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Thread: Tire Size and Overdrive Ratio in 1/8 scale Four Wheel Drive

  1. #1

    Tire Size and Overdrive Ratio in 1/8 scale Four Wheel Drive

    Remember that saying, A chain is no stronger than its weakest link? Well, that applies to R/C car racing. There are SO MANY things that have to be right in order for you to do well at a race.

    The suspension has to be free, the shocks have to work right, the center of gravity has to be low, the engine needs to be putting out enough power, the tires need to stick to the road (and to the rim), the camber, caster, toe-in, etc., have to be right, the radio has to be free of glitches, the carburetor has to be tuned properly, the servos have to return to center properly, the brakes shouldn't fade, the differentials (if you have one) shouldn't slip, the battery has to last for the length of the race, the drive belt shouldn't break, the fuel has to be good, the fuel-tank shouldn't have any air leaks, and on and on and on. Well, one new link in the chain you probably haven't thought too much about is overdrive ratio. If youíre racing 1/8 scale, this applies to you. If youíre racing 200mm sedan cars, itís still good to know, but you probably just want your tires to remain at a 1:1 ratio, with the front tires staying the same size as the rears.

    Many things in R/C car racing are intuitive. If you know how real cars work, then you might have a pretty good idea of how your R/C car works. However, very few of us are driving four-wheel-drive cars with smaller diameter front tires than the rear tires. That makes the answer to the following question far from intuitive.

    Here's a trick question for you: Do you want your front tires to wear at the same rate as your back tires? (In other words, if during a long race your front tires wear off .19" of foam rubber, and your rear tires wear off .19" of foam rubber, is this good?).

    Sorry, but if you answered yes, this may be the WRONG answer. All current four-wheel-drive 1/8 scale cars have larger diameter rear tires than front tires. That is a key point to keep in mind when thinking about overdrive ratios. Life would be simpler if the front and back tires were the same size, but who wants simple, right?

    First, some background information. On a four-wheel-drive car, the front tires PULL the car, and the rear tires PUSH the car. If the front tires are pulling just as hard as the rear tires are pushing, the overdrive ratio is considered to be 1:1, and you have a neutral handling car. You can greatly effect the way your car handles by making one end of the car work harder than the other. With the front wheels pulling the hardest, you almost have a front-wheel-drive car. It will be very stable, but with too much front wheel drive you won't be able to go as fast as other racers with properly set up cars. As a friend of mine put it, your car will push like a dump truck. On the other hand, with the rear wheels pushing the hardest, you have a car more like a typical two-wheel-drive car. Get on the throttle too hard, and the car will try to spin out. You'll probably find that your car will be a little more difficult to control, and again, you won't be able to go as fast as other racers with properly set up cars. My friend described this as the rear end will be very loose, especially when you accelerate. If you've ever lost a front or rear dogbone, you'll have an idea of how these two extremes may feel.

    There does exist an optimum overdrive ratio, and if you set up your car at that ratio, your car will be easy to control and very fast on the track. With this setup, your front wheels will be pulling just as hard as your back wheels are pushing. If you're smart, you'll probably set up your car like that before a race. You can get this information from the set-up sheets provided by the various car manufacturers or the people involved in racing the cars. If you're good at math, you can figure it out for yourself, based on the information provided in articles on four-wheel-drive car setup. With this setup, your overdrive ratio is described as 1 to 1, written as 1:1.

    It's VERY IMPORTANT that you realize that the overdrive ratio is directly related to the size of your tires. Remember though, that at a race, especially a long race, goes on, your tires will wear. They'll be smaller at the end of the race than at the start of the race. If you're an expert at this, you'll do your best to insure that the ratio between the size of the front and rear tires at the start of a race remains constant, so that at the end of the race, with both tires worn down, the new ratio of the size of the front and rear tires remains the same. If you're not an expert at this, you might think differently. For example, you might decide that the front end of your car doesn't have enough traction, and therefore put on softer tires on the front. Yes, you'll get more traction BUT the softer tires will wear faster, meaning that at the end of the race, your car might not be handling very well. This is especially true at long races!

