Physics Engineer

[NCJL]

I think I need a Physics Engineer.
Problem.
Trying to figure out the HP loss when increasing tire size and gear ratios together. Lots of calculators out there when increasing tire size without changing gears.

For example. I re-geared my JT to match a factory JT Sport with EcoDiesel. 3.73 gears with 31” tires.
My JT has 5.13 gears with 42” tires. The RPM’s basically match each other at speed.

How much HP did I lose? I had to lose something.
With all the rolling resistance talk going on. I found calculators showing HP loss when going from all-terrain to mud terrain, up to 5hp using the same size tire.
Can’t find anything addressing the diameter, width and weight change of a tire within the same RPM band.

Thanks.

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[ShadowsPapa]

I think I need a Physics Engineer.
Problem.
Trying to figure out the HP loss when increasing tire size and gear ratios together. Lots of calculators out there when increasing tire size without changing gears.

For example. I re-geared my JT to match a factory JT Sport with EcoDiesel. 3.73 gears with 31” tires.
My JT has 5.13 gears with 42” tires. The RPM’s basically match each other at speed.

How much HP did I lose? I had to lose something.
With all the rolling resistance talk going on. I found calculators showing HP loss when going from all-terrain to mud terrain, up to 5hp using the same size tire.
Can’t find anything addressing the diameter, width and weight change of a tire within the same RPM band.

Thanks.

You can't figure that because you don't lose HP. You take more HP with more WEIGHT, wind resistance (which varies with tires! as well as the Jeep height)
The HP is still there - it goes to increases in rolling resistance, height, frictional losses of various gear sizes and other factors.

You would have to know the rolling resistance of that TIRE BRAND, tread pattern, rubber compound and more. You can't figure that stuff.

We can take two "identical tires" in size/diameter, width and so on. both of them being A/T types - and tire A may have a very different rolling resistance vs. tire B. Even the state of CA is looking into this in proposed laws. Tire brands and rubber compounds and so on matter- not just sizes.

Unless you are racing, you are going down a deep rabbit hole and even then unless you know the exact rolling resistance of the MODEL of tire you are buying - it's all a guess. You can't plug in a tire size and know what you want.

What is the goal, seriously - why the need to know that number?

 

[Gvsukids]

Your engine produces the exact same horsepower at the crank, but what reaches the pavement during acceleration or shows up on a chassis dyno tells a very different story.
Because your effective final drive ratio matches almost identically (\frac{3.73}{31} \approx 0.120 vs. \frac{5.13}{42} \approx 0.122), your engine runs at the exact same RPM and power band at any given road speed. The losses you feel come entirely from physics downstream of the engine:

 

[Vtur]

No Physics Engineer would own a Jeep so you probably won't be getting an answer from one .

We are doing nearly everything against physics here.

 

[IRAH]

I'm certainly no engineer. Ex army truck driver/examiner here, but one thing I learned is there's a lot more people out there that know things I dont.

But I found this!

https://gearsmagazine.com/magazine/...ain-concerns-to-address-with-oversized-tires/

This seems to make sense to me. What do you think?

(TLDR, It was found that the old jeep 3.8 lost about 3.5% of torque per inch of tire size increase)

 

[Master_Cylinder]

I had physics in high school.

 

[RougeShot12er]

I stayed at a holiday inn

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