Wheel flop, fork length, and head angle

[Cheg]

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.

 

[A Muzi]

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

 

[Cheg]

"A Muzi" <[email protected]> wrote in message
news:[email protected]...
> 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.

 

[Zeeexsixare]

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

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

--
Phil, Squid-in-Training

 

[Cheg]

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

"cheg" <[email protected]> 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

 

[Chalo]

"cheg" <[email protected]> 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.

> 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.

> 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

 

[Cheg]

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

 

[Cheg]

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

 

[A Muzi]

> "cheg" <[email protected]> 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

 

[Cheg]

"A Muzi" <[email protected]> wrote in message
news:[email protected]...
> > "cheg" <[email protected]> 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]

"cheg" <[email protected]> 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

 

[Cheg]

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

 

[Chalo]

"cheg" <[email protected]> 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:

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: http://www.danscomp.com/cgi-
bin/hazel.cgi?action=DETAIL&item=444132

Chalo Colina

 

[Scott Hendricks]

[email protected] (Chalo) wrote in message news:<[email protected]>...
> "cheg" <[email protected]> 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)
>
> 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]

[email protected] (Chalo) wrote in message news:<[email protected]>...
> "cheg" <[email protected]> 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)
>
> 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.

 

[Mark Hickey]

[email protected] (Scott Hendricks) wrote:

>[email protected] (Chalo) wrote in message
>news:<[email protected]>...
>> "cheg" <[email protected]> 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)
>>
>> 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 http://www.habcycles.com Home of
the $695 ti frame

 

[Chalo]

[email protected] (Scott Hendricks) wrote:

> 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

 

[Scott Hendricks]

Mark Hickey <[email protected]> wrote in message news:<[email protected]>...
> [email protected] (Scott Hendricks) wrote:
>
> >[email protected] (Chalo) wrote in message news:<8-
> >[email protected]>...
> >> "cheg" <[email protected]> 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)
> >>
> >> 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 http://www.habcycles.com 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.