AG TIRE TALK KEY TAKEAWAYS:
TRELLEBORG, “There is not a tire combination that is a “one size fits all” situation and often different operations would require different sets of tires for the same equipment.”
TITAN, “The tire industry has developed the RCI (Rolling Circumference Index) system based on the OD (Overall Diameter) of tires to assist manufacturers in the proper of selection of tire size combinations that will work.”
CONTINENTAL, “The Rolling Circumference Index (RCI) is a grouping of tires based on the rolling circumferences. The next higher RCI is always ~5.4% higher (+/- .2 % Tolerance), which means a factor of ~1.054.”
ALLIANCE, “If you are working with the Rolling Circumference Index group system, most MFWD tractors need to have fronts and rears that are 5 steps apart in the RCI, like a 50 Group in the rear and a 45 Group for the fronts.”
MAXAM, “If you have too much positive slip ratio (above 5%) you will have “excessive” work by the front tires pulling the rear tires equates to a loss of efficiency and higher fuel consumption. If you have a “negative” slip result, the front tires will experience a braking effect that creates a loss of pulling power, more fuel consumption and reduced steering capability.”
PRECISION INFLATION, “It is important to remember that RCI is a great guide; however, sometimes a tire will have a rolling circumference between groups or have variation between manufacturers- that is why we recommend contacting your local tire dealer to ensure lead lag is correct based on ACTUAL Rolling Circumference.”
BKT, “Because of the increased air pressures and load carrying capacities of IF/VF tires, it would be wise to check the standard wheels being used. Make sure they are built to handle the higher pressures and loads these tires will carry.”
Trelleborg Wheel Systems
Norberto Herbener: OE Applications Engineer
Modern Agriculture requires different kinds of tires (size, tread pattern, technology) for different applications and conditions in order to increase productivity, reduce compaction, increase flotation, reduce equipment damage and increase operator’s comfort. There is not a tire combination that is a “one size fits all” situation and often different operations would require different sets of tires for the same equipment. A typical case is with sprayers, where during crop applications “skinny” tires are used to reduce crop damage and large flotation tires are used for broadband application without a crop standing.
Equipment like sprayers (same size tires all around the equipment) or equipment with hydrostatic drive (like combines) are not affected by mandatory equivalent rolling circumference when changing tire sizes. However, with front wheel drive tractors (MFWD) the relationship between the front and the rear axle, called interaxle ratio, must be maintained in order to have the best traction performance and to avoid transmission issues. The front tires must always turn faster than the rear tires as they are smaller and need more revolutions to cover the same distance. In addition to this “faster” front tire rotation (depending on the interaxle ratio), the front tires rotational speed needs to be 1 to 4% higher than the rears to assure the grip and to help with steering. This additional rotational speed is called front axle lead. If the front tires cover less distance than the rear tires, they would create a braking effect with potential transmission damage. This is called front axle lag (we need to avoid this).
In order to make life easier for producers looking to change tires to a different size, the Tire and Rim Association (TRA) created an index called Rolling Circumference Index (RCI) to assist in tire size selections. This Index is created using the tire’s Rolling Circumference (RC). According to the RC, each tire is then placed in a RCI group. Each RCI group has a logical, even progression of RC where each RC is 5.4% larger than previous one. What is important when selecting different tire sizes is that the difference between the rear tire RCI and the front tire RCI is the same as the tires being replaced. This will maintain the correct RC ratios to insure proper front axle lead. For example, a front RCI 38 and rear RCI 43 (43-38=5) should have the same ratio as a front RCI 43 and rear RCI 48 (48-43=5). This is the value referred to by some OEMs when they mention that the ratio between the front and rear axles is 5 steps. For a tire size to be considered within a specific RCI, the calculated RCI of the tire must be between -0.2 to +0.2 of the RCI for that group.
Here are some points to consider:
1) Tire size – Each standardized tire size has theoretical calculated dimensions called nominal values. These dimensions allow for +/- tolerances and depending on the manufacturer’s tire brand, the same tire size may have a different RC. These differences could allow, depending on the tire brand and their tolerances, the same tire size from one brand to be included in one RCI group and another brand tire to be included in a different RCI group. This is the reason why specific tire combinations are offered by OEM in some tire brands and not in others.
