Got the video up,

https://youtu.be/lO1jaMtfIUg

Don't trust me, search and read some, you might learn a little... ;)
Here is to get you started;
AdriansModelAeroEngines.com :: ABC Model Engine Break-in

Look what came in the mail today, guess they are still in business.

https://image.ibb.co/hMA6Ra/20170812_185352.jpg

That's exactly how mine was! Clutch juuuust starting to come on when the engine is already near redline.
Steel shoes fixed all that!

Also, holy crap Buku IS still in business? Good to know!

Some things I found out about the buku shoes. One they use bigger dowel pins then standard 3 shoe clutches. Also even though they are brass, there still not all that heavy. Thinking I'm going to need steel shoes, no way around that.

But was really bumed about the dowels. Would hate to modify my flywheel just for those shoes. When I had a lathe it would have been no big deal. Now I have to watch what I do, don't want to mess anything up. I did find some heavier aluminum shoes with set screws as weights. Probably try those for now.

https://image.ibb.co/eYv1sF/20170812_201527.jpg

If you can't raise the weight, lower the spring tension.
Although it's better to have heavy shoes with heavy springs, you can get lighter shoes and lighter springs to engage at the same rpm its just the total "clamping" force of the shoe will be much lower. (Heavy springs do not "cancel out" heavy shoes, due to the centrifugal forces not being linear)
But getting it to engage at the right rpm is still huge.
I know you know all this, Im stating more for the people who will come across the clutch conundrum in the future.

Try stretching the springs, and turn the shoes backwards.

I tried turning shoes backwards last night. Spring doesn't line up with clutch pilot grove after doing so. Don't have the tools as of now to modify grove so that's out unfortunately. I did have some what good results doing that on first 4 stroke build. So down the road that will be my setup with steel shoes instead.

Going to try those weighted aluminum shoes today. Not expecting a huge difference but even a little will help for now. I've also started looking for another mini lathe and mill.

A Lot of progress made today on the clutch setup. It's working great but think there is more left in it with steel shoes. Have any of you guys temped out your clutch bell while running? If so what kinda temps are they running? Mine was between 160-185f while bashing around, no jumps.

Same temps here.
More video? :D

Buy some copper-tungsten rod from Amazon and use it to fill the holes in those aluminum shoes. You'll probably double their weight. I did the same thing on the clutch shoes for a 2-speed transmission I'm working on:

http://i.imgur.com/wt2I0SR.jpg

This may be your preferred source for information, but that doesn't mean it is entirely accurate. Thanks for your concern, but between physics in college, the technical reading I do on a daily basis, and years of experimentation, I've already learned plenty.

Google "thermal expansion coefficients of metals" and you will get several authoritative lists that show aluminum expands more than brass. Pinch is reduced when an engine is running because the top of the sleeve reaches a higher temperature than the top of the piston, NOT because the entire sleeve expands more than the piston at the same temperature -- in fact, the opposite is true, thus overheating can cause the lower part of the piston to scrub against the lower part of the sleeve. Anyone who claims otherwise is wrong, even if they have their own website.

You keep saying that you understand, but at the same time you contradict yourself and try to reach for any straws you can find...
Forget I mentioned anything at all!
Lets get this interesting thread moving in the direction it should, sorry for any inconvenience! :)

Hmm, I'll look into that this week, Thanks for the heads up. Are you trying to make a 2speed trans shift correctly with a 4 stroke, is that why your adding weight?

Not specifically for a 4-stroke engine, but a similar application where the RPM of the center-gearbox driveshaft is too low for the 2-speed transmission to shift properly in its original configuration. I replaced the springs in the 2-speed clutch with softer springs and it still wasn't enough.

Either way the concept is the same -- if you can't soften the springs enough for the clutch shoes to swing out and grab the clutch bell at the desired RPM, then you can increase the centrifugal force acting on the springs by making the clutch shoes heavier.

