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Bracing Diff housings - under or over?
Posted: Sun Aug 09, 2009 10:21 pm
by dank
Ok some people say that bracing the diff on top is better than below. Others say that you should brace the underside as steel under extension stress is stronger than steel under compression stress.
In the interests of this question we only want to brace one side. I know bracing both would be the strongest but if you had to choose, under or over and why in relation to strength, not worrying about clearance and other associated factors.
What is better?
Posted: Sun Aug 09, 2009 10:25 pm
by BIG GQ
I will be interested to see the outcome of this thread and here from the likes of Greg (1madengineer) , etc as this is of interest to both myself and mate at work.
The obvious down fall of underside bracing it clearance but lets here it
Posted: Sun Aug 09, 2009 10:52 pm
by ajsr
hey dank
if this is on your zook I run a lowrange one on top of the housing it means good clearance from the sump(ext bumpstops) and from our big vic ruts.
hows your motor swap going?
Posted: Sun Aug 09, 2009 11:06 pm
by dank
Hey andrew, Making progress mate check my members thread soon i'll update it.
Those in the know lets here it!
Posted: Sun Aug 09, 2009 11:22 pm
by -Nemesis-
Does anyone know of a top braced housing (properly) that has still bent through the braced section?
You would 'assume' the clearance benefits of top bracing would outweigh the under side method even if it were stronger.
Though I'm no expert, will be interesting to hear the arguements.
Posted: Sun Aug 09, 2009 11:48 pm
by bazooked
i used to do this for quite sumtime before i changed jobs, we got the sheeties down the road to bend up a u channel was about 3mm from memory, had templates already made up, depending what the customer wanted, most of them were underneath, but for the people i new who were into sum hardcore 4x4ing it went on top, dont forget they will need to be straightend after the job is done as ours were fully welded, ask cass jones how well they went, we did a fair amount for him.
Posted: Mon Aug 10, 2009 12:06 am
by alien
my front housing is done on top for clearance reasons... i cant see why above or below would make a difference personally - although leaf spring leverage would affect it - mine SPOA so above would mean leverage is on the top end.
Posted: Mon Aug 10, 2009 8:09 am
by AFeral
Just to expand the debate, what about bracing the diff from the front. A brace on the front can also be shaped like a skid as too allow the diff to push over rocks etc.
Posted: Mon Aug 10, 2009 8:42 am
by nastytroll
depends how extreme your going and how much room you have available.
I have done the top on the front of the gq/gu and bottum of the rear because of all the coil and trailing arm mounts. Its a daily so not really getting thrashed cliff hanger style.
I think if you still bent the housings you would need to adjust your driving style.
Underneath would be stronger but by how much in the situation before other catastrophic failures came into place.
Posted: Mon Aug 10, 2009 11:06 am
by ISUZUROVER
No difference in strength (assuming bracing above/below would be identical).
Steel is just as strong in elastic tensile and compressive strength. No need to consider strength in the plastic region, because by then your housing would be well and truly banana'ed.
Question - I have a question about steel properties. Steel is
generally defined by tensile strength. Is the compressive capacity the
same as the tensile capacity because steel is a homogeneous substance?
-----------------------------------------------
Steel is more or less a linear elastic material. Unlike concrete, which is
much weaker in tension than in compression, steel theoretically responds the
same way in either tension or compression. However, with enough applied
force, steel and other metals will cease to behave elastically and begin to
behave plastically. When a material is linearly elastic, its deformation,
or strain, will be directly proportional to the applied force and it will
return to its original shape when the force is removed. A plastic material,
on the other hand, will permanently deform without breaking (think of taffy
or perhaps the stringiness of melted mozzarella cheese on a pizza).
In real life, of course, there is no such thing as a perfectly elastic or
plastic material. In the case of steel, structural engineers are concerned
about the tensile strength in terms of both the ultimate strength and the
yield strength. When a specimen reaches its yield strength, it will begin
to stretch and transition from elastic to plastic behavior. As more force
is applied, the steel will reach its ultimate tensile strength and break.
Structural engineers take advantage of this property in their designs. In
an extreme event, such as an earthquake or major structural failure, this
plastic phase is useful because it allows the structure to sag and absorb
extra loads. If steel is too brittle, it will shatter instead of
stretching, possibly causing the structure to come crashing down. In either
case, the structure is ruined, but the plastic behavior of the steel allows
time for escape.
In compression, steel can still behave plastically. Think about how a coin
looks when it is run over by a train. This property allows metals such as
steel to be pressed into things like car bodies. However, in some
compressive situations, shear forces can also develop inside the specimen.
While steel behaves equally under tension and compression, depending on the
type of steel, it can be substantially weaker under shearing loads. In this
case, the steel may suffer a shear failure before reaching an ultimate
compressive strength.
Posted: Mon Aug 10, 2009 11:28 am
by alien
am i not correct in assuming leverage would play a part though?
ie: a SPOA leaf sprung vehicle vs SPUA leaf spring - the leverage on the tube is on opposite sides - so you'd gain/lose strength in the bracing depending on which side the perches are welded??
or am i overthinking it? lol
Posted: Mon Aug 10, 2009 2:41 pm
by Bush65
The mechanical properties of steel are determined from testing a sample in a tensile testing machine (there are other tests, but this the one which is applicable in this discussion). Then failure theories are used in conjunction with the mechanical properties for strength design.
If you compressed a non slender piece of steel in a press and stretched a similar piece in another machine, it would require considerably more compressive force than tensile force to cause the pieces to fail.
