My bumpers project news

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Benoit,
You are right in assuming that a bumper that is stiffer than the
chassis rails may cause more damage than good.
This is what I have in mid for a new front bumper project - a strong,
heavy duty cradle for the winch, which will strengthen the front
chassis rails, and light sections on each side to give some protection
to the lights and front quarters. That way an impact to a side will
not pivot the chassis frame, although it will not entirely prevent
damage to the bodywork. But that's a lesser problem than a bent
chassis.
I am not, however, sure about mixing aluminium and steel for the
bumper mount. A heavy steel structure at the front will vibrate on
rough roads. Because aluminium is stiffer, compared to steel it is
also easier to break under stress. I think making some calculations
and analysing stress points would be a good idea for your experiment.
--
Rgds,
Roman (London, UK)
'92 HDJ80 (auto)
On 6/6/07, Benoit Bernard <[Email address removed]> wrote:
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Hi Guys
If the front chassis bends from an impact can it be repaired or is the
vehicle dead.???
How can you tell what size, or thickness bumper is suitable.???
Is it usual that if the bumper is made of thicker metal than the chassis,
the chassis will bend.???
How can you calculate the force needed to bend the bumper or the chassis.???
Would longer thinner brackets than the bumper is mad of work to protect the
chassis and if so how can you calculate what thickness brackets to use
compared to the??? bumper and chassis.
john 92HDJ 80 1HDT
 
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Hmmm, anyone with a degree in latticework dynamics?
--
Rgds,
Roman (London, UK)
'92 HDJ80 (auto)
On 6/6/07, John Byrne <[Email address removed]> wrote:
 
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John
You can definitely straighten out a bent chassis. A vehicle repairer
would put it on a jig which both holds the vehicle and provides a
template for the final position, he would then use hydraulic jacks
and/or turfors (or similar) and/or big hammers to pull and push it back
into shape.
How good is the end result? Well, that depends upon how badly damaged
it was in the first place and how skilfully it is repaired. A slight
twist can probably be corrected without any problems, but if the chassis
rails were buckled after a hard crash then it would be better to junk
the vehicle.
An insurance assessor would be the best person to ask - they make these
judgements every day.
As for calculating the force: you need the dimensions and shape of the
section, its wall thickness and the material properties. I could
estimate them for you for a given section, but there is little point
because it would only be an estimate and would depend very much upon the
direction of the loading and the restraints on the section.
Remember also that if you bolt a massive bumper onto the front any force
it collects has to go somewhere, and that will be the chassis rails.
Bumper is cheap to repair, chassis is expensive, so the ideal bumper is
one that is strong enough to take everyday loads, which could be quite
large if you plan to fit a winch, but not so strong that the chassis
will be damaged in a minor bump.
The traditional way to protect the chassis at the bumper connection is
to bolt the bumper into a slotted hole so that bumps simply push the
bolt backwards in the slot, absorbing the energy in friction. You could
make a DIY shock absorber out of all sorts of things - wood comes to
mind as the cheapest and easiest.
Dare I ask: have you bumped it (seems unlikely given the injection pump
problems...) or are you thinking of buying a crash-damaged one? If the
latter PLEASE get some professional advice!
Christopher Bell
|
| Hi Guys
| If the front chassis bends from an impact can it be repaired or is the
| vehicle dead.???
| How can you tell what size, or thickness bumper is suitable.???
| Is it usual that if the bumper is made of thicker metal than the
chassis,
| the chassis will bend.???
| How can you calculate the force needed to bend the bumper or the
chassis.???
| Would longer thinner brackets than the bumper is mad of work to
protect the
| chassis and if so how can you calculate what thickness brackets to use
| compared to the??? bumper and chassis.
| john 92HDJ 80 1HDT
|
|
| --
| European Land Cruiser Owners Mailing List
| Further Info: http://www.landcruisers.info/lists/
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Aluminium extrusion would make a better crumple zone but large section solid
aluminium end on (in compression) will transmit the forces back as well or
better than say steel box section of similar strength. Also consider
corrosion from dissimilar metals when bolting the al to steel.

Malcolm Bagley
Stafford UK
FJ45 '75 & FJ45 '76

From: [Email address removed] [mailto:[Email address removed]] On
Behalf Of Benoit Bernard
Sent: 06 June 2007 13:36
To: [Email address removed]
Subject: Re : [ELCO] My bumpers project news

Thank you Roman, I did not specify all the mounting idea yet but yes the
thinking you do below is come to me too. So I still have corner steel in 5mm
to support up/down bumper vibrations, but 10mm aluminium for any front
shocks loads. I here that aluminium is stong as steel but only for each
kilogram so that mean that if it's more lighter than it is not as strong.
----- Message d'origine ----
De : Roman <[Email address removed]>
=C0 : [Email address removed]
Envoy=E9 le : Mercredi, 6 Juin 2007, 11h36mn 49s
Objet : Re: [ELCO] My bumpers project news
I am not, however, sure about mixing aluminium and steel for the
bumper mount. A heavy steel structure at the front will vibrate on
rough roads. Because aluminium is stiffer, compared to steel it is
also easier to break under stress. I think making some calculations
and analysing stress points would be a good idea for your experiment.
--
Rgds,
Roman (London, UK)
'92 HDJ80 (auto)

