Understanding the dynamics of tire size?

Brian McVickers

Administrator
Staff member
Ok, question for all the brainiacs! (You know, those of you who can rattle off equations and scientific theory!)

Given the scenario below, how will a vehicles driveline / transmission / transfer case / differentials... react to the larger tire size, would it need to work harder or easier and to what if any significant measure? Furthermore, can a system that was originally paired with the stock tire size accomodate the larger tire size?

Stock tire size: 255-65-16
Sidewall: 6.5"
Radius: 14.5"
Diameter: 29.1"
Circumf: 91.3"
Revs/mi: 694.0

New Tire size: 265-75-16
Sidewall: 7.8"
Radius: 15.8"
Diameter: 31.6"
Circumf: 99.4"
Revs/mi: 637.0


Edit --- Let me know if you need more information...

:ylsmoke:
 

Robthebrit

Explorer
The rolling circumference of the new tire is around 9% bigger so you'll be going the same speed with 9% lower RPMs, you'll also have 9% less torque at the same setting.

Rob
 

Pskhaat

2005 Expedition Trophy Champion
And of course radius & diameter are linear against the circumference, so you can simply calculate the % difference using the total tire height.
 

goodtimes

Expedition Poseur
pskhaat said:
And of course radius & diameter are linear against the circumference, so you can simply calculate the % difference using the total tire height.

almost. Don't forget that tires are not "round" once they have weight on them.
 

Pskhaat

2005 Expedition Trophy Champion
goodtimes said:
almost. Don't forget that tires are not "round" once they have weight on them.
You know, I've heard so much debate on this I don't know what to believe anymore. One side says that the quantity of tire still must be turned underneath regardless of the pressure and eventual shape, thus one doesn't actually see fewer/more rotations per mile aired down than they do at full pressure. The other side of the coin maintains the opposite.

But for the sake of our quick and dirty calulations I think we could claim a linear relationship?
 

Pskhaat

2005 Expedition Trophy Champion
mcvickoffroad said:
Will the 265/75 stress the system significantly more or less than the 255/65?
Is there a way to measure this?

Mostly more, sometimes less. The computer and shift points in an auto are tuned for the stock tire size, as well as gearing and people claim that all the time. However, many vehicles with the same tire size are sold every day with varying differential gear options so mechanically speaking I feel that the drivetrain is certainly up to par with the new larger sizing.
 

david despain

Adventurer
Robthebrit said:
some of those ECU programming gadgets can adjust can for none stock tire sizes.

Rob

i dont think there are any available for the DII. ( i dont think that even the test book can do this but im not sure)

this seems to be one of the most popular tire size upgrades for the disco. i bet you will be fine. drive line stress seems to be more of a function of weight and driver style than tire size.
 

Brian McVickers

Administrator
Staff member
Now we're talking, at least in words that I can understand!
Thanks everyone for the input, keep it coming inf you can think of more...
 

michaelgroves

Explorer
Robthebrit said:
The rolling circumference of the new tire is around 9% bigger so you'll be going the same speed with 9% lower RPMs, you'll also have 9% less torque at the same setting.

Rob
Just keep in mind that the 9% less torque Rob is referring to is 9% less available torque. The actual torque on the drive-line will be 9% more, in almost any given circumstances.

Reason: the torque on any component is the "twisting force" it undergoes. It's a result of the engine/transmission applying a force, and the ground resisting it. The more the ground resists (i.e. a steep hill, or if you chain the vehicle to a tree), the higher the torque will be. The maximum torque experienced will be reached when either the wheel starts to turn (whether the vehicle moves or the wheel spins), or the engine reaches its maximum torque capacity (in the lowest possible gear). *

It is easier to overcome the ground's resistance with a small diameter wheel than a large one (it's a gearing/leverage issue), so a wheel with a 9% increase in radius will require 9% more torque to turn it against any given ground resistance.

It therefore follows that, other things all being equal, you need 9% more torque at all times, if you have 9% bigger tyres. But your engine hasn't changed, so at the extreme limit, you are 9% worse off. Hence Rob saying you have 9% less.

