The Ultimate Gearing Discussion

[luke91]

OK, I really don't know that much about gearing, and the ratios that describe them, so please enlighten me.  

For drag racing;
why do most people like to have a lot of close gears?  I understand the theory that it keeps you in the peak of the power band, but why doesn't anyone try this:

Three gears:

1st gear:  Short, to get the revs up quickly.  Shift at peak            torque.  
2nd gear:  Very long.  Picks up right where 1st gear left off, but keeps you in the peak torque range for a long time.  Rpm's increase very little, while speed increases a lot.  
3rd gear: same as 2nd, if needed at all.  

I feel like such a loser because I don't know anything about gearing, but why wouldn't this work?

Also, how do you describe gear size, and where one gear connects with the other.  Let me put it this way:  

1st gear remains constant, and there are two differrent 2nd gears:

you could have a very long or very short 2nd gear, and both could join with 1st at the same point.  Let's say in either case, shifting from 1st to 2nd at 7,000 rpm will land you at 5000rpm in 2nd.  How do you explain this, even though the two 2nd gears are different sizes?  I'm confused.  

--luke

[uiuc240]

You can't do what you're talking about.

Gearing is gearing.

In the standard 5 speed box, the ratios are as follows:

1st = 3.321:1
2nd = 1.902:1
3rd = 1.308:1
4th = 1.00:1
5th = 0.79:1

final drive is 4.08:1

So you take 4.08 and multiply that by the internal ratio to get the over all ratio.  Hence, 1st is really 13.55:1...meaning that 7000 rpms at the flywheel will get you 516 rpm at the rear axle.  If you have the stock tires, which have a circumference of roughly 76", you'll be going 37.13 mph.  I don't feel like typing out the math.  Trust me...or check for yourself.

Anyway, then, when you shift, you'll land at a given rpm.  Let's say 5000 for the sake of simplicity.  If you LOWER the number of the ratio, to effectively make the gear LOOOOONNNNNGGER...you will NOT land at 5000 rpm when you shift.  Just imagine going from 1st to 3rd instead of from 1st to 2nd.  That's effectively what you'd be doing.  And if you do that, you'll see that you no longer land at 5000 rpm when you shift.  Make sense?  Try it if you don't understand what I'm saying.

Point is, you can't have your cake and eat it too.  You either have long ratios spaced far apart that require a wide powerband and lots of horsepower...or you have close ratios, a short powerband, and a revvy motor.  You pick.

Personally, for me, I want the 3.92:1 rear end out of a J30 to mate to my SR.  That would lenghten the ratios by 4% or so, and give me just a tad more range in each gear.  Perfect.

End.

Eric

[adey]

That's why having 6 (forward) gears to work with is so great... you can have close gearing from 1-5 for more "pull" in each of those gears, then have a really tall 6th gear for cruising.
What you (luke91) are suggesting is having a tall second, which doesn't work because when you shift from your 1-2, you'll not be in your peak power band anymore.
think of it as a bicycle, it works exactly the same way. (i.e. easy to pedal in 1 gear, but you don't get very far, hard to pedal from a start in 5 gear but you can go fast w/ out peddaling too much.)  
It's near impossible to describe gearing in words, you have to go do some proper research (or improper research) at places like www.howstuffworks.com and similar (preferrably more automotive-related sites).

[luke91]

Gotcha
thanks guys

[DSC]

Here's a lot of reading for ya, some really great info.

gears

gear ratios

manual transmissions

differentials

They should be read in that order to better understand the whole system.

For more on engines, brakes, transmissions and other automotive stuff (even rotary engines) check out the howstuff works automotive section.