hamster

and please don't say "you tuck in".

It's probably a stupid question. During long descents, my speed tends to hover in the high 30's even riding in the drops. If it's really steep or I try to pedal, I can kick it up to about 42. I've been playing with this calculator

https://www.kreuzotter.de/english/espeed.htm

which gives me the coasting speed of 42-43 mph even for a triathlon bicycle going down a 8% grade (consistent with my experience). I assumed that 50 mph was impossible without a gnarly tailwind.

But looking around in Strava, I see people coasting down fairly sedate hills, e.g. 6-7% here https://app.strava.com/rides/85332#z415|434) at 50 mph. Down a 8-9% hill, a couple of guys manage to hit 58 mph:

https://app.strava.com/rides/8787667#z928|951
https://app.strava.com/rides/5754456#z1493|1524

How is that done?

AlbertaBeef

FYI strava isn't as accurate at maximum speeds as a properly-set cycle computer will be - so you can't take it at face value. GPS only updates every so often...

Second - to go faster downhill, gain weight. =) I'm 230lbs and was going just under 100km/h yesterday (a little over 60mph) on an 8.5% grade with 40km/h tailwind... Other people on the road that were more 'normal' sized (for cyclists) were doing the same downhill around 75km/h. I would've gone faster but had to turn off at the bottom... That "yee haa" moment was reward for climbing it ... It's a 4.2km hill and I had to climb it into the headwind.

AlbertaBeef

BTW I just played with the calculator you linked. At my current statistics, an 8% downhill choosing 'triathlon' gives me just shy of 53mph with no wind factored in, which is pretty close in my experience. I frequently do 100km/h (62mph) on the hill I mentioned with any kind of tailwind...

zaqwert6

I cant imagine that a bike computer, tailored for a sweetspot of probably 20mph is very accurate at 3 times that speed.

Cars aren't, motorcycles aren't.

My sportbikes are intended for speeds 150 and up and the speedos are off an all of them by at least 10mph by the time you get to only 100.

GPS, forget about it. The one in my car shows max speed right now, IIRC , of something like 208mph.

gregf83 

It's not hard to calibrate any bike computer to withing a few %. GPS speed is very accurate as well. There may be the odd point that's an anomaly but that would be obvious by looking at a graph of surrounding points. Most garmins are used in conjunction with a cadence/speed sensor in which case they would be very accurate as well.

To reach maximum speed you need to do more than going in the drops. A proper tucked position will be more aero than the default positions used by kreuzotter. If you want to use it the superman position might be close.

unterhausen

I think a tuck might get you there on a 9% grade, but I'm not sure. Other possibilities are traffic and tailwinds.

AlbertaBeef

+1

I'm faster with my hands gripping the top of the bar, just outside the stem,and my forehead to the bar. I look up every few seconds to scan for road debris, then head down again. Much like this position from this image, from gearinches.com

ericm979

Sorry, but that is the answer, after lack of turns, steepness, and tailwind. How areo you are largely depends on your position. Besides that, at the top go hard until you're spun out. If you start from a higher speed you can often reach a slightly higher max speed. Finally, if two riders take turns they'll be faster than a solo rider. The second rider slingshots past the first who catches the draft etc. You can do it at speeds where there's no pedalling if you time it right.

contango 

Only 208mph in your car? That's pretty weak, I managed 243mph on a mountain bike according to my GPS.

As long as you've got a decent satellite lock a GPS speed does seem pretty accurate. My 243mph was as I emerged from a long tunnel and the GPS had to figure where I was. As far as it was concerned it lost my position and I assume it figured I was still there, then I jumped by most of the length of the tunnel in a sampling period and so the speed showed something silly.

I've found using a cycling computer and a GPS side by side the speeds they show are invariably the same.

zaqwert6

Does a bike GPS adjust indicated speed to compensate for going down a hill?

No others do that I'm aware of.

Wheel sensed MPH has to be the worst over wide range. Set up standing still or even tweaked at a slow roll by tire circumference vs at speed, neverless extremely high speeds, with tire deformation, tire temp changes, etc. Forget it if you tapped your brakes and jacked your tire temp dramtically. Even the slight bumps in the road going to change indicated speed based on circumference.

I guess I'll add for effect...... IMO. YMMV

Carry on.

fordmanvt

I saw Peter Sagan doing it, but this is the first picture I found.

tfro

Or, it's actually fairly accurate. If the tire was compressed 1/2 of an inch(which is pretty extreme on most road tires) that would give an error around 1.8%. So, at an indicated speed of 60 mph would mean an actual speed of 58.9 mph. Sounds good enough to me.

How much do you think a tire expandeds or compresses during a descent?

gregf83 

No one is talking about .1% accuracy. Bike and GPS computers are accurate to within a few % which at 50mph is an error of less than 2mph.

GP 

Go down a straight hill like Sleeping Indian.

