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Old 03-14-2013, 01:52 PM   #16
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Here are two pictures, and the engines are OS 18TZs.



Well, I tried to upload two pictures but don't know how.
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Old 03-14-2013, 02:29 PM   #17
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Originally Posted by MantisWorx View Post
Maybe you are confused on what flex is?? I dont dis agree with the fact that you need a certain amount of flex but not on a crank, you will get more play from the bearings than you will from the crank. Exactly where do you think the flex will come from? think about it, your not going to get any flex between the bearings ( if you did the crank would bind)meaning it can only happen on the largest area of the crank!
I have seen a video about Cosworth engines where the engineers did calculate the zero headclearance at max rpm and indeed you could see a complete clear squish band on the piston. Flex is there and it is needed to stay alive. I just calculated it, at 44.000rpm the (lineair) acceleration of the piston in an half stroke a 3 gram piston can create about 30kg of force. With the conrod making 5 to 6 grams in total it can go up to 50kg, that is huge! It is for sure pushing and pulling hard on the crankshaft,

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Here are two pictures, and the engines are OS 18TZs.



Well, I tried to upload two pictures but don't know how.
You need to upload them....
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Old 03-14-2013, 04:09 PM   #18
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Originally Posted by Roelof View Post
I have seen a video about Cosworth engines where the engineers did calculate the zero headclearance at max rpm and indeed you could see a complete clear squish band on the piston. Flex is there and it is needed to stay alive. I just calculated it, at 44.000rpm the (lineair) acceleration of the piston in an half stroke a 3 gram piston can create about 30kg of force. With the conrod making 5 to 6 grams in total it can go up to 50kg, that is huge! It is for sure pushing and pulling hard on the crankshaft,



You need to upload them....


I understand what you are saying but that movement is not soley from the crank but more from the other moving parts. The crank is only one part of the equation and since the crank is the hardest and most unlikley part to actually "flex" the movement you see is from the soft piston deflecting,rod deflecting, dual bushings and wrist pin all of which will move/flex more than a crank. Why would the actual crank flex before any of the other moving soft parts? I want to bring up the fact that movement and flex are two completely different things. A resonance can and will cause flex where as movement is just simply from tolerances on moving parts. I am a little confused on which one you are attempting to explain!!
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Old 03-14-2013, 04:37 PM   #19
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Default Piston skirt

Just food for thought: I had the Same problem twice with same engine. Fortunately I was at a race which Rody Roem was attending. Rody explained to me that a broken skirt is usually caused by a worn crank pin. He explained that with a worn pin at high RPMs the resonance travels through the rod to the piston and skirt and will cause it to crack. Made since to me as it happened to twice to the same engine.
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Old 03-14-2013, 04:45 PM   #20
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Quote:
Originally Posted by Roelof View Post
I have seen a video about Cosworth engines where the engineers did calculate the zero headclearance at max rpm and indeed you could see a complete clear squish band on the piston. Flex is there and it is needed to stay alive. I just calculated it, at 44.000rpm the (lineair) acceleration of the piston in an half stroke a 3 gram piston can create about 30kg of force. With the conrod making 5 to 6 grams in total it can go up to 50kg, that is huge! It is for sure pushing and pulling hard on the crankshaft,



You need to upload them....
Quote:
Originally Posted by MantisWorx View Post
I understand what you are saying but that movement is not soley from the crank but more from the other moving parts. The crank is only one part of the equation and since the crank is the hardest and most unlikley part to actually "flex" the movement you see is from the soft piston deflecting,rod deflecting, dual bushings and wrist pin all of which will move/flex more than a crank. Why would the actual crank flex before any of the other moving soft parts? I want to bring up the fact that movement and flex are two completely different things. A resonance can and will cause flex where as movement is just simply from tolerances on moving parts. I am a little confused on which one you are attempting to explain!!

Flex is present and necessary. Every crank I've inspected that broke the crank pin was due to the hardening being done incorrectly. The crank pin was too stiff and did not flex enough when loaded by the piston assembly. If the piston assembly was a 'soft' as you're implying in your arguement, the bushings and piston would beat themselves to pieces in short order once the engine accelerated above 10000 rpm.

