Looking for advice: Gears, axles, tires, etc.

[Cooter_76]

Now that my Explorer 5.0/4R70W is up and running, I need to address the axles & tires. Just starting to dip into the planning stages now, and looking for some guidance.

Bronco currently has 2.5” suspension lift, 1” body lift, 32 x 11.50 BFG KM2’s on 15 x 8 aluminum wheels. Tires are at least 11 years old. Rear fenders are cut, with old-school flares. Front fenders are uncut. Axles are original Dana 44 up front and small bearing 9” in the rear with 3.50 gears & limited slips in both (and I think they’ve reached their “limit.”)

I’m 90% sure I’ll wind up with 33 x 10.50 tires. I can’t see myself going so far as to really need 35” tires. 35’s would also require more fender trimming and might be harder on other components (steering, etc?) I do plan to eventually cut the front fenders, regardless. I like the look of 33 x12.50, but would probably prefer the street manners of the 10.50. I better stop there, or I’ll wind-up talking myself into a Prius.

According to the door sticker on the Explorer, it had 3.73 gears and 235/75R15s. So the engine would be turning 2126 RPM at 70 MPH. Assuming I go with 33" tires, here are the theoretical RPMs at 70 MPH with a given diff gear:

4.10 gears - 2046 RPM
4.30 gears - 2145 RPM
4.56 gears - 2275 RPM

Based on that, it seems like 4.56 would put the engine operating pretty close to where it was designed to run, and account for the heavier tires. But would 4.88's be better?

So I need new gears, and like the idea of a gear-driven limited slip (TrueTrac.) Probably a good idea to step up to 31 spline axles. Any issues using the adapter bearings with a small-bearing housing? Would new 28 spline axles suffice? I seem to remember a thread recently where people were talking about how weak the small bearing housings are.

Window-shopping on WH’s site, it looks like I can get a built third member with Yukon gears and a TrueTrac for $1801, plus $325 for new axle shafts. Didn’t see the bearings on their site.
Then I see that Speedway Motors has a complete fabricated housing with gear type posi, fully built third member and new axle shafts for $1699. Am I reading this right? Would it basically just need spring perches and new brakes?

I haven’t started thinking about the front axle yet. Wanted to make sure I’m headed down the right path.

 

[SteveL]

If it's mostly street and some off road 4:56 should be good with 33s and the od tranny. Sure 4:88 will get you more power but might up the rpm more than you like on the highway. If you're not gonna step up to a selectable locker like arb a true trak would be my next choice. As far as axles if you go 31 spline big bearing at least it's done. Especially if you're paying labor on the 3rd member set up. If you start looking at street rod shops for a housing make sure they know it's for a 4x4 with 33s. When I was looking the couple street rod places I checked recommended one of their beefier housings to accommodate the bigger tires and abuse. Then the prices were higher than just calling curriewhich I can pick up to save shipping.

 

[Nothing Special]

On gearing it really comes to personal preference. I have 4.10s with my stockish 302, NV3550 and 33" tires and I love that combo because it's smooth and quiet (it was too "buzzy" for my liking when it had the same setup except still running the stock non-OD 3 speed). But there are also people who will think the 4.56 gears will still be a dog and will tell you "don't fear the gear." None of us are wrong, just different strokes for different folks.

 

[Broncobowsher]

Done this before.

Years ago I did the 4R70W with 33s. But I had a 351 that was built for some strong low end power (and mid range, and a bit of top end as well). I ran the calculators and made all the pretty charts, I had driven the engine behind a stick and knew where it was happy at. Figured 4.56 gears would be perfect. Found a deal on a set of 4.88s I could not pass up. So 4.88 gears it would be.

After driving it I concluded it was slightly over geared, but not enough to be worth changing. It would have been perfect if I had 35" tires, or I had only a 302, or I had 4.56 gears. With your combo, you have the 302 instead of my 351. Thus I will recommend based on a very near match to your combo to run 4.88 gears.

It's strange to think of 4.88 being a good highway gear, but they were. I did 3 Phoenix to Vegas round trips and it was completely happy with it. RPM was low enough it wasn't screaming, but high enough that you could drive it without having to drop a gear to gain any speed. It wheeled good, but I also ran extra low gears in the transfer case so I will just leave it there. In town it was a blast to drive around. Pulled from a stop really strong but would settle down and just cruise.

