Gearing Stock & Mod

This week we’ll go over gearing and how I personally look at it in both stock and mod as bigger classes like 4wd SC and 1/8 E Classes.

I often see stock cars at the local track being over geared for several reasons.  The biggest reason is generally because “It’s what the fastest guy is running” or “I was gearing for temps.”  Let me tackle these one at a time and explain why this maybe wrong for you and your car. First if you don’t drive like the “fastest” guy, have the same car as he does, run an ultra-free gear train like he does, or have a car with nearly identical parts as he does, your gearing maybe off. Often times the “fastest” guy will also likely be carrying more corner speed than most which may allow him to run a bit more gear than you. Second gearing for motor temps alone is never a good idea. Again car setup, driving style, drive train drag, car weight, battery quality can all play a part in the motors temp.

Personally I always tell our drivers to gear for performance in this order: 1. Lap times 2. Temperature 3. Lap Times.   Again let me elaborate a bit.

One, gearing for consistently fast lap times is critical for obvious reasons. You need to repeat that “hero” lap for 5-6 minutes, not just one time. Last time I checked there are no trophies for “Hero lap.”  This could mean that on the track layout, being a gear lower than you may think will help you clear jump sections more consistently or be quicker off the corner which allows you to pull on the over geared cars.

Two, temperatures are important but not in regards as to getting speed out of the motor. All electric motors run better the cooler they are, so why would you gear one up just to get it to 160f if you can get the lap times out of it when it’s 140?  I don’t know either!?  This is why getting #1 correct is important.

Three, the overall lap time rather for the lap time drop off of the car. If you’re running 14.3’s for the first 3 minutes then the car falls off to 15.3 for the last 3 minutes that’s not a great setup, it shows either the motor getting hot and fading, or the battery going soft, or a combination of the two. This shows a motor that is “pulling” the gear but is clearly over geared by fading that hard. In this case note the mah usage of the battery, drop a pinion or two and re-run the car.  What you’re looking for is laps in the 14.3 to 14.5 range but there is little to no fall off. An indication you’re change is also headed in the right direction is less mah usage as well. A more efficient setup will often use less power overall.  While sitting track side I can often watch cars and tell if they’re overgeared. I’m looking for the cars acceleration rate and it’s ability to get out of the corner strong.  If the car is never topping out on the straight away and it’s requiring a bit more run up room before jumps, it’s likely over geared. A properly geared car will just briefly stop accelerating or will taper off it’s rate of acceleration at the end of the straight away right when the driver is going to lift/brake for the turn. A properly geared car can navigate the jump sections with a tighter line and with greater ease than one that is overgeared. Remember, look at the track and layout. Does that extra 2 feet on the straight away you gain in top speed really show up in lap times on the track when you lose the ability to hold a tighter line in the jump and turn sections? You’ll have to test it to find out, again gearing always for 1. Lap Times, 2. Temperature, 3. Lap times.

Gearing can be altered by changing the pinion or spur gear. Pinion changes are often the first step as tooth for tooth, they make the biggest change. Changing the Spur for example from a 74 to a 72 is a small change compared to a pinion change of 28 to 29.  It’s time to learn the math of FDR or Final Drive Ratio.  First you must know your cars internal transmission ratio. Two popular ones are 2.6 or Associated, and 2.43 for TLR.  If I’m running a 22 2.0 using a 70T spur with a 30T pinion my FDR would be:  70/30 x 2.43 = 5.67    When is it best to change the spur vs the pinion?  I often use the pinion change to make larger jumps in gearing, and spur gear changes for “half” pinion changes. Why?  Well let’s do that FDR math again.  If I need to reduce gear from my 70/30 setup and go to a 70/29 setup my FDR goes from 5.67 to 5.86. Now a spur change to 72/30 would give me a 5.83 FDR. So you can see that 2 teeth on the spur is less of a change than one tooth on the pinion. When you’re dialing in that last .1 of a lap this is where optimal gearing can help you.  In general one pinion is 3-4 spur gear teeth when comparing the two.

A side effect of changing spur gears vs pinions that is often over looked is the movement of the motor location in the car. Now that mid motor is the popular layout moving to a smaller spur gear often moves the motor more towards the rear of the car.  This can affect weight transfer, pendulum swing of the car, etc.  Pinion changes do move the motor but nowhere near the amount a spur gear change will.


Now on the larger cars where 4 pole motors are often being used the gearing theory as a whole is accurate however there are some instances where they maybe not as strong.

Under gearing a 4 pole motor can and often will create more heat and mah usage than one that is over geared. 4 pole motors make gobs of torque and wasting that torque on an under geared car will mean you’re revving the motor past its optimal use range.  For example the Pro4 4600 motor in a TLR SCTE will often be geared in the 13-15T range, compared to a Pro4 4300HD on the same track being geared in the 15-17T range. While it’s only a few hundred kV the gearing is quite different.  Temperatures again are important and as always try to keep them under 180f for max life and to prevent fading at the end of a race.

1/8 E Cars are some of the most fun to run especially E Truggy. The size of the car and the speed at which is accelerates is simply a frown killer.  Going down the longest straight away your 1/8 car shouldn’t be topping out until right when you’re lifting for the turn at the end of the straight.  If you’re gearing is allowing the car to top out or isn’t accelerating hard to the end, try gearing up a tooth. You’ll likely always have too much bottom end anyhow so gearing up a tooth can help prevent wheel spin on loose tracks also. Again monitor your batteries mah, motor temp, and esc temp. All of which on Tekin equipment is easy to do. Don’t forget on your Rx8 Gen1 and Gen2 you have the onboard temp monitor of the esc which utilizes the 7 led tree. On the Rx8 Gen2 you can not only use the onboard monitor but you can also use the data logger which is even easier now since you now have the data logger on the Android app!

Again this is what I’ve found as an approach to gearing. If you have questions or a more detailed gearing question using any of your Tekin equipment, be sure to hit us up for help.

Good Racing,

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