Last night, I had a dream that I was riding in a car that was driving down a highway somewhere. At one point, we were going down a really steep incline—I mean, really steep. It looked like we were going straight down the side of a cliff. This is the first time I’ve had one of these dreams in a long time. I used to have them a lot as a kid, but it’s been about twenty years since I had one, I’d say.

Anyway, I was wondering: Where is the steepest incline on a paved road and how steep is it? I mean, I know there are mountain paths that are probably very steep, but I’m talking about U.S. highways and whatnot.

Also, how steep an incline could your average automobile drive down safely and with little or no problem?

Hmmm, interesting - I remember seeing a picture of the ‘steepest street’ in a copy of the Guinness Book of Records many years ago (possibly in San Francisco) - but I can’t find it on their website.

Some claim it’s Baldwin St. in Dunedin, NZ, and that wouldn’t surprise me.

Part of it would depend on the speed you drive down it, and how long it is. If you want to maintain a constant speed on a very steep incline, you probably need to use brakes to keep from accelerating, and you may burn the brakes out on a long enough trip (happend to my dad driving in the mountains).

It seems to me, though, that aside from that problem, as long as, looking from the side, a vertical line through the centre of gravity of the car never gets ahead of a vertical line through the front axle, there’s no problem.

Until you try to use the front brakes, though… then the situation is akin to going over the handlebars on a bicycle. To prevent that, the braking force must be balanced by the weight of the car behind the axle. Hmm. Depends on car geometry and C of G.

I’d have thought there would be a loss of sufficient friction well before that point, after which you’re going to start sliding, whether braking or not.

I agree. I would seem to me that once you get past a 100% grade (a 45% degree angle), the amount of friction on the tires would decrease pretty rapidly because gravity’s effectiveness of pulling the tires into the pavement is reduced.

Cool pic, Trunk!

I’ve seen different claims for the steepest street in San Francisco, but the two most often named are:

from here.

I’ve driven down Filbert between Hyde (top) and Leavenworth (bottom), and it takes a certain leap of faith. The first few yards going east from Hyde are flat, then you come to the “steep grade” sign but beyond it you just see… nothing. It isn’t until you’re committed that you can see the road surface because it’s such an abrupt gradient shift (it’s one of the streets on Russian Hill where long-wheelbase vehicles such as stretch limos occasionally “bottom out”).

Wonder what the gradient of Baldwin street is?

This is by a guy who tried to bike up Baldwin St. It has another picture like the one I posted earlier.

He says it reaches 38% grade and the street was the result of someone planning it with incorrect topographical information.

He also mentions several of the other steepest hills in the world.

The static coefficient of friction is the critical thing here. Roughly (heh), the *static coefficient of friction is equal to the tangent of the angle at which “breakaway” occurs. Once “breakaway” happens, the static coefficient is no longer of concern; we’re now looking at the dynamic, or kinetic coefficient of friction. All this assumes, of course, that your brakes are sufficient to keep the tires from turning. If they’re not, then we gotta look at the rolling coefficient of friction. In all of these types of friction, the value of the coefficient is dependent on the materials in contact.

You can find a table listing the coefficiencts of friction between various materials here: http://www.engineershandbook.com/Tables/frictioncoefficients.htm The static and dynamic values for rubber on dry and wet asphalt and concrete are near the middle of the page.

And here’s a pretty good page with some simple experiments you do at home to determine coefficients of friction. Quiz on Friday. Ya know, in case you thought you were just gonna be able to slide easily through this.
http://www.school-for-champions.com/science/frictioncoeff.htm

Before I saw this post of yours, I decided to measure Baldwin St’s grade myself using your photo.

Loading the photo into MS Paint, extrapolating the lower edge of the deck and one of the support posts to where they would intersect the curb of the street, and assuming a 90 degree angle where the posts and deck meet, I obtained a grade of 38.0% How pleasant it is when such numbers match up!

I’ve walked up Baldwin Street. I’ve also walked up Filbert Street in San Francisco. The street in Dunedin is definitely steeper.

i’ve been up the Chimney (also described on that page)! Mind you, I wasn’t driving, and was making fun of the French runabout being used at the time (I nearly got out and pushed)

According to the CIty of Dunedin’s web site , Baldwin Street has a gradient of 1 meter rise for every 2.86 meters of run. This makes the tangent of the angle equal to 1 / 2.86, or about 0.35. Taking the inverse tangent gives an angle of about 19.3 degrees, which is very, very steep. However, the site says only six meters out of the entire street length are this steep. Filbert Street’s grade is “only” about 17.5 degrees.

I need to go put as protrasctor to a street in Eureka Springs AR. USA

I think it is a bit steeper than that 19 - 20 degrees. Hummmmm

I searched an hour for a web site that lists the world’s or even the nation’s steepest paved roads, and I found nothing. Nada. Zilch.

Anyone know of a site?

The relevant quote from you linked page says:

I am unable to parse this in a way that makes sense. The high-gradient part of Baldwin must be longer than 6 metres. My post #12 above shows that, at least for the width of this house, the grade is 38%. This surely corresponds to more than 6m of the street length.

I’ve stood at the top of the Filbert Street grade in SF. Apart from a small horizontal part east of Hyde Street, the entire block from Hyde to Leavenworth is of uniform grade (which is apparently 31.5%). As I said in post #9, the transition from “flat” to “steep slope” is extremely abrupt. When you eyeball it from the top of the slope, it appears to be of totally uniform grade until a couple of metres from the bottom. Walking back and forth around the visual “event horizon”, the entire sloping block of roadway appears and disappears in one step. From maps, I estimate the horizontal distance of the slope to be 125 metres, so we’re talking about a 40-metre gain in elevation, which is supported by SF Topo maps.

Here is a sideways photo of the relevant block of Filbert St.

Is anyone else skeptical about the description of the guy’s tires slipping while simply trying to stay stationary on Baldwin Street? I’ve ridden up some pretty steep concrete embankments (e.g. the sides of storm drains), and traction on concrete is pretty damn good. And if bike tires couldn’t hack it, wouldn’t car tires be roughly the same?

I have a certain skepticism about this. If the postman slid to the bottom on his arse when there was a frost, surely that would place the entire street off limits to residents trying to get home, as either driving or walking would be impossible.

It’s an impressive street no doubt, but I reckon a bicycle could hold still on it.