# Thread: Need help to understand gear ratio/wheel diameter.

1. ## Need help to understand gear ratio/wheel diameter.

Hi everyone,
my name André (boilo56), i'm news to this forum thing, and i need help to understand a lot of things in the RC world. But now, i wish to know what gear ratio i must use in my merv, to run whit traxxas #6873 tires( BFGoodrich Mud-terrain). My merv is a 7105 model that i converted in a 7107(vxl model whit a velineon 380 motor).

Thank you for helping me!

2. Those tires are a bit big, so a small pinion should be use to avoid overheating issues, a 9-11t pinion should be a good start point. Also depends what batteries you plan to run.

3. and a 55 spur gear!

4. If you give me the OD of the wheels I can give you any gearing you want inside the available gearing combos.

Match stock or taler to a goal speed if your rig can make it.

5. The ones I have seen are 4.3" tall .you can gear down but still a lot of rotating weight that will break parts!

6. There is an RC speed calculation script on the net that gave me some pretty valuable gearing information...

http://scriptasylum.com/rc_speed/top_speed.html

Rollout is the ratio of how many revolutions of the motor causes the tires to move one inch.

If the ratio is 0.5:1, then 1 motor revolution rotates the tire 0.5 inch.
If the ratio is 3:1, then 1 motor revolution rotates the tire 3 inches.

Some people prefer a rollout of 1:1, which you can read about here
The "1:1 rollout rule" is a method used by some to find the "ideal" gearing such that one revolution of the motor causes the tires to move one inch.

Simply put, a 1:1 ratio means the total gear reduction equals the tire circumference. So, if you have 4" tires (circumference = 12.57"), the total reduction ratio of the differentials, transmission, etc should total 12.57.

This rule does not suggest the gearing to achieve maximum top speed, nor does it guarantee a cool running setup. It is simply a starting point. You should always use ESC, motor, and battery temperatures as a guide.
The calculator will give you your final rollout ratio. I find that 1:1.1 or lower will generally produce results without overheating. Hope this helps.

7. I like the looks of that info and I'm gonna give it a run through, thanks for the post.

Closest gearing to stock 50/28 with those wheels around 109mm would be 55/23

The extra weight is the real issue.

8. Dang Cipher, where did you dig up that information!?!? Is your Alias "Einstein" or "Neo"? Did you help design the matrix?

All kidding aside, thanks for the data, I never thought that much into ratios before. At times, I want to know this much about these cars but, geez, it takes so much processing power and I get lazy too fast!

If you can PM links to some of the technical data you have layin' around, that would be sweet - Thanks

"Red"

9. Hi, thank you all for the infos, since i'm totally new to the rc world, your advices are really valuable for me. i will experiment those as soon as my merv is repaired, i broke my front bulkhead, steering post and front of my chassis, by hitting a steel post while flying, or trying to. I will post my test results as soon as i can do them.

10. well, i'm running 2 traxxas 7.2v 1200ma in series, only to see what's Inside. but my goal with those things is for running in mud, as the tires are mud-terrain. But i'll surely do some speed runs just to see what it got!

11. thank you Cipher Jones, that link is great, i just put everything i know about my setup, and it said that: Theoretical Maximum Vehicle Speed =97.24 mph (156.2 km/h). but i'm not sure about the current draw in amp, and i don't know the motor resistance... But these are with stock setup exept for the 4.3in tires. i'm sure i can't get to that speed, (physics laws)!

12. Originally Posted by boilo56
thank you Cipher Jones, that link is great, i just put everything i know about my setup, and it said that: Theoretical Maximum Vehicle Speed =97.24 mph (156.2 km/h). but i'm not sure about the current draw in amp, and i don't know the motor resistance... But these are with stock setup exept for the 4.3in tires. i'm sure i can't get to that speed, (physics laws)!
The rollout on that setup looks like its 1.7:1 The theoretical top speed is "up in smoke". The vxl-3m ESC has a 50,000 RPM limiter, so the top theoretical speed is 79 mph. I got about 4 minutes of runtime with 14.4 volts at a 1.16:1 rollout with 1600's.

In other words, you need to adjust your pinion (much smaller) to compensate for the huge tires. Final rollout should be about 1:1

13. Using that Rollout formula I came up with a 10.15in circumference with stock Talons. That would require the total gear reduction to be 10.15, the closest I can calculate it to is 50/25 giving a 10.08 vs the stock 9 using 50/28. That cuts top speed from around 50mph closer to 43mph.

I run 3s and on a hot day with no sink or fan didn't get over 160f on the ESC and 150 something on the motor, with the heat sink and fan the 2 pair up around 140f at cut off. I don't see the stock gearing to be under gearing the truck at all, and if the larger heavier wheels cause heat issues after being geared to stock (or as close as possible) with the larger wheels, the only thing that would be the cause is the added weight.

Putting lighter wheels with a larger diameter screws that calculation. The increase in OD calls for a larger amount of reduction when them being lighter often times will reduce rotating mass that reduces load greater than the increase in diameter generates. This in theory would allow us to gear up as apposed to the calculation calling for gearing down. Even when going to smaller wheels like 65mm foams, both lighter and smaller, the calculation only accounts for the wheels size ignoring the weight, so again in theory I would again have more room for gearing up.

