pully sizes

Hi all
I know its probably been asked before but wanted recomendation on pully sizes I should source to make it go quite fast but still usable at a low speed 12hp B&S motor, 6 speed box , 18-950 x8 tyres.
cheers

A 1:1 ratio is quite reasonable. I run a 5 on the engine and a 4on the gearbox on mine. Does 22mph.

It probably has been asked 100 times I'm sure but no worries.  The answer to this will vary from tractor to tractor, depending on engine, transaxle, wheel diameter etc, however, to find the answer i would do this;

1) get a hold of a gps or gps app on a smart phone or some way of calculating speed.

2) put the tractor in 1st gear and drive it at IDLE speed - no throttle/tortoise - measure this speed.

3) put the tractor in top gear and drive it at full throttle - (hare) - measure this speed.

4) next comes the harder bit, you need to get under the tractor and measure the current front and rear pulley diameters.  To keep it simple, measure around the outside of the belt that is wrapped around each pulley.  TYPICALLY the engine pulley is small, about 3-4inches diameter and the transmission pulley is bigger - 8" or thereabouts.

from the sizes of these 2 pulleys and the current speed range of the tractor you can begin to calculate what pulley ratio you will need to obtain the speed you want.  HOWEVER - you do have a few limiting factors - engine power - and probably most importantly the safety of your tractors handling at speed.  An unmodified chassis will start to feel pretty unsteady when pushed beyond its design limit, so if you do want to make it fast you should really look into sorting out the front axle and steering as that is where most of the speed related problems would emanate from.

5) so for example, i have a 4inch front pulley, 8inch rear pulley, at in top gear at full throttle it does 10mph, and in first gear at idle it will do 2mph.  lets say i want to do 20mph top speed.  

first thing to work out is the current transmission ratio of the belt drive - ratio=(front pulley diameter/rear pulley diameter).  in this case 4/8= 0.5

i want to double the speed, so i have to double the ratio.  (front pulley diameter/rear pulley diameter)=1!

if you double your top speed - your slow speed will also double, so in this example the 1st gear idle speed is now 4mph.

the sizes of pulleys you choose depends on what space/clearance you have in your chassis and around your clutch, but i feel it is better to increase the size of one pulley rather than reduce the size of one as it gives more surface area for the belt to grip on - remember the belt will be under more load.  for the above example i could put an 8" pulley on the engine, (8/8=1), however that may not fit, so an alternative would be 6" pulleys front and rear, (6/6=1).

look at the other projects on here and see what chassis and steering mods the guys have done for higher speed projects, also take into account what terrain / use you have in mind to make a good choice on ratios, if you gear it too high it won't be as fun

did you make you own pulleys

the short answer i have found is to buy them, lol, depends on your personal fabrication skills i guess, i have made ones before - cutting centres from one pulley - welding into another etc, theres only so much you can do generally speaking, but its a pain to get it right and really buying them is easier / more cost efficient in the long run IMO

I could cut and weld but prefer to buy but cannot find anyone to get them from.do you buy them in uk?

they are easy enough got a hold of, you just need to know where to look, strangely, bearing shops tend to stock/sell pulleys and belts! a bearing/belt shop that caters for agricultural customers will usually have a better selection than an online shop in my experience.

the front pulley needs to have a 1" bore with a 1/4" keyway for a briggs engine, and the rear pulley bore will depend on your transmission, but is typically smaller. Typically the pulleys those guys sell are cast steel pulleys rather than the pressed steeliness that come stock on mowers. you will also need to specify what belt section you are running, it will more than likely be an 'A' section v belt, possibly a 'B' section v belt.

For buying pulleys, the diameters for most motors and transaxles (will vary from mower to mower but this is what i generally see)
MTD transaxle usually runs a 7-8" pulley with a 1/2 inch bore.
Most other transaxles (peerless foote etc.) will usually run a 8-10" pulley with 5/8 bore.
Most briggs flathead/OHV singles will run a 1" bore. 1/4 keyway for the pulley like stuntman jon said. And you are better off looking at small engine repair shops. Most of them should know where to get pulleys.
Probably not available in the UK, but if can fine Chicago die cast pulleys they are pretty good. Dont get a pulley thats cutout like a pulley for a clothesline. 3rd gear clutch bombs will destroy those pulleys like they were made of plastic. This is about all of my knowledge on pulleys. Best of luck!

"3rd gear clutch bombs" lol

Another source for pulleys would be furnace repair shops or furnace part stores. They won't have an engine pulley but will have pulleys to fit transaxles. Just stay away from aluminum pulleys.

I buy my pulleys from http://www.grainger.com  Good quality, fast shipping. Sometimes can be pricy but they will have the pully you need

I'd aim for 15-20mph. much beyond that and you'll start to lose torque without some supporting mods. with such a narrow and short wheelbase, that feels plenty fast without supporting mods to steering and suspension geometry