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This Tech Article is copied from Preparing T-Jets For Stock-Class Racing. Dr. Carl T. Dreher April 8, 1996 First, understand that the "stock" class can be the most contentious of any racing class because everyone has a different idea of what "stock" is. The suggestions here are a menu: choose what you think is legal for your group. Basically, turning a stock HO car into a successful stock-class racer consists of removing every possible bit of friction and making maximum use of manufacturing variations. (This is also known as "blueprinting".) None of these suggestions by themselves makes a big difference, but the sum of many tiny improvements is very noticeable. You must develop a critical eye and a critical attitude. No improvement is too small. I probably spent more time preparing stock-class cars than preparing unlimited class cars. Here's are some general rules: (1) Use fresh parts. No one wants to believe that plastic chassis and brass gears wear out, but they do. Once the bearing holes become enlarged, they are useless. (2) T-Jet parts were cheap and manufacturing variations could be wide. You'll need LOTS of parts to pick from to build ONE successful stock-class car. When stock T-Jets were selling for $3.00, I could count on spending $30 to build one stocker. (3) Improvements in handling generally reduce lower lap times than improvements in top speed. Obviously, this isn't true for drag-racing. 1. Body Choice: T-Jet bodies vary greatly in weight and more importantly, weight distribution. When I was preparing stock-class cars, I had access to every body style Aurora made and also to an analytical balance. I weighed a lot bodies! My favorite for GT racing was the Ford GT40. It is light, wide AND CARRIES IT'S WEIGHT LOW. This is VERY important! Most T-Jet bodies carry their weight up high. The Ford J-Car is also good but is a bit tail-heavy. The Camaro body is good for the TransAm class. Choosing the right body style can make quite a difference. Remember, we are talking about little DC motors. A fraction of a gram makes a difference. The windows of most T-Jet cars were held in place by flattening a plastic post with heat. If your rules allow it, you can grind off the excess plastic and glue in the window. This saves a bit of weight. You can also remove 5-10 thousands of an inch off each body post to lower the body very slightly. Again, check your rules. 2. Wheelbase: You can vary the wheelbase of T-Jets by moving the front axle forward or back. This really affects the handling. My driving style, coupled with fact that most of the tracks I raced on had long straights and fast corners, favored the long wheelbase. If you race on tight, twisty tracks, use the short wheelbase. 3. Gears: Here's where we spend some hours. Our goal is to reduce the friction of the gear teeth and the friction of the gears on the plate. Start by using #600 sandpaper and a hard, flat plate to reduce the thickness of the brass gears by a few thousands of an inch. (Or as much as you can get away with and still claim it is stock!) Then, get some #2000 sandpaper and polish the surface of the gears and the face of EACH TOOTH. You can also use a "burnishing tool", available from stores that service the watch industry. After polishing, I even went so far as to assemble the gear train on a junk gear-plate and spin them with a bit of toothpaste on the teeth. This finished the polishing. Don't forget to clean everything when you are done! Inspect the bearing hole of the idler gear, which tends to wear over time even when oiled. It should spin freely but with NO slop. Even a little wobble around the plastic stub consumes precious HP. If it's worn, toss it in the junk pile. 4. Rear gear and pinion: Polish the pinion gear shaft. It should slide in the gear plate bearing hole but, again, have NO slop. Place the pinion gear on the shaft, insert it in the gear plate and trial fit the gear plate on a chassis. You want the pinion shaft to extend into it's chassis bearing just enough to locate it securely. Any more than that adds friction. Adjust the pinion gear up/down as necessary. When assembling the pinion shaft on the gear plate, both gears MUST be absolutely parallel and perpendicular to the shaft. The tiniest bit of wobble is deadly. The best way of guaranteeing this is to use a "staking" tool, available from the watch repair trade. It is a precision set of fixtures used to insert a shaft into a watch gear. It is an expensive set of tools, but you can usually find used ones at watch and clock collector swap meets. There should be NO up/down play in the pinion shaft. Take your time here. Use a jeweler's hammer to gently tap the pinion gear onto the shaft. Check the fit as you go. 5. Axles: Test fit a number of axles. There is surprising variation in axle diameters. The one you choose must fit with no wobble or drag. 6. Chassis: Check that the chassis is square. Occasionally, I found T-Jet chassis that were warped. 7. Wheels and tires: I used to buy tires by the dozens and then sort them into wide and narrow set. There was easily a 20% variation in thickness. Choose a wide set for the rear and a narrow set for the front. Inspect the rubber. Rubber ages and gets hard with time. Fresh tires have more compliant rubber, which is what you want in the rear. Old, hard tires are great for the front. Inspect the wheels. Quite a few of the plastic wheels that came with the T-Jet were NOT concentric. If they are not round, toss them! At the rear, epoxy the tires to the wheels. When dry, chuck an axle in a drill, carefully press the wheel on the axle, make sure it is running true, and then sand the tire tread. We have two goals here. The first is to slightly reduce the diameter of the tire. This provides better acceleration and also lowers the chassis for better handling (at the expense of slightly reduced top speed.) The second goal is to increase the tread width. Stock T-Jet tires are tapered. Reducing the diameter moves the tread surface to the slightly wider area. Use a micrometer when