Blower pulley question
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Blower pulley question

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    Senior Member propless's Avatar
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    Default Blower pulley question

    This might be a dumb question, but something doesnt seem right here. Help me understand. I was looking at the pulley drive ratio chart on the BDS site and it shows the pulleys I have (61 and 54) to be 11.5 under driven with the big one on top. But 13 over with the big one on the bottom. How is that possible ?? Is there some kind of new voodoo math involved or is there a drive ratio change inside the blower and there comparing crank speed to the blowers rotor speed ?

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    Just Me snoc653's Avatar
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    It's simple math. With the big gear on the top, one crank rotation moves the belt 54 teeth making the formula 54/61=.885 (11.5% under). When small gear is on the top; one crank turn equals 61 teeth. 61/54=1.129 approx rounded up to 1.13 or 13% over.
    Last edited by snoc653; 12-29-2010 at 01:23 AM. Reason: put them on the wrong ends but the math was right
    So many projects, so little time

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    Senior Member propless's Avatar
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    Hmmm, OK, thanks snoc653. I can see it adds up now, and I'm sure its right. But it just doesnt seem right, seems like it should be the same (13 over or 13 under), theres somekind of voodoo going on here.

    So, if I had the big pulley on the top and turned the crank over by hand its going to turn the blower pulley 11.5 % slower than the crank. But with the pulleys in the same place, if I turned the blower pulley by hand it would turn the crank 13% faster than the blower pulley.

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    as stated, the math is correct, BUT, if you read the chart, you will notice that the closer the tooth count is to each other, the closer the ratio under/over is to each other, therefore, a 61/57 combo would give you a -7/+7 ratio
    larger pulley on top turns the blower slower (- under)...smaller pulley on top turns the blower faster (+ over)...i hope this helps

    FastRat

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    gn7
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    Propless, you dealin with inverse percentages. They will play hell with your mind if you let them. Try this:

    You buy a spark plug that normally sells for 2.00, but today you get it for 1.00. What percentage discount did you get.

    The next day you go back, and now the spark is the normal price of 2.00, and you buy it anyway. What percentage more did you pay for the second plug, than you did for the first.

    If the stock market goes down 50%, what percentage does it have to go up to get back where is was.



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    If the stock market goes down 50%, what percentage does it have to go up to get back where is was.[/QUOTE]
    NO NOT again PLEASE

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    Senior Member propless's Avatar
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    Yea, lets not use stocks as an example for anything. To many bad memories there.


    But, just for fun (because its winter) lets look at it like this. Lets say I have a bicycle that is direct driven by a blower belt with a 61 tooth pulley at the pedals and a 54 tooth pulley on the wheel (and no its not on a treadmill ). I'm pedaling along at exactly 10mph (GPS confirmed). Power is being transmitted from the 61 pulley to the 54 so the wheel will be 13 percent over driven, right ? Now I come to a down grade and can coast. I remove my feet from the pedals because its direct drive. Bicycle stays at exactly 10mph (GPS confirmed), so the wheel is turning at the exact same rpm. But now power is being transmitted from the rear wheel's 54 pulley to the pedal's 61 pulley and thats 11.5 under drive. So why are the pedals not rotating at a different rpm, its a different percentage.


    I know the math adds up and its right. And thanks guys for not calling me an idiot, I had to think on it a bit, but I finally got my little brain around it today.

    When did all the cool new smilies show up ??

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    gn7
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    Thats why I used the spark plug deal also. In case the stock market one made you cry. Point is, you only get a 50% discount on the first plug. and yet when you bought the second one, it cost you 100% more than the first one.
    And heres the bad news, if the market goes down just 50%, it has to increase 100% to get back where it was. When was the last time the market increased 100%



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    gn7
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    Quote Originally Posted by propless View Post
    Yea, lets not use stocks as an example for anything. To many bad memories there.


    But, just for fun (because its winter) lets look at it like this. Lets say I have a bicycle that is direct driven by a blower belt with a 61 tooth pulley at the pedals and a 54 tooth pulley on the wheel (and no its not on a treadmill ). I'm pedaling along at exactly 10mph (GPS confirmed). Power is being transmitted from the 61 pulley to the 54 so the wheel will be 13 percent over driven, right ? Now I come to a down grade and can coast. I remove my feet from the pedals because its direct drive. Bicycle stays at exactly 10mph (GPS confirmed), so the wheel is turning at the exact same rpm. But now power is being transmitted from the rear wheel's 54 pulley to the pedal's 61 pulley and thats 11.5 under drive. So why are the pedals not rotating at a different rpm, its a different percentage.


    I know the math adds up and its right. And thanks guys for not calling me an idiot, I had to think on it a bit, but I finally got my little brain around it today.

    When did all the cool new smilies show up ??
    When you were pedaling, it was the REAR wheel that was being over driven. When you coasted, it was the PEDALS that where being under driven. But the pedals are going around exactly the same speed they were when you were pedaling. You are comparing percentages of different rpms. The crank is spinning 11.5 slower than the blowers rpm, OR the blower is spinning 13% faster than the cranks RPM. the percentages are different because the rpms of each pulley is different. The closer you get to 1 to 1 the less difference in the inverse number


    If I take 10% hp from your 300 hp motor, you have 270. If I give you back 10% you now have 297.



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    One of my favorites is if you drive 2 miles and you average 30 miles an hour for the first mile, how fast do you have to drive the second mile to average 60 miles per hour over the entire 2 mile stretch.
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    2 oranges because motorcycles dont have doors..... Dam I get it

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    I see I was right. This place has gone nuts.

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    Senior Member propless's Avatar
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    Quote Originally Posted by gn7 View Post

    If I take 10% hp from your 300 hp motor, you have 270. If I give you back 10% you now have 297.
    But, if I turned the blower faster to get back the 10% that you took and then you gave back the 10% I would have 330.

    Quote Originally Posted by snoc653 View Post
    One of my favorites is if you drive 2 miles and you average 30 miles an hour for the first mile, how fast do you have to drive the second mile to average 60 miles per hour over the entire 2 mile stretch.
    Why would anyone drive 30mph ??

    60 mph = 1 mile per min, 2 miles at 60mph would take 2 min. Total trip time should = 2 min.
    30 mph = 1/2 mile per min, 1 mile at 30mph has already taken 2 min so it cant be done.

    Did I win something ??
    I hope thats right

    Quote Originally Posted by mdsheppie View Post
    2 oranges because motorcycles dont have doors..... Dam I get it
    No No, its 2.3 oranges because motorcycles have the big pulley in the back.

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    That is the correct answer. So I'm tot sure what you win other than another chance to tackle those blower pulleys. There is a geek answer to that, and it would be the speed of light; as it is believed by some that if traveling at the speed of light, time stands still.
    So many projects, so little time

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