Blade tip speed on a 21"

[Doogiegh]

Does anyone know the typical blade tip speed on a 21" mower, say a Toro 21" or a craftsman 20-21" homeowner grade stuff?

And lets say you are using blades that are 18 inches long.. How "MUCH" of the blade do you think actually does the cutting? The outside 4-5 inches on each end? so the middle 8-10 inches near the center hole really don't do much cutting, right?

How far down the ends do you sharpen? Gonna do some math on your numbers.. Thanks

Gary

[Bob Minney]

My ariens 21" approx 18,000 fpm @ 3,000 rpm
OEM blade has 3.5" cutting edge
Gators 5"

[barnard]

It's easy to figure on a walk behind with the blade attached directly to the crankshaft. Formula is blade lengthX3.14Xengine rpm. Example with 21 inch blade and governed engine speed of 3,600 rpm. 21x3.14=65.94 round to 66 this is inches traveled at blade tip in one revolution of engine. convert to feet by dividing by 12. This is 5.5feet per rev. Times 3,200=19,800fpm

[Likestomow]

I only sharpen the outer 1.5" of every blade I use.

[Lawn Specialties]

Advertised blade tip on a Toro 21" is 18500. I sharpen mine same as they come from the factory but have thought of going in further. Any body tried it?