Yanmar AF120 turf tyre retrofit

[pumasong]

Hi folks,

I've been talking with a bunch of helpful forum users in another thread but thought it might be useful to others if I posted a build thread for the work I'm doing on my Yanmar AF120 grey import. It's one of the rice paddy type compact tractors that can be had relatively cheaply here in the UK. I bought mine with a factory FEL and it's been a solid workhorse for the last year.... but..... the R2 ag tyres were causing no end of problems as most of the work is on turf.

I had a look at turf tyres to fit the factory rims but the sizes were tricky - 8.3 x 22 rears and 6.0 x 12 fronts. Also I wanted to keep the ag tyres useable so that if I did need to use the ag tyres, I would be able to without messing about removing tyres from rims. I looked for suitable rims with the same stud pattern but couldn't find anything.

After a while I found these rims and tyres on eBay- they were from a John Deere 2520:

I calculated the rolling circumference from the diameters of the existing rims, and the front to back ratio. As the machine has permanent 4wd it felt important to keep the ratio as similar as possible. The ratio was 1.50:1 front to back on the standard rims and 1.51:1 on the new ones, which couldn't be much more similar, so I felt I was onto a winner.

I bought the wheels, this is a photo of them next to the tractor but not yet mounted (as the stud pattern and hub diameter are quite different):

The sizes are so similar I think they will work great. So the next challenge is getting them mounted.

In the other thread (here: Still trying to find turf rims for AF120) bmaverick pointed me to another similar project undertaken to fit JD rims to a Yanmar. They fabricated wheel adapters to fit, like this:

This is as far as I have got at the moment. I've spoken to a local engineering firm who can manufacture the adapters for me, and I've been prepping the wheels for respray as they have a fair bit of surface rust and paint damage.

I'll update here once the adapters are made and report on how they perform!

 

[pumasong]

For reference purposes, here are the dimensions of the old wheels and how I calculated that the new ones would be similar enough to the originals. Since my machine has permanent 4wd I was concerned that the front-to-back rolling ratio be as similar as possible to the originals to avoid gearbox windup and other problems. The ag tyres were measured from the centre of the hub to the peaks of the tread as I figured this was the most accurate way to get the true rolling circumference, and in any event that would be the rolling circumference when on tarmac (which is also where gearbox windup would occur if it was going to, due to the wheels not being able to slip).

Original Ag Front
Radius 310mm
Circumference (calculated from radius) 1947mm

Original Ag Rear
Radius 470mm
Circumference (calculated) 2953mm

I believe this gives a rolling circumference ratio of 1.51:1. (e.g. 1.51 rotations of the front wheel = 1 rotation of the rear)

The new JD 2520 tyres + rims are the following sizes:

New Turf Front
Total height/diameter 24" = 609mm
Radius (calculated) = 304.5mm
Circumference (calculated) = 1913mm

New Turf Rear
Total height/diameter 36" = 914mm
Radius (calculated) = 457mm
Circumference (calculated from radius) = 2871mm

This gives a front to back ratio of 1.50:1 which is almost identical to the standard rims.

The next stage is getting the adapters machined, which should happen in the next fortnight. I am also going to respray the rims as they are quite pitted and have some rust.

 

[rScotty]

SNIP.... The ag tyres were measured from the centre of the hub to the peaks of the tread as I figured this was the most accurate way to get the true rolling circumference......SNIP

The most accurate way I know of to get rolling circumference is to put a blob of paint on the tire, drive down the road, and measure the distance between the paint marks on the tarmac. That gives a true rolling circumference - which is what you want.

The rolling circumference of an irregular oval shape with one flattened side can be quite different from the circumference of a circle calculated from radius. I was surprised at the difference.

But it could go the other way and maybe you will be as pleasantly surprised at how close they are.
When I did the job you are doing, I ended up having only to buy new fronts to make the ratio perfect. Compared with the cost of rear turfs and wide rims, the cost of the new fronts was a pittance.
rScotty

 

[pumasong]

The most accurate way I know of to get rolling circumference is to put a blob of paint on the tire, drive down the road, and measure the distance between the paint marks on the tarmac. That gives a true rolling circumference - which is what you want.

