Rear spring build questions

[dbkirkp]

I have had my MJ for 4.5 years now and I finally have some time/money to fix some things from the prior owner and have some questions. Thankfully I have only driven it sparingly on the weekends, but now I have the means to do something great with it. 

 

Previous owner put an XJ chrysler 8.25 under it and did spring over. They didn't move the spring perches and it rides horribly. The springs also bottom out easily when you load up the bed -It came with 33" wheels. I pulled a 4.10 Ford 8.8 from the junkyard and found a dana 30 from a '97 TJ manual 4 cyliner - Also 4.10. The truck is getting brand new axles! I am adding a lunchbox locker up front and looking at an OX locker for the ford 8.8. 

 

The current front springs have some spacers in them. I bought Rubicon Express 4.5" springs with lower control arm lowering kit to hopefully improve the ride.

 

I want to do the rear springs right. Do I keep the old twisted stock springs that were converted to spring over? Do I stay spring over with the new Ford 8.8? I don't mind buying new springs, but I wouldn't know which ones to get with the new axle so the height is right to match the front. Is there a way I can get an inch or so of rear lift with an add-a-leaf? Just put in a shackle? I want to use the bed fairly frequently as it is a long bed so I want the springs a bit stiffer. More of an overlanding build than a rock climbing build. 

 

PS, I plan on using the iron rock off road swap kit. https://www.ironrockoffroad.com/product/xj-ford-88-axle-swap-kit-w-truss.html There is an option that comes with a truss, but I will have to trim it down to fit the width of the MJ spring perches. I was planning on doing this. Any thoughts on why that wouldn't be a good idea?

 

[ghetdjc320]

First thing I’ll mention here is I’d recommend chucking that TJ axle. They are a low pinion Dana 30 and not up to big tires. The high pinion Dana 30 was found in XJs, MJs and YJs. I know you didn’t ask about it but try to find another axle. The 8.25 is a great axle too. Are you swapping it just to change gear ratios? 
Regarding the control arm drop brackets, although they can technically be used with that lift height it’s definitely not necessary and quite a bit of work. The RE 4.5” springs net about an inch less on the mj. I run their 5.5” springs and get about 4.5. Soa will give you about 5.5” but can vary depending on the leaf pack you use. 
What I’d recommend is let us know what you are wanting out of your truck. What size tires are you wanting to run and what engine and transmission are you running now. 

[dbkirkp]

37 minutes ago, ghetdjc320 said:

First thing I’ll mention here is I’d recommend chucking that TJ axle. They are a low pinion Dana 30 and not up to big tires. The high pinion Dana 30 was found in XJs, MJs and YJs. I know you didn’t ask about it but try to find another axle. The 8.25 is a great axle too. Are you swapping it just to change gear ratios? 
Regarding the control arm drop brackets, although they can technically be used with that lift height it’s definitely not necessary and quite a bit of work. The RE 4.5” springs net about an inch less on the mj. I run their 5.5” springs and get about 4.5. Soa will give you about 5.5” but can vary depending on the leaf pack you use. 
What I’d recommend is let us know what you are wanting out of your truck. What size tires are you wanting to run and what engine and transmission are you running now. 

 

Thanks for the info on the TJ Axle! I just read that they bolt in, didn't realize it was low pinion. My front pinion is leaking, thought a new one would be easier than rebuilding it. I haven't pulled it yet,  so I will keep looking for a 4.10 off an xj, or regear mine. 

 

Looking to swap the rear axle for the disc brakes from the explorer and for funsies. I work in finance,  I am doing this stuff because I find it a fun hobby, even if it doesn't come second nature to me. 

 

I want to keep the 33"x12" tires that came with it and size the lift to fit and ride nicely with that.  I have the 4.0 renix with the Peugeot 5 speed. 

 

The truck will be used for overlanding/hunting and an occasional trail in Utah as I am a few miles away. Mainly trips to the hardware store and taking the mountain bike to the trailhead.  

[AZJeff]

I don’t consider low pinion D30’s to be the kiss of death.  My 2000 XJ had one, and my MJ currently has one from a 2001 XJ in it.

 

I don’t do super heavy rock crawling stuff, so the small gain in ground clearance for the high pinion is not a major loss.

[ghetdjc320]

16 hours ago, AZJeff said:

I don’t consider low pinion D30’s to be the kiss of death.  My 2000 XJ had one, and my MJ currently has one from a 2001 XJ in it.

