8.75 bracing

I've already put a back brace on my 8.75,but I feel there could be more done to prevent the housing from flexing and cracking the caps.

I'm wondering what everyone is doing, besides "putting a Dana in it" or a "9 inch"

I stumbled upon this 8.8 brace setup on my buddies car,and I'm thinking of doing something similar that attaches to where the pinion snubber plate bolts to, and attaching it to the axle housing similar to this.


Has anyone ever done something similar or different.

I've had no problems with a full back brace.. not
stopping short at the leaf springs... this is a 9"
but the same style brace

I have yet to hear a good explanation for how a back brace makes an 8.75 CENTER live longer, and I have heard a lot of them. Regardless, you may want to review this thread for some ideas, there are about 5? pertinent pics starting at pic #44?.

https://board.moparts.org/ubbthreads/ubbt...actice-car.html

Proving these are related has yet to be achieved IMO:
" to prevent the housing from flexing and cracking the caps"
if that is your real intended meaning.

I've got the full back brace already installed.


I'm just kinda thinking out loud, but i feel directly connecting the actual center section via an old pinion snubber mount to the housing would help as well. I could be wrong, but it's just a thought.

You can put them on... sure wont hurt but I never
seen the need... JMO

If you plan on making enough HP to need the back brace on any rear end with stock cast iron caps please replace the stock drivers side cap with a billet steel cap and better fasteners
I bent the crap out of my original 8 3/4 housing wuith no back brace with steel caps and 35 spline axles and spools with ladder bars in my old pump gas street Duster

Id weld square tubing vs several things and bolted.

Your pick looks neat but I think it would allow too many ways for things to flex.

i've been hammering my 8 3/4 for years behind a big block. the trans brake when it was fitted did the most damage twisting the housing & eventually breaking a cap. i have a good strong brace on the housing now & i made caps for my caps to prevent another failure there. i did this mod before Williams made their fancy cap upgrade.

I will tell my age, but I attended a direct connection seminar years ago with Ronnie sox and they said the bracing was for cars that had the leaf springs moved in for tire clearance, since the housing had less support out at the ends the tube would flex and the axle would lever the side bearing and then pop the cap.

I don't see how something like in the picture will help what plagues the 8.75 by design. It's done that way on an 8.8 b/c you can't really put a backbrace on it. That just keeps the housing from flexing, not the centersection.
To "fix" the 8.75, you need to somehow keep the pinion gear from moving away from the ring gear under load.
Instead of spending money trying to make my 8.75 live just a little longer, I replaced it w/ something stronger.

Not my strong subject (is there one?) but it seems to me that to relieve its load the pinion gear tries to move backward (toward the D/shaft).
This pulls the tubes with it, so a back brace working in tension (stretched) would be useful.
I can't see what force the front-mount turnbuckles would counteract, since they work in tension and are nearly useless to resist bending forward.

An aside: unless there's space/package problem, the farther behind the axle the rear brace extends (like a "V") the stronger it is, which may allow less wall thickness or skeletonizing the walls to save weight.

....and lived happily ever after. THE END


We went S60 when we noted things twisting/breaking when we reached transbrake level. Being I like things "easy", I wish we would have gone a derivative of the 9". It's so much easier to set up gears on a bench and throw them in at will, but to be honest, with the S60 and it's side adjusters over the side shims of a "stock" passenger car/truck D60, now that I have two sets of gears and the accompanying pinion shims, I can swap gears just as quickly and at less cost than having to buy a whole pig.

They don't make the gear ratio I need with a dana, so that's out of the question.

You can get any ratio imaginable for a 9".

[quote=polyspheric]Not my strong subject (is there one?) but it seems to me that to relieve its load the pinion gear tries to move backward (toward the D/shaft).[quote]

But its reactionary force is against the Ring, so the ds sees nothing of this force battle. The ring is forced to the drivers side, and towards the rear, as the hypoid gears push away from each other, or try to. This rearward force of the ring from an overhead view is counterclockwise in direction, which is why the Drivers side cap is loaded in tension, and I believe always the first to fail. The pass side cap mainly is just for positioning and stabilization.

