Axle Conundrum..

[Dzimm]

So I'm trying to figure out what I want to do for a rear end for the autocross truck.  I am looking to switch to a stronger rear end, with lower gears for quicker acceleration, limited slip, preferably disk brakes, but still be able to get decent gas mileage on the highway.  I will be switching to slightly smaller tires but the size is still up in the air.  

 

The truck is a 92 4.0l AX15 2WD with a D35 3.07 gears and runs about 2500rpm at 70mph on 29" tires. 

 

I was looking at Ford 8.8 axles because they are strong and limited slip from the factory and seem to all be about $200-$250.  However I'm only finding limited slip with 3.73 gears.  By my math, that will put me at 3000-3500 rpm at 70mph depending on tire size (smaller tire = higher rpm).  I would also have to have a new driveshaft made so all in going with a Ford 8.8 I'm gonna be about $500ish.  

 

If I did a C8.25, I could do 3.07 or 3.55 gears and add a posi lock to it.  Going this route, I can keep the driveshaft.  The axle would be $200 and the limited slip $350 putting this at $550.  This however would leave me with drum brakes.  I could get a liberty C8.25 for disks but they are about $400 on the cheap side.  

 

Now I do have a 95 XJ with a C8.25 with 3.55 gears that I am going to pick parts from for the truck.  With 3.55 gears I'd be running about 3000 rpm with 29" tires so I'd end up probably around 3500rpm with smaller tires.  If I used this axle, I would only have to buy the posi lock so would cost about $350 all in.  

 

I want to get your guys opinions on what to do.  What gears/axle should I go with?  

 

 

 

 

 

[Pete M]

the pullaparts are full of libertys just waiting to give you their 8.25.    even car-part.com should be able to find you one for a lot less than 400.  

 

don't shy away from deeper gearing just for the sake of the freeway.  my Dakota came from the factory with 3.92s and 27 or 28" tires and a big honkin' thirsty v8.    wouldn't trade that ratio for anything. 

[Dzimm]

55 minutes ago, Pete M said:

the pullaparts are full of libertys just waiting to give you their 8.25.    even car-part.com should be able to find you one for a lot less than 400.  

 

don't shy away from deeper gearing just for the sake of the freeway.  my Dakota came from the factory with 3.92s and 27 or 28" tires and a big honkin' thirsty v8.    wouldn't trade that ratio for anything. 

Carpart has some listed but the only ones remotely close to me are $400.  Cheapest I saw was $300 but too far away to offset the cost difference.  

 

If I use the C8.25 I've got, I can do a brake swap and add the limited slip cheaper than anything else.  Just a lot more work.  

[bad_idea]

The 8.8 is the axle I would choose.  It's a strong axle with good aftermarket support.  The Explorers came stock with 3.55s, 3.73s, or 4.10s.  All with LSDs.  Not sure you would need to replace the driveshaft.  I don't believe the pinion length is that much more than the 8.25.  

[Dzimm]

1 hour ago, bad_idea said:

The 8.8 is the axle I would choose.  It's a strong axle with good aftermarket support.  The Explorers came stock with 3.55s, 3.73s, or 4.10s.  All with LSDs.  Not sure you would need to replace the driveshaft.  I don't believe the pinion length is that much more than the 8.25.  

The pinion length isn't the issue, the 8.8 has a mounting flange not a yoke like the driveshaft has so the end needs changed.  I think 3.55 is what I want to go with but I can't seem to find any 8.8s locally that are lsd and 3.55,  they all 3.73.  I'm keeping my eyes peeled and asking around tho. 

[Eagle]

3 hours ago, Dzimm said:

The truck is a 92 4.0l AX15 2WD with a D35 3.07 gears and runs about 2500rpm at 70mph on 29" tires. 

 

 

To begin, something is very wrong with your numbers. A 235/75-15 is roughly a 29-inch tire (28.9"). With 3.07 gears in overdrive, 235s should run roughly 1940 RPM at 70 MPH, not 2500.

