My 3.6 Turbo
12-08-2013, 05:25 PM
#107
Rennlist Member
The valve stem supports are there to impart a centripetal force against the valve stems, in the direction opposite that of centrifugal force.
The reactive centrifugal force at the valve stem, due to the rotating wheel/tire assembly, imparts a stress at the rubber base of the valve stem [classical loaded beam]. When that stress is great enough, the material [rubber] goes into elastic deformation. That is, if/when the stress is removed [e.g. vehicle slows down/comes to a stop], the material returns to its undeformed state.
In this case, stress increases with increasing reactive centrifugal force [e.g. wheel rotational velocity/vehicle speed]. If yield stress is exceeded, that material goes into plastic deformation and has been permanently altered. That is, if/when the stress is removed, the material does not return to its undeformed state. If ultimate stress is exceeded, that material goes into failure. To prevent the valve stems from failing, the valve stem supports are there to impart a centripetal force against ever-increasing centrifugal forces.
No, not tangential...orthogonal to the velocity vector [circular path] of the valve stem at the instant of failure/detachment.
Okay...I'll drink to that. 
The reactive centrifugal force at the valve stem, due to the rotating wheel/tire assembly, imparts a stress at the rubber base of the valve stem [classical loaded beam]. When that stress is great enough, the material [rubber] goes into elastic deformation. That is, if/when the stress is removed [e.g. vehicle slows down/comes to a stop], the material returns to its undeformed state.
In this case, stress increases with increasing reactive centrifugal force [e.g. wheel rotational velocity/vehicle speed]. If yield stress is exceeded, that material goes into plastic deformation and has been permanently altered. That is, if/when the stress is removed, the material does not return to its undeformed state. If ultimate stress is exceeded, that material goes into failure. To prevent the valve stems from failing, the valve stem supports are there to impart a centripetal force against ever-increasing centrifugal forces.
12-08-2013, 10:49 PM
#108
Three Wheelin'
Thread Starter
12-08-2013, 10:51 PM
#109
Three Wheelin'
Thread Starter
The valve stem supports are there to impart a centripetal force against the valve stems, in the direction opposite that of centrifugal force.
The reactive centrifugal force at the valve stem, due to the rotating wheel/tire assembly, imparts a stress at the rubber base of the valve stem [classical loaded beam]. When that stress is great enough, the material [rubber] goes into elastic deformation. That is, if/when the stress is removed [e.g. vehicle slows down/comes to a stop], the material returns to its undeformed state.
In this case, stress increases with increasing reactive centrifugal force [e.g. wheel rotational velocity/vehicle speed]. If yield stress is exceeded, that material goes into plastic deformation and has been permanently altered. That is, if/when the stress is removed, the material does not return to its undeformed state. If ultimate stress is exceeded, that material goes into failure. To prevent the valve stems from failing, the valve stem supports are there to impart a centripetal force against ever-increasing centrifugal forces.
No, not tangential...orthogonal to the velocity vector [circular path] of the valve stem at the instant of failure/detachment.
Okay...I'll drink to that.
The reactive centrifugal force at the valve stem, due to the rotating wheel/tire assembly, imparts a stress at the rubber base of the valve stem [classical loaded beam]. When that stress is great enough, the material [rubber] goes into elastic deformation. That is, if/when the stress is removed [e.g. vehicle slows down/comes to a stop], the material returns to its undeformed state.
In this case, stress increases with increasing reactive centrifugal force [e.g. wheel rotational velocity/vehicle speed]. If yield stress is exceeded, that material goes into plastic deformation and has been permanently altered. That is, if/when the stress is removed, the material does not return to its undeformed state. If ultimate stress is exceeded, that material goes into failure. To prevent the valve stems from failing, the valve stem supports are there to impart a centripetal force against ever-increasing centrifugal forces.
No, not tangential...orthogonal to the velocity vector [circular path] of the valve stem at the instant of failure/detachment.
Okay...I'll drink to that.
12-10-2013, 03:15 AM
#110
Rennlist Member
How important are they? Disregarding my diatribe on centripetal/centrifugal force, I suspect they were, yet, one more thing, over-engineered by the Factory.
Most aftermarket wheels I've seen, like Paul's RH 3.6s, do not have them. But, some, like RDH's speedlines, do. It would be interesting to note in which camp the factories from Ferrari, Aston, and Lamborghini sit. I suspect, for wheel aesthetics, they leave the stem supports off. And if that is indeed the case, then, they can't be that important in preventing valve stem failures. Any exotic owners here care to chime in for our edification?
Leave it to the Germans...
04-19-2014, 10:15 PM
#111
Three Wheelin'
Thread Starter
Finally shes taxed, tested & insured to use now on the road after last years overhaul. What perfect weather now ready for the summer in the UK.
I couldn't wait as you can see a drive out at night whilst everyones asleep after work
I couldn't wait as you can see a drive out at night whilst everyones asleep after work
05-12-2014, 11:10 AM
#114
Three Wheelin'
Thread Starter
The spring weather here in the UK is getting better and summer is around the corner. This year I promised to use the turbo a little more whilst shes fresh.
Just a few chores on the list to do, but whilst she had an oil change, I discovered the oil cap was a little worn on the inner plastic thread. So a new one was order from the Porsche dealers. Part number 944! is that right?
Just a few chores on the list to do, but whilst she had an oil change, I discovered the oil cap was a little worn on the inner plastic thread. So a new one was order from the Porsche dealers. Part number 944! is that right?
05-12-2014, 11:42 AM
#117
Rennlist Member
Check the fill neck. The sealing surface gets damaged by careless wrenches and then they'll leak. I replaced mine as the top part with the threads is sold for repair but it is brazed on. I carefully removed the old one and brazed in the new one while I had the engine out last year.
05-12-2014, 11:58 AM
#118
Three Wheelin'
Thread Starter
Check the fill neck. The sealing surface gets damaged by careless wrenches and then they'll leak. I replaced mine as the top part with the threads is sold for repair but it is brazed on. I carefully removed the old one and brazed in the new one while I had the engine out last year.
