[email protected] wrote:
>>> 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.
> A Muzi <[email protected]> wrote:
>> 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.
[email protected] wrote:
> 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
