VegasJen

So here's what I want to know about these posts. How are you getting any watts at these speeds? I mean, just on my little rides where I'm going down my 2% grade with a tailwind, I might push 40mph. But at that speed, pedaling has no input. My wheels are turning faster than I can pedal.

Ghazmh

Warp .0000000000001.

TK LP

64 MPH, totaling terrified, grinning ear to ear. Much younger.

ofajen

The fundamental equation for bike speed in relation to pedal power doesn’t have to worry about gearing. Pedal input is theoretically possible, but ordinary gearing doesn’t allow for it.

The equation just looks at conditions (wind, grade up or down, riding surface) and bike/rider data (total weight, frontal surface area, tire rolling resistance) and then figures out the power the rider needs to supply to make the equation work. On a down hill grade, the answer could be negative meaning that is the braking power needed to stay that slow and not reach terminal velocity. Typically it’s positive and means pedaling is needed.

Example, I can run the calculator to see I can coast my road bike down a 12% grade at 53.5 mph. That means I put in all the factors, and get roughly zero watts needed. In theory, if I pedaled at 300 watts, I should be able to nudge up to 56 mph and hold that for a while since I can sustain that power a long time.

However, as you point out, I likely don’t have relevant gears at that speed to let me add any power. Even at my top gear and spinning at 130 rpm, the bike is already going faster, so I just spin up and coast.

In theory, a really big chainring and really small cog or a compound drive with an internal gear hub as well could offer the ridiculously high gearing needed to spend your energy making tiny speed increases rather than just relaxing and enjoying the thrill of coasting down.


Otto

Jughed

53x11 gears (std road bike gears) = 40mph at 105 RPM. It can be done, I can put power down up to about 130RPM - not for a long time, but I can pedal that fast.

The 76mph down hill is very suspect on a road bike. On a -15% grade, you need a 60x10 gear ratio at 160 rpm just to keep up. And at sea level you also need to ADD 3330+/- watts to the equation. 1/2 of that a 5k elevation - about 800watts at 10k.

Assuming a road bike was being used - this feller would need huge gear ratios and have the ability to put down some serious power at 160RMP while controlling a bike at 76mph. Or he had a 50mph wicked fast tailwind.

jadmt

https://www.cyclist.co.uk/news/faste...h-on-a-bicycle

Jughed

Ive never had the opportunity to launch from a draft…

Alone I will crank up to 26 mph, not easy for me, settle in for a bit and do an all out sprint. Max flat ground speed was close to 31mph@1140w. I wonder what a launch,, if I could keep up with the lead out, would do…

Ive even tried to slow roll up to speed and I poop out between 30-31, even when trying to be aero as my big body will allow.

Im always amazed to see solo riders finish a long stage on a 34+ breakaway.

highandlowrpm

Maybe a little different for me, since now I only ride single speed analog and e-bikes, which in both cases limit me to the single gear ratio.

Every ride, get up to about 38-40mph. Max speed 53 mph, down a mountain. Got to really crank up the RPMs, then get in an aero tuck, since I’m geared out.

ofajen

Realistically we are discussing terminal velocity and coasting at such high speeds. That will be dictated by total weight, grade and rider position.

Weight has a significant impact. On a 30 pound total bike weight, my 180 pounds gives a terminal velocity of 53.5 mph on a 12% grade. If I weighed 250 pounds, my terminal velocity rises to 62mph.

I would need a 24% grade to reach 76 mph (and boy would that be hair-raising), while a 250 pound rider needs about 18%.

Oh my… two 200 pound riders on a tandem would need to brake slightly on a 12% grade to stay at 76 mph.

Otto

rsbob 

Internet search?

tomato coupe

I'm guessing your calculations are for sea level. If you're in the mountains, on a hot humid day, terminal velocity increases by a fairly significant amount.

growlerdinky

59 mph

Koyote

For instance, in the canyons up above Fort Collins, Colorado? In June?

I’m a bit amused by all these armchair physicists… None of whom were actually there.

ofajen

Yeah, just doing things for my reference first. I don’t know what Koyote weighs nor the relevant elevation but if I were cycling at, say, 8000 feet my terminal velocity on a 12% grade increases to 61.5 mph. For a 220 pound rider it increases to 67 mph. For a 250 pound rider it’s 71 mph. For two 200 pound riders on a tandem, it’s 88 mph.

It’s also not uncommon for a long descent to have a somewhat steeper section based on terrain that would increase the max speed. A quick dip to 14% makes a difference in max speed.

And these online calculators are just using standard formulas but any real life situation will be different in some way. A particular effective tuck posture could have a significant impact.

Otto

ofajen

Fair enough. I have no peer-reviewed publications in aerodynamics or fluid flow, only condensed matter experiment and theory and those were a long time ago.

Otto

jadmt

I did a search and downloaded it right after I posted that

Koyote

No, don't get me wrong -- I think your comments, below, are quite reasoned. They demonstrate more nuance than most others. You're pointing out that the online calculators can't account for everything - which is true.

rsbob 

And then again there is the terror of high speed wobbles. Must be something better than bracing the top tube with knees and raising your bottom off the seat.

Eric F

At 60mph, I was sitting on my top tube, with my chest on my stem (the now-illegal position in UCI racing). Maybe I could have gained a bit more speed if I didn't have my hands out in the wind on the drops, in easy reach of my brake levers.