Tire weight vs heights impact on inertia

SleepEatJeepRepeat

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So I see this talked about on the site often, and people regularly say that tire height effects breaking and stopping, toll on drive train more than weight because of moving inertia. They then recommend regearing if you go 37s. I am planning on replacing my 35” ridge grapplers with 37” ko2. I felt this gave me a light 69lb tire, taller tire, better snow performance, and a light better road ride and trail flex (c ply version comes out in April) The 35” grappler is e ply very stiff and very heavy weighting in at 75lbs (it has been a great tire, silent smooth and looks awesome) .. a few folks on here were telling me the larger ko2 would drive worse because of the large size and spinning inertia. I wrestled with for two days, I kept thing that the amount of weight spinning had to matter very significantly as well as the size of the spinning disc. So I found a spinning disc inertia calculator online and took the specs of the manufacturer website using inflated diameter and spec weight. It actually confirmed my suspicion that the amount of weight spinning has significant impact.. the shorter heavier ridge grapplers actually has slightly more rotational inertia than the taller lighter k02.. thought I would share, to help people get what they want out of there builds :)

1629DDD9-BF2B-441C-854F-90AB3601B0F6.jpeg


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Jebiruph

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I like this, but the problem is that the mass of the tire is not consistant from the wheel to the outer diameter. Each tire needs two calculations, one for the sidewall section and one for the tread section. Since you don't have separate mass calculations for the separate sections, it would probably be more accurate to recalculate the inertia assuming the mass is within 1" of the outer diameter. Reducing the diameter by the thickness of the tread might get a more accurate calculation for comparisons.

Edit: Instead of all the weight, estimate a percentage (50% ?) of the weight that's in the outer 1" of diameter.
 
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SleepEatJeepRepeat

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I like this, but the problem is that the mass of the tire is not consistant from the wheel to the outer diameter. Each tire needs two calculations, one for the sidewall section and one for the tread section. Since you don't have separate mass calculations for the separate sections, it would probably be more accurate to recalculate the inertia assuming the mass is within 1" of the outer diameter. Reducing the diameter by the thickness of the tread might get a more accurate calculation for comparisons.

Edit: Instead of all the weight, estimate a percentage (50% ?) of the weight that's in the outer 1" of diameter.
The calculator I was using was specific to tire usage
 
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I like this, but the problem is that the mass of the tire is not consistant from the wheel to the outer diameter. Each tire needs two calculations, one for the sidewall section and one for the tread section. Since you don't have separate mass calculations for the separate sections, it would probably be more accurate to recalculate the inertia assuming the mass is within 1" of the outer diameter. Reducing the diameter by the thickness of the tread might get a more accurate calculation for comparisons.

Edit: Instead of all the weight, estimate a percentage (50% ?) of the weight that's in the outer 1" of diameter.
But I also thought along the same lines my grapplers have very deep tread that means morr rubber on the outter edge , meaning more weight on the part of the wheel with the most inertia. Especially with off-road tires that is common. So the k02 per the two spec sheets does not have as deep of a thread, which is where the weight saving comes from (they are also thinner) And I don’t think the tire inertia calculator I used was set up to calculate design details like that. So my hope when I get everything installed, is that the weight savings will be more dramatic. But atleast from this I can feel pretty safe it won’t drive any worse than it does now.
 

Jebiruph

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But I also thought along the same lines my grapplers have very deep tread that means morr rubber on the outter edge , meaning more weight on the part of the wheel with the most inertia. Especially with off-road tires that is common. So the k02 per the two spec sheets does not have as deep of a thread, which is where the weight saving comes from (they are also thinner) And I don’t think the tire inertia calculator I used was set up to calculate design details like that. So my hope when I get everything installed, is that the weight savings will be more dramatic. But atleast from this I can feel pretty safe it won’t drive any worse than it does now.
I'm not a tire expert, but I'm assuming the highest density of weight is in the belts under the tread, that's why I suggested subtracting the tread.

I bought the white CJ replica wheels thinking my stock tires would fit, but I need wider tires. I found wider tires with close to the same weight and diameter to not adversely affect performance. But now that I think about it, with two tires the same weight and diameter but different widths, the wider tire will have a higher percentage of it's weight at the outer edge, so more inertia. Not a significant difference for my jeep, but people go nuts over this type of thing in the bicycle world.
 
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I'm not a tire expert, but I'm assuming the highest density of weight is in the belts under the tread, that's why I suggested subtracting the tread.

