So you’ve got the car shaped, sanded, and painted and you’re getting ready to install the wheels. You know they need to go straight. You know they need to be able to turn freely. But did you know that you should install one of them differently than the others?
Your car will be faster if one of the front wheels never touches the track.
The main enemy to speed is friction. The less friction your car has, the faster it will be. If your car is correctly aligned and rolls straight, most of the friction you’ll find is from the wheels rubbing against the axles and the car body. By keeping one wheel of the track, it doesn’t need to turn and won’t rub against an axle head and car body. You’ll have 25% less friction because you’ll have 25% less wheels.
There’s another important reason to run on three wheels. Your car has a limited amount of energy available. There’s energy stored up from the gravity that pulls your car down the track. You want as much of that energy to be used in actually propelling the car forward, but some of that energy is used to get the wheels rolling when the car starts out. If one of your wheels doesn’t touch the track, it doesn’t roll, and you’ll save that energy for the forward motion of the car.
If you have only three wheels on the ground, you need to pay a lot of attention to alignment. If your alignment is slightly off on a car with two front wheels, your car can still run relatively straight as one wheel will help make up for the other. But if you only have one wheel down and it’s poorly aligned, your car will drag against the guide rail.
For all the reasons a three-wheeled car is good, a two-wheeled car is even better. But that requires a balancing act that’s nearly impossible to accomplish.
Comments
18 comments so far ↓
KJD // Jan 29, 2006 at 7:22 pm
I think you are incorrect saying three wheels touching is better then four. I think this is a lucy myth. Friction is defined as FF=FC. Where U is the coefficient of friction and C is the force applied. Since C will be constant and F will be constant, then for four wheels FF = FC / 4 and for 3 wheels FF = FC / 3 (this implies even more friction). What I think may be happining with the three wheel myth is that there is less ROLLING friction. This would indicatate that the rolling friction plays a greater part than the wheel / axel friction component (which makes sense). Also some pinewood rule say that ALL 4 wheels MUST make contact (we would not want to cheat now would we
).
Adam Kalsey // Jan 30, 2006 at 10:15 am
Ah, but the force is not constant. You have less force with three wheels than with four. It’s true that the downward force due to gravity is a constant (about 1.4 Newtons), but this is the real world and not the ideal conditions set forth in a physics textbook. There are additional forces at play. The most significant of these is the force the wheel applies when it makes contact with the inside edge of the car or the outer edge of the axle. No matter how straight your wheels are, they will tend to slide and make contact with the car and the axle head as the car encounters imprefections in the track.
If all you were measuring was the friction on the inside of the wheels, then the number of wheels wouldn’t affect the friction of the system at all. If U is the coefficient of friction and F is the force due to gravity of 1.4 Newtons, four wheels would each have friction equal to UF/4 and three wheels would each have friction of UF/3. The friction of the overall system wouldn’t change, even if there were 100 wheels, as each wheel would have UF/100 friction, times 100 wheels, the system’s friction is still UF. But now you have 100 wheels bumping into the car body and the center guide rail instead of three.
All that said, the reduced energy required to spin three wheels instead of four is likely the biggest reason that a three-wheeled car is faster. Rolling friction is generally negligable in any system (think ball bearings), so it’s probably not much of a factor.
As for rules that require four wheels to touch, someone’s just being overly officious. It’s actually pretty difficult for a kid to build a car with four wheels that all touch the ground. How many children are going to be able to install all four wheels at exactly the same height and compensate for differing wheel diameters and wheels that are out of round? Most cars I see have only three wheels touching — even when it wasn’t done intentionally.
Adam Kalsey // Jan 30, 2006 at 12:47 pm
The word I was trying to come up with just came to me. The “other” friction I’m talking about is the braking action. Just like the caliper brakes on your bike, this braking action increases friction.
KJD // Jan 30, 2006 at 5:26 pm
Thanks Adam, I agree with most of what you say. My point being that if only three wheels are supporting the loan then the “downward” frictional force that those wheels must indure is increased by a factor of 25%. I also believe that rolling friction does play a major roll. A car with “outlaw” wheels will signifigantly perform better than a car using standard wheels. Yes from experience I know that it is almost impossible to get all four wheels touching ( I have tried). As far as braking friction, I think this could be minimized by utilizing an axel desgn that contains a very slight bevel towards the center of the tire (again not an easy feat to accomplish). Another interesting topic I would like to see is how axel diameter to wheel boar diameter affects the performance. Keep up the good work and thanks for you site I enjoy it very much!
Adam Kalsey // Jan 31, 2006 at 11:06 am
Outlaw wheels outperform regular wheels for a number of reasons, none of which have anything to do with rolling friction.
First, the energy required to get (and keep) a wheel rolling is less because the wheel weighs less. Razor-thin, lightened outlaw wheels have less mass and therefore less rotational inertia.
Second, and probably most significant, the wheels are round, balanced, and have a straight wheel bore. Stock wheels are mis-shaped, out of balance, and the wheel bore isn’t perpendicular to the tread.
Friction is independent of surface area, so stock wheels will have no more rolling friction than razor-thin outlaw wheels. Physicists consider rolling friction to be insignificant. Sliding friction is the problem with pinewood cars.
