genejockey 

Maybe it's just me, but if your hypothesis (that calculator) and the real world (post 23, among others) disagree, your hypothesis is wrong. It suggests that whatever modeling that calculator is based on is flawed.

RoadMike

Yeah, I can't even get close to that speed anymore, when I did that I weighted around 220 and must have had quite a tail wind as well. I'm a little over 6' 4" tall and 180 lbs now and the fastest I get moving without some assitance (tail wind, drafting vehicles, etc) is around 55 mph too. I like going fast up to a point, but when I start approaching the mid 50's it stops being fun and starts being a serious test of my concentration. I'll sacrifice the 9 mph in descent speed for the vast improvement in my climbing from the weight loss any day though.

contango 

What cassette do you have? I've gone over 40mph freewheeling down a 14% grade, hit 40mph pedalling quite fast in (I think) 50-13, and hit 38mph on a mountain bike pedalling in 44-12 down a big hill.

You are a lightweight compared to me, I tip the scales at more like 240lb. Sometimes that's what makes the difference, more weight to boost the speed but without a proportionate increase in frontal area for wind resistance. You'd probably smoke me going up the hills though.

redcon1

What about the pipe, does it stay lit above 40mph?

thump55

In regards to being passed by coasters:

Cresting speed will have a large impact on max speed on a straight line downhill (especially if it is shorter). If you are getting passed further down the hill by coasters, it could be that they were pushing faster than you over the crest and on the first 1/3 of the descent.

merlinextraligh

Hence, if you want to go faster than 40mph, with a compact crank, you pound over the top of the hill, hammer hard until you're in the low 40's, then tuck and coast. Pedaling hard to get up to speed makes a bigger difference, and gives a better return on effort, than trying to spin like a gerbil to raise your speed once you're already in the 40's.

FWIW, I've done 62mph downhill on a single bike, and 65 mph on a tandem.

Phil_gretz

Rode my touring bike this Saturday and we had some seriously long descents. Unloaded tourer ~30 lbs. I weigh 175, 6' 1". I glanced down (to the cyclometer) on one particularly fast section, and we were doing 48 mph in a group. Things were stable. PG

Excelsius

This thread made me curious about my own speed - since I got the Garmin edge four months ago, I don't have any other recorded speed before then. The Garmin Connect summary shows a max of 48.3mph, but after a closer examination using the Player function, there is no such data point! Ignoring that, my top speed is 46. But what this demonstrates is that you can't always trust your gizmo's recorded speed since apparently it can be much lower than recorded. I am not sure why this is the case because I thought that Garmin records the speed continuously. I guess it's possible for it to freeze sometimes or maybe the magnet sensor doesn't always work reliably, especially at faster wheel rotations. So if you want to know your true max speed, I think you need to look at the data more closely. The Player function in Garmin Connect has the feature to advance to the next data point. And by the way, I have set up my Garmin Edge 500 to record the GPS coordinates for every second, so it's the highest resolution that can be achieved on that device anyway.

As far as going over 40mph, I am able to do it on almost every ride. Even if the descent isn't steep enough, my 52x11 compact allows me to pedal hard enough to get there. It's like an opportunity for interval training that's actually fun. Other than what was said about cresting and aerodynamic position, it's also important how well you can corner with minimal or no braking. There are great youtube videos demonstrating the correct way to corner at speed.

Someone seemed to imply that aero wheels don't matter on the descent, but I disagree. The entire functionality of those wheels comes into play at speeds over 25mph, so a descent at over 40mph is the perfect place where an aero wheel can provide the most advantage.

jsutkeepspining

I don't think using the calculator is the ideal thing here. I routinely hit 50+ mph on short not too steep descents (7-9%) and i weigh a massive 155 lbs.

bikerjp

Assuming you are going downhill (not some beast like Cav) on a 50x11 you can spin up to 40mph at a cadence of about 110 or so. Not comfortable to hold for any length of time but once you hit 40 tuck and let gravity do its thing.

jsutkeepspining

110 rpms not comfortable? **** i used to race crits at 130 rpms for prolonged periods of time #juniorgears

banerjek

Calculators make assumptions that won't be true most of the time. 40 is really an easy speed to hit as others have mentioned -- I'll do that with one hand on the bars sometimes.

As far as the OP getting passed when he's pedaling and they're not, it's because aero rather than pedaling tends to be more relevant at higher speeds -- at least those achieved due to gravity. It can happen though. 2 weeks ago I was descending a two lane road, caught a car going 45, got bored (plus I knew I'd take a series of curves coming up faster than it could), and used the draft of the car plus a bunch of pedal power to slingshot myself around it. But except for the boost, it was still all about aero. Handling skills are also relevant to top speed when going down hills.

hamster

Name the location of the descent and the last time you hit 50 mph there and I can check wind speed/direction. We'll probably find strong tailwind.

