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Old 10-10-08, 02:26 PM   #1
GTALuigi
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Is there any Science behind this effect?

So i commute everyday.

i have a rear rack on my bike, and for my Long portion of the route i usually strap in my backpack to the rear rack, but there are days when i'm in a hurry, and don't have time to fiddle with the elastic to strap my backpack onto the rack.

So the question is why does it feels like "heavier" or feels like i'm "dragging" something behind, when the backpack is on the rack vs. when it is on my shoulders?

The backpack doesn't wait much, so i can't see that being the issue

the only think i can think of, might just be the wind drag, or perhaps it's just a psychological thing?

my average coast speed is 30 km/h ~ 35 km/h.

and when my backpack is on shoulders i feel the bike going faster, and i use less leg power to maintain the same speed.
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Old 10-10-08, 02:49 PM   #2
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It's probably related to your center of gravity moving. When I say "your," I mean the whole you + bike + backpack system.

Without the backpack at all, your center of gravity with the bike is pretty close to where your normal CoG is anyways. Certainly, it's shifted towards the CoG of the bike as it stands alone, but unless you're really light or your bike is really heavy, it won't be that much.

Now, when you wear the backpack, your CoG is also pretty much unchanged. In fact, it should put it closer to where your natural (sans bike) CoG is, once you factor in the bike as well.

When you put the backpack on the rack, you're throwing off your "natural" CoG even further than when it's just you and the bike. That should do two things. First, when you move the bike with relation to you (e.g. when you get out of the saddle), the increased mass will require you to exert more force for a given acceleration of the bike (e.g. when rocking it side-to-side). You can think of the bike as having a greater moment of intertia. Second, bicycles are examples of inverted pendulums (pendula?) and as a rider, you unconsciously constantly correct to keep from falling over because it's an inherently unstable system. When you put more mass on the bottom of that pendulum, the system becomes more stable (or at least less unstable), which means that you have to do more work to upset the pretty delicate balance. Upsetting that balance is what allows you to do important things like turning.

It could also be wind drag.
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Old 10-10-08, 02:50 PM   #3
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Quote:
Originally Posted by GTALuigi View Post
So i commute everyday.

i have a rear rack on my bike, and for my Long portion of the route i usually strap in my backpack to the rear rack, but there are days when i'm in a hurry, and don't have time to fiddle with the elastic to strap my backpack onto the rack.

So the question is why does it feels like "heavier" or feels like i'm "dragging" something behind, when the backpack is on the rack vs. when it is on my shoulders?

The backpack doesn't wait much, so i can't see that being the issue

the only think i can think of, might just be the wind drag, or perhaps it's just a psychological thing?

my average coast speed is 30 km/h ~ 35 km/h.

and when my backpack is on shoulders i feel the bike going faster, and i use less leg power to maintain the same speed.
racks are evil throw it in your nearest river.
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Old 10-10-08, 02:53 PM   #4
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I've noticed the same, I think it's the CoG thing. Like when taking off from a stop, normally I stand up a bit, and with the bag strapped on the rack the bike feels waaaay more different. Instead of the bike rocking back and forth as I pedal, it feels like it's standing still, fighting against my movements.

However, turning feels more solid with the weight on the bike.
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Old 10-10-08, 02:55 PM   #5
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Non-scientific SWAG answer:

When your backpack is on your back it's riding in the slip stream of your body, so no additional drag, and when your pack is on your back it's adding to your weight, increasing the force you are putting on your pedals.

Aaand I may be entirely full of bull, with the whole thing being psychological. I use my homemade panniers. If I need more room I tow a trailer.

My helmet light seems brighter on a foggy/misty night. I know it's not brighter. I know it's not putting as much illumination on target as it would on a clear, dry night, but it seems like it is because I can see the beam on the water droplets all the way to the target.
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Old 10-10-08, 02:58 PM   #6
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However, turning feels more solid with the weight on the bike.
That's part and parcel of the increased stability to which I was referring in point two. Harder to get into a turn, but easier to maintain it once you're there.
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Old 10-10-08, 03:00 PM   #7
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There are those that think in terms of the whole package - rider plus bike -when discussing weight. . And they wouldn't see a difference between whether the weight in on your shoulders or on the bike.

