Funny Bike Conversion

Quoted message said:

I've got a steel TT bike from back when funny bike configuration with
700c rear and smaller fronts were common. Given the front wheel is
650c, I'm thinking of converting it to a standard 700c fork. Anyone
tried this or know if the slight elevation in the front end will be as
minimal as I suspect it would be? I haven't quite decided if I would
use it as a road bike or a legal TT bike. (kinda hard before riding
it...) I'd appreciate some wisdom before going ahead with the HS/fork
replacement.

Ideally, you could do a drawing. But if a suitable fork is handy, just
try it. You'll most likely find your new lower head angle ( with a
taller fork) wants an excessively long rake to keep trial constant
(~~50mm). The long rakes are generally found on touring forks with even
taller clearance so it may be hard to keep up with the head angle change
looking for more trail. The result may handle poorly with skimpy trail.
Or not.

If you can experiment with stuff you already own this may be
interesting. I would not pay for any standard fork for such a project.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Quoted message said:

I've got a steel TT bike from back when funny bike configuration with
700c rear and smaller fronts were common. Given the front wheel is
650c, I'm thinking of converting it to a standard 700c fork. Anyone
tried this or know if the slight elevation in the front end will be as
minimal as I suspect it would be? I haven't quite decided if I would
use it as a road bike or a legal TT bike. (kinda hard before riding
it...) I'd appreciate some wisdom before going ahead with the HS/fork
replacement.

Josh

I've gone the other way (700C to 650C fork and tire). IIRC, I
calculated the head angle change as 4 degrees. It sure felt about that
-- in my case it went from slightly slow to very quick handling,
though you're going the other way so it "might" be tolerable. Andy's
right tough: look for lots of rake and don't pay anything unless
you've tried it first.

On the other hand, I've used a bumper jack to bend steel frames to
change head angle too...

Quoted message said:

Quoted message said:

I've got a steel TT bike from back when funny bike configuration with
700c rear and smaller fronts were common. Given the front wheel is
650c, I'm thinking of converting it to a standard 700c fork. Anyone
tried this or know if the slight elevation in the front end will be as
minimal as I suspect it would be? I haven't quite decided if I would
use it as a road bike or a legal TT bike. (kinda hard before riding
it...) I'd appreciate some wisdom before going ahead with the HS/fork
replacement.

Quoted message said:

I've gone the other way (700C to 650C fork and tire). IIRC, I
calculated the head angle change as 4 degrees. It sure felt about that
-- in my case it went from slightly slow to very quick handling,
though you're going the other way so it "might" be tolerable. Andy's
right tough: look for lots of rake and don't pay anything unless
you've tried it first.

On the other hand, I've used a bumper jack to bend steel frames to
change head angle too...

I assume you were _restoring_ the original head angle after a crash and
not 'upgrading' an otherwise sound frame ???

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

A Muzi said:

Quoted message said:

On Feb 7, 12:48 pm, [email hidden] wrote:

Quoted message said:

Quoted message said:

On the other hand, I've used a bumper jack to bend steel frames to
change head angle too...

I assume you were _restoring_ the original head angle after a crash and
not 'upgrading' an otherwise sound frame ???

Either way.... Yikes!

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

A Muzi said:

Quoted message said:

I've got a steel TT bike from back when funny bike configuration with
700c rear and smaller fronts were common. Given the front wheel is
650c, I'm thinking of converting it to a standard 700c fork. Anyone
tried this or know if the slight elevation in the front end will be as
minimal as I suspect it would be? I haven't quite decided if I would
use it as a road bike or a legal TT bike. (kinda hard before riding
it...) I'd appreciate some wisdom before going ahead with the HS/fork
replacement.

Ideally, you could do a drawing. But if a suitable fork is handy, just
try it. You'll most likely find your new lower head angle ( with a
taller fork) wants an excessively long rake to keep trial constant
(~~50mm). The long rakes are generally found on touring forks with even
taller clearance so it may be hard to keep up with the head angle change
looking for more trail. The result may handle poorly with skimpy trail.
Or not.

If you can experiment with stuff you already own this may be
interesting. I would not pay for any standard fork for such a project.

--
Andrew Muziwww.yellowjersey.org
Open every day since 1 April, 1971

Yes, I can experiment with parts I already have. My plan was to put a
Kinesis aero (aluminum) fork on it but the rake does not look very
long, at least by sight. The handling was indeed my main concern as
the only thing scarier than riding a TT setup with insufficient trail
would be riding no hands on a TT bike (which I foolishly did on my
first TT bike thinking it would be similar to a road bike)

I am confused by Andy's response as I thought that elevating the front
would increase the trail by pushing forward point B in the diagram :
http://www.prodigalchild.net/Bicycle6.htm#Shimmy

Thus in order to increase or maintain trail, a fork of the same or
lower rake/offset would be required. No?

Theoretically, I can modify the rake of a steel fork with the Park
fork-bending tool that I have, but don't want to be bending forks if I
don't have to.

Quoted message said:

Quoted message said:

Quoted message said:

I've got a steel TT bike from back when funny bike configuration with
700c rear and smaller fronts were common. Given the front wheel is
650c, I'm thinking of converting it to a standard 700c fork. Anyone
tried this or know if the slight elevation in the front end will be as
minimal as I suspect it would be? I haven't quite decided if I would
use it as a road bike or a legal TT bike. (kinda hard before riding
it...) I'd appreciate some wisdom before going ahead with the HS/fork
replacement.

Quoted message said:

A Muzi said:

Ideally, you could do a drawing. But if a suitable fork is handy, just
try it. You'll most likely find your new lower head angle ( with a
taller fork) wants an excessively long rake to keep trial constant
(~~50mm). The long rakes are generally found on touring forks with even
taller clearance so it may be hard to keep up with the head angle change
looking for more trail. The result may handle poorly with skimpy trail.
Or not.
If you can experiment with stuff you already own this may be
interesting. I would not pay for any standard fork for such a project.

Quoted message said:

Yes, I can experiment with parts I already have. My plan was to put a
Kinesis aero (aluminum) fork on it but the rake does not look very
long, at least by sight. The handling was indeed my main concern as
the only thing scarier than riding a TT setup with insufficient trail
would be riding no hands on a TT bike (which I foolishly did on my
first TT bike thinking it would be similar to a road bike)
I am confused by Andy's response as I thought that elevating the front
would increase the trail by pushing forward point B in the diagram :
http://www.prodigalchild.net/Bicycle6.htm#Shimmy
Thus in order to increase or maintain trail, a fork of the same or
lower rake/offset would be required. No?
Theoretically, I can modify the rake of a steel fork with the Park
fork-bending tool that I have, but don't want to be bending forks if I
don't have to.

That drawing is a great link for this project of yours.
When you raise the front (650C=>700C), the head angle goes lower (e.g.,
74 deg=>70deg). Point B in your drawing moves forward. Adding rake
brings it back, hopefully to about 50mm.

Generally steeper angles need less rake, lower angles , more rake. In
your specific case, since a typical road fork rake of 40~45mm will be
about the same rake as your typical 650C tri fork, your new head angle
will add way too much trail. A more ample rake will reduce that
hopefully back to the same 50mm value with which you began. Sketch it
and see! [p.s. don't bend that aluminum fork!] Before making a mandrel
to shape steel blades, see what free long-rake forks are available to
you. Curving the blades shortens the axle-to-brake bolt length.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971