Can't break 40mph downhill



Lonnie Utah

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Aug 21, 2004
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wiredued said:
I think my aerobars might be preventing me from laying on the handlebars the way the premier racers do on long descents can aerobars slow you down when you are coasting downhill? Getting my back flat seems difficult with them on. However I talked to someone who I think is a more experienced rider and he can't break 40mph either and he doesn't have aerobars.
You guys need a bigger hill. Come to Utah and you can go 50+. I can show you where.

L
 

cydewaze

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Jun 17, 2004
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Riders in a group should decend more quickly than a single rider alone. Frontal area is only part of the drag. There is also significant drag created by the low pressure in the void behind the rider (forgive me for not knowing the technical terms for this stuff).

In a group, the rider behind the leader lessens this void because the air isn't having to close back in behind the leader, if that makes any sense. Just like in NASCAR, a row of cars is faster than a single car.

I was in the Tour de Toona for a few years, driving the team vehicle for our team. One of the stages included a climb/decent that I had ridden the previous week, and I was able to hit around 54 on the decent. But when the men's pro/1/2 went down that hill, the lead motorcycle had to do 70 to stay ahead of the pack. Also, one of our cat 3 women who had managed 51-52 solo was riding the brakes at 56 in the peloton on that same decent.

I'm pretty sure a group decends faster than an individual.
 

frenchyge

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cydewaze said:
Riders in a group should decend more quickly than a single rider alone. Frontal area is only part of the drag. There is also significant drag created by the low pressure in the void behind the rider (forgive me for not knowing the technical terms for this stuff).
It's significant on a square-backed stock car with a spoiler sticking up, but I can't believe there's much effect from that on a biker's butt. I also have a hard time believing that another person riding a couple bike lengths behind you does much to disrupt that effect, unlike having the nose of another stock car literally 6 inches behind the rear bumper of the car in front. If you're going to ride that close to the rider in front, then you may as well put your hand on his butt and push him forward (effectively doubling the combined weight of the train, with no increase in frontal area) as you accelerate behind him, but I don't think that's really safe at those speeds. That'd be like bump-drafting for bikers. :eek:

I agree with your thoughts, in concept, but I just have a hard time believing that helps a group descend faster. The riders behind are going to have to ride the brakes just to stay behind the leader, since he's taking all the wind.
 

wilmar13

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Nov 30, 2003
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frenchyge said:
You'll probably need a decent tuck to break 50mph. 68mph sounds like an urban legend to me.

How would you know you live in Kansas :D ...

Yeah I agree 68 is pretty fast, possible, but not likely in a race without a very dangerous and steep decent. My fastest speed was reached down a 2 mile 15% grade after slingshoting around the group with a tailwind, in massive tuck after sprinting an 11: 63mph. I guess at higher altitudes thinner air would make it easier though (like above 8000ft).

To original poster if you can't break 40mph downhill, you aren't on much of a hill...down a 8% grade that is more than 1/2 mile long you can sit upright and break 40.
 

jbieryjr

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Apr 15, 2004
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What kind of bike and rims are you riding. The bearings in the rims have alot to do with top end speed. Example a bad bike with $1000 rims goes faster than a good bike with $200 rims.

I recently upgraded my rims to Mavic Ksyrium SSC. I see a increase speed and topend by 1-2mph and 7mph.
 

wilmar13

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jbieryjr said:
The bearings in the rims have alot to do with top end speed.
Yeah they do, almost as big of a difference as climbing with well-trimmed finger nails vs. without.
 

rayner

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Nov 1, 2003
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andrello said:
Wrong. Weight has a lot to do with overcoming drag. Objects only accelerate at the same rate - g. So in a vacuum they hit the ground at the same time but in air the object that can apply more force against the force of drag will hit the ground first. F=mg, i.e., a bigger m gives a bigger F which means it cancels more of the force due to drag.

Weight has zipp to do with overcoming drag it mearly increases the toatl positive force and so the drag is less in proportion to the total force in relation to a lighter person. I think this is what you are saying. Your formula is nearly correct. In a downhill situation F=m[gsin(pheta)] with pheta being the angle concerned as f=mg means that you are in a freefall situation.
 

andrello

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rayner said:
Weight has zipp to do with overcoming drag it mearly increases the toatl positive force and so the drag is less in proportion to the total force in relation to a lighter person. I think this is what you are saying.
What? If weight is a force opposite to the drag then weight does matter - which is basically what you just wrote above in mangled language.
rayner said:
Your formula is nearly correct. In a downhill situation F=m[gsin(pheta)] with pheta being the angle concerned as f=mg means that you are in a freefall situation.
Yes. Freefall is the example that was given and I was responding to. It's also simplified to illustrate a simple point. And it's not my formula - it's Newton's 2nd law.
 

rayner

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andrello said:
What? If weight is a force opposite to the drag then weight does matter - which is basically what you just wrote above in mangled language.
Yes. Freefall is the example that was given and I was responding to. It's also simplified to illustrate a simple point. And it's not my formula - it's Newton's 2nd law.

