Re-gearing question.

[BoostedToy]

Similar to me how the Ridge Grapplers did in snow and ice. They wore pretty well and were much more quiet than expected but they were no beuno in cold weather, slush, snow and ice (KO2’s are not good in ice either).

 

[Old-Raptor-guy]

First off, happy New Years everyone. !


Think I may throw some 39” K02s for a bit.

Just had a new 3rd member put in with the linking so rather NOT re-gear until its rebuild time.

I’m currently running 37” KM3s which weigh in at 77.4 pounds.

39” K02s are weighing in at 79 pounds.

So there’s a 1.6 pound discrepancy.

Truck is currently goose tuned and is QUICK.

Just looking for some real world feedback.

Thanks!

You don't re-gear because of the extra weight. You re-gear due to the loss of mechanical advantage from the larger tires.

A tire is a reverse lever (every time I say that someones head explodes, I believe it would be classified as an extreme example of a third class lever). The tire is driven from the center, the radius of the tire is the lever but it is being driven from the "wrong" end. Think of a 12" ratchet and a 20" ratchet, the longer the ratchet the more mechanical advantage you have right? Now reverse that, the 20" ratchet will take more torque to drive from the center.

lets do some math, it might make more sense. Bear with me it has been awhile since I have done this...................................thinking.............more thinking..............................................

3.5 EcoboostHO makes 510 torque,
that is input into a 4.69:1 first gear which is 2391.9 ft/lbs of torque to the transfercase, which in 2/4hi is 1:1 so 2391.9 output to the rear pinon, which then multiplies it 4:10 which puts 9806.79 ft/lbs of torque to the axle shaft it self.

I am going to use an axle diameter 1.5 inches, tire diameter of 35 and then 39.

the radius of the axle is .75" (assuming a 1.5" dia axle) convert that to feet is .0625
the radius of the tire is 17.5 convert that to feet is 1.4583333

9806.79 to the axle times the distance from the fulcrum (radius of the axle) .0625 = 612.92 ftlbs to the axle flange then from center of flange to outer edge of the tire is 612.92 / 1.4583333 = 420.29 ftlbs to the pavement (assuming no mechanical loss of course)

change the tires to 39 and the diameter becomes 19.5" which is 1.625 ft.

now 612.92 / 1.625 = 377 ftlbs to the pavement. a loss of 43 ftlbs. This is where the loss of power comes from, trust me it isn't an extra 5lbs per tire.

Now there is an advantage if you want to call it that, the tire will spin slower at a given speed which will follow backwards through the drive train, and result in lower engine RPMs at that speed. With 3 overdrive gears that really isn't needed, it isn't 1984 (well politically yea but that is a different thread).

If you rear gear correctly, you will regain the torque lost to the tires and the engine will be turning the same rpm and factory intended.

4:88 seems a bit to much, off hand, lets check the math.

torque to axle shaft goes from 9806.79 to 11672.47
to the ground with 35s 500.24ftlbs
with 39s 449 ftlbs That is a little much, a gain of 29 ftlbs which seems small but every ftlb is huge as you can see. I would have to do the math backwards, but the engine would probably be spinning about 300 rpm over stock at freeway speed.

is there a gear available in between 4:10 and 4:88?

I know, I know I bring up my F350 a lot but it is a great example. Back in the day, everyone and I mean pretty much everyone (around here) lifted their F350's to fit 35's (normally 6 in the front 4 in the back, that is what mine is) I call it the Utah suspension package. Well 22 years ago trucks cam from the factory with 245 and 265 tires on 16" wheels. Tires in the 30-31 " range. going to 35 cost a lot of power.

Everyone (but me) would "chip (called tune now)" their truck to get the power back. But that has a cost, of more load put on the drive train and we saw shit ton of transmission issues. The engine could handle the power for the most part if not abused 24/7, but the transmissions could not handle the added power.

knowing the powerloss was from the loss of mechanical advantage I rear geared my truck. Factory gearing was 3:73, my math said I needed 4:20 but the choice was 4:10 or 4:30. I went with 4:30. When people get in my F350 they comment on how many RPMS I am turning at 80 which is about 2250, which is about 250 more than they are used to, BUUUUUTTTT if you do the math I am only turning 50 more RPM than the factory tires with factory gearing, right where the engine was designed to run (250 rpm on a diesel is quite a bit). No I don't drive it 90, I am comfortable at 75-80 in it.

Gearing the truck made a huge difference, no one thought it would but when I tried to explain they didn't seem to understand. Most people who ride in it think I am tuned, and I can run with a tuned truck all day without overheating issues (mostly transmission).

all the math above was based on 1st gear on a gen2 with 10 speed.

if you want to add 4lo that is fun, 2.64:1 multiplier. 510 to 2391.9 to 6314.61 to the pinon then 25,899.92 to the axle is 1109.57 to the pavement in first gear 4 lo.

I will 100% admit that I might be slightly off by a % or 2, but I am 95-99% there and gives the idea of gearing, and tire size and how they go together. If someone wants to correct me, that is fine, but understand you are mostly doing it for your own ego (there is always that one guy, hell I am that guy from time to time)

 

[Old-Raptor-guy]

One more thing to kind of put the "tire weight" thing to bed. Yes the extra tire weight makes a difference, but that would become more of a thing as tire speed increases.

Do you believe a Jeep with 40s made to climb rocks in Moab, that is trailered to and from. That never goes over 20 mph was regeared to 5.10:1 because the tires are heavy?? (oh god please say no!)

 

[Nex]

That's a lot of math and thinking before coffee. Thanks for the lesson.

 

[Gumby]

The math seams good to me. Nice job @Old-Raptor-guy