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#21
January 8th 05, 02:31 PM
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If you have the clearance (another argument for finding a road frame
from the 70's for conversion), try 32 or larger tires. You would not believe the difference it makes on rough roads compared to 23's! A Muzi wrote: [SNIP} 2. I ride in NYC where there are thousands of road obstacles (potholes, manhole covers, etc.) and I'm finding the ride to be extremely harsh. Any suggestions (e.g. a good fixed gear saddle)? That sounds too tight. You'll feel lash no matter what when you reverse direction. http://www.yellowjersey.org/chainchk.html You'll learn to lift your butt off the saddle. No saddle will cushion a pothole with your full weight on it. 
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#22
January 9th 05, 02:24 PM
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Tom- In the past, Carl Fogel has posted links to pictures of motorcycles with
chain tensioners - a motorcycle is effectively fixed-geared when the clutch is engaged. BRBR OK, install a tensioner on a bicycle fixed gear and ride it and let me know what happens. Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302 (303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"
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#23
January 9th 05, 02:38 PM
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Peter Chisholm wrote:
[Dreadful AOL quoting format corrected] Tom Sherman wrote: In the past, Carl Fogel has posted links to pictures of motorcycles with chain tensioners - a motorcycle is effectively fixed-geared when the clutch is engaged. OK, install a tensioner on a bicycle fixed gear and ride it and let me know what happens. I was merely attempting to preemptively post what Mr. Fogel's response would be. The evidence [1] indicates that I did quite well. [1] http://groups-beta.google.com/group/rec.bicycles.tech/msg/85f8f73c79d7a014?dmode=source. -- Tom Sherman - Near Rock Island
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#25
January 9th 05, 07:45 PM
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#26
January 9th 05, 08:12 PM
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Several knowledgeable posters have reiterated that you can't/shouldn't
use a chain tensioner on a fixed gear bicycle. Carl Fogel, comparing apples and locomotives, demurs based on irrelevant motorcycle technology: During normal riding, a motorcycle's chain functions like a fixed-gear bicycle's chain. (The clutch can be used to freewheel, but massive engine braking through the chain to the point of skidding the rear tire is routine in trail riding.) A motorcycle chain is much heavier and wider than the bicycle chain. A motorcycle chain moves much faster than the biycycle chain (My trials machine has a fixed 14 x 52 front/rear ratio, as opposed to say a 42 x 16 of a fixed gear bicycle.) So, assuming equal tire diameters, for equal accelleration forces, the bicycle chain tension will be 3.25 (52/14) the tension for the motorcycle's. To get the same amount of chain tension, the motorcycle would need to be able to accellerate 3.25 times as fast as the bicycle. In fairness, this does omit the fact that the motorcycle is heavier, so the disparity is less. A motorcycle chain accelerates far more rapidly than the bicycle chain and brakes far more heavily. Speed doesn't matter, only tension. Single-pivot trailing-arm under-swing-arm chain tensioners have been installed on all trials motorcycles since 1976. front rear chain-stay/swing-arm bb---pivot-------------------------------axle \\ \\ \\ ------\\---------------------- lower chain run \\_pad_ trailing arm and pad or idler gear coil spring at pivot / forces pad up and back / I think that you're quite right about the strange tensioners inexplicably sold for bicycles. Let me explicate this for you. Motorcycles are _designed_ to have chain tensioners, so their chainstays have suitable attachment fittments. Bicycles are _not_ designed to have chain tensioners, other than derailers. There have been bicycle chain tensioners designed to clamp onto the chainstay, but these have not worked out well in practice. Clamping to a tapered, elliptical tube securely without crushing it is not so easy if any significant amount of load is to be withstood. The bicycle accessories ignore common sense and use a leading arm that would indeed be torn off by the slightest misalignment. This is an imaginary problem. I've seen lots of singlespeed MTBs with chain tensioners, but I've never seen nor heard of one getting torn off in use. But the chain constantly re-algins a common-sense trailing arm design. Remember, on a fixie and on a motorcycle, the chain is still going backward whether you're accelerating or braking. It does not reverse direction unless you stop and