Are people thinking about rc performance all wrong today?

[Jay Dub]

Firstly, when one scales weight, it is not a 2d variable that is scaled by the fractional amount (eg. a 3.25lb car would not be 3250lb in full size). The scaling of mass is a cubic function. The car would be much heavier.

Secondly, a lighter car will always be faster when properly tuned and driven. The phenomena we are currently seeing stems from 20 years of designing and tuning cars around a very specific weight envelope. When one deviates from that weight standard, there are bound to be issues until the chassis is returned to where is (through "breeding" so to speak) happiest. I guarantee that within the next 5 years, that all the factories will develop cars and parts to optimize their laptimes at the newer, lower weight limits.

I for one am all for the lower weight and less power argument. I recently stopped by my local track to watch the ROAR Off Road nats. The cars looked fast, handled well, and went through a set of tires EVERY run. The weight of the car, and the available power is why this is such an issue (I know tire compound has something to do with this btw). I think in the best intrest of the "racing" hobby, the cars should be much lighter, and have to use lower traction spec tires -smaller power plants will follow naturally. -Jeff

[fredswain]

I weighed my RC10 last night with body, wing, wheels and tires but without electronics. This time I used a digital scale. It was a 2.15 lbs. The body and wing together weigh .25 lbs and the tires I weighed were the Dirt Hawg tires that I use for bashing which are quite large and heavy compared to other tires. I think it's possible to get the car to 2 lbs without electronics. Keep in mind mine is a bit heavier in certain areas in that I am using all stainless screws rather than aluminum as well as locknuts and braces in several places for strength. The car can definitely be made lighter if I wanted it to be. The battery I've been running is the Gens Ace 50c 5000mah hard case lipo. It weighs .65 lbs by itself. If I were to use a 25c 2200 mah soft case lipo then I could get the battery weight down to .35 lbs. Yes the runtime would be less but I'm already getting over a half hour from my pack. The longest races we have are 10 minutes but typical is 7-8. I think that even with a 2s lipo and some careful selection I can keep the total ready to run weight of the car under 3 lbs. The nice thing about that is that I can set the shocks up softer again which is far less stress on them so hopefully the shock bottoms won't be as likely to come off. I should also be able to run a slightly higher turn (slower) motor which would mean more low end torque and more efficiency to help the lower mah battery run longer. The more and more I get into this the more I like it.

[Coelacanth]

I would be interested to know your weight with everything as it would be when running, but without battery. The battery itself can vary too much depending on the kind, size, number of cells, etc. but every car needs electronics--servo, speed control, wires & connectors in order to drive.

[fredswain]

That's what I was getting at. I can keep weight down just by selecting the proper battery. It and the motor are the items with the most individual weight. I may go back and replace screws at some point with all aluminum but in the end it's only going to save a couple of grams. As for wiring one thing to do is to keep it all as short as possible which means cutting it to length rather than twisting and tying the excess out of the way. It would look cleaner too.

[Coelacanth]

A car with weight pared down to less than 1000 grams is a pretty remarkable feat, but I'd like to know the weight with the entire car, everything exactly as how you'd run it, except battery.

[Lonestar]

interesting discussion - here's a similar one I started on oople a few months ago...

http://www.oople.com/forums/showthread.php?t=51872

There's some interesting input in there by some top european drivers

Paul

[Lonestar]

As for wiring one thing to do is to keep it all as short as possible which means cutting it to length rather than twisting and tying the excess out of the way. It would look cleaner too.

The wire you twist is servo wire... which weighs close to nothing. The heavy 12 or 14awg wire is the one that weighs a lot

PAul

[fredswain]

One interesting justification for doing things a certain way that I see made time and again on different forums is that if pro sponsored drivers do certain things then those techniques must be the best way. Everyone copies the pros as a result. My response to that is that my day job is mechanical design and engineering and theirs is driving rc cars! I'll take my chances with my own ideas.

[Coelacanth]

In general, people tend to copy what was successful or already done before. With that mind-set, small, gradual progress will be made. But breakthroughs only happen when people put the "improve what's been done before" attitude away and instead blaze their own trails.

[fredswain]

It seems like there are a couple of main points to adding weight, and valid ones at that. One is the argument of sprung vs unsprung weight. It seems to me that current car and track design are almost making it mandatory for more weight. What I mean by that is that modern tracks have lots of long smooth sections between corners or jumps whereas many old tracks were mostly constant terrain change that was handled at slower speeds and a lack of large jumps. Modern cars are designed around modern tracks meaning that they need to be fast but stable. Longer suspension arms are more forgiving and more stable at speed due to their lower roll centers as a result of their suspension geometry. A heavier car is also slower to react which makes it more inherently easy to control at high speeds. These modern cars would probably get their butts kicked by the old cars on the old tracks. The faster and smoother a track, the less agility seems to matter and the more pure stability does. It seems contrary then to be using modern servos that can go lock to lock in .06 seconds since they inherently speed response rather than slow it. Why not just run less weight and slower responding components? Everything is a balancing act and some of what weight is doing is nothing more than a bandaid solution to other problems. Keep in mind that heavier cars require even heavier components to compensate so suspension systems will need to be built heavier and stronger adding to unsprung weight. We have seen most companies move to big bore shocks now as well.

