Sixtysixdeuce

OK, guys, need some help brainstorming. My RC10GT street fighter project is coming along, but I'm up against a wall with gearing to exceed 100 MPH using the stock drivetrain configuration. I simply can't gear it tall enough.

As of right now, it has a 20T clutch, 65T RRP spur and running 5.5" 1/8 tires:



We were able to get 53 MPH with the 2.8" Talons I had on it before, could have tuned a little more out of it, but broke a CVD after run 3. Was going to try again yesterday, but didn't even have a chance to get dialed in when the RX started freaking out for some reason, and it was late, so we just packed it in. I put a 2.4GHz set up on it today, and I should see the 22T clutch bell by Wednesday, but even with the bigger tires and clutch bell, I suspect I'm going to top out at about 70 MPH. I also don't think the little Picco .21, good as it runs, can push this almost 6 pound car to triple digit speeds.

So, I have an OFNA LX2 rear chassis sub-assembly en route (diff, arms, axles, hubs, shock tower, etc), and I'm about to order an LRP ZR 32 Spec 2 engine to get the requisite power.

This brings me to my quandary: How to set this thing up with a dog bone coming directly off the clutch bell and into the diff. It'll have a really steep final drive ratio at 3.0:1, but I believe the ZR 32, with it's 4.3 claimed HP, should be able to motivate this lightweight 1/10 scale with that ratio, hopefully to well in excess of 100 MPH. The math says that, at peak RPM, it will top 200 MPH, but we all know it won't redline under such load, nor can these tires take that speed.

Anyway, if anyone has any better ideas than simply turning an outdrive hub on my lathe and locking it down to the clutch bell with set screws, I'm all ears!

rcpowerhorse

if it helps the Tamiya Nitrage had a clutch bell set up to accept a small dogbone

Anthoop

lol at those rear wheels.

I do not know the car at all, is it belt drive and 2WD?
Not posible to buy a smaller spur and make it fit?

t-max97

My Feedback: (9)

I get the theory of why you put those tires on there, but you should also consider the weight of the massive tires, the drag and the torque/lower gearing required to accelerate, if you look up video's of rc speed runs I doubt you will find many with massive tires on the back. If I was doing this, I would put normal tires on it that are light, try to make the car as light as I could, smaller battery pack, less fuel, lighter tires ect, tune just shy of peak rpm and gear it high. Eventually you will toast your engine doing this because of the load on the engine and the heat created. If you want all out straight line speed, an electric would be easier to achieve this. Either electric or nitro, once you get it to run that fast, that's all it will be good for.

nitrosportsandrunner

I believe that the speed record for nitros is only alittle over 100mph. I tried to google it but the results were all for the electric rc speed record...164mph.

anyway, it is safe to say that the guys who do break 100mph with their nitros have some serious money in them.

Problem is in the way nitros make power and the gearing. With an electric...which produces the same torque at 1rpm as it does at 30,000...allows for taller gearing as it still has the torque to get things moving.

Nitro motors do not produce alot of torque at low rpms. So as mentioned above, getting the tires rolling with the engine geared really high will be very hard on it...and likely to stall it out.

Not discouraging the OP from trying it. Id love to see a direct drive setup with a nitro. I cannot begin to calculate gearing ratios, rpms, torque and tire size to say for sure whether it will work or fail....I doubt anyone can.

If you have a welder, weld a drive cup right onto the cluch bell

Sixtysixdeuce

It's an Associated RC10GT RTR chassis. Drive is gear-to-gear. 22T bell and 52T spur is the best I can do with the physical proximity of the engine & transmission.

Agreed. The reason for it is the inability to gain and more inches of travel per motor revolution with gearing. That's why it's going to be set up with a longitudinally mounted engine and the OFNA 1/8 scale diff. I plan to put 4" 1/8 scale foam tires on it then, as even these large tires are ballooning pretty good at over 50 MPH.

Well, that Picco actually runs surprisingly cool. Even after a couple of back-to-back runs, I checked head temp and came up at 198*. TRX 33% fuel, med-cold plugs. I'm actually probably going to switch plugs to a hotter one, as this engine doesn't get good ignition below 200*

Yes, but electric is boring, and has even more heat issues in these applications. Nitros get hot running hard at low speeds; with the amount of air moving over the fins at high speed, they don't heat much. Though I've never personally built a fast electric, as I understand it, there are some serious cooling challenges for motor, ESC and battery under that kind of load.

I have considered putting a Mamba max & 6S set up in a 10GT for this purpose, but I'm just a nitro guy at heart.

There is one that bests 200 MPH, but not on the open road:

http://www.youtube.com/watch?v=T31juxlQuGA

That may have been the problem back in the day, but there are some seriously potent nitro engines out there now; the LRP .32 I'm going for is rated at 4.34 HP stock, easily modded to over 5. These engines accelerate 12+ lb 1/8 scale monster trucks impressively, so should have no trouble with a 5 lb 2WD 1/10 scale, even with stupid gearing. As well, these larger engines do generate quite a bit of low-end torque; the OFNA Force .32 I have in my other GT is considerably less potent than the LRP, and will still flip the truck over backwards at 30+ MPH if you hammer it on a high traction surface. And that GT tips the scales at 6.72 lbs! It's also geared fairly high, with a 14T bell & 48T spur. It's actually faster than the other one was with the 2.8" tires, but does not handle well at over 40 MPH with it's height, soft suspension and the really squishy Talon tires.



