Wheel flop, fork length, and head angle

I have a bike with a 65 degree head angle and about 1.5 inches of
fork rake. The trail is about 2.5 inches. It has 20" (406mm) wheels
front and back. It has a lot of wheel flop and cannot be ridden with
no hands. I would like to improve the steering stability. I'm not
real clear on what the effect of changing the geometry would be, but
qualitatively it seems to have less fork flop if I put the rear wheel
up on a block.

Would it help to run a larger tire or a larger wheel in the rear,
e.g. a 451mm wheel?

Would it help to make the fork shorter without changing the rake? The
length of the RST suspension fork on this bike can be adjusted by
sliding the fork legs an inch or two r up in the fork crown.above the
top of the fork crown.

Both of these things would steepen the head angle and reduce the
trail, but I'm not sure if that is the problem. Thanks for any
advice.

cheg wrote:

I have a bike with a 65 degree head angle and about 1.5 inches of
fork rake. The trail is about 2.5 inches. It has 20" (406mm) wheels
front and back. It has a lot of wheel flop and cannot be ridden with
no hands. I would like to improve the steering stability. I'm not
real clear on what the effect of changing the geometry would be, but
qualitatively it seems to have less fork flop if I put the rear wheel
up on a block.

Would it help to run a larger tire or a larger wheel in the rear,
e.g. a 451mm wheel?

Would it help to make the fork shorter without changing the rake? The
length of the RST suspension fork on this bike can be adjusted by
sliding the fork legs an inch or two r up in the fork crown.above the
top of the fork crown.

Both of these things would steepen the head angle and reduce the
trail, but I'm not sure if that is the problem. Thanks for any
advice.

You could do a drawing but, since the fork is adjustable,
why not adjust it and see what happens? I mean, how long
could it take to move a few bolts and go for a ride?

If a different wheel fits, and the brake is still
functional, do try that as well. And write back, please!
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

"A Muzi" wrote in message
...
cheg wrote:


You could do a drawing but, since the fork is adjustable,
why not adjust it and see what happens? I mean, how long
could it take to move a few bolts and go for a ride?

If a different wheel fits, and the brake is still
functional, do try that as well. And write back, please!
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971


Absolutely, will do both of those things and report back, but I was
hoping to get some understanding of the theory. If there is a
threshold I may or may not get there with these changes so I may not
know if I'm headed in the right direction.

ot
know if I'm headed in the right direction.

Is this a dual-crown fork with 20" wheels?!?!

--
Phil, Squid-in-Training

"ZeeExSixAre" wrote in message
...
ot
know if I'm headed in the right direction.

Is this a dual-crown fork with 20" wheels?!?!

--
Phil, Squid-in-Training




RST Sprite TL
http://www.gaerlan.com/bikeparts/frame/forks/fork.html, a little down
on the left side.

"Jeff Wills" wrote in message
m...
"cheg" wrote in message
news:oa56c.31292$KO3.81619@attbi_s02...
I have a bike with a 65 degree head angle and about 1.5 inches of
fork rake. The trail is about 2.5 inches. It has 20" (406mm)
wheels
front and back. It has a lot of wheel flop and cannot be ridden
with
no hands. I would like to improve the steering stability. I'm not
real clear on what the effect of changing the geometry would be,
but
qualitatively it seems to have less fork flop if I put the rear
wheel
up on a block.


Cheg-

Is this a recumbent? If so, I'd really doubt any changes you make
will enable you to ride "no hands". No-hands balancing and steering
is
accomplished by tilting the bike with your legs. On a recumbent,
your
legs are at the wrong orientation to do this.

Jeff

Its a folder: http://cheg01.home.comcast.net/r20.html

"Chalo" wrote in message
om...
"cheg" wrote in message
news:oa56c.31292$KO3.81619@attbi_s02...
I have a bike with a 65 degree head angle and about 1.5 inches of
fork rake. The trail is about 2.5 inches. It has 20" (406mm)
wheels
front and back.

That's a lot of trail for a 20" wheel bike. With a head angle that
slack, I imagine that you can observe a noteworthy amount of
chassis
drop at the front when you turn the wheel to the side. In my
experience from building choppers, it is this drop that stokes a
bike's tendency to flop. The rider's weight makes the front end
want
to turn, and straightening it out requires lifting that weight back
up. More fork offset counteracts the chassis drop and makes the
bike
more manageable.