 

See if this link will open for you: https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/edit#gid=0

 

Before I would venture to offer suggestions, I would like to know what size tire you actually plan to use (not just "smaller"), and how much of what kinds of driving you plan to do. And are you going to be autocrossing the tires you drive on the street every day? When I was autocrossing (back when Henry first invented the Model T), I had one set of tires for racing on Sunday, and another set for driving to work during the week. Mine happened to be the same size, but they don't have to be.

[Jeep Driver]

2 minutes ago, Eagle said:

 

To begin, something is very wrong with your numbers. A 235/75-15 is roughly a 29-inch tire (28.9"). With 3.07 gears in overdrive, 235s should run roughly 1940 RPM at 70 MPH, not 2500.

 

Before I would venture to offer suggestions, I would like to know what size tire you actually plan to use (not just "smaller"), and how much of what kinds of driving you plan to do. And are you going to be autocrossing the tires you drive on the street every day? When I was autocrossing (back when Henry first invented the Model T), I had one set of tires for racing on Sunday, and another set for driving to work during the week. Mine happened to be the same size, but they don't have to be.

 

 

Yes, of course. 

 

In his other thread.......round n round.....about height and width and wheel backspacing and wheel size and spacers and tire size.............

 

All that said.........yadda yadda......

 

 

Waiting for him to figure this out-

 

Is he going to trailer the truck? What about tools? Spare parts? Fluids? Fuel? Etc?

 

Two sets of tires is a nobrainer, par for the course, racing tires at the track and highway tires on the way home if he's going to drive it to the track. 

 

If you're going to race the truck you need to set it up for racing. 

 

 

Curious, what did you have planned if you roll a tire off the rim? What do you have planned for spare parts?

[Dzimm]

6 minutes ago, Eagle said:

 

To begin, something is very wrong with your numbers. A 235/75-15 is roughly a 29-inch tire (28.9"). With 3.07 gears in overdrive, 235s should run roughly 1940 RPM at 70 MPH, not 2500.

 

Before I would venture to offer suggestions, I would like to know what size tire you actually plan to use (not just "smaller"), and how much of what kinds of driving you plan to do. And are you going to be autocrossing the tires you drive on the street every day? When I was autocrossing (back when Henry first invented the Model T), I had one set of tires for racing on Sunday, and another set for driving to work during the week. Mine happened to be the same size, but they don't have to be.

My true rpm with 29" tires right now is more like 2250 at 70mph according to the tach.  I used some formulas I found online to get my numbers and they seem to be slight over estimates, which is good because lower rpm is better on highway.  

 

As for smaller tires, I can go as small as I want to but I figure I have more tire options than I do axles and gears options so if I setup the axles and gears to what I want, I can choose tires later based on what I have.  If I had to guess, I would say it will prolly be something like a 24"-26" tire.  The truck will see mostly highway driving hence why I want to keep mpg/rpms in mind.  It will see the track maybe 4-5 times a year so I'll be using the same tires for DD and track.  

[Dzimm]

2 minutes ago, Jeep Driver said:

 

 

Yes, of course. 

 

In his other thread.......round n round.....about height and width and wheel backspacing and wheel size and spacers and tire size.............

 

All that said.........yadda yadda......

 

 

Waiting for him to figure this out-

 

Is he going to trailer the truck? What about tools? Spare parts? Fluids? Fuel? Etc?

 

Two sets of tires is a nobrainer, par for the course, racing tires at the track and highway tires on the way home if he's going to drive it to the track. 

 

If you're going to race the truck you need to set it up for racing. 

 

 

Curious, what did you have planned if you roll a tire off the rim? What do you have planned for spare parts?

I am nowhere close to getting a spare parts bin setup for it.  It won't see the track until next year at the very earliest.  

 

As for the post about tire size and backspacing, I posted towards the end that it's basically impossible to get the truck within limits of stock street classes unless I ran on the rims with no tires so I gave up on that.  Now I'm setting up the truck how I want and will run in whatever class I end up fitting in because I'm not looking to be competitive as much as for the fun of it... And the fun of smoking the old rice burners from the stoplight. 

 

At this point, all I want is a DD that is setup to autocross as well.  That's what autocross is about, having fun in whatever vehicle you have no matter if it's bone stock off a dealer lot, or purpose built for racing.  I'm looking to be somewhere in between those. 

