Fun Calculations on Tire Choices

[jimmyjamm]

Engineering, well, you may notice at times like this (https://www.fordraptorforum.com/threads/max-boost-with-tune-and-stock-bov.83894/) that engineering implications are met with stiff opposition, just ask TwizzleStix

 

[K223]

I am overthinking this without a doubt, but let's see what y'all say...

The 35" KO2s are listed at 65 lbs and 37" are listed at 69 lbs. For these calculations, I am assuming the wheel weight is the same.

Although 2" diameter is only a 6% increase, the additional penalty comes into play due to larger weight being further outboard from the center of the axle.

If we do the simplest torque required calculations using Fc=mv2/r to arrive at centrifugal force numbers, it comes out that for the same amount of acceleration within the same duration of time, the 37" tires (which actually have 36.5" diameter vs. 34.4" for the 35" tires) requires 18% more torque to be applied compared to the 35" tires.

Another way to look at it, probably, is the 510 ft-lb torque the engine in the raptor will act at wheels as if it only has 418 ft-lb. That is a staggering drop in power, and that's only per wheel, not the impact on all four wheels.

My concern is more on the braking performance than acceleration frankly. I wonder if this means the braking distance will increase at least by 15-20% for the 37" package vs. base.

Although I realize the 37" tires are needed, and in some cases even bigger ones, depending on the driving conditions, but that probably applies to a smaller segment of buyers.

There is good news though: The 37" package only costs about 8.6% more

Thoughts?

I think your calculations are flawed. But you may not be lol

You are using the 510lbs-ft off of the crank measured output of the engine and not what the measurement would be at the rear wheels. I think in general that’s about 400 or so TQ.

So you may need to base your power loss on this number. But for example sake, we can get an idea of the power loss with the heavier tire.

Most all will tell you they have felt a decrease in acceleration with the larger/heavier tire. Many have tunes to compensate as well.

You would normally have to expect this decrease and some loss of mileage. But I find it interesting that someone mentioned the 37” KO2’s are listed at only over a pound heavier per tire. I would have to gather Ford requested BFG to produce a lighter tire for the Gen 3. If this is true, the 37” truck wouldn’t give up that much to the 35” version. And Ford can still keep mileage up amongst other things.

 

[spizike9]

I think your calculations are flawed. But you may not be lol

You are using the 510lbs-ft off of the crank measured output of the engine and not what the measurement would be at the rear wheels. I think in general that’s about 400 or so TQ.

So you may need to base your power loss on this number. But for example sake, we can get an idea of the power loss with the heavier tire.

Most all will tell you they have felt a decrease in acceleration with the larger/heavier tire. Many have tunes to compensate as well.

You would normally have to expect this decrease and some loss of mileage. But I find it interesting that someone mentioned the 37” KO2’s are listed at only over a pound heavier per tire. I would have to gather Ford requested BFG to produce a lighter tire for the Gen 3. If this is true, the 37” truck wouldn’t give up that much to the 35” version. And Ford can still keep mileage up amongst other things.

Yeah at only 1 lb difference I can’t see too much negative going with the 37 over the 35.

 

[slv_]

I think your calculations are flawed. But you may not be lol

You are using the 510lbs-ft off of the crank measured output of the engine and not what the measurement would be at the rear wheels. I think in general that’s about 400 or so TQ.

So you may need to base your power loss on this number. But for example sake, we can get an idea of the power loss with the heavier tire.

Most all will tell you they have felt a decrease in acceleration with the larger/heavier tire. Many have tunes to compensate as well.

You would normally have to expect this decrease and some loss of mileage. But I find it interesting that someone mentioned the 37” KO2’s are listed at only over a pound heavier per tire. I would have to gather Ford requested BFG to produce a lighter tire for the Gen 3. If this is true, the 37” truck wouldn’t give up that much to the 35” version. And Ford can still keep mileage up amongst other things.

