Frame geometry and neutral handling

On Mon, 15 Sep 2003 08:41:05 +0000 (UTC), Shane Halpin <[email hidden]> may

have said:

Can anyone advise how 1 frame geometry, 2 wheel & tyre size, and fork offset interrelate to affect
handling?

This to decide on how to make mine much less twitchy by changing 2 / 3

The axis of the steer tube needs to intersect the road surface forward of the center of the front
wheel's contact patch; how far forward this point lies will determine how much "caster" or "fork
trail" the wheel has. In extreme cases, where there's no trail at all, the fork will naturally try
to turn to the side as the wheel rolls; that's dangerous. With enough trail, the bike is stable even
on rough surfaces and at high speeds. Too much, and low-speed handling can get tricky.

Here's more data:

citybikes.comframe geometry.htmOpen ↗

There's probably more information available about this subject from motorcycle books since that's an
area where building bikes with custom frames designed to accomodate extended forks has been a common
practice for many years.

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

Quoted message said:

Can anyone advise how 1 frame geometry, 2 wheel & tyre size, and fork offset interrelate to affect
handling?

This to decide on how to make mine much less twitchy by changing 2 / 3

What size frame are you riding, and what is the head angle? What brand and material?

The general rule of thumb is that more stable handling is achieved with more trail. Trail is
increased with slacker head angles (mostly), or less fork rake (smaller effect among commonly
available forks). Tire size has very little effect (but larger = more trail). A longer fork will
raise the front of the bike, and decrease the head angle very slightly.

Someone at Serrota told me the "sweet spot" for trail is around 57-63mm, depending on the
desired feel.

Longer stems will also slow down the steering feel, but stem length should ideally be
chosen for fit.

Large frames (60cm+) tend to have steep (74deg) head angles, intended to keep the wheelbase short.
Between the resultant lack of trail, and structural problems, there seem to be a lot of problems
with twitchy handling and shimmy with bigger bikes.

If someone has found a solution, let me know!

-David

Shane Halpin said:

Can anyone advise how 1 frame geometry, 2 wheel & tyre size, and fork offset

"Trail" is the property that determines stability. More trail gives more stability. To find out
about trail, see:

phred.orgtrail.htmlOpen ↗

Art Harris

(David Mackintosh) may have said:

Tire size has very little effect (but larger = more trail).

Larger front = more trail (slightly). Larger rear = less trail (slightly).

Quoted message said:

A longer fork will raise the front of the bike, and decrease the head angle very slightly.

And thereby increase trail.

The same effect should be possible by spacing the steerer tube below the lower bearing, although
once the tube's been cut, that's no longer an option unless you want to switch to a threaded setup.

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Quoted message said:

Can anyone advise how 1 frame geometry, 2 wheel & tyre size, and fork offset interrelate to affect
handling?

This to decide on how to make mine much less twitchy by changing 2 / 3

Your first problem is accurately measuring head angle, fork lenger and rake. After you do that draw
it out on paper and figure out what fork rake will give you the trail you have and what trail you
desire. I design my bikes to 58mm of trail. Bill Boston (sorry, I can't find his webpage but you can
Google) likes to use castor angle on his bikes. But your big problem is measuring your bike. Good
luck, Phil Brown

Stupid me, Bill Boston is www.billbostoncycles.com

Phil Brown

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

Quoted message said:

The axis of the steer tube needs to intersect the road surface forward of the center of the front
wheel's contact patch; how far forward this point lies will determine how much "caster" or "fork
trail" the wheel has. In extreme cases, where there's no trail at all, the fork will naturally try
to turn to the side as the wheel rolls; that's dangerous. With enough trail, the bike is stable
even on rough surfaces and at high speeds. Too much, and low-speed handling can get tricky.

Here's more data:

citybikes.comframe geometry.htmOpen ↗

There's probably more information available about this subject from motorcycle books since that's
an area where building bikes with custom frames designed to accomodate extended forks has been a
common practice for many years.

Quote from the above site:
------------
The more rake a fork has the more "trail" there is in the steering, which makes the bike easier to
control over rough terrain and at high speed, but more side to side floppy at low speeds
------------

What a load; either they didn't proof their text or they don't have a clue what they're talking
about. Which unfortunately would jibe with what you find from the personnel in most local bike
shops. Anyone in the business *should* know that more rake = LESS trail, and it's MORE trail that
gives bikes more steering stability at speed.

SB

So here's a question.

It seems to me that rake is only part of the equation in handling. It's the factor that Jones
famously analyzed in developing the URB (unrideable bike). But surely a bike with a 75 degree head
angle and a bike with a 71 degree head angle will handle differently, even if fork rake is adjusted
so that trail is the same.

On Mon, 15 Sep 2003 13:06:29 -0400, "Steve Blankenship" <[email hidden]> may

have said:

Quote from the above site:
------------
The more rake a fork has the more "trail" there is in the steering, which makes the bike easier to
control over rough terrain and at high speed, but more side to side floppy at low speeds
------------

What a load; either they didn't proof their text or they don't have a clue what they're talking
about. Which unfortunately would jibe with what you find from the personnel in most local bike
shops. Anyone in the business *should* know that more rake = LESS trail, and it's MORE trail that
gives bikes more steering stability at speed.

I didn't notice that at first. You're correct about their error in the "rake" statement. However,
they got it right in the part about more trail providing greater stability at speed, and that was
what I had primarily been looking at.

