Sunday, 21 November 2010

Wisdom, the ultimate, and the infinite.

 If you've been on board this particular bus since the month of July, you may remember that I cut the original BMW brake pedal up and welded a chunk of steel to it so that it could act as an idler in the linkage between the brake pedal and the brake proper, serving to correct the pedal ratio and place the final operating rod in a position where it's arc of movement wouldn't conflict with the arc of movement of the brake and thus ensuring that the effect of suspension movement on the brake actuating mechanism was minimised to the point where it became negligible.

If you don't recollect the occasion, this picture should refresh your memory. While it all appears to work, with the wisdom conferred by hindsight I think it's fair to say that it's not the ultimate in infinite beauty. As you should be able to see in the picture to the right, taking at least one leaf out of Phil Irving's book (of which I own several, and so should you) I thought I'd add some lightness, though I settled for making it look a little more elegant rather than "simplicate" it, and in truth, I only added some lightness where it showed.

In my time I've seen seen quite a lot of motorcycle parts that have had lightness added through the ever popular medium of holes which have also had unsuspected quantities of weakness added that led in turn to them acquiring substantial degrees of brokeness. Drilling "speed holes" is largely a waste of time, in most cases you can easily engineer something of equal strength to, and far lighter than, the original, but it has to be said that drilling unnecessary holes in motorcycle parts practically screams "racing motorcycle".

Lest you think that all this is the lead up to another pathetic excuse for not progressing the work on the frame, I give you another picture that ought to dispel any such notion. Careful study of the afore said will show that it wasn't just the rear brake mechanism that caught a dose of lightness, as the crankcase is looking positively anorexic too. The other item in the illustration which is adorned with holes, is the new engine mount I've added. The thinking behind it is not entirely uninteresting I feel. It's fairly obvious that the frame has had some attempt at triangulating it in addition to the main tubes I bent up for it, but it's equally clear that the brace tubes aren't perfectly triangulated and therefore don't appear to offer the ultimate in stiffness.  Oddly, they probably do offer the ultimate in stiffness, what they don't offer is infinite stiffness.

Possibly you feel an explanation is in order? Then I'll attempt one. Considering the frame as a separate entity, it's subjected to assorted forces which are transmitted to it via the forks and the swing arm. If for a moment we simply assume the forks and the swing arm to be infinitely stiff, then the limit of the forces that can be applied to the frame is the limit of the forces that can be applied to the forks, and the swing arm. And those forces are applied by the tyres.

Which ought to make it obvious that the unrelenting pursuit of stiffness to resist forces much greater than those which the tyres can handle, is not only a waste of time, but adds weight. So the considered wisdom should be that the ultimate in stiffness is sufficient stiffness to resist all the forces likely to be applied this side of the ditch, while saving the weight that infinite stiffness would require...

With that in mind, I happily compromised the geometric purity of my additions to allow the fuel tank to fit over them without requiring modification.

I'm reasonably sure that the solution I've arrived at will require more force to deflect it from the true and righteous path than the skinny tyres are capable of generating. Sadly, BMW saw fit to mount the engine off of the lower rails, and my reinforcement of the existing structure does nothing for them, but at the same time makes it impossible to remove the engine. That whole pot of woes is further stirred by the engine being repositioned so that it is impossible to change the oil filter without removing the engine from the vicinity of the right hand lower frame rail.

Using the same logic as most other BMW modifiers, I'd elected to make the lower rail on that side removable which would not only facilitate removing the engine, but allow for removing the oil filter without taking the engine out of the frame as well. Sadly, it doesn't take Werner Von Braun, or any other rocket scientist, to realise that as the engine is held in the frame by mounting it to the lower rails, removing one of them is going to make for a fairly floppy engine. Not only that, but the engine experiences a torque reaction around the drive shaft, which causes the top of the engine to want to move to the right of the frame under hard acceleration.

Hence my top engine mount, which I freely admit to having copied in principle from somewhere German, although I forget where. As well as feeding the torque reaction into the additional bracing , the upper engine mount serves the purpose of supporting the engine while the right hand rail is removed for routine oil filter changes. Not to mention providing somewhere else to drill speed holes.

With everything as welded as it could get with the engine in the frame, I set about removing the engine from the frame. Somewhat unusually, I had to cut the rail off the frame to allow the engine to be removed. With the offending (lower right hand) chunk of frame rail removed, I was slightly alarmed to discover that it was what we in the trade like to refer to as "stupidly thick" in the wall department. So much so, that the Suzuki Bandit frame splices I hoped to employ wouldn't fit into the bore of the BMW frame tubes. After a quick visit to the lathe, the Suzuki splices slipped into the bore of the BMW tubing with only the lightest of hammer blows. As a job, perfectly doable with a file, should you wish to adopt a similar strategy at any point. Since the frame splices were something on the order of 65mm long when installed, I took the trouble to mark the frame with two lines around 65mm apart, before hacking the rail out of the frame, ensuring that the cut I made was between the marks that denoted the extent of the frame splice. Accordingly, once the chunk of rail was cut out, I trimmed the cut ends back to those marks, and reassembled everything with the inclusion of the frame splices.

Since I rambled on about artificial, self imposed dead lines in my last diatribe, it was at that point I decided to call it a day and head off to the wonderland that is Tesco in the run up to Christmas and lay in some provisions for the weekend, leaving the final fettling of the frame rail and the last of the welding for next week.


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