Well, calculating the theoretical speed is not difficult, but only real world testing will show what the true capabilities are. Just like electrics, you cannot calculate your real top speed based on motor RPM/gear ratio * tire rolling circumference, because the RPM under load will never be max. A 3,300 KV motor on 4S LiPo should turn 55,440 RPM, but it ain't gonna do that when it's actually propelling a vehicle. Similarly, a nitro that's rated to 38,000 RPM is not gonna hit that in a car at speed.

The simple formula for speed calculation is engine rpm divided by gear ratio, times tire rolling circumference. Using SAE, that's tire diameter in inches times 22/7 (or 3.14, roughly), multiply that by final drive RPM (engine RPM/gear reduction), divide the product by 12 to get feet, then again by 5280 to get miles, then multiply by 60 to get MPH.

So, assuming the ballooned diameter of these tires is 6.5", we have ~20.5" of ground covered per revolution. The transmission ratio is 2.6:1, and the bell/spur ratio is currently 65/20, or 3.25:1. So final drive is 3.25 * 2.6, or 8.45:1 Engine rated RPM is 38,000. So 38,000/8.45 = 4,497 wheel RPM (theoretical). 4,497 * 20.5" = 92,188" covered per minute. Divided by 12, we get 7,682 feet per minute. 1 mile = 5,280 feet, so 7,682/5,280 = 1.45 miles per minute. Times 60 shows us that if engine were able to hit max RPM, we'd get 87 MPH in a vacuum.

However, the engine cannot max out under that load, so that 87 MPH calculated top speed is rather worthless. Hopefully I'll get it on radar this coming weekend, and my guess I that the real top speed will be 20 MPH less than the calculated top speed; I'm betting on ~65 MPH.

Anthoop

Ah I see better now I checked the manual.
So how about a larger pinion then? You can always use a threaded clutchbell and use whatever size pinion you want.
I think you will have less transmission loss on your current setup opposed to the crownwheel and pinion Ofna option.....provided the trans. gears (plastic?) and housings can take the heat.

20 MPH is a good goal for the difference between theorectical and actual if you are using max quoted RPM to calculate with...

The nitro speed WR is 120...not a crazy expense from the look of it...just singing- http://www.youtube.com/watch?v=e-MAsj2cg7M
The speed records for different classes can be found here- http://www.rossa.org.uk/records.html and http://www.iscspeedrun.com/Records.html

t-max97

My Feedback: (9)

Just so you know, horsepower ratings on these engines mean about the same as what a politician says.

Sixtysixdeuce

They are, however, a good gauge of performance when you have played with quite a number of engines in various applications. It may not actually achieve 4.34 HP, but I can pretty well count on it being 30-35% more powerful than the 3.1 HP rated OFNA .32

I could, but even then, the largest size I'm aware of is 27T, which is only a 23% increase over the 22T. This Picco also doesn't have an SG shaft, and is 1/4-28 threaded, making SG adapters difficult. The current pilot shaft is a painstakingly retapped Associated 2219. The LRP is SG, but I'm pretty sold on the longitudinal layout for several reasons, not the least of which is better chassis balance.

I don't think there'll be much parasitic loss through the hardened steel OFNA ring & pinion. The Associated stealth transmissions are pretty darn tough, especially with RRP gears, but I've destroyed a steel input shaft and the case on my other GT. I now run a GPM alloy housing on that one. Bottom line, though, the OFNA diff is tougher and gives me the configuration I want, including larger 1/8 scale dog bones, hubs & wheel bearings.

nitrosportsandrunner

ill keep looking for the top speed record for a nitro.

you cant compare a full RC nitro truck to a tether car. There is soo little to those. 2 tires, no steering, aero body, direct drive screemer nitro.

when you figure that the top electric rc speed record is 164 and that was done with a 11 HP electric motor in a full carbon fiber car....you can see how 100mph with a heavier nitro (even with a 5hp motor) is a tough task.

Sixtysixdeuce

Well, we're gonna find out. LRP ZR-32 is en route, as is a TRX long threaded clutch bell-I'm hoping there's enough hanging off the end to cut dog bone drive notches in it, but if not, I'll cut them into another piece and thread it on.

I won't quit until I have a 100 MPH RC10GT. I'm about as stubborn and determined as one can be, sometimes to my own detriment, so it'll happen. Will it happen with this arrangement? Should know in about 2 weeks

t-max97

My Feedback: (9)

Lol keep is updated.

nitrosportsandrunner

my thinking is, if you want a nitro to hit those speeds you would need a pan car type direct drive setup. Pan cars are also super light. Would require VERY smooth pavement tho.

Anthoop

Look no further...

Yes...let us know how you get on...and good luck.

Maj_Overdrive

My Feedback: (11)

Something like a pan car would be your best bet for a single speed. Two speed would really be better. But if you're looking for more clutchbell options get a LST2 clutchbell that you can remove the gears. Rc-monster has gears for them all the way up to 30t.

