I’m checking out the repair manual for the MGB I’m buying. The MGB had two types of differentials. The early “banjo” type is “three-quarters floating”. The later “tube” type is “semi-floating”. I’ve heard, in reference to other vehicles, “full floating”.
What does “floating” mean in reference to differentials?
It has to do with how much the actual axle shafts support.
Obviously, you have an outer housing, and inside that ride the axle shafts. These transmit the power from the “pumpkin” in the center- the ring and pinion- to the outer flanges, where the wheels are attached.
In most standard automotive axles, there’s a bearing and oil seal at the outer end which supports the shaft in the housing. At the inner end, the splined end of the axle slips into the ring-gear carrier, itself supported on bearings. The axle is typically retained in the system, by a clip on the splined end, that holds the whole mess in place.
This is a “semi floater”. If the axle breaks in the middle- or really, anywhere outboard of the splines- the axle, and thus the rim and tire, can physically leave the vehicle, typically with semi-catastrophic results.
A “3/4” floater puts the retaining mechanism, usually a flanged bearing, at the outer end. So if the axle shaft itself breaks, the rim is not ejected and control is maintained.
A “full” floater goes one step more- it actually puts an entire freewheeling hub and bearing assembly, with inner and outer races on a hollow spindle, at the outer ends of the axle. This hub and spindle actually support the full weight of the vehicle. The axle shaft slides through the hollow spindle and engages the gear carrier with splines on the inner end, and typically a flanged plate on the outer end.
In this case, if the shaft breaks, the rim and tire are wholly unaffected, other than it’s not being driven anymore. You can, in fact, take one axleshaft out entirely (and assuming a locking differential or 4-wheel drive) still drive under power.
This site on rear axle tech explains the difference between semi-floating and full-floating axles on 4X4s. This site has diagrams. (it’s floating axles not diffs)
Okay, so if I’m interpreting the diagrams and descriptions correctly the semi-floating axle has a bearing inside of the axle tube that supports the differential inside of the “pumpkin”, and another bearing at the outboard end of the axle tube that supports the wheel hub. In a full-floating axle the differential is supported in the same way as the semi-floating one but the outboard bearing is in the wheel hub and there is another bearing between the hub and the axle tube. Did I get that right?
Now the 3/4-floater has another bearing that supports the differential. I’m not seeing it in the diagrams I’m looking at.
Here is the three-quarter floating “banjo” axle.
And here is the semi-floating “tube” axle. (Note: The first one is called “banjo” because of its shape. I’m not sure why the other is a “tube”.)
In the 3/4-floater, where is the bearing that supports the differential?
(See, this is why I have to pay mechanics to work on my cars. :o )
I hope I’m interpreting this correctly, Johnny. (Just an amateur mechanic)
In your link to the banjo type, part #8 is the bearing which supports the diff.
stems from this in the second site from my post above :
This is merely saying the bearing supporting the diff is the same as the one for the semi-floater and goes on to say the difference is in the outer bearing.
Damn Johnny, its been a while since I had to pull any MGB technical info out of this brain.
IIRC all roadsters used banjo axles until 1968 when they went to the tube axle. I’m not 100% sure but I think all GT’s had the tube axle from start of production (1965) If GT’s had banjos at the start then they too changed over in 1968.
On an MGB semi floater a break in the axle will not cause the wheel to come off. Looking at the picture in the link Johnny provided the axle shaft (86) has a flange on it near the outer end. The wheel bearing (88) cannot slide inward past this flange. The wheel bearing is captured into the cap (89) which is bolted onto the flange of the axle housing. A break in the middle of the axle would not cause the hub and wheel assembly to come off the car as the wheel bearing is captured by the cap and flange on the axle.
both axles are fairly reliable in service, the only time when banjo/tube is important is when buying parts for the axle (I don’t recall if the wheel bearings are the same, I suspect not.)
Anyway if you have any further questions about MGB’s Johnny, feel free to drop me a line. I’ve owned them and worked on a bunch of them.
Thanks, Rick. This really wasn’t an MG-specific thread; the MGB was just what got me thinking about the terms.
I may have to contact you eventually about the new acquisition though. In the 1980s I had a 1966 MGB that my mom bought new, plus two '77s I bought used. They do need knowledgable people around them! (And I’m not one!)
Come to think of it, I do have an MGB question, which I’ve asked [url=“http://boards.straightdope.com/sdmb/showthread.php?threadid=143650” in the other tread.
I’ve heard that your classic Chevy rear axle is a semi-floater, whereas the rear axle in my old Plymouth Duster (Chrysler corp) I know was a 3/4 floater.