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  • #16
    Thanks for the gear ratio calculations, Rich. I was curious about that.

    Tinkerers that tjet folks tend to be, a 9, 12, or 14 tooth drive pinion could be installed and used with an appropriately shimmed tjet 15t crown if one really wanted some gear ratio choices in addition to what OS3 provides with their 15 and 19 tooth crowns.

    OS3's gear changes are among the more interesting modifications incorporated into the TFX chassis.

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    • #17
      If you used the 15 tooth crown gear the gear ratio would be 2.61:1 with a 12 tooth pinion and 2.24:1 with a 14 tooth pinion. With the 19 tooth crown gear that would be 3.31:1 and 2.84:1.
      I had to think about how to calculate the gear ratios for cars with pancake gears when I wrote my T-Jet tuning article some years ago. I found that it was easier for me if I started with the armature and armature pinion turning at 1000 RPM, in that case for a TFX with an 11 tooth armature pinion the 26 tooth idler gear would be turning at 430.077 RPM, the 23 tooth driven gear and the 11 tooth drive pinion would be turning at 478.261 RPM and the 15 tooth crown gear would be turning at 350.725 RPM. To get the overall gear ratio 1000/350.725 = 2.85.

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      • #18
        OS3 says the TFX crown gears mesh well with their 11T pinion but not as well with Aurora's pinions so racers experimenting with these different combos have been forewarned.
        Standard tjet crowns have 15 teeth, so that's an easy substitute. The Super II had a 19T crown; not sure if another 19T crown was ever made for the tjet.

        Using a 9T pinion and a 15T crown on the TFX gives a ratio of 3.48:1.
        Using a 9T pinion and a 19T crown on the TFX gives a ratio of 4.41:1

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        • #19
          If I recall correctly AutoWorld ThunderJet 500 Ultras have an 18 tooth crown gear. If you wanted to use a brass pinion in a TFX car you might need to use a T-Jet crown gear as well if you are looking for a perfect mesh.

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          • #20
            My OS3 TFX car is in the mail, I should have it very soon. A fellow club member got several cars yesterday, he did not get a chance to run them yet, but he noted that the gear mesh is good. I expect to be at his place for a race on Sunday, so I can check the cars out in case my car had not arrived by then. I was wondering what the diameter of the rear wheels were. T-Jets have 0.170" diameter wheels.
            It is my intention to evaluate the car as it comes, then see if there might be any minor tweaks that might be done to increase its performance.

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            • #21
              My OS3 TFX arrived today, I oiled it, adjusted the pickup shoes, snapped in the guide pin and trimmed it to the proper length. The pickup shoe tension measured 2.8g, the tires were 0.352” in diameter as advertised. The axle, armature shaft and cluster gear shaft holes were the correct diameter and the idler gear was a perfect fit on its post. The armature measured 17.1 ohms at 75°F. The total weight with a Fray Lamborghini body and screws was 18.2 grams. The downforce measured 8.6 grams on my homemade tester compared to 24.3 grams for a 2nd generation T-Dash with Killer Bee magnets. I will measure the TFX magnets when I get a chance. At 18 volts the car read 2.40 on my VRP dyno, my regular cars were already prepped and packed away for tomorrow’s race, I will run some of the pancake cars on the dyno later on.
              I ran the car around my ~50 foot MaxTrax road course without a body using an OS3 All Pro Elite controller, a small amount of brakes was needed. The car ran smoothly with some humming from the gears at higher speeds. The car had good acceleration and top speed. Rear grip was a limiting factor, the car never deslotted, it would just spin out at the limit. The tires did seem to get better as they scuffed in. The best lap time was 5.745 seconds compared to a full race T-Jet SS with silicone on sponge tires at about 5.4 seconds. With an Auto World ’70 Camaro body the car did 5.985 seconds and I had to turn the brakes up a notch. With a Fray type Lamborghini body the best lap time dropped to 5.595 seconds. I intend to try different tires in the future, with more grip the car may need more weight up front, an inexpensive E-Fray type front end might do the trick.
              I was very pleased with the car, it runs great with no tweaks, if we do not run them in a new class we will probably have some for IROC style racing.

