(this is a slightly abbreviated version of update letters sent out to the sponsors of our grass-roots fundraising efforts for the next trip to the salt flats) Thanks for reading!
Low-Tech Traction Control
One of the problems we have struggled with is rear wheel spin. A consequence of the design of this bike is that without rear suspension, you don’t have the possible weight transfer to the rear wheel. I have watched slow motion videos of various bikes and the slow undulations that they get into with rear suspension can become an issue under the right conditions. Talking to a few of the experienced Bonneville riders, a non-suspended chassis relies more on good salt than a suspended one, but if the salt is right it is more stable. That was the basis of the decision to run a solid rear end, that good salt would be the limiting factor. Since then I have not really seen big differences in the bike’s behavior that was related to salt conditions, and the wheelbase being considerably longer than everything but a full streamliner, the hope was to reduce oscillation effects by reducing their percentage of total chassis deflection. A one inch deflection over a 68 inch wheelbase is a much larger percentage of chassis change than that of a 98 inch wheelbase, such as the long bike is running. Another advantage of the solid rear end is that there is a considerable increase in volume for fuel, batteries, components and ballast, due to the center of the bike not being used up by the swingarm and suspension.
After the first trip to Bonneville the bike was seen to throw a rooster tail when the turbo kicked in and throttle was applied. For the next year, steel ballast plates were bolted to the underside of the chassis and lead plates were put in a box in front of the rear wheel. This helped and allowed the bike to run over 200, but the rooster tail still came on.
In 2010 we added another 60 lbs. of weight in the form of more lead, and two more plates bolted on to the chassis just ahead of the rear axle. This made for a total of 160lbs of weight added. That year, with fuel injection, we ran 203 and 205. It was a lack of power that kept the speed down and the extra weight was adequate to the task.
In 2011 we added another 50 lbs. of lead in the ballast box, but were rained out, precluding the testing. With more usable power provided by the Motec ECU, wheelspin was anticipated to be more of an issue.
For 2012, we added a heavier battery. This was adding a total of 210 lbs. more than in 2009, and on the 213 mph run, the rear wheel speed was seen to be 245 mph. This was more of a speed differential than we wanted, so one of the projects for this winter was to add yet more weigh for traction control. I suppose we could do it with the ECU, but since the front wheel speed sensor was not behaving, that would make it difficult to accomplish predictably.
Having discovered the detriments of weight located high and to the rear in 2007 (wobble-city), keeping the weight down was desired. We already had weight bolted under the bike and with the ground clearance constraint of four inches, we couldn’t put much more down there. Looking at the platform, there was only one place to put it, and that was in the floor.
The idea was to fill in the area in the X-braced floor of the bay area behind the motor. The logic was that this was as low as you could get, and if the mold panels were working, the arc described by the tubing would act as a means of holding the weight in place, simplifying its installation.
Pouring the first segment went pretty well and there were few leaks and issues. The second segment was to prove to be the much more challenging as there was not as flat a surface to seal against for the sheet metal. Molten metal ran out and made a bit of a mess. Luckily this was to be corrected in the subsequent segments and the bulging material removed by paddling it off. The bike had to be inverted for this and it was a very entertaining process that involved pouring some of the metal back in after a bit too enthusiastic use of the torch allowed some of the metal in the offending panel to escape and run all over the floor, my shoes, and pants. Ultimately it was made flat enough to allow the bolting on of the steel ballast plates and not hit any of the poured-in ballast.
The weight of the segments can be seen in black marker and totals about 70 lbs.
As you can see, there isn’t a wire or computer to be seen in this traction device!