 

IF I end up deciding I want to become competitive and set it up strickly for racing, I would go all out with a racing setup but that's not what I want to do at this point. 

[Eagle]

1 minute ago, Dzimm said:

My true rpm with 29" tires right now is more like 2250 at 70mph according to the tach.  I used some formulas I found online to get my numbers and they seem to be slight over estimates, which is good because lower rpm is better on highway.  

 

As for smaller tires, I can go as small as I want to but I figure I have more tire options than I do axles and gears options so if I setup the axles and gears to what I want, I can choose tires later based on what I have.  If I had to guess, I would say it will prolly be something like a 24"-26" tire.  The truck will see mostly highway driving hence why I want to keep mpg/rpms in mind.  It will see the track maybe 4-5 times a year so I'll be using the same tires for DD and track.  

 

I don't believe your numbers. I think your speedometer is off. What size are the "29-inch" tires you're running?

 

I made up my spreadsheet because the on-line RPM calculators are so far off. They use round numbers like 29" for tire size, but almost no tires are actually an even number. And a 29" tire doesn't sit or roll with the centerline of the axle 14.5" off the ground. The spreadsheet I made up used actual revolutions per mile from tire manufacturers wherever I could find such data. And your numbers are way out of whack. 235/75-15 tires, which are almost exactly 29" in diameter, only turn 1940 RPM in 5th gear at 70 MPH with 3.07 gears. The same 235s will only turn 2356 RPM at 70 with 3.73 gears.

 

My '88 MJ is rolling 3.73 gears, and I wish all my XJs and MJs had that ratio. It makes 5th gear useful for something beyond the interstate.

 

My spreadsheet doesn't go any smaller than 215/75-15, because it was made up for street and off-road and I didn't think anyone would ever put smaller tires than that on an XJ (or MJ). 215/75-15 is roughly a 28" tire. With 215s and 3.73 gears, the engine would be turning 2460 RPM at 70 MPH. That's right where it wants to be, IMHO. You could easily run 27" tires with 3.73 gears and not be straining the engine at all. Remember, the Jeep 4.0L engine is basically an AMC engine that was developed back in 1964, decades before overdrive transmissions were anything but an expensive and very rare option. My family all had AMCs. The usual setup generated 2500 RPM for 60 MPH, and 3000 RPM was 72 MPH. And they got decent (or better) gas mileage -- without overdrive.

[Eagle]

By the way -- you won't be using the same tires for autocrossing and street driving. Autocrossing eats tires.  You'll be far better off using normal MJ size tires for daily driving, and keeping another set of wheels with your autocross tires on them just for event days. In my case, my brother worked for Firestone back when we were racing, and he got the store manager to give us tires for free. The poor manager didn't have any idea what he was getting himself into.

[Dzimm]

13 minutes ago, Eagle said:

 

I don't believe your numbers. I think your speedometer is off. What size are the "29-inch" tires you're running?

 

I made up my spreadsheet because the on-line RPM calculators are so far off. They use round numbers like 29" for tire size, but almost no tires are actually an even number. And a 29" tire doesn't sit or roll with the centerline of the axle 14.5" off the ground. The spreadsheet I made up used actual revolutions per mile from tire manufacturers wherever I could find such data. And your numbers are way out of whack. 235/75-15 tires, which are almost exactly 29" in diameter, only turn 1940 RPM in 5th gear at 70 MPH with 3.07 gears. The same 235s will only turn 2356 RPM at 70 with 3.73 gears.

 

My '88 MJ is rolling 3.73 gears, and I wish all my XJs and MJs had that ratio. It makes 5th gear useful for something beyond the interstate.

