What is a "descending radius spiral turn"?

[FLrider]

Picked up a book on the BRP, for the trip I'm taking next week.

 

In bold red letters, a warning to motorcyclists, to watch out for these. I've done a lot of riding all over the country and I will admit that I don't know what these are.

 

I may have very well been on a descending radius spiral curve and just been too stupid to know that it was dangerous? Now I'm psyching myself out.

 

What's the story?

[rad]

It is too dangerous to even talk about, I better stop typing now before I get hurt.

[ShovelStrokeEd]

There are none in Florida so you don't have to worry. In my experience, there are very few on the BRP either. A couple of the roads leading on/off have some beauties though. The warning is misleading. I think what they mean is a decreasing radius turn that may be on a downhill section. These can be tricky and do require a bit of restraint if you don't know the road.

[BanjoBoy]

You find those on mountain roads, and I kinda like 'em, they can be very challenging.

Last year while riding up a ascending radius spiral turn, a "Bro-ette" was coming down, over shot it, and nearly took me out. It seemed strange to me because she was in the middle of a 6 rider group. Why didn't she just follow the Bro in front of her?

I guess some of them freeway on-off ramps could be considered a descending radius spiral. Basically you're banked over for awhile, and the road's dropping away from you. For some reason it really freaks some people out?

And then there's the decreasing radius turn. That's where a spiral gets tighter, and if yer not look'in up the road, it will catch you out. (You'll over shoot the corner)

 

Capice?

[rad]

The most infamous one in my county, the "Fish Hook". Moving from right to left in the pic

[OoPEZoO]

LOL

 

 

The ones on the parkway are not bad. If you are sticking to the posted limit, you might not even notice them. They are one of those corners that seems like they go on forever and wrap around on themselves, but then they also throw in an elevation change. They are a bitch for trying to pick your apex and hold a steady line, but they are also excellent opportunities for practice. If you want some really good ones, You have to get out to the west coast and hit route 1 in Northern California.......they have a whole bunch of tight corners with large elevation changes

[ShovelStrokeEd]

That "fish hook" is typical of many of the on ramps here in S. Florida. Most of ours are ascending rather than descending as we tend to favor underpasses on the approach but the nasty part, that last little decreasing radius just before the straight section bleeding into traffic is always there. I tend to treat these as a double apex, taking an early apex on the first section to leave me wide for that last dip into the transition. They can be fun with a car behind you as you suck him in with your slow progress around the first section only to swoop and whoop in the last. Tee hee.

[Pilgrim]

U.S. Highway 16 (aka Iron Mountain Highway)

southeast of Sturgis, SD, makes a complete, tight, descending loop, spiraling back under itself through a tunnel cut through the rock.

 

The satellite shot from Google Earth does not do it justice, but if you look carefully, you can see what I'm talking about.

 

The tight confines (it's heavily wooded on one side and rock wall on the other side except in the tunnel) encourages vertigo as you lean and descend in a constantly-changing direction with no hint of a horizon.

 

Pilgrim

[Deadboy]

There is a left hand turn downhill decreasing radius turn on the way to my place and I think I have taken it "well" maybe once in 6 years....If I cut the corner it is easy but staying in my lane just never seems to work out well.

[tallman]

If the rockslide doesn't keep you off the BRP near Pisgah Inn, the first off north of the inn, Pisgah Hwy ( I think 151) has a spiral staircase.

[James Clark]

[motoguy128]

With a decreasing radius turn, like others have said, you can either treat it like 2 turns connected together, or just stay to the outside, until the late tunr-in point.

 

Ultimately, you have ot remain relaxed and trust your tires. If you're only riding at a somewhat relaxed 6/10th pace, if the turn tightenes, it's just a matter of pushing on teh bars, tightening yoru line and pushing closer to 8/10th or 9/10ths pace.

 

The challenge of a descending, decreaseing, radius turn is that your sight lines may be reduced and further, the road might be off camber, and worse your are going downhill os you're essentially accelerating through the turn. The downhill factor will cause you to trail brake, which further reduces you the traction available for cornering.

 

Yes... that's right, you might even find the worst possible turn of them all... a blind, descending, off-camber, decreasing radius turn... with a change in pavement type, a driveway or intersection at the exit, and a little gravel tossed in.

 

The only thing scarier are high speed sweepers over the crest of a hill where the apex or exit of the turn cannot be seen, so the turn-n point requres either expereince or a guess.... which isn't much fun at higher speeds since hte bike is less responsive.

[motoguy128]

 

At least you can see if traffic is comming the other way as you enter the decreasing section.

[gottabmw]

I've never heard them called "descending radius turns." Usually, they're called "decreasing radius turns," which means that the corner gets sharper as you move through it, causing the tendency to under-steer through the corner. Likewise, going the opposite direction through the curve, it becomes an increasing radius turn, meaning you will have the tendency, if you don't recognize what's going on, to over-steer. You can't just turn in and hold the angle; it keeps changing. You usually find these kinds of corners where the traffic engineers have to force a turn into a small area. Freeway off/on ramps are common locations for these. Also, tight mountain switchbacks. Think of a spiral; going one way, it gets sharper and sharper and going the other way, it gets less and less sharp. Descending or ascending would refer to going downhill or uphill. Throw slope (descending/ascending) onto a decreasing radius turn and you have a challenge. I hope that answers your original question.

