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Old 10-15-2009, 08:19 PM
  #11176  
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Originally Posted by Scottrik View Post
Previous poster is correct. Multiplication and division have the same precedence so they are performed left to right. Same holds true for addition and subtraction once all the other operations are out of the way.

P(lease)

E(xcuse)

M(y)

D(ear)

A(unt

S(ally)

is the pnemonic we use to teach the order of operations, generally 6th-8th grade math. P(arenthese or grouping symbols), E(xponents), M(ultiplication and/or D(ivision) left to right, A(ddition) and/or S(ubtraction) again from left to right.

Doesn't matter if calculating gear ratios or whatever. To experiment try your hypothesis out...do the indicated multiplication first and then the division. You will end up with a very different (MUCH smaller, something just a bit over 1.0 in fact) result.
I always calculate it Spur x internal/pinion.
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Old 10-15-2009, 08:20 PM
  #11177  
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Originally Posted by Nick Priest View Post
64/32 would give you 4
61/32 would give you 3.8
Those two ratios won't mesh at all, I've tried 63/35 and the was a massive gap between the spur and pinion, I had to go to 63/38 just to get some meshing.

For running a 17.5, especially without turbo/timing advance, you'll need some big pinions. I'm currently using 70/34 which as room to go either way with the ratios and I'm thinking of going 70/35. This is for a track that takes up two tennis courts. But I'm also using a Tekin RS with the 200 software.
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Old 10-15-2009, 08:27 PM
  #11178  
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Originally Posted by IIGQ4U View Post
I always calculate it Spur x internal/pinion.
Which is yet another way to Tipperary as long as you perform the order of operations as you've written them. Again, do them anything but left to right and you'll be at sea.

Especially with roll-out there are several base formulas you can start with depending what the variable is that you don't know...just keep manipulating the formula algebraically.
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Old 10-15-2009, 08:32 PM
  #11179  
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I thought it was (spur/pinion)x internal



Originally Posted by Scottrik View Post
Which is yet another way to Tipperary as long as you perform the order of operations as you've written them. Again, do them anything but left to right and you'll be at sea.

Especially with roll-out there are several base formulas you can start with depending what the variable is that you don't know...just keep manipulating the formula algebraically.
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Old 10-15-2009, 08:37 PM
  #11180  
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Originally Posted by revo61 View Post
I thought it was (spur/pinion)x internal
Your parens are redundant...you would perform the division first.

AGAIN--there are often several ways to express a formula that will get you to the same result.

Suppose spur 108

Pinion 46

Internal 2.0

spur / pinion x internal yields 1.348 x 2.0 = 4.7

OR

spur x internal / pinion yields 216 / 46 = 4.7
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Old 10-15-2009, 08:48 PM
  #11181  
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Who cares

Long as you get the answer it doesn't matter
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Old 10-15-2009, 08:59 PM
  #11182  
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But what did Aunt Sally do??

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Old 10-15-2009, 09:00 PM
  #11183  
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What you're playing with, algebraically, is across the equals sign. As long as I perform the same operation on both sides I can make an infinite number of equations that will get to the same point. If I multiply both sides by a gajillion the equation looks quite different but yields the same result, thus there are truly an infinite number of ways the formula could be written yet produce the same.

We were given two variants of the formula, though, that are wonderful if you have a bunch of bits you've thrown together already and want to know what the FDR is. I would argue that more often we know the target FDR we want, the internal ratio is a given (can't be changed), and we'd really rather not change the spur unless we absolutely have to. What we USUALLY need to find out is the pinion that will yield the desired FDR. To do so we need to manipulate either of the formulas we've been given (and shown to work) which segregate FDR on one side and get them to show the pinion as segregated on one side.

Let's start with the first formula we were given (easier to get the desired result) and see how we can manipulate it to churn out the pinion we need to hit a desired FDR given internal ratio and spur.

spur x internal ratio / pinion = FDR

Multiply both sides by pinion and we have

spur x internal ratio = FDR x pinion

Now divide both sides by FDR

spur x internal ratio / FDR = pinion

Which I would argue, again, is far more useful in more "real world" situations.

Try it with the numbers I used in my previous example (the stuff that's on my VTA car).
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Old 10-15-2009, 09:03 PM
  #11184  
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Originally Posted by Lebatek View Post
But what did Aunt Sally do??

See...NOW you're starting to sound like one of my seventh graders

I believe she farted. Why else would she need to be excused?
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Old 10-15-2009, 09:17 PM
  #11185  
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really guys??!!!
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Old 10-15-2009, 09:21 PM
  #11186  
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I know they are redundant, but I wanted to make sure they knew to do it first. I never knew you could do it the other way. Learned something new thanks.


Originally Posted by Scottrik View Post
Your parens are redundant...you would perform the division first.

AGAIN--there are often several ways to express a formula that will get you to the same result.

Suppose spur 108

Pinion 46

Internal 2.0

spur / pinion x internal yields 1.348 x 2.0 = 4.7

OR

spur x internal / pinion yields 216 / 46 = 4.7
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Old 10-15-2009, 10:10 PM
  #11187  
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Originally Posted by Scottrik View Post
See...NOW you're starting to sound like one of my seventh graders

I believe she farted. Why else would she need to be excused?
My physics teacher always said it was please excuse my dumb arrogant students
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Old 10-15-2009, 10:18 PM
  #11188  
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Seems kinda harsh...then again if he's saddled with PHYSICS students who need reminders re: the order of operations I can probably understand his frustration.
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Old 10-15-2009, 10:27 PM
  #11189  
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Originally Posted by geeunit1014 View Post
My physics teacher always said it was please excuse my dumb arrogant students
I had a science teacher who asked us to prove the law of gravity. We threw out the 2nd story window.
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Old 10-15-2009, 10:29 PM
  #11190  
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Originally Posted by geeunit1014 View Post
My physics teacher always said it was please excuse my dumb arrogant students
That's highly amusing.
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