Sounds a good idea, I will do this out of interest. That said, I'm not worried about slight differences. Truth be told the machine will see so little tarmac as to be irrelevant, and when I do sell it, I can put the old rims back on.

Nonetheless it will be interesting to find out the true rolling circumferences, I will do the experiment and report back!

 

[CoyPatton]

Sounds a good idea, I will do this out of interest. That said, I'm not worried about slight differences. Truth be told the machine will see so little tarmac as to be irrelevant, and when I do sell it, I can put the old rims back on.

Nonetheless it will be interesting to find out the true rolling circumferences, I will do the experiment and report back!

One issue with trying to determine rolling circumference is this was calculated from new tires not tires with unknown wear issues. An addition complication in this is if you can find rolling circumference data on tires, you will find that the same size tire from a different brand has a slightly different data point. One last huge issues is that of finding any data on R2 tread tires that most gray tractors were originally equipped.
All that to say that you will probably never be able to locate or calculate an exact data point for your tractor. Close is likely close enough! At worse case, you have to drive in reverse more often to assist in reducing any bind, although a life spend in loose soils, should allow ample slippage to reduce binding.
My old YM Yanmar was stuck in 4x4 when I got it and was bound up enough there was a occasional pop from the axle. As far as I can determine no long term damage. And it gas been freed up and now shifts between 2x4 and 4x4.

 

[pumasong]

Unfortunately mine is fixed 4WD, there's no option to run in 2WD so I do have to be conscious of binding/windup, but since it's going to spend almost all of its time (with me at least) on turf/soil etc I'm not overly worried. I've read that anything +/- 5% should be OK and I'm pretty sure I'm a lot closer than that.

 

[pumasong]

OK so here's a project update, there have been some ups and downs but long story short, the new rims are on the machine and working great

I had the adapters machined, the finish was excellent, if anyone in the UK is interested I can forward the details of the shop as they were super helpful and competitively priced.

Here are some pics of the adapters:

Rear

Front

(sorry I have no pics of the front adapters before they were on the machine)

I had a number of problems along the way which I worked around. I originally intended to bolt the rear adapters from the back of the hub, meaning that the bolts would not need to be recessed on the front surface. Unfortunately I didn't have a thread gauge and I managed to get the bolt thread pitch wrong, so the threads on the hub were different than those on the adapter. As a result I decided to drill out the adapter and then drill a 28mm recess for the bolt and socket, which worked fine. Boy was I glad that I specified aluminium rather than mild steel!

On the front, I found it very hard to line up the two sets of threads (coming out of the hub and into the adapter). The bolts didn't go in 100% cleanly but they are holding firm.

I had the machine shop make the adapters with an inner recess that matches the centre of the hub, and with a new protrusion that matches the centre holes in the new rims. That way, the bolts are just holding the two surfaces together, but the load is primarily on the centre recess/protrusion. This wasn't the case with the design I was using as inspiration, I'm pretty sure the way I did it is stronger.

I made the front adapters as thin as possible - we went for 17mm in the end - in order to have the least impact on steering geometry. The rears are thicker as it makes no difference and will aid stability having a slightly wider track.

These are the rims on the machine:

In practise I have found that there are no strange noises from the transmission with the new wheels, or at least none that weren't there before As far as I can tell, steering is unaffected by the increased track at the front (approx 35mm in total). Damage to the ground is drastically less than before, in fact I only see impact at the front on a full-lock turn. There is no wheel fouling anywhere on the machine, including at full lock.

Overall despite the setbacks I am very happy with the mod. I may have to get new adapters made for the front if the threads are badly damaged, but I'll find that out when I take them off. For the time being they are fine and I'm no longer getting in trouble for ruining the lawn!

 

[pumasong]

.... and yes, I am going to spray those new rims in the summer!

 

[rScotty]

Congratulations on a nice project. It looks very good. I'll bet you will be just as pleased. Here are some pictures of a similar job on a Yanmar we did back about 25 years ago.
It is striking just how much alike they are....even to the yellow wheels.
rScotty

 

[pumasong]

Thanks rScotty

Amazing how similar the two projects' outcome but your tyres/rims were in much better shape! I will budget for replacements as they become necessary.