Dragbike Chapter II 3/31/11

The chassis was destined to be a challenge as there were a few different jobs it had to do.

One of the requirements was that it had to accommodate both a turbocharged set-up and ultimately an injected nitro combination.  There would be some cross-over in component function, but the highest demand on a part would dictate that part’s configuration.

One of the first things to consider was what type of track was this bike liable to run on.  The static weight bias I set biased higher to the rear for slippery tracks that a lot of street- tired cars and bikes run on because the treaded tires strip the rubber off the starting line and for the first 20 to 30 feet.  This is often the case at a local track with a strong street car showing.  If we were only going to run this at national event caliber tracks, the superior starting line traction allows less rear weight bias and a lower crank centerline height.  While there is less weight on the rear wheel initially, the better traction allows the front end to start to come up and increase the leverage of the weight of the bike more effectively on the rear tire.  This makes for good 60 foot times and detection of weak chains, clutches and transmissions.

Because the primary testing would be at a local track, I decided on a slightly higher rear bias and the crank centerline raised 1” from where I ideally wanted it.  The adjustable rear axle blocks would add a bit of weight but allow the shifting of the rear weight forward and the use of either a conventional motorcycle prostreet tire like the Shinko 003 or conversion to the Mickey Thompson drag slick, which has a taller profile and more rollout.

This M/T tire I have seen go 6.98 with no bar, far in excess of the goal of this project.  The narrower tire doesn’t look as cool as a 10 inch slick but is a lot easier to ride and doesn’t bounce when you snap the throttle shut.  See the comparison photos.

The distance of the crank from the rear axle had to be determined as a series of compromises between the weight bias, the length of the chain (excess weight and whipping around under the right conditions) and how far down in behind the motor the rider could get.  I’d like to say there is a set of hard and fast rules, but nobody has done a purpose-built Ducati drag chassis before much less had to make it accommodate a shift in component weight of 24 lbs over a 40 inch span when the turbo was switched to a nitro setup.  You just put the motor, the steering head fixture, the rear axle fixture up on the jig and start moving them around.

The rake to be used in the frame was something to be considered as part of making the bike easier to ride for a beginner.  The trap speeds weren’t expected to be over 170 to 175 mph so the need for a 42 to 45 degree rake wasn’t expected.  To keep in the prostreet look a rake of 38 degrees was selected  for responsiveness at slow speeds but enough stability to work well at higher speeds at the strip and possibly Bonneville or Maxton.  The Hayabusas are running less rake than that and going over 200mph at the speed runs so this should be a reasonable combination.

More time with strings, tape measures, levels and straightedges.

The minimum seat height is determined at this time.  Some sanctions have a minimum seat height of 21 to 23 inches, which is a nominal height I like to start with.  Too low a seat height and you can’t move around effectively as needed to shift your weight and hang off.  Too high and you are up in the wind adding drag.  A sit-in rider’s footpeg position was chosen to work with the prostreet theme.  This is a position that is more instinctive with most streetbike riders, the “laydown” type of frame used in the Top Fuelers and a lot of Bonneville chassis going out of general use a decade or more ago.

We also aren’t having to straddle a wide motor package so the width advantage of the laydown is not as big a benefit.

It took a bit of rethinking to get in the dragrace frame of mind.  After building three Bonneville land speed bikes, the requirements for component location and stiffness are different.  You also have to go into lightweight mode again, rather than the ballast-oriented mindset of the Bonneville stuff.  You control wheelspin at the dragrace with weight transfer loading, both static and dynamic.   You control wheelspin at Bonneville with weight and more weight along with a recently discovered subtlety, engine management.  Thirty pounds of boost and 260 horsepower on the salt when suddenly and inexpertly applied makes for an interesting ride.  We carried 110 pounds of ballast in 2010 and expect to add 40 to 60 pounds more this fall on the Bonneville bike.  That kind of weight might control wheelspin at the dragstrip but would make for a rather poky ride.

There is traction at the dragstrip, not so at Bonneville.

Next up is actually doing something.  Time to decide where things are going to be and freeze them there.  That is often the most difficult part of the process because you are in a constant second-guessing mode (too much rear weight and the bike pogos or not enough and it smokes the tire, etc. etc. and freaking etc.).  Once the tubes are beyond the tacking-in stage it is a major issue to remove them and the imparted stresses they have in the chassis.

Starting to connect the dots is when it becomes fun and the image in your mind’s eye actually takes shape.  It is also a time for restraint because you can get ahead of yourself and you make something like the idiots in the manufactured vehicle world do.  By this I mean something nearly impossible and unnecessarily unpleasant to work on.  Coming from a drag racing background, servicing the vehicle, whether a car or a bike is paramount.  An impediment here can cost you a race or a championship, so you try like hell to get it right the first time.  An example is the GP bike we built that requires only 10 tools to service it completely and a motor change is 30 minutes.  You can also remove and rejet the carbs in 10 minutes with two of the aforementioned tools.  The guys like working on the bike when necessary and it wouldn’t involve any more cost to do a street bike (or car) the same way, but it seems that a lot of the manufacturers have their “B” or “C” teams doing the designing and obviously none of them have ever picked up a tool in their lives.  It does take a lot more thought and effort to do it right, and is a source of physical headaches for me as I am straining the little gray cells for all they are worth during this process.  Something as simple as having all the fairing and bodywork fasteners be the same length would seem pretty simple but after the Ducati 916, evidently that skill is lost in the sands of time.  Puleeeze!!   Now that I have that little rant out of the way, take that!

Not having things buried in layers is a start.  The more complicated the systems, the more of a challenge it can be, as seen on the Bonneville Ducati with the new Motec system and enough sensors and wiring to light up a hospital.

Since this bike started out with a carburetor and a fixed-timing Dyna fabrication, there is a bit remedial tidying-up to do. Avoiding the plumbing and wiring traffic jams is the goal.

Next up:  Laying in tubing!!

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