I hear people who run stock batts in series get heat issues, but I think that's from trying to get too much out of NiMH batteries not gearing down with the provided pinion when running in series / running in series too long and an overall excessive voltage on the ESC.

The calculation also doesn't account for battery choice, 2s I believe can safely be geared higher then the same RC on 3s etc.

The thing I notice is that it consistently undergears and not by very much, but enough to give an idea of a "safe" gearing, certainly not "ideal" IMHO this is reserved for Temp Gun tuning. I would hope enough R&D went into the stock gearing for it itself to be used as a general "safe" gearing when changing wheel size accommodating it through the gearing keeping the same overall reduction motor to road, taking into account the increase or decrease in wheel weight. Heavier wheels need gearing down, lighter Wheels may allow gearing up.

Temp gun n ur dun

14. You basically reiterated what I quoted from the site, except I can vouch that lighter tires with a larger diameter heat the motor up in proportion to the circumference gain. Lighter tires of the same diameter do not reduce heat either. I tested that with a temp gun. Also, the script gives you a warning when you apply too much voltage to the equation, I.E. try to run 3s on the same setup, or 14.4v...

The motor kv (3900) and voltage (11.1) you selected exceeds the general recommended maximum brushless motor speed of 40,000 rpm. Motor overheating may result.
And of course the Traxxas manual should be the first thing that one reads before running. Unfortunately it is not all too frequently.

15. Originally Posted by Cipher_Jones
You basically reiterated what I quoted from the site, except I can vouch that lighter tires with a larger diameter heat the motor up in proportion to the circumference gain. Lighter tires of the same diameter do not reduce heat either. I tested that with a temp gun. Also, the script gives you a warning when you apply too much voltage to the equation, I.E. try to run 3s on the same setup, or 14.4v...

And of course the Traxxas manual should be the first thing that one reads before running. Unfortunately it is not all too frequently.
I read my manual Cipher, but mine is for brushed model 7105(since i converted it in a vxl), the part you refer to was not in my manual. By reading your reply, i understood that i was missing something... sorry for asking something clearly written.
i will be more careful next time. thanks again!

16. I don't mean to hi-jack a thread here, but I was wondering why no one has talked about under-driving(?) the gears in the trans? I'm sure I read somewhere that if you run tires taller than 4", that you should flip two gears in the transmission.
So wouldn't that make the final gear ratio under 1:1, and then gear-up from there?
I myself, am trying to figure out the gearing for how to run the monster jam wheels/tires on my merv, for when the snow comes =)

17. Actually, we have the same goal, but i am in the unknown, i got my merv two weeks ago, and it's been 20years since my last rc... but i think (remember, i'm a noob) it is a good lead. (i hope this means what i think, **** i hate not being good in English!) if you find anything, please share it with us!

18. I wanted to share that I followed a suggestion on here and just got these springs on fleabay. Now my little merv is sitting high & tight!
I run 2x 1500mah 11.1v traxxas lipos with a parallel connector, to increase my run time. I haven't had alot of run time, but the only time my esc went to "thermal shutdown" mode was when I was running in semi-tall grass (6-9in).
I have since installed the 23 tooth pinion gear, but am wondering still about just under-driving the trans gears.
I have diff lockers tra7381(hmm out of stock), center diff tra7014, and the 1/16 grave digger wheels/tires tra7265. I will get to installing these when the snow comes! If all this isn't enough to get some traction in the snow, I would have to pull out the snow chains that someone made for the OE talon tires! Gotta love 'ole fleabay =)

19. Whit the 23t pinion, did you get some torc when you start from a still position? I only have a 26t pinion, and whit that, it is pretty weak on start.

20. With the 23t there is a bit more torque off the line
With a stock merv I used a 25t most of the time, that was more in the hotter summer days with 2s,sometime 3s,
All depends on where n what ur driving on. Use a temp gun or train ur finger to guess

21. Originally Posted by neu_merv_dude
I wanted to share that I followed a suggestion on here and just got these springs on fleabay. Now my little merv is sitting high & tight!
I run 2x 1500mah 11.1v traxxas lipos with a parallel connector, to increase my run time. I haven't had alot of run time, but the only time my esc went to "thermal shutdown" mode was when I was running in semi-tall grass (6-9in).
I have since installed the 23 tooth pinion gear, but am wondering still about just under-driving the trans gears.
I have diff lockers tra7381(hmm out of stock), center diff tra7014, and the 1/16 grave digger wheels/tires tra7265. I will get to installing these when the snow comes! If all this isn't enough to get some traction in the snow, I would have to pull out the snow chai

ns that someone made for the OE talon tires! Gotta love 'ole fleabay =)
Do you know the spring rate of those springs or the length?
If you go with the 2.2" rim you could try http://prolineracing.com/tires/sand-...ck-rear-tires/

22. Originally Posted by 50togo
Do you know the spring rate of those springs or the length?
If you go with the 2.2" rim you could try http://prolineracing.com/tires/sand-...ck-rear-tires/
From the ebay listing: 9/16" x 1" x .054"