Rear Axle Musings

[Robert J]

Ever since the Jeep dispatcher Dana 44 axle posted, I can't stop thinking about what rear differential I am going to use on my car.  By extension, I have been thinking of how nice it would be to improve the rear suspension (ala Fast Cars 3 link).  My budget may not quite allow that as it is already being healthily exercised on all other aspects of this build.  I know what several of the options are.  What would you do if you were building your rear axle and rear suspension today? I am running a bored and stroked SBF for the record.

[nobogez07]

I'm using Classic Conversions 4 link rear suspension in my build.  I like the suspension geometry better than a 3 link with no binding as a concern

Doug B

[MGB-FV8]

IMHO, the benchmark seems to be the Ford 8 inch set up; Ted also sells the housing.

[MGBV8]

What would you do if you were building your rear axle and rear suspension today?

I wouldn't need as strong a rear as you. You need something that will stand up to the power & how it will be applied. The 8" Ford, Ford Ranger, Toyota, & S10 rears are fine for the milder builds. Heavy hitters may need a Dana, 8.8 Ford, or even a 9" Ford. Especially for repeated hard launches.

Rear suspenson? If not going IRS, either a Classic Conversions four link or a good 3-link would be my choice.

[BlownMGB-V8]

I would look at several things:
-strength/reliability of all components
-range of gearsets/cost of parts
-efficiency
-sprung/unsprung weight
-intended use
-personal preference
-cost of conversion
-down time/delivery lead time
-size/fit in the car

It's a big list.

The stock axle has an 8" ring gear so it is strong enough in that area for any but the most extreme build (IIRC it measures 8" at the widest point and 7-7/8" at the mounting flange but feel free to correct me). However the spider gears and axle shafts are weak. Parts are scarce and expensive. Efficiency is good. It is somewhat heavy. It costs nothing to keep. So for the milder builds with the right transmission it remains a good choice.

The Ford 8" is strong in every component, parts are plentiful and cheap. It is light. Very good suspension packages are available at moderate cost with reasonable lead times and the conversion is not particularly difficult. Due to the low pinion placing and higher helix angle it is less efficient than a salisbury design, about a 5% difference IIRC. This is the most popular choice, falling short only on efficiency. It has not so far been shown to be inadequate for high horsepower applications but we may be approaching that point.

The (later) 10 bolt chevy and the Dana 44 both use an 8-1/2" ring gear. These are good for about 500 HP. They are common and cheap with plenty of parts selection. Both are Salisbury type so the efficiency is good but they are moderately heavy. The suspensions developed for the 8" Ford axle will work fine with these but may require a bit more fiddling to get right. Very good choices for high power applications.

The 8.8" Ford is common and cheap. Plenty of parts. Very heavy, and very large. It will not fit without removal of the battery boxes or very serious work. As a Salisbury type it is efficient. It is strong enough for the most extreme drag strip type application but is not a good choice for a street driven car.

The 9" Ford is still widely available but not as cheap as it once was. Plenty of parts. Very large but not quite as heavy as the 8.8" Ford. It has a high helix angle and is less efficient. Otherwise similar to the 8.8" above.

If you see a trend here, congratulations. Ring gear size determines the strength of the axle assembly. Provided that the other components such as spider gears and axle shafts are commensurate in size, this is worth repeating: RING GEAR SIZE determines axle capacity. As a rule banjo axles are lighter but not as strong as Salisbury types. However, most banjo axles do not have a high helix angle and do not suffer from the inefficiencies that the 8" and 9" Fords do. For all of the various axles not listed here you can determine their merits based on three factors. Ring gear size, housing type, and helix angle. But ring gear size rules. All manufacturers use the very best materials, heat treatment, and manufacturing methods and size for size they are equivalents. Do not be mislead. An import is no stronger than domestic or vice-versa, metric or inch size. Ring gear size rules.

So before you even get to the merits of live axle vs IRS you can decide what type is the best fit for your car based on the above considerations, and for an axle not mentioned you can do the exact same analysis. So for instance, a Mopar axle, a Nissan, BMW, or Roll Royce, whatever it is, the process is exactly the same and there is no mystery here.

