Lou Holtman said:Quoted message said:Lou Holtman said:[email hidden] wrote:
> On Sun, 03 Dec 2006 23:48:19 +0100, Lou Holtman
> <[email hidden]> wrote:
>
>
>
>> Tim McNamara wrote:
>>
>>
>>> In article <[email hidden]>,
>>> Lou Holtman <[email hidden]> wrote:
>>>
>>>
>>>
>>>
>>>> Jim Higson wrote:
>>>>
>>>>
>>>>
>>>>> HarryB wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>> How much drag is considered normal for a cartridge bearing BB?
>>>>>
>>>>>
>>>>> Don't know about normal, but if I give my cranks a good push
>>>>> with no chain
>>>>> connected they continue to spin for several minutes.
>>>>>
>>>>> Veloce cartridge BB (2006 model I think)
>>>>
>>>>
>>>>
>>>> Several minutes??? That's not normal.
>>>
>>>
>>>
>>> The Veloce bottom brackets are pretty smooth turning- smoother
>>> than most of Campy's other BBs including (the last ones I tried)
>>> Chorus and Record.
>>
>>
>>
>> We are talking of minutes. That is a very loooong time. I can't
>> believe that.
>> Last week I installed a Shimano XT hollotech II crankset (outboard
>> bearings). I gave it a swing and it wouldn't turn even one
>> revolution, but that's just the drag of the seals and you don't
>> notice that while riding.
>>
>> Lou
>
>
>
> Dear Lou,
>
> For fun, I tested a 1998 Schwinn Le Tour triple with 170 mm cranks,
> pretty much unused and bought for spares, a ten-year-old bottom of the
> line example of a crank.
>
> With no pedals, the level crank will support two dimes over the left
> pedal eye, but it starts to turn under the weight when a third dime is
> added.
>
> According to the US Mint, a dime is around 2.268 grams, plus or minus
> a smidgen for grime and wear:
>
> http://www.usmint.gov/about_the_mint/index.cfm?flash=yes&action=coin_specifications
>
>
> So the initial bearing resistance (higher than the moving resistance)
> is a bit less than 170 mm / 6 grams of torque, which works out to the
> surprisingly high value of about 0.1 foot-lbs of torque.
>
> Bracing the frame and giving the pedals a brisk turn produces about 13
> seconds of spin on average.
>
> Adding some light pedals increases the inertia and angular momentum
> more than the wind drag and produces about 20 seconds of spin on
> average.
>
> So maybe really smooth modern bearings can spin for minutes. I'd love
> to see some quick and dirty tests involving how many coins (or half or
> quarter coins) a level crank can support before it starts to turn.Carl, I gave my front wheel (with tire) with a recently overhauled
and very smooth running Record hub a very firm spin (I hurt myself)
and it stopped after 45 seconds. So I think 'several' minutes is a
ridiculous claim.Lou
Dear Lou,
Since a front wheel is not a bottom bracket, the two may behave
differently.
For one thing, the wheel has considerably greater wind drag from all
the spokes--you can feel the fan effect off to the side of a spun
front wheel.
The drag is partly due to the area of the spokes, which is
surprisingly large. With ~275 mm of exposed spoke at mostly 1.8 mm for
a butted spoke, each spoke has ~500 mm^2 of area. At 16 spokes, that's
8,000 mm^2. For 32 spokes, it's 16,000 mm^2.The two 170 mm pedal arms on the crank that I tested are about 15 mm
wide by 160 mm exposed length, so 2 x 15 x 160 = 4,800 mm^2. So the
crank has considerably less raw fan surface than any normal spoked
wheel.The other aspect of the wheel's increased wind drag is that the wheel
is much larger in diameter and therefore spins the spokes at a much
higher speed. About half the spoke length sticks out further than the
crank, so at the same rpm the spoke travels faster--and wind drag
rises wildly as speed increases.So wheels may not compare very well to cranks.
In any case, your 45 second front wheel hand-spin turns out to be well
under what can be achieved:"In one test, the wheel is accelerated to 70km/h before being
released; time is recorded until it comes to a complete standstill.
The best performing Cometes, used by Meares for her record, can spin
for over 12 minutes."http://www.cyclingnews.com/tech.php?id=tech/2006/features/meares_bt_stealth
So some front wheels can go over 720 seconds when spun up to about 43
mph.Sorry that you hurt yourself, but it's unlikely that your hand-spin
for 45 seconds reached even 20 mph.When I tested hand-spinning of front wheels, I found that about 15 mph
was the maximum, due to the extremely limited span and awkward angle
at which our arms engage wheels. Your hand must change direction 90
degrees in only a quarter-turn of the wheel, so your hand comes off
the wheel long before you reach even the speed of a casual
stone-throw:http://groups.google.com/group/rec.bicycles.tech/msg/9b36efb8133cd8a7
The current RBT record for hand-spinning a 700c front wheel [modest
cough] still stands at 16.2 mph.Anyway, my initial reaction was disbelief when a crank was said to
spin for minutes, but I'm willing to be convinced. A 2006 crank,
beautifully finished and designed and so forth, with heavy pedals,
should certainly be able to beat my cheap 1998 test crank's 20 seconds
with light pedals.I'd love to have someone with a nice new crank give it a whirl without
the chain and report actual seconds.Cheers,
Carl Fogel
OK I'm challenged. I now go back to my garage, remove the chain and spin
my 2006 Campagnolo Carbon Compact crank, with smooth running Record BB.
I'll be back..Lou
OK I'm back. 25 seconds the first attempt and 26 seconds the second. The
crankarm with the magnet for my cadence sensor ended up in the down
position twice. Pedals: speedplay zero's. I'm not a gorilla so there are
people who can spin a crankarm faster than I but 26 seconds is far
enough of 'several' minutes. I'm done.
Now I have dirty hands...
Lou
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