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A question for you math wizards and engineers

Discussion in 'Performance and Tuning' started by gosolo, Oct 23, 2016.

  1. Oct 23, 2016 at 2:06 PM
    #1
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    Isn't there a simple equation to determine power loss when upsizing tires?

    Initial HP is fixed but probably unknown.

    Height dictates circumference and distance traveled per revolution.

    Increasing diameter decreases torque by ? %.

    Wondering if someone with better math skills than mine can 'splain why this can't be a simple equation.
     
  2. Oct 23, 2016 at 3:19 PM
    #2
    T-Rex266

    T-Rex266 Elon approved Staff Member

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  3. Oct 23, 2016 at 6:04 PM
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    CM-LENNY

    CM-LENNY No Complaints

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    Hey gosolo, I am pretty good at math and the best answer I have for you is : Re-Gear. My formula for your equation : Bigger than stock tires = lower gears. You more than likely have 4:30 and the only gear swap for our trucks at this time is 4:88. I am running a 34.75" tall tire and put 4:88's in right after the tires. BEST decision of my life. Truck performs beyond expectations. Check with Kieth at Dirty Deeds and he will be the first to tell you, re-gear is the best route after tire increase. Best of luck to you.......Enjoy
     
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  4. Oct 23, 2016 at 8:35 PM
    #4
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    I have regeared trucks in the past and may do so again but for now it is not necessary. I'm actually trying to find out if there is a simple equation to determine the % of power lost by increasing tire diameter a known amount.
     
  5. Oct 23, 2016 at 8:46 PM
    #5
    Wynnded

    Wynnded What MPG...

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    I'm guessing that there isn't a simple formula. There are just too many variables. Each brand of tire is going to weigh different, aftermarket wheel weight, things like that. The fuel you're running, elevation, etc. That's probably where the dynometer came from, a measurement directly at the wheel.
     
  6. Oct 23, 2016 at 8:53 PM
    #6
    joonbug

    joonbug °°°°°°°°°°

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    You would definitely have to factor in weight not just diameter
     
  7. Oct 23, 2016 at 8:56 PM
    #7
    mudsweatngearz

    mudsweatngearz New Member

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    My theory:
    No power loss. The engine will still produce the same amount of power no matter what size tire you use. No torque loss either. Where in the torque curve power is produced may change depending on tire diameter.
     
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  8. Oct 23, 2016 at 8:57 PM
    #8
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    Most of the other potential variables go away if you're staying with the same vehicle. I suppose an average weight increase could be determined?
     
  9. Oct 23, 2016 at 8:58 PM
    #9
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    Sure the engine produces the same power but it isn't delivered at the asphalt.
     
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  10. Oct 23, 2016 at 9:02 PM
    #10
    mudsweatngearz

    mudsweatngearz New Member

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    Why not? Same amount of power reaches the rear axle no matter what.
     
  11. Oct 23, 2016 at 9:02 PM
    #11
    Nor7

    Nor7 Chilton Manual Guy

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    Bigger wheels and heavier tires means larger moment of inertia which mean they take more torque to turn. Think of it like trying to spin a steel rod on it's axis versus trying to spin it like a propeller.

    Also, if the truck is lifted the aerodynamics change, more area is exposed to wind resistance.

    You could do some quick and dirty calculations knowing the diameter and weight difference and get a percentage difference.

    To do more detailed calculations you'd need exact weights and dimensions.
     
  12. Oct 23, 2016 at 9:19 PM
    #12
    dirtydeeds

    dirtydeeds Exhaust Fabricator Vendor

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    This is the assumption that gets everyone into trouble. You're not taking into account the biggest variable.

    The tires are constant. But our trucks aren't carbureted, or the engine would be a constant. Since we have an ecu that is looking for 'load', you have to account for every degree of timing the ecu pulls and every point of afr the ecu adds.

    Figure 10-15hp lost for every degree of timing pulled.

    The ecu is constantly cross referencing throttle position, to mass air volume, to... you guessed it, wheel speed.

    By slowing the wheel speed in relation to the other 2 inputs you're making the ecu see a false level of load. With artificially high load the ecu pulls timing and adds fuel in an attempt to keep our 10.2/1 compression ratio motor from blowing since it assumes we are only running 87 octane.