As for IRS, I think you all know my position by now. The Dana 44 is an excellent choice for strength, availability and cost. It is a little heavy but in an IRS that is sprung weight and offsets the engine weight of a higher powered conversion so it is a plus. Equally important it is compact and fits the unmodified car. Unsprung weight is greatly decreased for a better ride and more consistent tire contact. This is a good start but the expense is a big deterrent. The conversion is usually more involved, and typically either requires fender flares or reduced suspension travel to retain ride height. However if fender flares can be tolerated this allows much wider rubber to absorb the higher power output, which then combined with more allowable suspension travel than stock simply transforms the car.

My choice, the Dana 44 Jag based IRS, 3" of flare per side and 315/35-17 tires with 8" of wheel travel. It doesn't get any better than that.

Jim

[MGBV8]

Let's not leave out the Chrysler 8 3/4" banjo rear, a quality rear that can survive under a '57 Chevy. Getting hard to find.

Also, consider modern IRS from Toyoto, Nissan, & Mazda.

Personally, I'm leaning toward the GM 9 Bolt Australian rear.

The BW 9-bolt was last installed in 3rd gen cars in 1989.
A performance rear with a 7.750 dia ring gear, no "C" clips on the 28 spline axle shafts, Cone type limited slip and larger side pinions & spider gears. Was available stock in 2:77, 3:27 & 3:45 with very few parts available on the aftermarket. None at GM Dealers. A stonger rear then the stock 10-bolt, however it was NEVER designed for more then 300 Hp and Drag Strip launches.

All I need.

[denvermgb]

Jim,

What about the Ford 7.5?  I understand it has the bearings of the 8.8, but not as heavy?  As you probably remember, I bought my Ford 7.5 from a Mopar guy who told me it was an 8.8, which is what I wanted.  The 7.5 has been in my car a few years and has been strong and trouble free.  I am by no means the rear axle guru, just throwing out another option.

Brad

[MGB-FV8]

IMHO, you have to take in consideration servicing issues along with many other factors.  I've purchased an aluminum Ford 8" center section (very light) as per the recommendation of Currie Enterprises; one of the Currie brothers stated that it was good for up to 700 horsepower and that most hot rodders and street rodders in California used the Ford 8" for its strength and unsprung weight.  Mine is also set up with a "True-Trac" clutchless posi-traction with 3:42 gears.

MGB is a very light car and that lower weight extends the punishment allowable from a high horsepower engine.  Mazda intended to fill the market void left by MG's closure and developed the Miata roadster nine years later; I own one and I must say that if my MGB project even comes close to the Miata's performance, I'll be very happy.  You could follow the design of the Miata by installing a rear independent suspension set up and ball joint/coil-over set up in the front but you have to do it within Miata's thinking of light weight and unsprung weight calculations.  Todd Budde offers an affordable IRS that mimics Miata's design; he uses a Nissan's set up that adapts to the MG.  The Nissan's rear differential is as stout as some of the better differentials mentioned above.  If it was available at the time I started my project it's probably the way I would have went.

Brad, the Turbo-Bird used the 7.5" for long time and also the Ford Ranger.  A friend of mine had a Turbo-Bird making around 450 H.P. and it held up amazingly well to the stick-shift abuse; the T-Bird was also a much heavier car than an MG.  The 7.5" is also one of the lightest choice for unsprung weight.  Listen to Jim's advice in his post as a strong rear is not the only factor to consider.

Jim, I'm curious as what you meant by the word "efficiency" of a type of differential such as the 8" and 9" having a slightly lower rating?  I always assumed that the 9" rear's reputation came from the extra tooth making contact accounting for its strength.

[Robert J]

Jaques, I wasn't familair with Todd Budde.  Do you have his contact information?  I am familiar with and appreciate Nissan build quality, so this sounds worthy of investigation;
"Todd Budde offers an affordable IRS that mimics Miata's design; he uses a Nissan's set up that adapts to the MG."

Thanks for your input as well Jim.  I have to say that the Dana 44 remains at the top of my list, especially since I can take the time to find a Jeep Dispatcher axle that is already the right width.  I am installing VW Rabbit flares in the rear and plan to run 16" wheels most likely.  I would love to go with an IRS setup if I can find one that I can wrap my head around.  My fabrication skills are fairly good, but this is obviously an area where everything need to be put together right.

