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# The Physics of Rollers

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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

# The Physics of Rollers

04-22-09, 11:29 AM
#26
Phantoj
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Originally Posted by KDTX
These sure are some goofy questions. What is the reasoning?
I think they are interesting. We learn by asking "why is it done this way?"

Here's another question: what if you DID cross the belt over on a standard set of rollers, so the front roller spun backwards. Would the rollers be unridable? Or would the rider just have to remember to un-countersteer?
04-22-09, 11:41 AM
#27
Brian Ratliff
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Originally Posted by Phantoj
Lol, no, not unless you crossed the belt. Think about it.
1+ Your contact point with the front wheel is in front of the steering axis. You couldn't keep it upright because it is unstable; instead of the front wheel wanting to return to be in-line, it will want to flip the bars around backwards. If you crossed the belt, the wheel would spin backwards and you'd have to steer opposite of normal, but it might work, except that...

...with three rollers, your bike is constrained in a plane parallel to the ground, but with only two rollers, it's not and is free to ride up over the front roller or back over the rear. It just won't work for several reasons.

On a normal set of rollers, the three rollers constrains the bike in a plane parallel to the ground and the contact point of the front wheel is the same as it is on the road. The front roller is directly under the axle of the front wheel, which makes steering feel normal. If you move the roller forward, the trail of the wheel is effectively shortened, and the steering becomes more twitchy.
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04-22-09, 11:43 AM
#28
Brian Ratliff
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Originally Posted by Phantoj
I think they are interesting. We learn by asking "why is it done this way?"

Here's another question: what if you DID cross the belt over on a standard set of rollers, so the front roller spun backwards. Would the rollers be unridable? Or would the rider just have to remember to un-countersteer?
It would be unstable unless you simultaneously moved the front roller so it's contact point is forward of the steering axis. That would make it stable; then you could steer it and keep it upright, but the steering input would be directly opposite of normal.
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04-22-09, 12:06 PM
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Originally Posted by Phantoj
I think they are interesting. We learn by asking "why is it done this way?"

Here's another question: what if you DID cross the belt over on a standard set of rollers, so the front roller spun backwards. Would the rollers be unridable? Or would the rider just have to remember to un-countersteer?
In addition to the steering axis issue Brian mentions, "remembering to countersteer" is a lot harder than it sounds. I read another article about a guy who put a set of gears in a headset so that turning the bars left turned the wheel to the right. He had a street carnival exhibit and offered prizes to people who could ride it. None of the riders could complete the distance because it was just so counterintuitive, except for one guy who rode the bike no hands.
04-22-09, 02:10 PM
#30
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Originally Posted by Phantoj

quoted for awesomeness. Fajans is a nice guy and his link = /thread
04-22-09, 02:16 PM
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It's been done.

Same with bicycles, just more weight and speed.
04-22-09, 02:29 PM
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Is the belt necessary in this setup?
04-22-09, 02:33 PM
#33
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Originally Posted by cinegabe
I meant one roller per wheel, with the rear drum behind the rear wheel and the front drum ahead of the front wheel. Can you visualize that? Why wouldn't it work?
Why? What is holding the wheels on the rollers? Whats stopping the back wheel from going down to the floor and the front going up over the top of the front one? If you remove the middle roller, thats what you'd get. the 3rd one holds the wheel in place front to back so that doesnt happen. You seem to assume that the bike will not tip up or down at all.
04-22-09, 02:35 PM
#34
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Originally Posted by cinegabe

Is the belt necessary in this setup?
No, I don't think so. Theres some resistance of the front roller, but thats about all. I'd think removing the band would net about a 0 change. (A loose bike on a roller is different tho)
04-22-09, 03:03 PM
#35
Phantoj
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Originally Posted by Grumpy McTrumpy
It's been done.

Same with bicycles, just more weight and speed.
What is the "it" that has been done? (What are you driving at? What conclusions should be drawn from that video and applied to this topic?)
04-22-09, 03:08 PM
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The front and back rollers have to spin at equal rates in order to have balanced angular momenti which allow for you to remain upright. Duh.
04-22-09, 03:15 PM
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Steering is not the key to balance, it's absolutely the gyro effect. When I'm trying to track stand, I can turn the handlebars and shift my weight all I want, I'm still going down. That doesn't happen when the wheels are in motion.
04-22-09, 03:18 PM
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But steering is key to the dynamic stability of which you speak.

