5.13 or 4.88??? Please Help.

[Ryanc]

I'm baffled, what does the gear ratio have to do with the pinion angle?

Im sorry I did not explain myself. With the higher RPMs of the driveshaft with the gear swap, a factor of 1.25 from 4.10 to 5.13, it is increasingly necessary to align the pinion with the driveshaft. The double carden is really considered a "near CV joint". You could take a rzeppa and run it like you want, the true cv will cancel out the vibes from the single ujoint on the pinion. Unfortunately, only the stocker uses this. AEV is still working on theirs and Jim Reel is in collaboration with RCV. If not, you may develop certain vibrations than can cause damage to your entire drivetrain.

 

[bob91yj]

Driveshaft vibration due to driveline speed has nothing to do with pinion angle. A properly built/balanced driveshaft shouldn't have any issues at the speeds most Jeep's operate at.

You want to talk driveline speeds, our race Jeep was running 6.19's in Diamond 9" housings on 33's (spec class tire size). We experienced some failures in the stock front driveshaft due to rotational speed, so I'm familiar with what you're talking about.

What people forget in their drivetrain numbers is the tires. They should be the final number in a driveline analysis, but for some reason they are not. With 315/75/16's (metric 35's) my FINAL gearing is about an 8% under drive of a stock configuration. At MAXIMUM designed road speed, my driveshaft would be spinning 8% faster than stock. It's a rare occasion that my junk is running within 8% of whatever the top speed is for my rig. Using just the gearing numbers it's a 20% under drive which could conceivably be a problem.

 

[Ryanc]

What people forget in their drivetrain numbers is the tires. They should be the final number in a driveline analysis, but for some reason they are not. With 315/75/16's (metric 35's) my FINAL gearing is about an 8% under drive of a stock configuration. At MAXIMUM designed road speed, my driveshaft would be spinning 8% faster than stock. It's a rare occasion that my junk is running within 8% of whatever the top speed is for my rig. Using just the gearing numbers it's a 20% under drive which could conceivably be a problem.

Would you mind elaborating on that? Interesting.

 

[bob91yj]

Is this strictly a JK issue? I've never heard of it being a problem with older generation Jeeps.

 

[Ryanc]

Is this strictly a JK issue? I've never heard of it being a problem with older generation Jeeps.

I could probably name at least 10 occurances that I know of. I think is is mostly rubi tcases, and maybe one atlas. It is allways the same speed, around 60-75mph.

 

[bob91yj]

Would you mind elaborating on that? Interesting.

The rotating circumference of the tire is the final "gear" in the equation. If I regeared to 5.13's and kept the stock 31" tires, I'd have approx 20% underdrive (not 25%). With the larger rolling circumference of the 35's you travel a longer distance per revolution negating some of the gear reduction.

 

[shovelbill]

yes.....effective gear ratio is quite different than "gear ratio".

your Jeep in stock form will spin 2280 rpms at 65 mph with the .84 OD.....
with 35's, it's now at 2150 with an "effective ratio of 3.87:1. with 4.56's it'll be at 2390, 4.88's it'll be at 2560 and with 5.13's it'll be at 2690.

if you weren't towing a trailer i'd suggest 4.56's for best economy, but that engine is a bit on the anemic side as i understand compared to the 4.0 I6. figure out where that motor makes it's best economy and target your cruising speed to that with a swap.......you'll never have the best of all worlds.

the increased drag from the lift and trailer, rolling resistance from the mongo tires and additional weight is what's really killing your gas mileage.

the higher number, numerically, is actually a LOWER gear not higher...fwiw.

 

[bob91yj]

I agree that incorrect pinion angle will result in driveline vibrations...my statement was gear ratio has nothing to do with pinion angle. Driveshaft vibration based solely on driveshaft speed can be corrected with balance/materials.

 

[Rubicon_Fan]

Appreciate your opinions!
I have heard about JK's with double cardin driveshafts having vibration issues at hwy speeds. Most people say it has to do with pinion and caster angles. To avoid these vibes it is recommended to stay below +4* caster. Now, from what I understood 4.88 will bring me back to stock, which is great, but I also want to mention that I'm In BC and most of our hwys are very hilly.
I like the idea of 4.88 and lower rpms, but would I regret not going 5.13 going up the Coquihalla (very steep hwy)???


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[Ryanc]

Appreciate your opinions!
I have heard about JK's with double cardin driveshafts having vibration issues at hwy speeds. Most people say it has to do with pinion and caster angles. To avoid these vibes it is recommended to stay below +4* caster. Now, from what I understood 4.88 will bring me back to stock, which is great, but I also want to mention that I'm In BC and most of our hwys are very hilly.
I like the idea of 4.88 and lower rpms, but would I regret not going 5.13 going up the Coquihalla (very steep hwy)???