2) Tread pattern – A R-1 tire’s RC is smaller than its R-1W counterpart of the same size and the same brand. It’s suggested not to mix tread patterns.
3) RCI spread – Not all tire sizes can be assigned to specific RCI group (remember the -0.2 to +0.2 spread of RCI) and some will fall outside this spread. For example, if the formula result for a specific tire size and brand is 47.5, it would not have an RCI (for RCI 47 you would include the range from 46.8 to 47.2 and for RCI 48 it includes from 47.8 to 48.2).
4) Load and inflation pressure – the RC specified for each tire size and model in the Tire Manufacturer Databook is measured when the tire is set at nominal inflation pressure and under nominal load. The tire RC will change (not too much) depending the inflation pressure used for the load on that specific equipment. OEM engineers physically test each tire combination using the correct inflation pressure for the load before they release these tire combinations and offer them for sale.
In order to be able to consider all tires (including the ones out of the RCI spread) and to assure we have correct front axle lead; we need to use the actual Rolling Circumference (RC) of both tires and the interaxle ratio for that specific tractor model for our calculations. This interaxle ratio will change depending the tractor model, type of transmission and the front axle configuration. Your equipment should be able to get this information and calculate if the desired tire combination will have an acceptable front lead.
RCI is a good initial guidance when changing tire sizes but always check to have the right interaxle lead percentage to avoid transmission damage.
Alliance Tire Americas
Nick Phillippi: National Product Manager
When it comes to a tire/wheel conversion, step one is to determine the goal or the need of the equipment. What isn’t it doing or what do you want it to do better? Then determine if you’re doing all you can with what you have. For instance, are you operating at the optimal inflation pressure? Is your machine ballasted properly? Lastly, seek out all the options to really determine which makes most sense. Although there can be benefits to switching to taller or wider tires, they are not always the best choice.
Determine if you already have the correct size, load index and type of tread on the unit before you move to make major changes.
If you’re looking for more footprint and thinking about a larger tire, you need to be sure you won’t have too much tire— if your tires are too big, you won’t be able to gain what you want as insufficient weight will not provide full footprint at low inflation pressures. Also be sure you understand the air pressure requirements at different operating loads and speeds and determine if you are going to be able or willing to make changes on air in the field. You may have a situation where you have the right tires, but need a central tire inflation system (CTIS) to take advantage of them.
Most tire/wheel conversions are done to gain footprint or increase tire volume (actually, they’re both the same thing), lowering air pressure and thereby reducing compaction. But, before you jump to expensive or complicated solutions, first determine if moving from a bias to radial, or from a standard radial to VF, or finding a tire with a larger overall diameter that can be mounted on the same rim can get you to the target goal.
One of my favorite up-fitment examples for a very popular tractor setup is switching out 480/80R50 and 380/80R38 sets to VF480/95R50 and VF380/95R38 sets. You don’t need a rim change, there are normally no clearance issues, and you can lower ground pressure by up to 52%.
Important: whenever you change sizes on MFWD units, you must maintain the correct lead/lag by keeping in mind the RCI—the Rolling Circumference Index, which factors in the length on the ground of one full rotation and the height of the tire (don’t worry, it’s on a chart from your tire manufacturer). The index is a grouping of tires that are all within about 4 inches of each other in outer diameter (OD).
Basically, your tractor is designed around a very specific ratio between the RCIs of your front and rear tires, so to avoid excess tire wear or even destroying your transmission or transfer case, you must change the front and rears in the same percentage.
In the above example, we took off rears that had a rolling circumference of 240.7 inches and replaced them with tires with rolling circumferences of 256.6 inches, a difference of 6%. Same way with the fronts: we moved from rolling circumference of 183.3 inches to 195.2 inches—or the same 6%, to maintain the same front-to rear-ratio. Be careful—even standard R-1 tires of a particular size from two different manufacturers—or different designs from the same manufacturer—may vary slightly on RCI, so double check them all. You need to keep the change the same or at least within 2%.
If you are working with the Rolling Circumference Index group system, most MFWD tractors need to have fronts and rears that are 5 steps apart in the RCI, like a 50 Group in the rear and a 45 Group for the fronts. At Alliance Tire, we prefer to provide the actual number of inches in the rolling circumference so you can be extremely accurate about your choices.