Copper-tungsten is a good choice for this mod, because it's so dense but still easy to work with. Brass is 8.7g/cc, copper is 8.9g/cc, and tungsten is a whopping 19.6g/cc. Copper and tungsten don't alloy together, so the rod actually consists of tungsten powder glued together with copper. The result is a metal-matrix composite that bends, cuts, and grinds like copper, but has a density of 13.8g/cc (for a 70% tungsten / 30% copper mix), which is 55% higher than pure copper. It's easy to work with, and its density makes it easy to add weight even if you have minimal space to work with. Those two little chunks of copper-tungsten (0.1cc total volume for both chunks) doubled the weight of the clutch shoe.

Seems like a good choice for adding weight. It's good to know, like I said above I'll be looking into this also soon. I was able to add softer springs to the 2 speed and it shifts fine now. But thinking a little extra holding force would be nice insurance.

Video
 

New video up with different clutch shoes.

https://youtu.be/eAjBD1x3Hb8

That's much better but still so bad. Do a stall test. Hold the truck still and give it wide open for a sec, see what kinda rpm the engine reaches. It still sounds like full rpm all the time to me.

Yeah it's still slipping bad that's for sure lol. At least it's getting up and moving now, but still so much left in it. I held truck yesterday while giving it gas and it won't stall out, slipping to much.

Not stall out, I mean clutch stall as in the highest rpm the engine can reach without the wheels turning.

I'd say around 5000rpm

it sounds better than your first video, sounds like the clutch might actually be grabbing now at high rpm, but the engagement point is still way too high.

just for a point of reference, a normal aluminum clutch shoe is 1.7 to 2.0 grams, 2.0 being a heavy shoe for a 2 stroke. my steel shoes are 3.8 grams, and i run 1.0 springs with them.

I got some .7 springs coming, well see if that helps. I sent ya a pm about those steel shoes. I'll take them if you still want to sell them. Kicking myself in the butt for selling my mill and lathe. Just had to much crap to move at the time, and no room in trailer to hual them.

Sign me up for a set of Sharkey Shoes, too.

i actually ended up finishing those shoes off and putting them in the xo-1 already. i can do a run of these at some point, but im unsure at this point when that will be.

Sounds good, hope to have a lathe and mill again here soon. So I may be good by time you make another set.

Are you sure the clutch bell is thoroughly degreased?

Steel-on-steel sounds like a bad idea -- not as bad as aluminum-on-steel, but still not great. Have you tried making bronze shoes? They should be even heavier than steel, which would allow for some end-user fine tuning by drilling holes through the shoes to lighten as necessary, and bronze is a much better choice for a friction material because it's self-polishing, unlike aluminum, and it can grip harder than steel.

EDIT: If you're using *stainless* steel, though, that would actually work pretty well, since it's softer than carbon-steel.

steel on steel isnt ideal, but its working. so far ive seen no damage whatsoever to my clutchbell and the shoes look fine.

when i looked into making shoes i looked at the coefficient of friction of several materials. bronze on steel has a static cf of 0.22. brass on steel is slightly better at 0.35. steel on steel however is 0.6-0.8. taking that into account, and what i had around the shop, i figured id give it a try, and so far its working great.

if someone was wanting heavier shoes i can add some material to them, i copied the ofna shoes as best i could but they ended up a little smaller. after running them with a couple different spring combos (1.0 is best for me, 0.9 i had some bogging as it was coming in too soon) i saw no need to add weight to them.

My steel shoes show almost no wear, same for the bell. Steel on steel is fine.

Steel is one of those metals that will tolerate scrubbing until it suddenly doesn't. Google "galling" for more info. Aluminum is really bad about it, and that's why I will never run aluminum clutch shoes no matter how much "punch" I might get from them, but steel does it too. Stainless steel is less prone to galling than carbon-steel (in my experience) because its chromium content makes it more slippery and self-polishing, but it still doesn't perform as well as copper alloys like brass or bronze.