In the configuration where the fork is secured, your balance is also secured, so no, the connection is unnecessary. It may mess with the power output, however, because you will only be turning 2 rollers instead of 3.
04-22-09, 03:23 PM
#39
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Originally Posted by n00bL35
Steering is not the key to balance, it's absolutely not the gyro effect. When I'm trying to track stand, I can turn the handlebars and shift my weight all I want, I'm still going down, because I'm not good at it. That doesn't happen when the wheels are in motion, even at 1 mph, because I can steer more effectively.
fixed
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04-22-09, 03:27 PM
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Originally Posted by AdamJaz
It may mess with the power output, however, because you will only be turning 2 rollers instead of 3.
How much so? 1/3 less? Please elaborate. Not just you, anybody...
04-22-09, 03:35 PM
#41
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Small rollers have more resistance because they deform the tires more.
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04-22-09, 03:36 PM
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Originally Posted by AdamJaz
The front and back rollers have to spin at equal rates in order to have balanced angular momenti which allow for you to remain upright. Duh.
Is this an attempt at subtle humor, or are you serious?
04-22-09, 03:37 PM
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Originally Posted by cinegabe
How much so? 1/3 less? Please elaborate. Not just you, anybody...
Well, you're turning 2 rollers instead of 3, no front wheel tire friction or spoke wind resistance .... so it will be less resistance. Doesn't really matter though. Ride it and get a workout, and adjust your gears as you see fit.
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04-22-09, 03:37 PM
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Originally Posted by cinegabe
How much so? 1/3 less? Please elaborate. Not just you, anybody...
I would expect 1/2 to 1/3 less power. I think most of the resistance from rollers is due to the deformation of the tire on the roller (thus, why small-diameter rollers offer greater resistance).
04-22-09, 06:06 PM
#45
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From Kreitler:

As the drum diameter decreases, the amount of resistance increases. This is due to bearing friction and tire friction. For a given wheel speed, smaller drums rotate at higher RPM’s than larger drums, producing more friction in the sealed cartridge bearings. Smaller drums also create more tire friction because the roller has a smaller contact patch and indents the tire more.

The 4.5 drums have relatively low resistance. Most riders can ride in larger gears and keep the wheel speed high with relatively low effort on the 4.5 drums. Higher wheel speed enhances balance and provides a greater coasting effect if you stop pedaling momentarily.

Combined with the Headwind Fan, the 4.5 models provide the widest range of workload of any other roller combination. For easy spinning or intense interval workouts, you can do it all with the 4.5 drums combined with the Headwind Fan. The 4.5 drums are the easiest for learning to ride rollers, too.

The 3.0 drums provide approximately 40% more resistance compared to the 4.5 drums, making harder workouts possible without the addition of the Headwind Fan but not allowing the easy spinning that the 4.5 rollers offer. The 3.0 rollers are a good choice when higher resistance is desired, and at a lower cost than the 4.5’s w/Headwind Fan combination. Some smaller, but strong riders (under 120 pounds) may still find that the 3.0's have too much resistance for warming up or for less intense rides. Higher weight riders have an easier time on the smaller drums because the inherent resistance of the drums is lower, relative to their power output.

The 2.25 drums have approximately 90% more resistance compared to the 4.5 drums. Only the strongest of riders are able to ride the 2.25 drums. The 2.25 rollers were originally designed for the USA Cycling Team when they were looking for smaller rollers for travel that also provided enough resistance for interval workouts or sprint warm-ups. If you do not routinely average over 20-25mph on your solo road rides, look to the 3.0’s or 4.5’s. Also, if you are a lighter weight rider (under 120 pounds), you may find the 2.25's too hard, regardless of your speed outdoors.

For a numerical comparison of the different Kreitler Drums and accessories, please refer to the Wattage Chart (will open in a new window). The Wattage Chart is for comparison purposes only. Your actual wattage produced will vary depending on body weight, tire pressure, wheel aerodynamics, and other factors.
04-22-09, 08:05 PM
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Originally Posted by AngryScientist
I just read an interesting few articles on bike stability, interesting stuff. learn something new everyday i suppose. looks like i'm done for the day.
And I stayed in a Holiday Inn a couple weeks ago. Interestingly, I feel that leaves me qualified to say that you're incorrect, especially as lean would change nearly nothing if you welded the headset strait (I'm also a year into a master's thesis on this topic).
04-22-09, 08:07 PM
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Originally Posted by Tulex
Ever see a bike on a tight rope?
No, but I've seen road bikes on the skinny side of 2x4s, that's pretty impressive. I know a few guys who don't do tight ropes, but do slack lines, maybe I can get one of them riding (not likely).
04-22-09, 09:21 PM
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Originally Posted by jccaclimber
No, but I've seen road bikes on the skinny side of 2x4s, that's pretty impressive. I know a few guys who don't do tight ropes, but do slack lines, maybe I can get one of them riding (not likely).
Point being, they don't turn for balance.
04-22-09, 09:22 PM
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Now if you really want to have fun with your buddy....put a figure eight in your friends belt and get ready to run when they try to ride it......and go flying off to the left when they turn the wheel right.
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04-23-09, 06:56 AM
#50
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Originally Posted by Tulex
Point being, they don't turn for balance.
How do they do it? Is it tricky?