Icons that make up a 911 icon: Part 1
05-30-2010, 07:04 PM
#46
Brett. The inertia of any part of the wheel is linear reletive of the axle. The dynamic occurs when the axle has velocity reletive of the ground(0 velocity). The ground has a 0 velocity as does the mass of the wheel in contact with the ground, and therfore the wheel at that point has 0 inertia.
Rusnak is right, 1000Gs at the top of the wheel reletive to 0Gs at the bottom-while in motion.
Rusnak is right, 1000Gs at the top of the wheel reletive to 0Gs at the bottom-while in motion.
Brett
Last edited by Brett San Diego; 05-30-2010 at 07:21 PM.
05-30-2010, 07:17 PM
#47
Rennlist Member
Ok, suffice it to say, the stem support is needed on 8's and 9's at a minimum, as the stem wants to bend outward when you drive fast. I lost a support at Texas Motor Speedway once, and my valve stem destroyed itself bending outward on the front straight prior to me entering turn 1. This was my RH rear tire, and scared the heck out of me. Thank goodness for high-banked turns.
05-30-2010, 07:53 PM
#48
Just my two cents worth.
The stresses on the valve stem at high speed have little to do with ground speed or the surface of the road. Once the centrifugal forces take over, gravity (the earth) has very little to do with the forces.
The motion path of the stem relative to the car, or more specifically, the wheel hub creates a constant force radial to the wheel that is significantly higher that any forces placed upon it from the earth.
The stem, at rest, sees 1G relative to the earth.
The forces on the stem in motion relative to Earth:
2500 RPM = 41.67 Revs per second.
Each rev produces 2 cycles of vertical acceleration relative to the earth.
88.33 cycles per second or inversely 0.012 sec per cycle.
Vertical distance traveled per cycle is 14 inches.
This works out to a maximum vertical acceleration of 1166 in/sec, or 29.66 meters/second.
1G = 9.8 m/s
At 60 MPH, the max vertical force (relative to the earth) on the valve stem is 29.66/9.8 3Gs
The forces relative to the wheel hub:
At a 7" radius you reach 1G at 71 RPM
At 60 MPH, the wheel is turning 2500 RPM
At 2500 RPM the valve stem is experiencing 1293 Gs.
At 120 MPH the wheel is turning 5000 RPM and the stem is experiencing 4956 Gs.
No wonder 1/2 an ounce has such a dramatic effect on wheel balance.
The forces on the valve stem wile driving down the road at 60 MPH are 1293 +/- 3 Gs.
:-)
The stresses on the valve stem at high speed have little to do with ground speed or the surface of the road. Once the centrifugal forces take over, gravity (the earth) has very little to do with the forces.
The motion path of the stem relative to the car, or more specifically, the wheel hub creates a constant force radial to the wheel that is significantly higher that any forces placed upon it from the earth.
The stem, at rest, sees 1G relative to the earth.
The forces on the stem in motion relative to Earth:
2500 RPM = 41.67 Revs per second.
Each rev produces 2 cycles of vertical acceleration relative to the earth.
88.33 cycles per second or inversely 0.012 sec per cycle.
Vertical distance traveled per cycle is 14 inches.
This works out to a maximum vertical acceleration of 1166 in/sec, or 29.66 meters/second.
1G = 9.8 m/s
At 60 MPH, the max vertical force (relative to the earth) on the valve stem is 29.66/9.8 3Gs
The forces relative to the wheel hub:
At a 7" radius you reach 1G at 71 RPM
At 60 MPH, the wheel is turning 2500 RPM
At 2500 RPM the valve stem is experiencing 1293 Gs.
At 120 MPH the wheel is turning 5000 RPM and the stem is experiencing 4956 Gs.
No wonder 1/2 an ounce has such a dramatic effect on wheel balance.
The forces on the valve stem wile driving down the road at 60 MPH are 1293 +/- 3 Gs.
:-)
I'm no expert, but what we're doing here is looking at the problem in different coordinate systems (cartesian vs. polar). It doesn't matter which coordinate system you use, the results should be the same with regard to the motion of the valve stem. The math gets much more complex in cartesian coordinates, which is why someone invented polar coordinates to handle rotational motion, thankfully.
I think you've put it well with regard to how to handle this physical problem. It simply makes much more sense to look at the physical system and realize that the valve stem is on the wheel and not the ground and pick the reference frame of the rotating wheel to consider the motions and forces involved in creating those motions.
I hated to take the thread off topic, but I don't think correct physical interpretations were being offered so I felt we should continue the discussion until we get it right. I'm not saying I'm right, but I do hope someone more expert than me can make sure that incorrect interpretations do not mislead anyone.
Love those iconic Fuchs. My 7 and 8 in Fuchs are missing valve stem supports, and this discussion reminds me that I should get some. In fact, it was Harvey Weidman himself who pointed that out to me during the Parade Concours in San Diego in 2007. My car still won. The judges weren't quite as astute. LOL
Brett
Last edited by Brett San Diego; 05-30-2010 at 08:11 PM.