Brian Ratliff

You sprint as hard as you can to get up to 40mph as quickly as possible, then you tuck in. If you start a 1500ft hill from stopped, it will take most of the hill to get to 40mph. If you get to high 30's in the first 200m of the hill, then you'll get into the mid 50s easily.

Homebrew01

Why do you think a bike computer loses accuracy at high speeds ? It's just math. If the wheel spins twice as many times in a minute, then it's going twice as fast.

AlbertaBeef

I know some riders that do that, and I've seen it multiple times in the tour this year - I can't personally get into that position... I'm too long-torso'd to fit like that unless I get a custom-built bike

From what I understand it's much more dangerous than the position I adopt - with that much body weight over the front wheel control would be compromised.

AlbertaBeef

... and if you're on a longer hill, a heavy rider in a tuck can really get moving - especially if there's a tailwind. The 4.2km (2.6 mile) hill I do while training allows scary fast speeds when the wind is really blowing here, which it often does... I can literally pass traffic and it's got a 100km/h speed limit...

mkmartin

I weigh 125 and regularly hit upper 40's, 48 or 49 being the fastest i've gone this year.

datlas 

I am in a similar situation. I top out around 42 or 43MPH on downhills.

A few weeks ago I hit 49MPH but I was drafting a tandem.

I say tuck as best you can while maintaining a safe position, and accept the fact that some folks who are heavier, more aero, or just more stupid than you may go faster.

Reynolds 

You can't beat this for aero:



WARNING: This is a skilled professional. Don't try this at home.

zaqwert6

Bicycle people are kinda odd I guess.

To recap , the OP was questioning disparity between realized Max speeds and those posted on the net.

Max speeds. Not averages, not cruising but Max indicated speeds.

Small calculation errors during user setup can equal large errors at max speed , this should be obvious...but maybe not. I know everyone here has their bike comps set up to .0001 resolution, thier tire pressures to .001 psi and comped for temps and atmosphere but I think there are many out there who don't. Many just bolt it on and go or base all setup on the numbers given in the booklet. Many riders are not at all consistant or critical about tire pressures and/or rider included compensations.

Then we take that potential for errors and add in relatively extreme decents which the action of, again, exposes these errors to a greater degree. And lastly when talking about GPS... GPS is NOTORIOUS at giving false MAX speeds. Again, this should not be a shock to anyone I don't think, but maybe it is. Also , GPS does NOT correct speeds relevent to elevation. Its point to point, birds eye view. If thats not understood by some, for example, jump off a building with your GPS. It will say you are traveling zero miles an hour. Are you ?

It all adds to reasons why my posted speeds might contradict a random users posted online. And lets not forget the BS factor, athough extremely rare but believe it or not, some may wish to 'enhance' thier personal data when posted online. Shocking again I know.


A 1/2 inch is nothing. Check out some high speed tire compression shots, you'd be amazed at how much a tire compresses even at slow speeds. 1/2" you might see rolling out of your driveway.


I thought we were talking about registering a max speed , not comparing average speeds on flat ground.



Its not about the math ,its about error set up at rest being multiplied at speed.

Oh well, sorry for the wasted effort in posting a possible explaination which clearly has no basis in reality.

I'm clearly out of my league here.

See ya'll.

datlas 

^^ Good points.

I agree you are probably casting pearls before swine, but maybe the OP will appreciate it.

ultraman6970

IMO and experience it depends on the guy. Personally if somebody tells you that he went down hill no pedaling at 50 mph (80 km/h) i would dare to say that the guy is teasing you. By experience... you can get 50 to 60 km/k (40 mph) almost w/o pedaling from almost any high gradient hill (have 2 stitches and a lot of flesh tattos to confirm this). At some point the guys have to pedal, is the only way to get over 50 mph...

The other difference is that serous racers dont tell the second part because people just see the speed or the speed in a instant. The guys at some point pedal with 53x11 like at 110 rpm... they do it for a few secs to gain speed then they coast. I assume the OP can do 42 but i doubt he is strong enough to pull over 80 rpms with 53x11 or 12... Not the same go 70 rpms than 110 rpms using the same gear multiplication.

To finalize the position over the bike helps too, the tall guy using a specialized with a front tube 50 inches long and a 14 cms stem looking up and with a position that pretty much is like riding in a BMX bike wont help at all to get higher speeds. The bike has to be well balanced (not wheels balancing), but the bike. Guys that are seated with the whole weight in the back usualy cant descend that fast because the position is too upright and the handling of the bike is really bad, the front of the bike goes like floating.

The bike helps too, some bikes downhill do just bad and other ones do really good.

Good luck op

tfro

Well, a tire is under 1". So 1/2" from nominal(i.e. when you setup your computer) is actually huge. But let's just say the tire is collapsed to the rim and you setup the computer for a fully inflated and unweighted tire. The error goes to a massive 2.5 mph at 60 mph. So your point still doesn't hold up.