Steering Geometry Explained

[John Morgan]

Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try
to explain.

By changing the headtube angle, what you are really doing is changing the distance between the
contact patch of your tire and the extended steering axis (this is called "trail"). Imagine a
straight line going through your steerer tube, past your fork and hitting the ground. If you
lengthen the "trail" distance your steering will be slower and more self-centering, and if you
shorten this distance your steering will be quicker and less self-centering.

So what about fork rake? Increasing the rake has the same effect as steepening your headtube angle.
In all the suspension forks I see, the hub is offset in front of the slider tubes to make up for the
fork legs being straight. This allows for less distance between the tire contact patch and the
steering axis intersection.

To answer the original question, the reason the headtube angle makes such a big difference is
because when the trail is large, the centering force is large (the force that makes the wheel want
to point straight forward), when the trail is small, the centering force is small. Now if you hit a
rock that deflects your wheel to one side, a bike set up with a slack steering angle will be easier
to handle because the force of the rock is counteracted by a greater centering force on the wheel.

So now the question I put to you all is this: why does a steep headtube angle enable you to make
sharper turns?

-John Morgan research: http://www.phred.org/~josh/bike/trail.html

 

[Bill Wheeler]

On Fri, 4 Apr 2003 00:13:16 -0700, "John Morgan" <[email protected]> wrote:

>Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try
>to explain.

[snip nice try]

too many words

Simply get used to the bike and it's handling capabilities, and adjust yours accordingly. Most
people should be able to handle any bike on any trial. It's a matter of learning to ride.

Peace, Bill The mind serves properly as a window glass rather than as a reflector, that is, the mind
should give an immediate view instead of an interpretation of the world.
:-]

 

[G.T.]

Bill Wheeler wrote:
> On Fri, 4 Apr 2003 00:13:16 -0700, "John Morgan" <[email protected]> wrote:
>
>
>>Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try to
>>explain.
>
>
> [snip nice try]
>
> too many words
>
> Simply get used to the bike and it's handling capabilities, and adjust yours accordingly. Most
> people should be able to handle any bike on any trial. It's a matter of learning to ride.

Yep, it's a complex subject which makes my head hurt. Over the years I've discovered what works and
what doesn't. Bike geometry is pretty similar across models so most can be ridden well.

Greg

--
"Destroy your safe and happy lives before it is too late, the battles we fought were long and hard,
just not to be consumed by rock n' roll..." - The Mekons

 

[Miles Todd]

On Sat, 05 Apr 2003 16:45:53 GMT, G.T. <[email protected]> wrote:

> Bill Wheeler wrote:
>> On Fri, 4 Apr 2003 00:13:16 -0700, "John Morgan" <[email protected]> wrote:
>>
>>
>>> Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try to
>>> explain.
>>
>>
>> [snip nice try]
>>
>> too many words
>>
>> Simply get used to the bike and it's handling capabilities, and adjust yours accordingly. Most
>> people should be able to handle any bike on any trial. It's a matter of learning to ride.
>
> Yep, it's a complex subject which makes my head hurt. Over the years I've discovered what works
> and what doesn't. Bike geometry is pretty similar across models so most can be ridden well.
>
> Greg
>

Years and years ago, Otis Guy (look him up if you don't know who he is) built a series of frames
incrementally steeper head angles, starting at 60 degrees and finishing the serias at 90 degrees. He
tested all of them extensively for rideability. His conclusion? You can learn to ride anything. The
angles that seemed to work best were in the 67-74 degree range, though.

Miles

--
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

 

[John Morgan]

> >Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try to
> >explain.
>
> [snip nice try]
>
> too many words
>
> Simply get used to the bike and it's handling capabilities, and adjust yours accordingly. Most
> people should be able to handle any bike on any trial. It's a matter of learning to ride.
>
> Peace, Bill

LOL... well thanks for at least calling my try "nice." I got thinking about the subject because I
need to do a math project. This seemed like a reasonable topic since it has something to do with
trigonometry and physics, so I did a little research. Now I just need to find a simpler way to
present it to the class.

-John Morgan

 

[Chris]

"John Morgan" <[email protected]> wrote in message news:Wcaja.631$6t.476@fed1read05...

snippity snippity

> So now the question I put to you all is this: why does a steep headtube angle enable you to make
> sharper turns?
>
> -John Morgan research: http://www.phred.org/~josh/bike/trail.html

John -

Look for this month's (or quarter's) Asphalt magazine...it's all road, and expensive, but there is a
bike gemoetry section that is very similar to your post, only even more in depth. And there's
pictures.

It's very, very deep...so much so, I can't imagine riders get anything from it - just fledgling
frame builders.

My opinion: do all the math regarding bike you want. In the end, in comes down to whether or not an
arbitrary tester (out of a 6B strong pool) has the proportions to agree with your desgin.

Chris

 

[Westie]

"John Morgan" <[email protected]> wrote in message news:Wcaja.631$6t.476@fed1read05...
> Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try to
> explain.
>
> By changing the headtube angle, what you are really doing is changing the distance between the
> contact patch of your tire and the extended steering axis (this is called "trail"). Imagine a
> straight line going through your steerer tube, past your fork and hitting the ground. If you
> lengthen the "trail" distance your steering will be slower and more self-centering, and if you
> shorten this distance your steering will be quicker and less self-centering.
>
> So what about fork rake? Increasing the rake has the same effect as steepening your headtube
> angle. In all the suspension forks I see, the
hub
> is offset in front of the slider tubes to make up for the fork legs being straight. This allows
> for less distance between the tire contact patch
and
> the steering axis intersection.
>
> To answer the original question, the reason the headtube angle makes such
a
> big difference is because when the trail is large, the centering force is large (the force that
> makes the wheel want to point straight forward),
when
> the trail is small, the centering force is small. Now if you hit a rock that deflects your wheel
> to one side, a bike set up with a slack steering angle will be easier to handle because the force
> of the rock is
counteracted
> by a greater centering force on the wheel.
>
> So now the question I put to you all is this: why does a steep headtube angle enable you to make
> sharper turns?
>
> -John Morgan research: http://www.phred.org/~josh/bike/trail.html
>
>
Thanks John. A nice little tidbit of information to get the ol' grey matter going. I'll have a think
about the question. So far I've got the answer to why you'd be able to turn faster, but I haven't
figured out sharper yet. Unless theres a smaller radius from the "virtual" pivot of where the back
tyre contacts the road through to "outside of the circle" that the front tyre makes..... or
something. LOL!
--
Westie

 

[Van Bagnol]

In article <Wcaja.631$6t.476@fed1read05>, "John Morgan" <[email protected]> wrote:

> So what about fork rake? Increasing the rake has the same effect as steepening your headtube
> angle. In all the suspension forks I see, the hub is offset in front of the slider tubes to make
> up for the fork legs being straight. This allows for less distance between the tire contact patch
> and the steering axis intersection.

Er, I think it's the other way around. A greater rake puts the contact patch _closer_ to the
steering axis, reducing the self-centering steering force.

A greater rake, however, increases the _gravitational_ self-centering force, where the downward
weight from the fork tube helps swivel the wheel straight by "hanging" from the axle hub.

Van

--
Van Bagnol / v a n at wco dot com / c r l at bagnol dot com ...enjoys - Theatre / Windsurfing /
Skydiving / Mountain Biking ...feels - "Parang lumalakad ako sa loob ng paniginip" ...thinks - "An
Error is Not a Mistake ... Unless You Refuse to Correct It"