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Hey Christopher
Thanks for that info.
As usual I am very curios when certain topics come up that I can get to
grips with.
I very much like the idea of the slotted holed brackets.
I already did my OEM bumper in last year when playing off road.
And as usual Toyota wanted 750 euro just for the centre bit without all the
other bits.
So i had a guy make me one so thats why im so nosey.
How strong is the front end chassis, would it bend in a minor bump.
Why dont they make them of stronger steel.
john 92HDJ 80 1HDT
 
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John
Glad to hear you aren't buying a wreck.
How strong? Strong enough for most people most of the time.
Why don't they make them stronger? Cost and weight.
"Cars" are limited to about 3.5 tons max gross weight in most places,
and the 80 is already at 3 tons. Also more weight needs heavier
springs, bigger brakes, bigger engine, more fuel, etc etc.
The trick is to build the lightest and cheapest structure to do the job,
which is why most modern 4x4s use a monocoque chassis rather than a
ladder one - it's much more efficient.
Also, even back in the 80s, cars had to pass crash test legislation
which involved driving head on into a solid wall at 30 mph. If you made
the chassis *really* strong it wouldn't give, resulting in the
unfortunate consequence that the car body would go from 30 to 0 pretty
much instantaneously, but the passengers wouldn't. So the speed
difference between occupant and dashboard when they met would be
extreme. For the passengers to survive you need to soak up the impact
in the engine bay, which necessarily means deforming the chassis.
I think you've got the wrong vehicle John, you'd be *much* happier with
a chieftain tank! They are a bit cramped inside, and heavy on fuel, but
strength isn't a problem.
CB
| Hey Christopher
| Thanks for that info.
| As usual I am very curios when certain topics come up that I can get
to
| grips with.
| I very much like the idea of the slotted holed brackets.
| I already did my OEM bumper in last year when playing off road.
| And as usual Toyota wanted 750 euro just for the centre bit without
all the
| other bits.
| So i had a guy make me one so thats why im so nosey.
| How strong is the front end chassis, would it bend in a minor bump.
| Why dont they make them of stronger steel.
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Never heard of a chieftan with vibrations on the drive system or brakes so
perhaps your on the right track. (opps sorry)
I suppose with a couple of hundred thousand euros work on the engine you
could run biodiesel (actually I suppose the MOD spec multiple fuel types
anyway).
Malcolm Bagley
Stafford UK
FJ45 '75 & FJ45 '76
I think you've got the wrong vehicle John, you'd be *much* happier with
a chieftain tank! They are a bit cramped inside, and heavy on fuel, but
strength isn't a problem.
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I used to have a brother-in-law who was a cavalry officer, and he took
me on a tour of "his" chieftain at Tidworth.
It was absolutely vast, yet when you climbed down into the turret basket
there wasn't room to swing a kitten. And the ergonomics were awful:
boxes with knobs sticking out everywhere, it must have been a nightmare
to travel in.
It ran on red diesel from a very ordinary looking pump, although I
*think* they had a "multi-fuel" engine that would burn pretty much
anything. Don't know quite how - spark plugs? Two-stroke? Anyone know?
Reliability was apparently well up to contemporary British Leyland
standards, and fuel consumption (when it ran) up to 5 gallons per mile.
CB
|
| I suppose with a couple of hundred thousand euros work on the engine
you
| could run biodiesel (actually I suppose the MOD spec multiple fuel
types
| anyway).
|
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Hey Christopher
So the passengers in a chieftain tank would fair less well than me in my old
cruiser because they dont have any crumple zones.
Can you attach alu to steel or weld together.
Just thinking if the bumper is steel and the brackets are alu they will bend
and absorb some impact.
So would it be best to make a bumper out of what ever the chassis is made of
with slotted bolt holes on the brackets or slight cuts so they will fold in
an impact.
Would steel brackets or Alu bend or break. and does the thickness matter to
when they will bend or break.
Is the chassis 5mm or 6mm, isn't that the thickness of the ARB or other
aftermarket bumpers or have I got it wrong.
john 92HDJ 80 1HDT
 
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John
Last year I had the pleasure of spending a week with a US armoured division
at Fort Hood in Texas. Their cruiser of choice is the Abrams tank. Cramped
like the Chieftan, hot inside, noisy when firing the cannon or machine gun
but amazingly agile and fast when it works. It's powered by a gas turbine
engine so whistles rather rattles but very impressive over rough ground at
40+mph.
One reassuring fact for you - they have more mechanical problems with their
Abrams tanks than you've ever had with your Cruiser - believe me.
Jeremy
On 6/6/07 15:33, "John Byrne" <[Email address removed]> wrote:
Jeremy Llewellyn-Jones
Mob: 07831 458 793
--
 
