4WD Front to Rear Tire Ratios

[dragoneggs]

Yeah, that hopping down the hill is not a good feeling. Really doesn't matter where in the driveline the brake application happens as long as it only affects the rear axle. When in 2wd applying the brakes going down a hill will only slide the rear tires. When in 4wd the rear axle and front axle rotate at the same approximate speed. So brakes applied to the rear axle will also slow or stop the front axle. So braking is dramatically improved.

Funny side note. When sliding down a hill in 2wd all that's required to reduce or stop movement is to drop the FEL bucket. But in the panic of it all, a guy never thinks to do that.

A very good point and advice!!! For some reason, when I am reversing and looking backward I tend to raise my bucket without noticing. I know... kind of scary. I need to think more about the FEL. I do use it to help push me backwards out of stuck moments though. Now if I can just remember when I am hopping to drop it! :laughing:

 

[Gigfy]

Hi,

I came across this forum via an internet search for sizing my tractor tires. Just want to say thank you for the discussion. Comments from ovrszd, rscotty, and sixdogs were very helpful (btw, I've never needed more than 3 dogs (or one wife) to keep warm on a cold night, so it stands to reason six dogs lives farther north

Really, the only thing I can add to this discussion is that I'm glad most 4wd tractors don't have a split (extra) differential b/t the front and back half of tractor. IMO, this would add a lot of unnecessary cost to tractors.

An "open" differential, as a split/third differential would mean the front wheel assist of the tractor wouldn't really work.

Remeber, for open differentials, the side with the least resistance is the one that "spins", while the other side remains more or less idle. Applying this to a FWD tractor with a differential between the front and back, would ultimately mean the tractor wouldn't be a true FWD. Imagine being stuck in the mud and seeing the front wheels of your FWD tractor spinning, while the back wheels remained stationary. Seeing that, anyone would be pissed (and probably write the manufacturer a "not so kind" letter).

Using a limited slip as a split/third differential would help, but these have more wear parts, and more parts to wear and replace over time (especially for those who like to drive their FWD tractors in FWD on hard surfaces). It's much easier for most folks to replace tires, than to dig into a differential (or pay someone to do so) than to replace clutches. Of course there are electronic means to achieve a limited slip, but these also wear parts. There are also auto locking differentials, but those have a lot of extra parts, and consequently more things to go wrong and replace.

I once owned a blade (i.e. road grader - "blade" is a synonym for a road grader in the excavation trade). It had a fixed differential so that all four back wheels turned at the same speed, no matter what. I once had to replace some parts in the final drive, and was very thankful for its simplicity. The simple design of the final drive was a very tough design, and only needed work because of extreme previous abuse. The blade would tear up ground on turns, but the tires didn't seem to mind. What the tires didn't like was a heavy load, under a lot of torque. In some cases, I could put enough load on the tires to tear off bits of tread. This was where you really saw a lot of wear on tires, not on the turns.

My Massey FWD tractor has a clutch engaged FWD. It's run off a solenoid which interrupts oil to the FWD clutch pack. In other words, the clutch pack is spring loaded so that if there is no oil flow, the clutches are locked (thus engaging the FWD). If the solenoid is activated, oil flows to the clutch pack and releases tension to allow the clutches to slip. The solenoid is activated by a simple toggle switch on the dash. In essence, it's "push button" FWD activated. Push button is so easy when you need FWD. Lovely design.

Massey also solved the problem of engaging the FWD for braking (BTW, I'm not a Massey rep, and I'm sure other manufacturers have overcome the same issue. I'm just talking about the design.) But for those interested, because the FWD clutch pack is spring loaded (which means the electronic oil solenoid is normally closed) it's easy to "disrupt" the oil flow to the FWD clutch pack. They simply put an extra switch in the brake pedal, so that when the brake pedal is pushed (engaged) the oil flow is cut-off from the FWD clutch pack, thus engaging the FWD, to allow for extra braking. Like I say, I'm sure other tractor manufacturers do the same thing, but I thought it was a clever and simple set up.

