calculating final ratio

[TwistedCreations]

ok, i have asked before and have been told..for the life of me, i can't remeber how to freagging do it.. so in dumbass terms how do you do it?


i have a Emaxx tranny which i found out from tower hobbies, that 1st ratio is 28.9:1.. then i have 9 tooth pinion, 72 spur, and running maxx diffs. also, i will be running a 3.8: 1 reduction unit..so how do you figure that all out..i think the maxx diffs are 2.28:1 if im not mistaken( which i probably am :roll: )

so you divide 72( spur gear) by 9 ( pinion) right? that would be 8. the multiply 8 by 2.28( diffs) which would equal 18.24...then multiply 18.24 times the trans ratio right? ( that 28.9:1 must be the final drive ratio and not just the trans ratio) if so, 28.9 x 18.24= 527.1 and theres noway it crawls that slow...( so whats a emaxx tranny?).. but then after you multiply the tranny by the 18.24 you would multiply that number by the 3.8( reduction unit) and that would be your final crawl ratio right??

so if that's basically the way you find it out, i need to know the ratio of the tranny..anyone know? if not, how do you go out about finding it..do yu have to multiply the gears inside the tranny? mines locked in 1st anyway, which is 36T for 1st gear,idler gears which are 20T,then on the input shaft you have 18T( i believe, because the top gear has 13T on it) so how would you figure out the ratio of just the trans?

anyway, hopefully someone can understand all this crap and give me some input( in dumbass terms) so make it easy

 

[BARK]

The final gear ratio is independent of the number of teeth on the idler gear. If I am understanding you correctly on what which gear drives what (I have never taken my maxx tranny apart) then divide 36T/18T to get a reduction of 2:1.
You have the right idea when it comes to calculating the final ratio, but I dont think the numbers that you are using are correct.
Does anyone have the 1st gear ration for the Emaxx tranny.

 

[TwistedCreations]

thats why i was wondering if anyone knew how to calculate what ratio a tranny is? noone seems to know the ratio of the Maxxy tranny..i know it sure aint the 29.1 that i posted earlier. i would say that is the final crawl ratio for 1st gear ( multiplying this and that and yadda yadda yadda) instead of traxxas giing you just a ratio of the tranny, they figure they would just add it all up for ya that way...

i thought i had my math correct when figuring up a final crawl ratio, but i wasn't sure what the Maxx tranny # was and wasn't for sure on where to add or multiply the reduction unit :roll:



cmon guys, someone has to know the ratio of the maxx tranny.. if not, someone tell me on what gears i need to use and how to do it. that way i can at lest try and figure it out... cmon grizzly, dirk,Toyboy or any other gear ratio wizard out there.. what's the answer

 

[wheeler]

i want to know what the ratio is too, if you got one couldn't you just give the spur gear a turn and count how many times the outputs turn?

 

[kona_joe]

I found this on Traxxas's web site...

Hope this helps
Joe
---------------------------------------
How do I calculate my gear ratio?

Use the following formula:
# of teeth on the diff gear = transmission ratio
# of teeth on the top gear

# of teeth on the spur gear X transmission ratio = overall ratio
# of teeth on the pinion gear

http://www.traxxas.com/support/faq/trx_faq_electricb.htm#gearratio[/url]

 

[Ace]

I'll take a stab at it:

This is from Traxxas pdf's of the tranny and parts list. I don't have a tranny here with me at work so I hope I'm right!

Stock pinion spur: 66/18=3.66

Tranny internals:

1st gear: 36/13=2.769
2nd gear: 31/18=1.722

axles: 37/13=2.846

SO, final ratios:

1st gear:
3.66 X 2.769 X 2.846=28.897:1

2nd gear:
3.66 x 1.722 x 2.846=17.971:1

72/9 spur/pinon gives finals of: 63.04:1 (1st gear) and 39.20:1

63.04:1 with 6" tires and say 18,000 rpm motor would give top speed of 5.09 mph in 1st and 8.19 in 2nd.

 

[Grizzly4x4]

You had the right idea Toyota. I am pretty sure the 13 tooth top gear is for first gear and the 18 tooth top gear is for 2nd gear. If I am wrong just change the number in my equation but like I said I am pretty sure of this.