Where the stressed part is slender, it will fail by buckling well before the ultimate compressive strength of the material is reached. This is where the notion that steel is stronger in tension comes from.
Most diff bracing that is done well, should not fail by buckling, except if it is thin and takes a direct hit from a rock or the like, but not from bending loads.
If you braced on top, and the wall thickness of the axle tubes is thick enough, then the extra clearance (compared to bracing underneath) could be advantageous.
The best way to be able to run the bracing from the pumpkin to the ends of the axle tubes/knuckles, is probably more important than issues of tension or compression.
Whether local areas of the axle housing are in tension or compression will change for different loading cases. Landing hard on the wheels will cause tension on the underside of the tubes. Landing on a rock, between the spring mounts will cause compression on the underside.
Posted: Mon Aug 10, 2009 7:15 pm
by ludacris
I braced mine on top near the pumkin with 6mm box cut to shape and gussets on the balls 6mm plate cut to shape. Has not bent so far. You can brace the top and laminate the bottom if you are going over a lot of rock to keep clearance.
At a guess the GU housing I cut up was about 5 - 6 mm thick close to the balls.
LudaCris
Posted: Mon Aug 10, 2009 7:23 pm
by WICKED
Steel is for pussy's!
Cardboard is the new "it thing"..... 'sept for the water......
Posted: Mon Aug 10, 2009 7:43 pm
by alien
^^ so are bent tie-rods =P
(or wide angle photos)
Posted: Mon Aug 10, 2009 7:52 pm
by WICKED
alien wrote:^^ so are bent tie-rods =P
(or wide angle photos)
No, No bent tie rods are the Bomb Diggity 'Yo!
Posted: Mon Aug 10, 2009 8:35 pm
by Gwagensteve
On top IMHO Dank.
There is a case for in front/out back for high speed hits - Baja race cars are nearly always braced in front/back (but they are always braced everywhere too)
Steve.
Posted: Mon Aug 10, 2009 10:20 pm
by bru21
I think top is better as clearence is a problem when braced on the bottom.
I braced my gu on the front and top.
This is my new baby on order:
Posted: Tue Aug 11, 2009 2:48 pm
by oops
If you braced on top you should also be able to get more triangulation, and any beam like structure will add more stifness if it is in tension or compression than bending eg top or bottom as opposed to front or back. If there is enough cross section in the brace it shouldn't fail by buckling, From memory, if you want to analyse it try seaching johnson buckling theories, I think euler was only for long coulmns. Another thing to think about is welds, I would think that below the welds will get tensile loads (weak) wheras on top they will get compressive spikes (good as you are trying to squish metal into metal). This only means you get get away with less welds, on top for the same strength. You also gotta think where the weakest points on the the diff is as there is no point adding a heap of extra weight where it is not needed, as it is all unsprung weight and therefore detrimental to your cars handling. I'm reckon the bracing at the front of the diff is for frontal impacts as i can't see it being very efficient at adding stiffness from up and down loads.
Re: Beam/Diff Bracing
Posted: Tue Aug 11, 2009 11:05 pm
by clive2267
I hope this contributes usefully on how to brace a differential housing
The key is to add stiffness to the beam by increasing its cross section area as far away from the centre of mass as possible. For ground clearance you would add it to the top.
Any metal plate you add to the top will carry the most stress in, either compression or tension, the top edge will be the most stressed in compression and may deform if it not stiff enough. This is called local buckling - the material will form small waves along its length. Grab a thin strip of steel and bend it thin edge up - watch the inner or compression edge forms a series of small waves or buckles.
The best way to deal with this to stiffen the plate with a return stiffener return lip if is is a single plate - or better still use a 'top hat' - upside down U. This stiffens the top edge of the hardest working material and allows it to carry load without local buckling.
The other advantages are that the, sometimes brittle,welds are closer to the centre or neutral axis (least stressed) part of the diff. Secondly adding the top hat shape section to the top increases the strength front to back as well, as the sides will 'work' as flanges when the diff. is pushed back
The order Bru21 has placed for his braced (stiffened) differential look extremely well thought through and follow what I have been trying to describe. These mods look and deserve a good price.
Perhaps cutting the bottom of a standard rectangular hollow section, welding it legs down and and grading/shaping it down from the centre to the hubs. This would achieve a fair amount without requiring extensive tooling and shop work. Look for a section that has a form factor (Kf=1) means the tube walls will not buckle under under load
Hope this helps your decision making
Re: Bracing Diff housings - under or over?
Posted: Sun Feb 20, 2011 9:28 pm
by hokey
i've got a gq front diff out atm that i'm rebuilding to replace my bent one. don't want to go to the extent of getting my housing straightened so was just going to put a couple of plates at the nuckle ends.
wondering if its really worth the extra effort of doing the axle tubes? will bracing the nuckles help alot? the nuckle seems lIke the weakest part of the diff.
thanks
Re: Bracing Diff housings - under or over?
Posted: Mon Feb 21, 2011 7:43 pm
by bushy555
Do both.
Me front GU diff: 8mm channel on top. Rear GU diff: 8mm channel on bottom.
Left the oil in the diff whilst welding with a GMC $99.00 stick welder. Job done.
Re: Bracing Diff housings - under or over?
Posted: Mon Feb 21, 2011 8:11 pm
by Matt_85Lux
anyone remember this effort?
Re: Bracing Diff housings - under or over?
Posted: Tue Feb 22, 2011 2:11 pm
by Roctoy
they could hav at least put some dimple holes in it...
