Matching Imperial dual wheels to Metric tractors.

[Dukes Fit]

Some real world advice sought and a lot of strange numbers involved.

Tractor is on 650/65R38 rear and 540/65R28 front.

Ideally I’d dual up with the same but can’t find any of that size and buying new is eye watering.

From some internet searching it appears that a 20.8x38 is equivalent to a 520/85R38 and a 16.9x28 equivalent to a 420/85R28

A 520/85R38 compared to a 650/65R38 is 2.1% taller or in simple numbers 1.5 inches (38mm)

A 420/85R28 is 0.9% taller than a 540/65R28 or 0.5 inches (13mm)

I know a dual shouldn’t be taller than a tractor tyre so my questions are .....

Will a worn 20.8x38 dualled to a decent condition 650/65 work without putting undue stress on the studs/hubs? Likewise a worn 16.9x28 dualled to a decent 540/65?

Is anyone running a similar set up to the one above.

Will the wider metric tyres fit on the same stocks dual rim so when they are sufficiently worn they can be put on the dual wheels when new ones are put on the tractor?

Or if anyone has the tractor sized duals for sale?

Apologies for the complicated explanation and thanks in advance

 

[Nearly]

I'm going to post my favourite chart.
The different sizes are designed to be a match.
There will be more difference between tyre brands and due to wear than you have calculated between theoretical sizes. Just my opinion.

 

[Mr Charisma]

I may have a set of 20..8 38 and matching fronts for sale

 

[Flat 10]

Some real world advice sought and a lot of strange numbers involved.

Tractor is on 650/65R38 rear and 540/65R28 front.

Ideally I’d dual up with the same but can’t find any of that size and buying new is eye watering.

From some internet searching it appears that a 20.8x38 is equivalent to a 520/85R38 and a 16.9x28 equivalent to a 420/85R28

A 520/85R38 compared to a 650/65R38 is 2.1% taller or in simple numbers 1.5 inches (38mm)

A 420/85R28 is 0.9% taller than a 540/65R28 or 0.5 inches (13mm)

I know a dual shouldn’t be taller than a tractor tyre so my questions are .....

Will a worn 20.8x38 dualled to a decent condition 650/65 work without putting undue stress on the studs/hubs? Likewise a worn 16.9x28 dualled to a decent 540/65?

Is anyone running a similar set up to the one above.

Will the wider metric tyres fit on the same stocks dual rim so when they are sufficiently worn they can be put on the dual wheels when new ones are put on the tractor?

Or if anyone has the tractor sized duals for sale?

Apologies for the complicated explanation and thanks in advance

That’s the standard way of doing it that you have worked out. All depends on brands etc. Don’t over inflate the duals.

 

[quattro]

Some real world advice sought and a lot of strange numbers involved.

Tractor is on 650/65R38 rear and 540/65R28 front.

Ideally I’d dual up with the same but can’t find any of that size and buying new is eye watering.

From some internet searching it appears that a 20.8x38 is equivalent to a 520/85R38 and a 16.9x28 equivalent to a 420/85R28

A 520/85R38 compared to a 650/65R38 is 2.1% taller or in simple numbers 1.5 inches (38mm)

A 420/85R28 is 0.9% taller than a 540/65R28 or 0.5 inches (13mm)

I know a dual shouldn’t be taller than a tractor tyre so my questions are .....

Will a worn 20.8x38 dualled to a decent condition 650/65 work without putting undue stress on the studs/hubs? Likewise a worn 16.9x28 dualled to a decent 540/65?

Is anyone running a similar set up to the one above.

Will the wider metric tyres fit on the same stocks dual rim so when they are sufficiently worn they can be put on the dual wheels when new ones are put on the tractor?

Or if anyone has the tractor sized duals for sale?

Apologies for the complicated explanation and thanks in advance

I think I’ve a set of 20.8-38and 16.9-28 for sale (duals)

 

[mf7480]

20.8 tyres are basically an 80 profile rather than 85 so if you can find some imperial tyres they will match better

 

[D14]

I'm going to post my favourite chart.
The different sizes are designed to be a match.
There will be more difference between tyre brands and due to wear than you have calculated between theoretical sizes. Just my opinion.
View attachment 859510

I don’t understand that chart because it’s got a 650 75 R38 tyres listed under the 42 rim size? So what size dual in old terms would run with the 650 75 38 wheel?

 

[Flat 10]

I don’t understand that chart because it’s got a 650 75 R38 tyres listed under the 42 rim size? So what size dual in old terms would run with the 650 75 38 wheel?

20.8 r 42..............

 

[D14]

20.8 r 42..............

Having a stupid moment as how can a 42 work with a 38?

 

[Flat 10]

Having a stupid moment as how can a 42 work with a 38?

Step duals.

 

[D14]

Having a stupid moment as how can a 42 work with a 38?

Yes but why on the chart is a 38 wheel listed under a 42 rim? Surely 38 is 38?

 

[Flat 10]

Yes but why on the chart is a 38 wheel listed under a 42 rim? Surely 38 is 38?

Because it’s equivalent to a 20.8 r 42. And then you use step duals (relatively uncommon I know) it’s all to do with varying depth of sidewalls. I’m not a very good explainer.

 

[Nearly]

Because it’s equivalent to a 20.8 r 42. And then you use step duals (relatively uncommon I know) it’s all to do with varying depth of sidewalls. I’m not a very good explainer.

And I just copy and paste the chart.

 

[quattro]

Yes but why on the chart is a 38 wheel listed under a 42 rim? Surely 38 is 38?

i think the chart relates to rolling circumference of the tyres
as it states on top of the chart

 

[Dukes Fit]

I'm going to post my favourite chart.
The different sizes are designed to be a match.
There will be more difference between tyre brands and due to wear than you have calculated between theoretical sizes. Just my opinion.
View attachment 859510

I'm going to post my favourite chart.
The different sizes are designed to be a match.
There will be more difference between tyre brands and due to wear than you have calculated between theoretical sizes. Just my opinion.
View attachment 859510

Agreed, I was thinking a set of 30% 20.8 would have a significant enough reduction in size to match up to the 650/65

 

[Dukes Fit]

Thanks for the replies, it confirms what I was thinking.
I did find a pair of 650/65R38 for sale on a dealers site but no wheels to suit as I reckon a 650 tyre would be a squeeze on an 18 inch rim? Unless anybody knows better.
I'm guessing there aren't that many 23inch wide dual wheel rims around

 

[s~~t shoveller]

You need to make sure you have enough clearance inbetween the tyre too,if you try to squeeze wide tyres on old dual rims you really need to widen the band that fits I the rim.I have done several myself by welding a flat strip on and bending it to a circle as I went.hope that makes sense!

 

[s~~t shoveller]

I have also cut some rims in the middle and widened them to accept wider tyres.this is the best way as you don't need to widen the spacer band but it's not for the faint hearted.and also would mean you can put your tractor tyres on when they're worn.local engineer will do it for you for sure.

 

[Akerman]

I may have a set of 20..8 38 and matching fronts for sale

Are they still for sale

 

[Akerman]

I think I’ve a set of 20.8-38and 16.9-28 for sale (duals)

Have you still got the duals for sale