Regardless of which RCI system you use, when you’ve picked out the options, it’s always good to get that second opinion from your equipment dealer or experienced farm tire technician. Be careful: “close” may not be close enough, and it will be an expensive lesson.
Some conversions are done to increase traction, reducing slip and thereby saving on fuel. Again, before you invest in new tires and rims, be sure you are already at the correct and lowest air pressure and see if moving to VF tires or perhaps just a slightly wider or taller tire on the same rim can meet your needs.
As with any investment, it’s not just enough to say you will save money (or fuel or time) or reduce compaction. You need to compare the likely savings to the cost and really see if the payback will be worth the payment.
Also, are you making the decision based on an unusual season, a unique event ,or small part of your overall operation? If so, be sure the investment will pay out over the normal operations and normal conditions that you will be farming in. Is there another way that might be less costly for those small windows of need, like leasing another unit for short periods?
Selecting the right tire/rim combination and using it to its best advantage can be a very powerful way to get the most performance and efficiency from your equipment. It can be worth every penny you invest, and then some. But before you make the leap, make sure you really need it, and talk to your tire dealer for advice so you know you’re making a great investment and using it well.
Precision Inflation, LLC
Ken Brodbeck, VP of Technology
When & How to Use RCI Chart
The large majority of tractors in the 100 to 400 hp range are set up to go between 30 inch-wide rows.
But what about 20” rows or bedded vegetable crops? Narrower tires are in order.
And what if the 360 hp Mechanical Front Wheel Drive (MFWD) machine is the big horse on the farm? We may need wider tires, not those wimpy 480 mm wide things!
So how do we find the new set of tires and keep the proper front/rear tire gear ratio required by the tractor manufacturer?
Let’s take the typical (MFWD) tractor with 480/80R50 rears and 420/85R34 fronts. This is defined as a “5 Step Tractor.” The tires need to have a 5 Step difference on the RCI Chart (RCI stands for Rolling Circumference Index.) RCI 48 – 43 = 5 Step difference between front and rear tires. Going to a smaller diameter front and rear will work as well by selecting any tire from RCI 47 rear – 42 = 5 Steps.
Alternate Section Widths (mm) Range
The chart below has popular 5 Step MFWD tractor RCI 48 480/80R50 rears and RCI 43 420/85R34 fronts, showing alternate wider and narrower rear and front sizes highlighted below.
Narrower Section Widths (mm) Range
If you are looking for narrower options, simply move to the left of the RCI 48 480/80R50 for rear tire options, then move to the left of RCI 43 420/85R34 for front tire options.
Wider Section Widths (mm) Range
If you are looking for wider options, simply move to the right of the RCI 48 480/80R50 for rear tire options, then move to the right of RCI 43 420/85R34 for front tire options.
Alternate Set Ups & Important Checks
Remember, tires are considered part of the tractor transmission. While most MFWD tractors have “5 Steps” between the front and rear tires, there are some 4 and 6 Step tractors out there. Always contact the tractor manufacturer to be sure. For some select large Deere tractors, you can look at the frame for indicator:
In the case of a 4-Step Tractor, you simply select a RCI 48 Rear and matching RCI 44 Front.
RCI 48 – 42 = 4 Step difference between front and rear tires.
It is important to remember that RCI is a great guide; however, sometimes a tire will have a rolling circumference between groups or have variation between manufacturers- that is why we recommend contacting your local tire dealer to ensure lead lag is correct based on ACTUAL Rolling Circumference.
Lastly, it is important to contact your local tire dealer before making final selection of tires to make sure equipment has sufficient clearances for desired options and that front and rear widths correlate for application.
For getting the first pass options narrowed down, nothing beats the RCI Chart!
Titan International, Inc. (Manufacturer of Titan and Goodyear Farm Tires)
Scott Sloan: Ag Product Manager / Global LSW
There are many occasions when an end user is looking to change the tire set up on their tractors, such as going from narrower row crop section widths to more of a floatation configuration. On the other hand, in the case of utility and compact tractors moving away from an Ag bar design to and industrial lug design, both of these are fairly common. The thing that the end user needs to consider is the gear ratio of the tractor and the lead/lag ratio. Front tires typically are spinning 1%-5% faster than the rears. This ratio is important in the performance of the tractor.
