - your initial question immediately jumps out as being all too broad to answer easily. Try here
school-for-champions.com/science/frictioncoeff.htm
Note - these are smooth surfaces for concrete and rubber.
The scope of the question is problematic and I think the minds here have already stressed/eluded to this point. If your bot is 5 lbs and some recommendations have been made based on this - go with it.
Real world experience is always better than book worm mumbo jumbo - when that tire hits that uncalculated pebble the coefficient of friction for the original two surfaces is rendered moot.
If you are developing a drivetrain go robust. People more appreciate over expression of power rather than weakness as long as no one gets hurt.
Man, by the sounds of it, you have a large undertaking on your hands - all this math, all this mechanical, all this social networking, all this human psycology. Don’t complicate things by mitigating the experience of those who answered. For these same people have lots of real world experience moving a 5lb wheeled object over surfaces. I smile as I say this - several of your first responders have been rolling through coefficient of friction forces with 5 lb bots for quite a long time.
harshly and hopefully humourously spoken,
Chris out
PS - dont forget to factor in air friction once underway (Im such a sarcastic as_)
I do hope you take this lightly yet thoughtfully
… and the coefficient is for sliding friction… 
anyway i found a link at the bottom of the page that would get me started 