then roll backward, and at any normal backward speed the arm still works fine. Existing chain tensioners have provision for alignment of the pulley. Once the pulley is aligned with the actual chainline, it stays there. Trailing-arm chain tensioners have been as reliable as bricks on motorcycles for over thirty years. Whether such tension is of any value on a bicycle is open to question, but they simply wouldn't be ripped off by the chain--they face the right way. As mentioned above, having the chain tensioner ripped off is a non-issue. The danger is having the chain derail from the chainring. It is my impression (though I make no claim to motorcycle expertise) that the front sprocket of a motorcycle chain is in an area of tight clearance, where there's no room for it to derail. Aside from the risk of derailment (and possible rear-wheel lockup) the other reason not to use a chain tensioner on a fixed gear bike is the excessive lash it would introduce to the drivetrain. Sheldon "Bicycles Are Not Motorcycles" Brown +-------------------------------------+ | Don't drink and drive: you might | | hit a bump, and spill your drink! | +-------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide http://captainbike.com http://sheldonbrown.com
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#27
January 9th 05, 09:52 PM
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Sheldon Brown wrote:
... +-------------------------------------+ | Don't drink and drive: you might | | hit a bump, and spill your drink! | +-------------------------------------+ Cyclists solved that problem a long time ago.  -- Tom Sherman - Near Rock Island
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#28
January 10th 05, 01:27 AM
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On Sun, 09 Jan 2005 15:12:11 -0500, Sheldon Brown
wrote: Several knowledgeable posters have reiterated that you can't/shouldn't use a chain tensioner on a fixed gear bicycle. Carl Fogel, comparing apples and locomotives, demurs based on irrelevant motorcycle technology: During normal riding, a motorcycle's chain functions like a fixed-gear bicycle's chain. (The clutch can be used to freewheel, but massive engine braking through the chain to the point of skidding the rear tire is routine in trail riding.) A motorcycle chain is much heavier and wider than the bicycle chain. A motorcycle chain moves much faster than the biycycle chain (My trials machine has a fixed 14 x 52 front/rear ratio, as opposed to say a 42 x 16 of a fixed gear bicycle.) So, assuming equal tire diameters, for equal accelleration forces, the bicycle chain tension will be 3.25 (52/14) the tension for the motorcycle's. To get the same amount of chain tension, the motorcycle would need to be able to accellerate 3.25 times as fast as the bicycle. In fairness, this does omit the fact that the motorcycle is heavier, so the disparity is less. A motorcycle chain accelerates far more rapidly than the bicycle chain and brakes far more heavily. Speed doesn't matter, only tension. Single-pivot trailing-arm under-swing-arm chain tensioners have been installed on all trials motorcycles since 1976. front rear chain-stay/swing-arm bb---pivot-------------------------------axle \\ \\ \\ ------\\---------------------- lower chain run \\_pad_ trailing arm and pad or idler gear coil spring at pivot / forces pad up and back / I think that you're quite right about the strange tensioners inexplicably sold for bicycles. Let me explicate this for you. Motorcycles are _designed_ to have chain tensioners, so their chainstays have suitable attachment fittments. Bicycles are _not_ designed to have chain tensioners, other than derailers. There have been bicycle chain tensioners designed to clamp onto the chainstay, but these have not worked out well in practice. Clamping to a tapered, elliptical tube securely without crushing it is not so easy if any significant amount of load is to be withstood. The bicycle accessories ignore common sense and use a leading arm that would indeed be torn off by the slightest misalignment. This is an imaginary problem. I've seen lots of singlespeed MTBs with chain tensioners, but I've never seen nor heard of one getting torn off in use. But the chain constantly re-algins a common-sense trailing arm design. Remember, on a fixie and on a motorcycle, the chain is still going backward whether you're accelerating or braking. It does not reverse direction unless you stop and then roll backward, and at any normal backward speed the arm still works fine. Existing chain tensioners have provision for alignment of the pulley. Once the pulley is aligned with the actual chainline, it stays there. Trailing-arm chain tensioners have been as reliable as bricks on motorcycles for over thirty years. Whether such tension is of any value on a bicycle is open to question, but they