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Old 09-27-12, 12:57 AM   #1
bikeme
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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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Old 09-27-12, 01:06 AM   #2
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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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Old 09-27-12, 07:01 AM   #3
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It could be a poor selection of tires.
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Old 09-27-12, 07:29 AM   #4
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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...
Also, is this something new - like it was coasting well before but then something happened?
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Old 09-27-12, 07:34 AM   #5
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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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Old 09-27-12, 08:02 AM   #6
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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?
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Old 09-27-12, 08:09 AM   #7
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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?
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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Old 09-27-12, 08:27 AM   #8
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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.
I agree with you on this. Years ago I was out in Calif ridiing from Rosemead down to the base at El Toro. I spotted another cyclist after turning a corner and gave chase. 10 miles later he turned and I realized it was They not he. tandems reall move along!
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Old 09-27-12, 08:52 AM   #9
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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?
Huh!

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.
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Old 09-27-12, 08:54 AM   #10
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Originally Posted by bikeme View Post
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?
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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Old 09-27-12, 09:14 AM   #11
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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.
Yep, tandems are effin' monsters going downhill. There is no way a normal cyclist could overtake a tandem on even the lightest inclination (at parity of cyclist strength and stamina). Same difference with coasting downhill.

Reference: I rode a tandem with one of my ex GFs.
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Old 09-27-12, 09:58 AM   #12
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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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Old 09-27-12, 10:05 AM   #13
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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
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.
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Old 09-27-12, 10:36 AM   #14
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Drag brake?
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Old 09-27-12, 11:30 AM   #15
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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.
Let me just add the fact that bobotech will not beat tandems going downhill, because if he is so heavy as he says, he is probably also large - and will have a larger aerodynamic profile in the direction of motion, than a tandem (with two people half his weight).

EDIT: this without even mentioning that smaller riders have a better power-to-weight ratio than bigger ones.
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Old 09-27-12, 04:27 PM   #16
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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?
The above is the sort of knee-jerk drivel that one finds on the interweb. On level ground and taking into account only aerodynamic drag and no wind a tandem will decelerate more slowly than a single; to first order if the team drag is 40% greater than a single bike and the team weight is 80% greater than a single, tandem deceleration will be (approximately) = (1.4/1.8) that of a single.
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Old 09-28-12, 12:26 AM   #17
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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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Old 09-28-12, 12:27 AM   #18
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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.
Thank you!
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Old 09-28-12, 12:28 AM   #19
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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.
HR, you are correct. They kind of bog uphill though unless the team are monsters.
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Old 09-28-12, 12:29 AM   #20
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Also, is this something new - like it was coasting well before but then something happened?
Exactly, occurred mid-ride.
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Old 09-28-12, 04:49 AM   #21
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They kind of bog uphill though unless the team are monsters.
Thanks the FSM for Wikipedia:
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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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Old 09-28-12, 04:54 AM   #22
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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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Old 09-28-12, 05:13 AM   #23
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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?
No, because you are comparing their "feeling before" to their "feeling after" .... with no actual facts or first hand information. Maybe they both had a bad day and were a bit slower .... nothing to go on.

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.
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Old 09-28-12, 06:25 AM   #24
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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.
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Old 09-28-12, 07:13 AM   #25
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Thanks the FSM for Wikipedia:
Ramen!

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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