View Full Version : Calculating Mph.

01-21-2005, 06:11 PM
how can you calculate mph.???


01-21-2005, 07:24 PM
I guess you're asking for an rc? Set up two lines on the ground where you're going to run, a measured distance apart (needs to be measured accurately) and use a stop watch. Make them far enough from where you start that you'll be at top speed. Have a friend stand where he can see both lines, then start and stop the stop watch as your rc crosses those lines.

Now its simple math - say your lines are 50' apart and it takes 1.13seconds to travel that 50 feet - 50feet/1.13sec = 44.25feet/sec. Handy conversion from feet/sec to mph - multiply feet/sec by 0.682, so 44.25ft/sec X .682 = 30.17mph. You can do it the long way using 5280 feet/mile, 60sec/min and 60min/hr, but it comes out the same. Make a few passes until you start getting speeds that are close together indicating you're making good measurements with the stop watch.

This is actaully an average speed for the 50 feet (or whatever). If you can get access to a radar gun, thats more accurate, but if you're careful doing this, you should be close. You want to strike a balance with your measured distance - too long and its too averaged, too short and you can't operate the stop watch consistently enough to get good times which leads to erratic speeds.

What I remember is 60mph is 88ft/sec and ratio from there - the .682 is actually 60/88.

01-21-2005, 07:28 PM
http://www.misbehavin-rc.com/pit-lane/speed/g-calculate1.asp easyer

01-21-2005, 07:34 PM
Good Answer.

In theory, you can guess how many RPM your motor will pull under load. Then figure out your gear ratio and your tire height.

Lets say 20,000 rpm, you're clutch bell is a 12 tooth, spur is a 30, and you diff has an internal ratio of 2.5:1 and maybe you're tires are 2.5 inches tall....

20,000 revs per minute is
1,200,000 revs per hour
times clutch teeth (12)
devide by spur teeth (30)
devide by diff ratio
192,000 tire revolutions per hour
tire is 2.5 inches tall, times PI gives
1,507,964 inches per hour
devide by 12
125,664 feet per hour
devide by 5280
23.8 mph!

Phew. Thats why I just keep it in Excel.

01-21-2005, 08:11 PM
always_opencarb - never have seen that on his site, cool!

Guess his selection of 100feet says thats probably the best distance to use and get accurate times. PIA to measure off 100' though, lol!

01-21-2005, 08:29 PM
yeah its in gereral RC hope he doesn't mind me linking it

01-21-2005, 09:52 PM
hey cool duster 360 i tried that and thats about how fast it looked with out it getting clocked by radar...

01-21-2005, 09:55 PM
hobby grade radar guns are relatively cheap nowadays.Bushnell sells a couple slightly different models that you can find on e ba y for about $100 USD