| Schumi |
08-03-2016 12:59 PM |
My wife is from Bretagne and we are moving back to closer to where she is from to look into some real estate renovation projects. I'm semi-retired and just looking for a new journey, living abroad seems like fun, and I need somewhere where real estate is cheap enough so I can outfit a proper workshop. France fits that bill.
As far as axles on the bikes go- Steel is used on the big bikes because they have forks and hubs designed for giant diameters. As you go up in diameter it makes sense to use a high strength, thin walled tube versus something else because it is so easy to get the stiffness you need, and steel is simple because fatigue is ignored and is one less thing to worry about. Also, real race bikes require frequent tire changes where damaging threads is a very real issue, so steel makes a good choice as the nuts/end bolts can be spun down quick and take large variances in torque while producing safe clamping loads without fear of cross threading or yielding AL threads ... or on titanium, galling between the Ti and steel nut/bolts
.
The rear axle on the wheel I am using needs to be 20mm, which is small- the stock Rs250 uses a simple low alloy steel axle. 6AL4V has a yield of at least 850 MPa or more, which will easily match the steel original for strength at a reducting in stiffness of about 50%, which, in my type of rear swingarm design, isn't much of a matter. Fatigue is also not an issue with titanium. There is no mathematical reason it cannot be used, it is often not used for race bikes due to cost and difficultly to machine- extremely difficult to machine with a good finish, and finish is key as the axles fit into bearing bores.
7050 Al is a more fatigue resistant 7-series similar to 7075. Increased copper content gives the increased fatigue life, at cost of slight decrease in corrosion performance IIRC. Again, the design is tailored to the application. The front axle is 25mm and the steel design to fit the original forks (GSX-R forks in this case) was semi-hollow. The Aluminum design needs to be solid most of the way through to keep stresses around the step-down into the bearings low enough to allow for it to have suitable fatigue life (and I'm using some pretty harsh street impact numbers driving those fatigue cases, which it really shouldn't see on a race track ever.) Even with the mostly solid design, the aluminum saves 240 gram over the OEM steel design, at the slight disadvantaged of reduced stiffness. However, with the ridiculously light curb weight of this bike, the stiffness requirements need not be as high as wheel loading is so much lower on track than a big sportbike. I am running very very stiff forks for such a light bike which carry a substantial about of my required CP stiffness in the total front end.
It's easy to move power on a two stroke to different areas of the band- by raising/lowering the ports you are effectively changing the timing, like advancing/retarding a camshaft does to intake/exhaust valves in a four. Putting extra thickness in the base gasket raises the jug, and then taking that extra off the head takes the head back to where it should be- this, along with a pipe tuned for it, raises the power band. This is really needed on supermotos due to the smaller rear tire vs. the stock dirtbike tire lowers the final gearing, and so a standard CR500 engine at 6000RPM in 5th gear with a super moto tire on the rear and a 15/45 sprocket combo will only do about 70mph.
|
