Opinions Tapered D. Head Rods

[Hot-Shot Motorworks]

Glens,    I am trying to find, without much luck, a moving cutaway illustration that may help you understand what the rod angles look like in relationship to the cylinder.  I will let you know when I do.  I noticed one on a web site some time ago but can't remember who's it was.

[Hot-Shot Motorworks]

Stability...

[glens]

There's http://www.animatedpiston.com/New%20AP%20Site/Twin%20Cam%20Button.swf but it's not terribly detailed.

My first profession decades ago was as a pre-computer detail draftsman and I worked for a large machinery manufacturer.  I have no problem visualizing this stuff.  It would pretty much require an "official" set of prints (at least the pertinent ones) to convince me there's an asymmetry in the rod angles front-to-back between when the crank pin is on the cylinder side of center and when it's on the other side of center.  But I look forward to anything you can find.  I realize it's really just academic but I have that mindset

On a separate note, I've had to fetch your referenced web page in a text-mode browser to be able to read it.  In both of the standards-compliant browsers I have/use, the images obscure the text quite inconveniently.  Microsoft is not known for caring about standards anyway, and "Office" really is a poor way to create a public web page.  Sadly, I primarily use MS Windows for interfacing with my TTS VCI.  Certainly not for connection to any public networks.  Please take this in the good spirit in which it was intended!

http://validator.w3.org/unicorn/check?ucn_uri=www.hotshotmotorworks.com%2Fdidja_know.htm&ucn_task=conformance

[ongrade]

Just a thought, based on engine rotation and firing order, doesn't the rear cylinder fire as the front is coming up on the compression stroke? If so then you've got the rear cylinder giving the crank and consequently the front rod a kick in the ass on its upstroke. This could cause a sudden shock load on the rod and piston driving it into the cylinder wall.

[Admiral Akbar]

Just a thought, based on engine rotation and firing order, doesn't the rear cylinder fire as the front is coming up on the compression stroke? If so then you've got the rear cylinder giving the crank and consequently the front rod a kick in the ass on its upstroke. This could cause a sudden shock load on the rod and piston driving it into the cylinder wall.

nope, exhaust stroke..max

[ongrade]

Okay, exhaust stroke, but the piston is still traveling upwards at a consistent speed when the rear cylinder fires and gives the front a sudden hit. Same effect some of the energy is expended pushing the rod forward instead of up due to the rod angle.

[1FSTRK]

I do not yet have enough information to come to a definative conclusion here. I tend to agree with Glens at this point. I also want to add this is exactly why I think this forum is the best on the net. We have a collection of the real Myth Busters. It can be a thread like this or the one on the SE comp sprocket but at the end there is usually enough well researched fact to come to a conclusion.

[04FLHP]

I have been at the bike shop when they were boring cases for oversized cylinders and the fixture they use sets up on the main bearing, they bore the spigot hole then rotate the case 45 degs. then bore the other hole. After that they dust off the base surface with a fly cutter to make sure it is square to the crank mains and spigots. They did not offset the bore from the mains at all. The rod is a piece of metal with two holes in it, regardless of it’s shape the top will be located in the cylinder by the piston pin and in this case TDC will be when the main shaft, and both holes in the rod form a straight line. With the cylinders square and centered to this line every crankshaft degree before and after will create the same angle for both cylinders and rods  regardless of direction of rotation.
I would like to see any drawings or math that can correct me on this, as I am always willing to learn something new and to my knowledge no one has ever brought any of this offset information up before.

[John D]

I have yet to see the bushingless tapertop rod in any stock configuration, if someone else has I would like to see it (somewhat of an urban myth that HD corp initiated with their literature, I do not believe any of them actually hit the production line), been looking forward to it as when you spin a wristpin bushing nothing good happens and you certainly can not replace with out complete dis-assembly of your motor.  I can not tell you honestly how many I have seen spun, but more than I would ever want in a product that came from our facility. 

The stock taper-top rod works reasonably well in the stock application it was designed for, but I do not want to take ownership of it in a performance application.  I always want to make sure people are aware of what we see, what they choose to do with this information is their business, but dont shoot the messanger. 

