XRAY - Model racing cars

Generally starting ratios are based on experience. If you're a newbie your best bet is to ask one of the locals for a starting ratio until you gain enough experience to make a good guesstimate yourself.

Once you have a ratio you run the car. The simple rule I allways use to get close to the final ratio is this:

- If the car appears to hit top speed before the half way mark on the main straight you are probably undergeared (gear ratio too high). So use a larger pinion to gear up (lower ratio; I know, confusing... ).

- If the car hits top speed after traveling 2/3s of the main straight you are definitely overgeared. Use a smaller pinion to gear down.

Once you manage to get the car to hit top speed between the 1/2 and 2/3 mark on the main straight it is time to fine tune the ratio. Now you look at the infield and determine whether your car is accelerating fast enough out of corners. This requires more experience and is usually not very important for newbies.

The trick is to balance the speed on the straights with the acceleration out of the corners to get the fastest laptimes. On tracks with a long straight but very challenging infield you will probably gear slightly lower to get the acceleration in the infield and as a result hit top speed on the straight at around the 1/2 way mark. If the infield is relative fast (flowing corners) you will want to gear higher as acceleration is not so much of an issue and as a result you will hit top speed at the 2/3 mark.

Hope this helps.

Tonyv,

Thanks for your quick response!!

I will try your advices out next time when I hit the track.

btw, what's a good "general' starting ratio for lower turn motors? ( just for reference.....)like 8T , 13T and 19T?

does the rule change when using brushless motor?

chi

I'm sorry to say there's no such thing as there are so many different tracks. On some of the large European tracks (250 meters or longer with 50 meter straights) we run 12 turns at ratios between 5.5 and 6.0 or thereabouts. But on short technical tracks (15 meter straight and 100 meter total) we go as low as 7 to even 8, especially indoors.

asking people at the track is a good starting point however start off one pinion size lower than they are suggesting, check the motors temp after 5min(normal racing conditions) if its cool then go up a tooth, then recheck. all motors are different even if they are the same brand i have a few monster motors and because of each motors characteristics can have 2-3 pinion sizes different between them. as mentioned track layout can affect your choice for either more torque or more rpm. if your looking for good advice on motor building and tuning check out www.rccars.com on this page you will see a section about big jim's motors. he is one of the best motor specialists in the world, and gives his knowledge for free

Yes, starting lower is a good idea. But checking motor temperature is not necessarily a good measure as the motor will overheat when you are undergeared just as much as when you are overgeared... So even if the motor is hot you may have to gear up rather than gear down.

Guys thanks!

Kerry: Just visited BigJim site, very informative! thanks. (that's gonna keep me busy reading for a while.)

Tonyv: btw, gearing up means using a larger(more teeth) pinion gear right?

thanks guys.

Chi

Right, larger pinion = gearing up.

another question: what's the difference between say, 100(spur):50(pinion) vs 50(spur):25(pinion), even though the gear ration is the same?

is the answer something about moment of inertia, hence result in different power consumption and affects the runtime?

will there be any difference in the driving performance of the car?

please don't mind my questions.......


chi

There are two answers to that question, depending on what you meant exactly.

First we should note that there are different sizes for the teeth on the spur and pinion that can be used. The kit comes with 48Dp spur and gear which is what most of the XRay team drivers use as well. However you can also get 64Dp which means the teeth are significantly smaller. Therefore a 64Dp spur that has the same diameter as a 48Dp spur will have a lot more teeth. I don't know the exact numbers but I believe a 64Dp spur with 124 teeth has about the same diameter as a 93 teeth 48Dp spur. In this scenario 124/41 is almost the same gear ratio as 93/31.

So what's the difference? The smaller teeth allow less play between the spur and pinion teeth, making them more direct (sligthly more acceleration) and efficient. Moreover due to the smaller 64Dp teeth a one tooth change in the pinion equates to a smaller change in gear ratio than you'd get with 48Dp allowing fine tuning of the gear ratio. However the drawback is that the 64Dp spurs and pinions will get damaged a lot more easy and the spur can be stripped in a matter of minutes if you're unlucky. As the 64Dp advantages are barely noticeable the latter is the reason why we usually don't run 64Dp.

The second answer to this question assumes that you are using 48 Dp and need to decide between using 93/31 and 90/30. There are two reasons why we would run 93/31. (1) the larger spur pinion combination will enable us to get a better gear mesh. (2) with the larger gears the motor will sit lower, lowering the center of gravity (CG) which improves car handling (as long as the motor does not stick out below the chassis off course).