What is static LOADED radius for a stock rear?

[Stranger in the Black Sedan]

Hey guys can anyone tell me what the static loaded radius is for a stock rear BFG? I accidentally used un loaded radius from a tire size calculator when I programmed the revs/mile for my MM and the speedo is reading several MPH fast. I will correct it tonight but need to know the static radius of my tires so I can calculate revs/mile for my 3.73s

Thanks

STeve

[Zack]

Sorry for being rude, but get a tape measure and measure

[Stranger in the Black Sedan]

Um it needs to be accurate enough for the programmer... I'll measure rollout if I have to but I'd like the manufacturer's spec if someone has it. If it were a current model tire BFG would have it on their site.

[Zack]

BTW, the Factory value is 723

[red]

For most reasonably sized passenger radials, the statically loaded radius used for calculation of revs/mile is about 1.5% less than the free hanging radius, which is caculated using the section width, aspect ratio, and wheel diameter. This varies slightly from tire to tire due to construction, sidewall height, etc. Your adjusted value can be off due to temperatures or if the tire is under/over inflated, low on tread, etc.

[red]

I will correct it tonight but need to know the static radius of my tires so I can calculate revs/mile for my 3.73s

What does having 3.73s have to do with the revs/mile? They are two separate factors the PCM uses for calculating speed, etc. You specify axle ratio and tire revs/mile separately.

[Marauderjack]

Regardless of loaded or unloaded you still have the same circumference so revs/mile will be the same unless you under inflate so badly the tread buckles....effectively reducing the circumference!!!

Marauderjack

[red]

Regardless of loaded or unloaded you still have the same circumference so revs/mile will be the same unless you under inflate so badly the tread buckles....effectively reducing the circumference!

I suggest observing your tires more closely. With the suspension and tires not in motion (i.e., static) and on the ground (i.e., loaded), do you notice how the sidewall bulges out at the bottom? Now, measure the static loaded radius, i.e., from dead center of the wheel to the ground, using a meter stick and straight edge. You'll find that it is less than the free hanging radius, which is what one would calculate using the section width, aspect ratio, and wheel radius. For very short and stiff sidewalls, you'll find less change between loaded radius and unloaded radius. For tall and soft sidewalls, you'll find more change. For most tires, 3% is an accepted amount of change in revs / mile and will yield results very close to the manufacturer measured revs/mile. As far as your argument about circumference, tire sidewalls are constructed of compounds and bands, neither of which are rigid. As each section bears the load of the vehicle, the sidewall flexes and the radius is reduced. So, the circumference is effectively reduced even under normal circumstances. For our stock rears, not considering the loaded radius change of about 1.5% will result in a change of over 20 revs/mile and consequently a MPH error of almost 2 MPH at 60 MPH. I hope this clears it up for you.

[Marauderjack]

I suggest observing your tires more closely. With the suspension and tires not in motion (i.e., static) and on the ground (i.e., loaded), do you notice how the sidewall bulges out at the bottom? Now, measure the static loaded radius, i.e., from dead center of the wheel to the ground, using a meter stick and straight edge. You'll find that it is less than the free hanging radius, which is what one would calculate using the section width, aspect ratio, and wheel radius. For very short and stiff sidewalls, you'll find less change between loaded radius and unloaded radius. For tall and soft sidewalls, you'll find more change. For most tires, 3% is an accepted factor and will yield results very close to the manufacturer measured revs/mile. As far as your argument about circumference, tire sidewalls are constructed of compounds and bands, neither of which are rigid. As each section bears the load of the vehicle, the sidewall flexes and the radius is reduced. So, the circumference is effectively reduced even under normal circumstances. For our stock rears, not considering the loaded radius change of about 3% will result in a change of over 20 revs/mile and consequently a MPH error of almost 2 MPH at 60 MPH. I hope this clears it up for you.

Red,

Just think about the track on a bull dozer....make it oval or round and the circumference will be the same (same distance over ground per revolution)......same with the tread unless the tire is grossly under inflated??

All I'm saying is the radius/diameter can change per measurement but the travel over ground will be the same...thus the revs per mile shouldn't change!!

I played with this from 20 PSI to 45 PSI and saw no difference in the speedometer and GPS......while the radius/diameter measurement changed!!

Marauderjack

[Stranger in the Black Sedan]

Hmmm you are right that my revs/mile should be the stock setting (which it isn't). Using the OOOOLD "extreme Tune" SCT software, when I entered 245/55/18 (stock) it replaced 723 revs/mile with a caculated revs/mile value assuming an unloaded tire. I will go plug the stock # back in. Problem solved
Damn I'm a dumbass

[red]

Just think about the track on a bull dozer....make it oval or round and the circumference will be the same (same distance over ground per revolution)......same with the tread unless the tire is grossly under inflated?

All I'm saying is the radius/diameter can change per measurement but the travel over ground will be the same...thus the revs per mile shouldn't change!