Makes sense, if the energy state of the bike is lower with the wheel
turned then it will turn. Unfortunatey the only way I can see to
change the offset is to change to a different fork and I like the one
I have in other respects.


If you slide the legs up in the crown more than a little bit, then
the
tire will bottom out on the fork crown, potentially sending you
ass-over-teakettle. Don't ask me how I know this. If you decide
to
slide the legs up, first remove the fork leg caps and release the
springs so you can collapse the fork to its stops. With the wheel
in
place, this will show you exactly how much you can raise the fork
legs
before encountering interference between tire and crown.


I understand the concern, that is good advice.


Both of these things would steepen the head angle and reduce the
trail, but I'm not sure if that is the problem. Thanks for any
advice.

It's not the trail per se that's the problem, but reducing trail by
either means will have the effect of reducing frame drop, and that
is
the cause of your wheel flop.


Thanks for the reply. It was vey enlightening. I'll see what I can do
without major surgery to the bike.

"cheg" wrote in message news:oa56c.31292$KO3.81619@attbi_s02...
I have a bike with a 65 degree head angle and about 1.5 inches of
fork rake. The trail is about 2.5 inches. It has 20" (406mm) wheels
front and back.
Would it help to make the fork shorter without changing the rake? The
length of the RST suspension fork on this bike can be adjusted by
sliding the fork legs an inch or two r up in the fork crown.above the
top of the fork crown.

Chalo wrote:
If you slide the legs up in the crown more than a little bit, then the
tire will bottom out on the fork crown, potentially sending you
ass-over-teakettle.

Yikes. I was wrong with my dangerous ("try it and see")
advice.
I skimmed past the "suspension" part of it, which Chalo noticed.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

"A Muzi" wrote in message
...
"cheg" wrote in message
news:oa56c.31292$KO3.81619@attbi_s02...
I have a bike with a 65 degree head angle and about 1.5 inches of
fork rake. The trail is about 2.5 inches. It has 20" (406mm)
wheels
front and back.
Would it help to make the fork shorter without changing the rake?
The
length of the RST suspension fork on this bike can be adjusted by
sliding the fork legs an inch or two r up in the fork crown.above
the
top of the fork crown.

Chalo wrote:
If you slide the legs up in the crown more than a little bit,
then the
tire will bottom out on the fork crown, potentially sending you
ass-over-teakettle.

Yikes. I was wrong with my dangerous ("try it and see")
advice.
I skimmed past the "suspension" part of it, which Chalo noticed.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971


I tried lowering the fork an inch (after checking tire clearance) and
installing a 451 rear wheel. The 451mm wheel fit in the frame just
fine but the rear brake was not usable in that setup. The head angle
came out to about 69 degrees. The changes seem to improve stability
and significantly reduced wheel flop while stationary, but no-hands
is still no-go. The 451 wheel has a 7 speed cassette while the 406
wheel has a Sachs 3x7, so I don't think I'll make that switch
permanent. One more change I'll try is a Maxxis Hookworm 20x1.95 rear
tire and a Primo Comet front tire to get some of the diameter
difference without having to swap hubs to get the 3x7 back.

With all these parts, maybe I need another Raleigh 20 frame...

"Chalo" wrote in message
om...
"cheg" wrote:

Makes sense, if the energy state of the bike is lower with the wheel
turned then it will turn. Unfortunatey the only way I can see to
change the offset is to change to a different fork and I like the one
I have in other respects.

If you can find or make (or have made for you) a fork crown/steerer
assembly that accepts you current stanchions but has some offset or
rake built into it, then you might be in business.

A lot of 20" suspension forks use the old Rock Shox Mag 21-type
standard of 1" leg diameter on 107mm centers. If yours is like that,
you are in luck-- Mag 21 crowns are raked to provide offset for
sliders that have no offset at the dropout. RST crowns have a touch
of offset but no rake. By switching to a crown from an old Mag 21 or
one of its clones, you will combine offset at the dropout with rake at
the crown for more total offset than would otherwise be possible.

Chalo Colina

Thats great, I may find a trashed Mag 21 at Recycled Cycles to part out. I know
the leg diameter is right but I have to check the spacing. The trick will be to
get a crown that will take a 1" steerer.