The great SFPM Debate

[rides20]

..Alright everyone bear with me here... I'm trying to get into the "nitty gritty" of polishing, sure I can make stuff shiny BUT!....I'm confused about the whole SFPM thing....
Caswell's manual shows a formula of (1/4 dia. of wheel X Motor RPM) =SFPM, and for best results use a SFPM speed of 3600-7500.....Here's where the confusion comes in: If you use a 6" wheel on a 3000 RPM motor, you're turning 4000 SFPM...if you use an 8" wheel on the same speed motor, you're turning 6000 SFPM. How can you have a higher Surface speed, if having a larger diameter wheel theoretically is turning slower than a smaller diameter wheel running with the same SPINDLE speed

[fly]

Your thinking of RRM's, it takes a 6" wheel longer to make a complete round per minute than it takes a 8". Look at the ratio, when you go to a 2" larger wheel you step up 2000 SFPM. Its not talking about RPMs its talking about SFPM..
Say the smaller wheel has to make two rounds to stay the same as the larger wheel making one round.

Does this make since (NEWBIE)
Just watch your rpms and you will be fine....

[fly]

Hey Newbie lets try the flash chat....

[rides20]

If you have 12" rims on a car, and you travel at X rpm...then you put on a set of 20"s (polished ofcourse!) at the same RPM, the 20s are going to GO farther in the same duration of time, but will actually make fewer rotations than the 12's...This sounds like the S.A.T's ! HAHAHAHAH..And ehhhhemmmmm....... I've been "upgraded" to Amateur. HAHAHAHAHAh

So all in all....the formula says Slower is better??

[fly]

Yea just check your mph and you will see, ask the cop next time he stops you....Ha Ha Ha.....

[sswee]

From years in the machine shop, let's see if I can clarify. The formula Caswell gives is a short cut for the calculations. The long way is to find the circumference by multiplying Pi times the diameter. Multiply the answer by the RPM running. This gives you the IPM (inches per minute). Divide this by 12" to get the SFPM (surface feet per minute).

On autos one tire size is approx. 3 mile difference on the speedo.

6" wheel at 3000 RPM's

3.14 * 6 = 18.8495" circumference
18.8495" * 3000 = 56548.6677" IPM
56548.6677" / 12 = 4712.3889' SFPM

6" / 4 = 1.5
1.5 " * 3000 = 4500

The Caswell formula is more closer to actual since when you bear into a wheel you are not running at the 6" diameter but at 4" to 5".

SS

[rides20]

But isnt it still spinning slower by using a larger wheel?

[pickleboy]

no bro you are still thinking in terms of rpm. the wheel is still spinning at the same speed. if you had a 4" wheel and turned it at 3000 rpm,it would be 3000 sfpm. and a 8" wheel at 1500 rpm would be 3000 sfpm. like your example of the wheels on a car. now spin them both at the same speed and you get 4"=3000 sfpm 8"= 6000sfpm. clear as mud right.

[sswee]

No, a motor turning 3000 RPM will turn 3000 RPM no matter what size wheel is on it. The larger the wheel the higher the SFPM.

If you put a larger wheel on a motor and it turns slower than with a smaller wheel on the same motor.
1. the wheel is too large for the motor. (a Chevy 350 in a Kenworth)
2. the motor is weak and fixing to burn up.

[pickleboy]

were you talking to me or him on this?