Most North American MFWD tractors have a gear ratio of approximately 1.332. The tire industry has developed the RCI (Rolling Circumference Index) system based on the OD of tires to assist manufacturers in the proper of selection of tire size combinations that will work. There are charts available that show the different sizes and their perspective RCI values. An example is the 480/80R50 rear and 380/80R38 combination on MFWD. The rear tire is considered a group 48 RCI and the front is considered a group 43 RCI. The difference in those two groups is 5. That is called a 5 step drop. The idea is as long as the rear and front RCI’s are 5 steps apart they will work on the tractor. You can match a group 49 with a group 44 or group 47 with a group 42 and they will work fine on the tractor. Fig. 1 is an RCI chart showing the different sizes and their perspective RCI’s. This is a great tool to use when selecting different options for your tractors.
Titan offers an online lead/lag calculator to assist in the selection of tires. By entering the published rolling circumference of the front, rear or both and the gear ratio of the tractor it will tell you the target rolling circumferences of the tires that are needed to make the combination work. https://www.titan-intl.com/en/resources/Lead-Lag-Calculator
If the lead/lag is not correct there can be issues with the tractors performance. For instance if the front tires are not changed the rear tires OD increases, this will put the tractor into a negative lead/lag meaning the rear tires will be pushing the fronts which could lead to the differential binding up and potentially locking up. If the front tires OD are increased without and change to the rear the tractor will have a higher over travel, which means the tires will be spinning too fast causing faster wear and potential issues with the tractor not disengaging from the front wheel assist without some work.
Whenever changing the fitment it is best to do the research and understanding what is currently on the tractor and what the RCI drop is from rear to front. Once that is understood it is relatively easy to identify sizes in the appropriate RCI that will work. Local tire dealers are also a great resource in navigating through the appropriate options for the best fitment. On a final note, when moving to a larger OD tire on a tractor although the lead/lag is correct, it is important to verify that the tires will clear all the fenders, frames, fuel tanks and toolboxes to ensure the new tire will fit into that particular envelope on the machine.
Michelin Ag
David Graden: Operational Market Manager – Agriculture
Most commonly, a producer or tire dealer will use lead/lag calculations or RCI (rolling circumference index) when converting tires and wheels from duals to wide singles, converting bias to radial, old standard radial tires to newer metric tires, etc. The purpose of this would be to increase flotation, reduce soil compaction, lower fuel consumption or improve productivity.
Rolling Circumference Index (RCI) is commonly used to match front and rear tires to each other within a given gear ratio. For example, a certain John Deere machine may have a “5 step” transmission. Well, a 480/80R50 Michelin Agribib 2 has an RCI of 43, therefore it will match several front sizes that have an RCI of 43 (13.6R38, 420/85R34, 380/80R38, etc.) If you look in the Michelin data book, however, not all tires have a published RCI. This is where you have to be careful. Michelin does not publish RCI for all tires because some tires may fall within a grey area, meaning depending on the load and inflation pressure, these tires will fall within one of two RCI numbers.
In the instance where RCI is not available, you may need to calculate the mechanical lead. Most tractor manufacturers impose a mechanical lead of between 0% and 6%. This lead is specific and may vary depending on the manufacturer and the vehicle. See the following calculation:
Example:
- You have an older Ford-New Holland 8970 equipped with 16.9R30 front tires and 480/80R46 rear tires
- All tires are worn out & customer wants to replace them with Michelin Agribib 2
- You look in the owner’s manual and find the tractor has a Mechanical Ratio = 1.35
- Will the Michelin 420/90R30 and 480/80R46 Agribib 2 give an acceptable lead?
Answer:
- 420/90R30 Agribib 2 has an RC = 177.7 in
- 480/80R46 Agribib 2 has an RC = 228.5 in
- Mechanical Ratio/inter-axle ratio = 1.35
- In this instance, yes; the proposed Michelin Agribib 2 tires will fit within the manufacturer’s specifications
It is very important you keep this in mind when are converting one size of tire to another, or in some cases, even between tire manufacturers. If your tires are mismatched with respect to the mechanical lead ratio of your machine, you would experience increased fuel consumption, erratic or rapid front/rear tire wear, wear and tear on your transmission, and overall poor tractor performance.