The coefficient of friction is only part of the story. A higher coefficient of friction will make the shoes bite harder assuming they weigh the same, but brass is more dense than steel, so the same-size shoes made of brass will weigh more and experience more centrifugal force at the same RPM. That higher centrifugal force translates to higher pressure on the interface between the clutch shoes and the clutch bell, which compensates for the lower coefficient of friction. Plus, with a coefficient of friction that high, the steel shoes will start galling eventually. Unlike aluminum which will gall without warning, at least the steel shoes will squeal when they start galling.

Consider that Teflon has a coefficient of friction of .04 -- nowhere near the coefficients of friction of brass or bronze -- but despite this my HPI Bullet ST with a Losi 3.4 engine and a 3-shoe Teflon clutch can pop wheelies often enough that it gets irritating. The amount of centrifugal force exerted on clutch shoes is immense, and that will ensure solid engagement unless you have a severely undersized clutch or it's contaminated with oil.

- - -

I wonder how much of this "my clutch is slipping" problem is actually the result of inaccurate expectations? Yes, 4-stroke engines produce more torque than 2-stroke engines, but torque is a measurement of instantaneous force -- it's the horsepower rating that tells you how the engine will perform on-average as it's running. Even then, the horsepower rating is *peak* horsepower, not *average* horsepower like you could calculate using an idle-to-redline dyno test, and I don't know if anyone ever dyno-tested these engines, so who knows what the *average* horsepower looks like.

A single-cylinder 4-stroke engine only exerts forward torque on the vehicle's drivetrain 25% of the time -- the other 75% of the time, during the other three strokes, the engine exerts *reverse* torque on the vehicle's drivetrain, drawing power out of the drivetrain's rotational momentum. To get power delivery that "feels right" if you're used to 2-stroke engines, you'd need a 2-cylinder 4-stroke, and at that point it would become completely impractical to install in a RC car.

Also, keep in mind that engines are rated according to the piston's displacement over a complete combustion cycle -- two crankshaft rotations, for a 4-stroke engine. So if you have a FS-26S, the cylinder size is not .26 cubic inches, it's .13 cubic inches -- roughly equivalent to a .12 engine, except it's rotating at half the speed of a 2-stroke engine, and it's only producing forward torque for .5 out of every 2 crankshaft rotations, whereas the 2-stroke produces forward torque for 1 out of every 2 crankshaft rotations. So the power delivery of a .26cu-in 4-stroke engine is only a quarter of the power delivery of a higher-revving 2-stroke engine.

To really benefit from the torque advantage of a 4-stroke, you have to get one that's rated at quadruple the displacement of the 2-stroke engine you're replacing. Only then will the 4-stroke engine produce enough power during a combustion cycle to compensate for having 1/4 as many power strokes as the 2-stroke engine you're replacing. (1/2 the RPM * 1/2 as many power strokes per crankshaft rotation.) To get the same power output as a .12 2-stroke engine, you'd need a .48 4-stroke engine, not accounting for improvements in volumetric efficiency thanks to having valves instead of ports, anyway.

Fyrstormer, you're "engineering" again. You always seem to think to far into literally everything man! Haha

Also, as for expectations:
Watch speedworks's video then watch Sharkey's. That's the difference a non slipping clutch makes.

Well, he did give one accurate advice, Google! :lol:

i agree, coefficient of friction is only one part of it, centrifugal force is the other. yes bronze shoes will be heavier, however thats going to mean heavier springs are needed to keep it grabbing at idle. i know the relationship between the spring force and heavier weight isnt linear, it would take some playing around to get it right. ive never said making bronze clutch shoes is a bad idea, and im not ruling out making some in the future. in the end i looked around the shop and i had a chunk of 3/8 thick steel and no 3/8 bronze, so i made them from steel. end result is they work, maybe not perfect from an engineering standpoint.

