Friday, January 27, 2017
I've been meaning to write something and I realize it has been over a week. Every time I start I realize that I want to add pictures to help tell the story better. I want to finish the with the gears but need to get a few images together. Sorry for the delay. I really haven't forgotten, and things are moving along again. Please stand by.
Friday, January 6, 2017
Currently residing in the where are they now file.
I Have a embarrassingly dusty collections of parts laying around my shop. there are everything from BMW Transmissions and final drive parts to smart engine bits and various sport bike shocks. As well as gears and intermediate shafts from bikes of various styles and from various manufacturers. All of this is kind of where I left off. I never really stopped though construction kind of ground to a halt since about September 2013. After I got the frame Mocked u and the DiFazio hub Center parts all finished I turned to mounting the engine and ran into a problem. The first problem was that I had start fabrication before I finished designing and I really should have known better. I was anxious to start making parts and got enough on paper that I thought that what was left was relatively simple and essentially detail work. Of course one mistake in placing the engine in the original design and the one in the front suspension compounded to create a few fairly big challenges. More important and even more fundamental was how to solve the problem of gearing. This I was planning to solve concurrently with designing the frame etc. However it turned out, that when I got caught up in over thinking the solution to the gearing problem, and then when the events of the last few years forced me to put life on hold the resultant mess caused a huge psychological barrier that kept making me procrastinate the restart (?) of this project.
The problem with the gearing is this: The CDI engine being a Diesel was designed to rev no more than 4.5K rpm. This is the practical speed limit of most Diesel engines and is limited by the speed at which flame cam propagate thought the compresses air fuel mixture in the cylinder. Diesel oil just doesn't burn fast enough to spin the engine any faster. The next issue is that most motorcycle transmissions are designed for engines that operate at least 2 times faster than the practical operation of the smart and are therefore geared much lower than I need for my application. these transmissions are also designed for a top speed that is again well below what I'm shooting for. Normally on a chain driven bike I could change the ratio of the front and rear sprockets, but even then with the existing transmission gear ratios I have to work with I would have had to over drive the final drive to even get close to where I wanted to be. To get the speeds I'm looking for, the front sprocket would have to be so large that I would have to drastically modify the transmission case and would still have trouble fitting it with the rear suspension. This lead me to explore two ideas. the first was to build a rear final drive like the one BMW uses but with my own gear rations. Having a custom set of bevel gears wasn't really an option so I tried to find existing gear pairs that were reasonable priced, designed to handle the power of an engine my size and lastly to give me a ration that would give me the speeds I was hoping to achieve. What made this a bit more difficult was that most manufacturers publish the gear ratios of their transmissions but often leave out the primary input ration which is crucial to calculation the rear wheel speed for any given engine rpm. I was forced to actually open one of my transmissions up and count the teeth on all the gears in order to put together an accurate spread sheet together of all 6 gear sets and the primary input ratio. In case you are wondering, The primary input ratio on these BMW transmissions is 1.8. Meaning the engines rotational speed is reduced by ~55% going into the transmission. Since the Primary gears and I have no experience with designing gears I dismissed the idea if changing the primary gears to get a ratio that worked. the next Idea was to build my own final bevel drive. replacing the BMW unit which on the R1150Rt had a ratio of 33:11. Making the spindle and the housing wasn't particularly daunting, I just need a gear set that again had a ratio that worked for me. Most Japanese shaft drive bikes use engines with a horizontally or transversally oriented crank. This requires what they call a middle drive gear on the output of the transmission to change the drive line direction 90deg to accommodate the drive shaft. These middle drive gears are usually found in ratios relatively close to 1:1.2, are easily able to handle 100hp or more and best of all are super cheap used on EBay. I was able to get a number of sets from different bikes like the Kawasaki ZG1200, Vstar 1500, Yamaha XS1100. Each gear set was bout $35 with the gear box and it allowed me to choice of similar ratios. After settling on the Kawasaki ZG gear set I designed a new gear box and spindle that allowed me to use the BMW rear wheel and also keep the BMW single sided swing arm configuration. What I