KJD // Feb 6, 2006 at 10:49 pm
Hi Adam, I agree with many of the issue you bring up with “outlaw” wheels. I still think that rolling friction (or could be “sliding” friction as you suggest) does play a major part. This is easy to prove by profiling a standard wheel so that it rides on a very small portion. This wheel will perform much better. Therefore with all things being equal the answer must be relevent to the surface area of the wheel. So we must next ask the question what forces are at play here. The only answer I can come up with is rolling (sliding?) friction. In addition I think there is a difference in the determination of “sliding” friction as opposed to “rolling” friction in reguards to surface area (although I have not researched this subject). Also you bring up a good point about the standard wheels being misshaped. Other than going through 50 or so to find a “good” set are there any other tricks to “true” a bad wheel? I guess I should also explain that my over analytical approach is due to having a degree in Physics.
Larry Phair // Apr 15, 2006 at 5:13 pm
In the discussion of 3 wheels vs 4, you have left out the point that lubricants perform better (less friction) when there is more pressure applied to the lubricant. This is the dominant reason for having only 3 wheels touching the track. (Think 3 guys carrying a mattress versus having 4. If there are only 3, they each have to carry more weight)
Glen // Feb 23, 2007 at 7:51 am
So how come you wouldnt want to change the camber of your wheels so insead of running on an area |__| that big you run on one _\ Just curious? If friction is my enemy then it would make since that less surface area = less friction?
Adam Kalsey // Feb 23, 2007 at 8:29 am
Friction is independent of surface area. So camber doesn’t matter. Canting the axles up or down won’t reduce friction. What it can do is reduce the surface area that’s in contact with the track, thereby reducing the probability that the wheel will encounter a track flaw or debris.
But canting the wheels, at least with BSA wheels that aren’t always round, can make the car run crooked as well. It also pushes the wheels to the axle head or the car body, adding braking friction.
In general, you want to run the wheels in the middle of the axle without touching any part of the car. You can’t do that if you cant the wheels.
freakfreak // Feb 17, 2008 at 3:16 pm
I’ve been down the track a few times, but I have never heard the expression, “lucy myth”. WHAT is this dude (or lady) talking about? I’m genuinly interested. PLEASE…SOMEBODY RESPOND here or the janeethel@yahoo.com
See Below (copied & pasted) :
KJD // Jan 29, 2006 at 7:22 pm
I think you are incorrect saying three wheels touching is better then four. I think this is a lucy myth.
DAH // Mar 16, 2008 at 11:13 am
Rolling weight is critical to reducing wasted energy release. In Cycling we talk about rolling weight a lot. Two wheels of identical weight but one requires more energy to rotate, why? It has to do primarily with placement of weight within the wheel system. More weight on out along the rim is a really bad idea, and as those crafty Boy Scouts don’t have requirements governing the thickness or mass of the wheels it is advisable to remove as much useless mass as possible.
Stephen // Mar 25, 2008 at 9:40 am
I wonder if the 3 wheels idea is good.
Because when the car is in the starting gate, its nose is down and the two front wheels are solidly on the track. - Right?
If this is true, then one of the back wheels are off the ground!
This means that when the car hits the level part of the track, a transition takes place: One of the front wheels shifts off the ground and both the back wheels are now touching the ground.
The energy in that front spinning wheel is lost as it comes off the ground. And the back wheel that was off the ground must start spinning when it touches the track, draining energy and speed from the car.
It sounds like a trade off with no advantage to me.
What say you?
ben // Apr 6, 2008 at 1:09 pm
the other advantage to 3 wheels is reduced wobble. It is virtually impossible to get the slop in the axle / wheel to be exactly the same in the two front wheels. this tends to cause the front of the car to wobble left / right loosing a lot of speed to this type of energy waste.
the winning cars in our pack all have used this trick. it seems to be the right thing to do.
Dave // Feb 11, 2009 at 9:45 pm
Answering Stephen’s question:
Since the majority of the weight is to the center and rear of the vehicle, the front wheel that is raised will be raised at all times.
I know because I have done this with my son for the past three derbies and have won all of them against 60-70 other vehicles. I attribute one part of our success to a raised front wheel. We keep coming up with better designs and more ways to reduce friction and improve tracking (keeping the vehicle straight). In fact, this year we broke the track record!
Our rules are fairly simple but restrictive enough to keep things like movable weights and springs out of the equation.
The pinewood derby is one of the most enjoyable events my son and I participate in because we can work together on many different areas: Art, physics, craftsmanship, woodworking, and competition.
Old Muley // Feb 20, 2009 at 2:56 pm
An interesting idea, but my son’s pack has specifically disallowed this practice. At inspection the check that all four wheels must contact the track surface, and all wheels must rotate when the car is tested. Any raised wheels much be adjusted down prior to final inspection or they cannot compete.
viper // Mar 27, 2009 at 6:48 am
hey good weight and three wheel advise im doing a school project on weight distribution an need some tips for it
thanks guys
i appreciate it
viper
Michael Blue // Jan 28, 2010 at 1:48 pm
Muley, this sounds overly judicious. Our car didn’t even touch the track in the inspection booth, but had already run several practice runs on the actual track. Why there was a clearance difference between the two boggles the mind. Of the 8 kids in our group, at least half wouldn’t have been able to compete if your officials were there.
Hyper-Tech Pine // Feb 24, 2010 at 10:52 am
lift one front wheel, it is faster for several reasons
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