simply wouldn't be ripped off by the chain--they face the right way. As mentioned above, having the chain tensioner ripped off is a non-issue. The danger is having the chain derail from the chainring. It is my impression (though I make no claim to motorcycle expertise) that the front sprocket of a motorcycle chain is in an area of tight clearance, where there's no room for it to derail. Aside from the risk of derailment (and possible rear-wheel lockup) the other reason not to use a chain tensioner on a fixed gear bike is the excessive lash it would introduce to the drivetrain. Sheldon "Bicycles Are Not Motorcycles" Brown +-------------------------------------+ | Don't drink and drive: you might | | hit a bump, and spill your drink! | +-------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide http://captainbike.com http://sheldonbrown.com Dear Sheldon, I'm simply pointing out that trials machines have had trailing-arm chain tensioners for over 30 years. Motorcycles function pretty much as fixed gear bicycles, only with considerably greater forces and speeds. They don't rip off trailing arm chain tensioners as Peter insists will happen. I have no idea what forces Peter thinks will rip trailing arm chain tensioners off--presumably speed, weight, tension, or the good fairies of legend, since the tension itself merely moves the arm a little further outward against the coil spring. And yes, motorcycles rather easily accelerate more than 3.25 times as fast as bicycles. I keep getting the feeling that a number of bicycle dealers have never even looked at a motorycle, much less ridden one. Consider that practically any 250cc 200+ pound motorcycle can spin its 4-inch wide rear tire with a 200-pound rider from a dead stop. Peter has not mentioned any fear of the chain flying off the chain ring (he talks about the bicycle ripping the tensioner off, a perfectly natural consequence of the strange bicycle designs). If you want to switch to that problem, what forces are going to cause the chain to fly off when a tensioned pad is pressing up against the bottom run of a normally tensioned chain? The motorcycle chain is moving faster than the bicycle chain and whipping around the much smaller, tighter curve of a much smaller, tighter front sprocket. You guys do understand that a trailing arm chain tensioner is simply adding spring pressure to a normally tensioned chain, don't you? That it's literally as sophisticated as a mouse-trap? That it requires a mount no sturdier than a flimsy luggage rack or a derailleur? I'm not arguing that it's a good idea, that it will improve a fixie, or that it offers perfect tension. I'm certainly not arguing that the strange bicycle designs work--I expect that they tear off, just as Peter says. (There were idiot motorcycle designs like that in the early 1970's, and they promptly destroyed themselves.) I'm just pointing out that there's no question that trailing arm chain tensioners have been used on trials machines by Honda, Yamaha, Kawasaki, Suzuki, Montesa, Ossa, Bultaco, Italjet, GasGas, Sherco, and every other trials manufacturer for the last thirty years without the slightest problem. They do not tear off the machines for the obvious reasons. Draw a simple picture, for heaven's sake, see which way the chain goes on a fixie and a motorcycle, and figure out whether the moving chain will tear a trailing tensioner off or brutally force it right back into aligment. What would you all say if a motorcylist insisted that there's no way that bicycle tires could be glued to rims and not tear off? Carl Fogel
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#29
January 10th 05, 03:42 AM
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Quoth Carl Fogel:
They don't rip off trailing arm chain tensioners as Peter insists will happen. I have no idea what forces Peter thinks will rip trailing arm chain tensioners off--presumably speed, weight, tension, or the good fairies of legend, since the tension itself merely moves the arm a little further outward against the coil spring. I don't know what led Peter to make that assertion. I would guess that he was referring to a rigid adjustable tensioner, not to a spring loaded one, but I could be wrong. Peter has not mentioned any fear of the chain flying off the chain ring That is the danger of relying on a chain tensioner for a fixed gear. (he talks about the bicycle ripping the tensioner off, a perfectly natural consequence of the strange bicycle designs). If you want to switch to that problem, what forces are going to cause the chain to fly off when a tensioned pad is pressing up against the bottom run of a normally tensioned chain? When reverse tension is applied, the lower run will straighten out (a spring loaded tensioner will permit this) and then the upper chain run will become slack. The motorcycle chain is moving