If you need assistance with this, all Michelin Ag reps are equipped with the tools and knowledge to walk you through this.
Continental Agriculture North America
Harm-Hendrik Lange: Customer Solutions Engineer for Continental Commercial Specialty Tires (CST)
The first thing to consider is the vehicle’s/tractor’s operators manual and what is written about tires and tire exchange. Many tractors have special requirements. While general recommendations vary in the percentage values of proper lead (often 0% – 5%), many manufactures have a smaller tolerance band for specific tractor types, so this needs to be considered when changing tires.
We recommend the check to confirm the lead and acceptable technical data for each tire change individually. Requirements for different tractors are individual due to the variation of technical data of a specific tire size produced by different manufacturers. Even the data from the same tire manufacturer may vary for the same tire size, if multiple tire lines exist or if there are tires with different load indexes within the same tire line.
For traction and soil compaction (and maybe also for optics), it’s recommended to always choose the highest RCI option the OE permits for the vehicle type, if there are not specific reasons to divert from this theory. Factors that could lead to smaller tire diameters:
– If the total height of the vehicle needs to be limited. For example, operating in low buildings or if a lower center of gravity is required (for use on steep inclines).
– If the turning diameter needs to be optimized. Smaller tires on a steering axle would allow for sharper steering angles.
Of course, more tire width and more section height normally mean more air volume = load capacity at the same inflation pressure. If there is no limitation from that perspective (like row applications or plowing in a furrow), maximum permitted values are recommended for maximum performance on the field.
If the tractor is mainly used for field work, lead percentage may be chosen on the upper tolerance of the permitted tolerance band, as little higher lead values allow sharper field end turning with engaged MFWD compared to lower lead values. If the lead is very low, a tractor tends to go straight even with turned steering wheels, like it does when diff lock is engaged.
On the other hand, if a tractor operates part time or is on road often, lower lead values may be considered. Even if the MFWD is switched off on road normally, many tractors engage MFWD while braking, so high lead values may create high tension in the drive line.
Example: OE allows 1.0 – 3.5% lead. One tire combination would end up in 1.1% lead and the alternative one on 3.4%. For a field work tractor, I would use the 3.4% option, for a tractor used often in road transport, I would choose the 1.1% lead option.
What´s behind the RCI values? The Rolling Circumference Index (RCI) is a grouping of tires based on the rolling circumferences. The next higher RCI is always ~5.4% higher (+/- .2 % Tolerance), which means a factor of ~1.054. The RCIs in the tractor´s handbook can give 2 different kinds of information:
- The maximum RCI that is permitted (including the front tires for pairing).
- What the RCI difference is between the front and rear axle, as this should be kept constant.
Example: The RCI ratio between 41 on the front and 46 on the rear is 5 (46 – 41= 5). If your tractor manufacturer alternatively allows you to increase to RCI 47 tires on the rear axle, the front axle should then also increase to 42 to keep the difference of 5 RCIs constant.
What does this example mean to the lead value?
The tire circumference difference can theoretically be calculated by the RCIs or the RCI difference:
- RCI difference = 5 = 1.054 x 1.054 x1.054 x 1.054 x1.054 = 1.054 ^5 = 1.30
- Old tires RCI 41 & RCI 46: (4275mm & 5550mm) = 5550 / 4275 = 1.30
- New tires RCI 42 & RCI 47: (4505mm & 5850mm) = 5850 / 4505 = 1.30
For example, if the transmission ratio is 1.27 and the tire circumference ratio is 1.30, this would mean 2.36% lead for all tire combinations with 5 RCIs difference (1.30/1.27 = 1.0236) on this machine.
Nevertheless, as the real exact rolling circumference values can vary from the theoretical RCI numbers, the most precise calculation is reached by using the real rolling circumference data from the tire’s datasheet instead of the RCIs.
CEAT Specialty Tires Inc.
Jim Enyart: Technical Manager
There are several situations when a farmer should consider converting his current wheel/tire setup to an alternative configuration. The main issue to consider is what are your goals? Typically, a farmer wants to produce the highest yield with the best quality at the least cost of production. This will maximize the return which is why farmers are in the business of making money!