But this never seemed right to me. I think one can really feel the difference in how a lighter bike responds to the inputs we give it.
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Old 10-10-08, 03:02 PM   #8
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I found an anology! Compared to a car, it's the difference between sprung and unsprung weight; your limbs being the suspension.
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Old 10-10-08, 03:05 PM   #9
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I believe it's the problem is the center of gravity has shifted from the center of the bike to the back. Essentially, you're carry more load on the back tire. It feels like you're dragging something because you are. Put the backpack in the front and it'll feel like you're pushing something.

Or it can just be wind resistance (you can test if it's wind resistance by riding with a brick on the rack so see if you get the same effect).
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Old 10-10-08, 03:17 PM   #10
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I found an anology! Compared to a car, it's the difference between sprung and unsprung weight; your limbs being the suspension.
Well that analogy certainly clears things up
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Old 10-10-08, 03:19 PM   #11
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I wondered about it being wind resistance. There's an effect where the wind rushing past you actually curls back just after it passes you. If that space is empty, it doesn't do anything, but if there's something there, it pushes it forward. There's an interesting picture in "Bicycling Science" of a guy wearing a tail shaped like an insect's tail to take advantage of this effect. It could be that you backpack give the wind something to push against, whereas on the rack it just adds more drag.
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Old 10-10-08, 03:41 PM   #12
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Another thing to consider is that when wearing the backpack it covers and protects a lot of skin sensor apparatus (nerves). When not wearing the backpack, these sensors send more information than the central processor normally handles, requiring a needed change in the central processing management system.
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Old 10-10-08, 03:52 PM   #13
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Daily I gotta bring my bike up/downstairs at work. The stairwell is tight and clogged with boxes.
I dumped the racks and frame pump to shoulder the bike easier. Since my pack is 10 lbs.tops.
My bike and I are a lot more nimble. I guess any bike would be like this if the riding weight was
center.
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Old 10-10-08, 03:56 PM   #14
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Quote:
Originally Posted by oakback View Post
I've noticed the same, I think it's the CoG thing. Like when taking off from a stop, normally I stand up a bit, and with the bag strapped on the rack the bike feels waaaay more different. Instead of the bike rocking back and forth as I pedal, it feels like it's standing still, fighting against my movements.

However, turning feels more solid with the weight on the bike.
I believe this is pretty close, but it's actually a "center of mass" thing, not center of gravity. You have increased the weight (mass) of your bike and relocated where that weight is (behind you now as opposed to what was basically right between the pedals), so now when you stand to pedal, instead of the bike rotating at your feet it now tries to rotate somewhere behind your feet, requiring more effort from your legs to counter the twisting force from your arms.

Quote:
Originally Posted by CommuterRun View Post

My helmet light seems brighter on a foggy/misty night. I know it's not brighter. I know it's not putting as much illumination on target as it would on a clear, dry night, but it seems like it is because I can see the beam on the water droplets all the way to the target.
It does seem like it's brighter because the reflected light travels less distance from light to reflection (rain/fog reflection) to your eyes, so the light that you actually see is brighter.

I'm not a physicist, but I did stay at a Holiday Inn Express last night.
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Old 10-10-08, 03:56 PM   #15
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While wind resistance might play a part, the CoG shift (as resipsa nicely explains) is likely the major effect at work here.
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Old 10-10-08, 04:19 PM   #16
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yes i think so the same.

But i'll take on the suggestion to do that simple test, about strapping on a brick, and ride, and see if it's wind resistance, or if it's more of a "center of mass" thing.

obviously both plays a role, however this test can at least answer the wind resistance "unknown" of the equation.

Technically speaking, i'm expecting the brick to draw less air resistance, even though it's a rectangular block shape.

Also as mentioned my backpack is rather light, but yes it weight similarly to One regular sized grey hollow brick used in walls construction.

So if it's not a wind drag issue, then i should still feel the pull of the weight behind me as previously discussed, which will point out it's a "center if mass" issue.

but if it actually a wind drag issue, then i should be able to move slightly better, due the brick design, since it occupies less space, i should feel less effect of the wind drag, compared to my backpack, which takes up more volume in shape.


very interesting science...
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Old 10-10-08, 04:43 PM   #17
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Dunno, I feel like wearing a backpack makes me feel slower and less manuverable, and definetly less comfortable.
Putting it in the rack means less weight shifting around on my shoulders, so less work keeping balanced, more focus on pedaling fast, etc...
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Old 10-10-08, 08:33 PM   #18
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Look up frictional forces, coefficient of friction ect. If the weight is on your back the weight is more or less evenly distributed over the 2 wheels. If it is over the rear wheel then 100% of the weight is over the back wheel. That has consequences as to the force the rear wheel can generate to propel you.
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Old 10-17-08, 12:05 PM   #19
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Good news, after some test, it really was mostly wind drag more than the Center of Mass or Center of Gravity thing.

