Wheel flop, fork length, and head angle

cheg said:

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" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

cheg said:

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

Quoted message said:

know if I'm headed in the right direction.

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

--
Phil, Squid-in-Training

"ZeeExSixAre" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

ot

Quoted message said:

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
gaerlan.comfork.htmlOpen ↗, a
little down on the left side.

"cheg" <[email hidden]> wrote in message news:<oa56c.31292$KO3.81619@attbi_s02>...

Quoted message said:

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

"cheg" <[email hidden]> wrote in message news:<oa56c.31292$KO3.81619@attbi_s02>...

Quoted message said:

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.

Quoted message said:

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.

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.

Quoted message said:

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.

Chalo Colina

"Jeff Wills" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

"cheg" <[email hidden]> wrote in message

news:<oa56c.31292$KO3.81619@attbi_s02>...

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

front and back. It has a lot of wheel flop and cannot be
ridden

with

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

qualitatively it seems to have less fork flop if I put
the rear

wheel

Quoted message said:

Quoted message said:

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

Quoted message said:

accomplished by tilting the bike with your legs. On a
recumbent,

your

Quoted message said:

legs are at the wrong orientation to do this.

Jeff

Its a folder: cheg01.home.comcast.netr20.htmlOpen ↗

"Chalo" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

"cheg" <[email hidden]> wrote in message

news:<oa56c.31292$KO3.81619@attbi_s02>...

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

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

Quoted message said:

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

Quoted message said:

to turn, and straightening it out requires lifting that
weight back up. More fork offset counteracts the chassis
drop and makes the

bike

Quoted message said:

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.

Quoted message said:

If you slide the legs up in the crown more than a little
bit, then

the

Quoted message said:

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

Quoted message said:

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

Quoted message said:

place, this will show you exactly how much you can raise
the fork

legs

Quoted message said:

before encountering interference between tire and crown.

I understand the concern, that is good advice.

Quoted message said:

Quoted message said:

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

Quoted message said:

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.

Quoted message said:

"cheg" <[email hidden]> wrote in message news:<oa56c.31292$KO3.81619@attbi_s02>...

Quoted message said:

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 said:

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" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Quoted message said:

"cheg" <[email hidden]> wrote in message

news:<oa56c.31292$KO3.81619@attbi_s02>...

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Quoted message said:

front and back. Would it help to make the fork shorter
without changing the rake?

The

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Quoted message said:

top of the fork crown.

Chalo said:

If you slide the legs up in the crown more than a
little bit,

then the

Quoted message said:

Quoted message said:

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...

cheg said:

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

"Chalo" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

cheg said:

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.

cheg said:

It has a lot of wheel flop and cannot be ridden with no
hands. I would like to improve the steering stability.

I encourage you to pursue the measure of adding fork offset
to normalize your bike's steering characteristics, but there
is one relevant fact which nobody has yet brought up:

No-hands riding is dependent on front-wheel gyroscopic
forces to provide steering corrections. Small diameter
wheels will have less gyroscopic stability at any wheel
weight and road speed than large diameter wheels, and its
wheels are apt to be lighter as well. Thus a small-wheeled
bike will be more difficult to ride without hands than a large-
wheeled bike even if there is nothing amiss with the
steering characteristics.

If you would like to add mass-- and thus gyroscopic stability--
to the perimeter of your wheels, and you have clearance to
fit them, I recommend these in the 20x2.3" size: danscomp.comcgiOpen ↗
bin/hazel.cgi?action=DETAIL&item=444132

Chalo Colina

[email hidden] (Chalo) wrote in message news:<[email hidden]>...

Quoted message said:

cheg said:

It has a lot of wheel flop and cannot be ridden with no
hands. I would like to improve the steering stability.

I encourage you to pursue the measure of adding fork
offset to normalize your bike's steering characteristics,
but there is one relevant fact which nobody has yet
brought up:

(sections snipped)

Quoted message said:

Chalo Colina

you've got it backwards. Increased trail adds stability.
Decreasing offset increases trail. Increasing offset would
decrease trail and make the stability issue worse.

[email hidden] (Chalo) wrote in message news:<[email hidden]>...

Quoted message said:

cheg said:

It has a lot of wheel flop and cannot be ridden with no
hands. I would like to improve the steering stability.

I encourage you to pursue the measure of adding fork
offset to normalize your bike's steering characteristics,
but there is one relevant fact which nobody has yet
brought up:

(sections snipped)

Quoted message said:

Chalo Colina

you've got it backwards. Increased trail adds stability.
Decreasing offset increases trail. Increasing offset would
decrease trail and make the stability issue worse.

(Scott Hendricks) said:

[email hidden] (Chalo) wrote in message
news:<[email hidden]>...

Quoted message said:

cheg said:

It has a lot of wheel flop and cannot be ridden with no
hands. I would like to improve the steering stability.

I encourage you to pursue the measure of adding fork
offset to normalize your bike's steering characteristics,
but there is one relevant fact which nobody has yet
brought up:

(sections snipped)

Quoted message said:

Chalo Colina

you've got it backwards. Increased trail adds stability.
Decreasing offset increases trail. Increasing offset would
decrease trail and make the stability issue worse.

Normally, but the bike in question has 2.5" of trail already
and its main handling issue is wheel flop. More trail would
only make it worse.

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home of
the $695 ti frame

(Scott Hendricks) said:

you've got it backwards. Increased trail adds stability.
Decreasing offset increases trail. Increasing offset would
decrease trail and make the stability issue worse.

As a rule of thumb, what you say is correct. In this case
you are wrong.

When the head angle is very slack, as on cheg's bike and
many choppers I have built, turning the front wheel away
from center causes the bike to drop down. This makes the
bike tend to fall or "flop" into a turn, since being turned
to either side is "downhill" from center.

When this is the case, adding fork offset reduces the amount
that the frame will drop when the front wheel is turned,
which has a stabilizing effect. The effect I describe is
stronger than the stabilizing influence of additional trail,
so it is of net benefit to add offset in such a case.

I have made fork crowns (for choppers with roughly 40-45
degree head angles) that are equilateral triangles, with
almost 4" of offset. Such crowns can transform a really raked-
out bike from barely rideable to pretty manageable.

Chalo Colina

Mark Hickey <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

(Scott Hendricks) said:

[email hidden] (Chalo) wrote in message news:<8-
[email hidden]>...

Quoted message said:

"cheg" <[email hidden]> wrote:

> It has a lot of wheel flop and cannot be ridden with
> no hands. I would like to improve the steering
> stability.

I encourage you to pursue the measure of adding fork
offset to normalize your bike's steering
characteristics, but there is one relevant fact which
nobody has yet brought up:

(sections snipped)

Quoted message said:

Quoted message said:

Chalo Colina

you've got it backwards. Increased trail adds stability.
Decreasing offset increases trail. Increasing offset
would decrease trail and make the stability issue worse.

Normally, but the bike in question has 2.5" of trail
already and its main handling issue is wheel flop. More
trail would only make it worse.

Mark Hickey Habanero Cycles habcycles.comhabcycles.comOpen ↗ Home
of the $695 ti frame

2.5 inches = 6.35 cm of trail. Not that much. If it's not
stable enough to ride with no hands, maybe the problem
isn't the steering geometry, wheel flop at slow speeds or
while stationary, notwithstanding.