What I was trying to say was weight cannot overcome drag. Weight increases drag. Where weight helps is in the the matter of increasing the ratio between forward motion and retarding force

Weight does matter but in your statement you said it can overcome drag. This is quite simply incorrect.
 

Lonnie Utah

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Aug 21, 2004
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rayner said:
Weight increases drag.
Last time I checked, surface area and surface "roughness" increases drag.

Now if you are trying to say that a heavier rider generally has a larger surface area and that increases drag, I'll buy that, but weight itself has nothing to do with drag.

L
 

rayner

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In a downhill situation weight=speed=drag. In a non downhill situation weight doesnt have anything to do with drag. The faster you go the more drag you have. I just didnt like the statement that weight can overcome drag as really weight is what is producing drag in the first place.

Essentially I agree with Andrello but some minor points were incorrect. Nobody riding on a road is in freefall and weight cannot overcome drag as essentially weight is creating drag in this case due to the fact that weight is proportional to speed which is proportional to drag. Not trying to start an arguement.
 

DannoXYZ

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Jul 21, 2003
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Air-resistance is a reality that we have to examine, our top-speed on a bike is dictated by it on the flats and on the downhills. It's a reality in motorcycles and autos as well. While power-to-weight ratio dictates acceleration, power-to-Cx*Cd determines top-speed.

It's not so much weight, but pounds/sq.in. of force pushing on the frontal area that overcomes the drag. The more pounds of force you have for each sq.in. you block, the faster you go, or for the same amount of force, the less sq.in. you have blocking the air, the faster you go.

For example, when I jump out of a perfectly good airplane and arch out flat with my arms & legs extended, I represent a certain cross-sectional surface-area. I have only my weight to push against the air that's flowing in that cross-sectional area, and it balances out at 125mph. Imagine that I'm carving a virtual-tunnel through the air by pushing it aside from the path that my body takes.

But if I straighten up and go head-first straight down, that same weight (force) against a much smaller cross-sectional area will allow me to push aside less air faster. I create a smaller tunnel through the air, have to push less of it aside and I end up falling at 200mph. Skin-suits and cone-head helmets can increase that to 220mph.

Same thing on a bike, you guys can test it out by cresting a hill at 10mph and sitting up and holding your arms out wide and you'll see a certain terminal velocity. Tuck down low the next time starting at that same 10mph and you'll get much more speed. The fastest I can hit on San Marcos Pass around here is 50mph on 10% grade in a tuck with my hands on either side of the stem with my face and teeth dangerously close. I can actually hit 52mph with a one-arm tuck, placing one arm behind my back with my hand directly behind my butt, and it's a little scary for control. Tucking in behind a car or truck on the downhill, I can get up as fast as them easily, 60-65mph. But most drivers freak out when I do that and they slow down, bastards!!! I have no idea how someone can reach 68mph. That requires 2.5x the power as 50mph and maybe if you're a 425lb guy, you might have enough mass behind you to push aside the air that fast... but I'd be damn hell scared of the brake-pads melting!!! :(

For the guy who can't hit 40mph on the downhilll, sprint as fast as you can as you crest the hill and start down. The faster you can get going at the top, the faster you'll be at the bottom.... and let go of the brakes! ;)
 

frenchyge

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wilmar13 said:
How would you know you live in Kansas :D ....
Because top speed is dictated by the speed of the riding lawn mower you're being towed behind, and it's really hard to get one of those suckers up over 60mph unless you've custom fitted it with a supercharger. :D
 

wilmar13

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jbieryjr said:
Have you not heard of roll resistance?
The % of total power it takes to overcome rolling resistance at speeds above 20 mph is negligible, and rapidly decreases as speed increases on an exponential scale. The % of that rolling resistance that comes from your bearings is almost nothing unless you have some serious problem with your hub, so you are saying that a very small % of a negligible small % gives you a 7mph boost in speed :confused: . Aero wheels will give a small bump and the effect will be increased with higher speed, but it has absolutely nothing to do with the bearings at all it is solely because they are more aerodynamic. So you were attributing the speed increase to the wrong thing firstly, and secondly by saying you gained 7mph by changing your wheels only you are implying that the parasitic drag of your wheels is not only greater than the drag of your bike/body, but can be improved on a dramatic scale...it will be improved but 7mph is totally unrealistic and you are making statements based on sketchy anecdotal evidence. Bicycling magazine did a "somewhat" accurate study a few years ago and found that deep rimmed reduced spoke count wheels like the Carbone will give you a solid .2mph at 25mph (disks gave .4mph). FWIW Carbones will be my next wheel purchase for several reasons...

