TOTM: Gear Tech 101

    Quote from "Da Mutt"

    I think but wont sware to it that the reason that the lower gears break easier is because the ring gear teeth are thinner/smaller trhan on the higher gears this is one reason that I went to and decided to stay at 3.73. hoepfully the v8 will help with my street performance



    Actually the ring gear is thicker when numerically higher. The problem is that the pinion gears get real small and the tooth contact between the r&p decresses. This causes a lot of force to be transmited through a thin contact patch. Keep in mind that if you have a larger diff like a d60 that when the pinion shrinks it still has a ton of contact. With a d30 it is kind of scary. If you have a 4 banger though you will not break because you don't have enough power.


    BTW, the v8 will have more torque and hp, but you still want to stay with in it's power curve.

    wait so bigger gears are gonna make me slower on the highway? or will i slow down with my 4.10's and 32 and need to go to 4.56's to gain my speed back and to get my mpg back as well?


    (getting confused again) :shrug:

    bLaTaNt DiSrEgArD<br /><br /><br />TJ 98 4 banger gone but not fogotten<br />WJ 02 limited- Do all vehicle &quot;QUICK THROW A LOCKER AT IT!!!&quot;

    numerically higher gears are going to increae your eng rpms and increase your vehicles mechanical advantage. I will try to explain..



    lets say when your jeep was stock it had 30 in tires and 4.10 gears and ran 2500 rpm at 65. this is where the factory set it up to run because it is at its optimum rpm for a balance between power and fuel economy.(ie within your power band)


    now lets say you have put on your 32 in tires your new rpm at 65 mph is 2100. this is now out side of you power band is one reason you feel a power loss.


    also the mechanical advantage that your tranny/ transfer case/ rear gear ratio has been decreased. this is called your final drive ratio. Let me see if I can figure out how to explain this.


    when your motor turns it makes the tranny turn for each revolution of the motor the tranny turns at that ever ratio it has for the gear it is in.


    for example drjs tranny ratios are
    1st = 2.84
    2nd = 1.57
    3rd = 1.00
    4th = 0.69


    so in other words for every revolution of the motor it turns the out put of the tranny 2.84 times when in first gear and 1.57 times in 2 and so on and so on. this gives you increased torque by means of mechanical advantage. a transfer case that is in 2 wd or 4wd hi has a 1 to 1 ratio so mechanical advantage there and then you get to the rear end. when you get to the rear end the rear diff does 2 jobs first it changes the direction of the rotational force of the drive shaft and transfers it to the rear axles it also multiples your torque again by what ever your rear ratio is so for every turn of your drive shaft it turns you axle shaft or tires 4.10 times.


    when you increase tire size you increase the circumfance of the tire and thus increase the distance required to complete one revolution and decrease your mechanical advantage in the process. that is why regearing gives you back the power in all gears and all while cruising at highway speeds and is also why increasing eng hp and torque although help wont replace gearing.



    if anyone sees anything wrong with this please point it out to me. debate is how we come to final answers. I hope this helps clear up some of your question omaha. if you have any more questions and are free this weekend I will be in the garage I know I can explain it much better in person

    wish i could come down i have to twork all weekend but i am beginning to see the light at the end of the tunnel so i will continue ot read the posts and gain everlasting jeep knowledge

    bLaTaNt DiSrEgArD<br /><br /><br />TJ 98 4 banger gone but not fogotten<br />WJ 02 limited- Do all vehicle &quot;QUICK THROW A LOCKER AT IT!!!&quot;

    Quote from "Da Mutt"


    so in other words for every revolution of the motor it turns the out put of the tranny 2.84 times when in first gear and 1.57 times in 2 and so on and so on. this gives you increased torque by means of mechanical advantage. a transfer case that is in 2 wd or 4wd hi has a 1 to 1 ratio so mechanical advantage there and then you get to the rear end. when you get to the rear end the rear diff does 2 jobs first it changes the direction of the rotational force of the drive shaft and transfers it to the rear axles it also multiples your torque again by what ever your rear ratio is so for every turn of your drive shaft it turns you axle shaft or tires 4.10 times.


    You've pretty much got it all backwards.


    In first gear there are 2.84 revolutions of the input shaft (engine) for each rev. of the output shaft.


    Same at the rear, there would be 3.73 rev. of the drive shaft for each rev. of the axle shaft.

