|Grand Prix Racing -||The Science of Fast Pinewood Cars|
The effect of drift is difficult to estimate in terms of time and distance. But by knowing how and why drift happens, we can develop defenses against it. Some of those defenses will be design decisions. Some procedures we must do before the race. Others things we must do at the race.
One of the best ways to avoid problems is simply the practice of good wheel alignment. But even the best alignment can be thwarted by a bump on the track. Here are some more methods that have been tried for rough tracks and other conditions. But remember to check with your race coordinator to see if these practices are allowed.
- Avoiding the median
- uneven tread friction (that is, variation in the texture of your wheel or track surface)
- uneven axle friction (all your wheels didn't get the same treatment or they have defects)
- "edging" - inner wheel edges digging into the track's wood grain
- How else can drift happen? - too much axle play.
- wheel vibration
Colliding with the lane median or another car is the most costly consequence of drift. It makes sense that there should be a few simple ways to avoid hitting the lane median or at least minimizing the worse effects. Here is a quick sketch of three of those methods.
One method of reducing the effect of median impacts is to bend the wheels slightly under the car. Besides giving more clearance, the wheels actually roll against the median if they get too close. There is still impact, but rolling introduces less extra rubbing and does little more damage than rolling on the track lane.
Wheels can be canted up as well, but the inner edges of the wheels scrape the top of the lane median. There is a trade off in either case because friction at the car body (down) or at the axle hub (up) is increased.
Another method is to put "curb feelers" or "whiskers" on your car. These are a pair or rod or flame-shaped, springy projections mounted below the car near the front. They ride on either side of the lane median. Usually, they touch the median lightly all the time. As the car veers to one side, the whisker on that side applies more and more force to the median until the car is coaxed back to the center. However, if the whiskers are attached too far ahead of the center of mass, the car will bounce back and forth across the lane (elastically) at the end of the track.
Wheel impact with cars on neighboring lanes can also be a problem on narrow tracks. Sand the lettering off the sidewalls of your wheels to prevent catching the opponent and slowing both. You can also design your car with a narrower wheel separation. This will also reduce the maximum median impact angle.
Bad road conditions can send the best aligned car into a tizzy. Two conditions most likely to affect your car's path are bumps or rough areas and smooth patches or slippery surfaces. Worse yet is when one of these occurs on one side of the median and not the other. When that happens, the tread friction on one side of the car differs from the other and a sideways force develops toward one side of the lane median.
In this way, an unevenly smooth track can cause your car to veer one way or the other. The side your car turns toward is the rougher side of the track assuming your wheels are flat on the track and aligned perfectly. Wood grain is a more organized kind of roughness that tends to steer the car along the grain lines, especially if the inner edges of the wheels are sharp. When the grain is opposite on each side of the median, extra drag results as if the wheels were misaligned in opposite directions. The behavior is like increased tread friction on the effected wheels.
As wheels ride over bumps, they jump up and down very quickly. Since the force holding the wheel to the track changes erratically from this, the force could become small enough to cause sliding. This is also possible where the track is slippery smooth or slick from spilled lubricants. When it slides, it can be treated as tread friction with a smaller coefficient than the static tread friction coefficient. If the force causes intermittent sliding and traction, the spin up of the wheel will only be a part of the Iw2/2 energy cost. You can see why it is difficult to estimate the effects of this type of friction.
To smooth out the ride on a rough area, make wheels as flat and wide on the track as possible. The flat, wide tread will "average" out the roughness and avoid many jolting jitters that can increase tread friction. They will tend to ride over pits like they are not even there.
On the other hand, the flat, wide tread will catch more isolated bumps, if they exist. A good strategy to avoid isolated bumps is to shape the wheel so it touches only on an edge or ridge. But then pits and wood grain become a potential problem. You must know your track to find the best solution for your car!
Rough areas cause axle friction to vary erratically over a small range of values. Each "bump" causes the wheel to be lifted and dropped some small amount. This causes the vertical momentum of the part of the car supported by the wheel to change twice; once up, once down. To estimate the average force on the wheel, multiply an average of the vertical momentum of the car by 2 and by the number of bumps. Divide by the time of contact with the rough surface. This force can now be added to the frictional normal forces to get a more accurate wheel friction force model.
But there is another way the axle friction can be uneven. It happens when you lubricate your wheels. Some wheels get lubed better than others for whatever reason. Then each wheel in a front or rear pair may create a different amount of drag. When two wheels in a pair produce different amounts of friction, the car will pull to one side or the other. If the side ways pull is strong enough, the car will slide over to the median and stay there the rest of the race. So again, we see one type of drift leading to another, even worse form!
To avoid this kind of problem, try to treat each axle and wheel exactly the same way using the same techniques applied for the same length of time. You may even have to practice something like spraying your lubricant evenly the same way a few times before anointing your axles and wheels with it.
Edging is something you can watch by rolling a good wheel on a rough, flat, wood grain surface. At low speeds, the wheel can suddenly roll off in a different direction, or wander aimlessly like it lost its way. At higher speeds, momentum helps it to stay straighter, but unexpected turns still happen at times.
A wheel will trace an arched curve on a perfectly smooth, flat table if it is not perfectly squared up on its tread. This is not "edging". The wheel needs to be sanded square with respect to its tread; one side of the tread has a greater radius than the other.
But the inner edge of wheel can ride a bit lower than a wood grain ridge that is nearly parallel to it. When the wheel edge begins to cross over it, it is pushed back. That little push causes the wheel to pivot a bit. If the crossing point is in front of the track contact point, the wheel is deflected toward its face. If the crossing point is behind the contact point, it turns toward its back. Both situations tend to turn the wheel to run with the grain.
On the other hand, the inner edge of the wheel can also ride a bit higher than the grain next to it. So exactly the reverse can happen and the tendency will be for the wheel to cut across the grain!
But to make matters less predictable, both can happen at the same time for different wheels of the same car and the same wheels at different times. All that can really be said without knowing details about individual tracks, is that having distinct edges on your wheels makes a rough ride rougher.
Round off the inner edge of each wheel to prevent them from "digging" into the wood grain and ricocheting off the bumps.
A good, stable wheel rolls flat, smooth and straight, even when bearing the load of its axle. An unstable wheel bounces around, tilts and sloshes from side to side on its axle. There are a number of reasons that an otherwise good wheel can become unstable. But the main effects are caused by uneven weight distribution, uneven roundness, uneven lubrication and unevenness in the wheel bore or axle. Of course, a rough track can make good wheels behave badly too.
Good construction techniques help to eliminate some of these problems. If the axle is pushed in so that it restricts side to side wheel motion, energy loss due to this slipping or screwing motion will be reduced. But if the ends of the wheel bore, axle and axle slot are not trimmed and polished, the wheel will rub more against the axle hub and car body.
The energy loss due to side to side wheel motion is like that of a wandering, zigzag path, but much less since the wheel can only move side to side less than a tenth of an inch or so. The energy loss could be much greater if this small wandering causes the whole car to wander.
If a wheel also tilts on its axle enough to torque it, then it also increases axle friction. Some cars do something similar on purpose for the advantage of thin wheels or canted wheels.
Various modes of wheel vibration may lead to drift as well. But for wheels spinning as slowly and with as much weight on them as the Grand Prix cars, vibration can probably not occur under race conditions. Follow the link and see why you should relubricate your axles/wheels or repolish your axles if this happens when you spin your wheels while holding your car in your hand.
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|Grand Prix Racing -||The Science of Fast Pinewood Cars|
|Copyright © 1997, 2004 by Michael Lastufka, All rights reserved worldwide.|