Friend of mine had a '96 Explorer 5.0 that he used back in the day to haul clients around showing real estate, and sometimes tow his race car. With stock size tires he put 4.30 gears in it. Towed better, drove better, still comfortable to haul clients around in, didn't loose any fuel economy either. Almost bought it as a parts truck. I also knew he drove it hard.

Also remember the Explorer has a lot of aerodynamic improvements over an early Bronco. A little more gear to help overcome that. The calculations for RPM are good, but that would be if the vehicle has the same drag. The Bronco has more.

Go 31 spline. 28 spline axles were questionable in stock form. Many people have bought upgraded 28 spline axles to later regret it. Even the Explorer you got the engine out of has 31 spline axles.

As for the Speedway axle. The wheel bolt pattern is wrong for one. Not sure if the width is correct (guessing not).

 

[bax]

I just changed from 4.11 to 4.56 same combo 5.0/ 4r70/ 33'' tires. I like the change. Highway rpm was a little low in overdrive and too much if I locked overdrive out. 4.30 I did not think would be enough change. the 4.56 is the correct combo in my opinion.

 

[Cooter_76]

Thanks for all the feedback so far. Sounds like 4.56 or 4.88 would be a good choice, depending on personal preference. The 4.88's might be better in the event I decided to step up to 35's.

As for the Speedway axle. The wheel bolt pattern is wrong for one. Not sure if the width is correct (guessing not).

Good point. I picked one as an example. They have widths from 54-60" and probably have different bolt patterns, etc. I saw David Freiburger use a Speedway assembly on one of his project cars (the '69 Mach 1, maybe?) and thought I'd look into it.

 

[blubuckaroo]

This is the gear ratio calculator I like to use, because overdrive ratios are different on different trannys.
https://spicerparts.com/calculators/...rpm-calculator
Also, the running tire diameter is significantly different when the weight is on the tire. Measure the radius to the ground and multiply times 2.

And yes, gear ratios are a matter of preference. Don't select a tranny or gear ratio by highway performance alone, unless that's all you'll be doing.

 

[Broncobowsher]

If you aim for about 2000 RPM @ 60MPH with a 4R70W, the around town performance is going to be impressive as well. And you want to really book it, 90 MPH is 3000 RPM which is where you will need to be in the powerband to push that brick.

Picking on highway alone with a lot of transmissions generally leaves a lot of potential on the table. But the 4R70W has enough spread and good spacing to allow a nice highway cruise and fun gears in town. Low range is pretty nice as well.
Picking gearing for a C4 or stock 3-speed based on highway driving, yea that is going to leave you with a pretty sucky in town gearing. 4R70W takes care of that. Just remember don't fear the gear. Broncos (especially with overdrive) LOVE low axle gears, even for highway cruising.

 

[Cooter_76]

Fair points, and good info. I'd just assumed a safe starting point was to try to match the RPMs/speed with what the power train was designed for.
I don't necessarily need a super-low crawler or a highway hyper-miler. Just a good all-around fun driver that isn't wailing at 70MPH. Leaning towards 4.88's.

So would y'all use the existing small bearing housing and order new 3rd member & axle shafts?
Order the differential parts and have someone local put it together?
Put it together yourself? I don't think I'd have the patience, based on what little I know about differential setup.
Or for $1800ish, get a whole new housing, plus new rear brake hardware?
Other?

I'm all ears!

 

[Broncobowsher]

The small bearing housing makes me lean toward a new housing. Not a big fan of the conversion bearings, not the little 10" brakes on the 33" tires. Yea, ends can be cut off and replaced, but you have get the alignment right. The stock Bronco housings are not great, I've welded a few of my own and some of my friends as well. All adds up to an aftermarket housing as a good choice.

Now how you do that, I don't have a good answer. Don't know what local options you might have. Plugging parts into a housing is easy. But this is when you need to put the options to the axle and get what you really want and not just compromise with what you start with.

 

[DirtDonk]

Probably a good idea to step up to 31 spline axles. Any issues using the adapter bearings with a small-bearing housing?

As far as I'm concerned, it's just the extra cost. A small bearing is actually more at home on the highway, but should have a lower load rating and that's not necessarily good for an off-road vehicle with big tires.
Not that yours is going overboard on the tire or off-roading standpoints, but I wanted to throw that into the mix anyway.

