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#422
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Alan Baker wrote:
In article , Wayne Pein wrote: I really find it hard to understand how he could justify a decreasing radius turn as being a reasonable thing to build. Just because a roading program can spit out the stakeout points for a particular piece of roadway, that doesn't mean that it is a good idea to build it. Such a turn could be designed explicitly for the purpose of slowing traffic. In that case, a sign can warn of it. Wayne An off-ramp from a freeway that has a stop light at the end of the ramp. One must slow or stop anyway, and the forced slowing with a decreasing radius geometric additionally sends the message that the road about to be entered is not a freeway. Wayne |
#423
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In article ,
Wayne Pein wrote: Alan Baker wrote: In article , Wayne Pein wrote: I really find it hard to understand how he could justify a decreasing radius turn as being a reasonable thing to build. Just because a roading program can spit out the stakeout points for a particular piece of roadway, that doesn't mean that it is a good idea to build it. Such a turn could be designed explicitly for the purpose of slowing traffic. In that case, a sign can warn of it. Wayne An off-ramp from a freeway that has a stop light at the end of the ramp. One must slow or stop anyway, and the forced slowing with a decreasing radius geometric additionally sends the message that the road about to be entered is not a freeway. Wayne Why not have a constant radius turn of the same radius as your proposed decreasing radius ramp at its tightest? What would be the disadvantage? -- Alan Baker Vancouver, British Columbia "If you raise the ceiling 4 feet, move the fireplace from that wall to that wall, you'll still only get the full stereophonic effect if you sit in the bottom of that cupboard." |
#424
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Wayne Pein wrote:
Alan Baker wrote: In article , Wayne Pein wrote: I really find it hard to understand how he could justify a decreasing radius turn as being a reasonable thing to build. Just because a roading program can spit out the stakeout points for a particular piece of roadway, that doesn't mean that it is a good idea to build it. Such a turn could be designed explicitly for the purpose of slowing traffic. In that case, a sign can warn of it. Wayne An off-ramp from a freeway that has a stop light at the end of the ramp. One must slow or stop anyway, and the forced slowing with a decreasing radius geometric additionally sends the message that the road about to be entered is not a freeway. Wayne Why would you want to do that? I would think that would lead motorists to misjudge the exit to be safe at a faster speed than if it were a constant radius the whole way through. I would think the correct ramp shape would be a constant radius curve, with an "stop ahead" sign over the advisory speed sign to alert motorists to the signal. nate -- replace "fly" with "com" to reply. http://home.comcast.net/~njnagel |
#425
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Frank Krygowski wrote:
Brent P wrote: In article , Frank Krygowski wrote: Brent P wrote: I would suggest Frank ride his bicycle through a decreasing radius turn that wasn't visable until he was in it such that it forced him to brake hard. This would probably be the best lesson as to why this sort of design should be avoided. Braking while turning is as ill-advised on a bicycle as it is driving. Probably more so. :-) Almost every time I make a turn on the bike, it's done with a decreasing radius, and with braking while in the turn! This is normal for a bicycle! Sheesh. Newbies! Not braking by coasting frank. braking with the brakes. Coasting is normal on the road, not squeezing the hand brakes. Yes, braking with the brakes, Brent. While in a curve. Every day. It's quite normal. google for "friction circle" to see why that's a bad idea (yes, on a bike too.) nate -- replace "fly" with "com" to reply. http://home.comcast.net/~njnagel |
#426
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Alan Baker wrote:
In article , (Nate Nagel) wrote: Arif Khokar wrote in message ... Alan Baker wrote: I've driven my brother's Nissan Pathfinder (even before it had its shocks replaced) and it can easily -- easily -- more than double the advisory speeds on most ramps. Advisory speeds are based on the comfort level of a driver driving a 1939 Ford Vehicle. The lateral force would be enough to have a "ball on a string" deviate 10 degrees from the vertical position. Most drivers take curves such that the deviation would be between 12 and 14 degrees, IIRC. found this site: http://manuals.dot.state.tx.us/dynaw...okTextView/400 9;cs=default;ts=default I don't see any mention of a 1939 Ford, but essentially that appears to be correct. They do apparently allow higher G-forces for very slow speed turns, but 10 degrees is the recommended value for 35 MPH or higher. In any case the maximum value allowed is 14 degrees, still far less than people seem to find acceptable in day to day driving. I wouldn't be surprised if a 10 degree ball bank indicator reading *was* perfectly safe and comfortable in a bone stock '39 Ford, honestly. Perhaps it's time to revisit these standards; how often is a vehicle in regular use anywhere in the US older than the mid-late 1960's? Key quote: "The speed to be posted on the curve should not be reduced arbitrarily below that determined by the procedures provided in this section." Hmm, looks like *that* recommendation isn't followed across the board... Note that there really isn't *any* hard standard for advisory speeds for exit ramps, although obviously I have no way of knowing if that section of this document is derived from the Green Book or is unique to the state of TX. nate It also shows how stupid the system is. A ball bank indicator? One big problem with it: in addition to the movement of the ball due to lateral g forces, you also get movement due to the roll of the vehicle. And since different vehicles roll different amounts, you automatically get inconsistent results. Maybe *YOUR* car has perceptible roll at under 0.5G G nate -- replace "fly" with "com" to reply. http://home.comcast.net/~njnagel |
#427
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Alan Baker wrote:
