29" vS 26" MTB

In article <[email hidden]>
Roger <[email hidden]> wrote:
<snip>

Quoted message said:

Yes, but this is not the scenario ;-)

Of course it is, you're just confused.

Quoted message said:

I have allready been here, and done that (drawings). Now, go and do
your drawings with the BB (and hence ideal rider position) at the same
height from the ground but with the different sized wheels. Tip up your
drawings and you will see that you can tilt the 29er further before the
Center of mass is over the axle, which is what you are pivoting around
when you pull a wheelie or go over the bars.

But the bike doesn't stand on its axles, it stands on its tyres. The
only important things are contact points: tyres, bars, seat, cranks -
how you join them together is irrelevant from a geometry point of view.

Simon Brooke said:

in message <[email hidden]>, tim

(') said:

Part of me is wanting to jump up and down shouting "No! No!
No!"
but my A-level physics is now too rusty to be sure I'm right🙂 My
understanding of physics suggests that for the rider to topple
backwards climbing or to go OTB descending is dependant (at a simple
level) only on the CofG of bike & rider and it's position relative to
the downhill contact patch. If the CofG is behind the downhill contact
patch the bike and rider are going to start to topple. The position of
the axle is irrelevant.

Could you explain to me where my understanding is wrong?

You don't start to topple providing there's no braking. You and the bike
just effectively freefall together.

I did point this out in the OP. The problems come when you hit an
obstacle such as a root or rut, or when you have to brake to make a
sharp turn. There is a track a couple of miles from here which is full
of these sudden drops with sharp turns at the bottom which I cannot
manage. Two weeks ago there was a regional championship race on that
track. The experts would literally brake on the front going down the
drops to the point of having the back wheel suspended in the air just
on the point of going over, then they would drop it just before going
into the curve. But I also noticed that they were so far back on the
bike that they had to drop the back and get forward in a single move in
order to be able to turn. The less expert riders simply failed to turn
correctly.

BTW, I know the guy who designed that track, he is very short ;-)

Simon Brooke said:

Are you sure? They're quoted as Etrto 54-622. 622 sounds like 700c to me.

700c is a tyre of OD 700mm and a specific width. The rims it sits on
are 622mm and are different to 700a and 700b, both of which are smaller
diameter as the tyre is fatter. The term 700c is obsolete but happens
to coincide nicely with 25-622 (IIRC).

So technically, 700c are 622 but not all 622 are 700c. The difference
between 29" and 'normal' road tyres is the width of the rim, the 29"
being more suitable for fat tyres and unsuitable for thin ones.

...d

Anthony Jones said:

Roger said:

Contact patch is directly below the axle.

Eh? Are you sure about that?

No :-) It is wrong, most of the time.

Quoted message said:

No, the contact point on your diagram is where the dotted black line
intersects with the ground. Perhaps you should draw the ground on your
diagrams to make this clearer.

Yes, I am realising there is a lot wrong with my explanantion. But
nontheless, the axle around which we are pivoting is further out and
higher up, the rider stays in the same position realtive to the ground
because the is determined by the BB position, which is the same height
from the ground as on a 26".

Quoted message said:

I suspect you're confusing yourself by making this far more complicated
than it is.

I think I am confusing people by giving a bad explanation. I wrote this
after reading many articles about 29er's were it was common knowledge
that 29" BB's are at the same height as 26" ones.

Anthony Jones said:

Roger said:

I have allready been here, and done that (drawings). Now, go and do
your drawings with the BB (and hence ideal rider position) at the same
height from the ground but with the different sized wheels. Tip up your
drawings and you will see that you can tilt the 29er further before the
Center of mass is over the axle, which is what you are pivoting around
when you pull a wheelie or go over the bars.

No it isn't, you're pivoting about the contact point.

Anthony

You know I think you are right!

Will go away and think some more. Part of my problem is I have
experimented in the garage, tilting the bike up on a horizontal floor,
likewise for my drawings. Pulling a wheelie is not the same as going
ENDO. Back to the drawing board!

in message <[email hidden]>, Peter Clinch

(') said:

Rob Morley said:

How many people ride 26" rigid off-road these days?

Those of us that bought our MTBs when suspension of any sort cost Real
Money, and don't do enough off-road to justify an upgrade! ;-/

I'm planning to go back to fully rigid off road (not all the time, just
some of the time) because fully rigid really shows up (and teaches) bike
handling skills.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; It's dangerous to be right when the government is wrong.
;; Voltaire RIP Dr David Kelly 1945-2004

in message <[email hidden]>, Roger

(') said:

Tim Izod said:

Your diagrams indicate to me that in both cases the contact
patches are the same and CofG is in the same place.

Contact patch is directly below the axle.

Not when you're on a slope it isn't. On a downward slope, the contact
patch is behind the axle by r * cos( theta). On an upward slope, ahead
of the axle by the same amount.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Usenet: like distance learning without the learning.

in message <[email hidden]>, Roger

(') said:

Simon Brooke said:

in message <[email hidden]>,

Roger (') said:

There has been a lot of discussion flying around about this issue,
often neglecting to mention perhaps one of the most significant
effects, the geometric differences on steep slopes.

This article attempts to redress the balance:

http://users.libero.it/irwin/C1/c1.html

H'mmm... this is nonsense, and not very useful nonsense either.