the clutch slipping problem isnt from inaccurate expectations, its actual clutch slip. i run an rpm sensor on my engine and i can view a data log as a graph on my radio. when i first ran the aluminum shoes on my buggy you could hear the engine sitting at a high rpm and the buggy was accelerating. the graph showed it was going to around 17,000rpm almost instantly and staying there, the graph looked like a box. after putting the steel shoes on you can look at the graph and you see a nice slope as it accelerates. the end result is using a clutch thats designed to engage at 20k rpm, putting lighter springs on it and putting it on an engine that maxes out at 20k rpm doesnt work.

im a little confused on where your headed with the "a .26 4 stroke isnt a .26 its a .13". engine displacement is bore x bore x stroke x 0.7854 x cylinder count. that doesnt change if its a 2 or 4 stroke. if your talking expected power output, you cant directly compare expected output difference between a 2 and 4 stroke that directly. saying we have 1/4 the power of a high reving 2 stroke is far from accurate, so is needing 4x the displacement to be the same. in practice, my .56 in my buggy compares fairly well to a friends .28 powered buggy, and im only double the displacement of it. i can also run longer on a tank of fuel, but thats a different conversation.

you can engineer something to death, but in the end its hard to argue with whats working just fine in practice.

RC is an engineering hobby, dude. Why do you think I got sucked into it? 58 vehicles in 4 years, with the 59th in testing, the 60th in planning, and the 61st and 62nd sitting in their boxes waiting for me to get around to them. And I can afford all of them because "thinking too far into literally everything" ensures that I am very well paid.

If you get anything in this hobby to work right without thinking it through, it's either because of good luck, or good advice from someone who *has* thought it through.

Fair enough. Work with what ya got.

Oh, wow, yeah that's definitely clutch slip alright. That kind of data is invaluable for sure. I've considered getting a receiver that supports telemetry for outside-the-box projects like this, but thus far I haven't actually needed one.

Have you sanded and polished the inside of the clutch bell? That will ensure maximum contact area.

Well...I tried to find sources to back up my claim, but I can't find any. I learned in physics that the displacement of 4-stroke engines is measured over two crankshaft rotations instead of one, because it takes two crankshaft rotations to complete a combustion cycle, but damn if I can find any references to that online. I'd probably have to go to the library and dig through old engine-design books. The reason for doing it that way is because the volume compressed by the piston over two crankshaft rotations more accurately reflects the engine's energy consumption as it continues to rotate between combustion strokes. A 2-stroke engine is only losing energy during the upward stroke, but a 4-stroke engine is losing energy during two upward strokes and one downward stroke. In any event, I did the math, and you're right that O.S. rates the displacement of their 4-stroke engines over a single crankshaft rotation, same as their 2-stroke engines. So, whether or not that's the "most correct" way to do it, it does seem I was wrong about their engines. I concede the point.

A piston engine regardless of whether it's a two stroke cycle or four stroke cycle, the cylinder can only displace what it can displace. Pi * R^2 * Stroke = cylinder displacement volume.

This isn't just how OS does it, its how every piston engine is measured and has for just about forever? An engineer should certainly know this?

Like I said, I learned it in physics class, and in physics, displacement is measured in a very specific way, which I suppose engine designers are not obligated to follow. Of course, if you want to get super technical, all engines have a displacement of zero, because the piston always ends up in the same place it started. ;)

- - -

However, I maintain my claim that a 4-stroke engine needs to be 4x the displacement to produce the same power output as the 2-stroke engine you're replacing. Take for example the Novarossi ISON .12 -- it is rated to produce 1.56hp at 41100rpm, and that means it produces .19ft-lb of torque at that RPM. Compare that to the O.S. FS-26S-C -- it is rated to produce 0.5hp at 17000rpm, and that means it produces .15ft-lb of torque at that RPM.

