working out speeds from motor rpm's

**muchalucha** · 29th August 2009, 19:26

Bit of background, i got 2 wheel chair motors (cheers woody

) , they where too weak for a HW so i want to use them in a FW, they where right angle gear head motors that had an output speed of 113 rpm, way too slow for a FW (without silly big wheels) that and the gearboxes where knackered.

It is a 30:1 gearbox so the motors are 3390 rpm (which sounds about right), I am getting an 8 tooth sprocket for the motor and a 40 tooth sprocket for the wheels , i put it all into this calculator and it says 8mph(8 mph is ideal BTW

) ???
http://www.diygokarts.com/speed-calculator.html

That cant be right, Im thinking ive missed something here because if this is about a 4:1 ratio and the old gearbox was 30:1 there would be a lot of difference but the difference is only a few mph (3 to be precise). Ive done something wrong here but for the life of me i cant see what, and i dont want to order stuff until i know its gona work

thanks

**RogueTwoRobots** · 29th August 2009, 21:01

The ratio is 5:1 for a 40 tooth wheel sprocket/8 tooth motor sprocket, which gives an output rpm of 678. If you were getting 5mph with 113rpm it doesn't seem right getting 8mph with 678rpm.

What is your wheel diameter? Put that and the 678rpm into here and see what speed you get.

I put in 113rpm and to get 5mph out the wheel diameter needed to be 375mm (which gave 30mph at 678rpm

)

**craig_colliass** · 29th August 2009, 21:29

Hi
Wheel rpm 678 for 8 mph wheel dia 102mm

way too slow for a FW (without silly big wheels)

Seraphim has nearly 500mm dia wheels. I'm hurt!

**craig_anto** · 29th August 2009, 21:56

Craig, Seraphim's wheels are bigger than Tanto

**muchalucha** · 29th August 2009, 22:09

well the garboxes where dead anyway , and these where for the saint like HW i asked you about so the wheels would be like 500 mm anyway

**muchalucha** · 29th August 2009, 22:13

i have spotted my mistake , the origional wheels where 12.5 inches and my wheels are 4 inches, this seems to have solved the problem . sorrry

.

**craig_colliass** · 30th August 2009, 16:22

Hi Craig

I don't think you have seen The Mighty Seraphim, here is a pic with Tron (75mm wheels)

http://i569.photobucket.com/albums/ss13 ... 1251645451

I measured the wheels and they are 49.5 cm! the prize is left unclamed Â£5 to the first to OOTA The Mighty Seraphim.