JohnP2

I love these speed run threads! Keep it up, guys. I always find this very interesting and learn things.

Sixtysixdeuce

Here we go!



Just waiting on the long threaded clutch bell, but I can get started without it.

supertib

FYI a LRP 32 is roughly 1.7 HP with a max RPM of 34 000( tho much lower underheavy load ) .........the factory is grossly overestimating its power and RPM output..............A fully modified OS 28 makes 2.6 HP........

electric motors make full torque at zero RPM and the higher the RPM goes the less torque they make............. they do not make the same torque at 30 000 RPM as they do at zero RPM...

Sixtysixdeuce

We all know factory power claims are higher than actual. Point remains that, unless something is vastly different about this one, I can expect it to outperform the OFNA .32 in my other 10GT, which still has plenty of power to pull the front wheels of the almost 7 pound beast off the ground at over 30 MPH on 2.8" Talons with 14/48 bell/spur gearing. Frankly, I think the OFNA motor makes enough power do accomplish my goal, but why not go for a little more power when the cost increase is just $40?

This is experimental, and I don't do all kinds of quantum calculations to be sure it'll work first. The math says this engine will only have to rap ~25K RPM to hit 100 MPH on 1/8 scale foam tires with 4" diameter using the 3:1 direct driven OFNA diff. If it can't get there, out comes the motor & it's off to the mod shop.

I may be a little further out on my timeframe than I estimated, though, but I'll keep everyone posted. It's summer, and I have a family, so hobby takes a back seat to entertaining small children. I have a lot of work to do, as actually having parts in hand shows that I'll need to extend the chassis about an inch & get creative with servo linkage to make things fit. This .32 is monstrous on the 10GT chassis:



I also still have to sort out brakes, which may happen in the form of a modified Venom GPV-1 motorcycle brake kit, if it's robust enough. The plan right now is to mount the rotors on the diff outdrives (both sides).

Anthoop

I guess the output shaft of the diff is 8mm? Then Hobao part number H84010 has a coupling for the drive shaft with flats for a brake disc or two....making something to hold the pads will not be so easy....how about an anchor instead?

Sixtysixdeuce

Well, we're coming along. I have the basic layout mocked, but still a long way to go. The OFNA diff is set back off the rear of the chassis, extending the wheelbase by about 1-1/4":



For braking, I chose to order a Venom 1/8 scale motorcycle brake kit for the rotors & cables, will use NTC3 brake pads and hand made 7075-T6 calipers (not yet fabbed). The rotors were 12mm hex drive, but I rounded 4 of the 6 flats, and ground flats on the OFNA outdrives to match the other 2:



Suspension will continue to be the NTC3 shocks. I got the longer rod eyelets by using the OFNA sway bar links. The upper mount is the old graphite mount off my other 10GT, cut off & inverted, then affixed to the OFNA tower:



I may fab an entirely custom mount later, but this was easy, and the cut GT tower is very light weight.

Still haven't fully figured out how to get a dog bone drive affixed to the clutch bell, but I have some parts here and I'm toying with several ideas. Biggest challenge is the bearing arrangement; the standard clutch bearings are 10mm diameter, which will not fit inside the outdrive. This means that I either have to use a smaller bearing and a spacer on a long screw, or find a way to attach the outdrive after the clutch bell is in place with the screw tightened. The latter is more appealing, mainly because the heat generated by a clutch when running gears this tall is tremendous, and I have ceramic bearings for the standard 5x10x4 clutch bearing size. Also have a vented flywheel and aluminum shoes en route. This was a hard lesson learned on my other GT with the 20T clutch bell & 48T spur; That clutch was getting to nearly 500*F, cooking shoes and steel bearings.

Also thinking I'm going to rearrange the steering servo to get the engine a little further forward.

So, still a lot of work to do, but it's starting to come together.

rcpowerhorse

For a clutch bell look up the tamiya nitrage

Sixtysixdeuce

Only took about 5 hours to make. Lol. Now I just have to reverse engineer it and build a mirror image duplicate [&:]










The housing is 7075-T6, the cam is some kind of very hard steel cut from a parking brake cam lever (I had to heat it red hot to push a carbide drill through!)

Thanks! That might just work! Gonna order one and see what I can do with it. Might have to open it up a bit, but it's closer than anything else I've come up with.

t-max97

My Feedback: (9)

Looking good! How did you make that brake caliper thing lol?

Sixtysixdeuce

Careful mill work. Lots of it.

I'm very fortunate; in our podunk little town, we have a plastics manufacturer (helluva nice guy). He makes all his own molds from aluminum, usually 7075-T6, sometimes 6061-T651. As such, he ends up with a lot of scrap pieces that aren't much good for industrial use, but great for hobby stuff, and he sells me what I need at VERY reasonable prices. Pieces as thick as 3", which anyone who has ever shopped aluminum alloys knows would be extremely expensive to buy on the open market in the sizes they come or even as custom cut smaller pieces. I also buy polymer & nylon scraps from him, and some of his surplus tooling.