Optimising Tyre Performance
Tyre Performance requires correct weight split and tyre matching steer to drive on Front Wheel Assist Tractors.
Tractor weight split by power group for maximum performance.
Tractor weight split by power groups are:
- 2WD 20-25% FRONT 80-75% REAR
- MFWA 35-40% FRONT 65-60% REAR
- E4WD 50-51% FRONT 50-51% REAR
Tyre diameters have to be matched so that the ratio of the front and rear axle speeds produces the correct ground speeds. Steer tyre lead between 0 to 6%.The Axle Speed Ratio may be written in the tractor handbook. It will be a figure of around 1.3 to 1.5 depending on tractor design. Diameters must be matched so that the ratio of the front and rear axle speeds produces the correct ground speeds.
Because the front wheels are smaller, they have to turn more quickly than the rear wheels to give approximately the same ground speed for both tyres. As well as this speed difference, the front wheels usually turn slightly faster than the speed that would give exactly the same ground speed as the rear tyres.
This ‘lead’ of the front wheels varies between models but is usually in the range 0-6%. The drive ratio of front axle to rear axle is not a standard for all FWA tractors, and this is evident by the varying tyre sizes used.
Problems Caused by Incorrect Lead
• When a front-wheel-assist tractor is operating with the incorrect amount of lead, the tractor will often bounce and vibrate as the front and rear tyres slip relative to each other to make up for speed differences.
• This is usually most noticeable on hard surfaces. It can be difficult to tell the cause of this without careful measurement.
• If lead is too low, the front wheels will not be doing their fair share of work.
• This can sometimes be detected during draft operation (particularly in soft conditions).
• Axle Speed Ratio [ ASR ] The Axle Speed Ratio may be written in the tractor handbook. It will be a figure of around 1.3 to 1.5 depending on tractor design.
Rolling Circumference by tyre size is necessary to place tyre into the right RCI Index Group as below:
Rolling Circumference by tyre size
RCI INDEX: Tyre set combinations will change for different crop operations and or soil type. Different tyre pressures for road and field.
Although tyre details are available from tyre dealers / manufactures, two tyres of the same ‘nominal’ size may have differing rolling circumferences. This is because of differing tread patterns, construction (radial or bias), or different manufacturers. It is important therefore to give details for the correct Technical data OD – SLR – RC – SI by type of tyre.
A tyre fitted to a non-standard width rim will also have different characteristics. Overall tyre diameters must not be considered in ratio calculations either, because the dimensions from the centre of the wheel to the ground are less than one-half the overall diameter due to tyre deflection.
Rolling circumference is used worldwide to select tyre ratios.
OD Outer diameter
SLR Static loaded radius
RC Rolling circumference
SI Speed Index