As it has been mentioned, the greater the spring/control rod inset on the housing, the more leverage there is from a dead weight, and any acceleration forces to act on the housing from the bearing ends towards the center section by increased leverage. The back brace helps slightly with the dead weight angle, and will be much more helpful with the acceleration bending forces. However I think those forces might be over rated, when at the power levels for an 8.75.

I also doubt a spline fit 20"+ axle can add that much load to a bearing cap from housing flex. So how much is this housing really flexing? It would also be odd that this housing flex transmitted thru the spline axle normally only breaks the Driver side center section cap. Now that could be because its a combination of the way the cap is loaded with the housing flex, but..... None of my reasoning here applies to tire shake conditions when the forces are chaotic and off the chart.

Now with the rotational twisting of the housing under acceleration, two things stand out to me, a back brace is a rather poor choice to resist torsional twist of the housing, but it does improve matters, and in the situation where the springs are relocated, and moved in towards the center, the housing by design is now much stiffer, in a rotational standpoint, as the dimensions are all shortened on the center section side, and not sure any twisting is taking place outside of the springs, in the first place.

I like it. I've thought about this very design in my mind many times. Will it make your 8-3/4 stronger than a 9"?... no, but it certainly can't hurt.

My 8-3/4" has a moser backbrace, Williams main cap, Dr.Diff spool & 35-spline axles. To the OP: If you build this, let me know with a PM. I'd like to see how it comes together and may be interested in one.

IMO and you can do whatever you want, you can brace EVERYTHING on that rear and have nothing move.
Your weakest link will be the teeth breaking.
I'm in the same boat and not going to mess with the 8 3/4 . It's hard do to the fact I have everything I need for it.
In the long run i will be getting a 9 inch. Dana or 9 for me is a tossup but the 9 gets the nod because of availability.
Matt

I braced mine in several ways. In the end I ended up with a strange ultracase 9".
I took the 8 3/4 as far as it would go. It was back braced, load bolt supported, and the most important(in my opinion) was the internal bracing that went right up to the caps so they could not spread. After the internal supports, the teeth didn't break anymore. got me through a season of racing. low 9s at 3700 pounds with a trans brake. Started working on the 60' and started shearing pinion shafts. No patch for that!
Went with a 35 spline pinion ultracase and put the whole rear end doubt right out of my mind.
That said, with a lighter car, I probably wouldn't have needed to switch. It was a lot of work though.
I don't have any pics of the internal bracing.

My thoughts exactly with the bolts. I would do a brace inside like a can with no top or bottom that spaced from front flange to the back one, then holes drilled for axels. If there where some bearing with less taper to replace the back pinion bearing, or even bronze bushings like a metal lathe, would help on pinion deflection.

I think I'm going to shorten a Dana over winter and go with a 3.54 ratio.I order ends from Dr diff today, and ill reuse my 35 spline axles in the cut down dana.and reuse my aerospace brakes. I currently run a 3.23 and really really didn't want to stray away from it. It might pick up some with the 3.54 anyways, who knows.

I figured bottom 9s at 3600lbs, it won't live much longer even with my slow 60', but for 2 years it's going strong lol

You're doing the right thing. With the tall gear it'll break the pinion first because it's got so much load.


I sheared of a 3.23 pinion myself on TB drag radial launch.

Dana has been flawless. I'm even still using the stock 1978 F250 3.54 that came in my junk yard axle.

Weld the Ford ends on it, then you can use the Ford Explorer disk brakes.

Yup I agree. Going to try to avoid catastrophic failures.

I've got aerospace 4 piston rears already, and I'm going to reuse my 8.75 35 spline axles.

Make sure and reuse those axles on the same side in the new Dana 60, do not swap or change sides on them I did the same thing as you are planning on my old Duster with Strange 35 spline axles when I swapped them from the 8 3/4 with a Strange 35 Spline spool to the cut down Dana 60 with a Moser 35 spline 3 sereis spool, I had to cut about 3/4 inch off the passenger side axle. I took it to a freind who has steel chop saw and we beveled the ends on his bench grinder when done
BTW I do have several different gear sets of use Dana OEM gears in decent condition that I'm willing to sell if your interested, if so PM me on here

Next dumb question,



I used drdiff's calculator for tube length with my axles, and subtracted a half inch per side for spool engagement.