[Robert J]

Ah,  I found Todd Budde, although his link to  "Custom Rod Works" is down.

http://www.britishv8.org/mg/toddbudde.htm

[MGB-FV8]

Robert, look above under contacts and click on British V8 Vendors; his banner is "Custom Rod Works"

[MGBV8]

Jim, I'm curious as what you meant by the word "efficiency" of a type of differential such as the 8" and 9" having a slightly lower rating?

Jacques,

Jim, is speaking of the parasitic horsepower loss. More efficient rears take less horsepower to turn. That is something that always bothered me about the Ford 8" & 9". It's not a lot, but still...    If I ever do build a 8", I would do it like yours. Lighter is better.

The Chevy rears are better for efficiency because the closer the pinion shaft is to the center line of the ring gear --the less drag it will have. The Ford 8.8 is also in this camp because it is essentially a 12 bolt Chevy copy.

Another rear axle to consider for milder swaps: Rx-7.

[BlownMGB-V8]

Brad, I believe the Ford 7.5 uses a 7-1/2" ring gear so intuitively it would seem that part of the axle (and possibly the carrier) would be weaker than the OEM axle. With about 300 hp I broke the spiders in one stock axle and wore out the ring gear in another so I wouldn't expect a tremendously long service life out of the 7.5, but depending on how you drive it may last long enough.

Guys, all else being equal, in an axle bigger is always stronger/more durable. So what is equal and what is not? For most axles (8 & 9" excepted) helix angle is more or less equal. Metalurgy is equal. Heat treat is equal within the two classes of street and race gears. Housing designs are not, and that really is about it. So let me say it one more time in case it hasn't sunk in. Ring size Rules!

Horsepower ratings from anyone except a certified testing lab combined with a durability rating is nothing more than marketing hype and cannot be relied on for anything. Just because X brand axle handled 2000 hp one or two times does not mean it will handle 200 hp over the long haul. Again, Ring size Rules!

But a couple of other considerations. The banjo housing is weaker than the Salisbury type because of flex. High horsepower drag racers learned that the 9" Ford housing flexes enough to change the backlash clearance during a run, to the point where some racers were getting only ten runs out of a set of gears where the Dana 60 guys were having no problems with gear life. The housing can be reinforced internally by adding gussets to the bolt flange to greatly improve the strength of the housing.

But there is a plus to go with this minus. The banjo is the lightest and also is the easiest axle to change the gearing, whether by swapping the center section (by far the easiest method) or by putting the center section on the bench and changing the gears, as they are very user friendly for setting pinion depth and backlash and require no specialized tools. So for anyone considering their first gear swap this is the way to go.

With those considerations I would say that for a solid axle a banjo type with an aluminum carrier and the pinion close to the axle centerline, reinforced housing flange, beefy spiders and axles and a reasonable, available, and competitively priced range of gears in the 8 to 8-1/2" range would be ideal. Maybe Toyota makes or has made something that matches those requirements but so far the 8" Ford seems to be the closest thing. I would look at the early Landcruiser axles, but I think the pinion is a little weak.

Todd Budde's design has improved but see if it still requires you to remove the battery boxes, and aside from that inconvenience don't forget that those are structural and removal should be followed by installation of gussets as the factory did it.

I'm a little surprised the bolt-in IRS we developed for the Roadmaster didn't catch on, but I guess nobody really likes flares that much.

Jim

[MGBV8]

Guys, all else being equal, in an axle bigger is always stronger/more durable.

You left off heavier, Jim.     Which is exactly why those of us in the under 250hp class should be looking real hard at the smaller diffs.

[BlownMGB-V8]

I agree, and the OEM axle was overkill for the stock MGB. At least failures were very rare, and there are a lot of them still around. Rumor is it was a lorry (truck) axle. In that power range a 7-1/2" axle should have no problems and a long life. Between 250 and 300 I would look for an 8" ring and over 300, 8-1/2".

What you are really looking at here is not so much breakage as durability over the long haul and microscopic incremental wear.

Jim

[theonlyiceman53]

It's hard to beat the ride of a Jag IRS. Mine is a late model so pretty heavy.   If you have a heavier V8 up front it tends to balance it all back out.   On an dollar to engineering level I would say a Corvette c4 or c5 rear would be a good choice and would probably offer the lowest weight.   Just depends on how much work you are willing to do. Chevy spent a lot of good money on Vette suspensions so why not get some benefit?  
Bill

[Robert J]

I've been looking long and hard at the Jag IRS conversion.  I haven't ruled it out anyway.

