Front to Rear tire raito

[Blake13]

I used the same technique of jacking up one side of the tractor, making marks on the tires, putting it in 4wd, and rotating the one while counting the other. Repeated it several times

rScotty,

Sounds like we are in a similar situation and proceeded in similar ways. To be clear, I put the all 4 corners in the air as opposed to one side. Then instead of turning a wheel by hand (which I believe I understood that's what you did), I engaged the engine to spin the wheels and recorded it all so I could watch the video and count the front revs first, watch again, and count the rear revs.

I went 28 front revs without any lining up exact (that's like driving about 70 yards). I figure I will repeat the exp. anyway, may as well go a bit longer and see what happens. From the math I showed it shouldn't matter anyway. Being off by a lug distance after 28 revs was is still 99.8 percent accurate.

The variation in results you observed when you did the paint/measure rolling distance trial is what I was anticipating. That's why I figure I can get closer with determining the exact gear ratio and using calcs to determine the correct tires at different sizes.
Thank you for your detailed post.

Blake

 

[Blake13]

https://www.tractorbynet.com/forums/tires/371468-tires-why-rolling-circumference-loaded.html
This is a 2016 thread discussing rolling radius etc.

Thanks for the link. In the thread you actually posted the information from tirerack reaffirming why I use rolling the circumference value provided by the manufacture to calculate lead/lag in my original post.

The commentary pertains to this discussion so if I will add it below:

I ran across the info below on tirerack.com which seems to coincide with your comments

Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire's published overall diameter (which would only reflect the tire's unloaded radius), calculating the tire's absolute rolling circumference isn't possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

One bit of information for the discussion not in this comment: rolling circumference is a function of revolutions per mile. They both measure the same thing but different units. Take the number of inches in a mile (63360) and divide over the rolling circumference in inches, you get revolutions per mile. As the comment states, this is not related to loaded radius.

Remember, I never claimed to calculate the rolling circumference, or use loaded radius at all. I stated that I used the literature value from the tire manufacturer for the rolling circumference.

 

[LouNY]

All of the math is a good start and will let you determine the gear ratio of your tractors front and rear gears.

The loaded radius and loaded rolling circumference is what will determine your actual lead/lag of your tires.

I have no clue as to how manufacture calculated the rolling circumference,if I was to guess it would have been maximum rated load and maximum pressure.
I tend to run my fronts close to or a touch over maximum recommended pressure as when the loader is used that end of the tractor is definitely being loaded heavily.
By the same token the only time my rear tires are even close to being loaded to the maximum is when I am using my 3 point forks with a heavy load which isn't even 1% of the tractors use so I am only running around 12-15#'s in the rears compared to 25-30#'s in the front with these pressures my front to rear lead feels good.
Varying your air pressure can and will change your loaded radius which in turn is what will be your loaded rolling circumference.
My 4wd is engaged and disengage with a switch which controls a multi disk wet clutch, when I disengage it while stopped I can feel and see a slight lurch in the tractor.

 

[npalen]

Its probably been mentioned but I think that actually measuring the rolling radius front and rear in actual loaded condition is important. This info combined with the actual gear ratio as the OP is determining above can give accurate lead/lag ratio. Keep in mind the load being carried whether it is on the FEL or 3PH as this will affect the rolling radius also.
A heavy load on the FEL, for instance, would tend to reduce or even negate any front tire lead. That scenario might be an issue when turning on pavement with a heavy load in the bucket.
I guess I wouldn't depend only on the info provided by the tire manufacturer to determine the lead/lag.

 

[rScotty]

rScotty,

Sounds like we are in a similar situation and proceeded in similar ways. To be clear, I put the all 4 corners in the air as opposed to one side. Then instead of turning a wheel by hand (which I believe I understood that's what you did), I engaged the engine to spin the wheels and recorded it all so I could watch the video and count the front revs first, watch again, and count the rear revs.

I went 28 front revs without any lining up exact (that's like driving about 70 yards). I figure I will repeat the exp. anyway, may as well go a bit longer and see what happens. From the math I showed it shouldn't matter anyway. Being off by a lug distance after 28 revs was is still 99.8 percent accurate.

The variation in results you observed when you did the paint/measure rolling distance trial is what I was anticipating. That's why I figure I can get closer with determining the exact gear ratio and using calcs to determine the correct tires at different sizes.
Thank you for your detailed post.