Will it hurt anything to have equal length tubes with a truck 60 pinion offset in a factory style chassis?

I would slide the rear end into the car and center the pinion yoke in the tranny tunnel and go from there on the axle tube lengths needed to make your old or new wheels fit into your car properly I think my truck rear end axle tube on the passenger side was 3/4 shorter than the left when I was all done with it
BTW, my axles extended past the splines in both spools into the center of the spools I figure that way didn't hurt anything and I knew I had max spline engagement on both the axles and spool splines

You can brace it like a tank and it will NOT fix the inherent weakness of the 8.75. Torque pushes the ring gear away from the pinion when the case flexes. You CAN'T fix that. They break caps because all the debris from the shelled teeth gets in there and it pushes the cap off. So you will spend a lot of time and money trying to fix a problem you can't fix and it will eventually break anyway.

Put a 9" inch in it and forget it

yes what he said👍👍👍

All stock engines with reg engine mounts are moved to the pass side, some quite a bit. 8 3/4 pinion offset is the most, dana is less, reason one tube is longer. The tunnel is also offset to the right. I centered the dana housing in my 64 and didn't have a problem, there is wiggle room to line it up, front to back. No more misalignment than the up and down movement of the drive shaft. I wouldn't cut any axel, unless they touch, since the splines go thru the spool

if you look at the pic i posted earlier we tried to address that very problem. the extra cap on the cap is designed to fit snuggly against the back of the housing so its harder for the caps to deflect & allow the failure as you described. it seemed to have held out ok as since i did this trick the 8 3/4 lived without a failure. i dont use a brake on this car anymore so i'm sure my upgrade with live as long as i'm abusing this car.
my other racecar has a 9". too easy & you trip over parts everywhere.

That does nothing to address the problem, which is that the CASE is weak. It lets the ring gear and pinion separate, which shells teeth.

Well it hasn't failed since we did this mod years ago which has lasted umpteen times longer than before when we ran it in stock configuration.

Yep. Anything you can do to keep the caps from spreading away from their intended position will help the durability. It's not the easy way, it doesn't make it bullet proof, but it does make a large difference.

I'm going out on a limb here and saying it isn't as much the caps moving as it is the deflection of the pinion shaft and the case which supports the pinion against the ring gear.

In most any gearing system it's the smaller gear, called the pinion, that's most highly stressed.

The 9" has a support bearing or bushing on the other side of the pinion and that maintains the alignment much better. It doesn't seem like much but it makes quite a difference.

One disadvantage of the support is it moves the pinion lower on the ring gear, which eats more power by friction. The support also robs space for a proper limited slip differential. 9" limited slips are kind of one-sided. Of course, spools don't mind at all.

All in all, the advantages outweigh the disadvantages when we move into higher torque loads.

I see that Dr.Diff has a nodular 8 3/4 case for sale. Nodular iron is 50% stiffer than gray cast iron. This will lessen the amount of deflection for a given load. The nodular case should be the first purchase if you're building a stout 8 3/4.

R.

That pinion deflection idea of the source of the problem, has been tossed around before. It is somewhat supported by the change in the pinion ,in the 489 case, to a larger diameter pinion, albeit with a ID smaller bearing. Just because they made that change does prove that was the problem, nor that it solved the problem. Why that may be the case, when Mopar brought out the Alum case decades later, with the availability of significantly greater computer design assistance, it was based on the previous 742 case, and advertised as being a 10% stronger design, in Alum. Personally I think the ring gear/mounting would have much greater deflection then the pinion anyhow, by design, and nothing to back that up. Case design of the alum 742 has added/enlarged external ribs, for stiffness?

PS Maybe I missed something here, not sure anyone is much concerned about "caps" moving, so much as just failing from overload, and whatever it takes to mitigate that overload.