The implication for vehicle stress isn't as bad as you might think, though. Yes, at all times, there is 9% more torque on your components, but to compensate for that, everything turns 9% more slowly. Wear-and-tear is therefore probably not changed drastically either for the better or for the worse. And at the extreme, the limiting factor on torque is what the engine can produce. If the vehicle is designed to to put, say 500N.m of torque through overall gearing of 50:1, it means the side shafts should be able to withstand at least 25000Nm. No matter how big the tyres are, you can't put more sustained torque on the shafts than the engine can produce.

(Having said that, it is almost always shock-loads rather than sustained torques that break things, and you will be more susceptible to shock-loads with bigger tyres. The extra "leverage" and the extra grip from big diameter tyres does pose significant extra risk of breakage, especially if you have locking diffs as well).


*Footnote: Torque is not only required to get the vehicle moving initially, but also to continue accelerating it. The torque peaks when the engine reaches its maximum torque capacity, at which time no further acceleration can take place. Again, 9% bigger diameter tyres will mean that any given acceleration will require 9% more torque. (But the same power, so you don't necessarily reduce your acceleration ability).
 

Pskhaat

2005 Expedition Trophy Champion
michaelgroves said:
he torque peaks when the engine reaches its maximum torque capacity, at which time no further acceleration can take place.

Please clarify? If I have a torque peak/apex at 1800 RPMs, I still have torque output at higher RPMs throughout the torque fall, and as long as that force is greater than my total resistance I should be theoretically able to accelerate, and equally that I have the proper RPMs with enough torque to perform work (power) I can continue without slowing.

Operating engines above the torque curve in off-road situations is very important with torque rise, in other words as your RPMs go down from getting bogged down or bumping a rock, you actually increase the torque from the engine. This is one of the reasons I get so frustrated with high RPM torque peak, understroked modern and V-configured engines in off-road vehicles...the designers seemed to ignore that very important concept in favor of street drivability.
 

DaveInDenver

Middle Income Semi-Redneck
pskhaat said:
Please clarify? If I have a torque peak/apex at 1800 RPMs, I still have torque output at higher RPMs throughout the torque fall, and as long as that force is greater than my total resistance I should be theoretically able to accelerate, and equally that I have the proper RPMs with enough torque to perform work (power) I can continue without slowing.

Keep in mind Scott that the torque to the wheels is what accelerates the truck, not the torque directly produced by the engine. All engines produce /some/ torque when running at all speeds. The key is that in first gear you can accelerate pretty much continuously until literally you start shooting pistons through the block. The engine's torque band is almost irrelevant since there is enough torque multiplication through the driveline to produce sufficient torque at the wheels to move the truck (not necessarily quickly). This is not the case in all gears. You reduce the multiplication by shifting into higher gears to increase your speed. Less and less advantage is then available to multiply the torque to the wheels and so the band in which the engine can accelerate and eventually just keep the truck moving becomes smaller and smaller.

This is part of the dynamics of tire size, a larger tire increases friction, so requires more torque to overcome. There's a larger footprint, more weight, lots of things, it's not a limitless system. Rolling friction is pretty linear, but air resistance increases exponentially and it's all speed dependant. It simply take more force to move a truck at higher speeds and at some point the multiplication of the engine's torque is too low to keep accelerating. That's the top speed of the truck, the point where the engine's torque peak multiplied by the driveline equals the friction required to be overcome. That point is obviously going to be higher if the engine (i.e. crate small block) has more torque to give the drivetrain, lower if the engine needs more multiplication to accelerate (i.e. 22R-E).
 

michaelgroves

Explorer
pskhaat said:
Please clarify? If I have a torque peak/apex at 1800 RPMs, I still have torque output at higher RPMs throughout the torque fall, and as long as that force is greater than my total resistance I should be theoretically able to accelerate, and equally that I have the proper RPMs with enough torque to perform work (power) I can continue without slowing.

Quite right, Scott. My mistake. Pretend I said acceleration will be greatest at maximum torque, and will tail off along with the torque curve. :) The footnote wasn't thought through very well at all!
 

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