Hi all,
I used to fly RC planes/helis but got out of it a few years ago, then a buddy convinced me to join him for a bash session at a local indoor carpet track. One thing led to another, and I ended up grabbing a Latrax Rally. Seemed cheap and basic, but for the money, it was a quick and easy way to join the fun. That was last August.....I now have about 7 cars and probably north of $1500 into this hobby. I have some decent cars, mostly offroad. But the little Latrax, for all its shortcomings, has been something of a go-to basher for me and my buddy (he ended up buying one too).
Well I like a project, so I decided to buy another Rally and completely rebuild it into something that retains the zippy 1/16th fun that I like about it, while fixing its shortcomings and making it faster. After some research, and a lot of trial and error, I'm starting to get somewhere, but there is one lingering issue that is driving me crazy.
First, here's a rundown of what I've changed:
- One of the worst parts of this car are the tires. I've got a couple different wheel/tire combos on order, but at the moment I've already seen a marked improvement by switching to some 1/18th wheels that are made for an HSP car. They are slightly taller, the tires have a tread pattern, and there's actually a foam core. Noticeable improvement.
- Switched to a brushless system, which was one of the main upgrades I wanted to try. 4800kv with a 25A ESC, and a 20t pinion
- Bearing upgrade (kind of a must-have for this car)
- RC18 ball diffs front and rear, which is turning out to be one of the best upgrades
- Metal Integy turn knuckles to cut down on some of the steering slop
- Oil shocks
- 2* camber links in the rear (ended up with stock camber links in front, for now)
- Futaba R2106GF receiver so I can use my 4PLS, this is a very small and very light park flyer RX.
One big change I've ended up with is the battery. I started out with the typical 2200mah 2S lipo, which is big and heavy. But even with the brushed motor, this gives rather long run times (like 40+ minutes). I decided to switch to a humble 800mah 2S, with a mini-Deans connector. This is probably the single biggest change you can make to cut weight on this car. It also goes a long way towards fixing the unbalance issue (too much weight on left side of the car). But it also provides enough space to fix ANOTHER big problem with this car: the new ESC can be placed in the bottom of the chassis, behind the battery, instead of perched high on top of everything like the stock one. Relocating the ESC fixed my traction roll issues, at least on asphalt (yes, this little dude would actually traction roll on asphalt). Also seemed to make it a little less squirrelly.
So, with tighter and smoother suspension, better tires, ball diffs, brushless motor, lower CG, much lower all-up weight, and a lot of tuning and improving over the last couple days, I have a car that is hitting 25mph, is reasonably controllable throughout the speed/power range, drifts nicely on smooth concrete, and actually takes off in a (almost) straight line without zipping all over the place when I punch the gas.
But some issues persist. The main thing I'm working on right now is what seems to be a known characteristic of the Rally: with power on, it oversteers in right turns, but understeers to the left. When on smooth concrete, punching the throttle and turning right results in tight donuts. But to the left, I was just getting big wide circles. My other (unmodified) Rally has the same issue.
After much testing, I believe the root cause has to do with power delivery to the front wheels. I tried running as RWD, by removing the pins that engage the outdrives in the front, and the problem was completely gone. I then reversed the test and made the car FWD. The turning problem returned with a vengeance. Even just taking off in a straight line resulted in a pronounced pull to the right (although, it must be said, the car became very stable and easy to drive, since the rear wheels had much better traction). Accelerating very slowly, or coasting, did not exhibit the right pull.
After restoring the original 4WD configuration, I started messing with the front diff. My rear diff is fairly loose, as was the front. After tightening up the front just about as much as I could and still have diff action, I found that, on smooth concrete, I can now induce crazy tight donuts in either direction (although donuts to the right are STILL a bit quicker and tighter). Actually, it's less a donut, and more spinning-like-a-top. I've seen other, higher-end 4WD RCs do this, which I hope means that I'm making progress. Out on the street, the car is more stable with the stiffer front diff, but still has a tendency to spin out to the right when making even moderate right turns under power, even as left turns continue to be big, wide, sweeping affairs. It's very difficult to induce a spin-out to the left. Letting off the throttle in a left turn tightens the turn up noticeably.
Something else I discovered: while doing similar tests with my original (brushed) Rally on concrete, when I was making left turns, the motor RPM was lower. It's like the tires break loose easier when turning right, but everything stays planted when turning left. Holding the car in my hand, there is no binding or RPM reduction when steering with power on.
It really seems like more power is going to the left front wheel, which makes absolutely no sense. Punching power while in a left turn tends to make the car push. Punching power in a right turn is likely to spin it out.
This makes even less sense considering that the original gear diffs have been replaced. There's got to be something about the basic design of this car that causes the problem, but I can't imagine what that would be. My next step will probably be to open up the front diff housing and see if there's something about the design that might cause the right outdrive to bind when power is applied.
I have to say, after test driving tonight, the car is orders of magnitude more stable than stock. If it would just oversteer a bit more to the left and a bit less to the right, I might have a winner. I just today got the HSP tires all around (only had two before), not sure if those made the big difference, or if it was the stiffer front diff. I think it's a combination of the diff adjustments, better tires, lower weight, lower CG, and my expo adjustments. If I can just get a little more traction with the new tires I've got coming, and fix this right-turn issue, I think I could actually go for 3S power.
Thoughts? Ideas? Abandon the project and build a proper 1/10th touring car?
Matt
