On Fri, 05 Mar 2004 20:28:05 GMT, "Greg Estep"
<
[email protected]> wrote:
>
>"slartibartfast" <
[email protected]> wrote in message
news:[email protected]...
>>
>> "Christopher Meyer" <
[email protected]> wrote in message
>>
news:[email protected]...
>> > I recently bought rollers and am enjoying them...but here's the question
>> >
>> > When I ride with Bike A., my speed at the same exertion (measured by
>HRM)
>> > as when I ride Bike B is about 5mph slower. Bike A has Rolf Seistre
>> wheels
>> > (with new and recently rebuilt freewheel) and Conti 3000 GP tires
>whereas
>> > Bike B has Mavic SSC SL wheels with Vittoria Corsa Evo tires. I
>switched
>> > the wheels and Bike gained a lot of the speed with the Mavic/Corsa
>combo.
>> >
>> > What fascinates me is why there would be such a large difference between
>> the
>> > two wheel/tire combinations. Those of you with more experience, please enlighten me. Are there
>> > other factors?
>>
>> Are the freewheel's the same on both bikes? how 'bout the chainrings? Is
>the
>> computer properly calibrated for each wheelset? are the tires pumped up to the same presure on
>> both bikes? a combination of all these factors could make quite a big difference.
>
>Even if you have accounted for all the above variables (and I'm sure there are a few others) I
>would not be surprised to hear that the tire/wheel combination still makes an observable difference
>in your effort required. I'm guessing that it's the tire, not the wheel, that accounts for most of
>the difference. The deformation of the tires where they meet the drums is responsible for the
>resistance you experience while ridding rollers. Therefore, a change in the tire can be looked at
>as being similar to adjusting the resistance on a mag/fluid/air trainer. Of course, if you have an
>external resistance device on your rollers (Kreitler rollers with a Killer Headwind, for example)
>the relative effect of a tire change would be reduced.
The aerodynamic drag from the spokes is quite large when on the rollers, probably about 25% of what
you get at the same speed on the road, somebody else can do the math if they want to. Given that
you'll probably be going faster on the rollers than on the road, spoke drag will become
increasingly significant. Rolling resistance is much higher on rollers than on the road, for
obvious reasons. If you want to experiment, swap the tyres from one set of wheels to the other so
that you can isolate these two effects. Also check your position on the two bikes, as postural
effects can have consequences on the rollers which are often cancelled or overridden by aerodynamic
effects on the road.
Kinky Cowboy*
*Batteries not included May contain traces of nuts Your milage may vary