More rake, as defined by their drawing, would *decrease* trail; not good in my opinion.

--
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don't respond to something, it's also possible that I'm busy.

In article <[email hidden]>, Shane Halpin

Quoted message said:

This to decide on how to make mine much less twitchy by changing 2 / 3

Touring bike = steering heaven.

Tom Gibb <[email hidden]

Tim McNamara may have said:

So here's a question.

It seems to me that rake is only part of the equation in handling. It's the factor that Jones
famously analyzed in developing the URB (unrideable bike). But surely a bike with a 75 degree head
angle and a bike with a 71 degree head angle will handle differently, even if fork rake is adjusted
so that trail is the same.

Trail is more important than exact head angle, for small changes in the latter. You can have
significant differences in steerer angle that produce similar rideability results if the trail
values are appropriate. (Note that "appropriate" is not the same as "identical".)

If your steering is too twitchy, the thing to do is increase the amount of trail in the front. This
is not a trivial task in most instances.

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don't respond to something, it's also possible that I'm busy.

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

Quoted message said:

(David Mackintosh) may have said:

Tire size has very little effect (but larger = more trail).

Larger front = more trail (slightly). Larger rear = less trail (slightly).

Quite right.

Quoted message said:

Quoted message said:

A longer fork will raise the front of the bike, and decrease the head angle very slightly.

And thereby increase trail.

The same effect should be possible by spacing the steerer tube below the lower bearing,
although once the tube's been cut, that's no longer an option unless you want to switch to a
threaded setup.

I had thought of that, but you can't just put a spacer of any thickness under the crown race, since
the steerer tube or crown has a step there that makes the race a press fit. Another possible
modification would be a head tube extension between the bottom of the head tube and the lower
headset cup. Serotta makes one for the top of the head tube. I'd be more worried about using one at
the bottom, from a structural standpoint.

-David

Werehatrack said:

On Mon, 15 Sep 2003 13:06:29 -0400, "Steve Blankenship" <[email hidden]> may

have said:

Quote from the above site:
------------
The more rake a fork has the more "trail" there is in the steering, which makes the bike easier to
control over rough terrain and at high speed, but more side to side floppy at low speeds
------------

What a load; either they didn't proof their text or they don't have a clue what they're talking
about. Which unfortunately would jibe with what you find from the personnel in most local bike
shops. Anyone in the business *should* know that more rake = LESS trail, and it's MORE trail that
gives bikes more steering stability at speed.

I didn't notice that at first. You're correct about their error in the "rake" statement. However,
they got it right in the part about more trail providing greater stability at speed, and that was
what I had primarily been looking at.

More rake, as defined by their drawing, would *decrease* trail; not good in my opinion.

The rake statement does make sense in a motorcycle context, where raking forks means to lay back the
head tube at a slacker angle.

In the old chopper days there was raking and extending of forks, any bending was by accident. More
cosmetic than functional longer forks required more rake to keep the height of the front end down
and reduce weight transfer back.

Front wheels would flop side to side at low speed, run straight at high speed and corner poorly at
all speeds.

Marcus

Tim McNamara said:

So here's a question.

It seems to me that rake is only part of the equation in handling. It's the factor that Jones
famously analyzed in developing the URB (unrideable bike). But surely a bike with a 75 degree head
angle and a bike with a 71 degree head angle will handle differently, even if fork rake is adjusted
so that trail is the same.

I remember reading about Gary Helfrich (godfather of ti) doing a test once. He built several bikes,
all identical, but with head tube angles ranging from 72 to 74 degrees. It turns out that no one did
very well at guessing which was which, but he could get most people to guess any of 'em were the 72
by letting 5psi out of the front tire.

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

Marcus Coles may have said:

The rake statement does make sense in a motorcycle context, where raking forks means to lay back
the head tube at a slacker angle.

That's the usage I'm familiar with as well; "rake" was a measure of angle from vertical, not axle
offset from steer tube centerline.

Quoted message said:

In the old chopper days there was raking and extending of forks, any bending was by accident. More
cosmetic than functional longer forks required more rake to keep the height of the front end down
and reduce weight transfer back.

There are still people building these, but not as often. It appears that either sanity may have
struck the world of aftermarket Harley fashion, or the old chopper enthusiasts have finally started
to die out.

Quoted message said:

Front wheels would flop side to side at low speed, run straight at high speed and corner poorly at
all speeds.

That's what I recall as well. Oh, and braking was a wish and a prayer; the weight transfer forward
onto the too-small wheel and flexy forks ensured that neither wheel would have a grip for stopping.

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(TBGibb) said:

In article <[email hidden]>, Shane Halpin

Quoted message said:

This to decide on how to make mine much less twitchy by changing 2 / 3

Touring bike = steering heaven.

Tom Gibb <[email hidden]>

Thank you all for your replies - particularly Phil. I've taken measurements of the critical points
and have come up with a calculation of trail of about 60mm (+/-2). Now I know its somewhere near the
putative "sweet spot" all signs of twitchiness have spookily disappeared - I can ride hands off and
get "eats" from my back pockets etc. all day long - the power of suggestion (and the feebleness of
my mind!!!) - and all this from a man who can accurately discriminate between frame materials,
tyres, even frame colours, in a blind test (but only when riding responsive & stiff boutique wheels)
- the irony, the irony! ;-)

Once again - many thanks

Shane