Boardtracker build concept part two, history and tech.

Part one:https://old.reddit.com/r/motorcycles/comments/cyhyde/boardtrack_bikes_history_and_notes_on_building_a/In our first chapter we looked at the history of the actual boardtracker bikes. I found a few things of interest. Pretty much everything from 1910 backwards, if it was at all performance-oriented, looked a lot like a bicycle whether for straight-line speed such as Glen Curtis's bikes going back to 1903, or European bikes used in the Isle of Mann in 1907 which was actually on dirt roads, not street.Everything in that era prior to WW1 lookS like a bicycle, and my theory is it's because bicycle parts were simply what was available. This included springs, seats, handlebars, rims and tires and even for the most part frames with modifications of course.We know that by 1907 Glenn Curtiss was packing 40 horsepower in a bike that weighed 257lb and did 136 miles an hour, with a shaft drive instead of chain because he was breaking bicycle chains.But then if we flash forwards in 1929, we see Harley-Davidson making the same race engine in both road race and boardtracker trim and to our surprise we find the boardtracker more less unchanged except the horsepower for a given engine size was way up.To recap, here's that matched pair of Harley racers from 1929 (very late in the boardtrack era), one a boardtracker and one a roadracer (mostly for the Euro market):https://stupiddope.com/2016/11/28/old-school-classics-1929-harley-davidson-peashooter/https://ironageauctions.com/auctions/238886/Compare these bikes carefully! The boardtracker version has the wrong tires - ignore that (although as we'll see later tire choices in these size rims is tricky).In this chapter I want to make some guesses as to what was going on with the board trackers, why they remained more or less unchanged in terms of weight, ergonomics and overall layout. I want to look at what is good about the design and what is bad.Later in this series I'll design a theoretical modern board tracker homage that can actually hang a serious corner. This is something that most attempts to build a boardtracker have either not accomplished or didn't attempt. They may have had good reason for that of course.Let's start with what a theoretically perfect frame looks like for a modern high performance motorcycle. I want you all the picture an egg. At one end of the egg that you're holding in your mind's eye, draw a cross, a perfect T shape with equal lines from the center point but lines going all four ways, not three. Now imagine drawing each of those lines all the way around the egg lengthwise in the four different directions so that you have a crossing point at each end of the egg.Do you see those lines in your mind's eye? Good. Leave them there and get rid of the egg. Put a swingarm pivot point on one end of the egg where the cross is and put a steering head at the other. As I'm about to show you, most modern sport bikes are built on this kind of shape.Here's the frame of an early GSXR.https://images.cmsnl.com/img/partslists/suzuki-gsxr750-1987-h-e01-e02-04-06-15-16-17-18-21-22-24-25-34-39-53-frame_mediumsue0068fig-34_186b.jpgAs you can see it has two big beefy support lines coming way out sideways from the steering head; they go backwards, outwards and then down to the swingarm pivot point. It's got two smaller but still decent supports coming down off the steering head and going out, down and backwards to the swingarm pivot point. With some bends of course it's more or less that same egg-shaped structure with a whole bunch of support in the middle cross-bracing everything called the engine. That's right, the frame is not working just on its own, it's all internally braced all through the center of this egg shape via the engine.Take another example from Eric buell's playbook. Eric took his egg and turned it a bit so there's a single bar on top, deleted that bar and massively beefed up the two "side bars" by doing two bars with cross-bracing between.https://i.pinimg.com/736x/f6/9e/3d/f69e3d787b4573cd24cf893ed338ec6f.jpgNow I can hear you saying wait where is the bottom line coming off of the egg along the bottom stretch? Well that's the engine. Even more so than the GSXR, Erik Buell uses the engine as a stressed member so the main support along the bottom that's keeping the bike stable in the vertical direction is the engine. The latticework of steel bars in the frame coming out towards the sides and dropping down to the swingarm pivot point is helping with horizontal stability, and of course some vertical stability too. The frame alone would not be nearly as stable as it is with the engine installed and acting as that big lower support brace.So as you can see, the modern concept is to brace a frame in the vertical direction and in the horizontal direction. They use slightly different approaches to get there but the principal is well established.In a real original boardtracker you only have support in the vertical direction, not in the horizontal. Except of course for the roughly inch and a half wide steel tubes forming the cradle under the motor and the main horizontal supports from the steering head to the seat post. But where somebody like Erik Buell is using horizontal bracing that is at least a foot and a half wide, the horizontal bracing on a real boardtracker is only about an inch and a half, the width of the steel tubes used for the bicycle frame. So if you tried to seriously corner in a real boardtracker frame, to the point of hanging off, your frame would flex alarmingly as you introduced side forces. These forces would build, start as a vibration and eventually get to the alarmibg fishtail point. Believe me, I have felt exactly that in the frame of a 1979 Yamaha XS650 which had only a bit more side to side deflection protection at best compared to an original boardtracker. It certainly isn't as good as something like a Norton featherbed frame with 1950s technology.I may not have made it clear yet but the XS650 is the bike that I first turned into what we would now call a cafe racer and I have about five years stick time on it from 1985 to 1989, doing serious high-performance driving. By which I mean canyon racing in the Santa Cruz mountains.So I full well know what frame flex is.I think the original boardtrackers and their ancestors going back to at least the Curtis 1903 bike didn't care about serious cornering because they were straight line monsters. Boardtracks may have been oval but the massive banks turned them into straight lines as far as the bikes could tell.Plus, the tire tech of 1910-1920 must have been pretty crappy :).Let's talk seat and bars. Boardtracker seats were set back, almost to the rear axle. The point is to lay flat and keep our of the wind. Control in the corners wasn't as big as issue. Wide low handlebars gave stability, helping to fight frame and fork flex.The upshot is this. If we take a modern boardtracker homage and put serious suspension parts on it and reasonably sticky tires in something like 23inch meant for a Harley front rim, we're going to get some hellish flame flex unless we are very careful.Is there a solution? Without fabricating an entire new frame and engine mount system and everything else, in a bike that a reasonable mortal with basic wrenches and metal fabbing could build? Yes, I think so. And we're going to dig deep into my answer, which for sure isn't gonna be for everybody.Stay tuned. Next chapter we're going to look at other people's builds of boardtracker homage bikes and explore what's good and bad in each. via /r/motorcycles https://www.reddit.com/r/motorcycles/comments/d1kfpn/boardtracker_build_concept_part_two_history_and/?utm_source=ifttt