Concerning your bulldozer comment, to an extent I agree that the run of its track (i.e., the distance traveled by a full revolution) will have negligible variances if slip and other factors are ignored. However, other than the fact that a tire revolves and is used on a vehicle, its design is nothing like a continuous track. Foremost, the form of a tire is largely supported by gas pressure not solely by the rigidity of its material. For radial tires, the layout of the belts causes deflection (bulging) of the sidewall under load, which is a reduction in the free / hanging radius. Your question appears to be "where is the circumference going if the radius is being reduced?". What I'm trying to explain is that as each section of the tire bears the load (i.e., rotates to a position--the bottom--where it has to support the weight of the vehicle), the tread compresses AND the sidewall deflects, increasing the contact patch. There's a good description available from Google books (link; Tire Tread and Tire Track Evidence by William J. Bodziak, Pages 4 & 5, "How a Tire Carries the Load"). With that said, there is no fixed revs/mile value that will be completely accurate for all speeds as centrifugal force will lead to some tire growth. This effect is negligible for most street tires under normal conditions, so static loaded radius is used for calculating revolutions per mile.

Also, a correction to my above posts; I stated ~3% loss in radius. It should have read a ~3% loss in diameter / circumference / revs per mile (which is equivalent to a ~1.5% loss in radius). I've edited the posts accordingly.

I played with this from 20 PSI to 45 PSI and saw no difference in the speedometer and GPS......while the radius/diameter measurement changed!!

Load and inflation both definitely effect the revolutions per mile, so I suggest you reevaluate your experimentation or measurement equipment. I've done a good amount of experimenting myself with this on the Marauder (four sets of wheels--3 I own--and many types of tires) and also on other vehicles.

[red]

Hmmm you are right that my revs/mile should be the stock setting (which it isn't). Using the OOOOLD "extreme Tune" SCT software, when I entered 245/55/18 (stock) it replaced 723 revs/mile with a caculated revs/mile value assuming an unloaded tire. I will go plug the stock # back in. Problem solved

As stated above, the calculated result assuming a loaded radius will be close to the manufacturer reported revs per mile. In this case, my calculations for our stock rear tires yields ~727 and the manufacturer reports 724 (see this link).

[Stranger in the Black Sedan]

Yeah red Extreme tune calculated for an UNloaded tire. The error from using the unloaded tire radius instead of loaded gives me 2-3 mph higher than actual speedo readout at 70 mph, which makes sense since a loaded tire will have a smaller radius than an unloaded (hanging) tire. Thanks guys for straightening this out.

[Marauderjack]

Concerning your bulldozer comment, to an extent I agree that the run of its track (i.e., the distance traveled by a full revolution) will have negligible variances if slip and other factors are ignored. However, other than the fact that a tire revolves and is used on a vehicle, its design is nothing like a continuous track. Foremost, the form of a tire is largely supported by gas pressure not solely by the rigidity of its material. For radial tires, the layout of the belts causes deflection (bulging) of the sidewall under load, which is a reduction in the free / hanging radius. Your question appears to be "where is the circumference going if the radius is being reduced?". What I'm trying to explain is that as each section of the tire bears the load (i.e., rotates to a position--the bottom--where it has to support the weight of the vehicle), the tread compresses AND the sidewall deflects, increasing the contact patch. There's a good description available from Google books (link; Tire Tread and Tire Track Evidence by William J. Bodziak, Pages 4 & 5, "How a Tire Carries the Load"). With that said, there is no fixed revs/mile value that will be completely accurate for all speeds as centrifugal force will lead to some tire growth. This effect is negligible for most street tires under normal conditions, so static loaded radius is used for calculating revolutions per mile.

Also, a correction to my above posts; I stated ~3% loss in radius. It should have read a ~3% loss in diameter / circumference / revs per mile (which is equivalent to a ~1.5% loss in radius). I've edited the posts accordingly.



Load and inflation both definitely effect the revolutions per mile, so I suggest you reevaluate your experimentation or measurement equipment. I've done a good amount of experimenting myself with this on the Marauder (four sets of wheels--3 I own--and many types of tires) and also on other vehicles.

I agree somewhat red and in post #7 I stated as long as the "tread doesn't buckle" the circumference should remain constant!! When I played around with it with the dang BFG's I saw virtually no speedo difference from 25 PSI to 40 PSI...that's all I'm saying!!

Worrying about 1-3 MPH at 60 MPH is futile as most speedos have a great deal of error everywhere except the speed they are calibrated at....mine seems dead on at 65 MPH (GPS) but off 4 MPH at 35 MPH??

I have actually put my GPS in front of my speedo....don't even use it anymore!!

Marauderjack

[Stranger in the Black Sedan]

if you are off by 3 mph that will be a pretty large error in the odometer, in this case showing the car racking up more miles than it actually has. I don't want that! New cars should not be off by more than 1 mph at any given speed. The magazines test for it w/ really expensive equipment. The garmin I have shows the exact same speed as the speedo on my other non tampered with cars so I know the MM is off.