Compaction: If a farmer determines that compaction is limiting his ability to produce the highest yields and quality, an assessment needs to occur to identify the cause of the compaction. An action plan to correct the compaction problem needs to be developed and implemented. Converting tire sizes to reduce compaction can help by reducing ground pressure from your equipment. If you run bias tires during field operations you can simply change to radial tires or to the “IF” or “VF” to reduce ground pressure as well as increase traction. If converting to these higher technology type of tires does not satisfy ground pressure reduction goals then you may opt for a wheel/tire conversion to tires with a larger air chamber. You can decrease ground pressure by increasing the air chamber which can allow for higher load carrying capacities. These conversions can be via larger diameter, wider section width tires or both but will typically need rim changes also. There are options available to decrease your ground pressures and the best option for your operation should be investigated. Keep in mind that when changing tire sizes on front wheel assist tractors you need to maintain your lead/lag ratios to enable the tractor to operate mechanically within the design parameters.
Traction: If you have trouble pulling implements in high torque applications, grain carts or any other application with a reasonable amount of slip you may need to increase your traction. You might need to replace worn tires that don’t have enough lug depth left to give you the traction required or you may need to change your tire set-up. If you run bias tires by chance you can make drastic improvements by switching to the same size in a radial tire. If you are running radials you may want to consider changing to an “IF” or “VF” type of tire of the same size. The “IF” and “VF” tires not only carry more weight than a conventional radial tire at the same air pressures, they have a larger footprint. The footprints are enlarged due to the sidewall deflection which will grow the footprints in length. Growing the footprint in length will increase traction. Tire/wheel conversions for traction reasons is not very common since these conversions would likely be to a narrower tire that will increase traction but also increases ground pressure along with compaction potential.
Flotation: Flotation problems are encountered at times and harvest time is certainly a critical time. If you can’t harvest your crops due to wet soils you probably need to convert your combines to a flotation setup. Combine conversions to a flotation setup is usually a drastic change to maximum flotation. How much flotation do you need? Who really knows until after the fact so consider the most flotation you can justify.
Summary: When wheel/tire conversions are needed or wanted, you need to consider the best options that will help accomplish your goals. Conversions are expensive and they need to be done correctly to get the most benefit you can get.
BKT USA, Inc.
Dave Paulk: Manager Field Technical Services
There are several instances where it might make sense to change or convert tire sizes and wheels on tractors.
Switching from bias tires to radial tires would be a smart move in the right circumstances. Radial tires generally provide better traction, less soil compaction, better comfort, and can be run at lower air pressures. Although many radial sizes match the bias sizes in a conversion, there are instances where they don’t. In this situation, a person may have to change wheels and tires when converting to radial tires.
If narrower or wider tires are wanted or needed on MFWD’s (Mechanical Front Wheel Drive), in order to stay within the correct lead/lag ratio it will probably require changing out tires and wheels. These size changes are often required when going from wider to narrower rows, or vice versa. MFWD tractors are 4-wheel drive vehicles with different sizes of tires on the front and the rear. The transmission must be geared so that both fronts and rears turn at basically the same speed, since the fronts will turn more revolutions. When changing sizes on MFWD’s, the +1-5% tolerance to the lead/lag ratio must be honored to alleviate any transmission/transaxle problems. By knowing what the front to rear gear ratio is on a tractor, a person can use the RCI (Rolling Circumference Index) data to stay within the required range for the tractor. The rolling circumference is the distance in inches that a tire travels in one revolution. When selecting a new tire size, a person must choose the same front and rear combination RCI that matches up with the original tires. On MFWD tractors, if the fronts are too big and/or the rears are too small, the fronts will pull the rears (lead). If the fronts are too small, the rears will push the fronts (lag). They should both be turning at close to the same speed. It is necessary to be very careful when changing sizes on these tractors.
Changing tire sizes on 2-wheel drive tractors is straightforward. If both tires on an axle is changed, there should be no problems. The rims may have to changed depending on the size and the width of the tires used.
Tire sizes can be changed on full time 4-wheel drive tractors. These tractors have the same sizes of tires all the way around like a 4-wheel drive truck. If the same sizes are used on front and rear, there are no issues. Wheels will have to be changed because of the sizes and widths of the tires used.