An off-ramp from a freeway that has a stop light at the end of the ramp. One must slow or stop anyway, and the forced slowing with a decreasing radius geometric additionally sends the message that the road about to be entered is not a freeway. Wayne Why not have a constant radius turn of the same radius as your proposed decreasing radius ramp at its tightest? What would be the disadvantage? Starting with a larger radius is more consistent with the high speed entering the off ramp. As speed is lost, the turn can be tighter to ensure just that. Wayne |
#428
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In article ,
Nate Nagel wrote: Alan Baker wrote: In article , (Nate Nagel) wrote: Arif Khokar wrote in message ... Alan Baker wrote: I've driven my brother's Nissan Pathfinder (even before it had its shocks replaced) and it can easily -- easily -- more than double the advisory speeds on most ramps. Advisory speeds are based on the comfort level of a driver driving a 1939 Ford Vehicle. The lateral force would be enough to have a "ball on a string" deviate 10 degrees from the vertical position. Most drivers take curves such that the deviation would be between 12 and 14 degrees, IIRC. found this site: http://manuals.dot.state.tx.us/dynaw...BookTextView/4 00 9;cs=default;ts=default I don't see any mention of a 1939 Ford, but essentially that appears to be correct. They do apparently allow higher G-forces for very slow speed turns, but 10 degrees is the recommended value for 35 MPH or higher. In any case the maximum value allowed is 14 degrees, still far less than people seem to find acceptable in day to day driving. I wouldn't be surprised if a 10 degree ball bank indicator reading *was* perfectly safe and comfortable in a bone stock '39 Ford, honestly. Perhaps it's time to revisit these standards; how often is a vehicle in regular use anywhere in the US older than the mid-late 1960's? Key quote: "The speed to be posted on the curve should not be reduced arbitrarily below that determined by the procedures provided in this section." Hmm, looks like *that* recommendation isn't followed across the board... Note that there really isn't *any* hard standard for advisory speeds for exit ramps, although obviously I have no way of knowing if that section of this document is derived from the Green Book or is unique to the state of TX. nate It also shows how stupid the system is. A ball bank indicator? One big problem with it: in addition to the movement of the ball due to lateral g forces, you also get movement due to the roll of the vehicle. And since different vehicles roll different amounts, you automatically get inconsistent results. Maybe *YOUR* car has perceptible roll at under 0.5G G nate *Every* car does. -- Alan Baker Vancouver, British Columbia "If you raise the ceiling 4 feet, move the fireplace from that wall to that wall, you'll still only get the full stereophonic effect if you sit in the bottom of that cupboard." |
#429
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Nate Nagel wrote:
Frank Krygowski wrote: Brent P wrote: In article , Frank Krygowski wrote: Brent P wrote: I would suggest Frank ride his bicycle through a decreasing radius turn that wasn't visable until he was in it such that it forced him to brake hard. This would probably be the best lesson as to why this sort of design should be avoided. Braking while turning is as ill-advised on a bicycle as it is driving. Probably more so. :-) Almost every time I make a turn on the bike, it's done with a decreasing radius, and with braking while in the turn! This is normal for a bicycle! Sheesh. Newbies! Not braking by coasting frank. braking with the brakes. Coasting is normal on the road, not squeezing the hand brakes. Yes, braking with the brakes, Brent. While in a curve. Every day. It's quite normal. google for "friction circle" to see why that's a bad idea (yes, on a bike too.) It's only a bad idea if you enter the turn at a speed where *all* of the available traction is used for cornering, i.e. too fast. But since many turns are entered before the driver can completely see the turning radius throughout the turn he should always leave sufficient margin so there is still traction available for braking in addition to cornering. Fortunately the mathematics of perpendicular vector addition help us out here. The equation of a circle is x^2 + y^2 = r^2 where we can use 'x' for the traction available for braking and 'y' for the traction available for cornering, and 'r', the resultant is the total available traction. Let's assume the total traction is 1. Then entering the turn so fast that cornering alone requires a traction of 1.0 would leave nothing available for braking. But entering even a little slower, say where cornering only requires a traction of 0.9 now allows us to use some braking up to a traction of sqrt(1-.9^2) = 0.44 Because the forces are perpendicular we can use our brakes up to almost 44% of maximum and simultaneously corner at 90% of maximum without exceeding 100% of the available traction. Even if the driver (or cyclist) cuts it closer and is cornering so fast that he's using 95% of the available traction he can still apply the brakes at 31% of maximum before risking a skid. And even the real thrill-seeker who corners at 99% still has 14% of maximum braking available to him before initiating a skid. So yes, the brakes should be applied carefully and with some caution while cornering, but there is nothing about "friction circles" (really just basic vector force addition) that says no braking should be combined with cornering. I live near a 3500' hill which I frequently ascend and descend on my bicycle, usually in the company of other cyclists. The road is very winding and, as is common of many mountain roads, has turns where the radius of curvature varies. I use my brakes on the way down in most of the turns and don't think I've ever seen anyone descend this hill without doing a substantial amount of braking while in the turns. |
#430
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Nate Nagel wrote:
Frank Krygowski wrote: Yes, braking with the brakes, Brent. While in a curve. Every day. It's quite normal. google for "friction circle" to see why that's a bad idea (yes, on a bike too.) I don't need to google friction circles. I know all about them. It's interesting that Brent claimed _I_ was talking about racing, i.e. limit of adhesion situations, now that you are using a term that really pertains only to that situation. If a vehicle is not undergoing extreme lateral acceleration, there is plenty of friction available for braking as well as turning. And, as on freeway exit ramps, I have sense to stay away from ten-tenths cornering moves. -- --------------------+ Frank Krygowski [To reply, remove rodent and vegetable dot com, replace with cc.ysu dot edu] |
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