The rod bushings are made of the material they are for long service life and lack of friction on the steel crank and wrist pin. Yes they are 'softer' than the parts they ride on but hardly 'soft'. They are intended to be the wear items in the assembly to prevent failure of the more expensive and harder to produce crankshaft and wrist pin.

Per my calculations, an engine running at 44000 rpm (Roelof's example) with a 18mm stroke (typical average for a .21), gives 36mm of piston vertical travel per revolution. That works to 1,584,000mm per minute. This is equal to 62,362 in. per minute approx. That gives you a piston speed of 5196.850 ft/sec or 3543.307 mph or 4.654 mach. That's just an approximation covering the entire travel. Don't forget the piston comes to a stop twice per revolution and then accelerates the opposite direction to peak speed about mid stroke before starting to decelerate at the opposite end. Add to that cylinder pressure on the compression side of the stroke and the spike at combustion. You can be fairly certain that EVERY part in the reciprocating assembly is flexing/bending/moving, whatever you want to call it most if not all of the time. That sound you hear when the car goes by is the exhaust note, sure. It's also the sound of the piston breaking the sound barrier many times a second. Find a video from Lostallo, Switzerland when they run 1/8 open. The amount of time spent at WOT per lap is around 13-14 secs. The front straight alone account for over half that time. Listen to the cars as they pass the camera. Amazing does not describe it.

Yes, rods do stretch, bushings deflect, wristpin bend, bearings have runout, but the crank also flexes both in whole and at the crankpin when running. I've been studying these engines for a long time and I'm still amazed they actually run and don't destroy themselves instantly, especially they way they are treated by the majority of the users that have them.
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Old 03-14-2013, 05:40 PM   #21
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I think you are wrong with your calculations... The max speed of the piston is the same as the rotating speed....

The stroke of current .21 engines is about 17mm, 1 round is 53.4mm in distance.
Lets say @46.000 rpm is 766rps. In 1 second it is running 766 x 0.0534 = 41 meter/sec which is 143km/u (no sonic sound)
I can remember a limit for lubrication has a magic number of 35m/s...

V=a x t , a = V/t

t is 1 round /4 (the moment from 0 to max speed) so that is 1/766 and again /4 = 0.326msec

a = 41/0.000326 = 125624 m/s^2

10m/s^2 = 1G so you are talking about 12562.4G

I know a piston+rod is about 7.5 gram so it will create 94kg force @46.000rpm

This is pure theoretic because a 90 degrees travel of the crakshaft is not giving a halfway of the piston travel. A 90 degree exhaust timing is somewhere arround 7mm so there is a 7mm and a 10mm acceleration and not exactly half. And these calculations are in a lineair movement but actually is is more like a sinus shape so actually the 94 kg is a bit higer.

As you can see with such forces manufacturers are searching the limits of the mechanical strength.

By the way, some of the noise of an engine is comming from the cooling head, the vibrating fins can produce some noise....
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Old 03-14-2013, 05:51 PM   #22
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The term soft is in relation to the crank which is WAY harder than the rods or piston which are both made of aluminum! So what i am trying to understand is regardless of the speed of the piston and what not, when you are at the point when the flex is supposed to happen for the crank to flex means that the rod and or piston is fully flexed/stretched?? reason being is that it seems to me that in order for the crank to flex via the force of the combustion cycle all of that force must travel through the piston and rod before it even gets to the crank, therefore THEY take a fair amount of the abuse. Now with the crank pin sticking out with no support obviously makes that the weak link more so than the say middle of the crank, so i can see how an overhardened crank can lead to destruction. I have been machining metal for over 25years from aluminum to inconel, nitronix 50 and anything in between! So what you know in nitro engines i know in the machine process of creating them( my 4 axis CNC mill is running as i type this!!)! taking a wild guess but i assume the pistons are a form of 6k series aluminum (6061) rod a 7k series (7075) the bushing are more than likely made of a self lubricating bronze-oil lite and since the cranks are being hardened I am sure they are some sort of carbon based alloy but I am not sure on that. Now taking all of that into consideration the crank is easily 10-15 times harder (maybe more) than the aluminum parts and being the "caboose" on the line of fire is why i am having a hard time seeing the crank flexing (not moving). I think the reason the over hardened crank broke is simply shear force and shock and the softer crank can absorb that shear force/resonance. For something to flex and break implies that it bent before breaking and everytime I have seen a crank break it is sheared off straight at the pin. Make sense? I am not saying that the crank never flex's but the amount it does is multiple times less than the rest of the assembly.
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Old 03-14-2013, 06:15 PM   #23
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Originally Posted by speedracer1bp View Post
Just food for thought: I had the Same problem twice with same engine. Fortunately I was at a race which Rody Roem was attending. Rody explained to me that a broken skirt is usually caused by a worn crank pin. He explained that with a worn pin at high RPMs the resonance travels through the rod to the piston and skirt and will cause it to crack. Made since to me as it happened to twice to the same engine.
I can see the logic in this and all of the motors were in need of new rods but I didn't think they were that bad. It's a shame no one make after market pistons and rods for these little motors.
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Old 03-14-2013, 08:11 PM   #24
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and max piston speed is also in relation to conrod length as to at what degree of rotation it is achieved and is also the point of maximum side load on the piston when rod angularity to crank centerline is at 90 degrees
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Old 03-14-2013, 11:18 PM   #25
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hi that was good reading but I am a carpenter/rc came later and I am just getting my head around changing /rod before it all goes bang one nats== new rod and since then no bang but it was good reading///64 not out but it still makes me think when you look what is in there what makes them go like hell and not explode
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Old 03-15-2013, 06:00 AM   #26
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Without getting in the actual 'fight'.
I used a quick calculator to get some basic numbers for a Novarossi, both 16.8 and 17.6 mm stroke at 45000 rpm.