When moving from a standard size tire to an IF/VF tire, manufacturers recommend a wider wheel to maximize the benefit these tires offer for weight carrying capacity and lower air pressures. BKT’s line of VF tires (V-Flecto) are designed as a Narrow Rim Option (NRO) and will work on the narrower standard rims. Because of the increased air pressures and load carrying capacities of IF/VF tires, it would be wise to check the standard wheels being used. Make sure they are built to handle the higher pressures and loads these tires will carry.
Maxam Tire International
Greg W. Gilland: Business Development & Ag Segment Manager
Ag tires are designed by size or footprint to fulfill following specific functions:
- Carry the Load
- Transmit the Torque (or driving power)
- Provide Direction
- Allow the Chosen Equipment to fulfill its purpose
As the Ag industry has evolved from animal driven to 2WD tractors to mechanical front wheel assist tractors (MFWD / MFWA), tires have also evolved in design, materials, functionality and performance. Ag practices around the world differ based on crops grown, soil conditions, moisture content and environmental conditions. In order to provide solutions that are flexible to an evolving global market, the equipment manufacturers or OEMs offer their machinery with tires of different diameters or widths for the same machine to adapt their equipment to the specific needs of that grower or crop. One of the key tools that is used to adapt or convert tires from crop to crop or grower to grower is the RCI or rolling circumference index. The RCI is the rolling circumference or the distance a tire travels in one revolution. The advent of powered front axle tractors or mechanical front wheel assist / drive (MFWD) tractors to provide more power (torque) to pull implements in the field requires a clear understanding of the RCI impact. The smaller front tires must travel faster to stay in the same ratio or time that the larger rear tires travel as they roll. In other words, the front tires will turn more times as the tractors moves forward for every rear tire full revolution.
Each MFWD tractor must maintain a balance between “positive slippage” or front tires pulling, and “negative slippage” when the front tires are slower or lagging therefore resisting the push from the rear tires. This balance is the tractor gear ratio also known as, “the lead or lag ratio”. As a rule, the “ideal” tractor gear ratio between the front axle and the rear axle (IAR or Inter-Axle Ration) ranges from 1.2 to 1.5. For the front tires to pull successfully and produce the right amount of pulling power with the right “slip” to stay within the tractor gear ratio, the proper tire slip needs to be between 1% to 5% = “positive slip”.
If you have too much positive slip ratio (above 5%) you will have “excessive” work by the front tires pulling the rear tires equates to a loss of efficiency and higher fuel consumption. If you have a “negative” slip result, the front tires will experience a braking effect that creates a loss of pulling power, more fuel consumption and reduced steering capability. Each OEM publishes their recommended Gear Ratio for each model MFWD or MFWA tractor they manufacture. Tire manufacturers should provide their tires’ rolling circumference (RC) or the RC Index (RCI) values in their published product data to allow growers to calculate the right size tire fitment from front to rear for their equipment to ensure the right amount of positive slippage or pulling power for the given gear ratio.
The Front to Rear Gear Ratio Tire Slippage Calculation is as follows:
(Front Tire RC x Tractor OEM Model Gear Ratio) / Rear Tire RC) – 1) 100 = Slip %
MAXAM publishes in our Ag catalog both the tires rolling circumference and the tire RCI value to assist growers in ensuring the tires chosen for their equipment can operate within their MFWD gear ratio limits or needs. Growers, at times, will also consider changing tires by either selecting a bigger tire combination or width to improve their traction, flotation or pulling capability. Below is the MAXAM example of tires having similar diameters but different widths within the same RC or RCI to determine what is the best tire size suited to meet their field or crop production needs. Using the above equation and the RCI data, as well as the OEM tractor gear ratio, any grower can quickly determine if their equipment will allow switching or evolving to larger tires without compromising their required gear ratio slip rate percentage.
The MAXAM Ag tire catalog provides our tires’ RC, RCI and diameter specifications, to assist growers in determining any technical information necessary to support their equipment needs or potential tire size evolution.
All information is provided in this blog solely to provoke thought. All deductions made from information on this site must be confirmed by Certified Ag Tire Dealer before use. Ag Tire Talk does not recommend anyone conduct tire service work with exception of Certified Ag Tire Dealer Professionals.