Subject:
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Re: Mini-review: 8457 Power Puller
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Newsgroups:
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lugnet.technic
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Date:
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Mon, 28 Aug 2000 04:11:58 GMT
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Viewed:
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1278 times
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I missed the original post, but FWD pullers wouldn't work well. There's
several things that are better about RWD for this type of competition.
Natural torque caused by the drive wheels causes them to want to lift the
front end, lessening their effectiveness. From a physics point of view, you
can think of the wheels trying to rotate the car around the driven axle. If
you are going forward, this means the tires are trying to lift the front of
the car and are trying to push down on the rear of the car (try it, make a
vehicle with only two wheels in the middle attached to a long beam and see
what happens when you try and go forward, the front of the beam goes up, the
back goes down, when the back of the beam hits the ground, the whole thing
then starts to go forward). If the drive wheels are at the back, this is a
good thing, the torque of turning the wheels causes the wheels to be pushed
down harder (if you've got enough power, you can lift the front wheels off
the ground this way). If the drive wheels are in the front, the is a bad
thing, the torque of turning the wheels causes the wheels to lift up off of
the ground (the front wheels start bouncing up and down and with each hop
you are loosing power)
Think about this, what happens when you are sitting in your car (FWD or RWD)
and stomp on the gas? The front end rises up and the car lunges forward ...
depending on the amount of power and FWD/RWD, you may break the tires loose.
It's much easier to break the tires loose with FWD because the front of the
car "gets lighter" and the rear "gets heavier" due to this effective weight
transfer. If your car is RWD, you can take advantage of this, put shocks on
the front that allow it to go up quickly, but come down slowly (also known
as 80/20 or 90/10 shocks, drag racers use these, they don't work well on
street cars) and you can effectively launch your car harder because more
downward force is applied to the rear wheels.
FWD *cars* are good because it reduces weight (no long drive shaft, nothing
of use at the back but two support wheels) and all the mechanical components
are in one place. But most passenger cars aren't designed for performance,
performance is 3rd or 4th or further down on the list of goals
FWD for just about anything else is a bad idea.
You don't see many FWD race cars (think about top fuel dragsters, or the
tractor pullers that got this thread started) for this very reason.
If anyone wants to hear more, reply. There are reasons why RWD tractor
pullers *try* to keep the front wheels off of the ground (all the weight is
on the driven wheels and none is "wasted" on the front wheels, they only let
the front wheels down when they need to steer). There are reasons why top
fuel dragsters are as long as they are, etc. All these and more go back to
the same argument, you want as much of the vehicles weight and created
forces pushing the driven wheels down to maximize the energy created.
Mike
--
Mike Faunce
mike at faunce dot com
LUGNET #96
"David C. Pyatt" <dcpyatt@netzero.net> wrote in message
news:FzzD5H.4Mo@lugnet.com...
> In lugnet.technic, Ran Talbott writes:
>
> > Is there a rules requirement for rear wheel drive? This seems like a • natural
> > for FWD, with any of the minimal steering required done by the rear • wheels.
> >
> > Any benefits to RWD I can think of (like, say, using the load to • increase
> > traction) seem to be outweighed by costs (like increasing the friction • between
> > the load and the ground).
> >
> > It'll be interesting to see whether a Technic equivalent of the NTPA • starts.
> > Especially if it tries to make the competition more realistic by dropping • the
> > wheels off the load. You want some seriously-tough competition? Try • dragging
> > the load across a field of baseplates ;-)
> >
> > Ran
>
> There are 4x4 truck and 4x4 stock tractor classes when the farmers bring • out
> their stock tractors... I don't think there is any rule against FWD. but I
> would think that the load shift up the transfer sled would pick the front
> tires off the ground before you could get a full pull. Figure that the • weight
> is transfering off the wheels on the sled and on to the hitch point and • the
> rear wheels...
>
> I actually wanted to build a tractor/sled with technic before the Power • Puller
> was announced... sort of bummed me out when they released it! :)
>
> Dave
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Message is in Reply To:
 | | Re: Mini-review: 8457 Power Puller
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| (...) There are 4x4 truck and 4x4 stock tractor classes when the farmers bring out their stock tractors... I don't think there is any rule against FWD. but I would think that the load shift up the transfer sled would pick the front tires off the (...) (24 years ago, 28-Aug-00, to lugnet.technic)
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