When comparing agricultural tyres one of the important things to consider is whether the tyres will change the clearance under the tractor. A practical example is if you’re changing to rowcrop wheels but don’t want to lower the clearance under the tractor. There is a very easy way to find out by comparing tyre data.
First of all – ignore any data which refers to the ‘Diameter’ or ‘Overall Diameter’ of a tyre. Diameter can be misleading as it doesn’t take into account how a tyre sidewall flexes under load.
The measurement we need to compare is the Static Loaded Radius
What is Static Loaded Radius (SLR)?
Static Loaded Radius or SLR (sometimes referred to as Loaded Radius, LR or R’) is the measurement from the centre of the axle to the ground while under load and set to the correct pressures.
It’s important to note that the Static Loaded Radius is not half the Diameter – the SLR will be less because of the flex in the tyre’s sidewall. The difference between SLR and the half the diameter is known as the deflection.
How can I use Static Loaded Radius to compare clearances between different sets of wheels?
In simple terms, look up the SLR for the two tyres you want to compare – the difference in the SLR is the difference in clearance between the two tyres.
Assume we have a tractor on standard wheels and want to know how much a set of rowcrops will change the clearance under your tractor. We could do the following:
Existing wheels on the tractor
650/65R42 Michelin Multibib SLR: 858mm
Potential rowcrop wheels we could fit:
380/90R54 BKT Agrimax SLR: 930mm
380/90R50 Mitas AC85 SLR: 888mm
380/90R46 Mitas AC85 SLR: 846mm
Then, subtract the current tractor wheel SLR from the new SLR to get the difference in clearance: 320/90R54 BKT would be 930mm-858mm=72mm So the 320/90R54 BKT rowcrops would lift the tractor (increase the clearance) by 72mm.
Differences in clearance would be as follows:
380/90R54 BKT SLR: 930mm +72mm
380/90R50 Mitas AC85 SLR: 888mm +30mm
380/90R46 Mitas AC85 SLR: 846mm -12mm
Normally it’s sufficient to just compare the rear SLR’s but you could also compare the fronts to be certain.
Where to find the SLR data on a tyre?
SLR data is available online for most tyre makes, but sometimes it can be a little tricky to find. Hopefully the following guides should help:
It can be very confusing to find the SLR on the Michelin website, so we suggest looking in their databook.
You can view and/or download the Michelin 201 databook here: https://bwt.uk.com/wp-content/uploads/2021/12/Michelin-AG-Databook-2019.pdf
Scroll down to find your tyre size.
The Static Loaded Radius can be found under the column called LR mm. (In this instance it is 949mm)
For trelleborg, click the link below to get to their tyre selector search page.
Choose the correct tyre model
Scroll down to the technical data table. Static Loaded Radius is the 5th column in. Not to be confused with Speed Radius Index.
Bridgestone tyre data is also really hard to find online – the easiest place (and most up-to-date) is on the Bridgestone app.
Input your tyre size, select the icon showing the tyre with a pair of callipers. SLR is the one on the bottom left, in this instance measuring 844mm.
- Visit: https://www.bkt-tires.com/gb/en/#findyt
- Input your tyre data; Select rim diameter, Select size & Select pattern
3. Scroll down and find the correct tyre size
4. The Static Loaded Radius is shown under the column called SLR
- Visit the Maxam databook online: https://maxamtirena.com/wp-content/uploads/2019/08/Maxam-AG-Catalog-619.pdf
- Scroll down to find your tyre size
- The SLR can be found under Static Loaded Radius (mm) – shown as 884mm in this example