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RE: (ET) Tractor Pulls
In the real world the coefficient of friction under both rear wheels is
never the same. Not only will the ground surface at the contact patch be
slightly different, but weighting, air pressure, and tire wear will come
into play. How do I know? I never spin both wheels at the same instant -
its always one or the other. This implies that the one with the least
traction gets all the power, and that it rarely if ever is evenly divided.
Once a wheel begins to spin, some other dynamics like melting tires or ice,
or digging a hole to climb out of, take over to keep it spinning. Auto
hillclimbing is very traction dependent, and all the cars I've seen have
either limited slip (clutch) differentials or have them welded. It works.
It guarantees that no matter what the tracion conditions, the available
power will be evenly divided between the rear wheels. The only problem is
increased friction and wear (and possible surface damage) when turning
corners, but that's not a problem in a straight line pull. Almost all ATVs
have solid axles - mine has a solid axle in the rear and a lockable
differential in the front - 2 solid axles if necessary! True 4 wheel
drive.
Steers like a stone with the front locked. ATVs "solve" the problem of
different rear tire speeds in corners by using relatively soft sidewalls
and
low pressures that allow the tires to change diameters as the bike leans.
As you lean an ATV into a corner, the weight transfer of the rider to the
inside causes the inside tire to assume a smaller diameter (at least the
bottom 1/2) than the outside, at least in part solving the part. The rest
of the solution is to scrub the tread around the corner - so that's why
they
cause environmental damage.
Braking of the rear wheels has a downside - stopping one wheel completely
will transfer all the power to the other wheel, but it also attempts to
increase the speed of rotation. I used to have an old Bombardier (makers
of
ski-doos) tractor that used an auto axle to turn a dual rubber tracked 3
passenger tractor (like a baby snow cat). However, if the operator locked
a
track, the other spun wildly at times. It was always best to apply the
brake something short of locking the track, and I imagine the same would
apply to a pulling tractor. It is very difficult for an operator to
accurately control independent braking for traction purposes, but old VW
dune buggies used this system for years sucessfully, jut by adding a master
cylinder and levers so that each rear wheel could be controlled with a hand
lever.
Old lawn tractors probably did not use straight axles due to the turning
issue - sharp turns with turf or ag tires would tear up the lawn too much.
My old Gravely (1950's vintage) walk-behind has a differential, and Gravely
used them as long ago as 1939 in that model. They offered dual tires and
chains when the going got tough. My dad had lawn sized tractors back in
the
early 1960s (Jacobsen and Wheel Horse) which I remember getting stuck
frequently due to the %^$#$ non-locking differentials. they were exactly
the
same setup as the ET - a brake on the driveshaft meant you could watch a
wheel turn backwards with the brakes locked as you skidded down a hill!
Take goodness for regenerative braking. I remember seeing cheap riding
mowers that had a chain drive to a differential that was part of the rear
axle shaft assembly and mounted the rear sprocket. In fact, I used one to
make a go-kart as a kid. I would guess the really cheap riding mowers
might
have made due with one wheel drive, like the cheap go-karts.
I wonder how much trouble it would be to make drums or disks that mounted
between the wheels and the hubs using the current wheel mounting bolts.
Even a simple external strap brake (a contracting band around the outside
of
a drum) would help transfer power. Not the best, but it might be an edge
sometimes.
Anyone know the rules for some of these pulling classes? Does "stock" mean
you can't use duals on the driving wheels?
-----Original Message-----
From: elec-trak-bounces cosmos phy tufts edu
[mailto:elec-trak-bounces cosmos phy tufts edu]On Behalf Of Dave Reuter
Sent: Thursday, March 25, 2004 8:52 AM
To: Elec-Trak Tractor
Subject: Re: (ET) Tractor Pulls
As long as the coefficient of friction is the same under both wheels you
really don't need to weld up the diff. If it is sweeped concrete or asphalt
surface you're pulling on then you would be fine. If it is dirt or sand and
stone on hard surface then the welded gear set would have a huge advantage.
Brake biasing is a good traction control which is used by the auto
companies now days but it is at the cost of power to the wheels. Granted
you are transferring power to the non-spinning wheel but you're also
absorbing power from the drive unit in to the braking wheel brake as well.
I would weld the gear set before modifying the brakes if it is allowed. Or
find an axle that was manufactured with out a differential set. I notice a
lot of old 3 wheeler and some of the 4 wheeler ATV don't have differentials
in the rear axle, so maybe there is a cheap lawn tractor that doesn't have
a diff too.
Again this is just my personal feelings, I do not mean to knock anyone's
opinions. I have designed 4WD and AWD systems for a few years in my very
distant past before my enlightenment of HEV's, FCV, and EV's
Dave Reuter
"Pieter
Litchfield"
<plitch@attglobal To
.net> "Elec-Trak Tractor"
Sent by: <elec-trak cosmos phy tufts edu>
elec-trak-bounces cc
@cosmos.phy.tufts
.edu Subject
(ET) Tractor Pulls
03/25/2004 08:03
AM
Here's a tip based on some hill climbing and racing experience. If you are
thinking about dedicating a tractor to pulling, open up the rear end and
weld the differential spider in place. You now have a solid rear axle and
two wheels driving rather than one.
If there is a rule against this or you want to preserve the differential
for lawn work, it might be possible to mount a brake drum on each rear
wheel and at least be able to transfer power manually from the spinning
wheel by applting the brake to it.
Just a thought.....
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