CliftonGK1

I don't question it. I question the sanity of it (but I also question the sanity of UMCA racers in general... Coming from an RUSA rider, that's saying a lot!)

The first hill on my morning commute is 2.25 mile long with a couple of 10% drops on it. I routinely hit 46 - 48mph coming down that road without even pedalling. If I had a gear high enough, I could put more speed on it; but I max out at a 48/11 which only takes me up to 48 - 49mph if I spin it out at 145rpm.

Barrettscv 

Altitude and air temperature play a role in terminal velocity. "Thinner" or less-dense air will not produce the same aerodynamic drag as the air that is denser at sea level and on cold weather days.

Warm air and higher altitudes produce higher speeds.

Tom Stormcrowe

Also bear in mnd the UMCA cyclists are generall down on aerobars, as well and very aerodynamic. I crewed RAAM for Rob Lucas and he topped 70 dropping down into Monument Valley, for example, and also topped 70 dropping down Wolf Creek Pass in Colorado. I was driving the chase vehicle in both instances.

Lord Chaos

Wildanimals, I think you really need to look at your whole braking system. I would not feel at all comfortable going down a hill on which I was unable to stop easily. You never know what's around the next corner; one day I was bowling along about 30mph on a nice little side street that usually sees no traffic... and found, around a corner, a whole bunch of bicyclists coming up, all over the road. I was really glad for disc brakes and confidence in stopping.

One thing to be aware of, that hasn't been mentioned, is that braking heats the rim, tire, and the air inside. If it gets hot enough the tires will blow off the rims. This happened to a friend of mine on a ride we were doing in the mountains. Fortunately he didn't crash.

So, make sure you have strong brakes, adjusted well. If your rims aren't straight enough to allow the brake pads to be close, then get the wheels trued. Make sure the cables move easily in the casing. If not, squirt some lubricant in there and work the brakes.

Then go out and practice. Find out what the limits of the system are, on a short hill. Find out what at what gradient you can stop easily.

People talk a lot about going fast--I've had bikes up to 50MPH when I had a good view ahead--but rarely about how quickly you can get into big trouble. Is the thrill of the hill worth the risk of serious damage if something around the curve isn't what you expect?

echotraveler

tail winds could also accel your downhill experience, no matterr 50mph on a bike with gravel is really freking scary!

CliftonGK1

50 - 51mph is my official white-knuckle death grip, chamois-staining speed. I did 54.something last summer and that was just too danged scary for me.

jesspal

Also alternate between front and back brake, on long descents you can overheat your pads. Alternating gives them a chance to cool off and they will then work more effectively.

Riverside_Guy

Ha, that reminds me that around 85-90 on my motorcycle, everything got super smooth. Stayed that way up to the century mark. It was all in the harmonics...

RiverHills

PS - The comparison of pinewood derby cars to bikes is apples to oranges. In pinewood derbies, the primary resistance force is friction, not air drag.

Anyway, I don't know what the actual terminal velocity of a bike is on a given descent or at free fall, etc. I was merely offering up the fact that both myself and a skinny friend of mine seem to max out somewhere just under 50 mph. Just a personal experience...if someone else is able to go a lot faster on a different hill, my "terminal velocity" is by no means backed by calculations of scientific evidence.

Mr. Beanz

Pfft! Work hard going up, then who cares about your downhill speed!...Take your time and be safe!

I think its more about balance and picking good lines. I rarely use my brakes on a mtn descent. Maybe lite feather of the breaks before hitting a sharp switchback. I dont brake in a turn or I lose the smooth turning effect.

I rarely pedal on a downhill mtn descent. So I find it funny that on the same ride, I hit 45 max and all my friends hit 55 but I'm usually the first one down!

Lance didn't win the TDF going dowhill!

LandKurt

Are you coasting or pedaling? I think most other clydes have a different experience. Personally I seem to out coast lighter folks on the downhills. I often have to apply brakes to keep from running them down, it's kind of annoying.

The science explanation lies in the square/cube law. An object that is doubled in all dimensions has eight times the volume and mass (the cube) but only four times the surface area (the square). So for increasing sizes the mass goes up faster than the surface area, which leads to a higher terminal velocity even with greater drag.

Pinyon

With gravel, I don't like to go much over 25 mph either. There is just no way to stop fast while going downhill on gravel. I would have stopped as well.

On pavement, there are a lot of hills around here where I hit at least 45 mph on a regular basis. I've hit 55-57 mph a few times. I don't feel safe spinning my pedals over about 95 rpm when I'm going over 40 mph down a hill, so all of the top speeds are with me coasting downhill in a tucked position (butt off the back of the saddle, feet at 3-o-clock and 9-o-clock, collar-bones hovering over the handlebar, hands in the drops, with a flat-back). My top speed so far is 57-58 mph. I could have gone faster on a straighter section of road, but I usually wimp out and apply the brakes on the roads that I've gone that fast on. I find even shallow curves intimidating at that speed. The prospect of running off the road, or crossing the lane and slamming into oncoming traffic is not worth the risk to me.

prxmid

I hear ya. It's not so much a fear thing as an "Is it worth it" moment. Above 30, I start considering what happens if I fall. I can handle road rash, but cycling is supposed to be fun.