In addition, I have got to wonder how many of those who are going from 35 to 37s are replacing it with the same style of tire. Going from a factory KO2 to a Toyo MT adds substantially more weight. Not to say that there isn’t a loss in Performance just from increasing the size, but how much of the change in performance can be attributed to the change in tire size vs the substantial difference from the increase in weight.

 

[Badgertits]

I don't think this is accurate, but my physics is rusty so I definitely could be wrong. My doubt comes mainly because if the drop in power was actually that big, no one would ever go with bigger tires, since performance would be very noticeably worse.

The other is that the equation your using is for centripetal force. Basically, the inward force (from tire edge to center of wheel) needed to keep an object traveling in a circle. (object in motion due to inertia). For example, it's the gravitational force needed to keep a satellite in orbit. Maybe the same formula applies here, and I just don't understand, but it does not seem right. For one thing, I would expected that you would need to take into account rolling resistance and other forces in a calculation of this type?

When it comes to braking, wouldn't the mass of vehicle be a much bigger factor than just the mass of the wheel/tire?

it is very noticeably worse, but guys running 37”s or very heavy D or E rated 35”s just refuse to report such b/c they like the look/extra offroad clearance etc so myxh

but the penalty is way exaggerated when only talking about extra 1-2 damn lbs, regardless of 2”

you guys realize for instance that the Nitto Ridge Grappler in 37/12.5R17 weighs 82 lbs!!!?!?!? - going from 65 to 82 lbs is where you’ll find a big difference

on my last GMC Sierra 6.2 going from 33” nitto terra grapplers to BFG K02s (an increase of 11 lbs per corner) resulted in an immediately noticeable drop in braking performance, a slightly noticeable drop in acceleration/pickup (in fact....w/ the 11 lb heavier KO2s it simply would no longer engage AFM & drop into 6 or 4 cylinder mode, just stayed V8 all the time so that’s fairly telling) & I lost about .7-1mpg hey & 1-1.5mpg city

I am not in the market for a gen3 right now, but if I were to trade my 2020 in on one it’d certainly be w/ the 37” package b/c I know the weight penalty is nil

the upgraded frame, shocks, ride height, look, etc. are all absolutely worth it to me

 

[greenraptor]

Interesting. This is based on the 'new' 37" tires BFG lists on their site, I do not know what is being used on the '21 Raptor. If the difference is only 1 lbs, there should not be any noticeable impact on any part of the performance.

That “new” 37 is the KO2 Ford is using for the 21+ Raptor. It is a C load rating (which is what the Raptor is using), has RBL (raised black letters) and its speed rating is S. The “old” 37 KO2 is a D load rating, has RWL (raised white letters) and has a speed rating of R. You used the “old” one for your example at 69.49 lbs per tire.

 

[cjmcb]

In the land cruiser world, the “jump” from stock 32” (I think) to a 34/35” gets pretty much unanimous agreement that a re-gearing of the diffs is needed to maintain acceleration and to stop the transmission from hunting between the top two gears when cruising at a steady speed on an undulating road.

 

[birdieman]

First off, I am not an engineer, but if you simply look at the EPA mileage ratings for a 35 truck vs a 37 truck, the 35 is rated at 16 combined with 15 city/18 highway, the 37 truck is rated 15 combined with 15 city/16 highway you can see that there is a penalty for the 37" tires. If it affects the gas mileage by 6-12%, it must also affect the acceleration and braking.

 

[spizike9]

First off, I am not an engineer, but if you simply look at the EPA mileage ratings for a 35 truck vs a 37 truck, the 35 is rated at 16 combined with 15 city/18 highway, the 37 truck is rated 15 combined with 15 city/16 highway you can see that there is a penalty for the 37" tires. If it affects the gas mileage by 6-12%, it must also affect the acceleration and braking.

First I’ve heard there are EPA ratings out on the new Raptor. Where did you find those?

 

[birdieman]

Looked at window stickers.....

35" truck - https://www.windowsticker.forddirec...cuSE3mwwz34ieI0Mv12+nmpJcdV5+cEAiSdD129LETz4=

37" truck - https://www.applesportford.com/api/...&dealerCode=02680&postalCode=76661&status=NEW