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John
| So the passengers in a chieftain tank would fair less well than me in
my old
| cruiser because they dont have any crumple zones.
Absolutely correct. The 30mph "head on into unyielding barrier" would
kill all the occupants. This is also why side-impact protection for
cars is not performed simply by welding armour plating into the doors:
the passengers would simply be hit by armour plate!
| Can you attach alu to steel or weld together.
Yes, using a bolt. Normal practice is to insulate the connection to
prevent electrolytic corrosion.
Also, apparently (I did some Googling) you can weld them if you use the
correct welding rod, although the process is more like soldering.
Doesn't sound like a terribly good idea to me...
| Just thinking if the bumper is steel and the brackets are alu they
will bend
| and absorb some impact.
Anything will bend if you push it hard enough. I don't see any
advantage in using aluminium since the size of the pieces means that any
weight saving will be minimal. You will get corrosion problems, and
aluminium is more expensive and less ductile than steel.
There's nothing magic about aluminium!

| So would it be best to make a bumper out of what ever the chassis is
made of
| with slotted bolt holes on the brackets or slight cuts so they will
fold in
| an impact.
That's what I would do. I'd design them so that under tension, ie pull
from a winch, the bolts were up against the stops so would transmit the
load with no give. Under compression they would slide backwards in
their slots.
| Would steel brackets or Alu bend or break. and does the thickness
matter to
| when they will bend or break.
Anything will break when pushed or pulled or bent beyond its failure
point. How it fails depends on the shape, thickness, material and the
loading ... that's what mechanical engineers (eg me) are paid to
calculate. (And no, sorry, I'm not volunteering to design a
connection!)
Broadly speaking in tension, straight pull, strength will be a direct
function of the smallest cross-sectional area of the connection, pretty
much regardless of its shape. So to calculate the tensile strength of a
bolt:
- Take its smallest diameter, which is the inner diameter of a thread,
and get area from Pi x D / 4
- Find the tensile stress of the material. For a grade 4.6 bolt that is
400 N/mm2
- Stress (which is force / area) times area =3D force, which is the
tensile capacity.
So a 12mm grade 4.6 bolt, which according to my section book has a shank
area of 113mm2, will fail at 113 x 400 Newtons =3D 45,200. 10 Newtons is
roughly the weight of 1 kg, and there are 1000kg in a tonne, so that
bolt will fail at about 4.5 tons in tension.
The ".6" in grade 4.6 means that the yield stress is 0.6 times the
tensile stress, ie 240 N/mm2, and normal practice would be to use half
of that as a safe working load. So although that bolt will fail at 4.5
tonnes in tension, the safe design load for it is 0.3 of that, or about
1=2E35 tonnes.
That's in tension (pull) remember.
Now let's turn to the slotted hole I suggested. In this case the load
is trying to slice the bolt in half, which is what an engineer would
case "shear", and the safe working stress for a grade 4.6 bolt in shear
is just 80 N/mm2, or 2/3 of its safe stress in tension, because shear is
a more onerous loading for steel. Do the maths (stress x area) and you
end up with a safe working load of 0.9 tonnes for our bolt.
Ooh-err, that's starting to look a bit weak, so either we need lots of
them, or larger bolts, or we need to move up to a higher grade of bolt.
Fortunately one can buy grade 8.8 bolts (failure stress 800 N/mm2, yield
0=2E8 of this, ie 640, working tensile stress 320 N/mm2, working shear
stress 187 N/mm2. So a grade 8.8 12mm bolt is good for about 2.1 tons
in shear.
I haven't seen the ARB bumper design, but I rather suspect that it is
attached at each side by two or more grade 8.8 12mm bolts.
I could bang on, but I hope you are getting some idea of the design
process for a connection. I haven't considered the parent chassis
material or the connection geometry - that would have to be analysed too
and possibly the chassis would need to be reinforced.
Sorry John: that's a small part of bolted connection design compressed
into a few paragraphs. The question may be simple enough, but the
answers aren't! Most home projects opt for major over-design because
the cost of the extra material is trivial for a one-off, it's only the
manufacturers who might churn out a million copies who try to optimise
these things.
Incidentally all the numbers above came from a standard "section" book
for structural steelwork.
| Is the chassis 5mm or 6mm, isn't that the thickness of the ARB or
other
| aftermarket bumpers or have I got it wrong.
Dunno. The last time I stuck my head underneath my chassis looked
thinner than that. Remember that the ARB bumper needs to spread the
load from a winch to the chassis (bending) which then takes it in
straight tension.
CB
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Hey Christopher
Or should I be calling you Saint Christopher.
I own you another pint if we ever meet without the cruisers of course ,
cant drink and drive, well at least not very well.
Thanks for your time and patience. as usual I have learnt a lot but some is
too techy for me so ill have to read it again and again.
john 92HDJ 80 1HDT
 
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