Advise on coversion to "funny bike"...

Advise on coversion to "funny bike"...

Post by SK8 D » Sun, 23 Jun 1996 04:00:00


My wife's Cannondale 600 has 700 wheels but a very small frame size. (48")
  Her wheels look enormous relative to the rest of the bike and her.  It
cannot be as aero as a bike with 650's.
I was toying with the idea of altering the front wheel set up to allow for
a 650 wheel,  and keeping the 700 in the rear.  
Would I need a new fork as well as wheel?
Would I be able to use the Shimano brakes that are mounted now?
Would this new config. handle OK?
Do you think its stupid to do this and instead I should sell her bike and
use the money to get a nice new tri-bike with the 650's front and back?
Thank You for all suggestions..........stay fit............

 
 
 

Advise on coversion to "funny bike"...

Post by Thomas Roe » Tue, 25 Jun 1996 04:00:00

: My wife's Cannondale 600 has 700 wheels but a very small frame size. (48")
:   Her wheels look enormous relative to the rest of the bike and her.  It
: cannot be as aero as a bike with 650's.
: I was toying with the idea of altering the front wheel set up to allow for
: a 650 wheel,  and keeping the 700 in the rear.  
: Would I need a new fork as well as wheel?
: Would I be able to use the Shimano brakes that are mounted now?
: Would this new config. handle OK?
: Do you think its stupid to do this and instead I should sell her bike and
: use the money to get a nice new tri-bike with the 650's front and back?
: Thank You for all suggestions..........stay fit............

I have been toying with the same idea for my Cannondale SR400, but have
not started the changes. Here is the what I have concluded:

1. You will need a new front fork and wheel.
2. Brakes should be fine, if you do step 1.
3. The effective geometry changes should balance out. The seat post
angle will rotate to about 75-76 degrees, from the 73 degrees it is
at now. The reduction in the head tube angle will help to reduce the rake
of the front fork, making it more stable for riding in aero position.
4. Disadvantage, you will have to carry two sizes of spares.

I think it's worth a try, I plan on trying a swap later on this summer.

Tom Roehr       "The Javanator"


 
 
 

Advise on coversion to "funny bike"...

Post by Andrew R. Coggan, Ph.D » Wed, 26 Jun 1996 04:00:00

Quote:

> (snip)
> 3. The effective geometry changes should balance out. The seat post
> angle will rotate to about 75-76 degrees, from the 73 degrees it is
> at now. The reduction in the head tube angle will help to reduce the rake
> of the front fork, making it more stable for riding in aero position.

        I'm not sure I follow...switching to a 650 C front wheel and fork will increase both the
seat tube angle AND the head tube angle, the latter of which (by itself) will tend to make the
bike LESS stable (i.e., more responsive). The way to counter this is to get a fork with less
offset ("rake"), so as to keep the trail within the normal range.

        The reason I'm up to speed on this (pardon the pun!) is that I am in the process of
making such a conversion myself. Fortunately, at the time I was considering this, Craig Calfee
of Carbonframes posted the following useful formula on rec.bicycles.tech (thanks, Craig!):

trail = (wheel radius * cos of the head tube angle - fork offset)/sin of the head tube angle

        Craig went on to suggest that 5.5 cm of trail is appropriate for the average road racing
bike. I did these calculations for my bike (head tube angle 72 deg, fork offset 4 cm), and came
up with a trail of 6.7 cm - which is consistent with the way the bike feels (slower than your
typical criterium bike, but perfect for long rides or the use of aero bars). Switching to a 650
C wheel and fork will increase the head tube angle to almost 75 deg (I did a scale drawing to
work this out), which BY ITSELF would reduce the trail to only 4.9 cm - more like a track bike
than a nice, stable TT bike! However, the smaller radius of the 650 C wheel and the lesser
offset (2.5 cm) of the 650 C fork means that the trail will in fact be 5.8 cm - still less than
it was, but more within the range of most road bikes.

        Another factor to consider in making such a conversion is that the bottom bracket height
will decrease by about 2 cm (more or less depending on the wheelbase). This may not be an issue
if you brake and coast through turns, but the first few times watch out trying to pedal through
those sharp bends, especially is your using long cranks! Fortunately, my bike has a
higher-than-average BB height to begin with, so I don't anticipate any problems there.

        I hope to have the new wheel and fork installed in the next couple of weeks. If anybody
is interested, I'd be happy to report on how the changeover "feels".

 
 
 

Advise on coversion to "funny bike"...

Post by Thomas Roe » Wed, 26 Jun 1996 04:00:00


:       I'm not sure I follow...switching to a 650 C front wheel and fork will increase both the
: seat tube angle AND the head tube angle, the latter of which (by itself) will tend to make the
: bike LESS stable (i.e., more responsive). The way to counter this is to get a fork with less
: offset ("rake"), so as to keep the trail within the normal range.
:
:       The reason I'm up to speed on this (pardon the pun!) is that I am in the process of
: making such a conversion myself. Fortunately, at the time I was considering this, Craig Calfee
: of Carbonframes posted the following useful formula on rec.bicycles.tech (thanks, Craig!):
:
: trail = (wheel radius * cos of the head tube angle - fork offset)/sin of the head tube angle
:

Oops, not enough sleep when I wrote that one.

I am still willing to try it, taking into account the handling issues.

Let me know how it goes for you, Andrew.

Thanks,

Tom Roehr       "The Javanator"