SoutherlyBuster wrote: m.klinge wrote:
Ok - ive been doin a bit of calculation on the rails
Assuming the point witch experiences the most stress is right where the front (Or back) mounting screw is - spartan's rails can withstand a vertically force of 107 kg and a horizontal force of 10 kg each. (Assuming he has not used any type of high grade steel).
To me that dosnt sound like much as he will put more force on them than his own weight when he lands a jump.
What stress allowable did you use? A yield stress (when it yields) or ultimate (when it breaks)? I don't have the MMPDS at hand at the moment, but for steel (not stainless), elcheap grade is 125 MPa yield, 250 MPa for the better stuff and 350 MPa for the good stuff. From memory stainless steel is pretty strong and that is what George used.
Trying to guestimate the loads that your skates or vehicle will experience is always a challenge. Personally I go for what my body is likely to endure and anything beyond that there is no point making it stronger. So if you go for 2g loads, ie twice your body weight would be plenty (and some think this is over kill, and others think this is not enough!). That is the approach I used when designing my kiteboard. If you are serious about it, then put on some strain gauges. With the simple rail system that George uses, it's much easier to just estimate the load, make it, ride it and see if your assumptions were ok. If OK, then you can start optimising the design (ie do not exceed the peak stress) but reduce the weight, this then results in a fully stressed structure.
Well ive made the calculation using yield strength as i dont like the rails to be permanently deformed
What you need to know to calculate the dimensions:
- Yield strength (Y) of the material you want to use.
- Distance (D) from the outer part of the axel to the inner part of the first mounting hole (Mine is 140 mm).
- Thickness (t) of the material you want to use.
- Height (h) of the material you want to use.
- What "shape" the material is made in, eg L-profile, U-profil.
For a normal rail like spartans the following is true:
Max strength (kg) = (((Y*h^2*t)/D/2)*1/3)/9,8
For a rail made from L-profile the following is true:
Max strength (kg) = (((Y*h^2*t)/D/2)*0,63)/9,8
For instance - using a 3mm L-profile (35x35 mm) made from 6063-T6 aluminum the following would be true:
Y = 172 (According to wikipedia)
D = 140
t = 3
h = 35
Max strength = (((172*35^2*3)/140/2)*0,63)/9,8 = 145 kg pr rail.
Normal feel free to comment any