Bike won't coast
Friends' tandem (circa mid 90s) won't coast normally, slowing down like a brake was rubbing but none are. They have to even pedal it downhill. There is no sound given off--no whirring, squeaking, etc., the bike is just fighting friction of some sort. What is it--bad bearings (sealed bearings hubs), freewheel?
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So what happens while it is on the bike stand? You aren't giving us enough information to really pinpoint the issue. When the bike is on a bike stand, what happens when you spin the wheels by hand? And so on...
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It could be a poor selection of tires.
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Originally Posted by bobotech
(Post 14779207)
So what happens while it is on the bike stand? You aren't giving us enough information to really pinpoint the issue. When the bike is on a bike stand, what happens when you spin the wheels by hand? And so on...
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Tandems will slow down slightly faster than singles due to the greater drag, but that doesn't sound like the case. Furthermore, it takes a good deal of friction to slow a tandem down. Surely, this should be evident on the stand?
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UH This may seen like a dumb question but what kind of tandem? A multispeed external gear road or mountain? An internal gear setup? a Recumbent?
It isn't a Cruiser with a sticky coaster brake is it? As Bobtech ask what happens in the stand or if the you lift the bike and spin a wheel? Are the bearings too tight? Do you a drum brake? Is the rear wheel cocoked? |
Originally Posted by rdtompki
(Post 14779845)
Tandems will slow down slightly faster than singles due to the greater drag, but that doesn't sound like the case. Furthermore, it takes a good deal of friction to slow a tandem down. Surely, this should be evident on the stand?
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Originally Posted by HillRider
(Post 14779977)
I disagree. Tandems are actually faster downhill and coast better than a single since they have more weight but the same aerodynamic drag (same frontal area) as a single.
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Originally Posted by rdtompki
(Post 14779845)
Tandems will slow down slightly faster than singles due to the greater drag, but that doesn't sound like the case. Furthermore, it takes a good deal of friction to slow a tandem down. Surely, this should be evident on the stand?
Under any normal circumstances, tandems have a higher mass to friction ratio (from all sources) and therefore will coast faster, and slow down slower. That's the advantage of tandems, and why tandems are usually faster than solos except in hard hill climbing which puts extra demand on riders unless they're well coordinated. The problem with tandems isn't about getting them going, it's getting them stopped. Meanwhile the OP should do some basic diagnostics, lift wheels and spin unloaded, check tire pressure, double check internal brakes (if any). If it has a coaster brake, make sure it runs free when coasting, etc. |
Originally Posted by bikeme
(Post 14779198)
Friends' tandem (circa mid 90s) won't coast normally, slowing down like a brake was rubbing but none are. They have to even pedal it downhill. There is no sound given off--no whirring, squeaking, etc., the bike is just fighting friction of some sort. What is it--bad bearings (sealed bearings hubs), freewheel?
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Originally Posted by HillRider
(Post 14779977)
I disagree. Tandems are actually faster downhill and coast better than a single since they have more weight but the same aerodynamic drag (same frontal area) as a single.
Reference: I rode a tandem with one of my ex GFs. |
In all seriousness, is that why I feel like I am a rocket when going down a hill? Since I weigh more than most tandem teams combined? Nevermind that I am barely above walking speed when going back up that hill. LOL
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Originally Posted by bobotech
(Post 14780382)
In all seriousness, is that why I feel like I am a rocket when going down a hill? Since I weigh more than most tandem teams combined? Nevermind that I am barely above walking speed when going back up that hill. LOL
Like falling bodies, bicycles coasting downhill accelerate until they reach a terminal velocity where the air drag equals the effects of gravity. Once this equilibrium is achieved, there is no net force to cause further acceleration, and velocity remains constant until something changes. Since heavier riders have a higher ratio of mass to drag, their terminal velocity is higher. |
Drag brake?