I bought the white CJ replica wheels thinking my stock tires would fit, but I need wider tires. I found wider tires with close to the same weight and diameter to not adversely affect performance. But now that I think about it, with two tires the same weight and diameter but different widths, the wider tire will have a higher percentage of it's weight at the outer edge, so more inertia. Not a significant difference for my jeep, but people go nuts over this type of thing in the bicycle world.
Ya well I would hate to ride a bike that was running 37” ridge grapplers lol
 
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I don't know if you've seen any of the fat tire bikes, but it looks like they're getting there.
True they have them all over heat guess they do good I. The sand in the beach..
 

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The calculator I was using was specific to tire usage
Cool find, I've added this to my list of calculators:
https://bndtechsource.wixsite.com/home/rotational-inertia-calculator

However, you were using the one for an annular disk (disk with a hole in the center). You needed to scroll down a bit and find the tire specific calculator. The big unknown is the ratio of weight in the tread compared to the weight in the sidewall. We have no way of knowing if this is higher or lower for off road tires until someone cuts a brand new tire up and weighs the parts. :)

I was surprised that the two tires you are looking at have nearly identical rotational mass, but the taller tire will still create less leverage for the brakes and engine, so you are going to see a difference no matter how much you sharpen your pencil...

Tire1.jpg


Tire2.jpg
 
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SleepEatJeepRepeat

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Cool find, I've added this to my list of calculators:
https://bndtechsource.wixsite.com/home/rotational-inertia-calculator

However, you were using the one for an annular disk (disk with a hole in the center). You needed to scroll down a bit and find the tire specific calculator. The big unknown is the ratio of weight in the tread compared to the weight in the sidewall. We have no way of knowing if this is higher or lower for off road tires until someone cuts a brand new tire up and weighs the parts. :)

I was surprised that the two tires you are looking at have nearly identical rotational mass, but the taller tire will still create less leverage for the brakes and engine, so you are going to see a difference no matter how much you sharpen your pencil...

Tire1.jpg


Tire2.jpg

Oh good catch :)

The numbers are so close I don’t think they are going to feel any different. And the taller tire actually has lower equivalent mass. So if anything it should feel better when I accelerate or decelerate the lower mass. But it nice to get the extra ground clearance and not loose anything.
 

rcr1340

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I felt this same way. Weight VS Height. Thats why I have 37x12.5x17 Mastercraft MXT. 69 lbs. The tires Im taking off are 315/70/17 KO2s which weight 64 lbs each. Only 5 lbs difference
 

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rotational inertia affects acceleration and deceleration (braking).

the gearing to a higher number would only help you take off quicker, from a stop.
this is minimal use in a street jeep, since you are not drag racing.

your RPMs will be higher for a given speed.
changing gear ratio's wont improve braking at all.

what the regear does help with is offroading, with very large diameter tires, having something higher that 4.11's (stock rubicon) will protect your axles and ring / pinion somewhat. in other words, for every spin of the drive shaft your tires turn 1/5 of a turn instead of 1/4 turn. so it's less stress on the axle shafts and ring and pinion, and locker.

however, at some point, depending on how heavy your jeep is and how large the tires, you want something larger than a Dana 44. (***hint for the 392 jeepers!)

If you are starting with a Wrangler Sport, the axles are not even Dana 44's. You could buy a set from a member taking the 44's off of a Rubicon as a first upgrade.

if you are serious about offroading, rather than insert money into your Dana 44's that will likely break eventually anyways, you should just upgrade the whole units to Dana 60 / Dana 80 / Ford 9'inch, big GM axles, etc. you can buy a matched set with the gearing you already need.

https://www.fourwheeler.com/how-to/154-0708-weakest-to-strongest-axles/

https://www.4wheelparts.com/b/drivetrain-differential/complete-axle-assemblies/_/N-cm6jh

Currie is some of the best available, for all-out competition:
https://www.currieenterprises.com/jl-bolt-in-axle-packages
 
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... 37” ko2 ... (c ply version comes out in April)
I've seen this load range C, 37" KO2 mentioned elsewhere. I even tracked down an exact model number, but the tire suppliers all stated it was obsoleted. Can you share your source for their re-introduction? I may need to wait on purchasing an upgrade if they are coming out soon.

Thanks for posting the inertia comparison; that's really useful when considering overall performance changes from oversized tires.
 
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I felt this same way. Weight VS Height. Thats why I have 37x12.5x17 Mastercraft MXT. 69 lbs. The tires Im taking off are 315/70/17 KO2s which weight 64 lbs each. Only 5 lbs difference
have you actually done the switch, how do you like the mxt saw them in person and they look like a nice tire.. and i love that they are light
 

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