My team is getting a new motor soon, this motor runs at about 90RPMs per volt, running a 36 volt system this is about 3300 RPMs. To keep our car at a reasonable speed we need a 96 tooth drive sprocket. We bolt our sprocket directly to the freewheel with a 2" ID and 4 bolt holes in a 2 7/8" circle. Does anyone know a place to get a sprocket this large? If so I doubt it has something that will mount this easily so what are some ways to mount it?
My school does have a CNC machine that I could use. I am fairly good at online 3D modelng like autodesk inventor but I don't know the specs on a 35 sprocket. Also what metal should I use?
Thanks for the help
Ron said
Mar 1, 2016
Ryan;
remember that you are not looking to keep your car at "a reasonable speed" you are looking to keep your electric motor at the most efficient RPM...
sooooo....
Find what is the most efficient (least amperage drawn) rpm for your new motor, find this out and then gear accordingly for any given track you will race at.
try to go as fast as possible while keeping the motor as close as possible to this most efficient rpm range.
This is why we gear at different "speeds" for different tracks out here in the northwest area..
on a course with bad hills (like at hood river oregon or at Lane Community college in Eugene) we gear to keep the motor at the "happy place" for rpms at a slower mph speed.
at a course like Portland international raceway (mostly flat and straight or high speed corners) the "happy place" will be at a higher speed in mph....
even the "short" and "long" track events at P.I.R. are geared differently to account for lower speed corners...
to give an idea.. Hood River might be geared for 25mph (there is a 40 ft difference between top and bottom of the track)
Lane community college might be geared for 28 mph (20+ ft elevation difference)
PIR long track might be geared at 45-50mph (less than 10ft elevation and all corners are over 35mph)
PIR short track same elevation change but several 25mph or less corners...
so in the long run don't worry about "how fast"... worry about "how efficient".
Ron
Nitoragro said
Mar 2, 2016
That is my problem, most of our tracks here are FLAT, and run from between 17MPH on our tightest track and 20MPH on hour 1/4 high school running track. But so we can gear our car to go that speed we need a 11 tooth sprocket on the motor and a 96 tooth on the drive wheel. This will give us the happy place. It's just that I can't find a sprocket big enough. I have been using the gearing exxcel sheet and changing everything and looking at the motor curve and the 11:96 is what gives us our efficient speed.
mhodgertt said
Mar 2, 2016
When we used to run Scott motor that turned about 3000 rpm we also had to run large sprockets for the driven sprocket on our rear wheel. For most of our races the key was a 7.2 to 1 gear ratio. To get those we would go to a local lawn mower shop that could order items for go carts with 35 pitch chain. In their catalogs they had very large gears available. We have a couple left over 100 tooth and others down to about 72. If possible you will need to get these a 'blanks'. These have a 1" center hole in them. Then you chuck them up in the lathe and machine the hole to fit the freewheel. The trick is to get the sprocket exactly centered on the chuck. We use thin strips of 16 gauge sheet metal between the chuck jaws and the teeth of the sprocket. Then we check for center with a dial indicator. After the hole is enlarged so the freewheel fits very snugly--almost a press fit--You just drill 4 -1/4" holes to fit the teeth on the freewheel. So the new sprocket works that same as the ones you have.
Nitoragro said
Mar 2, 2016
I never would have thought about a lawn mower shop. At my school we don't have a lathe, but we do have a CNC machine so I think I could program to cut it.
Thanks for the help
My team is getting a new motor soon, this motor runs at about 90RPMs per volt, running a 36 volt system this is about 3300 RPMs. To keep our car at a reasonable speed we need a 96 tooth drive sprocket. We bolt our sprocket directly to the freewheel with a 2" ID and 4 bolt holes in a 2 7/8" circle. Does anyone know a place to get a sprocket this large? If so I doubt it has something that will mount this easily so what are some ways to mount it?
My school does have a CNC machine that I could use. I am fairly good at online 3D modelng like autodesk inventor but I don't know the specs on a 35 sprocket. Also what metal should I use?
Thanks for the help
remember that you are not looking to keep your car at "a reasonable speed" you are looking to keep your electric motor at the most efficient RPM...
sooooo....
Find what is the most efficient (least amperage drawn) rpm for your new motor, find this out and then gear accordingly for any given track you will race at.
try to go as fast as possible while keeping the motor as close as possible to this most efficient rpm range.
This is why we gear at different "speeds" for different tracks out here in the northwest area..
on a course with bad hills (like at hood river oregon or at Lane Community college in Eugene) we gear to keep the motor at the "happy place" for rpms at a slower mph speed.
at a course like Portland international raceway (mostly flat and straight or high speed corners) the "happy place" will be at a higher speed in mph....
even the "short" and "long" track events at P.I.R. are geared differently to account for lower speed corners...
to give an idea.. Hood River might be geared for 25mph (there is a 40 ft difference between top and bottom of the track)
Lane community college might be geared for 28 mph (20+ ft elevation difference)
PIR long track might be geared at 45-50mph (less than 10ft elevation and all corners are over 35mph)
PIR short track same elevation change but several 25mph or less corners...
so in the long run don't worry about "how fast"... worry about "how efficient".
Ron
Thanks for the help