    I have seen 40 hp missing for a truck running 35's, and when we installed stock tires and drove the truck around the block and put it back in the dyno, it magically had an extra 40hp.

    While gearing doesn't directly add 40hp, it does fool the ecu into not pulling 40 hp;)

    Im sure Toyota engineer Mike Sweers knows better than me, but this is what I've seen in the last 13 years of 'hot rodding' Toyota Tundra's:)
     
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  13. Oct 23, 2016 at 11:23 PM
    #13
    mudsweatngearz

    mudsweatngearz New Member

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  14. Oct 23, 2016 at 11:37 PM
    #14
    dirtydeeds

    dirtydeeds Exhaust Fabricator Vendor

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    Newton had no idea engines would be tuned with electronics. Haha.
     
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  15. Oct 23, 2016 at 11:54 PM
    #15
    mudsweatngearz

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    Still does not change laws of nature....even Toyota can not beat the fundamental laws of nature..hahahaha
     
  16. Oct 24, 2016 at 12:16 AM
    #16
    ColoradoTJ

    ColoradoTJ Certified tow LEO Staff Member

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    Here you go, and this backs up what Keith was saying.

    http://www.4x4abc.com/4WD101/math_wheels.html
     
  17. Oct 24, 2016 at 3:37 AM
    #17
    Dustydirt19

    Dustydirt19 Hoghead

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    If goimg with bigger tires on a 4wd do you have to change front and rear gears?
     
  18. Oct 24, 2016 at 7:46 AM
    #18
    ColoradoTJ

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    Good question.

    Yes, you have to change both front and rear gear sets. Since you most likely have 4.30 gears, what do you think would happen if you installed 4.88 gears in the rear axle only and then placed the truck in 4wd at 10 mph? The front axle would be going faster than the rear right? The transfer case would not last long at all.

    Hope this answers your question.
     
  19. Oct 24, 2016 at 10:40 AM
    #19
    MK44

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    Torque at shaft remains the same. Let's say 100 ft.lbs and your radius is 1 ft (2ft dia tire) so the force applies is 100 lbf. Go with a radius of 2 (4 ft dia tire) the force to the road is now 50 lbf. Let's say friction is 75 lbf basically you do a burn out with little tire and nothing with big tire. By changing the gearing you adjust torque at axle thus equalizing the force for the tire. Again this assumes a lot of constants which doesn't exactly translate to the real world but the principal is correct increase diameter decrease force to road (this does not mean it will adjust horsepower as that is a different unit)
     
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  20. Oct 24, 2016 at 2:38 PM
    #20
    MK44

    MK44 New Member

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    To supplement the comment horsepower is ft-lb per second or torque divided by time... since the torque is fixed and tire diameter does not effect time... no change in horsepower. Just a change in force applied to pavement.
     
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  21. Oct 24, 2016 at 2:42 PM
    #21
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    A lot like what I said above in post 9, but my language lacks your sophistication!
     
  22. Oct 24, 2016 at 2:52 PM
    #22
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    Wow, I missed this aspect entirely. I was focusing only on the loss of mechanical advantage with larger tire diameter. The power loss at the wheel to road interface predates all of the electrical controls you describe. I really appreciate the resources available here on this forum!
     
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  23. Oct 24, 2016 at 4:41 PM
    #23
    Wynnded

    Wynnded What MPG...

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    Oooo! I just remembered another variable to consider with bigger tires......spring wrap....muhahaha!
     
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  24. Oct 24, 2016 at 7:40 PM
    #24
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    Yeah right. And don't forget the alignment of the planets and the occasional solar flare
     
  25. Oct 24, 2016 at 7:46 PM
    #25
    gosolo

    gosolo [OP] You Don’t Know Who I Am But I Know Where You Live

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    am sipping scotch now so I better quit posting. I have a tendency to get sarcastic and occasionally belligerent.
     
  26. Oct 24, 2016 at 7:53 PM
    #26
    Wynnded

    Wynnded What MPG...

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    Dark matter man, dark matter.☄
     
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