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Ensure adequate wheel clearance

January 15th, 2006 · 10 Comments

When you build your car, make sure there’s enough clearance between the wheels and the body that the wheels can freely spin. If you tighten the axles too much, your wheels won’t be able to spin without making contact with both the car body and the axle hub.

Don’t make those wheels too loose, though. If there’s too much of a gap between the car body and your wheels, the wheel will slide back and forth along the axle as the car rolls down the track. This will make it hard for the car to roll straight.

The easiest way to get the right wheel clearance is to make a wheel gauge. Take the lid from a margarin tub and cut a business card sized piece from it. In the narrow end of the car, cut a slit that’s about a quarter inch long and just a little wider than your axles.

Axle Gauge

When you install your wheels, put the axle in the car body and then set the gauge between the wheel and the car body with the slit around the axle. Tighten your axle until your wheel is tight against the gauge and then pull the gauge out. Your wheel will now have an optimal gap.


10 comments so far ↓

  • Bruce Edney // May 21, 2007 at 12:12 pm

    My experience is that more wheel clearance is better! I found early on in my tests on my own 39ft track that the maxium possible clearance is best. I set the heads of my axels to max width allowable of 2.75″ and the car body to minimum of 1.75″. Think of it this way - with more room to move laterally there is more time the wheel is not touching the axel head or the car body, therefore less time for slowing forces at either extreme. I have demonstrated this many many times on my track and with several cars. I suppose results may be different on other tracks and with different car wheel alignments but my results clearly favor more clearance!

  • Adam Kalsey // May 21, 2007 at 8:37 pm

    The farther out your axles are, the harder it’s going to be to align the wheels. Unless you’ve got a lot of experience, patience, and time, you’ll end up with better-aligned wheels if you keep the clearance tight.

    Additionally, by pushing the wheels out to their limits, you run a greater risk of the wheels becoming mis-aligned during the race. Bumps against the wheels and axles will put more torque on the axles than if the axles are shorter.

  • Hypertech Pine // Feb 12, 2009 at 9:35 am

    Actually current testing indicates narrow wheel clearance is better. If your concerned with wheel rub on the body just cant the axel up slightly to make the wheel run out a little. Our side by side testing indicated no performace drop versus level axels when the nail heads were properly prepared. Also the wheel bore is not actually level, there is a few thousands taper as a result of the mold injection release process. More energy is lost in the “wiggling” of the cars tracking then adjustments caused by the guide rail. Many people are currently utilizing the rail runner method to eliminate as much as possible tracking errors. This works by actually narrowing the front wheel positions to leave about 1/8″ total movement from the center guide rail thereby forcing minimum variation in drifting off track line. The blocks are actually cut thinner in the front to accomplish this.

  • Reed C // Mar 29, 2009 at 3:46 pm

    Rail running is fast if done properly.
    Proper spacing can also be done with the cardboard from a 12 pack of soda. Exactly the right width, plentiful supply. No one questions why you have a small piece of cardboard in the your kit.

  • Pinewood Mom // Jan 27, 2010 at 12:23 pm

    Thanks for all the tips, guys! I take it more seriously than hubby does, so finding free tips to keep my son from ending up in last place is much appreciated. Yes, my son is involved in the project, but hubby won’t let our 9-year-old use his pocketknife (hubby can’t stand the sight of blood), so I didn’t bother letting him use the Dremmel either. I shaped the car under my son’s instructions, he sanded, he painted.

  • Adam Kalsey // Jan 27, 2010 at 1:34 pm

    That’s exactly how it should be done. Know your child’s limitations and capabilities and help him through the rest. Each year, the boys can do more and more of it on their own.

  • Michael Blue // Jan 28, 2010 at 1:03 pm

    Search for “rail riding” and the camber/caster techniques used for this. I’m kind of new to this, but it sounds like the best overall approach for removing friction and wasted energy and travel.

  • tom // Feb 4, 2012 at 1:48 am

    I love reading about all the high-tech solutions to running a piece of wood down a short hill. For all the great advice it is interesting that those who speak the longest usually end by saying “there’s always next year”.

    Keep it simple for the kids. Make it look like a race car. Use the standard “tips” like polish axles and scuff the wheels. Place some weights like a skier going downhill (hint, they don’t sit on the back of their skis - they always lean forward).

    Make sure your child actually does some work and follow their advice on some items so they have a bit of “ownership”. After all, do you like it when your boss tells you how to do every aspect of your job? HAVE FUN!!!

  • Bill Klingler // Feb 11, 2012 at 9:58 am

    Buy the axle insertion tool. It will put your axle in straight and set the proper clearance at the same time. In BSA I would guess bending or tilting axles is against the rules. You actually want the wheels to run against the heads of the nails if you can accomplish this without getting caught. If you cheat you will have to live with it. Your Tiger can learn every aspect of building a car and his little hands can do most of the work. I know Tom means well but there is enough “Dumbing Down” of the children in our society. I recommend Tom forget about skiers and learn about the “Cycloid of Constant Time” regarding pinewood derby cars. I also say, “HAVE FUN”, and make it a fun learning experience for your scout.

  • Bill Klingler // Feb 8, 2013 at 5:56 am

    Hi tom,
    My scout took First place again this year. You wouldn’t believe the Physics involved in running a piece of wood down a short hill. Actually, if the car is built correctly, there really isn’t much wood involved. Think THIN to WIN !! The basic “piece of wood” should not weigh over 1/2 ounce. Now you’ve got a good start.

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