So far it sounds like the O.S. FS-26S-C is only falling short on torque by 22% compared to the Novarossi ISON .12, but to achieve the same top speed you will need 2.4x higher gearing on the O.S. FS-26S-C, and that will slash the torque to the wheels down to 0.06ft-lb. If you have a super-efficient chassis, that might be enough torque to overcome drivetrain friction and wind resistance to reach the same top speed, but it probably isn't. Either way, with the FS-26S-C installed, the the car's acceleration will be significantly reduced by having only 31% of the ISON .12's torque at the wheels.

To get the same torque at the wheels that the ISON .12 provides, you have to step up to the O.S. FS-91S, which is rated for 1.6hp at 11000rpm and produces .76ft-lb of torque at that RPM. Gearing-up to compensate for the dog-slow rotational speed of the FS-91S (assuming you could even find a way to install 3.7x higher gearing into a touring car), you'd get .20ft-lb of torque to the wheels, which is slightly more torque than the ISON .12 produces, but it requires eight times the displacement to achieve it. This is even higher than the 4x multiplier I originally stated, but I'm making allowances for the fact that the FS-91S is an airplane engine instead of a car engine. It's possible with sufficient modifications to a FS-40S-C, you could somehow increase its output by 64%, wringing-out 1.48hp at 17500rpm. After adjusting for the higher gearing necessary to reach the same top speed, that would give you the same torque to the wheels as the ISON .12. I'm not sure how you'd mod the FS-40S-C to make so much more power, though.

- - -

Granted, this analysis only takes into account the peak power output of both engines, not the full torque-curve, and we all know that 2-strokes tend to have very peaky torque curves because of their reliance on resonator exhausts. So it's quite likely that an O.S. 4-stroke engine would provide significantly more torque to the wheels at low RPM, and that would work great for off-road trucks which need low-end torque more than they need top speed.

However, I'm looking at it from the perspective of touring cars, because all the examples I've seen in the past few years have been installed in touring cars. For a touring car it's the torque available at high RPM that determines the car's performance. That's probably why your experience with your 1/8-scale buggy has been different than what my analysis suggests.

Anyway, I'm all nerded-out now, so I'm going to go cook dinner.

the only proper way to compare 2 engines is with a dyno graph, something not published for r/c engines because the power figures are all "calculated". i have yet to see a manufacturer release actual dyno charts for their nitro engines, the closest ive seen is when r/c car action had their engine dyno, and the results on that showed most of the italian race engines to make far less power than claimed. from what i can remember, every engine they tested was over rated, and os engines were the closest to what they claimed.

the 4 strokes we are using are fairly crude adaptations of a plane engine, and out of the box performance reflects that. if you read the early info in this thread a user had his own home made dyno, and he proved the fs26 that a better carb and exhaust almost doubled the power output. my fs40 responded the same way in my nitro tc3. i do remember one user did also have good results with raising compression. there are a lot of things that can be done to increase power and further refine these engines, and i dont think its out of the question for my .56fsa to be hanging with high end .21 race engines when im done with it. i already think in the hands of the right driver it would be very competitive on a tight track.

onroad isnt really the place for 4 strokes. in my nitro tc3 the fs40 was fast, with the tallest gearing i had it would accelerate as hard as the rossi .21 i previously had in it, but peak far sooner and top speed was more on par with a .12. i was geared 36-29 in 2nd, i just couldnt find stuff to go taller.

Sharkey is onto something - inflated power figures from the manufacturer. Trying to calculate anything from their claimed power figures is fruitless because in order to obtain those readings, they were using such a minimal load and likely non-standard fuels. Never trust or expect to get the manufacturer claimed power figures. It is also true that the FS26 is an airplane derivative; the main differences is the camshaft (car cam is timed hitter for higher rpm) and stiffer valve springs. Guys that want more power from their FS26 and FS30 engines would use the car camshafts and valve springs. Run the rpm too hot and they will drop a valve (springs break and/or valve retainers).

The closest power figures I've gotten was from Dub Jett engines - he publishes a minimum max rpm for a given load for his engines and test runs each engine to ensure it meets those criteria. These engines are made to order though, and are hand fitted (extremely tight!!)