Combines Adding rear wheel assist to 7720 9600

Marshall, The 4WD helps in the mud, but it causes a lot of damage to the field and robs your power. The older units also cut your ground speed. We got tired of ruts and switched to bigger tires. I think we got a lot more "bang for the buck" with the large, low pressure tires than with 4WD. Start with Michelin 1050_50 R 32 tires on the front (this is the radial equivalent to 73X44-32). I think you'll find you can get through most anything. If you want to stop the rear from falling in I'd recommend the 28lR26 (or 750_65 R 26 for metric). If you have to haul the machine on a trailer, the wide tires may not be acceptable. In that case the Trelleborg 414 in 850_60-38 would probably be about the best. No matter what tires your using, get the ratings from the manufacturer and run them at the lowest possible pressure for your load. That's where the radial tires really help you in the mud.

Tire size on the rear axel does not make a difference as you might think of in the case of FWA on tractors. Because there is no mechanical connection between the drive axels the is no problem. The hydraulic flow from the pump must equallize between the motor on each rear wheel and the motor driving the transmission. On any combine I am familiar with opening the valves to the rear axel will cut ground speed close to half. You can make up for this by shifting to a higher gear on the transmission. Obviously the system is pumping oil through several motors so the is likely more heat but I don't think there is any difference on a 7720 with RWA compared to one without when it comes to the hydrostat resivior size or cooling system.