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  1. #1
    Banned SSTibet's Avatar
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    Default Rear End Ratio & Acceleration

    Can someone here explain the following with numerical examples:

    1. Why does a higher rear end ratio result with better accelaration?

    2. Everything else being equal, does it suffice to change to a higher rear end ratio for better accelaration? Or is it also necessary to get larger diameter rear tires?

    Need education to understand the basics as usual

    Cheers all

  2. #2
    Rear End Ratio & Acceleration 67HEAVEN's Avatar
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    Quote Originally Posted by SSTibet
    Can someone here explain the following with numerical examples:

    1. Why does a higher rear end ratio result with better accelaration?

    2. Everything else being equal, does it suffice to change to a higher rear end ratio for better accelaration? Or is it also necessary to get larger diameter rear tires?

    Need education to understand the basics as usual

    Cheers all
    1. It provides for the multiplication of available acceleration torque at the expense of attainable top end mph.

    2. Larger diameter (height) tires will defeat the steps taken in #1. It's equivalent to lowering your ratio numbers again.

    ____________________________

  3. #3
    Rear End Ratio & Acceleration DRTH VTR's Avatar
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    The ratio describes how many times the driveshaft goes around for each revolution of the wheels. If you think in terms of a bicycle and it will make sense. If a gear that allows many pedal strokes for every revolution of the wheel is selected, it will allow better acceleration than a gear that allows only a few pedal strokes.

    I think it would cause problems to change the tire diameter very much. It would not be that easy to do that, anyway. Bigger wheels are usually fitted with lower profile tires, so the diameter of the end product is similar to what you get with the smaller wheels.

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    Banned SSTibet's Avatar
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    I am not getting this... A 3.5 (per say) rear end means that for every 3.5 revs of the drive shaft the wheels will rotate one time. Yes? And a 2.0 (per say again) rear end means that for every 2 revs of the drive shaft the wheels will rotate once... Okay???

    So, everything else being equal, the car with 2.0 ratio is going faster than 3.5 car at any given rpm... Yes? So, where is the better accelaration???

  5. #5
    Rear End Ratio & Acceleration 67HEAVEN's Avatar
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    Quote Originally Posted by SSTibet
    I am not getting this... A 3.5 (per say) rear end means that for every 3.5 revs of the drive shaft the wheels will rotate one time. Yes? And a 2.0 (per say again) rear end means that for every 2 revs of the drive shaft the wheels will rotate once... Okay???

    So, everything else being equal, the car with 2.0 ratio is going faster than 3.5 car at any given rpm... Yes? So, where is the better accelaration???
    Does your car accelerate better at 2,000 rpm or 4,000 rpm?

  6. #6
    Rear End Ratio & Acceleration 67HEAVEN's Avatar
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    Think back to the example of the bicycle. Is it easier to accelerate from a standing stop in 1st gear or in 4th gear?

    The higher the gear ratio (numerically), the easier the engine can turn those rear wheels.

  7. #7
    Gone but not forgotten Ken's Avatar
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    Quote Originally Posted by SSTibet
    I am not getting this... A 3.5 (per say) rear end means that for every 3.5 revs of the drive shaft the wheels will rotate one time. Yes? And a 2.0 (per say again) rear end means that for every 2 revs of the drive shaft the wheels will rotate once... Okay???

    So, everything else being equal, the car with 2.0 ratio is going faster than 3.5 car at any given rpm... Yes?
    In a word, yes.

    Now listen to Bob.

    Don't make me get the technical explanation out!

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    Quote Originally Posted by Ken
    Don't make me get the technical explanation out!
    Ken, please get the "technical explanation out". Pleaaaaase

  9. #9
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    Quote Originally Posted by 67HEAVEN
    Think back to the example of the bicycle. Is it easier to accelerate from a standing stop in 1st gear or in 4th gear?

    The higher the gear ratio (numerically), the easier the engine can turn those rear wheels.
    Hmmm, so you are making an analogy here . Okay, okay. Of course the car gets better accelaration at 1st gear than any other gear. That is where there is the highest reduction between the "engine rpm & transmission rpm". And the relationship is similar between "drive shaft rpm & rear wheel rpm".

    In other words, more reduction = more torque = higher accelaration. Hmmmm, sounds okay to me.

    So, if I were drag racing (per say.... never will ), and everything else being equal I would go to a higher rear end ration differential and I will beat the s**t out my opponent? And both cars will have same tire size?

    Cheers

    P.S. Ken, where can I read the theory for this???

  10. #10
    Gone but not forgotten Ken's Avatar
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    Quote Originally Posted by SSTibet
    Ken, where can I read the theory for this?
    I dunno... Right here?

    Let me try this and see if it helps any.

    Gearing is the key to delivering useable power from an engine through the drivetrain and to the ground. Gear ratio selection must take vehicle speed and engine speed into consideration and is always a compromise between peak horsepower, torque and fuel efficiency.

    Power is transmitted from the engine through the driveshaft to the ring & pinion gears in the rear end. The numbers in a differential gear ratio indicate the number of revolutions that the pinion gear makes (driveshaft - 1st number) for each revolution of the ring gear (axles/rear wheels - 2nd number).

    Larger tires on the rear are more difficult for an engine to turn because they cover more ground per-revolution than smaller tires.

    Gears cannot MAKE power. Any increase in low-end acceleration achieved by installing lower (higher numerically) gears will result in a comparably decreased top-end performance (lower speed due to higher engine rpms - the limiting factor). Running an engine at high rpm’s for extended periods of time will result in decreased fuel efficiency and increased wear on engine components.

    Numerically higher gear ratios are often referred to as ‘lower gears’ and both are correct. The confusion can be avoided by explaining that a numerically higher gear ratio effectively provides more gear reduction than numerically lower gears.

    The easiest way to check the differential gear ratio on any rear wheel drive vehicle is to lift the rear end of the vehicle with a jack and physically rotate the rear tires one complete revolution while counting the number of revolutions that the driveshaft makes. If the drive shaft rotated just over four times, your differential gear ratios would be 4.10:1, etc.

    Any better?

  11. #11
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    Yeah somewhat better... Thanks all.

  12. #12
    Rear End Ratio & Acceleration DRTH VTR's Avatar
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    Quote Originally Posted by SSTibet
    So, everything else being equal, the car with 2.0 ratio is going faster than 3.5 car at any given rpm... Yes? So, where is the better accelaration???
    Assuming adequate power to bring it to speed, the 2.0 gears set would produce a higher TOP SPEED than the 3.5 would. However, acceleration would just not be as good. I think in terms of how many times the power stroke on a piston has a chance to force rotation of the driveshaft per unit distance. If a vehicle was geared to cover 100 ft with 50 revolutions of the crankshaft and another vehicle was geared to have 500 revolutions of the crankshaft to go the same 100 feet, then the second vehicle would have ten times as many power strokes to apply to acceleration. More force due to more power strokes means more force to accelerate. F=MA Mass stays the same, so if force goes up, acceleration goes up as a direct result.
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