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              • #22
                Was that with a 15-tooth crown or 19-tooth?
                the hum was from the armature (not the plastic gears?)…
                swap out the grey dragon magnets for white or black dragon magnets should changes things up, too!
                Changing rear tire OD to nearly match the crown gear OD seem to be the way to go…

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                • #23
                  The car has a 15 tooth crown gear, going to a 19 tooth gear would possibly reduce the top speed and make it more difficult to drive, at least with the stock tires. Another guy in my club plans on comparing the performance with the 15 tooth VS the 19 tooth crown. I have not yet measured the magnets. I have matched pairs of magnets up to 930 gauss. The TFX had minimal magnetic downforce in spite of the fact that the magnets are closer to the rails, so I would expect that they are not terribly powerful. Stronger magnets might reduce the car's top speed and more downforce might bog the car down, we will have to see. Experienced tuners like to try different magnets.
                  My own testing with pancake cars indicates that there is a sweet spot with respect to tire diameters. Going up or down just one size can make a bigger difference than you would expect. I will have to order tires in the appropriate size range.

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                  • #24
                    Got mine today, so not long to wait. Seems nice. I look forward to getting it some track time. The guide didn't pop in as easily as I'd hoped. Good thing I got a bag of spares.

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                    • #25
                      I had no problem with the guide, the instructions say to push on the rear tab with a small screwdriver held at a 45 degree angle.
                      My track has shallow slots, when I am racing on tracks with deeper slots I usually switch to a longer guide pin. I glue the guide pins so I can have some body float.
                      I checked the car's track clearance and with smaller diameter rear tires the chassis would be dragging on the track rails. The 15 tooth crown gear does have plenty of clearance.
                      Last edited by RichD; 09-28-2021, 05:29 AM.

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                      • #26
                        My car arrived yesterday.. I haven't been able to get on the track with it, but I did the basic oiling, scoring of the brushes, and washing the tires. The bottom of the gear is above the bottom of the chassis so clearance for the gear, at least the 15 tooth gear, is never going to be a problem. The chassis is already sitting low, so I'm not sure how much smaller you can realistically go with the rear tire.

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                        • #27
                          I just got mine....I like the guide pin design. I thought it was going to be problematic, but it snapped right in. Threw a couple of drops of oil at it and ran a few laps. It's gonna need some break-in, but it's also surprisingly quiet. I got a spare axle with a 19 tooth pinion, and will try that next. Thinking about mounting a Dash McLaren body on mine....

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                          • #28
                            I checked the magnets yesterday, they measured 555 and 565 gauss. The strongest Aurora T-Jet magnets were the green/orange ones that measure 600-650 gauss. In the future I may try some ~850 gauss magnets, but I want to try other tires first. I checked the track clearance and found that a little nib on the bottom of the chassis was almost dragging on the track. I plan on ordering Pro Series Super Tires in both the regular silicone and Silver Edition formulations size 340R and 344R for 0.350" and 0.354" mounted on the stock 0.170" wheels.
                            In a fit of ambition I did up this Vonco Porsche Carrera GT body for the new car.



                            One other thing is that there is very little clearance under the front of the chassis. If you like to back off on the body screws to get a little float the front screw might drag on the track.

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                            • #29
                              The chassis as a whole is low which is great to get the center of gravity of the car lower. If you use a flathead screw you shouldn't have a problem unless you really back the screw off, but in my opinion that would be way too much float. I have a Dash Karma Ghia mounted with and the screw head does not protrude past the bottom of the chassis and I have sufficient float.

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                              • #30
                                Nice report Rich D!!
                                I am surprised at how close it got to a fray car lap time on your track with 1/3 the downforce!! Will love to give these a shot in the future...

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