Truck/SUV axle ratios

[drkucho]

Hello i am triyng to find information about what would be a common gear ratio for the bevel gears inside a tipical truck / Suv Axle,

i am designing a toy truck axle similar to this

http://hometown.aol.de/scaleoffroad/vorderachse_2_offen.jpg

any help apreciated

thanks

 

[patprimmer]

Somewhere between 3:1 and 5:1 depending on motor torque vs rpm, maximum sustainable rpm, tyre height, intended area of use and maximum load likely.

Regards

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[drkucho]

thanks for the reply!!!

5:1 is huge reduction, i wonder how can they put gears with such ratio inside a not big diffbox ....

please check this pic, i dont think there is room for more than 2:1 in there

http://www.bushwacker.com/images/uploaded_images/radC068D.tmp.jpg

 

[GregLocock]

well, they make the pinion small! Typically they have 13 teeth

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[drkucho]

13 teeth pinion, that was my next question

 

[drkucho]

hummm ... to make a 13 teeth pinion forces me to make a huge spur which is not good for the diffbox size

i am making a 1:10 scale axle so the size of teeth must be really small, but there is a limit , i cant make them too small because they are hard to manufacture and also because of precision loose issues

what do you think is better for high torque? , lets say i have two pinion/spur sets with same ratio , and same gear diameter, but diferent teeth size , for example:

smaller teeth with 13T pinion - 40T spur (3.07:1)
bigger teeth with 9T pinion -27T spur (3:1)

the goal is to have a nice mesh and high torque gears

i assume that bigger teeth are better for high torque but will 9T pinion give a nice mesh?

 

[GregLocock]

You don't need to scale the ratio exactly. I would run with 13 teeth if possible.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dgallup]

Less than 13 teeth will be a wear problem. Also, integer ratios (27/9) will create wear problems. I don't know if that is an issue for a scale model that probably won't run very far/long.

 

[drkucho]

thank you for your input dgallup!

i see why integer values can create wear problems, but about the wear created by less than 13 teeth , why this happens exactly ? is it because not smooth mesh?

 

[jetmaker]

why do integer problems cause problems??? is it because the same teeth combinations always mesh, so that if there are any miaslignment/matching issues that they average out?

 

[drkucho]

i gues it is because same tooth of pinion allways mesh with same teeth of spur so any problem with a specific tooth will become bigger

 

[RossABQ]

Drkucho, your initial inquiry was pretty broad; if you are interested in the ratio's of typical mid-sized American SUV's, the answer could be narrowed down much more. The Jeep shown typically has a 3.55:1 ratio, most similar American SUV's are between 3.25 - 3.73. If there is serious off-roading contemplated, or very large diameter tires used, it could go up into the 4.11 - 4.33 range.

 

[drkucho]

thanks for your answer RossABQ

now i wonder , is there a reason in making such big reduction in the axle and not in another point in the transmission train?

 

[patprimmer]

Yes.

The further back in the drive line the torque is multiplied, the lighter the components can be.

If you do it in the gearbox, the gearbox, tail shaft, universal joints, pinion and pinion carrier would need to be heavier.

Regards

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[drkucho]

i am confused ,i gues you mean "divided" instead "multiplied" on this sentence

"The further back in the drive line the torque is multiplied, the lighter the components can be."

not sure if i am right cause my english is not so good , but it doesnt make sense to me in the way it is said

 

[patprimmer]

The final drive multiplies the torque by it's ratio.

If the diff is 3:1 the crown wheel axles etc see 3 times the torque of the tail shaft.

If the gears in the gearbox were all 3 times lower and the diff were 1:1, then everything from the first motion shaft back would need to be able to withstand 3 times the torque loading.

Regards

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[drkucho]

with other words, the more close to the wheels there is more torque, and the more close to the motor there is less

im sure we are talking about the same thing but i still dont get your first sentence as if torque is bigger , gears should be heavier , not lighter

i gues i am getting it wrong because my bad english , sorry

 

[Tmoose]

Traditional 4-cyl Jeeps from the 40s and 50s came stock with 5 something axle ratios, but part of the original military spec was a working speed range of 3 to 50 mph.

Deep ratios are used in modern times by off roaders with the real tall tires that created the need for lift kits.

The typical 4wd truck/jeep transmission has a "low" range for off road work. Ratio is something like 2 or 2.5 to 1.

http://www.novak-adapt.com/knowledge/model_18.htm

 

[drkucho]

interesting, thanks

 

[patprimmer]

Power train between the engine and the torque multiplier can be lighter.

Power train after the needs to be heavier as it needs to handle the multiplied torque.

Regards

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