faster than the bicycle chain and whipping around the much smaller, tighter curve of a much smaller, tighter front sprocket. A motorcycle chain is much wider and thus laterally stiffer than a bicycle chain. In my previous posting I alsop supposed: It is my impression (though I make no claim to motorcycle expertise) that the front sprocket of a motorcycle chain is in an area of tight clearance, where there's no room for it to derail. Am I wrong about that? You guys do understand that a trailing arm chain tensioner is simply adding spring pressure to a normally tensioned chain, don't you? Only to the bottom run. When back-loaded, the top run will become slack. That it's literally as sophisticated as a mouse-trap? That it requires a mount no sturdier than a flimsy luggage rack or a derailleur? That's true for a spring loaded one, but not for a rigid one that is designed to maintain tension in both directions. I'm not arguing that it's a good idea, that it will improve a fixie, or that it offers perfect tension. I'm certainly not arguing that the strange bicycle designs work--I expect that they tear off, just as Peter says. No they don't. As I said, that's a non-issue, at least for the spring loaded variety. I'm just pointing out that there's no question that trailing arm chain tensioners have been used on trials machines by Honda, Yamaha, Kawasaki, Suzuki, Montesa, Ossa, Bultaco, Italjet, GasGas, Sherco, and every other trials manufacturer for the last thirty years without the slightest problem. So what? This newsgroup is about bicycles. Chain tensioners don't work for fixed gear bikes, and whether they're leading or trailing arm design doesn't matter diddly. What would you all say if a motorcylist insisted that there's no way that bicycle tires could be glued to rims and not tear off? It would depend on how big and ornery-looking he was. Sheldon "Push-bike" Brown +------------------------------------------------------+ | It were not best that we should all think alike; | | it is difference of opinion that makes horse-races. | | -- Mark Twain | +------------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide http://captainbike.com http://sheldonbrown.com
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#30
January 10th 05, 05:09 AM
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On Sun, 09 Jan 2005 22:42:03 -0500, Sheldon Brown
wrote: [snip] It is my impression (though I make no claim to motorcycle expertise) that the front sprocket of a motorcycle chain is in an area of tight clearance, where there's no room for it to derail. Am I wrong about that? [snip] Dear Sheldon, Well, yes, you were pretty much wrong about that. Sorry. There's generally room to derail the chain off the small front (countershaft) sprocket on a motorcycle. Loosen the rear axle, move the wheel forward, and pull the loosened chain off. Generally, however, this would be pointless. The rear sprocket comes out of the chain about as easily as a bicycle rear wheel comes out of the rear cluster. If we want to work on the chain itself or replace it, we usually undo the masterlink if we want to work on a chain. In the case of my machine, I used to swap the standard 14-tooth trials cog with a 17-tooth highway cog, a somewhat unusual practice. Obviously, there's room up front if a 17 can replace a 14 without any concern. Motorcycles emphatically do not rely on putting the front sprocket near the cases to stop the chain from coming off. If the chain ever touched the engine case, even at walking speed, it would leave deep gouges. (Honest, motorcycle engineering is not that crude. Sochiro Honda never told 'em to try moving the countershaft sprocket closer to the nearest engine wall to keep the chain in place.) However, the heavy chains are moving so fast around such a sharp corner that the end of a broken chain can fly off and smash through the engine case in front of the cog (or just wad up somehow and do the same thing), so sometimes there's a thicker case wall if the case is steel and sometimes there's a curved protector in front of more delicate cases. But these are firewalls, so to speak, not guides. Think of bicycles--they don't rely on metal restraints in front of the chain ring or behind the rear cog to keep the chain in place. Again, I agree that spring chain tensioners may well add nothing to fixies and that they seem to get alone fine without such gadgets. But any claim or even implication that a trailing-arm chain-tensioner would be torn off simply contradicts widespread experience with numerous brands of two-wheeled fixed-gear chain-driven two-sprocket devices with far more power, speed, and acceleration than bicycle dealers sell. (The comparison doesn't seem like apples and locomotives to me--sounds more like bananas and plantains.) Carl Fogel
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