The weight isn't 'hiding' anywhere, simply a fact of life with a counterbalanced single cylinder using a flat plane crankshaft. Multi cylinder engines with a flat plane crank also have more vibration than cross plane cranks for the same reason. As for 'side-to-side' imbalances, it's the same the 'up-down' imbalance. There is no other force acting on the the rotating assembly other than centrifugal. There is a very small side load as the rod transitions at TDC and BDC but, it is quickly gone as the crank rapidly accelerates in the opposite vertical direction. There have been many experiments in offsetting the cylinder in relation to the crank centerline to reduce the side load at transition. There have only been minimal improvements and, with your FS-40 it would require a custom crankcase, which is out of the question.

Reducing weight overall in both the crank and piston/rod will help due to the lower inertia. If your assembly is already near 50-51%, you could try taking weight out of all the parts and likely see an improvement in not only vibration but acceleration and peak rpm.

This discussion is fun, I haven't been able to talk engine design in quite a while. All the focus is on electric, which is expected but, it's still nice to turn fuel into noise sometimes.

Iv been thinking about doing weight reduction/balancing on my .91. Haven't had the time and maybe the motivation to do so yet though. But when I do I plan to start with piston/rod weight reduction. Then go to crankshaft as im looking for more rpm with vibration dampening. Ill be doing the razor blade setup that was talked about above. Really want 20k+ rpm out of this beast!

"Where is the excess weight hiding" was a figure of speech.

Understood. I like your avatar. I too am a fan of whirling doritos of death. I've built many of the O.S. version.

Guys been reading through the thread. Lots of great info. Trying to consolidate important stuff in on post. Missing lots let me know what and will add.

FS30 To car spec
Cam OSM45762010
Valve Spring x 2 OSM45760210
Intake Manifold OSM45269400
Carb Retainer OSM21481700

Other options
Complete OS26 Head OSM44104020
Compete Valve Assy x 2 OSM45760020

FS26 to 30 size
Liner OSM43003100
Piston OSM43003200
Ring OSM43003400
Pin OSM43006000
Gasket OSM22714100

FS40 To car spec
Cam OSM45262050
Valve Spring OSM45260250
Cylinder Head OSM45204150
Exhaust Nut OSM45169200

Other options
Compete Head OSM45204050



Flywheel
Heavy steel flywheel recommended for low idle.
Option 1
OS Drive Washer OSM 45208010
Standard Flywheel
Associated Clutch Nut 2312 (Drilled and tapped to Ľ-28
Option 2
Get Crankshaft ground by machinist to 7mm OS FS26S-C 4-stroke development
Then use standard collet like Associated part 7618 and standard flywheel
Option 3:
Get crankshaft ground to 7mm by machinist, then use BuKu 34mm brass flywheel and BuKu BBK shoes with standard springs.

Option 3:
Get crankshaft ground to 7mm by machinist, then use BuKu 34mm brass flywheel and BuKu BBK shoes with standard springs.

Speaking of BuKu BBK shoes: I've been having trouble with my Revo's clutch slipping and overheating while driving through grass with 2-gram "heavy" ABK shoes, so I ordered a set of 4-gram BBK shoes to get more grip. I was surprised to find that Dave also sent me a set of 5-gram extra-heavy BBK shoes! He guessed that I was working on something unusual and needed the heaviest clutch shoes around. Now I just have to decide which set of shoes to try first; obviously the 5-gram extra-heavy BBK shoes are the best bet, but I wonder if they're so heavy that I might encounter spring-tuning issues. The 4-gram heavy BBK shoes are still 2x the mass of the "heavy" ABK shoes I was using previously, so they ought to work pretty well too. Then again, I'm never going to run a heavier vehicle with this unusual engine setup, so maybe I should just go straight to the extra-heavy shoes and save the slightly lighter ones for a future project. Choices, choices...

He sent me the extra heavy ones also, as he knew the project. Truth is, I haven't tried them since the tungsten weighted aluminum shoes have been so successful. So I'm really interested in your results.

The extra-heavy BBK shoes DEFINITELY have more grip. The first time I blipped the throttle, the truck stood up on its rear bumper, and then fell over upside down. This is the first time I've had to adjust a BuKu clutch to make the shoes engage at different RPMs for smoother engagement.