doing this to insure the tires have the same diameter! Some rules allow you to treat the tires. An old trick was to put a coating of clear nail polish on the fronts to make them slick and to reduce rolling friction. Some people had secret formulas in which they'd soak the rear tires to soften them. I never found anything that worked better than sanding the surface and using fresh rubber. When assembling the wheels on the axles, be VERY careful that they run true. Also check that the smooth part of the axle rides in the chassis bearings, not the splines. Test fit the body on the chassis and press-on the rear wheels only enough to clear the wheel wheels. You want the widest track you can get for the best handling. At the front, you also want the widest track you can get. However, the front is different because nothing locates the axle left/right. This means the axle and wheels can slide to either side. This is good, BUT you want to insure that it can't slide so far as to rub against the body, creating friction. 8. Armature: There are many things you can do to an armature and still stay within stock-class rules. The first step is to pick a good one! The manufacturing variations are huge. Start by checking that the commutator is running perfectly flat and smooth and that the armature shaft is straight and has no nicks. The comm on a T-Jet armature is only clamped in place and may have a wobble. Fixing this without unsoldering the windings is almost impossible... you're better off starting with a good armature than trying to repair a bad one. Next, get an accurate ohmmeter and check the windings. The resistances should all be the same. Furthermore, you want to find an armature that has the LOWEST resistance, meaning that it has less wire and therefore, less turns. This produces less low-end torque but much improved high-rpm power. Again, the manufacturing variation is quite large. One you've found a good armature (about one in a dozen), resolder the windings to the comm. Use as little solder as possible. This is for reliability. Use a pencil-point soldering iron and high-temperature solder. The next step is to true and polish the comm. A jewelers lathe or Unimate gives the best result. Use #1500 sandpaper for polishing. Be very careful of the comm tabs! It is easy to break a wire at this step. The final step is balancing the comm. This can make a very noticeable difference in performance. The best tool you can buy for this job is called a "poising" tool, available from the watch repair trade. It looks like a tiny vice attached to a tripod. The tripod has a bubble and adjustable feet for making the tool sit level. The "jaws" of the vice are thin plates of polished, artificial ruby. Laying the armature across the jaws will quickly show which pole of the armature is heavy. You then grind off a bit of the pole face to compensate. This takes a long time but the results are worth the effort. If your rules allow it, you can also "dewind" an armature. This consists of unsolder the wires from the commutator tabs and unwrapping about 50 turns. This reduces low RPM torque but significantly increases top speed. Most stock-classes DON'T allow this. 9. Brushes: There isn't anything you can legally change here. I always use fresh brushes, since worn ones will tilt in the brush holder hole and then contact the comm at an angle rather than full-face. You can also seriously alter the motor performance by adjusting the brush pressure. Generally, a T-Jet needs a SMALL amount of increased brush pressure. 10. Magnets: I found that fresh, stock magnets worked best for stock armatures. I tried having the magnets remagnetized and found that they became too strong, limiting RPM. Obviously, this could be adjusted. If your rules allow it, you can trim about .005" from the inside edge of the part of the chassis that holds the magnets. This moves them in a bit towards the armature. You then need to place a shim behind the magnets to hold them firmly in place. Most stock-classes don't allow this modification. 11. Pickup shoes: These should be smooth and have no grooves worn in them. Use #1500 paper to polish them and also to remove any oxidation at the end where the shoe makes electrical contact to the chassis. The shoe itself should lie flat on the rail for maximum electrical contact. Check this on a spare piece of track. I found that stretching the springs under the pickup shoes VERY SLIGHTLY increased electrical contact. This is very delicate work. If you stretch them too much, the font end becomes light and the car deslots easily. 12. Body mounting: Here is a trick that works wonders. Use super-glue to glue the guide pin in place and then mount the body SLIGHTLY LOOSE! Back-off the body mounting screws so the body is free to move a tiny bit. What this does is decouple the body from the chassis so that all the little vibrations from the track sections are absorbed by the body, leaving the chassis with better grip. I know this sounds crazy, and I'm not sure my explanation about why it works is correct, but it DOES works. I've never told anyone else about this. It was one of my trade secrets! Again, the body is just SLIGHTLY LOOSE! 13. Lubrication: Needless to say, cleanliness is imperative. The gear train, all bearings and EVERYTHING must be keep exquisitely clean. Use just a tiny amount of very lightweight oil on the bearing holes. The bottom armature hole is critical. You want just a tiny bit here. Any oil that gets on the comm and brushes reduces electrical contact. I have heard about conductive oils, but never had a change to try them. This is a good area for experimentation. ----- OK, that's it. Building a stock-class racer takes lots of time, lots of parts and a patient hand. You are trying to find every tiny advantage. Now, there is one more trick that will win more races than ANY of the above techniques. Practice driving! A cool hand at the controller is worth everything else put together. Learn when to pass and when to stay back. Remember, a single deslot costs more time than you can ever make up with a faster car. |