 

I don’t do super heavy rock crawling stuff, so the small gain in ground clearance for the high pinion is not a major loss.

It’s not the clearance that makes the lp 30 a weaker axle. It’s how the pinion engages the ring gear. By having a high pinion front and low pinion rear you are effectively engaging the strongest part of the ring gear. That plus the HP doesn’t use a crush sleeve which makes future jobs like pinion seals much easier imho. 
 

i have seen plenty of TJs with big tires on lp Dana 30’s. If that’s what came in your rig then why not wheel what you have. Heck, i run a super Dana 35 after having run 8.8’s and 44’s over the years. But if your Jeep already has a Dana 30 HP then going to a LP is a step backwards. I understand not wanting to mess with gears but a used axle of unknown condition is already going to need bearings refreshed at some point. Plus 4.10’s aren’t that ideal for 33’s (not to bad though). It seems everyone tries to avoid a regear nowadays. 
 

As for the lift, soa will net you about 5.5” of lift minimum in the rear which will vary a bit depending on the axle and leaf springs used. The SD truck springs 5 leaf springs will met you about 2.5-3” setup as spring under axle. Hells creek still makes an MJ leaf pack I believe for a 4.5” lift spring under. Depending on if you are running the stock fender flares or if your wanting to open up the wheel well a bit with different flares, you can fit 33’s in nicely with 4.5” of lift. With aftermarket flares you could probably get away with even less lift.  Keep in mind also that the rubicon express front coils don’t always show the correct lift for an XJ. Most show the rated lift for a TJ amd in fine print say it raises XJ’s about an inch less. My 5.5” RE coils raised the truck about 4.5-4.75”. A small coil spacer evened up the lift though. 

[AZJeff]

5 hours ago, ghetdjc320 said:

It’s not the clearance that makes the lp 30 a weaker axle. It’s how the pinion engages the ring gear. By having a high pinion front and low pinion rear you are effectively engaging the strongest part of the ring gear. That plus the HP doesn’t use a crush sleeve which makes future jobs like pinion seals much easier imho. 
 

 

I see your point about the gear mesh.  I never thought of that....

[Pete M]

having said all qthat, care to guess what type of front axle was in the 98 Grand Cherokee with the 5.9L engine? 

[dbkirkp]

29 minutes ago, Pete M said:

having said all qthat, care to guess what type of front axle was in the 98 Grand Cherokee with the 5.9L engine? 

Weren't those aluminum dana 44s?

[dbkirkp]

6 hours ago, ghetdjc320 said:

Plus 4.10’s aren’t that ideal for 33’s (not to bad though). It seems everyone tries to avoid a regear nowadays. 
 

 

Any tips on swapping gears?  I wouldn't mind the effort if I felt comfortable with it. Looks like it involves a press, which i don't have. Since I have a manual, I think the 4.1 will be just fine with the 33's. 

[Pete M]

no 44 up front  

[derf]

Swapping gears is tedious and meticulous work.  It's not overly difficult though.  It does require some specialty tools beyond a press.

 

If you're going to do many axles the investment is worth it.  If you're only going to do it once or twice, it's worth paying a shop to do it.

[ghetdjc320]

13 minutes ago, derf said:

Swapping gears is tedious and meticulous work.  It's not overly difficult though.  It does require some specialty tools beyond a press.

 

If you're going to do many axles the investment is worth it.  If you're only going to do it once or twice, it's worth paying a shop to do it.

Agreed. I eventually bought my own tools to do it myself after building several axles but had a shop fo it for 4 axles and it was about $250/axle. That was about 4 years ago

[ghetdjc320]

1 hour ago, Pete M said:

having said all qthat, care to guess what type of front axle was in the 98 Grand Cherokee with the 5.9L engine? 

Had one of those for about 8 years. Loved it but off-roading with big tires can load up and shock your axles much more than even spirited daily driving would. However, the OP already has an HP D30. Can’t see swapping in a LP axle just to change gears especially if it’s going to be a weekend warrior. A reverse rotation front axle has significantly superior gear engagement. 

[Tex06]

Haven't done the math in a while but it's like a 7% difference in strength between HP and LP on the gearset alone.  You're much more likely to break an axle shaft or knuckle before the LP/HP is a factor. Unless you're running the axle to it's limits, there's no real difference (which is why Chrysler and other makes have used LP front axles on and off throughout the years).