    There are several reasons why one tire might wear faster than the other. The first thing to consider, is that softer tires wear faster than harder tires. Another factor is that the smaller the tire, the faster it has to be spinning, which means for covering the same distance on the track the smaller tire will wear out sooner. That's obvious when you think about it - if your tire was 1/2" diameter, it would wear out in just a few minutes, and if it was ten feet in diameter, the tire wear would be so small you probably couldn't measure it. (Wearing out the same volume of rubber would make a huge differnce on a tiny tire, and an insignificant difference on a huge tire). Then there's your driving style. Some drivers are very smooth, and their tires last a long, long time, while others are constantly spinning or sliding their wheels, due to excessive acceleration and harder than necessary braking. You can wear out a set of tires in one qualifier if you drive too hard, or you can make a set of tires last for a whole weekend of racing (or more) if you drive smoothly.

    Since soft tires typically wear at a faster rate than harder tires, you'll probably want to select suitable tire hardnesses such that your tires wear in a manner that the overdrive ratio remains the same.

    To re-emphasize the original question that this article is written about, you DON'T want even tire wear. What you DO want is for the RATIO between the size of the front tires and the size of the rear tires to remain the same.

    In actuality, the overdrive ratio is determined by much more than simply the sizes of the tires. You need to know the sizes of the various pulleys in the drive system, as well as the tire diameters, to calculate it. Once you do calculate it though, unless you change to different size drive pulleys, the ONLY thing that will effect the overdrive ratio is the tire diameters.

    Let me give an example of what you're trying to avoid: To exaggerate, let's say your car had a neutral overdrive ratio (1:1) when it was set up with two inch diameter rear tires and one inch diameter front tires. In other words, when you used tires of those sizes, the front end would be pulling just as hard as the rear end was pushing.

    Now, if you started out with this set of 2" diameter rear tire, and 1" diameter front tire, and over the duration of a race, each tire wore off 1/4" of rubber (which is half an inch on the diameter), let's see what would happen.

    • Back tires: original diameter 2" minus 1/2" wear = 1.5" new diameter
    • Front tires: original diameter 1" minus 1/2" wear = 0.5" new diameter

    The original ratio of rear tire size to front tire size was 2" to 1", which can be written as 2:1 (two to one). The final ratio of rear tire size to front tire size is now 1.5" to .5", which can be written as 3:1 (three to one).
    This ratio change has a direct effect on the overdrive ratio of your car. If the overdrive ratio was 1:1 when using 2" rear tires and 1" front tires, it won't be even close to that when you have 1.5" rear tires and .5" diameter front tires. Your car would handle very differently as the race went on, and if it was good at the beginning of the race, it would be terrible by the end of the race, when the rear tires would be pushing much harder than the front tires would be pulling.

  2. #2
    I suppose some of you might be wondering how you could set up your car to avoid this situation. One way would be to change the tires, so the overdrive ratio didn't get messed up. If the best overdrive ratio was obtained with a 2:1 ratio of rear tire size to front tire size, and if you know (or can calculate) that the rear tires will wear down to a 1.5" tire diameter, and you want the overdrive to remain the same, using your high-school algebra you can calculate that you want the front tires to wear down to 0.75" (half of the 1.5" rear tire size), which means you should use harder front tires (or softer rear tires), so that the tires wear at the desirable rate. See, that high-school algebra really does come in handy! The important thing, is that the ratio between the size of the tires at the beginning of the race remains constant over the duration of the race. Never mind that I'm using ridiculous tire sizes here, I'm just trying to show how the concept works.

    To get back to my original example, if you wore off .19" of rubber, the DIAMETER of the tire at the end of the race would be .38" less than the diameter of the tire at the beginning of the race. Take your chart of overdrive ratios (if you're racing a four-wheel-drive car, you better have one!), and figure out your starting overdrive ratio. After you've done that, decrease the diameter of the front and rear tires by that very same .38", and look up the new overdrive ratio. Unless your front and rear tires were the same diameter to start with, you'll find that at the end of the race you would have a very different overdrive ratio.

