Rear Tire size change on 4WD

[knightgang]

I am shopping for new tires for my L2950 4wd tractor. the original R4 tires are size 420/70/24. We found a 6ply of this tire at $852 each and about $160 shipping. Several size conversion charts that I have seen show that for low pressure applications a 17.5x24 tire can be substituted.

The overall diameter of the 420/70/24 is 46.4" and the overall diameter of the 17.5x24 is 48". Will this 1.5" of diameter and the rolling circumference of the 17.5x24 tire be an issue with the 4wd driveline?

Thanks

 

[North Idaho Wolfman]

It's off quite a bit in RC:
420/70/24 RC 136
17.5x24 RC 155

So with that alone, Not a suitable match

Next is rim width
420/70/24 13" rim
17.5x24 15" rim

Again not good

Also to get the big picture, Need to know what size front tires your running now to know if they are going to be compatible.

Is there a reason you want to stay with R4's instead of R1's?

 

[knightgang]

It's off quite a bit in RC:
420/70/24 RC 136
17.5x24 RC 155

So with that alone, Not a suitable match

Next is rim width
420/70/24 13" rim
17.5x24 15" rim

Again not good

Also to get the big picture, Need to know what size front tires your running now to know if they are going to be compatible.

Is there a reason you want to stay with R4's instead of R1's?

Wolfe,
I would like to stay with R4s because this is a multi use tractor. It is being used and will continue to be used on soft ground areas for bush clearing as well as maintaining a couple of acres of yards with a finish mower among other tasks.

I talked to my Kubota Dealer yesterday, it seems this tractor was delivered new with 420/70/24 R4 tires on 15" wheels (measured and verified the wheel size this morning) and it has the 10-16.5 skid steer tires on the front.

I spoke to a tire dealer yesterday and these are the dimensions he gave me on the two rear tire sizes that we are discussing.

420/70/24 - 16.5" Wide x 46.4" Diameter (23.2" Radius)
17.5x24 - 17.1" Wide x 48" Diameter (24" Radius)

This changes your calculations on Rolling Circumference to the following:
420/70/24 - RC 145.77
17.5 x 24 - RC 150.8

So, with a 10-16.5 front tire, is this 5" of Rolling Circumference going to be an issue. Can this rolling circumference be adjusted by adjusting tire pressures?

Even a 14.9x24 is 48.1" tall and should be mounted on a 13" wheel, so that tire will not work with the 13" Wheel but is generally the same height and RC and the 17.5x24.

 

[knightgang]

Additionally, the average Overall Diameter of a 10-16.5 tire is 30.78" giving that tire a rolling circumference of 96.69".

The ratios of front to rear tires given these dimensions and everything laid out in the post above are as follows:

Front to Rear Ratio on the 420/70/24 -- .663362

Front to Rear Ratio on the 17.5x24 -- .64125



I am digging into this so deeply as there is a huge difference in price and availability between these tires. So much so that the 420/70/24 will cost me $1100 more for two tires over the 17.5x24 that I can get for $225 a tire as manufacture blems and $160 shipping. But I don't wan tot do this to save money now if it will tear my tractor up. Since I do not know the tolerances that are built into the drive line of the tractor to account for a sutle change as this, I want to cover all the basis before hand.

Thanks

 

[85Hokie]

Knight,

the problem is like the ol' fram oil commercial - "pay me now or pay me later".......

there is an old calculation that said make sure the ratio stays inside the 2% high/low.

IN a perfect world the rears and fronts should spin at the EXACT same rate, seems perfect eh? Well, after a couple of years, the rears will have worn a bit more than the fronts and the ratio is getting off more and more daily!

On most 4wd tractors the fronts typically "pull" the rears by a smidge when going straight and in 4wd.

The thing about 4WD is - why/when do you "need" it, trucks on the road use 2wd 99.99% of the time, when they are thrown in a situation, that small %, the tires are in a situation that allows the tires to spin and not hurt anything! And dont confuse cars that have all wheel drive - that in itself is a whole different animal.

So - what I am getting to, if you are going to get the ratio out of whack, you best make sure that the situation allows the tires to spin or give, otherwise the internals will cost you a hellva lot more than the tires. Bottom line - never use your 4wd UNLESS you know that somewhere the tire(s) can give! Most of us dont use 4 wheel drive on hard pavement, it is not needed, but in the last snow storm here on the east coast, many of us came out of snow covered area onto pavement and never placed it back into 2wd.......