 

My spreadsheet doesn't go any smaller than 215/75-15, because it was made up for street and off-road and I didn't think anyone would ever put smaller tires than that on an XJ (or MJ). 215/75-15 is roughly a 28" tire. With 215s and 3.73 gears, the engine would be turning 2460 RPM at 70 MPH. That's right where it wants to be, IMHO. You could easily run 27" tires with 3.73 gears and not be straining the engine at all. Remember, the Jeep 4.0L engine is basically an AMC engine that was developed back in 1964, decades before overdrive transmissions were anything but an expensive and very rare option. My family all had AMCs. The usual setup generated 2500 RPM for 60 MPH, and 3000 RPM was 72 MPH. And they got decent (or better) gas mileage -- without overdrive.

Foot in mouth.. I see the problem, my blue truck has 29" tires, the red truck has 27.5" tires.    the exact size is p225/70 whatever the p means.  

 

I still don't understand how your math works tho.  The formula I found is (speed x gear ratio x 336)/tire size = rpm.  So by that math with the 27.5" tire, 3.07 gears, at 70mph it comes out to a little over 2600 rpm, which is close to what my gauge says on the highway.  

 

So by your numbers, say I went with a 24" tire and 3.73 gears, what would the rpm be at 70mph? 

[Eagle]

My math is based off tire manufacturers' specs. It's tough to find today, but they used to include in their specs things like "static radius" (which is the height the axle actually sits above the pavement when the tire is loaded to a normal load), and revolutions per mile. That second one is what I used.

 

Take, for example, the 235/75-15. Some manufacturer's spec sheet told me that tire in that size turns 698.5 revolutions per mile. 60 MPH is one mile per minute, so at 60 MPH the tire is turning 698.5 revolutions per minute. Now multiple that by the drive ratio ... 3.73 times 698.5 = 2605.4 RPM. But that's for direct drive, 1:1 -- which is fourth gear. Now multiply that by the overdrive ratio (which in the version of the spreadsheet on Google Docs is 0.75:1, but some years of the XJ/MJ had 0.72:1).

 

2605.4 x 0.75 = 1954.05 RPM at 60 MPH. To get any other speed, do a straight proportion. For 70, multiply by 7 and divide by 6 ==> 2279.73

 

So I completely ignore the mathematics of figuring out the tire diameter from the size, because it's meaningless. How meaningless? Back when I was doing the spreadsheet, one of the tire companies (Cooper, IIRC) offered the same tire and size in both a P version (Passenger) and an LT version (Light Truck). Same tire model, same size -- but the LT version had a higher static radius and turned fewer revolutions per mile. Why the difference? More plies ==> stiffer sidewall ==> less tire squat under load. Theoretical tire size calculations can't take that into account.

[Eagle]

49 minutes ago, Dzimm said:

So by your numbers, say I went with a 24" tire and 3.73 gears, what would the rpm be at 70mph? 

 

I have no idea whatsoever. Find me the revolutions per mile for the tire you have in mind and I'll have an answer for you.

[bad_idea]

Fitting the 8.8 to your driveshaft is easy.  Get the adapter flange from a Ranger.  It will fit the u-joint on your driveshaft.  

 

I have a 8.8 in my Cherokee.  I like the axle and know for a fact it is easy to swap in.  Assuming you can weld. 

[Eagle]

1 hour ago, Dzimm said:

 

I still don't understand how your math works tho.  The formula I found is (speed x gear ratio x 336)/tire size = rpm.  So by that math with the 27.5" tire, 3.07 gears, at 70mph it comes out to a little over 2600 rpm, which is close to what my gauge says on the highway.  

 

 

2600 RPM would be about right (2636) for 1:1 transmission ratio -- which is 4th gear, not fifth. Then you factor in the overdrive ratio. 2636 x 0.75 = 1977 RPM at 70 MPH. Are you not using fifth gear?

[Dzimm]

1 minute ago, Eagle said:

 

2600 RPM would be about right (2636) for 1:1 transmission ratio -- which is 4th gear, not fifth. Then you factor in the overdrive ratio. 2636 x 0.75 = 1977 RPM at 70 MPH.

Ohhh that makes sense.  

[Dzimm]

13 minutes ago, bad_idea said:

Fitting the 8.8 to your driveshaft is easy.  Get the adapter flange from a Ranger.  It will fit the u-joint on your driveshaft.  

 

I have a 8.8 in my Cherokee.  I like the axle and know for a fact it is easy to swap in.  Assuming you can weld. 

