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  1. #1

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    How to Calculate Blade Tip Speed

    For mowers with blades directly connected to the engine crankshaft, here's how to figure the feet-per-minute / blade tip speed:

    L = blade length in inches
    R = engine speed in RPM

    L x R / 3.82

    So, a 21" blade, with an engine spinning at 3,100 rpm means:

    21 x 3100 / 3.82
    65,100 / 3.82
    17,042 feet per minute

    FYI, most mower companies stay below the ANSI maximum standard, which is 19,000 feet per minute.
    - - -
    Robert@Honda
    Caveat: I work for Honda, but the preceding was my opinion alone.

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  3. #2

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    Re: How to Calculate Blade Tip Speed

    Quote Originally Posted by robert@honda View Post
    For mowers with blades directly connected to the engine crankshaft, here's how to figure the feet-per-minute / blade tip speed:

    L = blade length in inches
    R = engine speed in RPM

    L x R / 3.82

    So, a 21" blade, with an engine spinning at 3,100 rpm means:

    21 x 3100 / 3.82
    65,100 / 3.82
    17,042 feet per minute

    FYI, most mower companies stay below the ANSI maximum standard, which is 19,000 feet per minute.
    robert@honda,
    Thank you for that information!!!!! Blade Tip Speed has become a very debated subject in some of the threads here with some attributing it as the most important specification in regard to a professional cut quality. I, for one, consider it to be important but believe there is more to cut quality than simply the BTS. And can you clarify that in most Spec Sheets for a ZTR, the FPM numbers are indeed the calculation you gave above? And, that being said, in your opinion is there a specific or even ballpark BTS rating that could be considered the absolute minimum if you expect that ZTR to produce a professional cut quality?????? Some will say that if the BTS is < 17,000 fpm the ZTR is junk when it comes to cut quality. I think there are other considerations like deck design, pitch, leveling etc. Can you provide some feedback on this controversial rating?????

  4. #3

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    Re: How to Calculate Blade Tip Speed

    Quote Originally Posted by djdicetn View Post
    can you clarify that in most Spec Sheets for a ZTR, the FPM numbers are indeed the calculation you gave above?
    The formula I posted was strictly for blades mounted to the engine crankshaft. Most ZTR/riding machines have blades driven by pulleys, belts, chains, gears, etc. so the math would be different.

    Quote Originally Posted by djdicetn View Post
    And, that being said, in your opinion is there a specific or even ballpark BTS rating that could be considered the absolute minimum if you expect that ZTR to produce a professional cut quality?????? Some will say that if the BTS is < 17,000 fpm the ZTR is junk when it comes to cut quality. I think there are other considerations like deck design, pitch, leveling etc. Can you provide some feedback on this controversial rating?????
    Any mower engineer will tell you a faster spinning blade will almost always provide a superior cut and finish quality. You are also correct the deck design and internal aerodynamics (airflow under the deck) are equally important. I'm not aware of any mower manufacturer that ignores the ANSI standard, and all must follow the Consumer Product Safety rules governing thrown objects, which is directly related to blade speed.

    I think most consumers tend to make decisions based on any number they can use to make a comparison. Be it blade tip speed, cutting width, self-propel speed or horsepower, it is simpler to compare different products. If one ZTR runs 17,000, but another runs 18,000, the 18k must be better, right? Sort of depends on what you define as better and no doubt that's a whole other can o'worms.
    - - -
    Robert@Honda
    Caveat: I work for Honda, but the preceding was my opinion alone.

  5. #4

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    Re: How to Calculate Blade Tip Speed

    Quote Originally Posted by robert@honda View Post
    The formula I posted was strictly for blades mounted to the engine crankshaft. Most ZTR/riding machines have blades driven by pulleys, belts, chains, gears, etc. so the math would be different.



    Any mower engineer will tell you a faster spinning blade will almost always provide a superior cut and finish quality. You are also correct the deck design and internal aerodynamics (airflow under the deck) are equally important. I'm not aware of any mower manufacturer that ignores the ANSI standard, and all must follow the Consumer Product Safety rules governing thrown objects, which is directly related to blade speed.

    I think most consumers tend to make decisions based on any number they can use to make a comparison. Be it blade tip speed, cutting width, self-propel speed or horsepower, it is simpler to compare different products. If one ZTR runs 17,000, but another runs 18,000, the 18k must be better, right? Sort of depends on what you define as better and no doubt that's a whole other can o'worms.
    OK, I understand your algorithm was more appropriate for a self-propelled push mower, but one would think that the FPM specs for BTS that a ZTR manufacturer gives is adjusted appropriately for the spindle RPM...right??? And it seems like nobody is willing to "go there" on a definitive minimum number, as you re-emphasized the "can of worms" that opens up. But the deck "design"; including offset/overlapping blades that create overcutting, tunneled baffles to increase aerodynamics/airflow and hanging systems/anti-scalp wheels that ensure the deck stays level with the lawn surface are the "intangibles" that improve cut quality in my opinion. I'm also toying with the recommendation in my Gravely manual of "heeling the deck"(leveling the deck with the pitch reversed....blade tips lower in rear or North/South facing than the conventional lower in front) which purports a better cut quality but requires more power. It's not that I'm displeased with the cut quality I get now(my wife can't get over the difference in the appearance of the lawn versus the old Craftsman lawn tractor) I'm just curious as to whether I can make my lawn the envy of the golf course down the street from me:0)
    And........user Old Goat & I agree on almost everything about ZTR quality.......except BTS ratings:0)

  6. #5

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    Re: How to Calculate Blade Tip Speed

    Quote Originally Posted by djdicetn View Post
    OK, I understand your algorithm was more appropriate for a self-propelled push mower, but one would think that the FPM specs for BTS that a ZTR manufacturer gives is adjusted appropriately for the spindle RPM...right???
    Yes, published BTS numbers are calculated using RPM of the blade spindle vs. engine RPM. My point was for those mowers that don't publish a BTS number, one can be calculated if you know the engine speed, which is usually published.

    Quote Originally Posted by djdicetn View Post
    I'm also toying with the recommendation in my Gravely manual of "heeling the deck"(leveling the deck with the pitch reversed....blade tips lower in rear or North/South facing than the conventional lower in front) which purports a better cut quality but requires more power.
    This technique can often help the final finish a great deal; type of grass, cutting height, etc. all figure in. Just 1/2" one way or the other can be significant.
    - - -
    Robert@Honda
    Caveat: I work for Honda, but the preceding was my opinion alone.

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