After 50, discretion is the better part of valor

cyccommute 

Those are certainly factors, however, they'll only cause minor changes around the terminal velocity. Terminal velocity is going to be a lot higher than 46mph

Barrettscv 

https://www.denysschen.com/catalogue/density.asp

Sea level @ 30F = 0.080 Lbs/Ft^3
7000 feet @ 90F = 0.055 Lbs/Ft^3

LandKurt

So how does that affect terminal velocity? Since drag varies as the square of velocity a 45% change in density would give a 20% change in terminal velocity. That does seem like more than a minor difference.

But I think the real reason you hit higher descent speeds at high altitudes is because or the dearth of long steep downhill grades starting at sea level.

Pinyon

I hear about guys that don't apply brakes regularly hit 60-65 mph, going down the back-side of the local Rist Canyon loop (from the top of Rist Canyon, going down towards Stove Prairie: near Fort Collins, Colorado). Those guys are NUTS! Little dips in the road make both of my tires leave the ground on that road when I'm going over 50 mph, and you can't see around those shallow curves...its not safe.

Every other time that I'm up there, I see things in the downhill lane like slow-moving tractors, stopped vehicles (usually construction/utility or ranchers stopping to fix fences, phone lines, signs or reflector posts, etc.), deer eating the salty grass near the edge of the road, small groups of cattle that got out of the fence, etc. Not safe. Not safe at all.

Barrettscv 

Check your math, the change in resistance is not linier. 20% is not correct, even if it was, it would be significant.

Michael

wild animals

Here's the hill! https://maps.google.com/maps?f=q&sour...38495&t=p&z=15

It's JP West Rd. It doesn't look as curvy from space, but much* of the time you can't see what's coming up. You guys would probably think it is baby stuff! But I found it harrowing. haha.

I think my brakes are okay. They aren't in a state of disrepair or anything. They are very close to the rims, I have the levers adjusted so I don't have to pull them too far to get the brakes fully engaged, and the pads are older but not worn down to a fraction of a mm or whatever. My wheels are true! It was just so steep. I'd get myself down to 3mph by not-quite-fully engaging the brakes, and when I'd let go again, I'd be going 30 in like 5 seconds (please don't do the math on this), without pedaling. Out of control! I'm going to try again and see what happens, but I'm too busy for the next couple days :/

genejockey 

+1.

This is why the big guy leads the train downhill.

LandKurt

OK then, lets walk through the math. I'm sure it will be riveting.

0.080 / 0.055 = 1.4545 i.e. a 45% difference in density.

square root of 1.4545 = 1.2060

You got me, I should have rounded that to a 21% increase in terminal velocity given a non linear squared drag function. Why did you think I was talking linear? A 20% increase is the difference between 40 and 48 MPH, that sounds significant.

Do you have a different answer for the increase in terminal velocity based on the density changes you mentioned? You seem certain 20% is wrong. I'm not trying to be argumentative, nor am I in a huff at having my math skills questioned (well, maybe a little), just wondering.

Barrettscv 

It's too late in the day, but I still don't buy the 20% is insignificant.

Hill-Pumper

That is no small hill there young lady. If you climbed that, I am quite impressed. I mapped the hill and it looks like about 2 mile with about 900 feet of decent. Parts of it are showing a 19 percent grade, so it is indeed steep and cause you to gain speed quickly.(The mapping site may not be totally correct, but it does give a general idea of what the hill is like) I have a similar hill down here that I ride the brakes on.

On a side note, we are doing a Clyde/ Athena ride down in Albany on September 5th if you would like to join us. It will be a 40-50 mile no drop ride, just so you know.

LandKurt

Spoil sport.

Oh what the heck. 30 MPH in 5 seconds is an accelleration of 8.8 ft/sec, which is about one quarter gravity. It's probably not a simple as that implying a 25% grade, but might not be far off.

From that terrain map it looks like the steep section drops 320 feet in altitude in about 2500 feet of road. That computes as a 12.8% grade if I read the map correctly. While that's not insane it's pretty steep. My claim to fame is once climbing 400 feet in a mile even. That 7.6% average grade was plenty enough to give me a bad case of the willies going back down. But I have a problem with heights and the descent set off my acrophobia.

youcoming

To the OP if you want to stop fast the easiest way is to shift wieght to the back of your seat or even better get off seat and have wieght over the rear wheel and then brake, you will stop in a hurry.