[Agent_Orange]

A communist plot.

[FLrider]

... I think I'll just go slow..er

[dhanson]

Slow in fast out, but be sure you are on the way out!

[EddyQ]

That "fish hook" is typical of many of the on ramps here in S. Florida.

 

Most all ramps in Ma. are decreasing radius "fish hooks". We have at least one decreasing radius ramp here that doesn't decend much but has a nasty reverse bank when you really don't want it. The grass doesn't grow too well outside that turn . . .

[AviP]

I've never heard them called "descending radius turns." Usually, they're called "decreasing radius turns," which means that the corner gets sharper as you move through it, causing the tendency to under-steer through the corner. Likewise, going the opposite direction through the curve, it becomes an increasing radius turn, meaning you will have the tendency, if you don't recognize what's going on, to over-steer. You can't just turn in and hold the angle; it keeps changing. You usually find these kinds of corners where the traffic engineers have to force a turn into a small area. Freeway off/on ramps are common locations for these. Also, tight mountain switchbacks. Think of a spiral; going one way, it gets sharper and sharper and going the other way, it gets less and less sharp. Descending or ascending would refer to going downhill or uphill. Throw slope (descending/ascending) onto a decreasing radius turn and you have a challenge. I hope that answers your original question.

+1

 

Put simply, if you were driving a car into a decreasing radius turn, you would not be able to hold the steering wheel at a constant angle. To stay on the road, you would have to tighten your turn continuously.

 

Decreasing radius turns are dangerous because it's very easy to run out of room fast, especially on a motorcycle. You enter the turn and think you have found the apex, but the turn just gets tighter and tighter. The apex of a decreasing radius turn is usually at the very end of the turn.

 

This was my most feared and favorite turn because it was close to home. It's the lazy looking turn from about top center to bottom center and not the tight looking ones. My first time, I entered it really hot doing about 75 mph on my R1 left knee down. I ran out of room, locked up the brakes, rear wheel slide into the curb still upright. The rear wheel hit the curb first at about 50 mph at an angle and the impact caused my right hand to wring the accelerator to full throttle. Surprisingly, the bike just launched out of the turn at about 75mph. It was weird and I was lucky. I was really close to going over the overpass.

 

This particular turn is sick for 3 other reasons:

1. It's blind and usually has cars backed up at the yield up ahead.

2. There is a nasty bump just before the apex that launches my front wheel in the air when I need it the most.

3. The portion of the turn just before the apex and beyond it is off-camber.

 

As you can see I studied this turn and was determined to master it. Eventually, I was able to do it at about 65 mph with the right line.

 

 

[Rinkydink]

The majority of freeway on/off ramps in Arkansas are decreasing radius turns. I can't imagine why they would do this as I thought the whole point was to make traffic flow on and off of the thoroughfare. A lot of the retaining walls look like remnants of a demolition derby event.

 

I discussed this on a call in talk show with the Arkansas State Highway Commisioner and he said it's purpose is to save real estate. Evidently eminent domain seizures aren't as entertaining as they used to be???

[gottabmw]

The majority of freeway on/off ramps in Arkansas are decreasing radius turns. I can't imagine why they would do this as I thought the whole point was to make traffic flow on and off of the thoroughfare. A lot of the retaining walls look like remnants of a demolition derby event.

 

I discussed this on a call in talk show with the Arkansas State Highway Commisioner and he said it's purpose is to save real estate. Evidently eminent domain seizures aren't as entertaining as they used to be???

Indeed, they are meant to save real estate, not always a choice where the topography won't cooperate. A decreasing radius turn is meant to gradually introduce a vehicle to a confined space. Even though it seems like a dangerous conspiracy by traffic engineers, it is better than a sudden circular curve that would fit in the same space.

[Rinkydink]

The slowing down part(off ramp)I can live with but when getting on the freeway isn't accelerating to matching speed to merge the objective?

[gottabmw]

Yes, and the on-ramps are increasing radius, allowing you to safely speed up as you exit the curve and enter traffic.

[Mister Tee]

Decreasing radius turns are frowned upon by most highway design manuals. Here is the Caltrans manual chapter that discusses curve design standards. Compound curves start at page 17.

 

http://www.dot.ca.gov/hq/oppd/hdm/pdf/english/chp0200.pdf

[Todd_Z1]

This is when you see a good looking women and your head attempts to to rotate in much the same manner as a hoot owl in a 365 degree radius when you ride by. This is often followed by the need for a visit to the chiropractor.

 

Todd

[JayElDee]

U.S. Highway 16 (aka Iron Mountain Highway)

southeast of Sturgis, SD, makes a complete, tight, descending loop, spiraling back under itself through a tunnel cut through the rock.

 

The satellite shot from Google Earth does not do it justice, but if you look carefully, you can see what I'm talking about.

 

The tight confines (it's heavily wooded on one side and rock wall on the other side except in the tunnel) encourages vertigo as you lean and descend in a constantly-changing direction with no hint of a horizon.

 

Pilgrim

 

Oh, yeah...been there done that. A complete 360. I did it in the rain with leaves all over.

 

Another famous m/c road with these is State Hwy 16 in WV.

 

John