    Did you ever have a 10-speed bike? (please say yes or this explanation wont work) In LOW gear, you could spin the pedals very fast, and climb any hill, but you could only go about 2 mph because you couldnt spin your legs any faster. If you put the bike in high gear (closer to a 1:1 ratio) It was really hard to get the bike started, you had to apply alot more power to get it moving, but you could max out your speed at 60mph.


    Dont worry, I dont think anyone here was explained this just once and was like ,"Oh yeah, I get it" Just retain as much as you can about this. One of these days this will turn on like a light bulb for you, and THEN youll understand it. Did you look at the animation on my earlier post?

    Quote from "Keith_C"

    You've pretty much got it all backwards.


    In first gear there are 2.84 revolutions of the input shaft (engine) for each rev. of the output shaft.


    Same at the rear, there would be 3.73 rev. of the drive shaft for each rev. of the axle shaft.


    :doh: your right thats what I get for tring to explain things at 1 in the morning. thanks for correctin my mistake

    Quote from "quadna71"

    besides, my experience has shown that having the rpm range slightly lower at highway speeds in fact makes your gas mileage worse. the engine is working hard to try and sustain that freeway speed. but when you run the rpm's higher - it may be revving alot...but not requiring alot of work to keep the speed up. that make any sense?


    I noticed that too. I havn't exactly been able to figure out my gas mileage at different speeds yet, but I know that when I went from my 31" (which were just like the 30" tires) to my 33" MT/Rs, I havn't been able to get above 14.5 mpg on the highway.


    The http://www.howstuffworks.com is a great web site to learn the basics. However, I havn't been able to find anything that specifically says (and if I did I forget) whether or not the amount of gas inputted during the intake stage ever changes. I always thought that the amount of gas put into the cylinder was the same. (like, you're not using more gas when you put your foot down, you're just using the same amount of gas more often, and when going uphill you're not really using more gas, but when your auto downshifts and the rpms fly up you are) Which leads me to believe that your best gas mileage is gotten at lowest speed (rpm) in top gear. The HowStuffWorks website throws in a couple other factors into the equation but I've basically always tried to follow the lowest rpm thing when I want better gas mileage.


    The thing is, with that said, you would think that since I put bigger tires on, I would be getting slightly better gas mileage now that each rotation actually takes me that much farther. But, the tires obviously weigh a whole lot more than the old ones so the amount of power needed to turn the tires is greatly increased. I thought that throwing in slightly lower gears (numerically higher, go from stock 3.73 to 4.11) would help, but throwing in those gears will raise the Rs (and gas mileage?) for the same speed as before.


    Likewise, if I were to throw 4.56s in wouldn't that mean there'd be a lot less stress on the engine but to go anywhere I'd be using a lot more gas for those higher rpms?


    Someone with a jeep with 33s (mud terrains) and 4.56s (who changed the speedo gear also, as I have) said he gets the same gas mileage he did when it was stock, on the highway too. I almost have a hard time believing him because it seems weird that he gets better gas mileage than me when I have 3.73s and he has 4.56s. I also have a hard time believing him because he said one of his family members has run a stock Dana 30 in a number of races while putting over 700hp to it and hasn't broken it yet. (Maybe the stock housing?)


    So, does anyone really know what's going on with the gas mileage?

    Quote from "Solomi"

    I also have a hard time believing him because he said one of his family members has run a stock Dana 30 in a number of races while putting over 700hp to it and hasn't broken it yet. (Maybe the stock housing?)


    It would depend what type of racing. To be running 700HP I'd guess some type of drag race or maybe thru mud.


    Because of the weight transfer, most of the traction will go to the rear.

    Just a helpfull tip from someone who has set up hundreds of gears. Never buy used gears unless your willing to replace them in a nother year or less, You can never get the same contact pattern and you end up wearing a new one into old gears leaving 2 patters and your .003 - .006 will just keep getting bigger and bigger and louder and louder.
    Tip # 2 (then I will shut up)
    Always re-use the pinnion shim that came out of rear your taking appart. This shim on your new pinnion will 99% of the time give you your desired pinnion debth.
    #3 C clips Kill they were outlawed in dragracing many many moons ago. (well if your going fast enough) so upgrade your axles and put a c-clip eliminator kit on. (new housing ends and bearings for the axles)
    Speeking of which anyone have any 30 and 35 gears in the 4.11 range?

    I think i have a set of Superior 4.11s from my Super 35 sitting in my basement with 150 miles on them.

Participate now!

Don’t have an account yet? Register yourself now and be a part of our community!

Register Yourself Login