    When you're at a race, measure the size of your tires before and after each qualifier, and record this information along with the type and hardness of tires that you used. Once you have this information, based on past experience, and knowing how long the race will be, you will be able to select tire types, hardnesses, and diameters, such that your car will be at its best throughout the duration of the race.

    Since overdrive ratio is one of the most important factors in how your car handles, you MUST be aware of this in order to be able to do well in a long race. There isn't any alternative. If you don't want to be bothered by it, go by the car manufacturer's recommended settings.

    If you're reading this and trying to make sense out of it, maybe an example at the track would be far more meaningful than anything I can write here. The next time you're at the track, take a few laps with tires set up at the recommended size. You can find this in the set-up sheets for most cars. Drive at least ten or so laps, so you get a pretty good feel for the way your car handles. If you can, take a few lap times.

    Pull into the pits, and change your tires such that the front tires are too big, and the rears too small. To make sure you find out the effects of this incorrect setup, exaggerate - put tires on the front that are way too big, and tires that are way too small on the back. If you'd like to make the test a bit more fair, set the ground clearance and other things back to the way they should be, so you will only notice the effects of using tires with the wrong overdrive ratio. Once you're going around as well as you can, record your lap times.

    When you're done with this, set your car back to the recommended setup, and see how much nicer it is. Take enough laps so you get back to being used to how your car should feel. Again, record your lap times.
    Now, pull into the pits, and set your car up at the other extreme, with front tires that are way too small, and rear tires that are way too big. Again, take enough laps so you can find out exactly how the car differs from the correct setup. Record your lap times.

    When you're done with this other setup, set your car back to the recommended setup, and see again how much nicer it is. Take enough laps so you can identify exactly how your car's setup feels different. Again, record your lap times.

    Assuming that you do this properly, you should enjoy your car the most at the recommended setup, running tires of the appropriate size to create the recommended overdrive ratio. You'll know how your car feels at the wrong setups, and hopefully you'll realize how much better your car will be, if you select the appropriate tires so that the overdrive ratio remains the same over the duration of your race. This isn't too important in a five minute qualifier, but it's extremely important in an hour-long race. To find out how important it is, compare your lap times with each setup, and see how much faster you are with your car set up properly.

    To make the most use out of this information, you'll need to do a few things you might not have done before. For example, you might start measuring tire wear over the duration of a race, and use that information to predict how your tires will wear in a long Main Event. If the tires at one end of your car are wearing too quickly, you can switch to a different hardness or tire compound, and hopefully find tires that wear at the correct rate for the track that you'll be racing on. If you're smart, you'll figure all this out during qualifying, so you can select the best combination of tires for the Main Event.

    You'll also want to get a tire truer, so you can true your tires to the appropriate size before a race. Using your tire wear notes, and an overdrive ratio chart for your car, you will be able to start every race with a good set of tires, that you've set properly to give you the best overdrive ratio.

    You'll also probably start accumulating a large collection of tires, so for any situation you come across, you can select a combination of front and rear tires that will give you both the desirable wear rate and the proper overdrive ratio over the duration of the race. While you're at it, make sure your tires are big enough so that the chassis won't be dragging along the ground when your tires are worn down near the end of the race.

    By the way, the information in this article is just one small piece of the pie. Knowing and understanding it is not going to change you from a middle of the C racer to one of the top racers in the A-Main, but if you race in long races, it will definitely give you an edge on the competition, especially when they ignore things like this. If you use a car setup from someone who's aware of this, and provides a recommended setup that takes tire wear into account, you can blindly follow their setup, and hope that you do well. On the other hand, if you take your own tire wear measurements, and set your car up so that the overdrive ratio remains optimal over the duration of a long race, you might be amazed how easy it is to do better than much of your competition, many of whom have foul-handling cars as the race approaches the end.

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