have a wide variety of battery layout and placement
options depending on class type. Some of the common
battery placement designs center all the batteries toward
the middle, use saddle pack configurations, or position
the battery to one side with electronics on the other.
To give you an
idea of weight differences between the two battery
technologies, let's use some common packs. The weight
difference alone between 2-cell LiPo (Orion 3600 at 224
grams) and 6-cell NiMH (Orion 4200 SHO at 428 grams) is
quite significant. The difference (204 grams) needs to be
addressed (weight will vary depending on packs used).
placed battery layouts like the ones in offroad 2wd
buggies and trucks, things tend to be slightly easier. It
can just be a matter of working on a new set-up that
reflects the new lighter weight, but as many of you know
finding that "magical" set-up isn't always that
easy. Almost everything is affected to some degree
including suspension, aerodynamics, tires and wheels,
acceleration, braking, etc.
A good example
of change is Ryan Cavalieri's AE B4 set-up. His buggy has
always been set up more or less the same with just a few
minor exceptions, and has always run with NiMH until
recently. As the rules for weight minimums change and his
choice of power shifts to LiPo, Ryan has made some unusual
changes to his buggy and will continue to make more. For
example, rear shock location has changed and there is an
inner hole drilled in the tower, and Ryan now uses a
spring that is lighter by one rate. Since the rear end of
the car is lighter and has less weight to stabilize it, he
also uses .5 degree hubs to compensate. Ryan still has to
add weight to meet the new rules, and the placement of the
weight is showing up in different areas, but overall his
buggy is lighter.
touring car chassis can be more of a challenge with most
designs sporting six rectangle spots (five in European
designs) all in line on one side of the car for your sub-c
cells (there are some exceptions, but for 99% this holds
true), with electronics and everything else on the other
side. There is a reason for this and you have to remember
that most touring cars were developed back when NiMh was
the standard. Now that LiPos are becoming standard, racers
have had to improvise to maintain the same balance. They
have used a wide variety of solutions to accomplish this.
It's not unusual to see weight added behind the battery,
in front of the battery, or directly to the battery with
double-stick tape or other fabricated piece (in this case
a scrap piece of lexan designed to work with the standard
strapping tape). One common and possibly easiest solution
is to add weight to the cut-outs in the chassis. This
keeps the center of gravity low and makes use of unused
space, but also has some limitations depending on car
design and clearance.
Some cars even
have specific places for adding weight to balance. The new
TOP Racing "Photon" takes this to another level
and incorporates the need to add weight by strategically
positioning chassis-specific weights in key areas. The
point is, you can't just switch batteries and run that way
and expect similar handling. Instead, plan your transition
to LiPo carefully and realize that the change will have
some side effects. You will also need to plan for a bit of
trial and error with your set-up since many of the factory
set-ups are based on the NiMH heavier cars.
Rules for events and organizations often play a key role
in planning out your solution to the weight problem. If
you plan to do big races where rules dictate the direction
of your set-up, you may want to keep that in mind and
practice with the same restrictions you will encounter.
Most of our current rules are based on cars running with
NiMH, and only recently have the rules begun to change to
reflect the use of LiPos with only limited changes to
bridge that weight gap. Weight rules for electric are
changing fast and can be confusing with their variances.
Remember too, rules are constantly changing, so check
often and check with the governing body if you're unsure:
Example of Minimum Weights for ROAR 2009
Touring Car Foam 1418 grams
Touring Car Rubber 1500 grams
2wd Buggy 1499 grams
4wd Buggy 1613 grams
Power-to-weight ratio (specific power) is a calculation
commonly applied to engines and mobile power sources to
enable the comparison of one unit or design to another.
Power-to-weight ratio is a measurement of actual
performance of any engine or power sources. It is also
used to measure performance of a vehicle as a whole, with
the engine's power output being divided by the curb weight
of the car to give an idea of the vehicle's acceleration.
The power-to-weight ratio (Specific Power) formula for an
engine (power plant) is the power generated by the engine
divided by weight of the engine as follows:
P-to-W = P/W
A typical turbocharged V-8 diesel engine might have an
engine power of 250 horsepower (190 kW) and a weight of
450 kilograms (1,000 lb), giving it a power to weight
ratio of 0.42 kW/kg (0.25 hp/lb).
Knowing this, if you were to take a 6-cell NiMH pack and a
2-cell LiPo pack with the same power output, considering
that a LiPo weighs significantly less, you would have a
big advantage bettering terms of power-to-weight ratio.
Ah, but this is where things can get even more interesting
since we all know the new LiPo technology means an
increase in power as well.
Recently I was able to catch up with Atsushi Hara and get
his thoughts about the state of touring car design. Over
the last few years, touring car design has started to
plateau with most designs seeing the battery and
electronics in basically the same configuration. With LiPo
battery technology, Hara feels that we are on the verge of
a big change in car design. With dropping weight minimums,
he feels we could see some pretty unusual stuff or may
even adopt some older designs that didn't work quite as
well with NiMH. As he points out, there really isn't a
reason why we should be carrying around so much "dead
weight" in order to balance out our cars. Will we see
touring cars with batteries in the center again, or even
have them running sideways? Time will tell, but it could
make for some interesting designs in the near future.
OK, now that you know a little bit about what to expect,
get to it. Yes, you will feel a difference, but you can
always start by trying to replicate the weight and
placement of your car in NiMH form and make the transition
to the lower weight through testing. Ask any engineer�he'll
tell you that light is faster on a track, so take
advantage of it. It's only a matter of time before minimum
weight rules drop and you'll be ready.