Optima Truck Build

[Basher67]

Well we're back on track. I was starting to think that a baja bug was the way to go but there's no way I could have done a better job than Mr. Ed's. That thing is absolutely badass. I started to build an optima TRUCK and I'm sticking to it. I've been thinking of picking up a Toyota baja truck conversion body for one of my RC10 T's and I think it might just work with this build. Wheel base is very close. Cant seem to confirm the width, but if it doesn't work, I'll just use it on my 10T. Just ordered it from TBG so I'll be looking around for some more appropriate size wheels and tires while I'm waiting for the body.

[Basher67]

Got some pics of the progress. I picked up these wheels and tires off of ebay. I wasn't sure because the tires weren't a "name brand" and the wheels were a brand I've never heard of. I couldn't be happier. The wheels are "Gmade" and seem to be good quality. They are a 3 piece wheel and the 6 "lug nuts" are the fasteners that hold the wheel together. It came with extra hardware, a set of longer starter screws to help suck together the wheel during assembly and a little micro socket for the "lug nuts" that fits on the end of a 2.5mm driver. The tires were no names but on arrival I could see they looked like scale BFG mud terrains. At about $25 for the wheels and less than $17 for the tires they are hard to beat. I don't own any crawlers and I don't know about the durability of these parts yet, but my first impression was definitely positive. Anyway, enjoy the new pics, throw some opinions at me and I'll do another update when I get the body in the mail and I start working out the body mounts.

[Coelacanth]

I'm interested to know what pinion gear would be considered optimal for tires with that large diameter. If you let me know the tire diameter, I'll post the results from my Optima Gear Ratio Calculator. I assume you'll be using the stock Optima spur gear?

[Basher67]

Tire diameter is 100mm. I believe it is a stock spur. I have a 9t pinion in there as a starting point and the motor is a 19t.

[Coelacanth]

Tire diameter is 100mm. I believe it is a stock spur. I have a 9t pinion in there as a starting point and the motor is a 19t.

That's pretty close to what I came up with. I'd go either 10T or 11T as both are within 10% of a 1.00 rollout ratio (between 0.90 and 1.10). 10T works out to 0.996 and 11T to 1.095. Theoretical optimal RR would be 1.00. This is just theory, but it's good to have an educated guess when significantly changing tire sizes. Let us know your gear ratio observations, when it gets down to a test drive. See the Sample column.

[Basher67]

Nice. Thanks for the info. I was thinking that I might set up some electrics in it and do some living room test runs while i'm waiting for the body to arrive. I'll let you know how it goes.

[Basher67]

I threw a traxxas XL5 esc and a Speed Gems Arconite 19t motor in it with a Spektrum receiver and quickly realized it's way too fast for the living room. I'm going to have to lower the suspension. The center of gravity is way too high. It wheelies and flips over way too easy when "rock crawling" the living room pillows. Also, while the extreme angle of the suspension doesn't seem to bind at these angles, there is a slight vibration that goes away as soon as the suspension is compressed a little and the steering binds a little. Some internal limiters should solve this problem.

[Basher67]

Instead of trying to make the shocks work. I swapped them out for a set of Ultima 2 shocks. The travel dimensions and spring rate were a much better match for this build than the associated shocks. The result was a dramatic change for the good. No more binding in the steering. I have come across a small problem some of you maybe are familiar with. This is the first Optima I've run with an esc that has reverse. The drive chain does not seem to like reverse. I believe it is adjusted properly, and is smooth and quiet in forward, but in reverse it sounds like it's skipping every few links. Anyone else run into this with the chain drives?

[Coelacanth]

There should be no issues with the chain-drive in reverse, it was originally meant to go both ways, even with the original MSC. A couple points come to mind, though...

1) Are you still running the 9T pinion gear? That seemed a tad low for the size of tires you're running, maybe it's too torquey. Try a 10T or an 11T pinion and see if that reduces the tendency to pop wheelies and make it behave better when driving.

2) A lot of people built the Optima (myself included, I'm embarrassed to say) with the chain oriented the wrong way. The chain should be mounted with the link ends facing to the outside, not to the inside/toward the sprockets. Pay close attention to the manual, it clearly mentions this and shows the correct orientation. I'm not sure if this would exacerbate that problem, but it's something to consider.

3) Make sure you've tightened the chain per the manual instructions. The chain DOES stretch over time and may require removal of a link or two.

EDIT: Some additional tests you can try to help troubleshoot the chain noise in reverse:

-Does the car roll smoothly with power off, forward and reverse?

-Does the car roll smoothly forward/reverse with the pinion gear removed?

Those things help eliminate the motor & ESC as possible culprits.

[Basher67]

I was hoping I would hear from you on this as it seems you have a few optimas yourself. Everything you listed are all good points but unfortunately, All appears to be in order. I am going to try bumping up the pinion size tonight. I rechecked the chain tension and I believe it's spot-on. Virtually no slack and there is still room for adjustment before I remove a link. I was watching/listening carefully as I ran it forward and reverse and I don't think it's slipping. There are two spots in the chain that are not perfect. I straightened them out pretty good when I assembled the chassis but they still could be the problem. Without the motor the whole drivetrain rolls very free and smooth with no binding forwards and backwards. Under power in forward it appears flawless. Even when I'm rolling backwards and hammer it full throttle (I know this is begging to break something) nothing skips or makes any noise. but anything more than just the slightest bit of throttle in reverse and it makes this popping noise every foot or so. I'm going to see if I can straighten this spot in the chain a little better. It's the only thing I can find that looks suspect.
Note: this is why optimas can suck. R&R of the chain is a complete teardown of the car.

[GoMachV]

kind of, back in the day we would open a link, remove it thru the top, and thread the new one Its a pain, but not nearly as bad as a complete teardown. The first time it will take you just as long as disassembly, but the next couple times you get better at it

[yellowdatsun]

BITD I built an Optima truck, I'll see if I can find pics of it, but that might take some digging.

It was basically a short course truck before there was ever that class.

[Coelacanth]

Your test showing the car rolls forward and backward smoothly without motor seems to indicate the issue might not be the chain.

Assuming the bend in the chain IS the issue, though, you could pop off the top chain cover, slip thin hex L-wrenches through two links of the chain, on either end of the link you want to remove, and snip off the wonky link...and hopefully have just enough slack to bend the two chain ends back together.

What you're describing though almost sounds like cogging...a hiccupy, jerky sort of acceleration caused by some ESC & motor combos, usually noticed at low speeds. Usually goes away under hard acceleration or when cruising at speed.

What motor did you buy? Is it sensored or sensorless? I've had great luck with sensorless motor/ESC combos with one exception...an older Castle Sidewinder ESC I had was real bad for cogging with sensorless motors. The Bullistorm/Tacon combo and Leopard Toro combo have worked beautifully, nice and smooth all the time. My buddy's Optimas running EZRun combos also run fine.

Can you try any other motor just to see what happens?

[Mr. ED]

I think the chain is catching on something when running in reverse, and sliding over when running forward

[Basher67]

I agree Mr. Ed. I think I have it narrowed down to exactly that. Holding it in place while put it in reverse on my desk makes happen. It's not skipping. That link that is slightly off is catching something in reverse inside the front diff housing. If I can't straighten it out, I'll just order a new chain.