With technology comes change, and today's switch from NiMh to LiPo is a big one. Unfortunately, in many cases making the switch hasn't been that easy, hasn't been that easy, because it affects the car in terms of weight distribution. It more or less affects the handling of the car based on how much it relates to weight distribution. What makes the task even more difficult is our current cars original design. Most cars have been engineered with the long held tradition of sub-c cells, and rules for the most part still follow those guidelines. As we have seen over the last few years, rules have been changing (albeit rather slow) and there have been many fixes to try and accommodate the new technology with only a certain degree of success. In order to make the switch easier, you have to understand what you are dealing with and how to respond. OK chubby, let's lose some weight, add some power, and drop those lap times.

Issue 163 (June 2009) - Words: Carl Hyndman

Today's cars 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).

For center 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.

Today's 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 Weight
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
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.

Hara's View
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.