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You can keep your stock driveshaft on. Just keep an eye on it.

 

[JPK]

..... You could take a rzeppa and run it like you want, the true cv will cancel out the vibes from the single ujoint on the pinion. Unfortunately, only the stocker uses this. AEV is still working on theirs and Jim Reel is in collaboration with RCV. If not, you may develop certain vibrations than can cause damage to your entire drivetrain.

You can currently send your front driveshaft to AEV for their rebuilding to a thinner profile which eleminates the boot tearing (and so eventual front DS death) on skids, exhaust cross over or auto tranny pan.

Another alternative is to replace the JKU's skids with DS freindly skids, River Raiders Off Road and others offer skids with plenty of clearence.

As far as pinion angles, using AEV's front control arm relocation brackets helps limit drive line vertical movement when the front end flexes and so pinion angle changes, either droop or stuff. Moreover, and counter to any other control arm set up, pinion angle is reduced on droop by repositioning the mounting points of the upper and lower control arms relative to each other.

For the rear, not pushing the axle back with longer lower control arms without compensating with longer upper control arms should keep a rear DS happy, at least on a JKU.

I've got 19,500 miles on my oem front and rear DS with no issues with AEV's 4.5" suspension.

As I wrote earlier, if drivabilty is an important goal, then go 5.13's. Returning to "stock" final drive ratio might be ok, but you've got the extra weight and the trailer to consider as well. The weight's there 24-7-365, but the trailer is there ?. If you're pulling the trailer a lot, then I would go 5.13's for sure, but the drivablity would be my goal over milage. Saving some $'s on fuel at the expense of constantly struggling up the highway would rain on my days of adventure/fun/freedom to get the heck out of Dodge City.

BTW, a change in milage of 1mpg and assuming 20k miles/year results in increased or decreased fuel use of 114gals/year.

JPK

 

[htek]

5.13s with 35s will be overkill and will break axle shafts if you wheel it.

That is a ridiculous statement. I have yet to break an axle shaft with 5.13's and 35's... and yes, I do actually wheel; in the rocks no less. Gear ratio has much less to do with axle failure then other things, like flogging your rig through an obstacle, or binding up the tire.

I run 5.13's on my xj with 35's and have no complaints - and absolutely would NOT go numerically lower. With that said, my jeep was built more as a wheeler, and it doesn't see a ton of mileage. Additionally, I have an auto tranny (and a totally different vehicle), so I can't directly compare driveability with your JK. People constantly sqawk about not having enough gearing - how often do you hear about people complaining they went too low?

-jm

 

[JPK]

I've been something of a muscle car guy. You can definitely go too low, when you do you loose top speed and some drivability. For a Jeep, going lower improves drivability and top end ain't a consideration!

Moreover, muscle cars aren't shaped like flying bricks - iirc wind resistance squares with speed, aren't hauling 75lbs winches, snatch blocks, tow/tree straps and shackles, maybe a roof rack, 2x 100lb bumpers (oems go ~35lbs,) camping/hunting/fishing gear, or towing a trailer. While lower gears are an improvement for off road use, hill climbing and rock crawling in particular, their main contribution to the happy life of almost all of our Jeeps which spend a heck of a lot more time on the road than offroad is improving the ability of the Jeep to maintain highway speeds and make OD and the cruise control useful again.

As far as breaking axles with 5.13's, well you can break anything if you're reckless, **** or try hard enough.

JPK

 

[Hilldweller]

So that was an easy question, eh?

I'd do 4.88 if you really want the mpgs and 5.13 if you want something easier to drive.

I had 4.56s and 33s with an autotrans on my JK and was always wishing I'd gone with 4.88s. Better yet, I still curse Jeep to this day for the 3.8L mill and lack of torque.
My new truck has a 5.7L Hemi, 4.56s, and 33s. Nice...

 

[Ryanc]

So that was an easy question, eh?

I'd do 4.88 if you really want the mpgs and 5.13 if you want something easier to drive.

I had 4.56s and 33s with an autotrans on my JK and was always wishing I'd gone with 4.88s. Better yet, I still curse Jeep to this day for the 3.8L mill and lack of torque.
My new truck has a 5.7L Hemi, 4.56s, and 33s. Nice...

I wonder what 5.38 would have been like with that trailer. Your RPMs at 70 would be still less than 2700. But that is moot, since, and I think, your front axle was a dana 30. I think 538 for D30 just came out. Talk about a tiny pinion gear:Wow1: I agree the 3.8 sucks, but that auto trans was complete ****.