 

[knightgang]

So, based you your reply, that 2% is the margin I am looking for. I am right on that margin. That said, this tractor is almost never run on pavement and agreed, when doing such it should be in 2wd. Mostly we only run it in 4wd when clearing the the wet areas, there is plenty of opportunity for slippage.

I have 4 four wheel drive trucks and I agree with you, 99% of the driving in HWY in 2WD. And I have been guilty of coming out of the woods needing 4wd to the pavement and not switching back. Luckily I can shift on the fly and eventually do, but the hubs stay locked (different conversation).

But yes, I think that 2% is what I have been looking for as this will be 2% different than stock. when delivered.

 

[85Hokie]

Not sure where the 2% came from .......that to me is like I have a 2% heart attack...... what that REALLY means is the pressure on the drive system is there, but NOT that bad. Then again bad is.......wear! Glad you fall into that magic area. Cause the sad part - there is nothing at 0%!

 

[knightgang]

Not sure where the 2% came from .......that to me is like I have a 2% heart attack...... what that REALLY means is the pressure on the drive system is there, but NOT that bad. Then again bad is.......wear! Glad you fall into that magic area. Cause the sad part - there is nothing at 0%!

There is going to be pressure on the drive system anyway, because you are, well,... driving...

I think the question is does that 2% overwhelm the normal pressure creating be the driveline.

Another point, until the Diff Lock is applied, there is a normal amount of slippage built into the drive line anyway to account for the turning radius, etc. The only time the Diff Lock is applied to to engage all 4 wheels equally to escape a stuck situation. Then, at least my tractor, disengages the diff lock automatically when it knows it id no longer needed which is a mechanical function of pressure being taken off that section of the driveline.

Did I make sense, or take a left and end up in Albuquerque?

 

[85Hokie]

With the diffy lock engaged - the rears are locked ...period, the drive from rear to front still will have the strain on it, no matter what ratio is applied to the tires - the front and rears both can spin at different revs due to each ones diffy's but front end to rear end has NO way way of slipping other than tire movement.

 

[knightgang]

Thanks 85, was not aware of that.

I have been doing some more searching in relation to the actual dimension of these tires to try to confirm my calculations. That said, I have come across 3 different references where 17.5x24 rears are coupled with 10-16.5 front tires. One primary example of this is on a John Deere 110 TLB.

85Hokie, you said that 4wd models typically will pull the rears just slightly with 4wd engaged. So, if the back tire increases diameter slightly, you take that front pull out.

Would there be enough difference in gearing between manufacturers of 4wd tractors to necessitate different combinations of tire sizes?

 

[North Idaho Wolfman]

Backing up a little, where or how are you getting the RC #'s?
I'm getting them from the manufactures of the tires.
And our #'s very greatly.
Your saying the RC of the average 10-16.5 is 96.69 and manufactures call it as 91, BIG difference.
The 420/70/24 your calling 145.77 RC and the manufactures are calling it average 136 RC again big difference.
The 17.5 x 24 your calling RC 150.8 and the manufactures are calling it an average of 155 RC.

If you arriving at your RC #'s off of a math formula, I think something is off.

Stock the tires 12.4-24 136 RC and 7-16 89 RC and had a ratio of .654 via manufactures listed RC's.
The 420/70/24 136 RC and 10-16.5 91 RC is very close ratio at .669.
This is little more than .02 % so almost an exact match.
Now changing from that to:
17.5 x 24 RC 155 and 10-16.5 91 RC is a ratio of .587 way off at 8%.
So you rears are going to out drive the fronts by a ton, thus forcing the front end down the road.

Remember, I'm not a salesman, this is not a paid for by Firestone / Goodyear / Michelin / or any other manufacture, and I have nothing to gain by passing on misinformation, I take what I research and pass it on.

All I can say is I know that that combination of tire is very likely (as nothing is 100% certain with tractors), going to cost you in repairs to the 4wd system down the line if not immediately as I know the ratio's wrong.

Edit for your edit:
I wouldn't compare another brand of tractor to yours as there is no relation at all, just as there is no relation to other models of Kubota, they all have different ratios hence different tire sizes!
Yes the front should pull the rears.

