Gearing...

[Daddy's Addiction]

I'm hearing mixed information on xmaxx gearing. I'm looking for a ratio for more low end torque for backflips. It's a completely stock xmaxx. So I want to adjust the gearing before a new ESC and motor. Any advise on a choice for ratio and brand?? Thanks

 

[Rep732]

 

[RcJunkie13]

eh that gearing chart is playing really safe. I used to run 19or 20/50 just fine
i can confirm that stock gearing sucks tho

 

[Rep732]

ever put it in the red junkie? I have it doesn't like it and goes into limp mode fast, don't forget tires and gearing play a role with or against eachother.

And I believe it's mutually understood stock gearing sucks across the board.

 

[RcJunkie13]

19/50 is in the blue and i found that to be the best

 

[Rep732]

Oh so the gearing chart you dismissed yesterday as "meh" is accurate? Wow who would of thought traxxas included a chart with factual gearing combinations . And I run a 16/54 with a hardend tekno pinion and hot racing hardened spur.

 

[RcJunkie13]

Oh so the gearing chart you dismissed yesterday as "meh" is accurate?

not necessarily… the blue implies that it is strictly for speed runs on hard surfaces however i bashed mine off-road. It’s a good guide however it does not need to be followed very strictly. Understandably traxxas is playing the gearing very safe.

And I run a 16/54

how do you find that? i would assume it’s quite limiting when it comes to speed

 

[Rep732]

Judging by your answer you don't know how to use the chart do you? If you knew how to read it you would find that I'm at the lowest of the increased speed across all surfaces with my combination. This is how you properly use the chart

 

[RcJunkie13]

i understand how to use the chart
i am trying to convey to you that sticking strictly to this chart is limiting yourself a lot

 

[Rep732]

Stock motor/esc.. your pretty limited.. as the chart.

 

[RcJunkie13]

.

 

[RcJunkie13]

Throughout my ownership of the xmaxx from the release date (the 6s release date that is, and then I upgraded to 8s), I have tested many gear ratios and learned that the chart is best used as a general guide rather than be followed precisely. Yes, obviously you are limited to what's on the chart however it is best used as a ballpark guide kinda thing. I would suggest trying 19/50 and seeing how you like it

 

[Daddy's Addiction]

It seems we are in a bit of a disagreement here...
Throughout my ownership of the xmaxx from the release date (the 6s release date that is, and then I upgraded to 8s), I have tested many gear ratios and learned that the chart is best used as a general guide rather than be followed precisely. Yes, obviously you are limited to what's on the chart however it is best used as a ballpark guide kinda thing. You seem to be following the chart exactly, therefore limiting yourself. I say this as respectfully as possible, but my age plays no factor in this conversation especially considering my experience with this rig is much greater than that of yours. I would suggest trying 19/50 and seeing how you like it
not sure if you removed the previous or if it was a glitch

Thanks for the info... do you think a 15/54 is to big of a jump?

 

[RcJunkie13]

I think the best solution here is to buy a few different pinions and a 50t spur (no need for 46 plus it’s alr included) that way you will have all 3 spurs and a variety of pinions and just experiment to what best suits your driving styles and conditions

 

[A.Lunatic]

Hi.

I’m new here but thought I could help as I struggled to find useful info on the subject of gearing / motors / kv / wheel size for the xmaxx.

Intro - I'm a mechanical engineer. I’ve been bashing for around 40 years, mainly truggies. Never raced as there are no tracks near me. After much research I finally gave in and bought myself an xmaxx for Christmas this year. Unfortunately I’ve had nothing but trouble with it! Long story short: Pinion came off on first run (first 6s pair on road outside my house) Loctited it, Pinion stripped on second run (same 6s pair), put a steel 19t on and it stripped the spur (second 6s pair). Put on the supplied 46t spur… stripped on 3rd run (still second 6s pair). Bought a hardened steel 46t spur and new 18t pinion. Mesh was way too tight to run it on suggested pin config (E), so I fitted a 19t with pins in A (19/46). Mesh was perfect.. Shaft came out of the motor rotor on 3rd 6s pair. Glued shaft back into the rotor but the fabric that holds the magnets in place on the rotor was damaged from the 3rd run so on the first acceleration the magnets disintegrated, motor seized and the ESC went pop. I have had less than 3 runs on 6s with my brand new truck and it is dead! I haven’t even been off road with it yet. I must have been unlucky and bought a lemon!

So I have just ordered a HW max 6 / 1100kV combo to replace the Traxxas rubbish, and bought mod 1.5 Vitavon gears with the input shaft support bearing. I’m hoping this will solve these issues. I’m not looking for speed figures, I just want it stock(ish) power / speed but reliable so I can bash for at least a whole battery pack without it tearing itself apart! I intend to use dual rates to nerf the throttle and make power delivery suitable for the specific terrain I am at, whilst not stressing the electronics / vehicle too much.

Anyway, I was looking for info online on recommended motors / gearing and I found it all a bit hit and miss, so I did some calculations and thought I would share my findings. I appreciate this is VERY subjective but it does gives you a rough idea of what to expect as you change these components and it doesn’t look a million miles off what you could expect with different combinations. Again, please bear in mind this is all theoretical and calculated in a no load / friction scenario..

