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tomahawk crank

Started by graeme, 27 Aug 2008 at 12:41

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I was talking to an owner of a DT Douglas the other night, and he mentioned that he had a spare crank of a type known as a "tomahawk" crank. Anybody care to tell me how this differs from the standard crank?




That is a new one on me. Sounds like a local euphemism! I wonder if it just refers to the shape of the detachable counterweights? They were narrower and a more pronounced 'hammer-head' shape on the earlier engines like the S1. There was less of a lozenge shape to the earlier center webs as well, and they were generally lighter and of 68mm stroke. A picture ought to sort it out.


Alan Cun

Hello All, This one caught my attention because I could remember way back having a crank which I referred to as an axehead crank which had in fact balance weights the shape of an axe head. Now I have taken a pic with two types of balance weights. The welding on one may not be obvious but has been changed from an axe head to what I would  refer to as a broad head. The little example is from a S1 crank. I have heard in the past the words crankshaft couple and this must have been played with by Douglas to eliminate or enhance couple for whatever reason. I did however realize from a note I have left on a OC/OB motor of which I have put axehead crank from DT. I have shown this motor because days ago I mentioned the use of a Yamaha two stroke oil pump and this is where I fitted it. I was short of a timing cover for this one and made a steel one. This motor will probably end up in the OC as the motor I used I fitted with a BM R50/1 crank and at that stage I couldn't arrive at a suitable piston height as the BM pistons to me were too heavy for the Doug motor. regards Alan    pics to follow


Photos added to Alan's post above.


I am trying to remember what shape the weights on mine were - can't find any picture but I think they were symetrical.

Re the oil pump that Alan has - when I got mine the timing cover was set up to take a pilgrim pump in a similar position.

How does it run with the two carbs connected like that ? Would certainly eliminate the problem of the long manifold !

Alan Cun

Hello Ian, This was one of those projects that was fired up by heaps of inspiration that again took me 80 percent of the way and put aside and never run. From what I remember I would have used Kawasaki pistons and as you can see the push rods are AJS twin. The rockers have been made up and rewelded from I dont know what and the oilers not yet fitted. I can report however we have fitted and raced carbs on short stubs of this configuration on 2 DT motors and they worked good. They were Amals though but the B&B,s shown should also work OK. Am hoping that someone can comment on what couple means for the flat twin motor. Can only guess it it what happens in relation to momentum between firing strokes?????regards Alan



Rocking couple on a Douglas flat twin exist because the cylinders are not exactly opposite to one another, but offset to the right and left hand side of the machine. The offset on a DT is something like 1-5/16". So even if the rods, pistons, and rotating mass were all in balance, the fact the pistons are not directly opposite introduces a rocking couple about a vertical axis. Not much, and the frame and the engine mounting does a pretty good job snubbing any such vibration.

Counterweights typically are present just to take some of the strain off the main bearing, by balancing out some of the reciprocating mass of the piston and connecting rod, and the centrifugal loading of the lower portion of the connecting rod and big end. But in the Dougie, they also help to counteract the rocking couple.

In the following illustration, the rocking couple of the pistons and connecting rods is in red, and the rocking couple of the counterweights is in blue-


Alan Cun

Thank you again Doug, The word couple has intrigued me for many a year and now I can clearly understand. So the balance weights are basically eliminated. I guess what I consider the axe head weights would show less resistance but later models i believe were just squared off did this create more oil splash?????Alan



The big end of the connecting rod projects out a little further than the swing of the butt end of the counterweight. So they would 'splash' first. In theory the later squared-off counterweights would create more windage and so throw more oil mist about. But indicators that they were coming to grips with oil control- such as better oil control rings, elimination of the anti-fouling pockets for the sparkplugs, and discarding the total loss oiling system- all came long after the change to squared counterweights. Since the squared counterweights came along about the time of the switch from 68 to 82mm stroke cranks, I suspect it had more to do with needing more mass to increase the balance factor. Possibly even increase the counter-torque for the rocking couple, but that might have just been an side benefit.

Determining the balance factor at that time was likely a matter of trial and error. So there will probably be examples of early square, and late chamfered counterweights to muddy the waters. I know recently a fellow chamfered a set of mid-thirties counterweights; perhaps thinking it was a racing modification. It would indeed drop the inertia of the rotating mass, at the detriment of upsetting the balance factor.

Speaking of splash lube, I do not think many realize the importance of the dividing rib cast in the bottom of the DT type crankcases. Often you see these broken or even purposely chiseled out. The intention was to create two separate pools of oil, with a small hole connecting the two. The small fins or dippers on center web straddle this divider. When the dipper strikes the pool of oil, it knocks most of it out off the way, with a little hopefully being driven into the sides of the big end bearing where it might do some supplementary good. Then along comes the other throw, and smacks into the pool of oil again. Were it all one common pool, the level would only have half a crank revolution to recover. With two separate pools, each side gets one full revolution of the crank to recover, or twice as long. Not very long at 5500rpm, but every little bit helps. So goes the theory. In practice there were other engines they made that did not, or could not (ones with sumps), have this divider.