FYI in case english is not your first language: your rims don't have any bearings unless you have some strange centerless wheel where the hub is your rim, I assume you meant your "wheels" or more accurately the hubs of your wheels.
 

wilmar13

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Just a quick comment on the developing psuedo physics debate... funny how many times the same subject will get rehashed over and over by different people and how it almost always goes the same way. DannoXYZ do not forget that weight is related (somewhat proportionally) to surface area because the density of human beings does not vary that much. ;)
 

DannoXYZ

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Jul 21, 2003
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"DannoXYZ do not forget that weight is related (somewhat proportionally) to surface area because the density of human beings does not vary that much. "

Ah yes, but it's really the frontal cross-sectional surface-area which faces the wind that we're concerned with. And this frontal surface-area does increases with bigger human beings. This increases with the square^2 function of size, yet the mass will increase to the cube^3 power of size. This means that a human that's twice as big (blocking twice as much wind) will weigh four times more... :) So big people will go down hills faster because they have more pounds pushing each sq.in. of surface-area that's facing the wind.

"Have you not heard of roll resistance?"

Heh, heh... work out the math (it's in any high-school physics book, you don't even need to know calculus ;)). Going twice as fast will maybe increase rolling resistance by 2x at most, in many cases, the higher velocities across the bearing surfaces will result in less friction than before, however the frictional drag on the tires will go up linearly with speed. So overall total roling-resistance increase will be less than 2x with 2x the speed. However, the air-resistance (drag) on that object will increase by a factor of 4X and the power required to overcome that extra drag will be increase by a factor of 8X...

Which brings up an interesting concept... what if you can remove the air-resistance from in front of the moving object??? On the chunnel they're planning across the Atlantic, the air-space in front of the train's going to be sucked out into a vacuum and pumped in behind the train... they say it's gonna be good for 600mph with very little power needed other than to accelerate the thing up to speed... :)

"Because top speed is dictated by the speed of the riding lawn mower you're being towed behind, and it's really hard to get one of those suckers up over 60mph unless you've custom fitted it with a supercharger. "

Good idea!!! I plan on doing that exact mod! To go along with my turbocharged barstool and my turbo+nitrous scooter:
 

rayner

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Nov 1, 2003
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rayner said:
In a downhill situation weight=speed=drag.

Was putting this in laymans terms for you. Any idiot could tell you that this is not a correct stetement. Weight is proportional to speed which is proportioanl to drag. Just havent got an alpha on my keyboard.

To get back to topic id try getting lower, getting faster before you start descending, don't wear falppy jerseys, knicks etc and keep your elbows in.
 

DannoXYZ

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Jul 21, 2003
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Yeah, it's just the terminology used and how it was presented that made it confusing for people because you got cause & effect mixed up. Maybe it can look like this, with X amount of power availble, you hit a Y speed which is a limit imposed by Z-drag:

power == speed (~drag)

Speed and drag as close analogues, at 50mph, a human body in semi-prone position will have a certain amount of drag. It will require a certain amount of power to push that body through the air at 50mph. HOW you generate this power is independent of the speed and drag. It can come from carb-burning pedal-power, it can come from igniting methane in farts, or bum-drafting from behind, or the downward pull of gravity on a hill, doesn't matter. It's not that weight causes drag, more weight just generates more power for higher speeds and it's the higher speeds that causes more drag and requires more power to push at that speed.

And yes, converting potential energy of falling objects into kinetic energy (falling/coasting) IS a valid way of generating power, think water-mills, dams and electric generators. The more weight you have falling, the more kinetic energy & power it generates.

A 400lb guy rolling down a 10% grade is gonna be hitting the air with A LOT more power than a 110lb guy (who blocks close to the same amount of air if they're both tucked). So at 45mph that 400lb guy's gonna have roughly the same amount of drag as the 110lb guy who may already be at an equilibrium speed with his power vs. drag. But 400lb guy's got a lot more power in reserve and he may hit 55mph before his power & drag equalizes.

What rayner was trying to point out was the relationship between power generated in falling vs. the increase in drag with increased speed. The power is represented by momentum mv or acceleration mg or kinetic energy 1/2 mv^2. However, that power is always smacked down by wind-resistance which goes up as a cube-function, v^3.

Another way to get some more speed on the downhills is to just get more weight without increasing drag. Put a rack on the back of your bike and strap a couple hundred-pound lead-weights to it. Place the weights high so that it's flowing in wind that's already parted by your body & butt. You'd be amazed at how much faster you'll go on the downhill.... just be careful about stopping at the bottom....