I can't do an adapter since I am lowering the truck, I will actually lose travel in the driveshaft so the extra length of the adapter would be too much.  My driveshaft is also the double walled one that can't be modified so I have to have one built if I go that route. 

 

I wonder tho if I could take an XJ shaft and the adapter if it would be long enough.. How long is your adapter from center of u-joint to matting surface of the flange? 

[Eagle]

Here ya go -- a 195/60-15 has a theoretical diameter of 24.2 inches. I have a spec for that tire -- 833.1 revolutions per mile. You can run the math on it and see what kind of RPMs you get.

[bad_idea]

It's a common flange.  I have no idea the length of it, sure google can dig it up.  Or a trip to the junk yard.

[Dzimm]

5 minutes ago, Eagle said:

Here ya go -- a 195/60-15 has a theoretical diameter of 24.2 inches. I have a spec for that tire -- 833.1 revolutions per mile. You can run the math on it and see what kind of RPMs you get.

For that tire it comes out to be 2331rpm for 3.73 and 2218.13rpm for 3.55 at 60mph.  I do like your math a lot better.  The formula I was using does give a close answer as long as I do the conversion to 5th gear.  I didn't even think about the trans ratio.   Thanks for the help!  I think I wanna go with the highest gears I can so I'm gonna probably snag a Ford 8.8 with 3.73.

[DirtyComanche]

Highest ratio numerically?

 

I was going to recommend an Izusu D44 or 12 Bolt.  Or an 8.8 with 4.10 gears.

[gogmorgo]

In terms of axle choice, you may want to consider width, and what that'll do to your handling. It also affects the width/height ratios that you'll be looking at working around. The 8.8 is narrower than the Jeep axles. Most people get around that by running spacers, but you'll have to run that past the SCCA rule book. A beefier axle also adds unsprung weight. 

 

I've done a small amount of suspension setup for autocross but it was long enough ago and for an entirely different setup so specific details is useless. But there are some mantras that do apply. 

 

The first is that without data you don't know what you're doing. Doing $#!& just for the sake of doing $#!& results in $#!&. You need to establish a performance baseline to compare everything you do. You need to get out and turn laps in your vehicle until you've reached a consistently good time, then you can start playing with stuff to improve it, otherwise what you're doing is a waste of time.  I realize this can be difficult when the series won't let you play, but some clubs lay out the course with chalk, and you may still be able to turn laps on a circuit after the club has moved on. A buddy with a stopwatch is enough. Once you know where you're at, you can establish whether or not your changes are for the good. Lowering tire size effectively increases final drive ratio... Small tires are lower gears. Lower gears are good, but they mean you're more likely to spin tires at launch, and you're going to spend more time shifting more often. You don't really know what ratio is going to work until you've settled on a tire and run some laps. You're building a custom turbo engine package, too, IIRC, and you won't know what that'll play like until you've turned some laps with it either. Again low gears are great and all, but you don't want one that'll have you constantly shifting back and forth.

 

The second is that the recipe for success is "simplify and add lightness". You may not need to add a beefy axle, which brings more unsprung and rotational mass, which will slow you down. The d35 gets a lot of smack talked about it, but it's still going strong at 330,000 miles in my MJ, and it was the only axle available in the early ZJ, even behind the 5.2, and could be rated for up to 6500lbs towing, for over 10,000lbs vehicle weight. I'm not saying it's the strongest axle out there, but you don't know yet that it won't hold up under your application, while we can predict something heavier will inevitably slow you down some. 

 

The third mantra is that the rule book trumps everything. You need to know the rules of the class you're racing. You already know you're going to need to get either wider, lower, or both, just to be able to run. If the axle you choose is too narrow, can you make up for it with wheel choice? Can you add spacers to make up for any difference? Does a beefier axle have a taller differential that will limit the amount you can lower your vehicle?

