Some cheapo tandems use a chain tensioner on the chain that goes between the two crank sets as a way of adjusting the chain tension. Often these bike are fitted with coaster brakes so the bottom leg of the chain sees loads that are on par with pedaling; or holding back on a fixed wheel. Like Charlie says, the thing has to be heavily built. The actual idler wheel is made of steel and runs on ball bearings. All in all a pretty bad design...
Jamie Swan Cell - 516-238-6782 Centerport Cycles Inc. 245 Main St. Northport, N.Y. 11768 631-262-0909 http://www.centerportcycles.com (mapped) http://www.cabinfeverauctions.com http://www.cabinfeverexpo.com http://www.limws.org http://www.liatca.org
On Aug 18, 2006, at 9:25 AM, Charlie Young wrote:
> It will work until you exert any braking to the drivetrain via the
> crankset. At that point, the forces on the tensioner will increase
> significantly and cause it to bend upwards (or be ripped off). If
> you were to build something to accomplish the task it would need a
> very strong (and probably triangulated) attachment to the chainstay.
>
> Tensioners can work with freewheels, non-fixed planetary hub gears
> and cassettes because the resistance of the ratcheting mechanism
> and the tension in the drivetrain necessary to maintain chain
> tautness are the primary upward forces. These forces are much
> smaller that the forces exerted by leg braking.
>
> Pure speculation: I'm wondering if the tensioner on the JT that
> initiated this discussion was intended to allow the use of a double
> chainring with a non-fixed SA hub. Positioning the tension arm
> idler at the center of the two chainrings would probably prevent
> the chain from unshipping.
>
> Charlie Young
> Honey Brook, PA
> ----- Original Message ----- From: "David Bean" <beandk@rcn.com>
> To: <classicrendezvous@bikelist.org>
> Sent: Friday, August 18, 2006 8:59 AM
> Subject: Re: [CR]Jack Taylor Tourist on ebay
>
>
>
>> It looks to me like the tensioner is not spring loaded. I.e. to
>> adjust the
>> tension, you would simply adjust and tighten the tensioner to take
>> up the
>> slack in the chain. Wouldn't a fixed tensioner work just fine with a
>> fixed-gear setup?
>>
>> David Bean
>> Arlington, MA