Pinewood Freak

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Three wheels are better

January 22nd, 2006 · 36 Comments

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

36 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

  • Hypertech Pine // Sep 23, 2010 at 12:02 pm

    Oh, and its not 25% reduction in friction its only about a total 10% reduction but the benefits in stability and tracking as a result of not having 2 loose front wheels fighting each other is also a huge plus.

  • Mike // Dec 16, 2010 at 12:31 pm

    Three wheels vs Four Wheels superiority is also somewhat dependent on the track configuration. 2 tracks, with same total length and height, 1 with steep drop, more transition and long-ish horizontal run and the other with less steep start, less transition and shorter horizontal run.

    On tracks with steep drops I’ve seen four-wheel cars overtake and pass three-wheel cars. Same cars on a less steep incline, and short runout, the 3 wheel cars stay in front.

    The three-wheel cars get up to speed faster on the descent, but they drop their speed fairly quickly after the transition to runout. The four-wheel cars take longer to get up to speed, but with 4 wheels carrying the built up rotational energy, and overall weight distributed over 4 points instead of 3 they take longer to drop their speed.

    My 3 wheel cars consistently win on our home track: less steep, gradual transition, short run-out. However, the same cars have been soundly beaten several times by 4-wheel cars on steep incline tracks…nearly every time in the run-out a few feet from the finish.

    At SHAC Scout Fair 2010, on the Mega-Track (100 ft long, 30 foot drop), the 3 wheelers consistently led on the drop, but after transition to horizontal, the 4 wheelers quickly gained ground and won frequently.

    Build your car to take advantage of the track configuration(s) you expect to be racing on.

  • Mikem24 // Jan 24, 2011 at 2:17 pm

    Lots of info here, both usefull, and not…
    But a lot makes sense. 3 wheels, vs 4 wheels….
    The fact that these “wheels” are not anywhere near perfect make it very difficult to even begin to predict their actual performance on a given lane.
    Then you have to factor in the lane changes. I have watched my son’s car do great in one lane, but not so good in another.
    3 wheels touching, generating less friction sounds great! I go with the 3 wheels only because of the “fighting” between the two that occurrs because of the imperfections, and the simplicity of them. Hey, they are only plastic / nylon and “nails” right?
    You don’t really have to “lift” one front wheel so high to be obvious either, just enough to almost eliminate contact.

    Commom sense dictates what is needed.
    * The car must be balanced correctly,
    * The wheels need to track as straight as possible

    • Correct Lubrication

    I fly r/c planes. I understand weight / balance and their effects on the planes.
    That helps in the understanding and only the understanding of the cars. Though they are a completely different thing.
    Aerodynamics do play a small part in the cars, as they are not flying, but rolling, and for a short time.
    As we all know, race cars (the real ones) are designed by science…. Be it Indy or Stock cars.
    And that science involves Aerodynamics..
    keep in mind that the pine cars are a lot different, they are so light, and short lived.

    You are all correct to a degree… Because it ALL matters.
    If I was only able to have THE track in my basement when he builds his little car, we would be able to test it as we tweak it…..
    The track matters, guys! keep THAT in mind.

    Yes, the weight / mass / location all matter!
    Friction (drag) is the enemy!, So… Aerodynamics, weight & balance, weight all matter!
    The best set-up car WINS! That’s the one that incorporates all of these factors and is done correctly…
    Getting them to all work together is the trick…

    The straighter the tracking (wheels) the better the speed, the lesser the friction (lube), the better the distance / speed.

    Graphite lube , polished axels, cleaned wheel holes, smooth paint, and no hanging things at the bottom. Use epoxy to keep weights in…
    Use a dremel tool to carve out an area if you must put weight in the bottom, never let anything go beyond the surface of the underside!

    Better to drill from side right in front of the rear wheels, and insert weights.

    Use a pencil to apply graphite to the inners of the wheels, and around the area the wheel touch the car body. the inside edge of the wheel is also a place for it, so when it strikes the rail, it doesn’t grab so much.

    Read the other tips everyone has entered, and apply them all…. Well, have your kids read them, I think with the more understanding they have about it, the better they will appreciate the win !
    tha’s all for now,
    Mike M /\/\ /\/\ —out

  • jimbo // Feb 4, 2011 at 11:16 am

    25% less friction ?? i dont think so .. because you are adding that 25% onto 3 point not 4 so the weight will be distributed to less contact bunts but pushing harder onto the track so instead of having the 25 % of the friction on 4 wheels you will have 33.33 percent on 3 not so good

  • Adam Kalsey // Feb 4, 2011 at 3:57 pm

    That paragraph is talking about breaking friction — the friction of the wheels against the body of the car and the center rail, primarily.

    Reading later on, it says “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.”

  • Brad // Feb 7, 2011 at 8:43 am

    I have been a strong proponent of the 3 wheel set up for 5 years. In that time, we set the track record at Pack and District and were top 15% at Regional. The region is fed from 43,000 Scouts.

    Our Pack has since changed to a steep drop / long run aluminum track.

    This year I watched my kid’s car get passed at the 39th foot of a 40 foot track. Was it a 4 wheel car? I have no idea!

    I really, really want to hear more about the
    combo of steep drop and 4 wheels.

