Another gearing question...I need help!
 

I've searched and searched and read a lot but still can't quite find my answer...

On a full Supermaxx with 1/8 diffs, Ofna 51 spur, 40 series tires, and 4s lipo, what would be good gearing for 7xl and 9920 or 16016?

I've been trying to understand gear ratios but have only become more and more confused!!:C:

Another retailer told me I need something like a 50:1 ratio and that the 51 spur is too much and should go back to stock. A lot of things he told me seems suspect, so I am ready to start over from scratch with advice from here instead. ( I only went there cuz he had the 9920 in stock, but I wish I didn't)

I have 10, 12, 14, 15, and 16t 5mm Mod1 pinions and only a 16t 32p 5mm pinion -- what would be the best choice? I have 51t Mod1 spur and both 66 and 72 32p stock spurs. I would like to stick with the lipos if possible, and am more interested in durability than hi speed. Wheelies would be fun too. I will either use ubec or receiver pack, and can go back to standard size wheels if needed.
Thanks so much for your help!
--Jonathan

I would start with the 14 or 15 tooth pinion. The 7xl seems more forgiving on high geared verse the smaller motors. a 50:1 ratio is crazy. I could see if you going rock crawling. Wheelies should not be a problem. Not doing a wheelie may be a problem. :) Hope this helps you out.

forget about this 50:1 idea!
and about the suggested bigger spur: Maybe your dealer had no idea how big mod1 gears are:C:

Try the 14 and see how you like it. This should put you in the save range (on 1/8 diffs). Always keep an eye on the temps, though. Especially when you try a new gearing.

Hope this helps

That's a good point Dafni -- thanks for that! I just compared the stock 66t spur to the 51t Ofna and they are almost the same size. The 1/8 diffs should make up for the larger tires and I should have enough pinions to find one that works.

I'm going to pick up the 7XL shortly, hopefully will be running tomorrow.

From thinking on this some more, if I had started with the 6XL and 12 cells gp3300 I might have been ok, but not with 4s lipo--just too hot. After running lipos though, it is hard to go back to nimh.
Thanks again!
--JD

You're welcome!

Hey, once again I feel the temptations of LiPo!

OK here's some calculations for ya.
Assuming you have a center diff/slipper and 1/8th F/R diffs
If you have a transmission or other gear reduction, the gear ratio will need to be re-calculated.
Here's a gear ratio break down
Center diff/slipper F/R diffs
14t pinion 12t pinion
51t spur 40t spur
51/14=3.64 40/12=3.33

So 3.64*3.33 gives you a final gear raitio of 12.14 to 1

Now for the other variables,
4s lipo = 14.8v
7xl kv = 2382
At this voltage the motor can spin at 14.8*2382 = 35253 RPM

So divide your motor's max RPM by your gear ratio to get the wheel's max RPM;
35253/12.14 = 2903RPM

The 40 series tries are around 18inches around (circumference)

So 2903*18=52254 inches per minute
Which is 4354.5 feet per minute
Which is 261270 feet per hour
Which is 49.48 mph

The 12t pinion looks like it will push your truck around 40mph, and the 10t looks like its around 35mph.

Of coarse the size of the truck (weight etc) play a role in the performace and speed the motor can produce.

I'd start with the 10t and work your way up. I would error on the side of more gear reduction than less, especially starting out. You can always gear higher :)

I listen to Dafni about motor temps. Heat gun's are definetly worth thier weight...

Oh ya, a 50 to 1 ratio would put you at 12mph ;)

Cheers

@ Starscream
What is the gear ratio for the gmaxx single speed?

Thanks starscream!! I also have the gmaxx single speed tranny, but even without that your calcs help a lot. I got as far as the 3.64 number, but didn't know what to do after that.

The 50:1 ratio does seem a little ridiculous, eh? :rolleyes: I guess that's what I get for going to a boat place...won't be doing that again.

I should be gettin' my 7xl in this week from mike..

I'm goin' to try 22/66 gearing... 6S lipo, 22.2 Volts

51000rpm (motor)
3:1(Spur to pinion)
2.77:1 (1st Gear)
1.81:1 (2nd Gear)
3.31:1 (1/8th diff)

multiply 3 x 2.77 x 3.31 to get final drive ratio for 1st gear
multiply 3 x 1.81 x 3.31 to get final drive ratios for 2nd gear

I have an ms excel sheet that does the calculations, PM me if you'd like me to send it...

Papa, the gmaxx single speed locks the tranny in second gear. It doesn't effect the gear reduction so it remains the same 17.9 to 1.

Multiplication occurs when a gear is added to the same axis point such as the emaxx input shaft. This shaft shares the 66t spur and the 13/18t input gears. Since gears such as the idler gears do not introduce additional gearing on the same axis they do not effect the final gear ratio.

Here's how to calculate the stock emaxx gear ratio:
Lets take second gear for example.
We have the following gears
18t pinion
66t spur
**Change occurs so Multiply current ratio by the next ratio**
18t input
20t idler (doesn't introduce new gearing so doesn't really count)
31t output
**Change occurs so Multiply current ratio by the next ratio**
13t diff pinion
37t diff spur

66/18=3.67
Current ratio is 3.67 to 1

the 66t spur gear shares the axis with the two speed gears (2nd being 18t) so this is where a multiplication occurs after we divide the following:
31/18=1.72

3.67*1.72=6.31
current ratio is 6.31 to 1 next reduction...

The outputgear shares the same axis as the 13t diff pinion.
The f/r diffs have the following ratio
37/13=2.85

2.85*6.31=17.9

So the final ratio 17.9 to 1

You must also factor in the circumference of the tires to get the actual output to the ground shown in the previous example.

You should be able to figure out any gearing scenerio this way.

Sorry for the book but I thought it would be helpful.

Yeah... that's rright starscream

Wow aqwut, you'll be pushing your truck into the 50 mph range with that gearing. Do you think the 7XL can handle that?

Let us know how it goes.

I don't know, but it should be able to... because the lehner 1930 can take my truck to 50mph and it's only 1150Watts...

This is a bigger motor, but it's only a fraction of the price...

I'm goin' to push taht 7XL as much as it can take..

I'm a firm believer of "you get what you pay for", but I hope this Feigao Xl motor prooves me wrong....

Just watch your temps. I ran a 7xl with 22/66 first and second gear in tranny with stock rears 2.92 I think. ran it on 18 cells. It was kind of quick. It thremaled a few times a hot days. I wanted to try a 24 66 gearing. Just never did.

really?.. that is wierd, my 1930s didn't even have a problem with that kind of gearing....

thanks for the scoop.. It's gotta be able to handle that.. that 7xl is almost 400grams... :)