The issue of wheel and tire weight vs total weight is a good one though so let's look at it a bit closer. It is argued with merit that a lighter car will have a harder time compensating for greater unsprung weight through suspension tuning. However what if we just go back in time 25 years and use shorter arms, which could be lighter than longer ones, as well as the old jellybean sized wheels and tires of the original RC10? Less unsprung mass. Obviously a 3 piece jellybean wheel is heavy for it's size but the point is clear in regards to weight and diameter. But by doing this we go back to the shorter arms which are less forgiving due to their higher roll centers. Yet another trade off.

Modern tires are also very different from the old ones. The old tires were made of a stiffer rubber but lacked foam inserts. It's not to say you couldn't buy foam, since you could, but very few ever ran them in off road. Now everyone does. If you squeeze an old mounted tire vs a new one, even one with soft foam in it, you'll notice that the old tire flexes more under weight due to the lack of inner support but the spikes are stiffer due to the harder compounds. The modern tires don't compress as much and the smaller spikes of today are also softer. It flat out takes more weight on them to get them to bite. Those old stiffer tires with larger spikes would be much better on loose dirt, even on a light car, than modern tires would. Yet another example of car design evolution following track design evolution.

I've seen people mention that adding weight helps hold the front end down during acceleration. I see that. I could also see just allowing the slipper clutch to run a little looser to accomplish the same thing. It would also reduce on power understeer in corners. An interesting thing is that people complain about mid motor setups in off road due to less forward traction during acceleration. If you've got so much forward traction that you are pulling the front wheels off the ground, you can stand to lose some forward traction! Keep in mind that modern mid motor cars utilize the rotational inertia of the motor to help apply force to the wheels during acceleration. Adding weight to those cars adds weight that the motor inertia must overcome. In other words the heavier the car, the less effective this is in aiding in traction and the more dependent you are with adding weight back to the rear of the car. Isn't that the whole point of moving away from rear motor in the first place though?

I honestly then see adding weight as a compensation to current design rather than a pure clean paper design to work around max performance with modern technology. It would require thinking a bit differently from the norm which I think is the biggest hurdle. The pros don't think this way so why do it? The optimum lightweight design I see in my head is looks vastly different from current car design but is something of a cross between cars of 25 years ago and cars of today, but with a different motor and battery placement. The car in my head would be as light as possible. It would be designed around certain electronic components and a certain weight battery. The motor would be mid mounted with it's rotation in the same direction as the wheels. It would be placed at the exact center of gravity in the chassis so that it's inertia from rotation has the most leverage on weight transfer both during acceleration and deceleration. This means it would be mounted about where a cross mounted battery is currently mounted. The battery would be placed immediately behind it. Don't ask me how to transfer power back to the wheels though. I haven't figured that out yet! The electronics would be immediately in front of the motor. The wheels would be smaller in diameter like the old RC10 wheels. The body would be kept as narrow as possible. That's the image I have in my head right now and it's a weird one!

[SoloGraphix]

Interesting conversation here. My brother and I recently built a SC truck out of an RC10GT converted to electric and our main focus was on keeping the wheelbase relatively short, weight as low as possible, and weight distribution to be perfect 50/50 f/b l/r so far the biggest hurdle has been rethink the suspension tuning because we went way to light in the front shock department due to not knowing how the larger front weight ratio would react. We ended up with a truck that had way to light of front suspension and you could visibly see the front tires and a-arms come up and back into the body over the bumps. I think I might even try this setup with 0 front kickup and get better results than the stock gt kickup.

Crappy phone pic attached.

[fredswain]

For off road I don't personally think 50/50 weight distribution is optimal. At least not for 2wd. I prefer sometime more along the lines of 35/65 but with the weight within the wheelbase.

[Coelacanth]

I too would agree that a bit more weight behind center, towards the drive wheels, is the ideal situation for *most* conditions--for 4WD too. With my upcoming Barney build, I'm doing the opposite of what I did with CYANide; instead of narrowing the track, I'm widening it for a more stable stance for offroad use.

[fredswain]

I'm not actually building this car for lightness. At some point I may take up that challenge. I've just been weighing cars to see how much they weigh before and after electronics and the electronics seems to be a very large percentage of the total weight. If I ever do decide to attempt an ultralight car, it'll be a clean sheet design. It would be nice to have my cnc router operating for that though as I could make whatever parts I'd need.

[Lonestar]

I honestly then see adding weight as a compensation to current design rather than a pure clean paper design to work around max performance with modern technology.

yep - my point exactly when I started that thread on oople.

Paul