Tippy Feeling (Pucker Factor)

Date: Wed, 9 Apr 1997 9:17:43 -0500
From: billm@usa.compass-analysis.com
Subject: That Tippy Tire thread...
To: toy4x4@tlca.org

>Go for the 35's.  I have 35's on my toy with 7 inches of lift and it is
>perfectly stable, it barely even leans on winding back roads doing at
>least 40mph.

In my opinon, the size of the tires really have nothing to do with how
stable the truck is, it's the stiffness of the suspension. With my
soft springs, the body on my truck LEANS. It's not really that big a deal,
it really more of a psychological thing.

 - -----------------------------------------------------------------
 Jack Alford              Off-Road.com - The best dirt on the net!
 jalford@off-road.com              http://www.off-road.com/
 Decatur, AL


This is not to incite flames or anger but:
Actually the tires have a lot to do with stability Jack.

When you raise a vehicle, you immediately change it's center of gravity.
A body lift changes it the least as the heaviest parts of a vehicle are
not raised (engine, tranny & xfer case, AXLES).  A suspension lift raises
everything a little, except for the axles.  Putting on the big tires
though... that's the killer as this is the ONLY modification to a vehicle
that does two things; 
1) raises the diffs for better ground clearance and
2) Raises every single component of the vehicle.  What this does is
removes "X" number of pounds below the originally engineered "center
line" of the before and moves it up making the bottom lighter and the
top heavier, exponentially.  It's a little like sprung and unsprung
weight in a suspension.  Now,.. if the tires turned at the same speed,..
the gyroscopic effect of them would assist in stabilizing the vehicle a
bit, at high speeds, but they spin a lot slower, so you don't even get
that.  There is a point in tire size where you can have your 4runner or
whatever just tip over at slow speeds.  Witness Monster Trucks.  and
quick moves and they're flipping, even with axles and assemblies that
weigh literally HUNDREDS if not THOUSANDS of pounds heavier.
FWIW
 -BillM
PS If anyone thinjks this sounds obnoxious or anything,.. it's not (i've
seen people here flamed for less) and if you must flame me.. flame me at
my email address as this isn't the forum.
Thanks
 -Bill Miranda

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Date: Wed, 9 Apr 1997 12:25:09 -0500
From: Jack Alford 
Subject: That Tippy Tire thread...
To: toy4x4@tlca.org

I wrote:

>In my opinon, the size of the tires really have nothing to do with how
>stable the truck is, it's the stiffness of the suspension. With my
>soft springs, the body on my truck LEANS. It's not really that big a 
>deal, it really more of a psychological thing.

Bill Wrote:

>This is not to incite flames or anger but:

None taken...

>Actually the tires have a lot to do with stability Jack.

I will agree with you that adding a larger set of tires does raise
the center of gravity but I will say that I've ridden in plenty of trucks
with BIG (36"+) tires and the truck felt much more stable than my truck
with my relatively small 33" tires. While raising the center of gravity
does play some part in it, like I said before, the softness of the suspension
cannot be overlooked either.

 -----------------------------------------------------------------
 Jack Alford              Off-Road.com - The best dirt on the net!
 jalford@off-road.com              http://www.off-road.com/
 Decatur, AL

             '86 Xcab Toyota Pickup -  33x12.50 BFG MT
         Solid Front Axle - Marlin Crawler - ARB's - 4.88's
         SFWDA  -  TLCA #3415  -  Rocket City Rock Crawlers

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Date: Wed, 09 Apr 1997 16:41:37 -0400
From: WartHog 
Subject: That Tippy Tire thread...
To: Toy4x4@tlca.org

Jack Alford wrote:
> 
> I wrote:
> 
> >In my opinon, the size of the tires really have nothing to do with how



> 
> Bill Wrote:
> I will agree with you that adding a larger set of tires does raise
> the center of gravity but I will say that I've ridden in plenty of trucks
> with BIG (36"+) tires and the truck felt much more stable than my truck



My question(s) is/are:

Were these tires installed on rims with standard width/offset, or were
they installed on rims of greater width and/or less back spaceing?

Going from, say, a 6" rim to an 8" rim with simular back space will
increase track 4" ( depending on tires, keeping same rim to tread width
aspect ). 

Going from a 6" to 8" rim, and increasing back space 1" ( ok, so I'm
cheating on the math here ;) will still increase the track 2".

Going from stock rims to a 10"er ( a guess on 36" tires ), I feel the
track can't help but be increased. The ratio between the raising of cg
and track increase is the key here. 


IM(V)HO

Mark

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Date: Wed, 9 Apr 1997 14:29:34 -0700
From: Eric Johnson 
Subject: That Tippy Tire thread...
To: "'Toy4x4@tlca.org'" 

I think this discussion of stability could borrow from the physics of 
involved with another hobby of mine, sailing. (bare with me!)

When discussing sailing yachts, there is often the discussion of 'initial' 
and 'ultimate' stability. Initial stability is a boats resistance to 
rocking from side to side. Boats with a high initial stability tend to 
beamy (wide), with flatish bottoms. A pontoon boat or catamaran, for 
example, generally has great initial stability.