Bingo........The CASE is what flexes, allowing pinion to move away from ring gear. The cap has zero bearing on this. It pushes the cap off when all the tooth debris gets lodged in there and SOMETHING has to give, that's the cap. A better cap may not break, but something else will. I have seen 8.75s with GOOD caps still shell teeth. When it cant break the cap, I have seen them break the pinion clean off or split the case. You get all that junk lodged in between SOMETHING is going to give

Yes the case flexes. The issue is, it flexes at the caps. The distance between the driver cap and the passenger cap increases. Internally bracing the housing at the backlash setting adjusters GREATLY strengthens the assembly. The rub marks on the braces proved to me the load that the cap takes. A look at the pig, and it is obvious where the case will move given enough force. I personally proved it to myself. When you make a change, and the problem moves elsewhere, you've "fixed" that problem. Now, did it stop the ultimate demise? No. I ended up with a race intended 9". My only reason for posting this over and over is that the actual reason needs to be disclosed.

Been running Dr Diffs nodular iron 8 3/4 w/stout Ford type caps for several months now and all`s well but only 2 passes a 1.37 and a 1.33 60`s so far and a few more soon I hope.............many, many years on the stock 742 in the 9`s and 1.3`s in the 60 and back braced.......

It seems to me that you put your Dart on a serious weight loss program and it could be that is one of the reasons your 8.75" still lives...JMHO.

Most of those were at 3200+ lbs. and in the 10`s but the last 7 years have been 9 second passes and several 1.3`s but on Hoosier QTP`s and my last two passes were w/28x10.5 w Mickey slicks getting me the 1.33 60.............Guessing I`m at 3050 or close but still haven't weighed it yet.............

I am not clear on what you are observing regarding the caps distance moving apart and internally bracing preventing and reducing that. I can see the carrier moving in the sheet metal housing, likely with both caps leaving a witness mark from the pre load bolts(?), but not sure what internal bracing improves that, or extends cap longevity. I often feel these issues often arise not in the observation, but in the conclusion drawn from them. I agree this road has been traveled many times here, and its still bumpy. Your comment on making a change fixing the problem, needs to be careful, a broken leg can be fixed with an amputation. . Related also somewhat, the comment often made on this topic, and I can't dispute, and have yet to see/hear conclusive proof of, is caps fail because of broken teeth jamming the gears, and then over stressing the cap(S). I am not clear how one definitely determines that order of failure.

This reminds me of comments this weekend in the pits. A couple of guys looked over the car and then watched it run.

They came back and said they new it was a serious piece and a "sleeper" when they saw the Dana 60. LOL!

quote=jcc]

I am not clear on what you are observing regarding the caps distance moving apart and internally bracing preventing and reducing that. I can see the carrier moving in the sheet metal housing, likely with both caps leaving a witness mark from the pre load bolts(?), but not sure what internal bracing improves that, or extends cap longevity. I often feel these issues often arise not in the observation, but in the conclusion drawn from them. I agree this road has been traveled many times here, and its still bumpy. Your comment on making a change fixing the problem, needs to be careful, a broken leg can be fixed with an amputation. . Related also somewhat, the comment often made on this topic, and I can't dispute, and have yet to see/hear conclusive proof of, is caps fail because of broken teeth jamming the gears, and then over stressing the cap(S). I am not clear how one definitely determines that order of failure. [/quote]

The internal bracing had the backlash adjusters supported. This is "side to side" support, not fore and aft like the load bolts. This caused the case from spreading. That stops the backlash from getting wide enough to start chunking off teeth.
Before doing this, the teeth would break and get jammed in between the gears. After doing this, the teeth never broke. The next issue was shearing off the pinion at the yolk. I went through 3 gear sets before doing this(teeth broken) to shearing the pinion with no broken teeth(twice before throwing in the towel)

OK, I understand better your point, I have not seen this side to side internal bracing, only the pre load bolt stuff, any pics posted? Also if I understand the concern here, only the drivers side cap would be moving, which could be thought of as the "caps" spreading. Sounds like a carrier cap girdle. And there is no primary thrust force acting on the pass side cap, as I understand it.

How much stronger are the nodular cases?

http://www.iron-foundry.com/grey-iron-vs-nodular-iron.html

Not sure this will answer questions specific to the 8-3/4 casings, but this shows nodular iron strength to be vastly superior to grey iron. It says however that some of this additional strength is due to its ability to flex, as opposed to grey iron which is characterized as more brittle. Not sure flex is what we're looking for when searching for a stronger 8-3/4 case. It's notable, however, that nodular iron is used by Strange in their 9" case.