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Originally Posted by FBinNY
(Post 14780404)
Yes, falling speed (in air) is determined by the Reynolds Number, or ratio of weight to the air resistance factor. This is why a squirrel can jump or fall from great heights without getting hurt, cat's rarely get hurt, and bigger creatures go splat when they land. Simply put, small fluffy objects fall slower than large dense ones.
Like falling bodies, bicycles coasting downhill accelerate until they reach a terminal velocity where the air drag equals the effects of gravity. Once this equilibrium is achieved, there is no net force to cause further acceleration, and velocity remains constant until something changes. Since heavier riders have a higher ratio of mass to drag, their terminal velocity is higher. EDIT: this without even mentioning that smaller riders have a better power-to-weight ratio than bigger ones. |
Originally Posted by rdtompki
(Post 14779845)
Tandems will slow down slightly faster than singles due to the greater drag, but that doesn't sound like the case. Furthermore, it takes a good deal of friction to slow a tandem down. Surely, this should be evident on the stand?
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It's a 7 spd. canti-braked Santana. They haven't had it on a stand yet. They've ridden it for years with no probs--they know what it should feel like. It just started to do this on a ride--when they stopped pedaling, the bike felt as though someone was applying a brake. On a downhill, they normally FLY even coasting, but now had to pedal least they slow to a crawl. Does this info help?
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Originally Posted by cny-bikeman
(Post 14780155)
There is no way to tell remotely what the cause is. If the freewheel/cassette is dragging the chain will sag at the bottom. Take off the wheels and check the bearings. If the hubs have cartridge bearings (some hubs say sealed mechanism but just have rubber seals over standard bearings) replace the cartridges. Otherwise overhaul the hubs, inspecting for pitting, bent axle, etc. Neither procedure is terribly expensive and can't hurt.
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Originally Posted by HillRider
(Post 14779977)
I disagree. Tandems are actually faster downhill and coast better than a single since they have more weight but the same aerodynamic drag (same frontal area) as a single.
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Originally Posted by DiabloScott
(Post 14779827)
Also, is this something new - like it was coasting well before but then something happened?
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Originally Posted by bikeme
(Post 14783049)
They kind of bog uphill though unless the team are monsters.
Originally Posted by http://en.wikipedia.org/wiki/Tandem_bicycle#Performance
Compared to a conventional bicycle, a tandem has double the pedalling power with only slightly more frictional loss in the drivetrain. It has about the same wind resistance as a conventional bicycle. High-performance tandems may weigh less than twice as much as a single bike, so the power-to-weight ratio may be slightly better than that of a single bike and rider. On flat terrain and downhill, most of the power produced by cyclists is used to overcome wind resistance, so tandems can reach higher speeds than the same riders on single bicycles. They are not necessarily slower on climbs, but are perceived as such, in part due to the need for a high level of coordination between the riders, especially if the physical abilities of the two riders are very different, requiring a compromise on cadence.
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Need to get it on a stand to find out the problem. It may be hub bearings & you son't want to try to keep riding it like that.
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Originally Posted by bikeme
(Post 14783046)
It's a 7 spd. canti-braked Santana. They haven't had it on a stand yet. They've ridden it for years with no probs--they know what it should feel like. It just started to do this on a ride--when they stopped pedaling, the bike felt as though someone was applying a brake. On a downhill, they normally FLY even coasting, but now had to pedal least they slow to a crawl. Does this info help?
It's like going to the doctor and saying "my friend doesn't feel as good as he did before .... what's the matter with him?" ...... pretty hard to diagnose that. |
I would lift the front wheel and spin it. If it spins freely and with no grinding, I would then take my hand and move the crank backwards. If the crank moves freely, then the free wheel is probably ok. Now lift the rear wheel and pedal it forward. If the wheel stops faster than normal, then the wheel hub needs to be looked at.
That is all I got. |
Originally Posted by wroomwroomoops
(Post 14783209)
Thanks the FSM for Wikipedia:
It would be interesting to see how a tandem team's combined power to weight ratio varies from the individual ratios. |
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