I've wondered how long this (extra) clutch pack will last, as it's a "wear" part, which I don't like. But I do like the versatility of push button FWD. No shifting, no stuck into FWD. It's instant, on the go FWD, when I need it. They do the same with a differential lock on this tractor. Push button, when I need it. I rarely use the Diff. lock though. It doesn't steer worth a darn, and unless you are driving straight, the tires spin so much, it doesn't give you any extra traction above the FWD.

 

[ovrszd]

Gigfy, welcome to TBN. You are almost a Missourian...

The old timers say a true four wheel drive tractor can not be disengaged and thus, is only the Center Pivot designs. They say everything else is Front Wheel Assist.

There's a lot of technology built into the newer large FWA AG tractors.

The JD blade I run has auto disengage on the Diff Lock. A sensor on the front wheel tells the computer when the front wheels are turned. At a certain degree of turn the computer automatically unlocks the Diff Lock. I used it once. Didn't like it. It waited too long in my opinion and banged loudly when it unlocked.

In any design, when engaged caution needs to be applied by the operator to avoid abuse. Examples would be running on solid traction in 4wd when not needed. Turning sharply on any surface when in 4WD. To name a couple.

 

[Gigfy]

Thanks ovrszd for the welcome,

I hope I didn't convey a third/split differential was always a bad thing. I just think some of these differentials are pretty complicated, and to me a small tire loss/wear is well worth replacing some of the internal wear parts of these more complicated designs.

Sounds like you run a pretty new/sophisticated blade. The blade I owned was an 80s model. It was a big blade, but everything was hydraulic. Nevertheless, it had a fixed frame. An articulating blade would turn sharper and have more need for operator option, or an automatic option, of an open differential. I've heard those articulating blades can turn so sharp, they can make a U-turn on the width of a typical gravel road. You definitely wouldn't want both sides turning at the same speed on that turn.

 

[Gigfy]

" You are almost a Missourian..."


BTW, I should mention I feel more like a Missourian than a Kansan. I used to have a livestock operation in Lafayette county MO (many years ago) but got hurt and sold out to move to the KS side. My wife and I re-purchased some farm ground in MO a few years back to start a peach orchard, so in essence, I'm still a "show me" type of guy. My oldest daughter was raised inher youth in MO, and I'm glad to see she still has the "prove it to me" discernment. From living in both states I can say each has it's advantages, but IMO, overall MO is the better of the two.

 

[sixdogs]

[U]Below is a cut and paste from another thread with rScotty on the Kubota M7040 that I thought would be helpful here. Especially helpful is the link to the calculation. Ignore the parts that are M7040/M7060 specific. [/U]




Here's another way of using the "front axle gear ratio" to compute the acceptable range of rolling circumferences for the M7040/M7060. Using the supplied ratio of 1.4383 provided by lakeridge (from Kubota) and using this site here

Mounting Tires on Four Wheel Drive Tractors

this is what I came up with. Using the OEM 16.9-30's with a rolling circumference of 173", the OEM 9.25-24 fronts and 123 rolling circumference provide around a 2% lead.

A 1% lead would equal a 121.48 front tire rolling circumference.
2% " 122.69
3% " 123.89
4% " 125.09
5% " 126.29

So, the 9.25-24 is perfect as is the 20" OEM metric option in Canada and Europe . The 11.2-24 tires some have used as a front tire option are too big with their rolling circumference of 127.9 exceed the desired range of 2 to 5% with it's more than 7% "lead". If guys are using 11.2-24's and haven't had troubles yet, it must be because they are in 2WD a lot or in very soft dirt that lessens the damage. Really, the only two choices for the M7040/M7060 are the OEM 9.25-24 and the metric 3.20-20's and bias 12.4-20's. The rolling circumference is very close to OEM in these.

Anyway, that's what we have so far. For me, I have Firestone 16.9-30 radials on the rear now and plan to order the radial equivalent of the 9.5-24's for the front and live with it. That is, unless someone finds a lower priced maker of 20" wheels for the front. Then I would use the Europe/Canada metric OEM 320-20's choice and be happy with it.

 

[rScotty]

There's a new thread just started today in the General Forum Section under Owning/Operating labeled: "4Wheel Drive and Tire Ratios" It starts out with thoughts on a simple way to measure tire rolling circumference (RC) and how the OverDriven ratio can be calculated directly from the RC. Hopefully that thread will benefit from some comments as this older one did.
rScotty