47.5 mph

That's an issue of terminal velocity. Near terminal, wind resistance climbs very fast.... though it's ALREADY related to the square of your speed, you can eventually get things like increased flapping of clothing/hair, which can affect your coefficient of drag (also related to square of speed).

Shape has a lot to do with it. more aero = faster. Drag from clothing can be rediced via skinsuits etc.

However, you should see at least SOME increase in speed. Are you saying that your terminal max speed is the same on a 1 mile 10% grade and a 1 mile 4% grade? If so, you must be pedaling. if you're pedaling--and if the slope is minimal--then most of the driving force is coming from your legs and not the hill, at those speeds.

i'm not saying that i "always" max out there on shallow downhills, but on steeper downhills where i really get moving i never get moving above 47.5.

for a little background i just (last fall) moved from chicago where i was a roadie/commuter for years, to Colorado, so its only since i've been here that i've had any experience climbing or descending (chicago- as you may know- barely even has speed bumps, much less hills).

i dont have a level on my bike so i'm not really able to judge just how steep i'm diving, i've just noticed that on those really sweet zippy, aero crouch- no pedaling mountain descents, regardless of the afforementioned factors, i always come home to a max speed of around 47.5.

and like i said, i'm not really too concerned about it, from a parctical standpoint since thats plenty fast for me, but i thought it was interesting that it seemed independent of slope/bike/etc...

does anybody else have a consitent "speed limit"?

The equation for terminal velocity is sqrt( 2W / Cd r A )

Where ...
W is weight
Cd is drag coefficient
r is density of the air
A is the area of the mass moving into the wind (your body's area)

I assumed that the average cyclist weighs 140 lbs
and area for cycling position is 2768 square inches

Cd according to a website is 0.9 for a cyclist, and r is 1.29 kg/m^3

after the unit conversions, I got 19.88 m/s which is 44.5 mph

Chances are that I over estimated the area of the body a bit, but the answer is comparable to what you've measured.

Aww, put him on a good 'bent and he will go faster, maybe even a lot faster.

West Texas Jim said:

The equation for terminal velocity is sqrt( 2W / Cd r A )

Where ...
W is weight
Cd is drag coefficient
r is density of the air
A is the area of the mass moving into the wind (your body's area)

I assumed that the average cyclist weighs 140 lbs
and area for cycling position is 2768 square inches

Cd according to a website is 0.9 for a cyclist, and r is 1.29 kg/m^3

after the unit conversions, I got 19.88 m/s which is 44.5 mph

Chances are that I over estimated the area of the body a bit, but the answer is comparable to what you've measured.

Nice work!

[quote="West Texas Jim"]The equation for terminal velocity is sqrt( 2W / Cd r A )

Where ...
W is weight
Cd is drag coefficient
r is density of the air
A is the area of the mass moving into the wind (your body's area)

I assumed that the average cyclist weighs 140 lbs
and area for cycling position is 2768 square inches

Cd according to a website is 0.9 for a cyclist, and r is 1.29 kg/m^3

after the unit conversions, I got 19.88 m/s which is 44.5 mph


I thought there was to be no math involved in this forum.

bentbrian said:

Aww, put him on a good 'bent and he will go faster, maybe even a lot faster.

down the hill maybe. but going up? :p

so like as i watch the tour i notice that phil and paul always mention that they're probably doing 70-80kmph on the downhills, which, if i have my math right is around 45-50 mph-ish... so i'm getting the sense that theres somethign to this terminal velocity thing.

badhat said:

so like as i watch the tour i notice that phil and paul always mention that they're probably doing 70-80kmph on the downhills, which, if i have my math right is around 45-50 mph-ish...

From stage 10 (velonews' live coverage)...
Pereiro and Jaksche are flying down the hill... clocked at 105kph right now.

That's upwards of 65 mph!!! Nutty!!!

L

All of you physics majors forgot a few a few variables in your equation:

S(f)=Shimmy factor - How much you shake going downhill, usually attributed to

P{C}= Condition of pavement -or- "potholes comin' at you!"

Let's not forget the most important variable....

PFT - Pure Friggin' Terror. I'm not ashamed to admit that when I begin to approach 50 mph, a combination of fear, self preservation and a strong gene pool tends to make me slow down a bit.

There, I said it. Now all of you can laugh. I'd rather go up than down....uhhh, that didn't come out like I thought it would...cyclingforums.comicon6.gifOpen ↗

dbianucci said:

All of you physics majors forgot a few a few variables in your equation:

S(f)=Shimmy factor - How much you shake going downhill, usually attributed to

P{C}= Condition of pavement -or- "potholes comin' at you!"

Let's not forget the most important variable....

PFT - Pure Friggin' Terror. I'm not ashamed to admit that when I begin to approach 50 mph, a combination of fear, self preservation and a strong gene pool tends to make me slow down a bit.

There, I said it. Now all of you can laugh. I'd rather go up than down....uhhh, that didn't come out like I thought it would...cyclingforums.comicon6.gifOpen ↗

I'm with you on this one, My definition of terminal velocity is the speed at which my fear factor tells me that any mistake/malfunction and the result could be terminal!

West Texas Jim said:

The equation for terminal velocity is sqrt( 2W / Cd r A )

Where ...
W is weight
Cd is drag coefficient
r is density of the air
A is the area of the mass moving into the wind (your body's area)

I assumed that the average cyclist weighs 140 lbs
and area for cycling position is 2768 square inches

Cd according to a website is 0.9 for a cyclist, and r is 1.29 kg/m^3

after the unit conversions, I got 19.88 m/s which is 44.5 mph

Chances are that I over estimated the area of the body a bit, but the answer is comparable to what you've measured.

You assume the average cyclist weighs 140 lbs???? Are you talking about tdf riders or the average cyclist on area roads?

I'm 6', 185 lbs. Other than my 14 year old daughter, I don't know any cyclists who weigh less than 175 lbs.

Alpha said:

You assume the average cyclist weighs 140 lbs???? Are you talking about tdf riders or the average cyclist on area roads?

I'm 6', 185 lbs. Other than my 14 year old daughter, I don't know any cyclists who weigh less than 175 lbs.

Hi, I'm Lonnie. Nice to meet you. I weighed 136.5 after my AM ride. However, I wouldn't call myself "average". Male 5' 8"....

L

I reckon there's something in this terminal velocity thing.

I pedalled as fast as I could down a steep hill and clocked 49.5mph (or at least that's what my computer said). Remember: I was pedalling!

Another time, (and don't try this at home) I was slipstreaming a minibus on a downhill (into Beddgelert for you NW folks) and the guys in the back were waving at me and gesturing 'Fifty five! Fifty five!' (i don't know whether they were telling the truth, as I had no computer at the time and therefore no way to verify my speed).

I'm not as daft as I used to be though, my MTB has a Shimmy Factor of just over 0.3 and my PTF quotient went through the roof when I hit 30 years old. 20mph is too fast now.😉

Whatever, I reckon with larger cranks you can pass 47.5. I guess that's how the TDF fellas manage it.

With all of the technology to determine ideal positions, and top of the line aero components it is not surprising that TDF riders go faster down hill than the average (plus the very favorable tremble factor).

I suspect that it is not just for the sponsors that all of the riders zip their jerseys after they crest a hill to go sown the other side. Wind resistance is impacted by flapping material.

Has anyone ever done mdel rocketry? Many use nothing but a streamer to slow the descent of the rocket to a safe speed.