[BlownMGB-V8]

My plan on the Jag is to do a gen-II version of the MGB bolt-in conversion using built-up aluminum lower control arms which will lower unsprung and overall weight considerably, plus possibly air bags for active level control. My memory on the weights is less than perfect but I believe that will put the assembly pretty close to stock weight and it is a reasonable expectation to even get below stock weight. Certainly the unsprung weight can be reduced to the lowest of any comparable axle since there is no need for an upper control arm and the new lower control arm will eliminate the forward link arms. The gen-I version had a much heavier cross member than is required also so weight can be shaved there too. That is what comes of using materials that are on hand instead of special ordering them but the up side was lower cost and a shorter lead time.

I didn't see any advantage in using the C4 vette rear. Less available, and much more elaborate mounting would be required if it even fits at all without modifying the car. Since the goal was a bolt-in assembly and the forward links make that impossible that was the end of it.

The Jag has a lot going for it. Wheel travel of up to 8" means a really good ride. To get that you have to do the flares but at the same time you get more track width for better stability, which to me seems a pretty good trade off. I've run a 6" wider track for decades now and have never regretted it. And I've done two cars now with nice curvy 3" steel flares that I think really compliment the contours of the car while not mimicing anything else out there, and providing plenty of room for those big meats to tuck all the way up in the wheel wells. Rabbit flares are cheap and easy but I always thought they looked a bit angular, not at all in keeping with the rest of the body contours.

Jim

[roverman]

Clan, C5 beware ! The Vette C5 set-up is a transaxle, with 23.5" of axle centerline, to front mounting face. This means lots of cutting/welding for any lbc. This is why I chose Porshe Boxter "S", with only 8" of centerline dimension. For drag racing a 8" or 9" ford, the ring gear "deflection", is the primary shock-load problem.This is because the ford designs have a very shallow cone effect, of the ring gear carrier, to allow for a stradle mounted pinion gear(stronger). The cure, in extreme aplications,(drag racing), is to install a ring gear thrust button, that will limit ring gear deflection. This only touches during extreme launches. Another plus of the bajo,(removable 3rd member) rear end, is serviceability. Anyone who has changed ring and pinion gears, in the vehicle, on an integral design rear, knows of the displeasure compared to bench work banjo types. You must spread/distort the integral housing, to remove the carrier. Spread it too much, and it's scrap, rhymes with &*#)@ !  Using an overly heavy IRS, for ballast, seems counter productive, where as, regular balast can be put exactly where you want it . Onward, roverman.

[MGB-FV8]

Art, I thought that the Ford 9" had one more tooth in contact with the ring gear giving it its ring/pinion gear strength?!?!?!?!

[MGB-FV8]

After reading this article, I think that I would stick with the Ford 9" for extreme performance.  Good reading that explains a lot.....

http://www.truehi9.com/gears2.html

[roverman]

Jaques and clan, strength is largely a function of ring gear diameter, as Jim B. states,(as long as companion pieces are also adequate). Number of teeth, in contact is a function of of pinion to ring tooth count. Higher numerically, will have "less" tooth contact, as pinion gear gets smaller, with fewer teeth,(higer psi loading of fewer teeth in contact). Second problem is distortion under extreme shock load. The teeth are very hard and do not tolerate much deflection, before they crack and then break. The 8"/9" ford designs are stronger per pound, because of "pinion drop" is greater, this means higher tangient angles of tooth contact. This means stronger, but more drag,(approximately 5 hp. between a 9" ford and a 10 bolt gm).  Onward, roverman.

[nobogez07]

the 8.8 is tough, cheap, and easy to install with today's kits!

It can certainly handle the 250 to 350 HP engines that go into B's.  More HP than that and it's a waste of time and money unless of course you're a straight liner in which case a completely new axle is in order!!

[BlownMGB-V8]

Have you guys lost your minds? Seriously, what sort of monster motors do you envision installing that you need that kind of heft in the rear axle?

Jim

[roverman]

Easy Jim- buddy, as we have previously learned, "some" consider the rear end/t-axle, a good source of fiber and ballast/lol. If a 8" is good then, math included, would not a 8.8 be 10% better ?  Cheers, roverman.