Friday, April 12, 2002

Is 1° Costing You The Win?

By: Tom Cole
Do you REALLY know if advancing or retarding the ignition timing of your engine 1° will increase horsepower? I think most of us can agree that there is a good chance that we would find some improvement, if we checked. How easy is it to experiment with your motor? As I have said before, I am no expert. I am just a father like many of you who likes to build his son’s racing motors. I love the feeling when a motor I built takes the checkered flag, and I have felt the heat and disappointment from the driver (and the wife) when it is obvious that MY poor performing engine cost us the race.
One fact I have learned is that perfect timing is when your motor runs at the proper temperature, responds the way you want it to respond, and you win races. There are no magic numbers or exact calculations that can tell you where timing needs to be. There are only beginning reference points. Just like the carburetor, timing must be tuned to the conditions and variables specific to each motor, track, driver, and kart. Throw in the altitude and the weather conditions, and you can see that the ability to adjust timing quickly and accurately is something that can set you apart from the crowd.
Because the degree wheel infuriates me, and offset keys are a joke, I set ignition timing by aligning the trailing edge of the flywheel magnet with the middle of the coil trigger button while the top of the piston (before top dead center) is at a measured depth from the top of the cylinder. The depth of the piston in the hole varies with many factors, but your cam manufacturer will usually give you a good starting point. (30° BTDC = .2115” in the hole when popup is 0.000” on a stock engine) Once I have everything aligned, I carefully hand tighten the starter nut. Then I recheck my measurements, torque the starter nut down to somewhere around 1,000,000 in./lbs. (I give it all I’ve got with hand tools) and recheck my measurements again. Also note that I do not use an offset key. I lap the flywheel and crankshaft together and then apply Locktite™ to the taper (not the threads) on the crankshaft before installing the flywheel. Now I reassemble the motor, mount it up to the kart, and give it a test run. This whole process takes about 30 minutes.
Now how many of us are going to take that motor and go through that same process at least three more times to check and see if +or- 1° makes a noticeable difference? The method I use to set the timing has a margin of error of at least +or- 2° because I don’t know the exact firing point of the coil trigger button. I know I need to experiment, but what a pain! And how much stress am I putting on the stock flywheel each time I knock it off and torque it back down. Haven’t they been known to fly apart? And how well is that flimsy stock animal flywheel going to hold up? I have heard of engine builders breaking them when they are first installed!
It wasn’t until I faced all this trouble with that touchy little blue-plate slapper-cam motor that the value of ARC’s billet finned flywheel with its adjustable timing hub really came into view. I can literally be finished getting the motor’s ignition timing right with the ARC flywheel, before I can test the first timing change on a stock or fixed hub flywheel. How much time do you have at a race between practice and the first heat? I am too busy changing oil, cleaning and prepping tires, and making chassis adjustments to tear down a motor.

Thursday, January 17, 2002

Rolling Resistance

By: Tom Cole Date: January 17, 2002
Every rear wheel drive car or truck on the road or racetrack uses tapered roller bearings in their front hubs. The reasons for this are simple, less friction and combined radial (perpendicular to the axial) and thrust (parallel to the axial) load capacity. So why are kart racers using ball bearings which are designed only for radial loads on their karts? I can understand why Jr. Drag racers don’t care about thrust load. They just go fast in a straight line. But kart racers go fast and turn fast. Usually, the one who gets through the turns the fastest wins. So again, why are so many kart racers using ball bearings? Endplay! Somebody has actually sold people on the idea that karts with tapered roller bearing hubs have so much endplay that you can’t set the toe-in. What a pile! How is it that those NASCAR and F1 boys can set the alignment on their cars? Never mind that endplay tolerance for a wheel with tapered roller bearings is only .002” (that’s half the thickness of a piece of notebook paper). How precise are the widths of the inner and outer races of a .65-cent ball bearing? How precise is the relationship between the inner and outer races? Are they parallel? Are they precisely on the same plane? How precise is the bore of the hub? Is the center sleeve exactly long enough? Well guess what! None of this matters if you use a properly installed tapered roller bearing. The guy beating you doesn’t give a rip how the wheel rolls on the rack. How the wheels roll on the track, in the turn under thrust load and down the straightaway under radial load is what matters in wheel bearings. And he is not going to tell you why he’s beating you.

All ball bearings roll on a “point” of contact between the ball and the races. A tapered roller bearing distributes the load over the length of the roller in a “line” of contact. This greatly reduces the friction coefficient allowing tapered roller bearings of the same diameter as comparable ball bearings to carry a greater load and achieve a much greater fatigue life. Simply put, it will roll easier than a ball bearing. The angle of the races along with the taper of the bearing rods allows a tapered roller bearing hub to roll equally well in the turns or on the straight-aways. Standard ball bearings DO NOT roll as well through a turn as they do down the straightaway, and they do not handle the demands of a kart racer as well as a tapered roller bearing.