 

D30 is a great axle for trails and daily driver (which it sounds like you're gonna be doing). I'd throw the LP D30 under the truck, replace the seals & fluid, and run it. If it gives you issues, you can always rebuild your HP out of the vehicle and at your own leisure and it'll bolt right back in. 

[ghetdjc320]

It’s nice to have a “fuse” in the axle. U joints can hopefully be that fuse before more serious damage occurs. 
 

If you know that LP axle is 100% then maybe that’s best option. I would never “downgrade” something that’s already on the truck though. 


 

 

[ghetdjc320]

40 minutes ago, Tex06 said:

Haven't done the math in a while but it's like a 7% difference in strength between HP and LP on the gearset alone.  You're much more likely to break an axle shaft or knuckle before the LP/HP is a factor. Unless you're running the axle to it's limits, there's no real difference (which is why Chrysler and other makes have used LP front axles on and off throughout the years).

 

D30 is a great axle for trails and daily driver (which it sounds like you're gonna be doing). I'd throw the LP D30 under the truck, replace the seals & fluid, and run it. If it gives you issues, you can always rebuild your HP out of the vehicle and at your own leisure and it'll bolt right back in. 

Chrysler also interchanged the Dana 35 throughout the years but that doesn’t mean its the same strength wise as an 8.25. It is interesting that the TJ  came with the D35 and LP35 while the XJ was 8.25/HP30 for several years. Wonder if they ran out of HP 30’s toward the end of the XJ run???

 

The one scenario where a lp 30 would be better is if your wheeling backward up obstacles 

Just because the HP 30 was replaced with a LP 30 doesn’t mean they are the same strength wise. It’s true neither should grenade while daily driving but while wheeling the HP is more than 7% superior. The pinion clearance, lack of a crush sleeve and gearset engagement where of a far superior design than the LP. 
 

LP was made for he rear and HP was made for the front. 

[Jesse J]

5 minutes ago, ghetdjc320 said:

 

 

The one scenario where a lp 30 would be better is if your wheeling backward up obstacles 

 

sounds like a blast actually

[ghetdjc320]

Here is a nice post (not mine) for reference along with a visual:

 

The coast side of the gear is the side that's beveled relative to the base of the gear. The drive side is the other side and it's face is perpindicular to the base of the gear. 
A low pinion in the front drives on the coast or beveled side of the gear and under higher loads, the pinion teeth are forced out of the contact area into the higher thinner edges of the teeth and that's what tends to destroy the gear teeth. A high pinion on the other hand drives on the flat side of the tooth and the contact pattern tends to remain static regardless of the loads placed on it. High pinion= driven on the drive side of the ring gear teeth. Low pinion= driven on the coast side of the ring gear teeth.

[Tex06]

19 minutes ago, ghetdjc320 said:

Chrysler also interchanged the Dana 35 throughout the years but that doesn’t mean its the same strength wise as an 8.25. It is interesting that the TJ  came with the D35 and LP35 while the XJ was 8.25/HP30 for several years. Wonder if they ran out of HP 30’s toward the end of the XJ run???

 

The one scenario where a lp 30 would be better is if your wheeling backward up obstacles 

Just because the HP 30 was replaced with a LP 30 doesn’t mean they are the same strength wise. It’s true neither should grenade while daily driving but while wheeling the HP is more than 7% superior. The pinion clearance, lack of a crush sleeve and gearset engagement where of a far superior design than the LP. 
 

LP was made for he rear and HP was made for the front. 

 

The 7% is just from an engineering/gearset POV (I deal with gear design in my day to day). The clearance is a definite nice upgrade but the gearset engagement only makes a very slight difference. Currie's most robust front axle is actually a LP design, a Ford 9" design (yours for only $17k and change!) and many of their axle combos are high pinion both front and rear (a high pinion rear uses the "coast" side of the gear to drive like a LP front would). I don't know why people still use crush sleeves when better options exist, Ratech's smart sleeve is an awesome replacement I've used in all my GM axles, and a number of companies sell crush sleeve eliminator kits for the LP D30.

 

Other than the clearance, I've yet to see a mathmatically better reason for a HP over a LP. Esp considering the price difference in a lot of places. Just my $.02

[derf]

58 minutes ago, ghetdjc320 said:

It’s nice to have a “fuse” in the axle. U joints can hopefully be that fuse before more serious damage occurs. 
 

