Sunday, 28 November 2010

By all that's Holy....

It's all very well to blithely say things along the lines of "drill some speed holes in it.", but that ignores the time and effort that goes into working out the positioning of the holes, and then measuring and marking out whatever scheme of spacing you arrive at.

Drilling some holes in an existing part, or a part that has to fabricated, but has a size and shape that is going to be defined by the job that it does, such as the new rear upper engine mounting plate that I've concocted, imposes some limitations, and provides a place to start if nothing else. Since I'd decided that I wanted speed holes in the gussets that I was going to add to the frame, and the size of the gussets in two of the places where I felt a little stiffness could be added wasn't critical. That being the case it made as much sense to size the gusset to suit the speed holes, as to take any other approach.

Because the main loop of the BMW frame is more or less a rectangle, it will be prone to lozenging. Since the front down tubes are attached to the top if the headstock and the top tube to the bottom of the headstock, what will tend to happen is that under braking when the bottom of the headstock wants to move backwards, the only thing stopping the top tube moving relative to the down tubes are the gussets from the down tubes to the headstock. Much like the Norton Featherbed frame which is said to have inspired BMW's engineers in the design of the Airhead frame, racers tend to adjust the steering head angle by the simple expedient of crashing the motorcycle gently into a wall, which is enough to "tweak" the frame and reduce the headstock angle, and in turn quicken the steering. Good isn't it?

I feel it's obvious that adding a gusset between the top tube and the down tubes will stop most of that from happening, but to my mind it also makes sense to box in the bottom of the factory headstock gussets as well, since that would stop them from flexing, which is what they'd have to do to allow the sort of motion I've been talking about.

The other area where I feel the frame is lacking is the swing arm pivot area. The anecdotal evidence would suggest that a beefier sub frame that was welded to the main loop would improve the handling. For that to be so, then the problem must lie not with the sub frame, but with the rigidity of the swing arm pivot area of the main loop. The tubular bracing should address some of that, but adding some gusseting to the area should not only enhance the stiffness, but provide yet another place to drill some speed holes.

The gusset that will run from the brace bar to the lower rail presented a slightly different design problem. The rear most edge of it had to be the shape that it had to be, but the leading edge wasn't quite so constrained, with the overall extent of the thing being more severely defined than for the headstock gussets. In essence, I traced the shape of the rear of the gusset onto some card, fiddled around with some hole layouts, and allowed the sequence of holes to define the leading edge of the gusset.

In the last picture, you can see three pairs of gussets. To the left are the swing arm pivot gussets I just described, the pair in the centre are the down tube/top tube gussets, and the last pair are to tie the cross tube beneath the swing arm to the lower frame rails. The point of the last pair being that they should contribute to reducing any tendency towards fore and aft movement in the swing arm pivot points.Especially if one side is heading fore, and the other aft....

That of course leaves the boxing plate for the bottom of the standard headstock gussets, which I haven't made yet, and may lead you to enquire just what it is I've been doing all week? Well, just making the headstock/top tube gussets consumed a day of my time, and the other day of this week that I could devote to things BMW was consumed in making the swing arm pivot area gussets. 

Aside from the entire thing taking for ever and a day to carry out, I'm quite pleased with the look of the gussetry I've made, and at the risk of belabouring the point, you are likely to spend more time looking at a project once it's completed than the amount of time that you are going to spend working on it, so rushing through things makes very little sense.

Talking of looking at motorcycles, I wonder if you noticed the curious thing about the headstock/top tube gussets? More on that later perhaps....

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