Ultimate stability is the ability of a boat to right itself when pushed 
over hard onto its side. Boats with deep, heavy keels have great ultimate 
stability, because their center of gravity is well below the center of 
buoyancy. Catamarans generally have lousy ultimate stability.

There exist boats with all four combinations of high and low initial and 
ultimate stability.

Why am I bringing this up? I think we can apply these concepts to trucks. A 
luxury car with an ultra-soft suspension and no sway bar is likely to have 
low initial stability; i.e. it will lean in corners, sway when the wind 
blows, etc. But it is likely to have high ultimate stability (be difficult 
to roll). On the other hand, a sky high pickup with RS9000s set for 5 is 
likely to feel real stiff (high initial stability), but with its CG so 
high, wouldn't be as hard to roll on an off-camber hill.

I'm not drawing many conclusions from all this, but I think it is a useful 
way to look at the problem. Any time you raise a vehicle (all else being 
equal, especially track) you are decreasing its ultimate stability, 
probably without affecting its initial stability. Does this make sense?
- --
- -- ej@blarg.net

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Date: Wed, 9 Apr 1997 15:40:25 -0700
From: Locke Christman 
Subject: That Tippy Tire thread...
To: "'Toy4x4@tlca.org'" 

Eric Johnson wrote:

>Any time you raise a vehicle (all else being 
>equal, especially track) you are decreasing its ultimate stability, 
>probably without affecting its initial stability. Does this make sense?
>--
>-- ej@blarg.net

Yeah, it does make sense.  Now if you relate this to Jack's comment
about his softer suspension, his rig may have less initial stability
(can feel body roll in the corners) but better ultimate stability than a
stiff sprung rig with equal center of gravity height and track width.

my $0.02 worth

Locke
>

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Date: Wed, 9 Apr 1997 16:33:10 -0700 (PDT)
From: Jonathan Albrecht 
Subject: That Tippy Tire thread...
To: Toy4x4@tlca.org

> >Any time you raise a vehicle (all else being 
> >equal, especially track) you are decreasing its ultimate stability, 
> >probably without affecting its initial stability. Does this make sense?
> 
> Yeah, it does make sense.  Now if you relate this to Jack's comment
> about his softer suspension, his rig may have less initial stability
> (can feel body roll in the corners) but better ultimate stability than a
> stiff sprung rig with equal center of gravity height and track width.

I don't agree.  If a truck has low 'initial stability', it is easier for
it to lean.  Period.  If you go around a corner at 'x' mph in two
identical center-of-gravity/height/width trucks, one with a low 'initial
stability' and one with high I.S., look at what happens:  The stiff sprung
one doesn't lean very much, and the center of gravity remains near the
center of the truck (side to side).  The softer sprung truck will lean
farther over, and hence it's center of gravity will move farther to one
side, and make it easier to tip. 

But a high center of gravity never helps things.  For instance, if you
take a short truck (like a CJ-5) and lift it 6", and put 35" tires on it,
but then put some really, really wide rims, and really, really stiff
springs and shocks on it, it will *feel* extremely stable.  The problem is
that the truck is very short, and the wider track actually reduces
stability at speed (increases the tendency to fishtail).  So in this case,
instead of tipping over, the jeep will fishtail very easy, and once it's
spinning, it'll roll over *really* easy, despite the stiff suspension and
wide tires.  And yeah, I've seen it happen. 


Jonathan Albrecht
albr9619@uidaho.edu
http://www.uidaho.edu/~albr9619/

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Date: Thu, 10 Apr 1997 12:04:50 -0500 (CDT)
From: twogrls@ix.netcom.com
Subject: Stability
To: Toy4x4@tlca.org

You wrote:
>Actually the tires have a lot to do with stability Jack.

>I will agree with you that adding a larger set of tires does raise
>the center of gravity but I will say that I've ridden in plenty of trucks
>with BIG (36"+) tires and the truck felt much more stable than my truck
>with my relatively small 33" tires. While raising the center of gravity
>does play some part in it, like I said before, the softness of the 
suspension
>cannot be overlooked either.

I would add that the width of the tires and the offset of the rims can more 
than compinsate for the additional height of the tire (Not excessive 
lift/tire size) A wider footprint will change your rollover piont the same 
way that the lenght between you axles (wheelbase) changes where you can go. 
As with everything about four wheeling you need to gently/carefully test 
your limits and then live within then. 

Footnote: I dont try a dangerous new hill with my kids in the truck and go 
gently the first time. Body damage can always be pulled -- kids cannot be 
replaced.
Also It may not look cool to walk up the hill or accross the river and see 
what you are getting into but it is always smart.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Mike Pelland					1994 Red Extracab Toyota
twogrls@ix.netcom.com				31X10.5 BFGs, Lockrite, RS9000;s            
San Antonio, TX 					4.88's,Downey SC clutch,
Tread lightly, TLCA Member, SWFWDA			Ramsey Winch,
 http://pw2.netcom.com/~twogrls/mptruck.html
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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