After snapping a crankshaft, we decided we had better call in the experts to help us decide exactly what failed. I contracted fellow AFM racer, and Mechanical Engineer, Ryan Tiexiera to help us with some FEA analysis of the failed crankshaft. Ryan took the broken crankshaft and created a solid model in SolidWorks. The model had to be as accurate as possible in order to assure we could design the changes to eliminate the stresses that were plaguing us.
He then ran a stress test of the model and the software showed a failure point exactly where our crankshaft had failed. See the red area in the CAD picture to the left? This is the area that the stresses are higher than the material's maximum strength. When designing a part, you want to design with a margin for error, a cushion if you will. This is known as the Factor of Safety (FOS). Our crankshaft had FOS<1.
When FOS = 1, the calculated stresses are = to the maximum strength of the material. So, a FOS greater than 1, preferably closer to 2 is desired. Ryan ran an entire battery of studies finding the ideal solution would be to increase the fillet at the crankpins to .0875"R and add a fillet of the same size at the mains. This increased our FOS from <1 to approximately FOS=1.2. We were still on the weak side according to the calculations, but the material selection doesn't take into account heat treating and nitriding of the crankshaft.
Also, we were hoping our estimated stresses were on the high side as well. Just to be safe, we changed the process a bit and had a special heat-treatment applied to the cranks and had the fillets shot-peened for even more strength and fatigue resistance. This is a trick used in aircraft engines for durability. The process was done to FAA specs by an aircraft engine machine shop.
As another stress reduction path, we decided to go back to Ti Rods. After a year delay, Carrillo was ready to make the rods. Like every other custom piece for this engine, the rods took a very long time to make. Also, we had the price go up on us at least once during the design phase due to a major material price increase. Made from 6AL4V (6% Aluminum 4% Vanadium) Titanium base material, the rods were forged to the basic shape, then machined to final size. So, the entire process was quite involved.
The price went from the originally quoted $2500/set to approximately $3200/set. And we were told this was still at a financial loss (remember I said that Carrillo was using us as a test case for the Ti process. Even with the rods built on the over-engineered side, the rods were incredibly light compared to stock. We measured a 10% weight reduction on the reciprocating end (I was hoping for more), but an astonishing 23% weight savings on rotating mass over stock rods.
As with all prototype parts, we ran into some small glitches. Carrillo left out the oil jet notches from the thrust faces of the rods (an oversight in trying to get us the rod as quickly as possible). These are not present on the stock rods because Honda incorporated an oil jet into their beam design to spray the pistons with oil to keep them cool.
The result was seized pistons in both of our new Ti-rod motors. Since we made several changes at once, we weren't certain if the oil jets were the problem, or the fact that we had used pistons designed for Aluminum/NikaSil liners back in the original cast-iron liners (with modified clearances to suit). Either way, we seized both motors the same day. So, we had to make some serious changes quickly. We cleaned up the liners by honing out the mess and adding extra clearance, as well as having Carrillo add the oil jets to the thrust faces (as seen in the photo above). The combination seemed to have done the trick as the motors ran flawlessly for some time after that.....
Regardless of our efforts, we still ended up snapping another crankshaft. I can't even describe the sick feeling in my stomach when I got the phone call from Lohmeyer. I was so upset, you might as well have told me my dog had died. I was so certain we had this beat that the news made me really question my motor-building capabilities at that point.
When I had disassembled the motor for the "autopsy", I was even more sick to my stomach. The crack looked as though it had initiated from the peen marks I had put in the crankshaft to secure the oil galley plugs. I though I had caused the stress-risers in the crankshaft that took out another motor, another couple of pistons and 3 out of 4 Ti Rods. The rod pictured received a small nick, but it's enough to create a very expensive paper-weight.
It was at this point we parked the project for some time while we regrouped, rethought and reconsidered why we were doing this. This was the point I thought we would honestly pack it up, cut our losses and walk away from the project. And, for nearly a year, we did.
Despite our struggles, this was a breakthrough seaon for us at the same time. We were able to move forward with R&D this year, learn a few more things and get past some other plaguing hurldes to get the highest output to date of over 88hp. It was also the first season that Lohmeyer won the 450cc Superbike Championship, and Paul Yoshimune finsihed 2nd overall to tie up the top two podium spots. So, we did feel like we were finally somewhat rewarded for our hard work.
Bore x Stroke: 57.0mm x 44.5mm
Pistons: REC/G-Force Design
Rings: Yamaha FZR400 (3-Rings)
Bores: Bored Stock Cast Iron Liners
Rods: Carrillo Ti "H" Rods
Crank: CCR Weld-up stock crank. 2nd Generation
Cams: Stock VFR Cams with RLR Slotted Adapters
Max rpm: 14,500rpm
Carbs: VFR 32mm CV with HRC F-III kit set-up (no airbox)
Race Fuel: VP Ultimate 4 (oxygenated)
DEMISE: Snapped Crank #2