So lets take the standard gearing / motor / wheels as an example:

The manual states that the x maxx final drive (wheel speed) can be calculated by spur/pinion x 8.11. (I have checked this and it is correct) So then by imputing figures for the gearing, motor kV, voltage and tyre diameter, you get a rough estimate of the final drive RPM and thus speed:

So the std. ratios are 50/18 x 8.11 = 22.53:1 (the motor turns 22.53 revolutions for every 1 revolution of the wheels)

With the stock motor and (fully charged) 8s batteries you get an estimated (no load) motor RPM of:

8 x 4.2V = 33.6V, x 1275kV = 42840rpm (motor RPM = kV x Volts)

So 42840rpm / 22.53 = 1901.5rpm (no load wheel rpm when on full throttle)

Now this is where it gets even more subjective as the tires are 8” but as we know they expand under centrifugal force, probably to at least 10”, but for the sake of these calculations (and as there is a load and the main driving tires at the rear will be compressed under acceleration) lets estimate this as 8.5”

8.5” = 0.216m, x π = 0.678m (the distance the truck will cover in one revolution of the wheels in meters)

So the (theoretical) distance the truck will travel per minute is:

1901.5rpm x 0.678m = 1289m.

So per hour that is 1289m x 60min = 77353m

Convert to miles – 77353m/1609 = 48.1mph.


If you do this calculation with optional 18/46 gearing it comes out at 52.3mph (50mph claimed by Traxxas with optional 46t spur gear), so not far off. I would surmise that once you get over 50mph the effect of wind / rolling resistance will start having an exponentially detrimental effect on top speed.

As a general rule, and as I am sure you all know, the bigger the pinion or smaller spur gear the faster the truck, and more load you will put on the electronics and create more heat. (like 18th gear on a pushbike) With smaller pinions and bigger spur gears you will get more torque (faster acceleration) and less top speed. (like 1st gear on a pushbike) This will also de-stress the electronics and shift the load onto the drive mechanics.

I have attached a chart with some different gearing / KV options. (1100kV and 800kV calculations are with mod1.5 gears, but the math is the same as it is the ratio between the two that is relevant). I tried to attach the excel file but unfortunately I can't. If you would like me to calculate a specific config for you please let me know, or use the calculations above.

I hope this helps! Sorry for the essay!

 

[Clubin]

Need to dumb it down.

 

[A.Lunatic]

Sorry, didn't want to appear patronising. It's honestly not as complicated as it looks. I would be more than happy to explain anything if anyone is genuinely interested? I think It is useful to understand the effect of different motor KV's and gearing (as well as tire diameters) when choosing upgrades. This can be applied to any vehicle as long as you know the motor kV or engine RPM, The chassis drive train gear ratio(s) (input shaft - spur gear to drive wheel ratio) and tyre diameters.

For example with the 1100kV motor I am buying for the xmaxx, the smallest pinion / spur set I could buy is 20/30. This will give a theoretical top speed of 76mph (if the motor is at its maximum speed, which is actually unlikely due to variables such as electrical resistance, friction in all the bearings, gears, the friction between the tyres and the ground, inertia, battery condition, wind resistance etc.)

I want to be closer to stock (50mph) so I have also bought a 16t pinion which will be closer to stock (61mph) with a bit leeway to down tune and keep the temperatures lower and put less stress on the electronics improving reliability. (although that's not hard with the experience I have had so far!)

Just out of interest, Attached are the ratios stated on the Traxxas Gearing Compatibility Chart for the xmaxx being discussed and the top speed you would get if the standard motor is flat out at its maximum rpm on fully charged 8s batteries and the tyres are 8.5" diameter.

Let me know if you would like me to explain anything

 

[Rep732]

 

[Cemo66]

Sorry, didn't want to appear patronising. It's honestly not as complicated as it looks. I would be more than happy to explain anything if anyone is genuinely interested? I think It is useful to understand the effect of different motor KV's and gearing (as well as tire diameters) when choosing upgrades. This can be applied to any vehicle as long as you know the motor kV or engine RPM, The chassis drive train gear ratio(s) (input shaft - spur gear to drive wheel ratio) and tyre diameters.

For example with the 1100kV motor I am buying for the xmaxx, the smallest pinion / spur set I could buy is 20/30. This will give a theoretical top speed of 76mph (if the motor is at its maximum speed, which is actually unlikely due to variables such as electrical resistance, friction in all the bearings, gears, the friction between the tyres and the ground, inertia, battery condition, wind resistance etc.)

I want to be closer to stock (50mph) so I have also bought a 16t pinion which will be closer to stock (61mph) with a bit leeway to down tune and keep the temperatures lower and put less stress on the electronics improving reliability. (although that's not hard with the experience I have had so far!)

Just out of interest, Attached are the ratios stated on the Traxxas Gearing Compatibility Chart for the xmaxx being discussed and the top speed you would get if the standard motor is flat out at its maximum rpm on fully charged 8s batteries and the tyres are 8.5" diameter.

Let me know if you would like me to explain anything

Can i do a 25/30 gearing on my stock 1275kv motor?

 

[RcJunkie13]

no