by compacting everything into the smallest possible size in a tube like position, the Drag behind sensation has almost completely gone.

as soon as i put the bag, back into its normal position, it did the same drag feel as before.

it's almost incredible a normal size backpack can exert so much force in wind drag.
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Old 10-17-08, 09:49 PM   #20
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Dunno, I feel like wearing a backpack makes me feel slower and less manuverable, and definetly less comfortable.
Putting it in the rack means less weight shifting around on my shoulders, so less work keeping balanced, more focus on pedaling fast, etc...

Same here.

If I put a backpack or camel pack on, I feel heavier and slower. So it must be in your head.
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Old 10-18-08, 08:07 AM   #21
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There are those that think in terms of the whole package - rider plus bike -when discussing weight. . And they wouldn't see a difference between whether the weight in on your shoulders or on the bike.

But this never seemed right to me. I think one can really feel the difference in how a lighter bike responds to the inputs we give it.
This is the sprung/unsprung weight thing. Your body and anything your carry on it is sprung in the sense that your limbs and even your spine to some extent act as a suspension, - they flex.

Having a higher ratio of sprung weight vs unsprung is better in terms of performance (all else being equal and it never is) because it provides less rolling resistance and better traction.

Whether we realize it or not, most of us learn this at a young age. For example, if you see some sort of bump or pothole in the road in front of you, most of us know that we will hit that bump or pothole with less force and will travel over the obstacle easier if we get our butt off the saddle. By doing that we're increasing the sprung weight and decreasing the unsprung weight. We're making the bike seem lighter to the road even though total weight of rider + bike hasn't changed.

Even a smooth road is full of imperfections and you're body is acting like a suspension (to a much lesser extent) even when you're on the saddle.

Anything strapped directly to your bike becomes part of unsprung weight and has a greater impact on performance than weight attached to your person.
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Old 10-18-08, 11:35 AM   #22
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Depending on the shape of the backpack it may well be improving the airflow around your upper body. It does this not by being in the airstream but by filling in the void where air forms a turbulent vortex. Typically this is known as the "wake" of the object, your body in this case.

At first I poo-poo'ed your thread as just a psychological thing but then I saw the speeds you're riding at. It may well still be at least partly psychological as well as largely a question of where the mass is located. But it's quite possible that it's also an aerodynamics issue at play as well.

In the drawing attached you can see how the air creates a turbulent wake behind your chest. It is this wake that generates the drag. Streamlining yourself by adding a big and roughly half circular backpack fills in a lot of that wake area and allows more of the air to flow around you without breaking down into turbulent back eddies. You can see in the second sketch how much smaller the wake is. Smaller wakes suck away a lot less energy. Now this is obviously just a best guess at the airflow but it'll be fairly close based on what I've seen for similar objects in wind tunnel videos.

The way to actually prove it would be to fill your backpack with something super light like bunched up newspaper or plastic bags full of air then go find a fairly steep hill that will let you coat down at around your 30kph speed. Using a speedometer coast over the top at around 25 kph and coast down in your usual riding position. Note the final speed at some landmark near the bottom. Then do it with your light but fluffed out backpack. Likely you'll want to do two or three runs each way but by the time you've repeated your runs a couple of times there should be some solid difference to pack vs no pack. If you only see a kph or so and there's more error than that in the trials then it's psychological.
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Old 10-18-08, 08:05 PM   #23
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BC rider is correct. A correctly shaped backpack will reduce drag. Google Oscar Egg to read out a French rider that had a backpack like fairing built and he set numerous world records on the track that were later disqualified.
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Old 10-19-08, 08:06 PM   #24
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weight is farther back on the rack = bigger rear tire contact patch

weight is farther back on the rack = more weight on back tire = bigger contact patch = more drag on that tire? Sure the front will be lighter with a smaller patch, but I don't think the relationship is linear.

Put a little more air in the back tire when you rack mount your pack and see if it makes a difference.
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Old 10-19-08, 08:14 PM   #25
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I think there is a significant psychological effect of having a heavier bike. With a lighter bike, you push the pedals and you can feel the bike exerting more force on your behind. The overall effect is probably not all that significant as far as power output goes. I know that when I had a really light bike, the only place I could really feel the difference was standing up accelerating in a sprint.
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