Blake

That's a real clever idea by taking a phone video. I believe you said that you locked the (rear) differential? I didn't lock mine, being concerned about locking the rear but not the front & not sure what that might do to the count. Accidently engaging the planetary on one diff but not the other was why I rotated by hand, I wanted to rotate very slowly and smoothly so I could tell by the friction if some gearset inside the axle started to move. At least I hoped I would be able to tell. Trying to v]isualizing planetary gearset motion while trying to hold the ratios straight in my head always makes my mind start to stutter and panic.

My manual rotation & count method must have worked, since I did get the same number for the overall gearing that was listed in my workshop manual.
I used tape on the tires with an inked line and for the front and rear pointers clamped some welding rod to concrete blocks. Repeatability probably less than 0.1 inch.
Repeated the complete test several times & there was no change.
BTW, I also found in the workshop manual specs there were two different overall ratios listed. Apparently the gearset was changed during production.

Well, we've had visitors all day and I haven't had time to look through my 20 year old notes on the process like I promised. Maybe tomorrow - though I won't post unless something interesting comes up.

On Rolling CIrcumference and measurement vs mfg. specs. Clearly wear, pressure, rim width, sidewall elasticity (age), temperature and load will all change the rolling circumference. Many of those are going to be specific to any particular tractor.
I measured the rolling circumference as I did on the road with paint blobs partly to make sure that the measurements I got were for my own tractor..... but honestly a lot of the reason was just curiosity to see how close the tire manufacturer came to getting it right. FWIW, they came remarkably close.
rScotty

 

[rScotty]

Oh, I forgot to mention that loaded radius is a special case and not arithmatically related to loaded (or unloaded) rolling circumference. So be aware of the definitions.

It all comes down to tire slippage & traction. How much and when. Think about what happens in soft soil when the tread sinks down in the muck to the basic bald tire shape, rolling circumference falls down off a cliff, driveshaft stress spikes up and traction goes to near infinite. Super stress....
The best measure driveline stress that I know of is by how hard it is to shift it OUT of 4WD.
rScotty.

 

[Blake13]

Well, we've had visitors all day and I haven't had time to look through my 20 year old notes on the process like I promised. Maybe tomorrow - though I won't post unless something interesting comes up.

On Rolling CIrcumference and measurement vs mfg. specs. Clearly wear, pressure, rim width, sidewall elasticity (age), temperature and load will all change the rolling circumference. Many of those are going to be specific to any particular tractor.
rScotty

I would be curious if you are able to dig up what you calculated for your rolling circumference. I am curious how that compares to the literature values. Thanks again for the detailed posts.

 

[npalen]

Would it be fair to say that the actual effective rolling circumference can only be determined by measuring the actual rolling radius in a given situation?

 

[Blake13]

So I retested again today. I discovered a couple interesting things. First, I'm sure some of you already knew this, but like i said i am not a differential expert. With an open differential, as one wheel is dragged to zero velocity, the opposite axle increases in velocity proportionately. When the axle is stopped completely the opposite axle doubles in velocity.

At this point I am pretty sure my first test was not accurate. Like I mentioned in my original post, I am suspicious that the front left and right axle may not be turning at precisely the same rate when suspended. I believe that the minute differences in internal friction don't allow the axles to turn at exactly equivalent rates. This time I jacked the tractor up, locked the rear diff and blocked the front right axle. The front turned about 63 revs per 20 rear revs. Since the front spins double velocity when the opposite side is blocked, half the revs to 31.5 revs. Then take rear / front = 20 / 31.5 = .6349 This new ratio is much closer to what i was expecting from the beginning. This suggests that the jacked up front open differential was allowing axle velocities to creep.

I tried once more a different way, to hopefully corroborate my new result. This time jacked on side like rScotty had done. This locked the left front and rear axles so that i would not get axle velocity creep. This time about 41 front revs for 26 rear. This works to .6341, very close tho the last calculated ratio. It is close enough for me to feel satisfied with the result, however I wonder what is going on in the gears that allow there to be ANY discrepancy. Perhaps spinning them by hand as rScotty did would prove more accurate. I will go back and add this info to the original post.

 

[Blake13]

Would it be fair to say that the actual effective rolling circumference can only be determined by measuring the actual rolling radius in a given situation?

Practically , the rolling circumference can only be determined by measuring it. This would be done by measuring the distance the tire travels per revolution while under load. The rolling circumference could be calculated but very complicated. Loaded radius nor rolling radius are a function of rolling circumference.