How does gear ratio affect strength?

[PNW_LJ]

I'd like to raise the question of how diff gear ratio is correlated to their strength.

I know that gear breakage is basically non-existent with TJs, even on 5.38s, provided: a competent install, max 35" tires, and avoiding wheel spin.

On the JK forums, however, going over 4.88 is often considered risky, and many members post pics of broken gearsets. I thought JK gears are significantly larger than TJ gearsets? Or is it that more JK guys go above 35s on stock axles?


So what affects gear reliability most? Is it the tooth count or size of the R&P (would a 5.13s in a Dana 30 = 5.38 in a Dana 44), or something else entirely?

 

[PNW_LJ]

JK R&P size comparison:

I'm also trying to find info online on tooth count differences, if anyone has that info please do share!

 

[KennethS]

I think the width of the teeth has a lot to do with the strength. More teeth on the same diameter gear (higher ratio) means each tooth is narrower; fewer teeth on the gear (lower ratio) means the teeth are wider (and stronger due to their larger width).

 

[PNW_LJ]

I think the width of the teeth has a lot to do with the strength. More teeth on the same diameter gear (higher ratio) means each tooth is narrower; fewer teeth on the gear (lower ratio) means the teeth are wider (and stronger due to their larger width).

Hm, so wider teeth -> more tooth to tooth contact going on -> less chance of gears deflecting/shredding?

 

[PNW_LJ]

Another often repeated claim is that a heavy-duty diff cover can "stiffen" the housing and help prevent gear deflection. Anyone have direct experience with this?

 

[JustDandee]

Another often repeated claim is that a heavy-duty diff cover can "stiffen" the housing and help prevent gear deflection. Anyone have direct experience with this?

not a jeep dif but are you referring to something like this- note the left and right "bolts" that aiding to reducing carrier deflection at the bearing caps

 

[PNW_LJ]

not a jeep dif but are you referring to something like this- note the left and right "bolts" that aiding to reducing carrier deflection at the bearing caps

View attachment 420130

Never seen that before. The claims I heard were around an aftermarket cover like ARB, a thicker cast iron diff cover.

 

[JustDandee]

conceptually i understand the design that I have on my chevelle could help with the carrier support. Just a stiffer cover I think would work with the idea that all sides of the opening are boxed & robust material vs one side being thin tin. How much it makes a difference, i will defer to someone with breakage experience.

 

[jjvw]

The deeper the gear, the less stress on the gear. The mechanical advantage is increasingly in favor of the gear set, the lower over goes. You know this from riding a multi speed bicycle.

 

[machoheadgames]

I don’t think there is any real proof of the ratio contributing to the strength. People have broken teeth of all ratios.

It mostly comes down to tire size, abuse with the throttle in 4LO, and poor gear setup. I think you see more aftermarket gears bust simply because people are getting into harder offroading with them and you also have humans setting them up. Some humans are better at setup than others.

Something that helps the lower ratios actually suffer less stress is the fact that they are low ratios. Teeth may be smaller, but there is less stress going into each tooth due to the ratio. Think of it like a bike in gear 1 or 10. In gear 1, the gears are so low that they turn with such low resistance your feet can’t hardly put any effort into them. In gear 10 you could stand on the pedals and yank on the handlebars and still take forever to get moving. Obviously on bikes you have different size sprockets to keep the same tooth size for the chain, but imagine if all the sprockets were the same, just reworked with different tooth counts and sizes. The 10th gear would have larger teeth, but those teeth would see much higher load from you putting all your effort into them.

At the end of the day, the gear sellers still sell them and warranty them, so I wouldn’t worry too much.

For what it’s worth, my truck (Tacoma) has 5.29 gears on 29.5” tires. I romp on it hard and they have yet to give up the ghost. 7 and 37 teeth so while the teeth are larger than a 53 tooth Dana 44 5.89, more force has to be put on those teeth for one revolution because one revolution of the tires is broken into less “steps” (less teeth around the circle).

No matter how you slice it, there are pros and cons. I wouldn’t worry too much. Use a good heavy 85W-140 conventional to provide a bit of a cushion to the gear teeth.

I wouldn’t use JK owners as examples. Heavier rigs, slightly larger gearsets, many absolute dumbass owners, and many of them run 37-40” tires. I don’t think it’s apples to apples.

 

[JustDandee]

Well they do make a dana 35 with the -girdle support-which is the key search word to see these.

 

[PNW_LJ]

The deeper the gear, the less stress on the gear. The mechanical advantage is increasingly in favor of the gear set, the lower over goes. You know this from riding a multi speed bicycle.

I don’t think there is any real proof of the ratio contributing to the strength. People have broken teeth of all ratios.

It mostly comes down to tire size, abuse with the throttle in 4LO, and poor gear setup. I think you see more aftermarket gears bust simply because people are getting into harder offroading with them and you also have humans setting them up. Some humans are better at setup than others.

Something that helps the lower ratios actually suffer less stress is the fact that they are low ratios. Teeth may be smaller, but there is less stress going into each tooth due to the ratio. Think of it like a bike in gear 1 or 10. In gear 1, the gears are so low that they turn with such low resistance your feet can’t hardly put any effort into them. In gear 10 you could stand on the pedals and yank on the handlebars and still take forever to get moving. Obviously on bikes you have different size sprockets to keep the same tooth size for the chain, but imagine if all the sprockets were the same, just reworked with different tooth counts and sizes. The 10th gear would have larger teeth, but those teeth would see much higher load from you putting all your effort into them.

At the end of the day, the gear sellers still sell them and warranty them, so I wouldn’t worry too much.

For what it’s worth, my truck (Tacoma) has 5.29 gears on 29.5” tires. I romp on it hard and they have yet to give up the ghost. 7 and 37 teeth so while the teeth are larger than a 53 tooth Dana 44 5.89, more force has to be put on those teeth for one revolution because one revolution of the tires is broken into less “steps” (less teeth around the circle).

No matter how you slice it, there are pros and cons. I wouldn’t worry too much. Use a good heavy 85W-140 conventional to provide a bit of a cushion to the gear teeth.

I wouldn’t use JK owners as examples. Heavier rigs, slightly larger gearsets, many absolute dumbass owners, and many of them run 37-40” tires. I don’t think it’s apples to apples.

Okay, that makes sense! So the amount of tooth contact doesn't play a role here either? I wonder how it differs between say 5.13, 5.38 and 5.89.

 

[LONGJP2]

Or is it that more JK guys go above 35s on stock axles?

If there is an increase in failures, it's probably the abundance of oversized tires (and lack of skill).