05-30-2010, 11:30 PM
#49
Rennlist Member
You've calculated the average velocity required for the valve stem to traverse the linear distance across the wheel (29.66 m/s) and then compared that to acceleration (G) whose units are m/s2 (meters per second squared). That comparison is not really relevant.
Brett
Brett
My math is minimalist at best.
The rotational forces are still way beyond the gravitational though.
05-31-2010, 09:38 AM
#53
First of all, thank-you w00t for starting this thread, I'm learning a ton reading through it and thoroughly enjoying the discussions that are taking place! I am also looking forward to the next 8 installations, and trying to guess what they might be!
As for the valve stem problem, the force on the valve pin of a spinning wheel is relative to the mass of the valve stem, the rpm of the wheel, and the size of the wheel (ie distance from the centre of the wheel to the mass of the valve pin). If we ignore the effect of gravity on the vertically rotating wheel, and other external forces such as drag from passing through the air, I see the problem as follows:
Assume the valve stem is 7 inches from the centre of the wheel (radius), weighs 1 gram, and the wheel is turning at 5000rpm. I'm going to convert the radius to meters, so that's r = 0.1778m.
Angular velocity w = 2*pi*5000/60 = 523.60 rad/s
centrifugal force = m*r*(w^2) = 0.001*0.1778*(523.6^2) = 48.745 Newtons ~ 11lbs.
So if I haven't messed this up too badly, the spinning valve stem is experiencing the same force as if you hung an 11lb mass from it.
To take this to G-forces (and this may be incorrect):
Force of gravity if the valve stem is sitting on the ground, is F = m*a = 0.001*9.81 = 0.00981N
Thus, while spinning the valve stem is experiencing 48.745/0.00981 = 4968.91 g's. I think we've seen a similar number before, from Ed Hughes. (Our assumptions on valve stem mass were likely different).
Sorry to bring the thread back to math, but I had to know!
Edit: It was irobertson that had a similar answer to me.
As for the valve stem problem, the force on the valve pin of a spinning wheel is relative to the mass of the valve stem, the rpm of the wheel, and the size of the wheel (ie distance from the centre of the wheel to the mass of the valve pin). If we ignore the effect of gravity on the vertically rotating wheel, and other external forces such as drag from passing through the air, I see the problem as follows:
Assume the valve stem is 7 inches from the centre of the wheel (radius), weighs 1 gram, and the wheel is turning at 5000rpm. I'm going to convert the radius to meters, so that's r = 0.1778m.
Angular velocity w = 2*pi*5000/60 = 523.60 rad/s
centrifugal force = m*r*(w^2) = 0.001*0.1778*(523.6^2) = 48.745 Newtons ~ 11lbs.
So if I haven't messed this up too badly, the spinning valve stem is experiencing the same force as if you hung an 11lb mass from it.
To take this to G-forces (and this may be incorrect):
Force of gravity if the valve stem is sitting on the ground, is F = m*a = 0.001*9.81 = 0.00981N
Thus, while spinning the valve stem is experiencing 48.745/0.00981 = 4968.91 g's. I think we've seen a similar number before, from Ed Hughes. (Our assumptions on valve stem mass were likely different).
Sorry to bring the thread back to math, but I had to know!
Edit: It was irobertson that had a similar answer to me.
Last edited by porsche0nut; 05-31-2010 at 11:56 AM.
05-31-2010, 11:06 AM
#55
Parts Specialist
Rennlist Member
Rennlist Member
BINGO - that was my original question 3 pages of math ago
and now a second question (which I have asked before)
Knowing ALL THAT THERORY and MATH... how do you ever get ANYTHING done!!
(haha)
Knowing ALL THAT THERORY and MATH... how do you ever get ANYTHING done!!
(haha)
05-31-2010, 11:22 AM
#56
Actually: It's true that you look at things in a different way after studying physics/solid mechanics/material science/etc... I do my best not to bring it up on first dates!
Back to the Fuchs, I have a question:
If I'm correct on the timeline, why did they stop manufacturing them after the 3.2?
05-31-2010, 11:41 AM
#57
Instructor
05-31-2010, 12:23 PM
#58
Rennlist Member
The stems on the smaller front wheels stick up perpendicular to the ground. It is when the stem sticks out at a angle when you need the supports. This is all way off topic now.
05-31-2010, 01:00 PM
#59
Race Car
The idea I am trying to support (poorly at that) was presened to me by a Bridgestone engineer. By no means am I trying to affirm the idea he postulated - he presented it to me as I do here- as food for thought. In fact, I enjoy the discussion and it sends me searching for answers. So thanks for a pleasant exchange. I am going to research it further, but it is strikingly similar to the "airplane on a conveyor belt" snafu. One thing we can all agree upon is that proper tires/wheels is critical to safety and performance.
Back to w00t's discussion of the Fuchs.
What's the opinion here on painted center crests? I REALLY like them on SOME 911s: the all-black Fuchs (Ivangenes avatar for example) and would be curious to see them on w00t's wheels.
Back to w00t's discussion of the Fuchs.
What's the opinion here on painted center crests? I REALLY like them on SOME 911s: the all-black Fuchs (Ivangenes avatar for example) and would be curious to see them on w00t's wheels.