 

If it were me, I would get the ride height down and the track width out as best as possible just to be able to see what's going on when you run, before doing anything else. There are a couple lowered MJ builds on here that could give you suggestions to accomplish that. I'd say just lower it and find a good wheel/tire combo that makes the track wider, then work on making it faster one step at a time, not by doing a bunch of things all at once. It would be pretty aggravating to me knowing I'd just put a ton of time and effort into a project only to find out a bunch of the things made the project more complex without really accomplishing anything. Of course, you wouldn't know you'd accomplished nothing without prior testing data...

[Dzimm]

18 minutes ago, gogmorgo said:

In terms of axle choice, you may want to consider width, and what that'll do to your handling. It also affects the width/height ratios that you'll be looking at working around. The 8.8 is narrower than the Jeep axles. Most people get around that by running spacers, but you'll have to run that past the SCCA rule book. A beefier axle also adds unsprung weight. 

 

I've done a small amount of suspension setup for autocross but it was long enough ago and for an entirely different setup so specific details is useless. But there are some mantras that do apply. 

 

The first is that without data you don't know what you're doing. Doing $#!& just for the sake of doing $#!& results in $#!&. You need to establish a performance baseline to compare everything you do. You need to get out and turn laps in your vehicle until you've reached a consistently good time, then you can start playing with stuff to improve it, otherwise what you're doing is a waste of time.  I realize this can be difficult when the series won't let you play, but some clubs lay out the course with chalk, and you may still be able to turn laps on a circuit after the club has moved on. A buddy with a stopwatch is enough. Once you know where you're at, you can establish whether or not your changes are for the good. Lowering tire size effectively increases final drive ratio... Small tires are lower gears. Lower gears are good, but they mean you're more likely to spin tires at launch, and you're going to spend more time shifting more often. You don't really know what ratio is going to work until you've settled on a tire and run some laps. You're building a custom turbo engine package, too, IIRC, and you won't know what that'll play like until you've turned some laps with it either. Again low gears are great and all, but you don't want one that'll have you constantly shifting back and forth.

 

The second is that the recipe for success is "simplify and add lightness". You may not need to add a beefy axle, which brings more unsprung and rotational mass, which will slow you down. The d35 gets a lot of smack talked about it, but it's still going strong at 330,000 miles in my MJ, and it was the only axle available in the early ZJ, even behind the 5.2, and could be rated for up to 6500lbs towing, for over 10,000lbs vehicle weight. I'm not saying it's the strongest axle out there, but you don't know yet that it won't hold up under your application, while we can predict something heavier will inevitably slow you down some. 

 

The third mantra is that the rule book trumps everything. You need to know the rules of the class you're racing. You already know you're going to need to get either wider, lower, or both, just to be able to run. If the axle you choose is too narrow, can you make up for it with wheel choice? Can you add spacers to make up for any difference? Does a beefier axle have a taller differential that will limit the amount you can lower your vehicle?

 

If it were me, I would get the ride height down and the track width out as best as possible just to be able to see what's going on when you run, before doing anything else. There are a couple lowered MJ builds on here that could give you suggestions to accomplish that. I'd say just lower it and find a good wheel/tire combo that makes the track wider, then work on making it faster one step at a time, not by doing a bunch of things all at once. It would be pretty aggravating to me knowing I'd just put a ton of time and effort into a project only to find out a bunch of the things made the project more complex without really accomplishing anything. Of course, you wouldn't know you'd accomplished nothing without prior testing data...

You bring up a lot of good points.  I'm really more in the research stage for what I want to do and to price things out based on what I will accomplish with them, that way i have an idea moving forward what I want to do and need to plan for, and some of those will be subject to change as the build goes on.  I can't run it as is due to the height/width thing and I cannot use the "tracks" after events as they are privately owned parking lots.  

 

The big thing I'm worried about is breaking the D35 because I need to be able to drive the truck as I have no trailer and when I bought the truck, the diff was very low on fluid so I have no idea what kind of damage was done as I have yet to have time to pull the cover.  

 

Do you happen to know the names of the lowered builds?  I searched around quite a bit but didn't come across any. 

 

 

 

 

[HOrnbrod]

Look for build threads from "Darren" or "GoBlazers" or shoot him a PM. He did a good job lowering an MJ for autocross as I recall, the best I've seen and had lots of pics.