Possible "Piston-Rock???"-novarossi-mps.jpg
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Old 03-15-2013, 07:24 AM   #27
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Nice! Which program is that?

With a simple straight calculation I was not far off
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Old 03-15-2013, 09:47 AM   #28
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Nice! Which program is that?

With a simple straight calculation I was not far off
You where quite close!

I used this (you find it if you scroll down); http://2.3liter.com/Calc2.htm

Im not sure how accurate it is, but for a quick calculation I think its accurate enough.
For more in-depth analysis I use other programs.
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Old 03-15-2013, 11:07 AM   #29
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Quote:
Originally Posted by Roelof View Post
I think you are wrong with your calculations... The max speed of the piston is the same as the rotating speed....

The stroke of current .21 engines is about 17mm, 1 round is 53.4mm in distance.
Lets say @46.000 rpm is 766rps. In 1 second it is running 766 x 0.0534 = 41 meter/sec which is 143km/u (no sonic sound)
I can remember a limit for lubrication has a magic number of 35m/s...

V=a x t , a = V/t

t is 1 round /4 (the moment from 0 to max speed) so that is 1/766 and again /4 = 0.326msec

a = 41/0.000326 = 125624 m/s^2

10m/s^2 = 1G so you are talking about 12562.4G

I know a piston+rod is about 7.5 gram so it will create 94kg force @46.000rpm

This is pure theoretic because a 90 degrees travel of the crakshaft is not giving a halfway of the piston travel. A 90 degree exhaust timing is somewhere arround 7mm so there is a 7mm and a 10mm acceleration and not exactly half. And these calculations are in a lineair movement but actually is is more like a sinus shape so actually the 94 kg is a bit higer.

As you can see with such forces manufacturers are searching the limits of the mechanical strength.

By the way, some of the noise of an engine is comming from the cooling head, the vibrating fins can produce some noise....
You are correct, I did those calculations in a hurry last night and forgot to do the last conversion. That gives me a more realistic piston speed of 25.226 m/sec.

I know the fins do resonate. More of the noise anymore is not from the exhaust but the intake, even with INS boxes. Does EFRA still require the 3 chamber mufflers? I remember Frankie doing noise testing at Lostallo at the WC in '09. Didn't see much difference in dB output and the engines ran worse if I remember correctly.
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Old 03-15-2013, 12:29 PM   #30
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Intake does make some noise but mounting an extra mufler on the already excisting exhaust system is doing more to lower the sondlevel than an insbox.

The 30xx 3 chamber exhaust were crap, no one is using them in the onroad although the 3036 I still have was a good exhaust. Now we have still the old 20xx type 3 chamber (with internal extra holes and larger holes) but some were taken off the list. There were some experiments with a small extra snap on silencer but no succes and now all eyes are focust on the new Hipex exhaust wich seems to have a dubble wall acting as a 4th chamber.
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