The only method to replace a wristpin bushing in this application is with a tobin-arp pin boring machine with the rod out of the flywheel assembly.  We have a pin-boring machine, but have yet to find anyone with enough stones to actually replace one when the 1st has failed them.

JMHO, Thanks, John

[LilEvilAmy]

This is an Evo, but it may help to visualize what's going on and when ...

[Admiral Akbar]

Okay, exhaust stroke, but the piston is still traveling upwards at a consistent speed when the rear cylinder fires and gives the front a sudden hit. Same effect some of the energy is expended pushing the rod forward instead of up due to the rod angle.

Doubt it's much of a force compared to the thrust of the rod pushing the piston down from the power stroke.. The rear cylinder also has to accelerate the flywheels so the forces is less than the power stroke.. The angle of the rod won't be that much either.. Peak pressure during combustion is what 10-15 deg ATDC so the front will be 30 to 35 BTDC.. Not much angle there,,

We can get back to my oil theory.. The extra pressure the rod sees helps to squeeze out the oil between the rod bearign and the pin. There is not enough oil to recover.. Should show up as a wear spot on the rod side of the pin hole..

[Admiral Akbar]

I have yet to see the bushingless tapertop rod in any stock configuration, if someone else has I would like to see it (somewhat of an urban myth that HD corp initiated with their literature, I do not believe any of them actually hit the production line), been looking forward to it as when you spin a wristpin bushing nothing good happens and you certainly can not replace with out complete dis-assembly of your motor.  I can not tell you honestly how many I have seen spun, but more than I would ever want in a product that came from our facility. 

Maybe they got a really big box of left over rods.. They had more in inventory than they thought. Or the supplier had a ton of stock based on a bloated delivery schedule from HD?

The stock taper-top rod works reasonably well in the stock application it was designed for, but I do not want to take ownership of it in a performance application.  I always want to make sure people are aware of what we see, what they choose to do with this information is their business, but dont shoot the messanger. 

The only method to replace a wristpin bushing in this application is with a tobin-arp pin boring machine with the rod out of the flywheel assembly.  We have a pin-boring machine, but have yet to find anyone with enough stones to actually replace one when the 1st has failed them.

JMHO, Thanks, John

I would think that the non bushed rod would be better in the case of the tapered rod.. No bushing to come loose..In a tapered rod bushing fit is probably only as good as the narrowest part of the rod..  Since you've seen a bunch come loose, do you see a trend more towards the front coming loose or the rear??

Thanks,

Bruce

[gabbyduffy]

                          a question for the fellows who have actually seen bushing failure in tapered rods... What type of motors are you seeing the failures in? 88", 96", 103", 107", 113",120", or maybe it would be easier to answer this question in terms of horse power and footpounds of torque..... Failure over 120HP?.... I know this is a difficult question to answer because of all the variables but do you see a pattern to the failures of any kind? Thanks.

[glens]

This is an Evo, but it may help to visualize what's going on and when ...

That's useful.  I yanked out some of the individual images and reassembled them forming two, attached below.

In the one called "front.gif" the crankpin is roughly 90° to the front bore axis and in the one called "rear.gif" the crankpin is roughly 90° to the rear bore axis.  As nearly so in both cases as the images portray.

What has been suggested in this thread is that in those two positions (each cylinder) the angle of the connecting rod vs. the bore axis is different from front to back (of its bore axis).  In order for this to happen, either (both) the bore axes do not pass through the center of the crankshaft, or one piston wristpin is offset forward of its bore axis and the other rearward of its bore axis.

It has also been suggested (countered) that both bore axes pass through the center of the crankpin and that if the piston wristpins do not lie directly on the bore axes that they are offset both the same, not opposite each other.

[attachment removed after 60 days by system]

[BVHOG]

       Never herd the term "slipper skirt pistons" ? .... Can somebody give me a description. Thanks.

Slipper skirts  http://www.patentstorm.us/patents/5058489/description.html