 

[knightgang]

I calculated my RC from the diameters listed for the said tires by the manufactures dealers. Math should be math, but I am not sure how the manufacturers do it.

Thing is, that using tire size calculators and looking at the offerings in these sizes from different suppliers, I have found different numbers.

For example: the 420/70/24, I was given a diameter of this tire by one dealer at 46.4", other information have this tire listed at 47.1 while another lists it at 48".

Same with the 10-16.5. I saw diameters of 30.1 to 31.4 listed over the course of about 15 tire listings. So I took an average.

I am finding very little information on a 17.5x24 as far as dimensions, but they are readily available for sale.

Wolfe, I understand everything you have said and that is why I am so concerned on this and want to put the right amount of money in the right place, but I also don;t want to spend more than necessary. You would think that when you looked up the size of a 420/70/24 tire that all of the dimensions would be the same given it is supposed to be the same size. Given the amount of variation that I have found, even replacing with the same size from a different manufacturer could cause an issue.

What about the cases of the John Deere Backhoe that are running the 10-16.5 / 17.5x24 tire combination? Just curious as to your thoughts here? Is this going to be a case by case by manufacturer about the right combination?

 

[North Idaho Wolfman]

I have found that doing the math off of diameter is never right.
The reasons for this is the diameter changes with load, weight, air pressure, and ply count that changes tire swash.
When reading specs for a tire, don't look at the diameter as it will throw you, just look at the RC specs.

As I noted in the edit of the last post, avoid comparing Green or Yellow Apples to Oranges (he he he)
Just because it works on a JD does not mean a thing!

 

[85Hokie]

And this too is where the rubber meets the road - pun intended!!

NIW - "The reasons for this is the diameter changes with load, weight, air pressure, and ply count that changes tire swash."

in simple terms, the diameter stays the same - but the RC changes with every bounce!

 

[knightgang]

And this too is where the rubber meets the road - pun intended!!



NIW - "The reasons for this is the diameter changes with load, weight, air pressure, and ply count that changes tire swash."



in simple terms, the diameter stays the same - but the RC changes with every bounce!

This makes me wonder if the RC can be "tuned" with tire pressure....


Sent from my iPhone using Tapatalk

 

[North Idaho Wolfman]

This makes me wonder if the RC can be "tuned" with tire pressure.

No not really, it's more of a tire structure dependent.

I understand that you want to get cheaper tires for the rear, don't we all!
You could do that with the rears and just Up size the front tires, there are options for that.

 

[knightgang]

It was more of a rhetorical comment.


Sent from my iPhone using Tapatalk

 

[knightgang]

So, I have been looking around and have hit on Titan Tires website. It appears that the 14.9-24 R4 in 6,8 and 12 ply all have a Rolling Circumference of 143.

Taking the into account the following:

Front tires: 10-16.5 R4 - RC = 91
Rears: 420/70/24 R4 - RC = 137
Ratio = .6642

Substitute in the 14.9-24 R4 and you get this:
Front tires: 10-16.5 R4 - RC = 91
Rears: 14.9 - 24 R4 - RC = 143
Ratio = .67132

This is only a 1/2% difference.

Now, the 14.9-24 is supposed to go on a 13" wheel, however the 420/70/24 is also supposed to go on a 13" wheel, but Kubota mounted them at the factory on a 15" wheel and it has been riding like that for 23 years.

So, in your opinion, would a 14.9-24 R4 tire work in place of a 420/70/24?

Thanks and I am still looking at options.

 

[85Hokie]

This is only a 1/2% difference.

Now, the 14.9-24 is supposed to go on a 13" wheel, however the 420/70/24 is also supposed to go on a 13" wheel, but Kubota mounted them at the factory on a 15" wheel and it has been riding like that for 23 years.

So, in your opinion, would a 14.9-24 R4 tire work in place of a 420/70/24?

Thanks and I am still looking at options.

The diameter of the wheel really has nothing to do with anything........you could place those on a 6" rim - ifn there was enough tire around to give you the RC!!!

If you are that close (1/2%) then shoot - buy em!!!!

 

[knightgang]

I think you meant the width of the wheel.

Now that I have a viable option, I just have to find some at a good price.