wound up with was buildable but wasn't ideal. The most nagging issue to me was what if the gear ratios didn't work out once I actually got out on the salt?. What If I got out to Bonneville and found out that the rear drive was either over geared or under geared for what the engine power, salt conditions etc turned out to be in actuality? Choosing to solve the gearing problem with a new final drive pretty much locked me in to one gear ratio. To change it would require finding a new gear set and completely designing and building a spindle and housing to fit. From there I played with the idea of creating an intermediate drive to convert the longitudinal output of the BMW transmission to a transverse shaft. Something that would allow me to run a chain final drive and from there vary the final ratio as needed by changing the sprockets. The problem was now one of having a front sprocket that was too big to be practical. Even though there are front sprockets made for Iron head sportsters that are as big as 26-28 teeth these would be difficult to fit without having t cut away a large part of the transmission case (which I was still planning on using as a swing arm mount like the BMW's) or fit the intermediate shaft assembly completely behind the transmission thus extending the wheelbase about another 15" from what the frame was already designed to be. There are a couple of examples of people who have made similar setups on both BMW's and Moto Guzzi's but with two major points. first is that the gearing they needed was very close to what a typical street bike would need so the size of the front sprocket was more or less "normal". The second was that these intermediate setups didn't work very well under high stress. Even for normal street use they had problems. It was also another layer of complication that I really didn't want to add.
This problem vexed me for much of the last two years the project was dormant. Eventually I decided that I had to start working again on the bike and that starting to do anything, even if I would up changing it later was important just to get the ball rolling again, so I tackled the gearing question again. This time in earnest and with a resolution that whatever I decided, I would build it, move on and worry later if it was the right solution. Luckily Chris convinced me to have another look at changing the primary input ratio of the transmission. Again, I had no practical experience with engineering gears but after some simple explanation and reassurance from Chris it didn't seem insurmountable. This of course would be the simplest solution by far if it could be done. I have three BMW R1150 transmission I have gotten on EBay over the last few years so I took the one that had the highest miles and the most knackered input splines (a common problem on these transmissions) and took it apart again (I had had it open once to count the teeth to confirm the actual gear ratios inside the box. As it turns out changing the primary ratio is very simple in theory. The R1150 transmission has three shafts in it. The primary input shaft which only has one gear on it. the intermediate shaft which has seven (the mating primary gear and half of the numbered gear sets. And finally the output shaft that has the other half of the number geared sets or 6 gears. The input and output shafts are serviceable items according to BMW and the gears are held on the shaft with retaining clips at one end of each shaft. These are pretty easy to disassemble and reassemble. However the intermediate shaft is never meant to be serviced. BMW considers it a monolithic part and if it is ever damaged or worn, BMW requires you to replace the entire shaft with gears. This is because of the way the shaft is assembled. The shaft and 6th gear (which is one of the outermost gears) are machined as one piece. All the other gears slide on to the shaft with some fixed to the shaft with splines. The last two gears at the opposite end of the shaft from 6th are pressed permanently on to the shaft thus capturing all of the other gears. As it turns out the two pressed on gears are 2nd and the driven primary gear. In order for gears to be pressed on to a shaft with no splines or no keys and be expected to never move, even over thousands of miles and under the torque of a 11500cc twin, the interference fit between the gear bore and shaft must be serious. This would also require a massive amount or pressure to get the gears assembled. I was hesitant about trying to get the intermediate shaft apart but Chris didn't think it was that big of a deal. We have a 25ton press at the shop so I plasma cut a split fixture plate out of 1/4" steel and we put the shaft in the press.
It took a lot of force. The pump handle to the 25ton hydraulic cylinder could practically hold my entire wait off the ground before the pressed on gears finally let go. it made a very loud bang but we got them off. Once off I mic'd the shafts and the bore diameters and drew new gears with the help of a gear generator in Solidworks. I'll pick up with those details and my trip to State Tool and Gear in the next episode. Stay Tuned!
|The Input shaft (Left) and the Intermediate shaft|
|The Stock Primary Input Gears|