Need belt sizing help on my Dynamark pulley swap.

Well as you can see from my youtube videos I took the rearend out of my Dynamark and packed it full of grease and oil then replaced the 7-1/2 pulley with a 3-1/2 pulley. The motor is an 11hp briggs motor. The stock belt was 87-1/2 inches so I picked up a 83 inch belt and that was still to big. I am going to try an 81? Does anyone know how much faster I will be going?

http://www.csgnetwork.com/pulleybeltcalc.html add roughly 1 inch to allow for the belt tensioner or just round the number up

looks good!!

You will be going roughly two times as fast ( probably about 1.7x)

to find out what size belt on a pulley swap, find the differance in circumferance of the new and old pulley (dx3.14) , divide that by 2, and add or subtract depending if you are going bigger or smaller.
the new belt should be about 6.28" shorter or 81.22"
round that down to 81 and you should be good.

Ditto on the 2X speed increase

Yea! but bad problem... lol went and got an 83 inch belt then they did not have an 81 at Tractor Supply CO in stock so I picked up an 80.. lol that did not work. So I need to find an 81..

Try napa autoparts - they carry lawntractor stuff.

Dont think the belt number ( B81) is the size, usually the size and the belt number are a few inches off.
a B81 belt is actually 83"

END PRODUCT!! https://www.youtube.com/watch?v=x6usyI4zC-4

OK so now the belt seems to be slipping a lot. Anyone know how to fix this? Bigger pulley on front perhaps?