Would new 28 spline axles suffice?

With an open diff and mild use, sure. But as soon as you add a limited-slip to the mix I start getting jitters. It's probably not an issue if you don't use the vehicle hard, but I still remember back in the old days where it seemed like every other Bronco that went off road with a limited slip or locker broke an axle or three.
Eventually Detroit, ARB and probably others simply stopped making 9" diffs for 28 spline axles to make the point.

...plus $325 for new axle shafts. Didn’t see the bearings on their site.

The bearings are already included and pressed on to the axle shafts when you get them.

For this discussion, since the large/small subject was brought up, the large bearing 31-spline axles from us are $75 less expensive than the small bearing version. Our Super-9 housing is $525 with free shipping right now.
Granted, that's 450 bucks more than you were about to spend, but when building up a rear end I don't even consider 28spl axles an option anymore, and for the amount you're already spending that extra 400+ can get you a lot of benefit now and in the future.
Easy for me to say, since I like recommending that housing and our big bearing axles. But there are definite benefits.
And I neglected to add the extra cost for brakes. If you opt for the axle shafts for a '76/'77 Bronco (no change in price, just flange/bearing offset) you can use a ready-to-run big brake kit for $325.

On the other hand, with all you're already contemplating spending on the rear end build, saving a few hundred dollars (roughly 700) can be a big bonus for your wallet.
It's just that when building one up, it's great to build it up with all the benefits included.

And the side benefit I almost forgot is that you now have a complete 9" Bronco rear end for sale. Fully ready to run with 3.50's, factory limited-slip and all the way out to axles and brakes.
Clean it up, paint it, make sure it works well and you can probably get a few hundred for it.

Paul

 

[Cooter_76]

This is the gear ratio calculator I like to use, because overdrive ratios are different on different trannys.
https://spicerparts.com/calculators/...rpm-calculator
Also, the running tire diameter is significantly different when the weight is on the tire. Measure the radius to the ground and multiply times 2.

And yes, gear ratios are a matter of preference. Don't select a tranny or gear ratio by highway performance alone, unless that's all you'll be doing.

Have you played with the Grimm Jeeper's calculator? It has a lot of common transmission gear ratios loaded, or you can customize. Same with transfer case ratios. And you can compare two different setups side by side.

I understand that the actual tire sizes vary from the listed size, but without all the different tires mounted on the specific vehicle(s), the listed size is probably close enough.

 

[jw0747]

You know me and my light blue EB from SA. I've been watching your engine install which is an awesome project and your work is outstanding. You're kinda at a point where I should provide some info about my EB.

My offroading used to include some heavy duty trails along with some lesser tough ones. Lately I've slowed down some as I've moved into the more senior ranks. My axles have 4.56 gears and 35"x15" tires. Back axle is 28-spline w/small bearings and holds a Detroit locker. In 15+ years with this setup I've bent and broken one axle. Front axle has a ARB and in 10+ years with this setup I've broken one axle u-joint. Before the ARB I had a Detroit Truetrac and went for the ARB because I wanted a selectable locker.

Unless you plan to do some fairly heavy duty trails I wouldn't spend a large sum of money on a complete new rearend. All things considered I've had good luck with my setup and don't think having big bearings and 31-spline axles would have been worth spending the money to convert.

Hope this helps and good luck with your "new" rig.

I'm watching,
JW

 

[Cooter_76]

You know me and my light blue EB from SA. I've been watching your engine install which is an awesome project and your work is outstanding. You're kinda at a point where I should provide some info about my EB.

My offroading used to include some heavy duty trails along with some lesser tough ones. Lately I've slowed down some as I've moved into the more senior ranks. My axles have 4.56 gears and 35"x15" tires. Back axle is 28-spline w/small bearings and holds a Detroit locker. In 15+ years with this setup I've bent and broken one axle. Front axle has a ARB and in 10+ years with this setup I've broken one axle u-joint. Before the ARB I had a Detroit Truetrac and went for the ARB because I wanted a selectable locker.

Unless you plan to do some fairly heavy duty trails I wouldn't spend a large sum of money on a complete new rearend. All things considered I've had good luck with my setup and don't think having big bearings and 31-spline axles would have been worth spending the money to convert.

Hope this helps and good luck with your "new" rig.