Let us assume for the sake of argument that we have two XC bikes, both
of which have minimal chainstay length, one of which has '26"' wheels
(559mm), and one of which has '29"' (622mm). When the bike is
horizontal, the hub of the '29" bike is 31.5mm - an inch and a quarter
- behind that of the '26"' bike. But when the bike is tilted up at 30
degrees it is 31.5mm * cos( 30) = 27.27mm or just over one inch. More
significant movements to the centre of gravity can be made by
adjusting your saddle clamp, or, as other people have pointed out, by
lengthening the chain stays (few if any mountain bikes have
minimum-length chainstays, anyway).

Like everybody else, you are looking from a horizontal viewpoint. The
difference is in the vertical. I took it for granted that everybody
knows that the BB height on a 29" is the same as a 26".

Yes, we know that. It doesn't help your argument any.

Quoted message said:

Given that the rider is to be put in his ideal riding position then his
weight will be lowwer wrt the axle height.

Yes, but the axle height is irrelevant for these ascending and descending
problems. What matters is the contact patch, and the contact patch is
always at a height of, errr, zero. D'oh!

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/
Wannabe a Web designer?
<URL:http://userfriendly.org/cartoons/archives/97dec/19971206.html>

Simon Brooke said:

I'm planning to go back to fully rigid off road (not all the time, just
some of the time) because fully rigid really shows up (and teaches) bike
handling skills.

I think a concentrated bout of rigid is what you need. And I'll swap
you my EBC Contour 400 for your Jekyll just to make sure you don't
slack! ;-)

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

David Martin said:

Simon Brooke said:

Are you sure? They're quoted as Etrto 54-622. 622 sounds like 700c to me.

700c is a tyre of OD 700mm and a specific width.

Actually, they're not... the 700 doesn't refer to real millimetres,
they're 'pseudo-millimetres', 1/25 inch. 700C was a way of producing a
metric-looking measurement for 28" tyres. 28 x 1-3/4 in fact.

Quoted message said:

The rims it sits on
are 622mm and are different to 700a and 700b, both of which are smaller
diameter as the tyre is fatter.

No, wrong way round, A and B are smaller tyre sections, so they use
larger rims. 700B is the same as the old roadster 28 x 1-1/2, which is
a 635 rim.

Quoted message said:

The term 700c is obsolete

Try telling the French that!

Quoted message said:

but happens
to coincide nicely with 25-622 (IIRC).

That'd be 700x25C. Straight 700C would be 45-622 (because C relates to
a section of 1-3/4"😉.

Quoted message said:

So technically, 700c are 622 but not all 622 are 700c.

Well, all 622 are 700x(something)C

Quoted message said:

The difference
between 29" and 'normal' road tyres is the width of the rim, the 29"
being more suitable for fat tyres and unsuitable for thin ones.

But I think this all means that, at last, I will be able to get some
tyres to fit the original rims of my 1921 James tandem - time to think
about putting them back on instead of the more modern 635s it's fitted
with at the moment.

--
Andrew

David Martin said:

Simon Brooke said:

Are you sure? They're quoted as Etrto 54-622. 622 sounds like 700c to me.

700c is a tyre of OD 700mm and a specific width.

Actually, they're not... the 700 doesn't refer to real millimetres,
they're 'pseudo-millimetres', 1/25 inch. 700C was a way of producing a
metric-looking measurement for 28" tyres. 28 x 1-3/4 in fact.

Quoted message said:

The rims it sits on
are 622mm and are different to 700a and 700b, both of which are smaller
diameter as the tyre is fatter.

No, wrong way round, A and B are smaller tyre sections, so they use
larger rims. 700B is the same as the old roadster 28 x 1-1/2, which is
a 635 rim.

Quoted message said:

The term 700c is obsolete

Try telling the French that!

Quoted message said:

but happens
to coincide nicely with 25-622 (IIRC).

That'd be 700x25C. Straight 700C would be 45-622 (because C relates to
a section of 1-3/4"😉.

Quoted message said:

So technically, 700c are 622 but not all 622 are 700c.

Well, all 622 are 700x(something)C

Quoted message said:

The difference
between 29" and 'normal' road tyres is the width of the rim, the 29"
being more suitable for fat tyres and unsuitable for thin ones.

But I think this all means that, at last, I will be able to get some
tyres to fit the original rims of my 1921 James tandem - time to think
about putting them back on instead of the more modern 635s it's fitted
with at the moment.

--
Andrew

Roger said:

There has been a lot of discussion flying around about this issue,
often neglecting to mention perhaps one of the most significant
effects, the geometric differences on steep slopes.

This article attempts to redress the balance:

http://users.libero.it/irwin/C1/c1.html

All I get is a page that says
"Article supressed because it is wrong!!! "
I find this somewhat disturbing.
--

Martin Bulmer

Martin Bulmer said:

Roger said:

There has been a lot of discussion flying around about this issue,
often neglecting to mention perhaps one of the most significant
effects, the geometric differences on steep slopes.

This article attempts to redress the balance:

http://users.libero.it/irwin/C1/c1.html

All I get is a page that says
"Article supressed because it is wrong!!! "
I find this somewhat disturbing.
--

Martin Bulmer

It was auto suppressed. Look back in the the thread and you will se
were Antony Jones finally convinced me that there was something wrong.

Not that I have abandoned the concept, or rather I am still determined
to find out the optium criteria for good (steep) climbing/descent. My
own experience with two bikes with different wheelbases tells me it is
not simply a case of lenghning the chainarm. Allthougth perhaps next
time I arrive at a genial conclusion I shall do a bit of peer revue!