If you know that LP axle is 100% then maybe that’s best option. I would never “downgrade” something that’s already on the truck though. 


 

 

My preference is that the driveshaft ujoints are my fuse.  First off, they're less likely to take out other parts when they let go (disclaimer, I've seen exceptions to that general rule).  Second, they're a lot easier to fix/replace on the trail than interior axle bits.  And unless you have a slip yoke, you can do without one of your two driveshafts in a pinch,  Dealing with broken axle guts is more likely to put you in a "bad" situation.

[Tex06]

53 minutes ago, derf said:

My preference is that the driveshaft ujoints are my fuse.  First off, they're less likely to take out other parts when they let go (disclaimer, I've seen exceptions to that general rule).  Second, they're a lot easier to fix/replace on the trail than interior axle bits.  And unless you have a slip yoke, you can do without one of your two driveshafts in a pinch,  Dealing with broken axle guts is more likely to put you in a "bad" situation.

100% agree. Part of the reason I don't go bigger on my ujoints unless I'm repeatedly breaking them...I want that to be my failure point if I have the option.

[ghetdjc320]

39 minutes ago, Tex06 said:

100% agree. Part of the reason I don't go bigger on my ujoints unless I'm repeatedly breaking them...I want that to be my failure point if I have the option.

Definitely agree with the u joints being the fuse in the setup. Don’t want a shaft or especially a R&P to be the week point. That being said, the OP already has an HP30 with a leaking pinion seal. Plus he has an 8.25 which has a nicer width and slightly better clearance than the 8.8. The axle he picked up is a used tj d30 which is at least 15 years old and of unknown condition. Being that it’s only a 4.10 gear set, I’d recommend just upgrading gears in his existing axles. Just my $.02 

[dbkirkp]

Hey, all the info on the HP vs. LP is much appreciated, but I still have questions about leaf springs.

 

Currently it is SOA on a Chrysler 8.25 - it still sags a bit compared to the front. Should I do an add a leaf or a shackle to straighten it up. Should I put a load of stuff in it for camping I would like to not have to cut the fenders for it to not rub on 33s. 

 

Looking at Bilstein 5125 shocks. How do I make sure I have the travel and length measurements correct?

[Pete M]

longer bumpstops are key to preventing rubbing.    even with stiffer springs it's the bumpstops that prevent damage once the physics of motion come into play. 

 

I would do an AAL if the springs are saggy.  or get new packs.

[ghetdjc320]

6 hours ago, dbkirkp said:

Hey, all the info on the HP vs. LP is much appreciated, but I still have questions about leaf springs.

 

Currently it is SOA on a Chrysler 8.25 - it still sags a bit compared to the front. Should I do an add a leaf or a shackle to straighten it up. Should I put a load of stuff in it for camping I would like to not have to cut the fenders for it to not rub on 33s. 

 

Looking at Bilstein 5125 shocks. How do I make sure I have the travel and length measurements correct?

interesting. Are those stock MJ leaf packs your running? Any pics? The stock 2wd mj leaf packs I run give me about 5.5” of lift and enough room to clear 35’s easily with flex. You can add a leaf as Pete mentions but I’d be curious to see if the main leaf is bent. The only new “stock” spring packs I know of are from SD springs but will jack you up another 2” over the 5.5 soa lift. They are good for an sua lift of between 1.5-3” depending on which packs you choose. I had them on my truck for a while soa and it sat with a good 4-5” rake. if your main leaf is bent or deformed I’d try to find a good set of used springs. 
 

For shocks, measure the distance eye to eye (or stem to eye) while level. Depending on your driving style and how much load down your bed, you may want to keep about 60-70% of the shock range for up-travel. I set mine just below the middle with about 55-60% up travel. So basically, if there is 24” from eye to eye, an the shock has 10” of travel, they you would want something that is about 18” compressed and 28” extended. That would give you a 60% up travel and 40% down travel to account for some weight. It does depend a lot on where you drive though. On Jeepspeed applications they bias them with a lot mor up travel to account for all the harsh landings and high speed “woops”. Rock crawling you want your wheels to drop on the unloaded side of that axle so you can use your unsprung weight to give traction to the low tire.  
Valving is another story and will depend again on the weight of your rig with accessories like bumpers winches etc. plus an average cargo weight. Accutune shocks can get you a custom set if you give them the info. Not terribly prices either. Last I checked they could do simple mono tube fox or bolstering starting about $150 a corner and they come tuned for your rig.