determining 4wd axle gear ratios

[case685]

Hi I'm looking at buying a Iseki TA357 4wd. It currently has 13.6-26's on the rear and 8-18's on the front that will need replacing. There are several sets of used 18.4-26's available locally which I would probably go with assuming they would fit on the rims. I would need however to find out the front to rear axle ratio to determine which size fronts to use. This being a grey market I won't be able to get that from the manufacturer so I'm hoping someone can outline for me a good and simple ( I aint no math whiz) way to determine that. Thanks. Gerry

 

[dbdartman]

Lift ONE tire, turn said tire TWO TIMES & count number of times drive shaft turns. If the drive shaft turns 5 times for every two revolutions of the tire, your ratio is 5:1. This applies to open differentials only. With a limited slip diff or solid axle you only turn the tire 1 revolution. Turning the wheel two revolutions simply makes up for the differential action. You could rotate the wheel one time, but then you'd need to double the drive shaft number to make up for the differential action.

 

[case685]

DB, thanks for the reply. I guess I should have spelled it out more clearly. What I'm looking for is a way to determine front axle to rear axle speed ratio. I'm thinking its as simple as putting the tractor on axle stands, putting the tractor in low gear and comparing revolutions. Is it that easy or am I missing something.

 

[AndyMA]

DB, thanks for the reply. I guess I should have spelled it out more clearly. What I'm looking for is a way to determine front axle to rear axle speed ratio. I'm thinking its as simple as putting the tractor on axle stands, putting the tractor in low gear and comparing revolutions. Is it that easy or am I missing something.

Yes that is what you are trying to match. From a practical standpoint when you change tire sizes you need to maintain the circumfrence ratio of the rear to front tires. Since circumfrence is a linear function of diameter times pi, it is also a direct ratio of the tire outside diameter that you need to maintain. If you do not maintain this ratio the tires will be forced to slip and excess forces will be applied to the driveline. You can easily measure circumfrence by marking a spot at the bottom of the tire and on the ground and driving slowly until the spot touches the ground again. More than one revolution just improves the accuracy.

Andy

Andy

 

[BotaBigot]

What AndyMA said...maintain the ratio of the front-to-rear circumferences.

That's assuming that the tires that are on it are the original sizes...

 

[Mike058]

This might be an interesting read for you if you haven't already seen it. http://tinyurl.com/2mfqn5

 

[case685]

Thanks Andy and Mike for the info. John, you brought up a point that does concern me a bit, knowing that usually tires and rims are removed from greys when they are put in containers. This tractor is a cab model though and I don't think the tires were removed in this case.

 

[Eagleview]

Take the numbers from the side of the tyres to a tyre centre , one of the numbers will be the aspect ratio of the tyres . Get the numbers off of the tyres you want to purchase and do the same to make sure the aspect ratios are still correct . This is not hard for them as they do it often when someone wants to change to turf tyres etc or vice versa . Rolling the tractor etc to make assumptions of the ratios is not accurate as tyre wear and tyre pressures come into play . As you are aware if you do'nt get it perfect the front and rear diffs will fight each other and wind up will occur .

 

[AndyMA]

Take the numbers from the side of the tyres to a tyre centre , one of the numbers will be the aspect ratio of the tyres . Get the numbers off of the tyres you want to purchase and do the same to make sure the aspect ratios are still correct . This is not hard for them as they do it often when someone wants to change to turf tyres etc or vice versa . Rolling the tractor etc to make assumptions of the ratios is not accurate as tyre wear and tyre pressures come into play . As you are aware if you do'nt get it perfect the front and rear diffs will fight each other and wind up will occur .

Where does aspect ratio come into the equation? You could have tires with widely different profiles (aspect ratios) but yet have the correct rolling diameter ratio and work just fine.

Andy

 

[Eagleview]

Front and rear tyre sets have "corresponding" aspect ratios so that they go on a 4X4 and will run as a matched set . If you want to go up a size or down a size , the front and rear tyres go on as a set to keep the ratio correct . You would need a degree wheel marked in .5 degree increments on the tyres to get it close by rolling the vehicle and as i said tyre pressures throw that out the door as your rolling a tyre on the tractor with the weight of that tractor causing flat spots on the bottom of the tyres reducing the rolling diameter and then measuring tyres probably not even mounted on rims never loan mounted on a tractor and trying to get it correct . Not to mention it's easier to ask the seller to get the numbers off the tyres so you can check rather than driving across the country hoping when you get there with your tape measure they will be what he needs . I was'nt trying to prove you wrong i was simply giving my opinion , as you have given yours .