  • D. Shawn Racing // Feb 16, 2011 at 11:11 am

    Seems like the track of choice at major competitions are these steeper drop designs (Best Track) now and that the 3 wheel design might not be in question? We were a disapointing 10th a few years back at a very competative Council race, but our issues were more with COM and wheel wobble than anything. I believe most of the fastest racers in the country are utilizing the 3 wheel design as they are rail ridding. I’m with Brad, I’d like to see some more discussion here on 3 vs 4. I’m sticking with the 3 until I hear more. Lastly, Track design, length, metal vs. wood, age has got to be a huge factor in alot of Pack races. Our track is 60 ft long with a gradual slope to a 35 ft flat section. Alot can happen on the flats. Dumb luck if you ask me for a track that long and beat up. Kids enjoy the crashes more than anything. :)

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  • Gonzo // Jan 20, 2012 at 4:39 pm

    If you execute a three wheeler well it should be worth about 1/3 a car length at the end of the track if all else is equal. I base this estimate on the the pizza cutter cheat seen in this video: (start at 3:49)

    http://www.youtube.com/watch?v=tC5qRw7QhLA

  • Michael Z. Williamson // Jan 22, 2012 at 8:02 pm

    Not to mention that a 3 wheel car requires perfect balance and juggling, and INCREASES drag against the rail as at least one wheel is in constant contact.

    A 4 wheel car will oscillate slightly. Too much adds drag. A little back and forth with light contact means less contact time and less drag.

    My son has consistently been in the top 4 with proper distribution of weight and proper polishing/matching of wheels and axles.

  • shayan // Feb 10, 2012 at 10:26 pm

    thats som BS

  • Bill Klingler // Feb 11, 2012 at 5:26 am

    WOW!!! It’s not that complicated folks. 3 wheelers are faster. It has nothing to do with friction. That’s a Pinewood Derby myth. When raising one front wheel off the ground the friction is simply transferred to the other 3 wheel/axle assemblies. However, 25% of the rotational energy formerly shared by all 4 wheels is now available and the linear velocity increases. The non-dominant wheel is still going to hit the guide strip unless you’re running a rail rider. If your in BSA you’re probably cheating because you have to either bend the axles or tilt the axles to achieve a rail rider and change the toe to toe-in on the dominant front wheel. A hard wheel rolling straight on a hard surface does not create friction.
    You buy wheels from Hodges Hobby House or Maximum Velocity to get a round wheel if you don’t have a lathe. You then buy a Pinewood Derby wheel balancer to balance the wheel if you are really that intense about it and have lots of time - which I do because I’m retired.
    I recommend reading the article on “Rail Riding” which is on the Internet and it will answer a lot of your questions. Yes, it does work. I also would recommend you buy the book “The Physics of the Pinewood Derby” by Dr. John Jobe. You will be very difficult to argue with after reading his book. Again, if you are that intense about it. Have Fun. I certainly have for the past 27 years and still having fun.

  • Bill Klingler // Feb 16, 2012 at 9:38 am

    Michael Z. Williamson
    Perfect balance and juggling? Not so. 3 points make a plane. Constant contact? Why would there be constant contact unless you’re running a Rail Rider? There is very little friction with the inside edge of the wheel rubbing the rail, with the wheel hub riding against the nail head and the wheel hub riding against the car body. That’s why people who say raising one wheel off the ground reduces friction are wrong. It is a Pinewood Derby Myth. It is actually a 25% savings in Rotational Energy. Also, a car bouncing back and forth off the rail actually runs a longer race, among other nasty things, than a car that is sliding along the rail the whole race. That’s one of the things that makes a Rail Rider so fast. You actually want the wheels to ride against the head of the nail if you can accomplish this without cheating. With 2 “perfect” cars a 3 wheeler will beat a 4 wheeler every time.

  • Nedd // Feb 9, 2013 at 4:40 am

    My son’s derby car won the pack derby on three wheels last year, he’s setting it up the same this year, enough said

  • John // Mar 8, 2013 at 3:23 pm

    We’ve been making cars with a raised front wheel for years and always finish in the top 3. The others in the top 3 also have a raised front wheel. They do, however, need to alligned, but you can get a tool to help you do that.

    The biggest factor is making sure you are at maximum weight. It gives you more push; however, location of weight placement is crucial. Put 80% of your weight on/behind the back axle. You can have the best 3 wheeled car with its weight in the front and it will loose to an average 4 wheeler with the weight in the back.

    Lifting a front wheel does help, but so does polishing the axles, reducing wheel weight, coning the hubs, lubricant…there are a myriad of little things that add up to big time differences.

    Aerodynamics, however, is not a factor unless you are using a sail to slow you down. You would need to go much further/faster for it to be a factor, so don’t spend all your time on that bit - work on your weight, axels and wheels. Your axels, by the way, should be as near the front and near the back as you can while still coming in at 7″.

    We just got our car kit - my daughter can’t wait to start polishing the axels. The shape will put the weight at the back.

  • Prof. Ollie // Jun 8, 2013 at 1:36 am

    Oh yeah well my son comes first every year and then I win all of my races too and my wife wins all of her races and then my brother won the olympic games so suck on that!!

  • Bill Klingler // Jun 19, 2013 at 3:39 am

    Congratulations on the wins, Prof. Ollie. Are you a League Racer? If so, what League, or Leagues, do you race and under what name?

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