Alas, even the extra-heavy BBK shoes can't withstand the BONE-CRUSHING POWERRRRR of the FS-40S-CX. While the shoes no longer slip constantly, they do start slipping as soon as the transmission shifts into second gear. When the transmission shifts, the engine RPMs drop and the centrifugal force on the clutch shoes decreases, and at that lower RPM they just can't transmit the engine's full torque without the extra leverage that first gear offers. I'm just going to have to reduce the pinion/spur gear ratio and accept the lower top speed; I suppose it's not too much of a loss since I rarely drive the truck on pavement anyway.

- - -

Oh, and the first time I ran the truck with the extra-heavy shoes, about halfway through the run the engine stopped and wouldn't start again. I didn't have time to figure out why, but when I went back and inspected the truck later that night, I discovered the flywheel was spinning freely on the crankshaft. The extra-heavy shoes were resisting slip so effectively that the next-weakest joint started slipping instead -- the crankshaft spun inside the flywheel collet and ground it up. I spent 15 minutes cleaning brass shavings off the engine and clutch before reassembling it with a new collet, plus a few drops of Red Loctite between the collet and crankshaft to hopefully prevent it from slipping again.

On a related note, is there a special tool used for holding a flywheel while tightening the nut that holds it in-place? For the past few years I've used a pair of parallel-jaw pliers to brace the pivot pins on the front of the flywheel while tightening the nut, but every now and then I wonder whether this improvised method is someday going to bend the pins on one of my flywheels.

Yeah, there are special tools. I've used channel lock pliers for 18 years.
Cool update! Not sure why you would have slip on those monster shoes though. My setup has more power, pushing an 11lb truggy against a tall ratio to hit 46mph, and it doesn't slip. Maybe your shoes need to wear in?

Wouldn't that flywheel breaking free issue be the reason your truck isnt doing well in second gear? Did you rerun it after the red loctite? Or does engine bog down really bad once it shifts and rpm never recovers? Mine did that with the .30 in the heavy truggy. Was just to much gear, clutch never slipped and only had 2.6g weight on them.

Your shoes shouldn't be slipping unless those springs are just to much. Or those composite shoes don't work that well. There is talk on other forums here about not using those shoes in heavy truggy only buggy. I have some to try also just holding off for now.

I guess I don't know for sure that the flywheel didn't slip a little bit before it finally let go completely, but I assume the D-cut in the crankshaft acted like a machining bit as soon as it was able to turn at all and it reamed-out the collet pretty fast. I don't see any rub marks on the crankshaft that would suggest the collet was slipping for a long time before it failed.

The truck bogs down only in 2nd gear. But also I'm driving it in grass that only gets mowed every 3 weeks or so, and that causes an enormous amount of extra drag. There's no problem at all when I run it on dirt or pavement.

I've thought about whether solid brass or stainless-steel shoes would work better, but I have no machining capability so I'm at the mercy of what the market offers. The BuKu clutch is the closest I can get to a perfectly designed package, because it has a heavy brass flywheel, heavy brass shoes, and adjustable springs available in multiple strengths. (I'm using the lowest-strength springs so the clutch will drag for the least amount of time as the engine revs-up.)

I haven't had a chance to drive it again. Partly I've been busy with other things, and partly I'm waiting for a 20t clutch bell to arrive in the mail. I'm going to drop the gear ratio down to 20/38 from the 22/36 I was using previously; if that is sufficient to fix the problem, then I can start investigating ways to regain the lost speed by modifying the engine for higher RPMs instead of relying on high gearing.

Sounds like a good plan dropping the ratio. I think your setup will be happier topping out around 28-30mph or so. The 40sc is a car engine and revs well, yes, but it's still just a 40. And you truck weighs what, 12lb? Imagine it in a 6-7 lb stadium truck.

"Just" a .40, LOL. It's double the displacement of any of my other engines (or more!), including the Losi 3.4 engine in my T-Maxx, which is only 290g lighter than my Revo. Sadly, the parasitic losses from driving a 4-stroke valvetrain appear to sap a significant amount of the power it could make if it didn't have that extra hardware. (or maybe it can't breathe well enough.) I suppose that's why 4-strokes never caught-on for ground vehicles.

Haha. Yeah... "just" a .40 sounds weird. But you're right about the losses. When you go up some in displacement though, the power comes on pretty good. My 62 is a beast... and speedworks even has a .91 build. I haven't seen that one in person yet, but its gotta be insane.