knife edge the crank???

[thehelix112]

While we're on the topic of knife-edging, the shape of the knife itself I find interesting. For example, johnc's crank does not like it is particularly aerodynamic. Sure its a lot more aerodynamic than a stock crank, but it would seem to me that the cross section of the counterweight should essentially be a tear-drop shape, though maybe an assymetrical cross section does nasty things to dynamic balance I don't know. In any case, a look at the actual profile that is passing through the air/oil would probably be of further benefit.

 

Additionally, I would think that (depending on the operating speeds), this profile should change as you go `out' (further away from the main journal). The profile closer to the mains should be more rounded where as the profile towards the extremities could taper down and require a different shape. This is obviously because that `outwards' edge of the counter weight is going (much) faster than the inside edge.

 

Dave

 

*maybe* the outside of the

[johnc]

You have to be careful with the shape of the counterweights. If you end up cutting a wing shape the "lift" will increase the loading on the thrust bearing and cause it to fail.

[TimZ]

While we're on the topic of knife-edging, the shape of the knife itself I find interesting. For example, johnc's crank does not like it is particularly aerodynamic. Sure its a lot more aerodynamic than a stock crank, but it would seem to me that the cross section of the counterweight should essentially be a tear-drop shape, though maybe an assymetrical cross section does nasty things to dynamic balance I don't know. In any case, a look at the actual profile that is passing through the air/oil would probably be of further benefit.

 

Additionally, I would think that (depending on the operating speeds), this profile should change as you go `out' (further away from the main journal). The profile closer to the mains should be more rounded where as the profile towards the extremities could taper down and require a different shape. This is obviously because that `outwards' edge of the counter weight is going (much) faster than the inside edge.

 

It should be possible to shape the teardrop such that is symmetrical and does not create "lift". Of course if you want to start worrying about lift, then you need to take into consideration the effect of the proximity of the crank throws and the rods which periodically pass by creating ground effects and causing lift imbalances (good luck figuring that out). That said, we don't really know that the design that JohnC showed doesn't create lift. It looks like it probably doesn't create much, though.

 

It strikes me that if this is the approach you want to take (and it does follow the theory of the "need" for knife-edging much more closely) that you would want to put the knife edge (or teardrop) shape on the leading edge of the counterweight, not on its end. This is where all of the supposed drag would be occurring.

 

Personally, I'm dubious on how big of an effect this is in the first place.

[thehelix112]

Tim/John,

 

Thanks for the replies, will post some more in depth thoughts later, but for now.

Very good point about lift, I was planning on a symmetrical design as Tim mentions.

 

Tim, I when I say end, I was not referring to leading/trailing edge, but more the outer extremity of the counterweight (where its airspeed is the fastest).

Excellent point about the `ground effects' when the counterweight is spinning past the throw/rods, but lets not also forget the effects when the counterweight is spinning past the main towers.

 

I would suggest that this effect would be cancelled side to side? As we always have two throws/counterweights in the same position, and each side of the weight/throw will be symmetrical and hence undergoing the same ground effects but on opposite sides. Or have I forgotten something?

 

Dave

[johnc]

If you end up cutting a wing shape the "lift" will increase the loading on the thrust bearing and cause it to fail.

 

[TimZ]

If you end up cutting a wing shape the "lift" will increase the loading on the thrust bearing and cause it to fail.

 

I understood what you were saying, but an airfoil is more like a teardrop cut in half - flat on one side, curved on the other. If you made it curved on both sides, there should be no lift. Of course you'd have to be careful to keep it exactly symmetrical, which might be difficult if you were doing it by hand.

 

I think if you actually did this shape you would end up taking off way too much mass on the trailing edge. What I had in mind was more like rounding the leading edge, and maybe a slight taper to the trailing edge, leaving it flat instead of pointed - think "Kamm back"

[johnc]

It... was... a... joke...

[katman]

Yeah, I believe worrying about the lift of a crank throw would be polishing turds to a very high lustre.

[TimZ]

It... was... a... joke...

 

...whew!

 

[thehelix112]

I don't really think its a joke. At 45mm from the crank centreline, the throw/counterweight is doing about 100kph at 6000rpm. Thats more than enough airspeed to create enough longitudinal force to slide the crank fowards or backwards I think?

 

Dave

[Tony D]

Oh come on now, guys! How many counterweights do you have?

 

Airfoil shape the first one to lift one way, then shape the next to lift the other way....thrust bearing wear is therefore balanced!

 

The odd man out on odd-counterwieghted crankshafts would always be the forward most one, where you do the conventional knife-edging. It will only splash through the oil on heavy braking annyway, so you want the engine braking that oil provides anways...

 

Where did I put my boots?

[johnc]

Here's the size wing you need to generate about 400 lbs. of lift (downfoce) at 100 kph. How much lift will a wing that has a chord of about 2" and a width of about 3" generate at 100 kph?

 

[JMortensen]

Not to mention the crank is spinning in a vacuum or at least low pressure in a properly vented crankcase...

[thehelix112]

JohnC, Not sure right at the moment, have the equations in a book at home. Unless your question was rhetorical, in which case I won't bother.

 

If I'm interpretting you correctly, you're essentially saying why worry about it? I would say most people could ask the same question of knife-edging at all. My thoughts were simply if you are going to do it, why not do it properly?

 

I was also wondering, and this would seem a no brainer but I just want to check, any weight that you take out of the counterweight really should be taken out of the rod/piston assembly on the other side yes? If so, then you really should get the rod/pistons before you give the crank to the machinist and tell them how much to take out.

 

Although, with the LD crank, I have heard that the LD rods it was designed to run with are less than petite. Did you measure the difference between those rods/pistons and what you were running?

 

Also, what balancer did the ROD run?

 

Thanks,

 

Dave

 

PS. SOrry for the hijack.

[johnc]

Unless your question was rhetorical, in which case I won't bother.

 

Rhetorical.

 

If I'm interpretting you correctly, you're essentially saying why worry about it? I would say most people could ask the same question of knife-edging at all. My thoughts were simply if you are going to do it, why not do it properly

 

In the engine I had built the knife edging was done properly by one of the best engine builders in the country with an essentially no expense spared budget. The same basic crank setup (knife edging, radiusing, shot peening, polishing, nitriding) is performed on the championship winning Speed World Challenge GT/Touring engines that Sunbelt builds.

 

I was also wondering, and this would seem a no brainer but I just want to check, any weight that you take out of the counterweight really should be taken out of the rod/piston assembly on the other side yes? If so, then you really should get the rod/pistons before you give the crank to the machinist and tell them how much to take out.

 

Makes sense to me.

 

Although, with the LD crank, I have heard that the LD rods it was designed to run with are less than petite. Did you measure the difference between those rods/pistons and what you were running?

 

Don't know. The rods in my engine were Carillos, pistons were JE custom forged, and the crank was lightened by 17lbs over the stock diesel crank.

 

Also, what balancer did the ROD run

 

ATI Super Damper with a 3lb flex plate/flywheel.

[thehelix112]

Thanks.

 

Dave