I'm watching,
JW

Hey JW, thanks for the perspective! I've always considered your "Baby Blue" ;D Bronco to be a nice, all-around capable rig. Two years ago at the LEBC RoundUp, I did the Fenceline trail for the first time. With 32's and failing limited slips, I narrowly avoided body damage in a few places. Would like to be able to tackle that level trail with confidence. That's about as heavy-duty as I'm shooting for.

 

[Cooter_76]

I'm finally getting some traction on this project. A buddy found a spare big bearing 9" housing in his shop, that he'll sell me for $100. I'm planning to pick it up this week. He said it has a third member with unknown internals, no axle shafts or brake hardware.
With this in mind, it sounds like I'll need a set of '76/77 31-spline axle shafts, this 11" drum brake kit, and a new 3rd member. For the 3rd member, I'm planning to go with 4.56 gears and a TrueTrac.
Tires will likely be 33x10.50 BFG KM3.
I don't have time to learn how to setup gears properly, so I'll have a local shop setup the front Dana 44 once I have the rear squared-away. Considering a Detroit locker and chromoly shafts for the front. With twin sticks for the transfer case and manual locking hubs, I'm hoping I'll be able to use this setup without getting in too much of a bind offroad.
Does this sound like a decent plan?

 

[blubuckaroo]

With your gear calculations being based on highway use, it's fair to assume you plan mostly street/highway use. If you plan to use this Bronco on winter icy/snowy roads, I'd recommend leaving the front differential open. Any kind of locker will make the car want to keep going straight and hard to drive.

 

[Cooter_76]

With your gear calculations being based on highway use, it's fair to assume you plan mostly street/highway use. If you plan to use this Bronco on winter icy/snowy roads, I'd recommend leaving the front differential open. Any kind of locker will make the car want to keep going straight and hard to drive.

Thanks for the feedback. I don’t foresee any icy/snowy road use here in Central TX. In the off chance that I did, could I unlock one of the hubs, or is that a recipe for breakage?
I was pretty happy with the performance of the original limited slip differentials, until the clutches started wearing, which is why I’m leaning towards a Truetrac in the rear. I could be convinced to put one up front, as well.


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[markw]

I’ve used Eaton E Lockers in a couple of Broncos and have been happy with them. Just another option I don’t see mentioned very often.


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[rguest3]

I had a 77 with a Nice 351W / 4R70W and 4.56 gears. I also ran 285/75/17s which are taller than your 33/10.50 you are looking at. The 4.56 was not enough gear for my combo even with the torque of the 351W.

The 33/10.50 is probably closer to 32" tall with a tape measure when mounted on the truck. The 4.56 would be my recommendation.

31 Spline axles are a little bit of insurance, not needed, but a good idea with the TruTrac. Maybe Open or factory limited slip up front.

 

[Nothing Special]

Thanks for the feedback. I don’t foresee any icy/snowy road use here in Central TX. In the off chance that I did, could I unlock one of the hubs, or is that a recipe for breakage?
I was pretty happy with the performance of the original limited slip differentials, until the clutches started wearing, which is why I’m leaning towards a Truetrac in the rear. I could be convinced to put one up front, as well.


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An automatic locker on snowy freeways is dangerous. If you're on the gas at all when one front tire hits snow the vehicle will pull that way HARD. As in the vehicle moves 10' to the left before you can catch it.

Unlocking one hub would make it a handful, but likely a more predictable handful. I certainly wouldn't recommend it from a driveability / safety standpoint though.

A TrueTrac in the front is reported to be invisible on snowy freeways. I have no experience with them there so I can't say more than that.

I do have a TrueTrac in the rear axle of my pickup. I'm a little disappointed in it in the snow. It doesn't have the bad manners of a clutch-type limited slip, but it's not very effective when one tire is on ice. You need both tires to get some traction for a TrueTrac to work well. That is usually the case, but not with one tire on ice (or one in the air when rock crawling).

.... I'm hoping I'll be able to use this setup without getting in too much of a bind offroad....

I don't have much experience with an automatic locker in the front off-road, but the little I've had, and the reports I've heard say that bind isn't that much of an issue. You do get significant torque steer, which can be annoying and might seem like bind (when you are on the gas or compression braking it will fight to keep the tires pointed straight ahead). But generally that's not too hard to overcome with power steering.