We have hills here as far as the eye can see. I think I exceed 40 mph once every 10 hours or so on the bike. According to my logs, I've exceeded 45 mph on three occasions since March. Two were in the mountains at 6000' altitude and one was going down 11% grade. I don't think I can say with confidence that I ever got to 50.

wphamilton

The calculator is consistent with "real world", making assumptions that are generally but not always true. One of the assumptions is that the grade is fairly consistent with the average grade, and that it's long enough at that grade to attain the terminal velocity. And of course very general estimates of the surface area that affects drag.

Post 23 for example is surely correct, as long as you "find the right road" as he suggests.

In my experience, as an "average sized cyclist" at 5'10" and 150 lbs, with the short hills at about 8% average grade on one of my commuting routes, hamster hit it dead on. These are rolling hills. I'm frequently at 35-38 mph but the top speed recorded on my computer for several months is 38.5 To hit over 40 I either have to work at it or find a better hill. Or use some aero gear. I'm pretty aero as well, and I sometimes find myself on the other side of OP's equation, coasting up to a larger guy who's pedalling downhill. I suspect that the calculator is more broadly consistent with the real world than are the counter-examples.

genejockey 

You realize the counter-examples take place in the real world, unlike the calculator.

DXchulo

Yeah, I think the calculator is better over a long period of time for an average speed on a longer descent. The thing is that even on descents around here that tend to maintain a relatively constant gradient, there are still going to be some sections that are steeper than others and there are going to be times when the wind is perfectly at your back as opposed to a crosswind or a headwind.

I went to Strava and found a recent descent of Geiger Grade (7.5 miles at -4.8%). There wasn't much wind that day and it was towards the end of a long ride, so I was just coasting. My average speed was 32.5. The calculator came up with 32.4. That's pretty damn close. Here's the thing, though: there were two different times when I was over 40mph. However, according to Strava there are a few sections where the descent is over 6%.

Meanwhile, if I select "Superman Position" instead of "hands on the drops," the speed goes up quite a bit. If I want to go fast on a descent, I'm usually in a position somewhere in between the two. I never go all the way to where my balls are on the top tube (not brave enough for that), but I do get low with my face close to the stem, arms in close, and back as low as possible.

So yeah, it's easier to go faster than the calculator suggests, but variables are constantly changing as you're going down the hill.

wphamilton

Sure, and I gave a counter-counter example, just as real as the others I'm trying to say, the math on which the calculator is based IS representative of the "real world" where one or two examples are not.

genejockey 

A model has predictive value, or it doesn't. There have been lots of counter-examples, not just one or two. And again, the examples take place in the real world.

You can't say it's representative of the real world, if the real world keeps doing something different.

wphamilton

That isn't true - a model predicts within certain parameters. Outside of those parameters, the model may falter. Yet the model may still be more representative of the real world - particularly since it has greater predictive value than the exceptional anecdotes.

genejockey 

So, it's predictive for two, maybe three people on this thread, and not for a larger number?

Okay, then.

BTW, I routinely hit 42 mph on a 0.4 mile stretch with an average grade of about 6%. I'm 6' and weigh 225, and I ride in the drops. Dead calm. And what limits me to 42 mph is running out of hill, not terminal velocity. That's 5 mph faster than the model predicts.

hamster

Counter-examples to what?

There have been counter-examples to the 43 mph rule, which is OK because the rule was specified with caveats about wind, altitude, and body weight. There haven't been any counter-examples to the general validity of the calculator itself. If someone comes and says "I weigh 160 lbs and I can hit 50 on a 8% slope at sea level with no wind", and it can be verified that he in fact hit 50 with no wind, that would be a counter-example and would indicate that something is wrong with the model.

TromboneAl

In a very rough way, that calculator is pretty easy to test. Today I went down a hill with a grade of 6% without pedaling, and used the calculator to predict my terminal velocity (before looking at my data). I stopped at the bottom and estimated the wind speed. It predicted 26.9 MPH, and that was in the right neighborhood. The peak shows 31.4 MPH, but the values around it are near the predicted value.

genejockey 

I just posted one. 6'; 225#; drops; 6% slope; dead calm. Calculator says 37.8mph. Try it.

I routinely - as in nearly every time I ride it - hit 42mph, and still accelerating when the road flattens out and starts going up.

hamster

Can I have the location of the slope and the last time you hit 42 there?

genejockey 

San Mateo, CA. Alameda de las Pulgas southbound between Nevada and Seville. Every Sunday at about 2:00 PM.