Author Topic: Rear end gears  (Read 11682 times)

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Offline SPARKY

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Re: Rear end gears
« Reply #15 on: April 24, 2013, 02:20:11 PM »
there are lots and lots of ways to skin a slow cat ---the faster the cat goes-  :roll: - the fewer options!
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

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Offline fastman614

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Re: Rear end gears
« Reply #16 on: April 24, 2013, 02:31:53 PM »
As a newbie when it comes to gear calculations I have a question or two. I plugged in some random numbers into the squirrel deal. Im basing thse calcs on running a powerglide with no converter with a 1.76 low and high gear of 1.0. Plugging in an 8000 redline, 29 in tall tire, final drive ration of 2.42 its coming up with max rpm of 162 in low and 285 in high. Does that sound right? I understand that its not taking into consideration vehicle weight, frontal area or any of that. Just seemd kinda high

The calculations are a series of equations that not everyone understand some of the "whys" of.....
You will first have to know some data about your running gear combination - or, at least, the one you are "wishing for".... The engine's maximum horsepower and at what RPM is important as the ultimate top speed of the vehicle will be in the vicinity of 3% to 10% beyond that..... The transmission's top gear - is it direct or overdrive?...... The tire diameter (don't worry about growth factors until you are getting past 250mph)

Starting with the tire diameter - you multiply 29" X 3.1416 (this number is designated by the Greek letter Pi - a mathematical "constant" which is the ratio of a circle's diameter to its circumference).... You will arrive at the amount of 91.1064". Divide this by 12 (making the amount into feet) - You will get 7.5922'... Then divide a mile - 5280' - by the tire's circumference... 5280 / 7.5922 = 695.45 - Tire revolutions in a mile. This is the amount of tire revolutions per minute that will be required to go 60 MPH.

Now, in your case, Frankie, my way of structuring the equation to determine top speed will be like this.

285 mph is 4.75 times faster than 60 MPH. So, 695.45 X 4.75 = 3303.39 Tire revolutions per minute. Now, multiply that amount by the gear ratio - 2.42 - 1.... Like this - 3303.39 X 2.42 = 7994.2 RPM.

This gets interesting when you "play" with it.... You can divide the end result by any one of several of the variable and then remultiply by other variable and come up with multiple determinations.... For instance - 7994.2 / 2.42 = 3303.39 X 2.75 = 9084.3 RPM...ooops! way past the redline! So, take 9084.2 / 4.75 = 1912.04 X 4.2 = 8030.6 RPM... 4.2 (60 x 4.2 =) 252 MPH

In order to factor in lower transmission gear ratios, you need to multiply the transmission gear ratio by the rear end ratio and then recalculate as if that is the overall ratio (which, while in that gear, it is)... so, with your powerglide, 2.42 X 1.76 = 4.2592 overall ratio... 162 MPH is 60 X 2.7....So, 695.45 X 2.7 = 1877.72 Tire revolutions... 1877.72 X 4.2592 = 7997.6 RPM....

I hope that explains it clearly enough....
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Offline Frankie7799

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Re: Rear end gears
« Reply #17 on: April 24, 2013, 05:04:23 PM »
Fastman, thanks for the explanation, I appreciate it!

Didnt mean to hijack the thread, carry on folks  :-)

Offline jimmy six

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Re: Rear end gears
« Reply #18 on: June 20, 2013, 11:19:44 PM »
Go to our section on formulas and you will find gear calc. It's a way to see what you may need. You can set it for torque and HP and RPM. I happen to use a Pontiac and have all the ratios from 2.56 up. I have 4 different tire sizes from 26 to 32. I also have trans with a .96 overdrive. Not much I can't do with a pumpkin and tire change.......best to be prepared.....good luck
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Offline SPARKY

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Re: Rear end gears
« Reply #19 on: June 21, 2013, 01:06:21 AM »
JD  :cheers: :cheers: :cheers:
Miss LIBERTY,  changing T.K.I.  to noise, dust, rust, BLUE HATS & hopefully not scrap!!

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

We are going to explore the racing N words NITROUS & NITRO!