Tyre Size Question

Longford Leader

I currently have 13.6 R24 (340/85 R24) firestones on the front, they have around 20% left.
The backs have 16.9 R34 (420/85 R34), they have around 60% left.

I can get two 14.9 R24 (380/85 R24) Michelins locally, they have approx 70% left.

Can I put the 14.9's on without having to change the backs to 18.4's ???

croot

I did that this spring. I replaced the two cracked front 340/85 R24 with two new Coulter 380/85 R24 which cost €860. Back ones are still 420/85 R34 but I will change them when I win the lotto.

Zr105

Longford Leader wrote: »

I currently have 13.6 R24 (340/85 R24) firestones on the front, they have around 20% left.
The backs have 16.9 R34 (420/85 R34), they have around 60% left.

I can get two 14.9 R24 (380/85 R24) Michelins locally, they have approx 70% left.

Can I put the 14.9's on without having to change the backs to 18.4's ???

You could but you'll mess up the drive train, the ratios will be all out of sync and each time you touch the brakes or put it into 4wd the loading on the shafts will be way higher than they should be and will eventually go bang.

blue5000

But the michelins are only wider, they are not higher. If the circumference is the same as your old ones you should be ok. There must be a table somewhere for compatible sizes?

Old thread here
http://www.boards.ie/vbulletin/showthread.php?t=2056063753

mythos110

Zr105 wrote: »

You could but you'll mess up the drive train, the ratios will be all out of sync and each time you touch the brakes or put it into 4wd the loading on the shafts will be way higher than they should be and will eventually go bang.

Exactly right. Don't even consider it...

mythos110

blue5000 wrote: »

But the michelins are only wider, they are not higher. If the circumference is the same as your old ones you should be ok. There must be a table somewhere for compatible sizes?

The /85 means the wall height is 85% of the width.

i.e.
340*0.85 = 289mm
380*0.85 = 323mm

That leads to a very different circumference so as stated, it puts a lot of pressure on the drivetrain and diffs.

Longford Leader

Thanks for the replies.

I had a look in the tractor manual and it recommends to use 18.4s on the back if 14.9's are on the front.

I presume the ratio's are set to a fine tolerance and I don't want to be tempting fate down the line. There are enough bills to pay without adding to the pile.

I will stick with 13.6's on the front for now.

croot

Balls. It was the tyre guy that suggested to me to put on wider ones on the front and I asked was it ok to do that without changing them all and he said its fine.....baxtard

Nekarsulm

Get the wider front tyres only choose a 75 or 70 ratio side wall. Any decent tyre place will have charts which show the rolling circumference in mm and will tell you what matches the currently fitted tyres.
I replaced a set of 16.9 x 34 r85 tyres on the back of a Fiat 90-90 with 600 x 38 r50 and they match perfectly.

Nekarsulm

You can fit wider tyres no problem, you just need to go "down" a size on the aspect ratio.
Any decent tyre place will have charts which show the rolling circumfrence in mm, and will show what matches the original fitted size.
So if your current tyres are 85 aspect, fit wider ones with , perhaps, a 75 or 70 aspect sidewall.

I replaced a pair of 16.9 x 34 85 tyres with 600 x 38 50 tyres and rims, as they match the fronts perfectly.

patsy_mccabe

As the tyres wear, won't the back to front rolling circumference ratio be thrown out some but too. I suppose it can tolerate some deviations.

Nekarsulm

I think there is a slight amount of "lead" built into most 4wd systems, where the front wheels are geared fractionally faster than the rears. So a difference of 1 or 2 percent shouldnt present problems.

Nekarsulm

http://www.vredestein.com/agricultural-industrial-tyres/conversion-table

Longford Leader

If I buy new I will probably move to the 70 aspect ratio and get the 380/70 R24.

I was just considering the 14.9's because I could have got them for €100 for the pair.