Spur and pinion ratio: 72/9 = 8

Tranny ratio: (20/13) x (36/20) = 2.769

Differential: 37/13= 2.846

So total reduction is: 8 x 2.769 x 2.846 = 63.045

With a gear reduction unit: 63.045 x 3.8 = 239.57

 

[Grizzly4x4]

If I wouldn't have double checked the tranny ratios I'd have beat ya, Ace. :lol:

 

[TwistedCreations]

thanks griz.. so your saying that the ratio of the tranny is 2.76 right? then the diff would be 2.84?

so since im locked in 1st and have a 7" tire... would the final ration of 239.5 be too slow ya think? i guess i could always change the pinions back to larger ones if need be


cool deal, thanks for the help

 

[BARK]

Tranny ratio: (20/13) x (36/20) = 2.769

This is why the idler gear is not considered when finding gear ratios. You could have a 1,000 tooth idler and the gear ration would still be the same. To simplify the math, dont even consider the idler.

And thanks for getting those tranny numbers (T/T).

 

[BARK]

so since im locked in 1st and have a 7" tire... would the final ration of 239.5 be too slow ya think? i guess i could always change the pinions back to larger ones if need be

The tire size does not change you crawl ratio, just your crawling SPEED.

 

[TwistedCreations]

i knew that. thats why i asked if 239.5 would be too low/slow of a gearing choise for running a 7" tire.. maybe i just worded it wrong. i haven't been able to type worth a shit 2day :roll:

 

[Ace]

239.5:1 and 7" tires with 18,000 rpm would gibe ya 1.5mph max!

Sounds a mite slow!

BTW speed is linear with gear ratio change, so if you halved the 239:1 you would double your speed (119:1 would give 3 mph)

I'd say 6mph or so is as slow as you want to go...

7" tires, 18,000 rpm ya want about 63:1 for 6 mph.

 

[Ace]

How are you getting 239:1 ???

 

[TwistedCreations]

So total reduction is: 8 x 2.769 x 2.846 = 63.045

With a gear reduction unit: 63.045 x 3.8 = 239.57

here is how im getting it Ace :lol: i thought grizzly was the math wizard around here. i have been outta school for so long, i keep forgetting all of it :lol:

i just checked him on my super duper blues clues calculator and he seems to be right 8)

 

[Grizzly4x4]

Sorry guys, I had meetings this morning.

to calc vehicle speed:
(motor rpm x tire dia.)/(gear reduction x 336)
with 20,000 rpm motor, BFT tires(6.625" tall) and 239.5 gearing:
(20,000 rpm x 6.625")/(239.5 x 336)=1.64 mph

I agree with Ace, might be too slow to "bump" obstacles.
My clod is 48.75:1, so my top speed is : 8.09 mph and I think I am too fast (not enough reduction) but not by much.

If you want me to take a guess at what a good speed would be I would say that I am shooting for a gear reduction of about 84:1 on my truck. This would give me a top speed of 4.69mph with BFT's and 5.67mph wif Kongs :twisted: . I dont know what the perfect ratio is but I do know that ALOT of the obstacles I climb require a good "bump".

BTW, I am the math wiz. :roll: :wink: :lol:

 

[Grizzly4x4]

This is why the idler gear is not considered when finding gear ratios. You could have a 1,000 tooth idler and the gear ration would still be the same. To simplify the math, dont even consider the idler.

And thanks for getting those tranny numbers (T/T).

I just wanted to show where the numbers were coming from without getting into alot of explanation. I could have left it out but I am wierd like that. :wink:

and yer welcome.

Remember guys BFT's are 6 5/8" (6.625") tall, not 7". :wink: :lol: :lol:

 

[TwistedCreations]

Remember guys BFT's are 6 5/8" (6.625") tall, not 7". :wink: :lol: :lol:

wouldn't want to bet on that would ya

ohh sorry, i just double checked mine and they are 6 7/8" tall :lol: i like what a skinny rim will do :lol:

 

[Grizzly4x4]

Ah yes, I forgot about a narrowed rim , good call Toyota. 8)

I should measure my "cut and shut" BFT's when I get home and see what they are.

Let us know what you decide to do with gearing and how it werks. Do you agree that you need some bump speed?

 

[TwistedCreations]

yes i need to be able to bump. thats why i was kinda thinking 239.5 was a bit too slow..i was gonna shoot for around 80:1.. maybe just use the reduction unit and then just go bigger pinion and smaller spur.. my 1:1 crawler is at 108:1 and 1st gear barely has enough to bump. i usually have to bump something hard in 2nd gear