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Best Speed

mitch66's Avatar

Join Date: Mar 2005
Posts: 62

06-08-09, 10:11 AM   #1  
Best Speed

I have a Craftsman 18.5HP 6 Speed Rider. What is the best speed to cut at & does it matter?


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Gunguy45's Avatar
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Join Date: Dec 2007
Posts: 20,655

06-08-09, 10:19 AM   #2  
Heres what I've seen mentioned on here several times....

You should run the engine near max throttle and adjust your ground speed using the transmission.

As I understand it, it's because the blade speed is directly proportional to engine speed and they are supposed to spin fast to be effective. Just like a circular saw...they cut great at 5000 RPM...but if you slowed it down to 2000, it wouldn't be nearly as efficient.

I could be wrong...but it's what I've read /heard more than a few times.

"I sometimes wonder how some people ever made it to adulthood..."

cheese's Avatar
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06-08-09, 09:13 PM   #3  
Correct, run the engine at full throttle. 4th gear is usually good for maintenance cutting (when the grass is not too high). The ground speed depends mostly on how hard the grass is to cut, how dull the blades are, how high the grass is, and if you're mulching or not. Basically, go as fast as you can without sacrificing quality cut and bogging the blades or engine too much.

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DaveC72's Avatar

Join Date: Jan 2009
Posts: 549

06-08-09, 09:28 PM   #4  
Thats why I wish my lawn tractor had a tach. Maybe some do ?

I know, on a boat, you find out your WOT (wide open throttle) rpm as per design.. and if you're running WOT at the control but your engine isnt making spec rpm .. then you are overloading the engine (will eventually plug up the ports and plug with carbon crap). In the case of a lawn tractor, you would need to slow down your ground speed (or sharpen blades etc).

I hope my next lawn machine will have a hydrostatic drive. I usually find the 'best' speed seems to be always between gears.

mitch66's Avatar

Join Date: Mar 2005
Posts: 62

06-09-09, 08:13 AM   #5  
Thanks to all of you!!!Beer 4U2

indypower1's Avatar

Join Date: May 2009
Posts: 634

06-09-09, 04:53 PM   #6  
Dave, you can add a tach fairly inexpensive. These are very easy to install. I put 1 on my new Poulan to keep track of the hours. WOT should be 3600 rpm, plus or minus, depending the actual engine.

Tiny-Tach: Gasoline Tachometers

DaveC72's Avatar

Join Date: Jan 2009
Posts: 549

06-10-09, 06:55 AM   #7  
Cool, I didnt know they make small tachs like that. Not sure about the lifetime battery thing.. Id have to snap out the unit each winter (my lawntractor gets stored outside in the -40 during winter).

geogrubb's Avatar

Join Date: Jun 2006
Posts: 2,333

06-10-09, 09:34 AM   #8  
Here is a little Vibra Tac from Germany I use it all the time and it's only about $10-12. Have a good one. Geo
Tecumseh - Vibra Tach / Tachometer 670156

Here is the formula for cutting grass(I stole it from someone).

Formula for mower blade speed
RPM's on lawnmowers are actually set for maximum blade tip speed of 19,000 feet per second, and of course should not exceed the engine mfg specs. for the engine.

To calculate blade tip speed you would have to take the length of the blade say 21" and multiply that by the engine or spindle RPM's say 3200 then take the result (for this example 67200) then multiply this by 3.14 (pie) (211008) and then divide by 12 (inches in a foot) = 17,584 Feet Per Minute.

The maximum blade tip speed as set by government standards is listed as 19,000 feet per minute.

Blade length X RPM's X 3.14 / 12 = Blade Tip Speed in Feet Per Minute.

If you want to figure out maximum rpm's based on blade size then you could do this

19000 (divided by) the result of (Blade length X 3.14 (divided by) 12) = Max RPM's

For a 21" blade the formula would look like this:
19000 / ((21 * 3.14) / 12) = 3457
It carries out several decimal points, but you can just round down for the maximum blade tip speed.

The maximum RPM's you should set a 22" mower for is 3300.

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