The larger the rear wheel diameter, the larger the contact patch of the tyre with the ground, giving better traction.
Also lowers ground pressure which may or may not be useful depending on the circumstances. My mate says on his farm in Warwickshire they sometimes have to put tonnes of ballast on the tractors to get traction.
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All these suggestions make sense. But notice that steam traction engines intended solely for road use also invariably had big rear wheels too, so clearly fundamentally it must be to do with traction rather than ground clearance or minimising soil compaction.
Also note that early tractors had metal spiked wheels, not pneumatic tyres.
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and to carry (not drag like the steam traction engines) very heavy hydraulic operated implements on its 3 point linkage at the rear
Edited by massey on 31/12/2008 at 14:58
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Ground clearance must be a big factor.
In nearly all tractors, the crank, gearbox, rear diff and half shafts are at the same height.
The simplest way to get ground clearance is to bolt large diameter wheels onto the end of the half shafts.
Some of the tractors brought over from USA/Canada after the Second World War had smaller, but still over-sized, rear wheels, giving slightly less ground clearance.
The mudguards fitted quite closely, too, which suggests to me you rarely get quagmire conditions in tbe wide open spaces over there.
Edited by ifithelps on 31/12/2008 at 15:08
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The tyres are bigger at the rear for many reasons, most of which have already been mentioned.
The main reasons are, better grip, higher ground clearance and the ability to carry a greater weight. Some large ploughs and seed drills are very heavy, bigger tyres can carry more weight.
The smaller front tyres are smaller for a better turning circle and they do not have to carry such a large weight as the front ones.
Tractors without 4 wheel air brakes and no suspension are not supposed to do more than 20mph on the road, but most tractors will do 25mph, so you have to throttle back a bit!!!
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Movilogo - your thread got me searching Youtube for 'Muir-Hill' , the old equal 4 wheel drive tractors I used to drive as a boy on the farm I worked at. Everything seems to be bigger in my memory, and they don't look quite as large now, but some lovely footage of restored Muir-Hills in action. If you like that sort of thing!! I recall having to almost stand on the clutch and learn double de-clutching when corn carting as no synchromesh. Ear defenders a necessity. And people say my Golf is 'gruff'. Nothing's gruff when you grew up on open top tractors. Ah, nostalgia, and I'm only 44!!
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This thread and the present cold spell reminds me of the time when I went to a local national tyre firm to collect one of our tractors that had had new rear tyres fitted. It is fairly common practice to put air/water in the rear tyres to add weight and increase traction, usually anti-freeze is added at the same time. Now whether the tyre firm didn't realise this, or they plain forgot, i'm not sure, but i had a very unusual feeling tractor on the run back to the farm! couldn't quite fathom what was causing the surging sensation, until the boss realised that the water in the rear tyres had frozen absolutely solid!
Billy
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