There is a common misconception in the micro-crawler community regarding how to calculate total overdrive when running modified gear ratios in both the front and rear axles.

A frequent assumption is that if you run an advertised 23% overdrive up front and an 11% underdrive in the rear, your total overdrive is simply 34%.

Mathematically, this is incorrect.

Here is a breakdown of how these percentages are actually derived and how to calculate the true operational overdrive of your rig.

How Manufacturers Derive Gear Percentages:

To understand why adding percentages fails, we first have to look at how manufacturers establish these numbers. All advertised overdrive and underdrive percentages are calculated using the factory axle ratio as a baseline.

The stock Axial SCX24 utilizes a 2-start worm gear and a 16-tooth ring gear (2/16).

When a manufacturer develops an option gear, they calculate its performance increase or decrease solely against that stock 16T baseline:

13T Front Overdrive: 16 \div 13 = 1.2307 (A 23% speed increase relative to stock)

18T Rear Underdrive: 16 \div 18 = 0.8888 (An 11.1% speed decrease relative to stock)

These percentages are accurate if you are only changing one axle and leaving the other stock.

However, the moment you run both aftermarket gears simultaneously, the stock 16T baseline disappears. You can no longer calculate your ratio relative to a stock gear that is no longer in the vehicle.

Calculating True Overdrive:

True overdrive is defined as the percentage of increased wheel speed at the front tires relative to the rear tires. Because the worm gear inputs spin at the same RPM, the actual distribution of speed is dictated entirely by the relationship between the front and rear ring gear tooth counts.

To find your true overdrive percentage, use the following formula:

Rear Gear Teeth ÷ Front Gear Teeth - 1) x 100 = True Overdrive %

A Real-World Example: 13T Front / 18T Rear

Let’s apply this formula to the common configuration mentioned above:

1. Divide Rear Teeth by Front Teeth: 18 \div 13 = 1.3846

2. Subtract the Baseline (1): 1.3846 - 1 = 0.3846

3. Multiply by 100: 0.3846 \times 100 = 38.46\%

By compounding these gears, your front tires are actually pulling 38.5% faster than your rear tires—nearly 5% higher than the 34% total you get from simple addition.

Summary:

When tuning your chassis geometry and weight bias for high-level climbing performance, precision matters. When mixing and matching axle gears, disregard the promotional percentages on the packaging. Instead, divide your rear tooth count by your front tooth count to pinpoint exactly how your drivetrain is performing on the rocks.

Yep, this is what addiction feels like.

Prepping for a local comp this Friday.

Swapped from steel high clearance front links to aluminum straight links. This removed a few grams from the front and shifted the weight bias from 64% to 63%. Small change but did remove some weight above the axle line. While this probably didn’t improve the COG, the straight front links make for a much smoother transition to the skid plate.

With the cab only body there is very limited space for the wiring to live. Shaved a few grams of wiring by simply cutting to length. The wiring is currently the highest thing on the chassis.

Lowered the carrier bearing down a set of holes, lowering the center and rear driveshafts.

Lowered the rear upper links down a set of holes which actually made the biggest positive change on this one.

Previously the rear chassis brace would rest on the rear upper links. This small change lowered the rear slightly, and allows the rear links to sit up higher in the chassis rails. Gained a little more rear clearance.

Shaved 5.6g off the right slider moving from the unnecessary 6ch RX down to the 4ch RX.

Also flipped the shocks, although I don’t think this yielded any measurable change. Mainly wanted to fix my “automatic preload adjuster” (the tire rubbing the pre-load ring). Now going forward it just tightens the o-ring nut at full lock instead of randomly adjusting my pre load lol. Not a bad band-aid while I work on a lower shock mount to move shocks forward and out of the way of the tire.

I think this one is about as dialed as it can get for now.

Moving on to the class 2 rig.

Most recent project.

Been wanting to try something with a more aggressive skid angle, so I gave this printed chassis a try. Not sure I ever want to go back to a 0-7 degree skid again. The extra clearance once it breaks over is hard to beat.

Modified NOD-24 transmission to accept a 55t spur gear. I found the 32t spur gear it came with to greatly reduce the torque. It would cause the motor to jitter if the tires got in even a slight bind. It wasn’t possible to tune this out of the ESC (escape32) so I decided to go with a larger spur gear.

The 32t spur gave it a ton of wheel speed, but I don’t think it was worth the loss of torque. Since this transmission is designed for a small motor (nanobam, micro Komodo) the larger spur gear helped bring back the torque I felt it was missing.

Now it has plenty of torque and still enough wheel speed to bump over obstacles.

Torq MLS-1022 servo, my favorite servo for the money.

Injora 39mm shocks with 40wt oil.

OGRC stamped steel wheels with brass front rings and printed rear rings.

JConcepts 63mm megalithic tires with MEUS pink inserts.

Dinky R/C ESC

MEUS Nylon V2 axles with 2/12t front and 2/16t rear gearing.

Ramp crab stainless front links (c10 length)

MEUS double bend 90mm rear links.

Parted out a broken stock transmission to use the bearings and shaft for the carrier bearing.

MOFO RC nanobam motor

This started from some parts I had leftover from other projects, didn’t know it would turn into my favorite rig (for now)

I used some stock plastic driveshafts for the center and rear, I plan to replace those with mofo RC driveshafts. I like the d-shaped holes on the ends, and they seem to be one of the few I can find with d shaped holes AND a wider range of collapsed and extended lengths.

The length of the center shaft is deadbolt front and the rear shaft is c10 front.

Really happy with the performance, thought I’d share the details in case someone wanted to build something similar.

Here are some links to the printed parts:

Chassis:

https://makerworld.com/models/1390060?appSharePlatform=copy

Carrier bearing:

https://makerworld.com/models/2201771?appSharePlatform=copy

Body hinges:

https://makerworld.com/models/2480290?appSharePlatform=copy

Body:

https://www.thingiverse.com/thing:7113242